|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online 21 May 2008
doi: 10.1242/dev.017905
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
,
1 Gonda Department of Cell and Molecular Biology, House Ear Institute, 2100 West
3rd Street, Los Angeles, CA 90057, USA.
2 Department of Cell and Neurobiology, Keck School of Medicine, University of
Southern California, Los Angeles, CA 90033, USA.
Author for correspondence (e-mail:
akgroves{at}bcm.edu)
Accepted 12 May 2008
 |
SUMMARY |
|---|
TOP
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES |
|---|
Key words: Mouse, Otic placode, Wnt, β-Catenin, Notch1, Jagged 1, Inner ear
|
INTRODUCTION |
|---|
|
TOP
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES |
|---|
;
Groves, 2005;
Riley and Phillips, 2003;
Torres and Giraldez, 1998).
Members of the fibroblast growth factor (FGF) family play a crucial role in
inducing the otic placode in all vertebrates examined
(Friesel and Brown, 1992;
Ladher et al., 2005;
Leger and Brand, 2002;
Mackereth et al., 2005;
Maroon et al., 2002;
Phillips et al., 2001;
Vendrell et al., 2000;
Wright et al., 2004;
Wright and Mansour, 2003). FGF
signaling induces the expression of genes, such as Pax2 and
Pax8, in a broad region of cranial ectoderm stretching from
rhombomeres 3 to 6 (Maroon et al.,
2002; Martin and Groves,
2006; Wright and Mansour,
2003). Evidence from lineage tracing in chick
(Streit, 2002) and mouse
(Ohyama and Groves, 2004b;
Ohyama et al., 2006) suggest
that this broad Pax2+ domain, which we have referred to as
the `pre-otic field', contains cells fated to become otic and epibranchial
placodes, as well as cranial epidermis.
We recently showed that Wnt signaling plays an important role in defining
the size of the otic placode within this Pax2+ pre-otic
field. Wnt signaling is activated in a medial subset of the
Pax2+ domain closest to the hindbrain
(Ohyama et al., 2006).
Inactivation of Wnt signaling in Pax2+ cells by
conditional deletion of the β-catenin gene (Ctnnb1; also known
as Catnb) leads to a large reduction in the size of the otic placode
and a corresponding expansion of cranial epidermis. Conversely, activation of
Ctnnb1 in Pax2+ cells expands the otic placode at
the expense of cranial epidermis (Ohyama
et al., 2006). To date, however, it is not clear how Wnt signals
direct cranial ectoderm towards an otic fate. It is possible that
Lef/Tcf/β-catenin transcriptional complexes activated by Wnt signaling
directly regulate otic genes. Alternatively, Wnt signals might act indirectly
by upregulating short-range signals that partition cranial ectoderm into otic
placode and epidermis.
There is growing evidence that Wnt and Notch signaling pathways co-operate
during cell fate determination in many tissues
(Crosnier et al., 2006;
Estrach et al., 2006;
Fre et al., 2005). Notch
signaling plays various roles in patterning the inner ear, ranging from
specification of neurons and prosensory patches to the generation of the
stereotypical pattern of mechanosensory hair cells and supporting cells
(Adam et al., 1998;
Brooker et al., 2006;
Daudet et al., 2007;
Daudet and Lewis, 2005;
Haddon et al., 1998;
Kiernan et al., 2005;
Lanford et al., 1999;
Shi et al., 2005). Both the
Notch1 receptor and several of its ligands, such as jagged 1
(Jag1) and delta-like 1 (Dll1), are expressed in the otic
placode from very early stages (Abello et
al., 2007; Adam et al.,
1998; Daudet et al.,
2007; Groves and
Bronner-Fraser, 2000; Haddon
et al., 1998). Notch signaling might therefore also have an early
function during otic placode development. We now provide evidence that
elements of the Notch pathway are positively regulated by Wnt signaling, and
that Notch1 signaling can in turn modulate the canonical Wnt signaling
pathway. We also show that while some aspects of otic placode identity are
regulated only by Wnt signals, other features of placodal differentiation can
be regulated independently by Wnt or Notch pathways.
|
MATERIALS AND METHODS |
|---|
|
TOP
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES |
|---|
)
(available from the Mutant Mouse Regional Resource Center;
www.mmrrc.org/strains/10569/010569.html);
conditionally activated Notch1 intracellular domain [cN1ICD
(Murtaugh et al., 2003)];
Notch1-null mutants (Conlon et
al., 1995); conditionally activated β-catenin
Catnblox(ex3) [cAct
(Harada et al., 1999)];
conditional β-cateninfloxed mutants
[β-cat-CKO (Brault et al.,
2001)]; Tcf/Lef Wnt reporter
(Mohamed et al., 2004);
conditional Rbpj/Rbsuh mutants
(Tanigaki et al., 2002); and a
GFP-expressing Cre reporter [Z/EG
(Novak et al., 2000)]. To
generate cN1ICD animals, N1ICDfloxed homozygotes
were crossed with Pax2-Cre animals. Age-matched heterozygotes and
wild types were used as controls for Notch1 mutant embryos. Detailed
mating strategies for cAct and β-cat-CKO mice have been
described previously (Ohyama et al.,
2006). To generate Notch1; cAct mutants, a line that was
heterozygous for Notch1; Pax2-Cre was crossed to animals that were
heterozygous for Notch1; cAct. To generate cN1ICD;
β-cat-CKO mutants, a line that was heterozygous for
β-cat-null; Pax2-Cre was crossed to animals that were
heterozygous for N1ICD and homozygous for a floxed allele of
β-catenin. For each mutant genotype, at least three embryos were
analyzed, except for Notch1; cAct mutants (n=2). All animal
experiments were done in accordance with the guidelines of the institution's
Animal Care and Use Committee.
Whole-mount in situ hybridization, immunostaining and detection of β-galactosidase
Whole-mount in situ hybridization was performed as previously described
(Ohyama et al., 2006). The
following probes were used: Notch1 (Jeffrey Nye), Dll1
(Achim Gossler), Jag1 (Tim Mitsiadu), Hes1 and Hes5
(Ryoichiro Kageyama), lunatic fringe (Lfng; Thomas Vogt) and
Wnt6 (Andrew McMahon). Probes for Pax2, Pax8, Foxi2, Dlx5,
Krox20, Hoxb1, Fgf3, and Epha4 have been previously described
(Ohyama et al., 2006). Embryos
were embedded in 15% sucrose and 7% gelatin in phosphate-buffered saline
(PBS), as previously described (Groves and
Bronner-Fraser, 2000), and 15- to 30-µm thick sections cut
using a Leica CM 1850 cryostat. Immunostaining and detection of
β-galactosidase on cryostat sections and embryos was performed as
previously described (Ohyama et al.,
2006). The following primary antibodies were used: β-catenin
(Zymed) at 1:200; activated Caspase-3 (R&D Systems) at 1:1000; green
fluorescent protein (GFP) conjugated to fluorescein (Abcam) at 1:250 to 1:500;
β-galactosidase (ICN/MP Biochemicals) at 1:100; jagged 1 (Jag1; Santa
Cruz) at 1:50 to 1:100; Pax2 (Zymed) at 1:500; and phospho-histone-H3 (PH3;
Upstate/Millipore) at 1:1000. Secondary goat anti-rabbit antibody conjugated
to Alexa 594 (Molecular Probes) was used at 1:200. Sections were
counterstained with the nuclear marker DAPI (Molecular Probes). All images
were captured using a Zeiss Axiocam digital camera and Axiophot2 or
M2 Bio microscopes, and were processed using Adobe Photoshop CS
software.
