1 Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD,
UK
2 Netherlands Institute for Developmental Biology (NIOB), Hubrecht Laboratorium,
3584CT Utrecht, The Netherlands
Author for correspondence (e-mail:
s.p.hoppler{at}abdn.ac.uk)
Accepted 6 October 2005
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SUMMARY |
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Key words: Lef, Mesoderm, Signalling, Tcf, Wnt, Xenopus
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Introduction |
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Early development of Xenopus is the best understood model system
for tissue- and stage-specific Wnt signalling
(Darken and Wilson, 2001;
Hamilton et al., 2001
;
Roel et al., 2002
;
Schohl and Fagotto, 2003
).
Wnt/ß-catenin signalling mediates three separate responses during the
early developmental stages leading to gastrulation. First, from cleavage stage
to early blastula (stages 3-8), maternal Wnt/ß-catenin signalling
establishes the dorsal axis of the embryo by lifting the transcription
repression imposed by Tcf3 on dorsal genes such as siamois
(Houston et al., 2002
;
Yang et al., 2002
). This early
function of Wnt/ß-catenin signalling is still reflected by the expression
of later dorsal genes such as chordin in dorsal cells during
gastrulation. Second, during slightly later blastula stages (stages 8.5-9.5),
Wnt/ß-catenin signalling is also active all around the marginal zone
(equatorial region), and is required upstream of zygotic FGF and nodal signals
for mesoderm induction (Schohl and
Fagotto, 2003
). The role of Wnt/ß-catenin signalling in
mesoderm induction is revealed by the expression of the pan-mesoderm marker
brachyury (Xbra). Third, subsequent to mesoderm induction,
zygotic Wnt8/ß-catenin signalling promotes ventral and lateral, but
restricts dorsal, mesoderm development
(Christian and Moon, 1993
;
Hamilton et al., 2001
;
Hoppler et al., 1996
;
Hoppler and Moon, 1998
). This
Wnt/ß-catenin signalling activity is best analysed during gastrulation by
the expression of ventrolateral mesoderm marker Xpo, and the
dorsolateral mesoderm marker XmyoD. As nuclear ß-catenin is
present all around the marginal zone during blastula stages
(Schohl and Fagotto, 2002
),
the question arises how gene expression is regulated tissue- and
stage-specifically downstream of Wnt/ß-catenin signalling.
Wnt/ß-catenin signalling is mediated by protein complexes of
ß-catenin with individual members of the Tcf/Lef family of DNA-binding
factors. The vertebrate Tcf/Lef family consists of four genes, Tcf1, Lef1,
Tcf3 and Tcf4, which give rise to many different splice variants
(e.g. van Noort and Clevers,
2002). Without ß-catenin, they all inhibit the transcription
of target genes in association with co-repressors (e.g.
Brantjes et al., 2001
).
Wnt/ß-catenin signalling stabilizes ß-catenin, which then forms a
complex with Tcf/Lef factors to permit or actively promote activation of
target gene transcription. Members of the Tcf/Lef family are highly homologous
in the N-terminal ß-catenin-binding domain and the high mobility group
(HMG) DNA-binding domain, which is located more towards the C-terminus of the
protein. Except for these two short domains, their amino acid sequences are
diverse, and some functional motifs, including a CtBP-binding motif
(Brannon et al., 1999
), a p300
interacting domain (Hecht and Stemmler,
2003
), or an E-tail motif (CRARF motif)
(Atcha et al., 2003
), are
present only in certain isoforms of Lef/Tcfs. Analysis of knockout mice
phenotypes indicated that Lef/Tcf gene function may not be fully
interchangeable or redundant (Korinek et
al., 1998
; Reya et al.,
2000
). This finding could reflect a difference in the temporal or
spatial expression pattern of Tcf/Lef genes, or indicate a functional
difference in the protein products of these genes. Recent reports showed that
different Tcf/Lef proteins when ectopically expressed have different
activities (Gradl et al.,
2002
) and may exert distinct functions on different promoters of
target genes (Hecht and Stemmler,
2003
).
