Center for Basic Neuroscience, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA
* Author for correspondence (e-mail: jane.johnson{at}utsouthwestern.edu)
Accepted 19 November 2003
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Spinal cord development, Roof plate, Homeodomain factors, Basic-helix-loop-helix transcription factors, Chick in ovo electroporation, Math1/Cath1, Mash1/Cash1, Bmp signaling, Dorsalventral patterning
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Three populations of cells develop from the dorsal neural tube/lateral
neural plate. Neural crest cells that give rise to the peripheral nervous
system are generated from the border region between the neural plate and
adjacent ectoderm and migrate out of the neural tube before or during neural
tube closure (Le Douarin,
1982). Roof-plate cells develop at the dorsal midline of the
neural tube and are a group of specialized glial cells. Both neural crest and
roof-plate cells are induced by Bmp signals from the epidermal ectoderm
(Liem et al., 1995
). The
roof-plate cells themselves subsequently become a source of several Bmps,
which can promote the differentiation of a third population of cells, the
dorsal spinal cord interneurons (Lee et
al., 2000
; Lee et al.,
1998
; Liem et al.,
1997
). Genetic studies of zebrafish mutants with different
components of Bmp signaling pathways disrupted (swirl/bmp2b,
snailhouse/bmp7 and somitabun/smad5) provide direct evidence for
the involvement of Bmps in the specification of neural crest and dorsal
neurons (Barth et al., 1999
;
Nguyen et al., 2000
).
It is not clear how Bmp signaling can mediate such a diverse array of
activities in the dorsal neural tube. It has been suggested that the
competence of neural progenitor cells in the lateral neural plate/dorsal
neural tube changes so that early progenitor cells generate neural crest and
roof-plate cells in response to Bmp signaling whereas late progenitor cells
generate dorsal interneurons, and in vitro explant studies provide evidence
consistent with this hypothesis (Liem et
al., 1997). Nevertheless, the question remains of what downstream
effectors mediate these activities of Bmps, and whether different downstream
mediators display a different subset of Bmp initiated activities. One group of
candidate effectors for Bmp signaling is the Msx family of transcription
factors.
Msx genes encode homeodomain transcription factors related to the
Drosophila msh gene (for a review, see
Cornell and Ohlen, 2000).
These genes have been implicated as downstream targets of the Bmps because
members of the Msx family are induced in regions where Bmp signaling
is active, such as the hindbrain, spinal cord, telencephalon, tooth, facial
primordium and limb (Barlow and
Francis-West, 1997
; Bei and
Maas, 1998
; Furuta et al.,
1997
; Ganan et al.,
1996
; Graham et al.,
1994
; Liem et al.,
1995
; Shimeld et al.,
1996
; Timmer et al.,
2002
; Vainio et al.,
1993
). The mouse Msx family has three genes, Msx1, Msx2
and Msx3 (reviewed by Davidson,
1995
). Msx1 and Msx2 have largely overlapping
expression patterns in a variety of tissues including the roof-plate and
adjacent cells in the dorsal neural tube and neural crest
(Wang et al., 1996
).
Msx3, however, is expressed exclusively in the dorsal neural tube
(Shimeld et al., 1996
;
Wang et al., 1996
).
The functions of Msx1 and Msx2 have been studied
genetically and biochemically. Targeted inactivation of Msx1 in mouse
reveals a role in the development of the molar tooth and palate
(Chen et al., 1996;
Satokata and Maas, 1994
), and
a role in the development of the midline structure in the forebrain
(Bach et al., 2003
).
Inactivation of Msx2 causes defects in calvarial bones, skin and
mammary glands (Satokata et al.,
2000
). Genetic mutation of Msx3 in mouse has not been
reported. Gain-of-function analysis of Msx1 function in cell culture and in
vivo model systems suggest that Msx1 acts as a negative regulator of
differentiation (Bendall et al.,
1999
; Hu et al.,
2001
; Song et al.,
1992
; Woloshin et al.,
1995
). This is achieved through its ability to repress the
transcription of differentiation genes such as MyoD
(Bendall et al., 1999
) and to
regulate the expression and activity of cell cycle molecules such as cyclin D1
and CDK4 (Hu et al., 2001
).
Biochemical studies show that Msx1 and Msx2 are potent transcriptional
repressors (Catron et al.,
1996
; Catron et al.,
1995
; Newberry et al.,
1997
; Zhang et al.,
1996
). Although optimal DNA-binding sites for Msx proteins have
been identified, in certain contexts, the repressor activity does not require
DNA-binding of these factors. Even though less is known about Msx3,
biochemical studies suggest that it is also a transcriptional repressor
(Mehra-Chaudhary et al.,
2001
).