Quantification of thickened placode and average placode cell density in Notch1 mutants
The thickened otic placode was defined as the two- to three-cell layer of
ectoderm located adjacent to rhombomere 5/6 (as identified morphologically
with DAPI staining and/or by lack of for Foxi2 expression).
Quantifications of placode size were made from 15-µm serial sections from
Notch1 mutants and age-matched control embryos. Length measurements
were made using Image J software. To allow for direct comparisons along the
anteroposterior (AP) axis of control and mutant mice, measurements were binned
into five categories: 0-20% (being the most anterior sections), 21-40%,
41-60%, 61-80% and 81-100% (being the most posterior sections). For a given
genotype, each bin consisted of multiple sections from several embryos. The
mean and standard error of the mean (s.e.m.) were calculated for each bin.
Non-parametric Mann-Whitney U-tests were performed to test for
significance between genotypes. The cranial region of Notch1 mutants
was comparable in size to controls. To confirm this, we measured the
dorsoventral (DV) length of the neural tube adjacent to the otic placode. The
measurements were processed as described for the otic placode. We found no
differences in neural tube length between Notch1 mutants and controls
(data not shown). For average density measurements, serial 15-µm thick
sections stained with DAPI and/or hybridized with Dlx5 or
Foxi2 probes were used. The cell density for each section was
calculated as follows: number of cells/µm2x500 and was
pooled for each genotype.
Quantification of cell proliferation, otic cup length and Wnt reporter domain length in cN1ICD mutants
Cell proliferation counts were performed as described previously
(Ohyama et al., 2006). To
account for variations in the staging of embryos, the mediolateral length of
the otic cup or Wnt domain was standardized against the DV neural tube length
adjacent to the otic cup and expressed as a percentage. Only mid-sections from
otic cups were used for quantification and Student's t-tests were
performed to test for significance between genotypes.
|
RESULTS |
|---|
|
TOP
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES |
|---|
; Ohyama et al.,
2006)]. Pax2 expression later becomes restricted to the
otic placode, which is morphologically visible as a thickening patch of
ectoderm next to rhombomeres 5 and 6 from the 8ss (E8.5) onwards
(Ohyama and Groves, 2004b). To
see whether elements of the Notch pathway were expressed at an appropriate
place and time to participate in otic placode induction, we compared the
expression patterns of Notch1, Jag1, Dll1, Hes1, Lfng and
Hes5 to Pax2 (Fig.
1). At 0-1ss, no Notch pathway transcripts were detected in the
pre-otic field (Fig. 1B,C).
Onset of Notch1 expression was observed as early as the 4ss, becoming
stronger by 5-7ss (Fig. 1C).
Scattered cells in the anterior Pax2 domain adjacent to the neural
tube expressed the Notch ligand Jag1 from around 5ss, although
posterior cells did not express Jag1 until 8-9ss (arrowhead,
Fig. 1C,D). Dll1 was
also expressed adjacent to the neural tube at 4-5ss
(Fig. 1B,C), and was restricted
to the otic placode from 9ss (Fig.
1C). Between 12 and 14ss, Dll1 expression was gradually
restricted to differentiating neuroblasts in the anteroventral placode (data
not shown) (Adam et al., 1998).
We were unable to detect Lfng at the pre-otic field and placode
stages in whole mounts (data not shown), although, at later otic vesicle
stages, Lfng is expressed in the anteroventral portion of the otic
cup destined to produce the vestibuloacoustic ganglion and the utricular and
saccular maculae (Morsli et al.,
1998; Raft et al.,
2004).
Hes1 and Hes5 are effectors of the Notch pathway that
function in many processes, including in the regulation of cell fate decisions
(Bray, 1998;
Kageyama et al., 2007;
Lai, 2004). Hes1
expression was scattered throughout the pre-otic field and by 10-11ss was
restricted to the otic placode (Fig.
1C). We found no evidence for Hes5 expression in the
pre-otic field (data not shown). These data suggest that at least some
transcriptional targets of the Notch pathway are expressed during early phases
of otic placode development.
We previously used a transgenic Wnt reporter mouse line
(Mohamed et al., 2004) to show
that the canonical Wnt signaling pathway is activated in the pre-otic field
between 3 and 5ss (Ohyama et al.,
2006). Several Wnt family members are expressed in an appropriate
location to trigger the observed Wnt reporter activity - for example,
Wnt8 is expressed in rhombomere 4
(Ohyama et al., 2006). We also
observed Wnt6 expression in the Pax2+ pre-otic
field at 0ss. It continues to be expressed in the neural folds at 5-7ss and in
the dorsal-most region of the otic placode at 11ss
(Fig. 1C)
(Lillevali et al., 2006). As
the onset of Notch pathway gene expression closely corresponded to Wnt
reporter activity in the pre-otic field
(Fig. 1B,C), we hypothesized
that the Notch1 pathway might interact with the canonical Wnt signaling
pathway in mediating the fate decision between otic placode and epidermis.
Notch pathway components are positively regulated by canonical Wnt signaling in the developing otic placode
Previous studies suggest that Notch pathway components can be regulated by
β-catenin (e.g. Estrach et al.,
2006; Katoh and Katoh,
2006). We therefore examined expression of Notch pathway genes in
embryos carrying gain- or loss-of-function mutations of the canonical Wnt
pathway in the Pax2+ pre-otic field. We crossed
Pax2-Cre transgenic mice (Ohyama
and Groves, 2004b) with mice in which β-catenin is
constitutively activated in Cre-expressing cells [cAct
(Harada et al., 1999)], and
examined expression of Jag1, Notch1 and Hes1. In
cAct mutants, Jag1 expression was expanded ventrally to the
level of the pharynx at the 9-10ss (bracket,
Fig. 2A) and this ectopic
expression continued until at least E9.0. Jag1 is thought to be a
direct target of β-catenin, as its promoter region contains five, three
and six consensus Tcf/Lef-binding sites in mouse, human and rat, respectively
(Estrach et al., 2006;
Katoh and Katoh, 2006). The
domain of Notch1 and Hes1 expression was also expanded,
although only after a delay (from the 14-15ss;
Fig. 2A, brackets). Such a
delayed induction of Notch1 and Hes1 relative to
Jag1 has also been observed in epidermis in which β-catenin is
activated (Ambler and Watt,
2007). Other Notch pathway genes, such as Dll1, Hes5 and
Lfng, were not expressed in cAct mutants (data not
shown).
|
)]. The
Jag1 domain was significantly reduced at 10-11ss, as we have
previously reported for Pax2 and Pax8
(Ohyama et al., 2006). Many
cells within the vestigial β-cat-CKO otic vesicle were
β-catenin-;Jag1-, suggesting that Wnt signaling is
directly responsible for Jag1 induction in the placode (bracket,
Fig. 2B). Close examination of
the vestigial mutant vesicles at E9-E9.5 revealed that cells expressing Jag1
were β-catenin+ and had therefore failed to undergo
Cre recombination (Fig. 2B).