In Xenopus, all four members of the Tcf/Lef family were recently
cloned. Tcf1 and Tcf3 are both maternally and zygotically
expressed, while Lef1 is expressed only after the onset of zygotic
gene expression at the mid-blastula transition (MBT)
(Molenaar et al., 1998;
Roel et al., 2003
). Maternal
Tcf3 is ubiquitously present in early embryos, while zygotic
expression of Tcf3 appears only much later in the anterior region of
the late gastrula. Tcf1 RNA is detected at high levels in the animal
hemisphere of cleavage- and blastula-stage embryos; at early gastrula stages,
Tcf1 is highly expressed in the animal cap and most of the marginal
zone except for a narrow domain around the blastopore. Low-level transcripts
of Lef1 become detectable in the mid- and late blastula. In the early
gastrula, we also detected an elevated expression of Lef1 in the
ventrolateral marginal zone (data not shown). The expression of Xenopus
Tcf4 is reported to be detectable from late neurula stages in the
midbrain region (Konig et al.,
2000
), but another investigation indicates that maternal
Tcf4 expression is detected by RT-PCR
(Houston et al., 2002
).
The functions of Lef1 and Tcf1 in the early development
of Xenopus embryos are still unclear. Considering the significant
difference in structures and expression patterns between Lef1, Tcf1
and Tcf3, it is reasonable to assume that these Tcfs may be involved
in mediating tissue-specific responses downstream of Wnt/ß-catenin
signalling. In support of this notion, we have previously demonstrated that
Wnt/ß-catenin signalling mediates tissue-specific Wnt signalling at
different stages of early Xenopus development by engaging different
Tcf-mediated nuclear mechanisms (Hamilton
et al., 2001) and that constitutively repressing constructs of
Tcf3 and Lef1 have the capacity to interfere specifically with
Wnt/ß-catenin signalling-mediated processes in different tissues and at
different stages of early Xenopus development
(Roel et al., 2002
).
Here we show by gene knockdown, rescue and overexpression experiments in Xenopus, that expression of genes encoding different Tcf/Lef transcription factors are required to mediate distinct responses to Wnt signalling. In particular, we show that Tcf1 and Tcf3 are non-redundantly required for mesoderm induction, and that for subsequent ventrolateral mesoderm patterning, both normal levels of Tcf1 and Lef1 gene expression are required. Further analysis indicates that different molecular functions of these Tcf/Lef factors are determined by LVPQ and SXXSS motifs in their central domains. This is the first systematic comparison of endogenous functions of Tcf/Lef genes in early Xenopus mesoderm development, which yields interesting and novel conclusions that are important beyond the context of early Xenopus development.
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Materials and methods |
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Whole-mount in-situ hybridization
Whole-mount RNA in-situ hybridization was performed
(Harland, 1991) with
modifications as described in McGrew et al.
(McGrew et al., 1999
). The
digoxigenin-labelled antisense RNA probes used were Xbra
(Smith et al., 1991
),
Xpo (Sato and Sargent,
1991
), XmyoD (Frank
and Harland, 1991
) and Xenopus chordin
(Sasai et al., 1994
). All
experiments were repeated independently at least once.
In-vitro transcription and translation (TNT)
TNT Quick Coupled (Promega, Madison, WI) in-vitro transcription and
translation reactions (25 µl reaction volume) were used to test the
efficiency of MOs. One hundred nanograms of pCS2+-based vector DNA
(Turner and Weintraub, 1994)
(see also
http://sitemaker.umich.edu/dlturner.vectors)
encoding the 5' sequences complementary to XlTcf1, XlLef1 or
XlTcf3 MOs were used as gene-specific templates. Additionally, a
control DNA vector template (100 ng) encoding Luciferase was used to monitor
independently the enzymatic reactions and subsequent gel loading. MOs were
added to the reactions (see below). The reactions were performed in the
presence of 35S-Methionine to radioactively label the protein
products. Following incubation, reactions were run on 10% acrylamide gels
(Nu-PAGE, Invitrogen Life Technologies) and the results were visualized by
exposure to radioactivity-sensitive film. MO titration (50-250 ng per
reaction) produced gene-specific inhibition of protein synthesis to different
extents. While keeping other conditions of the assay unchanged, we found 50,
100 and 150 ng to be sufficient for Tcf1 MO, Tcf3 MO and Lef1 MO,
respectively, to inhibit protein synthesis of their specific target to
undetectable levels. We chose to use MOs at 100 ng per reaction in the
representative experiment shown in Fig.
1A.