In the developing mouse spinal cord, all three members of the Msx family
are expressed in the dorsal neural tube from E9.0 or earlier
(Hill et al., 1989;
Robert et al., 1989
;
Shimeld et al., 1996
;
Wang et al., 1996
). As
development proceeds, Msx1 and Msx2 expression becomes
restricted to the dorsal midline roof-plate cells while the expression of
Msx3 is in the ventricular zone of the dorsal one-third of the neural
tube but excluded from the roof plate. Loss-of-function studies have not been
informative on the role of the Msx factors in spinal cord development possibly
owing to redundant activity and the overlapping expression of Msx1, Msx2, and
Msx3 in this tissue.
To begin to address the roles of Msx factors in the developing spinal cord, we have analyzed the fate of neural progenitor cells in chick neural tube upon overexpression of the mouse Msx genes by in ovo electroporation. For comparison, we also examined the phenotypes of chick embryos with the Bmp signaling pathway activated at different stages by overexpressing the constitutively active forms of Bmp receptors. We show that Bmp signaling and Msx factors have stage-dependent activities in determining dorsal cell fates. Activation of Bmp signaling in HH10-12 embryos resulted in an increase of roof-plate cells with a concurrent increase of apoptosis and repression of neuronal differentiation. This set of phenotypes was mimicked by overexpression of Msx1, but not Msx3. By contrast, when activated Bmp receptors were introduced into later stage neural tubes (HH14-16), dorsal interneuron cell fates were induced rather than roof plate. In this case, Msx3 but not Msx1 induced the same phenotypes. Together, these results demonstrate that Msx1 and Msx3 have differential functions in spinal cord development and each may mediate a subset of Bmp activities in patterning the dorsal neural tube.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Chick in ovo electroporation
Fertilized White Leghorn eggs were incubated at 39°C and embryos were
staged according to Hamburger and Hamilton
(Hamburger and Hamilton,
1992). Stage HH10-12 chick embryos used for electroporation were
analyzed after 24 hours (HH15-18) or 48 hours (HH20-22). Stage HH14-16 embryos
used for electroporation were analyzed after 24 hours (HH20-22). Plasmid DNA
was injected into the lumen of the chick neural tubes at a concentration of
2-3.5 mg/ml (except for ca-Bmpr1b, which was injected at a concentration of
0.3-1.2 mg/ml). A CMV-EGFP expression plasmid (Clontech) was co-injected at a
concentration of 2-3 mg/ml for visualization of the electroporated cells.
Electric square pulses were applied five times at 25 volts for 50 mseconds
with an electro-square porator T830 (BTX, Genetronics). Only embryos with high
levels of GFP fluorescence in the neural tube were processed for each
experiment. Each embryo was fixed in 4% paraformaldehyde/PBS at 4°C for 1
hour, washed extensively in PBS and cryoprotected in 30% sucrose/PBS at
4°C overnight. Embryos were embedded in OCT and cryosectioned at
20-30µm. Sections were taken in between forelimb and hindlimb levels and
only sections with high levels of GFP were used for analysis. All results were
repeated in at least four embryos.
In situ hybridization and immunofluorescence
In situ hybridization was performed as previously described
(Birren et al., 1993). Chick
Bmp4, Wnt1, Cash1, Ngn1 and Ngn2 antisense probes were
labeled with digoxigenin and hybridized to frozen sections of the embryos in a
concentration of 1-2 µg/ml. Bmp4 plasmid was obtained from
University of Delaware chick EST database (clone pgf2n.pk004.m4).
Immunofluorescence was performed as described
(Gowan et al., 2001
) with the
following antibodies: rabbit anti-Math1
(Helms and Johnson, 1998
),
rabbit anti-Lhx2/9 (L1) (Liem et al.,
1995
), rabbit anti-Pax2 (Zymed Laboratories), rabbit anti-Dlx
(Panganiban et al., 1995
),
mouse monoclonal antibody Tuj1 (Lee et
al., 1990
) and monoclonal antibodies obtained from Developmental
Studies Hybridoma Bank (DSHB), including anti-Msx (4G2)
(Liem et al., 1995
), anti-Pax7
(Ericson et al., 1996
),
anti-Lhx1/5 (4F2) (Tsuchida et al.,
1994
), anti-Isl1/2 (39.4D5)
(Ericson et al., 1992
),
anti-Lmx1 (50.5A5) (Riddle et al.,
1995
) and anti-Mnr2 (81.5C10)
(Tanabe et al., 1998
).