The domains of Notch1 and Hes1 were also significantly
reduced, although the resolution of the whole-mount in situ technique made it
difficult to determine whether all Notch1- and
Hes1-expressing cells also expressed β-catenin protein
(Fig. 2B).
Wnt and Notch signaling pathways differentially regulate expression of otic markers
The expression of Notch pathway genes in the pre-otic field and otic
placode, together with the regulation of these genes by Wnt signaling
suggested that Notch signaling might participate in the fate decision between
otic placode and epidermis. To test this, we conditionally activated Notch1 in
the pre-otic field using mice in which the active, intracellular domain of
Notch1 receptor (N1ICD) was knocked into the ROSA26 locus with a
transcriptional STOP cassette flanked by LoxP sites
(Murtaugh et al., 2003). We
drove expression of N1ICD in the Pax2+ pre-otic
field using Pax2-Cre mice (Ohyama
and Groves, 2004b). The Pax2-Cre mouse line expresses Cre
recombinase in the midbrain and rhombomere 1 (R1) of the hindbrain
(Ohyama and Groves, 2004b).
Conditionally activated N1ICD (cN1ICD) mutants displayed an
open neural tube phenotype at the level of the midbrain-R1 region, which is
likely to result from overproliferation of precursor cells induced by Notch
activation. However, the patterning of the posterior hindbrain next to the ear
was normal at E8.5-E9.5, based on the expression of Hoxb1 (rhombomere
4), Fgf3 (rhombomeres 5 and 6), Epha4 and Krox20
(rhombomeres 3 and 5; see Fig. S1B in the supplementary material), suggesting
that any otic placode phenotype in cN1ICD mutants is not due to
changes in the adjacent hindbrain.
|
). By E8.75-E9, Foxi2 expression was reduced
dramatically in cN1ICD mutants when compared with controls (dotted
outline and brackets, Fig.
3C,C'), complementing the expansion of the thickened
epidermis. To determine whether cell proliferation was responsible for the
expanded placode, we examined expression of the M-phase marker
phosphohistone-3 (PH3) in cN1ICD embryos (n=10 placodes) and
control embryos (n=8 placodes) produced by crossing the
Pax2-Cre line with the Cre-inducible Z/EG GFP-expressing
line (Novak et al., 2000). We
saw no significant differences in total PH3+ or
PH3+;GFP+ cell counts per section (see Fig. S1D in the
supplementary material).
The expansion of Pax8 at the expense of Foxi2 in
cN1ICD embryos is strikingly similar to that seen in embryos in which
the canonical Wnt pathway is activated [cAct embryos
(Ohyama et al., 2006)].
However, in contrast to cAct embryos, we saw only a modest expansion
of the Pax2 domain (bracket and arrowhead,
Fig. 3B,B'; see also Fig.
S1C in the supplementary material), and no expansion of the otic markers
Gbx2 or Sox9 (Fig.
3D). Finally, a marker of the dorsolateral otocyst, Hmx3,
which does not require either Wnt or Hedgehog signaling for its expression
(Ohyama et al., 2006;
Riccomagno et al., 2002) was
also not expanded in cN1ICD mutants
(Fig. 3D).
These results suggest that different aspects of otic placode development
are differentially regulated by Wnt and Notch signaling. Placode markers such
as Pax2, Gbx2 and Sox9 appear to be regulated by Wnt
signaling (Ohayama et al., 2006;
Saint-Germain et al., 2004),
but not Notch signaling, whereas markers such as Pax8, the
morphological thickening of epithelium and the repression of the epidermal
marker Foxi2 can be regulated by both Notch and Wnt signals. To
determine whether Notch signaling can regulate these markers independently of
Wnt signaling, we analyzed β-cat-CKO;cN1ICD mutant embryos in
which β-catenin was inactivated and Notch1ICD was activated throughout
the pre-otic field. Mutant embryos displayed greatly expanded regions of
thickened placode-like epithelium that expressed both Pax8 and
Jag1 (Fig. 3E). This
expanded region of thickened epithelium was largely devoid of Foxi2
expression (Fig. 3E), although
occasional Foxi2+ patches of cells could sometimes be
detected. These results show that Notch and Wnt signals can independently
regulate some aspects of otic placode development.
Inactivation of Notch1 reduces the size of the otic placode
Our results show that Notch1 activation throughout the
Pax2+ pre-otic field expands some otic placode markers at
the expense of epidermis. In complementary experiments, we examined
Notch1 mutants, in which a substantial portion of the Notch1
gene is deleted [amino acids 1056-2049
(Conlon et al., 1995)]. This
deletion encompasses RAM and Ankyrin repeats required for RBPJ
signaling (Conlon et al.,
1995; Fortini and
Artavanis-Tsakonas, 1994;
Kurooka et al., 1998a;
Kurooka et al., 1998b;
Lamar et al., 2001;
Nam et al., 2003;
Tani et al., 2001). We
confirmed that posterior hindbrain patterning was normal in Notch1
mutants by assaying for Hoxb1, Fgf3 and Krox20 expression
(see Fig. S2A in the supplementary material). All three genes were expressed
normally, suggesting that any defects observed in otic placode development are
due to deficiency in Notch1 signaling in the placode, rather than in the
hindbrain.
|
The reduction in the size of the otic placode in Notch1 mutants
may result from increased apoptosis, increased cell density or a change in
cell fate. We measured the size of the placode by examining Foxi2
expression, which is precisely excluded from the thickened placode region. The
Notch1 mutant otic placode was indeed smaller at 9-13ss, on the basis
of Foxi2 expression (dotted outline,
Fig. 5A,B). We compared the
mediolateral extent of the thickened otic placode in Notch1 mutants
and controls at 9-11ss and 12-13ss (see Materials and methods;
Fig. 5C,D) Notch1
mutants (9-11ss, n=25 placodes; 12-13ss, n=10 placodes) had
significantly smaller placodes than did controls (9-11ss, n=13
placodes; 12-13ss, n=6 placodes), regardless of the axial level of
the section (P<0.05-0.005; Fig.
5C,D). There were no significant changes in placode cell density
at 9-11ss (n=10 mutant placodes; n=6 control placodes) and
12-13ss (n=5 mutant placodes; n=4 control placodes;
P>0.05; Fig. 5F),
or in apoptosis when analyzed for activated caspase 3 expression
(Fig. 5G)
(Conlon et al., 1995;
Del Monte et al., 2007). We
also confirmed that the smaller placode was not caused by the precocious
generation of neurons by analyzing Ngn1 expression (data not
shown).
|
|
; de la Pompa et al.,
1997), confirming that Notch signaling can modulate the size of
the otic placode but is not necessary for its induction.