MO and mRNA injections
MOs targeting Xenopus laevis Tcf/Lef factors were designed by and
purchased from Gene Tools (Philomath, OR). The MO sequences were:
Lef1 MO: 5'-CTC CAG AGA GCT GAG GCA TGG CTC C-3';
Tcf1 MO: 5'-CGG CGC TGT TCA TTT GGG GCA T-3';
Tcf3 MO: 5'-CGC CGC TGT TTA GTT GAG GCA TGA-3';
Tcf4 MO: 5'-CGC CAT TCA ACT GCG GCA TCT CTG C-3'
(Kunz et al., 2004); and
control MO: 5'-CCT CTT ACC TCA GTT ACA ATT TAT A-3'. To examine
the phenotypes produced by the knockdown of individual Tcfs, we injected MOs
individually into the prospective mesoderm (marginal zone) of Xenopus
embryos. MO titration produced the phenotypic series of overt effects:
none-mild-substantial-toxic at 10, 15, 20 and 25 ng/cell for the Tcf1 MO and
30, 45, 60 and 75 ng/cell for both Lef1 and Tcf3 MOs. We chose 15-20 ng/cell
for Tcf1 MO and 60 ng/cell for Lef1 and Tcf3 MOs. Note that the observed
relative efficiencies of the Tcf1, Tcf3 and Lef1 MOs in the embryo correspond
well with those assayed in the in-vitro TNT reactions (see above).
|
All capped RNAs of each of the Tcf/Lef constructs were injected alone or with MOs into one side of the LMZ of embryos at the 2-cell stage. MO rescue experiments were always performed with mRNAs that lacked the target sequence recognized by the particular MO at the starting site of translation. The injection doses of these Tcf mRNAs were titrated to find a concentration that does not affect expression pattern of Xbra, Xpo or XmyoD when injected alone.
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Results |
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MO-mediated knockdown of different Tcfs during Xenopus laevis
development produced significantly different phenotypes. The induced
phenotypes were also generally different if any given Tcf was
tissue-specifically knocked down in dorsal tissue as opposed to lateral or
ventral tissue. Targeted dorsal MO-mediated Tcf3 knockdown caused a
complete headless phenotype (Fig.
1L), similar to that caused by the hdl (zebrafish Tcf3)
mutation or hdl MO injection in zebrafish
(Dorsky et al., 2003); dorsal
Tcf1 knockdown caused a severe bend in the dorsal axis at
approximately the position of the hindbrain
(Fig. 1F), and dorsal
Lef1 knockdown caused only a slight dorsal bend in the dorsal axis
but also an apparently mild patterning defect in the forebrain region
(Fig. 1I). Targeted ventral
knockdown of Tcf1, Lef1 and Tcf3 affected ventral
development to different extents (Fig.
1G,J,M), while Lef1 knockdown also affected tail
development, consistent with the results of Lef1 gene knockdown in
Xenopus tropicalis (Roel et al.,
2002
). Targeted lateral knockdown of Tcf1 with 20 ng Tcf1
MO, which was usually used throughout this investigation, caused a severe
phenotype in 90% of embryos with much delayed gastrulation movements,
typically followed by developmental arrest and widespread apparent cell death
at late gastrula and early neurula control stages (not shown). In the
remaining 10% of embryos it caused a combination of dorsolateral and
ventrolateral phenotypes (Fig.
1E). Less complete lateral knockdown of Tcf1 with just 15
ng Tcf1 MO showed the same combination of dorsolateral and ventrolateral
phenotypes in the vast majority of embryos
(Fig. 1S). Generally, lateral
targeted knockdown of Tcf1, Lef1 and Tcf3 impaired both
dorsal and ventral developments to a lesser degree than targeted knockdowns in
the dorsal or ventral mesoderm (Fig.
1E,H,K,S). These similar yet distinct phenotypes indicate that the
gene functions of Tcf1, Lef1 and Tcf3 in early development
of Xenopus embryos may be both overlapping and unique.
MO knockdown of Tcf/Lef gene expression does not affect the establishment of the dorsal axis
In our experiments, none of the Tcf/Lef MO appeared to either inhibit the
development of dorsal trunk axis structures when targeted to the prospective
dorsal side (Fig. 1F,I,L), or
to induce axis duplication when targeted to one cell of the ventral side (data
not shown), or even to affect the expression of the organizer gene
chordin (Fig. 1N-R).