Immunofluorescence images were taken on a BioRad MRC 1024 confocal
microscope.
BrdU labeling and TUNEL assay
BrdU (100-150 µl of 5 mg/ml in PBS) was injected to the vicinity of the
heart of the embryo 1 hour before harvesting. Incorporation of BrdU was
detected with mouse anti-BrdU (Beckton-Dickinson). TUNEL assays were performed
using the In Situ Cell Death Detection Kit (Roche) according to the
manufacturer's instructions.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
|
We measured neural progenitor cell proliferation by examining incorporation of the thymidine analog BrdU. In contrast to the dramatic reduction of the overall size of the neural tube, the size of the ventricular zone of ca-Bmpr1- or Msx1-expressing neural tube was only slightly reduced (Fig. 3F-J). We observed a 20-30% reduction of mitotic cells on the electroporated side compared with the control side both 24 hours and 48 hours post electroporation (Fig. 3J). However, most of the reduction can be accounted for by the increase of apoptosis in the neural tube, therefore it is not likely that ca-Bmpr1 or Msx1 has a direct effect on proliferation in this assay.
Altogether, these results demonstrate that increased cell death and decreased neuronal differentiation contribute to the small neural tube phenotype caused by ca-Bmpr1 or Msx1 overexpression. The fact that expression of ca-Bmpr1a/b cause ectopic Msx expression (Fig. 1A',B') suggests that Msx1 is an effector in the Bmp signaling pathway for these specific phenotypes in the developing neural tube.
Overexpression of Msx1 but not Msx3 promotes roof-plate cell fate in a stage-dependent manner
Bmp signaling is involved in the generation of dorsal cell types including
neural crest and roof plate, and dorsal interneurons (reviewed by
Helms and Johnson, 2003;
Lee and Jessell, 1999
). To
determine whether Msx1, Msx3, or both, are involved in the generation of any
of these cell types, we examined markers for each of these cell types in the
electroporated neural tubes. Expression of the ca-Bmpr1 constructs
resulted in an increase in the number of cells migrating from the dorsal
aspect of the neural tube expressing Dlx, a marker for neural crest
(Fig. 4A,D). The ca-Bmpr1
constructs also induced the expression of the neural crest marker
Slug on the electroporated side of the neural tube
(Fig. 4E), consistent with the
ability of Bmps to induce Slug expression in chick neural plate
explant culture (Liem et al.,
1995
). Electroporation of both mouse Msx1 and
Msx3 resulted in an increase in the number of Dlx-expressing cells
migrating from the neural tube (Fig.
4B-D). However, in contrast to the activity of ca-Bmpr1, there was
no induction of Slug in the neural tube of Msx1- or
Msx3-expressing embryos (Fig.
4F,G). Thus, Msx1 and Msx3 can phenocopy some aspects of
activating Bmp signaling, but they cannot fully mimic these activities in
induction of neural crest in this assay.
|
|
Msx3 and late activation of Bmp signaling induce dorsal interneuron cell fates
Activation of Bmp signaling in late chick embryos (HH14-16) induces neural
progenitor cells to adopt a dorsal neuronal cell fate marked by Math1/Cath1
(Lee et al., 1998;
Timmer et al., 2002
).
Therefore, we examined Cath1 expression when Msx1 and Msx3 were overexpressed
at this later stage. No significant change in the number of Cath1+
cells between the control and electroporated sides was detected when Msx1 was
electroporated into HH14-16 embryos and analyzed 24 hours later at HH20-22
(data not shown). By contrast, a twofold increase of Cath1+ cells
was detected in embryos overexpressing Msx3 at HH14-16
(Fig. 6D,E,G). In fact, a small
increase of Cath1+ cells was also detected when Msx3 was
electroporated into HH10-12 embryos and analyzed 48 hours later
(Fig. 6A,B,G). Using double
label immunofluorescence we found many Cath1 and Msx double positive cells
(Fig. 6B,E, arrows), suggesting
a cell-autonomous mechanism. These results indicate that even though Msx3
exhibits similar DNA-binding and transcriptional repression activity in vitro
as Msx1 (Mehra-Chaudhary et al.,
2001
), the two proteins have very different in vivo activities in
terms of regulating Cath1 expression. Induction of Cath1
expression is dependent on the N-terminal arm of Msx3 homeodomain because
Msx3a did not significantly increase the number of Cath1+ cells
(Fig. 6C,F,G).