Daudet and colleagues recently suggested that initiation, but not
maintenance, of Jag1 expression in the chick otic placode is
regulated independently of Notch1 signaling
(Daudet et al., 2007). We
confirmed this result in mice: Jag1 continued to be expressed in the
placode of Notch1 mutants, but the intensity of expression was
reduced when compared with controls (see Fig. S2B in the supplementary
material). It has been previously reported that Jag1 continues to be
expressed in a morphologically distinct otic placode in mice carrying
mutations of Pofut1, an O-fucosyltransferase essential for
Notch signaling (Shi and Stanley,
2003). We confirmed that Jag1 and Hes1
expression can be initiated in the absence of canonical Notch signaling by
examining conditional mutants of Rbpj/Rbsuh. Both genes
continue to be expressed in a morphologically visible otic cup, although
Hes1 was expressed at significantly reduced levels compared with
controls (see Fig. S2C in the supplementary material). This is consistent with
Hes1 expression being initiated by Notch signaling, but Jag1
expression being initiated independently of Notch signaling.
|
).
Similarly, conditional activation of Notch1 in the Pax2+
pre-otic field expands some, but not all, otic markers at the expense of
epidermis (Fig. 3).
Additionally, Notch pathway gene expression can be activated by canonical Wnt
signaling (Fig. 2). These
results suggested the possibility of reciprocal interactions between the Notch
and Wnt pathways. To test whether Wnt signaling is modulated by the Notch
pathway in the developing otic placode, we crossed Wnt reporter mice
expressing a β-galactosidase reporter gene under the control of six
Tcf/Lef DNA-binding sites (Mohamed et al.,
2004) to either cN1ICD or Notch1 mutant
lines.
Surprisingly, although the thickened Pax8+ placode was
dramatically expanded to the level of the pharynx in cN1ICD embryos
(Fig. 3A), Wnt reporter
activity showed a much more modest expansion, extending a little beyond the
lateral edge of the otic cup (Fig.
6A). We observed similar results with Dlx5, a known
Wnt-responsive marker of the otic placode (bracket,
Fig. 6A). To verify these
results, we made use of the fact that cN1ICD mutants also express
nuclear GFP after Cre recombination
(Murtaugh et al., 2003). We
co-immunostained cN1ICD;Wnt reporter embryos with
anti-β-galactosidase and anti-GFP antibodies to mark the extent of the
Wnt reporter and the expanded otic placode, respectively
(Fig. 6B). By E9-E9.25, Wnt
activity was elevated in the lateral regions of the mutant otic cup, which
normally demonstrate moderate or low Wnt activity (red arrowhead;
Fig. 6A). Furthermore, the otic
cup region was larger in cN1ICD mutants than in controls (see Fig. S3
in the supplementary material, n=13 mutant placodes, n=14
control placodes; P<0.005). However, the ectopic placode region
lateral to the otic cup, which expressed N1ICD and GFP, did not express
β-galactosidase (bracket, Fig.
6B). These results suggest that Notch signaling can augment Wnt
signaling, but that the active Notch1 ICD does not directly regulate
Wnt-responsive genes containing Tcf/Lef DNA-binding sites.
To test whether Wnt signaling can also be modulated by loss of Notch1 activity, we examined Notch1 mutant mice crossed to a Wnt reporter mouse background. As expected, Wnt reporter activity was detected in Notch1 mutant placodes (Fig. 6C). However, the intensity of Wnt activity, as measured by time-matched β-galactosidase reactions was weaker than in controls. Additionally, the mediolateral extent of the Wnt reporter and expression of the Wnt-responsive gene Dlx5 was slightly reduced (Fig. 6C), reflecting the observed reduction in the placode size caused by Notch1 deficiency (Fig. 4). Taken together with our data showing that Wnt signaling can upregulate Notch pathway components, our results are consistent with a model in which the Wnt pathway can positively regulate components of the Notch pathway, and can, in turn, be augmented by Notch signaling. One prediction of this model is that maximal activation of Wnt signaling by a constitutively activated β-catenin mutation will be unaffected by a Notch1 mutation. To test this, we analyzed Pax8 and Foxi2 expression in Notch1 mutant embryos that also carried the activated β-catenin (cAct) mutation. As expected, the size of the expanded Pax8 domain seen in cAct embryos was not significantly different from that in Notch1; cAct mutants (Fig. 6D). Similarly, the reduced domain of epidermal Foxi2 expression seen in cAct mutants was not significantly different from that in Notch1; cAct mutants (Fig. 6D).
|
DISCUSSION |
|---|
|
TOP
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES |
|---|
; Brooker et al.,
2006; Daudet et al.,
2007; Daudet and Lewis,
2005; Haddon et al.,
1998; Kiernan et al.,
2005; Lanford et al.,
1999; Shi et al.,
2005). Here, we have uncovered new roles for Notch and Wnt
signaling in the early development of the ear. Conditional activation of
Notch1 in the Pax2+ pre-otic field causes the
expansion of some, but not all, otic markers at the expense of epidermis.
Conversely, in the absence of Notch1 signaling, the otic placode is
significantly smaller. We have also shown that Wnt signaling regulates
components of the Notch pathway, such as Jag1, and that Notch
signaling positively regulates Wnt signaling. Our results suggest that Notch
augments the Wnt signaling pathway to help define the size of the otic
placode.
|
). Expression of these Notch pathway genes occurs only within
the region of the medial Pax2+ pre-otic field that
responds to Wnt signaling (Fig.
1C). Consistent with previous reports
(Duncan et al., 2005;
Espinosa et al., 2003;
Estrach et al., 2006), we
found that ectopic activation of the canonical Wnt pathway induced the
expression of Jag1, Notch1 and Hes1
(Fig. 2A), whereas conditional
deletion of β-catenin greatly reduced their expression
(Fig. 2B). Wnt signaling can
control the transcription of Notch pathway genes by directly acting on
elements located in their promoters
(Duncan et al., 2005;
Espinosa et al., 2003;
Estrach et al., 2006;
Katoh and Katoh, 2006). In the
case of the Jag1 promoter, there are multiple Tcf/Lef-binding sites
that are conserved between mouse and human
(Estrach et al., 2006;
Katoh and Katoh, 2006).
Putative Tcf/Lef-binding sites have also been identified in the
Notch1 promoter (Galceran et al.,
2004); however, although the Dll1 promoter also has
Tcf/Lef-binding sites (Galceran et al.,
2004), its expression was not expanded in embryos expressing
activated β-catenin. Recent evidence suggests that factors distinct from
Notch signaling are required to initiate Jag1 expression in the chick
otocyst (Daudet et al., 2007),
although maintenance of Jag1 is Notch dependent. Our results suggest
that Jag1 initiation in the developing ear might be regulated
directly by Wnt signaling (Fig.