This result may at first seem surprising, as maternal Tcf3 is
required for repression of organizer gene expression in early Xenopus
embryos (Houston et al., 2002)
and MO-mediated ß-catenin knockdown inhibits the establishment of the
dorsal axis (Heasman et al.,
2000
). Possible explanations for our results include: (1) that
there are sufficient maternal Tcf proteins in eggs and early embryos before
expression of Organizer genes, which last long enough in order to mediate
Wnt/ß-catenin signalling function in establishing the dorsal axis; (2)
that the MO-mediated knockdown is insufficient to inhibit the protein
synthesis from maternally or zygotically expressed Tcf mRNA in early embryos
(despite evidence that it is efficient at only slightly later stages, see
below); or (3) that there is comprehensive redundancy between the different
maternally or zygotically expressed Tcf proteins in early embryos (despite the
fact that co-injections of MO targeting different Tcf genes does not affect
axis development or chordin expression). We favour the first possibility and
think it most likely that MOs can effectively knock down only zygotic
expression of Tcf/Lef genes and is therefore insufficient to inhibit maternal
Tcf expression, which functions to mediate dorsalizing Wnt signalling before
the MBT (Darken and Wilson,
2001
; Hamilton et al.,
2001
; Yang et al.,
2002
). We conclude that MOs are not suitable reagents to
investigate Tcf requirements in dorsal axis establishment and have therefore
focused our investigation on studying the function of Tcf/Lefs in later events
of mesoderm induction and patterning.
Tcf1 and Tcf3 are non-redundantly required for mesoderm induction
It was reported recently that Wnt/ß-catenin signalling is required for
early expression of pan-mesoderm markers, such as brachyury
(Xbra), through FGF3 and Nodal signalling in the prospective mesoderm
(Schohl and Fagotto, 2003). To
investigate a potential role of Tcf/Lef molecules in mesoderm induction, we
analysed MO-mediated knockdowns of each Tcf factor by detecting the expression
of the early pan-mesoderm marker Xbra
(Fig. 2). We found that
Xbra expression was totally abrogated by Tcf1 knockdown
(Fig. 2D,L) and significantly
reduced by Tcf3 knockdown (Fig.
2G,M) in a dose-dependent manner
(Fig. 2L,M,N), but it was not
significantly affected by knockdowns of either Lef1 or Tcf4
(Fig. 2B,C,J). These results
are consistent with the temporal expression pattern of these Tcf/Lef
molecules, i.e. Tcf1 and Tcf3 are expressed before the
beginning of Xbra expression (at midblastula stages), while
Lef1 and Tcf4 are mainly expressed later. To test whether
the molecular functions of Tcf1 and Tcf3 proteins are interchangeable in this
event, we attempted to rescue the effect of the Tcf1 knockdown with
mRNA-mediated Tcf3 overexpression and vice versa. The effects of
overexpression of Tcf1 or Tcf3 on their own on Xbra expression were very
dose-dependent (Fig. 2K).
Absence of Xbra expression in Tcf1 knockdown was rescued by
an appropriate dose of Tcf1 mRNA but was not rescued by Tcf3 mRNA at any dose
(Fig. 2E,F,L); reduced
Xbra expression in the Tcf3 knockdown was rescued by an
appropriate dose of Tcf3 mRNA, but was not significantly rescued by Tcf1 mRNA
at any dose (Fig. 2H,I,M).
Furthermore, the combination of Tcf1 MO and Tcf3 MO attenuated their effects
on Xbra expression (Fig. 2N). These results show that Tcf1 and Tcf3 are non-redundantly
required for mesoderm induction for what appears to be antagonistic roles.
Both Tcf1 and Lef1 are required for ventrolateral mesoderm development
Zygotic Wnt8 signalling promotes ventral and lateral, but restricts dorsal,
mesoderm development through a ß-catenin-dependent pathway
(Christian and Moon, 1993;
Hamilton et al., 2001
;
Hoppler et al., 1996
;
Hoppler and Moon, 1998
). To
investigate the role of Tcf/Lef molecules in ventrolateral mesoderm
development, we analysed the expression of ventrolateral mesoderm markers
Xpo and XmyoD mRNA by in-situ hybridization after knockdowns
of each Tcf factor. We found that XmyoD and Xpo expression
were reduced significantly in knockdowns for Lef1, Tcf1 or
Tcf3, but were not affected by either Tcf4 MO or control MO
(Fig. 3C,D,F,K,P and
C',D',F',K',P'). As the mesoderm
induction is a pre-condition for later dorsoventral mesoderm patterning, and
because Xbra function is required for expression of later mesoderm
markers, including both ventral and dorsal markers
(Giovannini and Rungger,
2002
), it is necessary to test whether the apparent requirement
for Tcf1 and Tcf3 function for the expression of later
regional mesoderm markers is only a consequence of their prior requirement for
mesoderm induction or whether they are directly involved in dorsoventral
mesoderm patterning. We rescued Xbra expression (by Xbra mRNA
injections) in these knockdowns to restore mesoderm induction. We found that
Xbra mRNA did restore Xpo and XmyoD expression in
the Tcf3 knockdown (Fig.