2B, Fig. 7),
whereas Notch1 and Hes1 expression might be initiated by Wnt
signaling, and possibly also by FGFs
(Norgaard et al., 2003;
Zhou and Armstrong, 2007), in
addition to by Notch signaling itself.
Overlapping and distinct functions of Notch and Wnt signaling in the otic placode
We recently showed that the mouse pre-otic field defined by the expression
of Pax2 undergoes a fate decision to give rise to the cranial
epidermis and the otic placode (Ohyama et
al., 2006). The placode-epidermis fate decision is mediated by the
canonical Wnt pathway, such that conditional deletion of β-catenin in
Pax2+ cells drastically reduces the otic placode and
expands the epidermis, whereas conditional activation of β-catenin in
Pax2+ cells expands the otic placode at the expense of
epidermis (Ohyama et al.,
2006). In the light of the expression of many components of the
Notch signaling pathway in the developing otic placode
(Fig. 1), we hypothesized that
Notch signaling might act with the canonical Wnt pathway to specify otic
placode identity.
The activation of Notch1 signaling in the pre-otic field leads to a massive
expansion of thickened, placode-like epithelium expressing Pax8 at
the expense of Foxi2+ epidermis, in a manner very similar
to the activation of β-catenin (Fig.
3). By contrast, although Pax2 expression can be expanded
by the activation of Wnt signaling (Ohyama
et al., 2006), it showed only a modest expansion in
cN1ICD mutants compared with Pax8 (compare
Fig. 3A with
3B). Pax2 and
Pax8 are known to be differentially regulated by FGF signaling and
the foxi1 transcription factor during induction of the zebrafish ear
(Hans et al., 2004;
Nissen et al., 2003;
Solomon et al., 2003;
Solomon et al., 2004), and our
results suggest that these genes might also be differentially regulated by
Notch signaling. In particular, Pax8 can be regulated either by
canonical Wnt signaling or by Notch signaling. However, it is not clear
whether the two pathways regulate Pax8 in entirely different ways or
whether they converge on a nodal point, such as the binding of Lef/Tcf
complexes to the Pax8 promoter
(Schmidt-Ott et al., 2007).
Pax8 expression correlates with epithelial thickening in all
experiments in our study. However, further experiments are required to
determine whether Pax8 is directly responsible for regulating this
morphological change in the otic placode.
The examination of Notch1 mutants consistently showed a
significant reduction in the size of the otic placode
(Fig. 4B,C;
Fig. 5). This small reduction
is unlikely to be due to redundancy with other Notch genes, as there is no
detectable expression of Notch2-Notch4 in the otic placode
(Lewis et al., 1998;
Williams et al., 1995). A
similar persistence of the otic placode is seen after treating chick otic
ectoderm with DAPT, a 
-secretase inhibitor that abolishes Notch
signaling (Abello et al., 2007;
Daudet et al., 2007), in mice
carrying mutations in Pofut1, an O-fucosyltransferase that
is an essential component of the Notch pathway
(Shi and Stanley, 2003)
(C.S.J., unpublished), and in mice lacking Rbpj/Rbsuh/CSL
(Oka et al., 1995;
de la Pompa et al., 1997) (see
Fig. S2C in the supplementary material). In all of these experiments, any
reduction in placode size in the absence of Notch signaling is much more
modest than that seen in mice in which Wnt signaling is blocked by the
conditional deletion of β-catenin
(Ohyama et al., 2006)
(Fig. 2B).
Our results suggest a model (Fig. 7B,C) in which both Notch and Wnt signaling can specify the size of the epithelium destined to form the otic placode by virtue of their regulation of Pax8, Foxi2 and Jag1, and by the induction of a thickened epithelial morphology. Our data from mice in which Wnt signaling is activated in the absence of Notch1 (Fig. 6D), or in which Notch1 is activated in the absence of β-catenin (Fig. 3C), show that the two pathways can regulate these genes independently of each other. However, unlike the Wnt pathway, Notch signaling does not regulate the expression of otic placode-specific genes such as Gbx2, Sox9 and Hmx3, as these are unchanged in cN1ICD mutants (Fig. 3D). In addition, our results, taken together with previously published studies, suggest that Notch signaling also acts to augment Wnt signaling during otic placode induction, rather than being absolutely necessary for placode induction
Notch signaling acts to augment Wnt signaling during otic placode induction
To integrate our gain- and loss-of-function experiments with the Notch and
Wnt pathways, we propose a model in which some Notch pathway components, such
as Jag1, are induced by Wnt signaling. Subsequently, activation of
Notch1 by Jag1 feeds back to augment the Wnt response
(Fig. 7B). This feedback
activity has no effect on the most medial regions of the pre-otic field -
which receive the highest levels of Wnt signaling - but acts to increase Wnt
signaling in mediolateral regions of ectoderm that receive modest to low
levels of Wnt signaling. Thus, Notch-mediated feedback serves to sharpen and
refine the initial mediolateral gradient of Wnt activity during the pre-otic
field stage (Ohyama et al.,
2006) into a more binary pattern at the otic placode stage, where
Wnt signaling is either active (giving rise to the otic placode) or silenced
(giving rise to epidermis; Fig.
7C).
Our data support this model in four ways. First, Notch1 deficiency
causes a reduction in the area and intensity of β-galactosidase activity
in Wnt reporter mice, and a reduction of the domain of the Wnt-responsive gene
Dlx5 (Fig. 6C).
However, loss of Notch1 does not abolish the expression of either
marker, consistent with the notion that Notch1 signaling augments the Wnt
response but does not initiate it. Second, the reduction in Wnt signaling
resulting from the loss of Notch1 (Figs
4,
5) causes a consistent
reduction in the size of the otic placode, but does not eliminate it entirely.
The otic placode also forms in mice lacking other crucial components of the
Notch pathway, such as Pofut1 or Rbpj/Rbsuh/CSL
(Oka et al., 1995;
de la Pompa et al., 1997;
Shi and Stanley, 2003). Third,
mutation of Notch1 has no effect on the size of the otic placode in
embryos also expressing constitutively active β-catenin in the entire
pre-otic field (Fig. 6D),
presumably because cells expressing artificially high levels of activated
β-catenin are not dependent on Notch1 function for the stabilization of
otic fate. Finally, artificial N1ICD activation throughout the pre-otic field
greatly expands Pax8 to the ventral pharynx, but this is not the case
for Dlx5 or Wnt activity (Fig.
6A). This suggests that ectopic activation of N1ICD in regions of
the pre-otic field that receive no Wnt signals is insufficient to augment or
initiate the Wnt response (Fig.