3E,E',P,P') but did not rescue the expression of these
two regional mesoderm markers in the knockdowns for Tcf1
(Fig. 3G,G',P,P')
or Lef1 (Fig.
3L,L',P,P'). These results indicate that only
Tcf1 and Lef1 are required for promoting ventrolateral
mesoderm development, independent of any role in mesoderm induction. To
investigate the specificity of Tcf1 and Lef1 protein function in this process,
we expressed Tcf1 or Lef1 mRNAs in either the Tcf1
knockdown or the Lef1 knockdown. We found that Xpo and
XmyoD expression in Tcf1 or Lef1 knockdown were
both rescued by either Lef1 or Tcf1 mRNA
(Fig. 3H,I,M,N and
H',I',M',N',P,P'). Tcf3 was not only
unable to rescue Xpo and XmyoD expression in Tcf1
or Lef1 knockdown, but appeared to downregulate their expression even
further (Fig.
3J,J',O,O',P,P'). Moreover, overexpression of
Tcf3 alone significantly downregulated Xpo and XmyoD
expression (data not shown). These results show that although normal level
expression from both Tcf1 and Lef1 genes is required for
ventrolateral mesoderm development, the molecular roles of their protein gene
products in this particular process are interchangeable. In other words, the
function of Lef1 and Tcf1 in ventrolateral mesoderm
development is qualitatively redundant but quantitatively non-redundant.
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Similarly, the effects of Lef1 and Tcf1 gene knockdown on
Xpo expression were rescued by active forms of Tcf3, such as TVGR and
Tcf3grg
C, but were not rescued and were even exacerbated by the
constant repressor form of Tcf3, Tcf3
N
(Fig. 4J-Q,S). These findings
were confirmed in identical rescue experiments analysed by XmyoD
expression (data not shown). These results show that in ventrolateral mesoderm
patterning, Tcf1 and Lef1 are required to mediate the
ß-catenin-mediated activation of target genes.
|
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Discussion |
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Qualitatively different functions of Tcf1 and Tcf3 are required for mesoderm induction
Our gene knockdown experiments show that Tcf1 and Tcf3
are non-redundantly required for mesoderm induction, while Lef1 is
not required (see Fig. 6). This
role of Tcf1 is consistent with its high expression all around the
marginal zone (Roel et al.,
2003). We showed that the effect of Tcf1 gene knockdown
was predominantly caused by lack of transcription-activating ß-catenin
signalling, while the similar but seemingly milder effect of Tcf3
gene knockdown was predominantly caused by insufficient repression. These
results suggest either that these Tcf transcription factors, through as yet
unknown mechanisms, regulate a different set of downstream genes, which are
eventually responsible for mesoderm induction, or that a careful balance
between repressive and activating Tcf factor function is required for the
regulation of precise levels of expression of a common set of downstream genes
responsible for mesoderm induction. Whatever the precise downstream
mechanisms, our current findings suggest that mesoderm induction is mediated
where an appropriate intensity of Wnt/ß-catenin signalling-mediated
transcriptional activity is tightly regulated by the expression level and
ratio of Tcf1 and Tcf3.
Ventrolateral mesoderm development is promoted by Tcf1 and Lef1
Gene knockdown of Lef1 and Tcf1 affects ventrolateral
mesoderm development, independent of mesoderm induction, and through a loss of
ß-catenin-mediated transcriptional activation (see
Fig. 6). Moreover, at doses
that are not interfering with mesoderm induction, overexpression of repressive
forms of Tcf factors (Tcf3, Tcf4A, Tcf3N) represses ventrolateral
mesoderm development, while active forms of Tcf factors (Tcf1, Tcf4C, Lef1) do
not interfere with it (data not shown). These results show that
Wnt8/ß-catenin signalling, which promotes ventrolateral mesoderm
development, is mediated by the ß-catenin-dependent transcriptional
activation function of Tcf1 and Lef1. Unlike the earlier mesoderm induction
event, the repressive role of Tcf3 is not required, indicating that the
mechanisms of Wnt/ß-catenin signalling in these two events are different.