7C). Furthermore, Wnt reporter expression is enhanced in regions
receiving moderate levels of Wnt activity in cN1ICD mutants
(Fig. 6A). Although, the
mechanism of how Notch signaling augments Wnt activity is not clear, this
result suggests that it is unlikely that N1ICD can directly activate
transcription of Wnt-responsive genes by itself. A growing body of evidence
suggests that Wnt and Notch pathways interact during cell fate determination
(Aoyama et al., 2007;
Arias and Hayward, 2006;
Crosnier et al., 2006;
Estrach et al., 2006;
Fre et al., 2005). Notch
signaling can act upstream of the Wnt pathway
(Balint et al., 2005;
Johnston and Edgar, 1998;
Neumann and Cohen, 1996), or
downstream (Estrach et al.,
2006). Stimulation of the Wnt pathway can either antagonize or
activate the Notch pathway in different contexts - for example,
dishevelled can antagonize Notch signaling
(Axelrod et al., 1996), whereas
the downregulation of Gsk3 activity by Wnt signaling stimulates the Notch
pathway (Espinosa et al.,
2003). The Notch receptor is also able to antagonize
β-catenin activity (Nicolas et al.,
2003), sometimes in an NICD-independent manner
(Hayward et al., 2006;
Hayward et al., 2005).
Taken together, our current and previously published data suggest a model of otic placode induction whereby FGF signaling initially establishes a Pax2+ pre-otic field that is then patterned by a gradient of Wnt signaling arising from the midline. Wnt signaling upregulates components of the Notch pathway, which then act locally to augment the Wnt response and to mediate the placode-epidermis fate decision in the pre-otic field.
Supplementary material
Supplementary material for this article is available at
http://dev.biologists.org/cgi/content/full/135/13/2251/DC1
|
ACKNOWLEDGMENTS |
|---|
|
Footnotes |
|---|
Present address: Department of Molecular and Human Genetics, and Department
of Neuroscience, Baylor College of Medicine, Houston, TX 77030, USA
|
REFERENCES |
|---|
|
TOP
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES |
|---|
Abello, G., Khatri, S., Giraldez, F. and Alsina, B.
(2007). Early regionalization of the otic placode and its
regulation by the Notch signaling pathway. Mech. Dev.
124,631
-645.[CrossRef][Medline]
Adam, J., Myat, A., Le Roux, I., Eddison, M., Henrique, D.,
Ish-Horowicz, D. and Lewis, J. (1998). Cell fate choices and
the expression of Notch, Delta and Serrate homologues in the chick inner ear:
parallels with Drosophila sense-organ development.
Development 125,4645
-4654.[Abstract]
Ambler, C. A. and Watt, F. M. (2007).
Expression of Notch pathway genes in mammalian epidermis and modulation by
beta-catenin. Dev. Dyn.
236,1595
-1601.[CrossRef][Medline]
Aoyama, K., Delaney, C., Varnum-Finney, B., Kohn, A. D., Moon,
R. T. and Bernstein, I. D. (2007). The interaction of the Wnt
and Notch pathways modulates Nk vs. T cell differentiation. Stem
Cells 25,2488
-2497.[CrossRef][Medline]
Arias, A. M. and Hayward, P. (2006). Filtering
transcriptional noise during development: concepts and mechanisms.
Nat. Rev. Genet. 7,34
-44.[CrossRef][Medline]
Axelrod, J. D., Matsuno, K., Artavanis-Tsakonas, S. and
Perrimon, N. (1996). Interaction between Wingless and Notch
signaling pathways mediated by dishevelled. Science
271,1826
-1832.[Abstract]
Balint, K., Xiao, M., Pinnix, C. C., Soma, A., Veres, I.,
Juhasz, I., Brown, E. J., Capobianco, A. J., Herlyn, M. and Liu, Z. J.
(2005). Activation of Notch1 signaling is required for
beta-catenin-mediated human primary melanoma progression. J. Clin.
Invest. 115,3166
-3176.[CrossRef][Medline]
Barald, K. F. and Kelley, M. W. (2004). From
placode to polarization: new tunes in inner ear development.
Development 131,4119
-4130.
Brault, V., Moore, R., Kutsch, S., Ishibashi, M., Rowitch, D.
H., McMahon, A. P., Sommer, L., Boussadia, O. and Kemler, R.
(2001). Inactivation of the beta-catenin gene by
Wnt1-Cre-mediated deletion results in dramatic brain malformation and failure
of craniofacial development. Development
128,1253
-1264.[Abstract]
Bray, S. (1998). Notch signaling in Drosophila:
three ways to use a pathway. Semin. Cell Dev. Biol.
9, 591-597.[CrossRef][Medline]
Brooker, R., Hozumi, K. and Lewis, J. (2006).
Notch ligands with contrasting functions: Jagged1 and Delta1 in the mouse
inner ear. Development
133,1277
-1286.
Conlon, R. A., Reaume, A. G. and Rossant, J.
(1995). Notch1 is required for the coordinate segmentation of
somites. Development
121,1533
-1545.[Abstract]
Crosnier, C., Stamataki, D. and Lewis, J.
(2006). Organizing cell renewal in the intestine: stem cells,
signals and combinatorial control. Nat. Rev. Genet.
7, 349-359.[CrossRef][Medline]
Daudet, N. and Lewis, J. (2005). Two
contrasting roles for Notch activity in chick inner ear development:
specification of prosensory patches and lateral inhibition of hair-cell
differentiation. Development
132,541
-551.
Daudet, N., Ariza-McNaughton, L. and Lewis, J.
(2007). Notch signalling is needed to maintain, but not to
initiate, the formation of prosensory patches in the chick inner ear.
Development 134,2369
-2378.
de la Pompa, J. L., Wakeham, A., Correia, K. M., Samper, E.,
Brown, S., Aguilera, R. J., Nakano, T., Honjo, T., Mak, T. W., Rossant, J. et
al. (1997). Conservation of the Notch signalling pathway in
mammalian neurogenesis. Development
124,1139
-1148.[Abstract]
Del Monte, G., Grego-Bessa, J., Gonzalez-Rajal, A., Bolos, V.
and De La Pompa, J. L. (2007). Monitoring Notch1 activity in
development: evidence for a feedback regulatory loop. Dev.
Dyn. 236,2594
-2614.[CrossRef][Medline]
Duncan, A. W., Rattis, F. M., DiMascio, L. N., Congdon, K. L.,
Pazianos, G., Zhao, C., Yoon, K., Cook, J. M., Willert, K., Gaiano, N. et
al. (2005). Integration of Notch and Wnt signaling in
hematopoietic stem cell maintenance. Nat. Immunol.
6, 314-322.[CrossRef][Medline]
Espinosa, L., Ingles-Esteve, J., Aguilera, C. and Bigas, A.
(2003). Phosphorylation by glycogen synthase Kinase-3{beta}
down-regulates Notch Activity, a Link for Notch and Wnt pathways.
J. Biol. Chem. 278,32227
-32235.
Estrach, S., Ambler, C. A., Lo Celso, C., Hozumi, K. and Watt,
F. M. (2006). Jagged 1 is a {beta}-catenin target gene
required for ectopic hair follicle formation in adult epidermis.
Development 133,4427
-4438.
Fortini, M. E. and Artavanis-Tsakonas, S.
(1994). The suppressor of hairless protein participates in notch
receptor signaling. Cell
79,273
-282.[CrossRef][Medline]
Fre, S., Huyghe, M., Mourikis, P., Robine, S., Louvard, D. and
Artavanis-Tsakonas, S. (2005). Notch signals control the fate
of immature progenitor cells in the intestine. Nature
435,964
-968.[CrossRef][Medline]
Friesel, R. and Brown, S. A. (1992). Spatially
restricted expression of fibroblast growth factor receptor-2 during Xenopus
development. Development
116,1051
-1058.[Abstract]
Galceran, J., Sustmann, C., Hsu, S.-C., Folberth, S. and
Grosschedl, R. (2004). LEF1-mediated regulation of
Delta-like1 links Wnt and Notch signaling in somitogenesis. Genes
Dev. 18,2718
-2723.
Groves, A. K. (2005). The induction of the otic
placode. In Development of the Inner Ear (Springer
Handbook of Auditory Research, Vol. 26) (ed. A. N.
Popper, M. W. Kelley and D. K. Wu), pp. 10-42. New
York: Springer Verlag.[CrossRef]
Groves, A. K. and Bronner-Fraser, M. (2000).
Competence, specification and commitment in otic placode induction.
Development 127,3489
-3499.[Abstract]
Haddon, C., Jiang, Y. J., Smithers, L. and Lewis, J.
(1998). Delta-Notch signalling and the patterning of sensory cell
differentiation in the zebrafish ear: evidence from the mind bomb mutant.
Development 125,4637
-4644.[Abstract]
Hans, S., Liu, D. and Westerfield, M. (2004).
Pax8 and Pax2a function synergistically in otic specification, downstream of
the Foxi1 and Dlx3b transcription factors. Development
131,5091
-5102.
Harada, N., Tamai, Y., Ishikawa, T., Sauer, B., Takaku, K.,
Oshima, M. and Taketo, M. M. (1999). Intestinal polyposis in
mice with a dominant stable mutation of the beta-catenin gene. EMBO
J. 18,5931
-5942.[CrossRef][Medline]
Hayward, P., Brennan, K., Sanders, P., Balayo, T., DasGupta, R.,
Perrimon, N. and Arias, A. M. (2005). Notch modulates Wnt
signalling by associating with Armadillo/{beta}-catenin and regulating its
transcriptional activity. Development
132,1819
-1830.
Hayward, P., Balayo, T. and Martinez Arias, A.
(2006). Notch synergizes with axin to regulate the activity of
armadillo in Drosophila. Dev. Dyn.
235,2656
-2666.[CrossRef][Medline]
Johnston, L. A. and Edgar, B. A. (1998).
Wingless and Notch regulate cell-cycle arrest in the developing Drosophila
wing. Nature 394,82
-84.[CrossRef][Medline]
Kageyama, R., Ohtsuka, T. and Kobayashi, T.
(2007). The Hes gene family: repressors and oscillators that
orchestrate embryogenesis. Development
134,1243
-1251.
Katoh, M. and Katoh, M. (2006). Notch ligand,
JAG1, is evolutionarily conserved target of canonical WNT signaling pathway in
progenitor cells. Int. J. Mol. Med.
17,681
-685.[Medline]
Kiernan, A. E., Cordes, R., Kopan, R., Gossler, A. and Gridley,
T. (2005). The Notch ligands DLL1 and JAG2 act
synergistically to regulate hair cell development in the mammalian inner ear.
Development 132,4353
-4362.
Kurooka, H., Kuroda, K. and Honjo, T. (1998a).
Roles of the ankyrin repeats and C-terminal region of the mouse notch1
intracellular region. Nucleic Acids Res.
26,5448
-5455.
Kurooka, H., Kuroda, K., Honjo, T., Tani, S., Kurooka, H., Aoki,
T., Hashimoto, N., Honjo, T., Nam, Y., Weng, A. P. et al.
(1998b). Roles of the ankyrin repeats and C-terminal region of
the mouse notch1 intracellular region. Nucleic Acids
Res. 26,5448
-5455.
Ladher, R. K., Wright, T. J., Moon, A. M., Mansour, S. L. and
Schoenwolf, G. C. (2005). FGF8 initiates inner ear induction
in chick and mouse. Genes Dev.
19,603
-613.
Lai, E. C. (2004). Notch signaling: control of
cell communication and cell fate. Development
131,965
-973.
Lamar, E., Deblandre, G., Wettstein, D., Gawantka, V., Pollet,
N., Niehrs, C. and Kintner, C. (2001). Nrarp is a novel
intracellular component of the Notch signaling pathway. Genes
Dev. 15,1885
-1899.
Lanford, P. J., Lan, Y., Jiang, R., Lindsell, C., Weinmaster,
G., Gridley, T. and Kelley, M. W. (1999). Notch signalling
pathway mediates hair cell development in mammalian cochlea. Nat.
Genet. 21,289
-292.[CrossRef][Medline]
Leger, S. and Brand, M. (2002). Fgf8 and Fgf3
are required for zebrafish ear placode induction, maintenance and inner ear
patterning. Mech. Dev.
119,91
-108.[CrossRef][Medline]
Lewis, A. K., Frantz, G. D., Carpenter, D. A., de Sauvage, F. J.
and Gao, W.-Q. (1998). Distinct expression patterns of notch
family receptors and ligands during development of the mammalian inner ear.
Mech. Dev. 78,159
-163.[CrossRef][Medline]
Lillevali, K., Haugas, M., Matilainen, T., Pussinen, C., Karis,
A. and Salminen, M. (2006). Gata3 is required for early
morphogenesis and Fgf10 expression during otic development. Mech.
Dev. 123,415
-429.[CrossRef][Medline]
Mackereth, M. D., Kwak, S. J., Fritz, A. and Riley, B. B.
(2005). Zebrafish pax8 is required for otic placode induction and
plays a redundant role with Pax2 genes in the maintenance of the otic placode.
Development 132,371
-382.
Maroon, H., Walshe, J., Mahmood, R., Kiefer, P., Dickson, C. and
Mason, I. (2002). Fgf3 and Fgf8 are required together for
formation of the otic placode and vesicle. Development
129,2099
-2108.
Martin, K. and Groves, A. K. (2006). Competence
of cranial ectoderm to respond to Fgf signaling suggests a two-step model of
otic placode induction. Development
133,877
-887.
Mohamed, O. A., Clarke, H. J. and Dufort, D.
(2004). Beta-catenin signaling marks the prospective site of
primitive streak formation in the mouse embryo. Dev.
Dyn. 231,416
-424.[CrossRef][Medline]
Morsli, H., Choo, D., Ryan, A., Johnson, R. and Wu, D. K.
(1998). Development of the mouse inner ear and origin of its
sensory organs. J. Neurosci.
18,3327
-3335.
Murtaugh, L. C., Stanger, B. Z., Kwan, K. M. and Melton, D.
A. (2003). Notch signaling controls multiple steps of
pancreatic differentiation. Proc. Natl. Acad. Sci. USA
100,14920
-14925.
Nam, Y., Weng, A. P., Aster, J. C. and Blacklow, S. C.
(2003). Structural requirements for assembly of the
CSL.intracellular Notch1.Mastermind-like 1 transcriptional activation complex.