It was reported recently that in zebrafish Wnt/ß-catenin signalling
activates ventrolateral mesoderm genes directly in combination with BMP
signalling (Szeto and Kimelman,
2004
), where both BMP and Wnt signalling are active at submaximal
levels. This finding fits well with our model that the suboptimal
Wnt/ß-catenin signalling required for ventrolateral mesoderm development
is mediated only by the more active forms of Tcf factors (Tcf1 and Lef1), and
would be interfered with by overexpression of more repressive forms of Tcf
factors (i.e. Tcf3).
Repressive role of Tcf factors is mainly determined by LVPQ and SXXSS motifs
Our investigation into the role of Tcf/Lef genes suggests a requirement for
two fundamentally different types of Tcf/Lef factors in Xenopus
mesoderm development: a Tcf3-like predominant repressor and a Tcf1/Lef1-like
predominant activator. In our experiments, mutation or deletion of two short
motifs, LVPQ and SXXSS, in the central domain of Tcf3 dramatically changed the
activity of Tcf3 from transcriptional repressor to activator, which made it
function more like Tcf1 and Lef1 in mesoderm development. However, although
Tcf3 is the only known Xenopus Tcf factor that interacts with the
ubiquitously expressed transcription co-repressor CtBP through its C-terminus,
deletion of the C-terminus from Tcf3 did not change its repressive nature
significantly in mesoderm development. These results show that LVPQ and SXXSS
motifs are crucial for the repressive function of Tcf3 in mesoderm
development. LVPQ and SXXSS motifs do not appear to affect the interaction
between Tcf/Lef factors and transcriptional co-repressor Grg
(Pukrop et al., 2001), and the
mechanism of repression via these two motifs is still unclear. One possible
mechanism is that phosphorylation regulated by the SXXSS serine-rich motif
could prevent the formation of a ternary complex between DNA, Tcf and
ß-catenin, as has been shown for Xenopus Tcf4
(Pukrop et al., 2001
). We tend
to support this model, as it would explain the dominant role of LVPQ and SXXSS
motifs in determining the repressive function of Tcf factors in mesoderm
development.
Tcf/Lef function in aberrant Wnt/ß-catenin signalling in colorectal cancer
The primary molecular cause of colorectal cancer (CRC) is thought to be the
abnormal activation of the Wnt/ß-catenin signalling pathway. Wnt
signalling in normal colon tissue is mainly mediated by Tcf4, which
is absolutely required for maintenance of a mitotically active stem cell
population in the intestine (van de
Wetering et al., 2002). Tcf1 is also present in normal
colon tissues, but mainly functions as a tumour suppresser to prevent aberrant
Wnt signalling in the gut [probably mainly present as truncated constitutively
repressive isoforms (Roose et al.,
1999
)]. Human Tcf4 consists of various isoforms, among them the
LVPQ, SXXSS or Exon IVa motifs are either present or absent
(Duval et al., 2000
). Our
results here show that different Tcf/Lef factors trigger distinct effects in
vivo, and point mutations in some crucial motifs are sufficient to change
their functions completely. This finding suggests that mutation or aberrant
expression of Tcf/Lef factors may contribute to the progression and
maintenance of CRC and that these aberrantly expressed Tcf/Lef factors would
be potential drug targets for treating and preventing CRC by specifically
interfering with aberrant Wnt signalling.
Conclusion
Tcf/Lef genes encode a variety of DNA-binding factors with different
molecular functions. Tcf/Lef factors mainly differ in the way they mediate the
activation of target genes by Wnt/ß-catenin signalling. Short polypeptide
motifs within their central protein domain are crucial for determining this
difference in function. The expression level and ratio of Tcf/Lef factors with
these different functions modulates the intensity and outcome of
Wnt/ß-catenin signalling-controlled gene expression in the embryo, which
makes specific responses possible. These conclusions are important beyond the
context of Xenopus mesoderm development.
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ACKNOWLEDGMENTS |
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Footnotes |
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