J. Biol. Chem. 278,21232
-21239.
Neumann, C. J. and Cohen, S. M. (1996). A
hierarchy of cross-regulation involving Notch, wingless, vestigial and cut
organizes the dorsal/ventral axis of the Drosophila wing.
Development 122,3477
-3485.[Abstract]
Nicolas, M., Wolfer, A., Raj, K., Kummer, J. A., Mill, P., van
Noort, M., Hui, C. C., Clevers, H., Dotto, G. P. and Radtke, F.
(2003). Notch1 functions as a tumor suppressor in mouse skin.
Nat. Genet. 33,416
-421.[CrossRef][Medline]
Nissen, R. M., Yan, J., Amsterdam, A., Hopkins, N. and Burgess,
S. M. (2003). Zebrafish foxi one modulates cellular responses
to Fgf signaling required for the integrity of ear and jaw patterning.
Development 130,2543
-2554.
Norgaard, G. A., Jensen, J. N. and Jensen, J.
(2003). FGF10 signaling maintains the pancreatic progenitor cell
state revealing a novel role of Notch in organ development. Dev.
Biol. 264,323
-338.[CrossRef][Medline]
Novak, A., Guo, C., Yang, W., Nagy, A. and Lobe, C. G.
(2000). Z/EG, a double reporter mouse line that expresses
enhanced green fluorescent protein upon Cre-mediated excision.
Genesis 28,147
-155.[CrossRef][Medline]
Ohyama, T. and Groves, A. K. (2004a).
Expression of mouse Foxi class genes in early craniofacial development.
Dev. Dyn. 231,640
-646.[CrossRef][Medline]
Ohyama, T. and Groves, A. K. (2004b).
Generation of Pax2-Cre mice by modification of a Pax2 bacterial artificial
chromosome. Genesis 38,195
-199.[CrossRef][Medline]
Ohyama, T., Mohamed, O. A., Taketo, M. M., Dufort, D. and
Groves, A. K. (2006). Wnt signals mediate a fate decision
between otic placode and epidermis. Development
133,865
-875.
Oka, C., Nakano, T., Wakeham, A., de la Pompa, J. L., Mori, C.,
Sakai, T., Okazaki, S., Kawaichi, M., Shiota, K., Mak, T. W. et al.
(1995). Disruption of the mouse RBP-J kappa gene results in early
embryonic death. Development
121,3291
-3301.[Abstract]
Phillips, B. T., Bolding, K. and Riley, B. B.
(2001). Zebrafish fgf3 and fgf8 encode redundant functions
required for otic placode induction. Dev. Biol.
235,351
-365.[CrossRef][Medline]
Raft, S., Nowotschin, S., Liao, J. and Morrow, B. E.
(2004). Suppression of neural fate and control of inner ear
morphogenesis by Tbx1. Development
131,1801
-1812.
Riccomagno, M. M., Martinu, L., Mulheisen, M., Wu, D. K. and
Epstein, D. J. (2002). Specification of the mammalian cochlea
is dependent on Sonic hedgehog. Genes Dev.
16,2365
-2378.
Riley, B. B. and Phillips, B. T. (2003).
Ringing in the new ear: resolution of cell interactions in otic development.
Dev. Biol. 261,289
-312.[CrossRef][Medline]
Saint-Germain, N., Lee, Y. H., Zhang, Y., Sargent, T. D. and
Saint-Jeannet, J. P. (2004). Specification of the otic
placode depends on Sox9 function in Xenopus.
Development 131,755
-763.
Schmidt-Ott, K. M., Masckauchan, T. N., Chen, X., Hirsh, B. J.,
Sarkar, A., Yang, J., Paragas, N., Wallace, V. A., Dufort, D., Pavlidis, P. et
al. (2007). beta-catenin/TCF/Lef controls a
differentiation-associated transcriptional program in renal epithelial
progenitors. Development
134,3177
-3190.
Shi, S. and Stanley, P. (2003). Protein
O-fucosyltransferase 1 is an essential component of Notch signaling pathways.
Proc. Natl. Acad. Sci. USA
100,5234
-5239.
Shi, S., Stahl, M., Lu, L. and Stanley, P.
(2005). Canonical Notch signaling is dispensable for early cell
fate specifications in mammals. Mol. Cell. Biol.
25,9503
-9508.
Solomon, K. S., Kudoh, T., Dawid, I. B. and Fritz, A.
(2003). Zebrafish foxi1 mediates otic placode formation and jaw
development. Development
130,929
-940.
Solomon, K. S., Kwak, S. J. and Fritz, A.
(2004). Genetic interactions underlying otic placode induction
and formation. Dev. Dyn.
230,419
-433.[CrossRef][Medline]
Streit, A. (2002). Extensive cell movements
accompany formation of the otic placode. Dev. Biol.
249, 237.[CrossRef][Medline]
Tani, S., Kurooka, H., Aoki, T., Hashimoto, N. and Honjo, T.
(2001). The N- and C-terminal regions of RBP-J interact with the
ankyrin repeats of Notch1 RAMIC to activate transcription. Nucleic
Acids Res. 29,1373
-1380.
Tanigaki, K., Han, H., Yamamoto, N., Tashiro, K., Ikegawa, M.,
Kuroda, K., Suzuki, A., Nakano, T. and Honjo, T. (2002).
Notch-RBP-J signaling is involved in cell fate determination of marginal zone
B cells. Nat. Immunol.
3, 443-450.[CrossRef][Medline]
Torres, M. and Giraldez, F. (1998). The
development of the vertebrate inner ear. Mech. Dev.
71, 5-21.[CrossRef][Medline]
Vendrell, V., Carnicero, E., Giraldez, F., Alonso, M. T. and
Schimmang, T. (2000). Induction of inner ear fate by FGF3.
Development 127,2011
-2019.[Abstract]
Williams, R., Lendahl, U. and Lardelli, M.
(1995). Complementary and combinatorial patterns of Notch gene
family expression during early mouse development. Mech.
Dev. 53,357
-368.[CrossRef][Medline]
Wright, T. J. and Mansour, S. L. (2003). Fgf3
and Fgf10 are required for mouse otic placode induction.
Development 130,3379
-3390.
Wright, T. J., Ladher, R., McWhirter, J., Murre, C., Schoenwolf,
G. C. and Mansour, S. L. (2004). Mouse FGF15 is the ortholog
of human and chick FGF19, but is not uniquely required for otic induction.
Dev. Biol. 269,264
-275.[CrossRef][Medline]
Zhou, Y. X. and Armstrong, R. C. (2007).
Interaction of fibroblast growth factor 2 (FGF2) and notch signaling
components in inhibition of oligodendrocyte progenitor (OP) differentiation.
Neurosci. Lett. 421,27
-32.[CrossRef][Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  S. Freter, Y. Muta, S.-S. Mak, S. Rinkwitz, and R. K. Ladher Progressive restriction of otic fate: the role of FGF and Wnt in resolving inner ear potential Development, October 15, 2008; 135(20): 3415 - 3424. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