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The chick in ovo electroporation system provides a unique advantage to study stage-dependent activities of signaling pathways and transcription factors. Consistent with the explant studies, our results provide in vivo evidence for a temporal switch in the competence of neural progenitor cells in response to Bmp signaling. Overexpression of ca-Bmpr-1a and ca-Bmpr-1b in chick neural tube before or during neural tube closure (HH10-12) increased the number of cells expressing neural crest and roof-plate markers, while repressing neuronal differentiation and inducing apoptosis. By contrast, activation of Bmp signaling after neural tube closure (HH14-16) promoted the dorsal progenitor cells to adopt dI1 and dI3 interneuron cell fates rather than neural crest or roof-plate cell fate.
In this study, we found that mouse Msx transcription factors displayed a
similar set of activities in a stage-dependent fashion. Overexpression of
Msx1 in HH10-12, but not HH14-16 embryos, induced the expression of
roof-plate markers, caused ectopic apoptosis and repressed neuronal
differentiation. On the contrary, dorsal interneurons were induced by both
early and late overexpression of Msx3. Although the functions of Bmp
signaling in the development of dorsal midline have been established by both
loss-of-function and gain-of-function studies
(Hebert et al., 2002;
Liem et al., 1995
;
Panchision et al., 2001
), a
role for Msx1 in dorsal midline development has only recently been
demonstrated in mice (Bach et al.,
2003
). Msx1-/- mutant embryos lack a
functional dorsal midline in prosomere 1 of the developing diencephalon.
Roof-plate defects were not detected in the hindbrain and spinal cord of
Msx1-/- mutant or
Msx1-/-/Msx2-/- compound mutant
embryos, possibly owing to compensatory activity from Msx3
(Bach et al., 2003
). However,
in our chick electroporation assay, Msx3 displayed minimal activity in
inducing the roof-plate markers. Because the competence of dorsal progenitor
cells to respond to signals changes rapidly from stage to stage, it is
possible that Msx3 expressed at a different stage could induce midline gene
expression, and, thus, compensate for the absence of Msx1 and Msx2. How
molecular mechanisms control the temporal switch of neural progenitor cells to
respond to activating Bmp signaling pathway, and to activating Msx
transcription factors, remains an important question in dorsal neural tube
development.
The role of Msx factors as downstream effectors of Bmp signaling is
supported by experiments in multiple systems. The activation of the Bmp
signaling pathway is sufficient to induce the expression of Msx genes
at different sites (reviewed by Davidson,
1995), and genetic ablation of the roof plate in mouse resulted in
the loss of expression of Msx1 and Msx3
(Lee et al., 2000
) (K. Lee,
personal communication). The Xenopus Msx1 gene acts downstream of Bmp
signaling in epidermal induction and inhibition of neural differentiation in
early Xenopus embryos (Suzuki et
al., 1997
). Our data support a model in which Msx factors mediate
the stage-dependent activities of Bmp signaling in patterning the dorsal
neural tube. However, in some instances, overexpression of Msx genes does not
fully recapitulate the activities of Bmps. Thus, although Msx transcription
factors appear to mediate multiple aspects of Bmp signaling, there are likely
other parallel pathways required to account for the whole program induced by
Bmp signaling.
Distinct functions of Msx genes
The biochemical properties of two Msx family members, Msx1 and Msx2, have
been compared (Catron et al.,
1996). These two proteins bind a common consensus DNA site and
exhibit similar DNA-binding site preferences. Both factors function as
transcriptional repressors independent of DNA-binding in transfection assays.
There are subtle functional differences between the two transcription factors,
including a higher DNA-binding affinity of Msx2 and a greater potency of
repression by Msx1. Msx3 has also been shown to be a transcriptional repressor
in transfected cells (Mehra-Chaudhary et
al., 2001
). It has not been previously shown whether different Msx
factors have similar or distinct biological functions in any given
developmental system in vivo.
Our analysis of the functions of Msx1 and Msx3 in neural
tube development allowed us to uncover the distinct functions of these two
genes. We found three activities of Msx1 that are not shared by Msx3. First,
early overexpression of Msx1 but not Msx3 promotes
roof-plate cell fate. This is consistent with the observation that in E10.5 or
older mouse embryos the expression of Msx1 in the neural tube is
restricted to the roof plate, whereas that of Msx3 is in the dorsal
ventricular zone but devoid from the roof plate
(Wang et al., 1996). Second,
Msx1 represses neuronal differentiation along the entire dorsoventral axis of
the neural tube, whereas Msx3 induces the differentiation of dorsal
interneurons at the expense of ventral neurons. Inhibition of terminal
differentiation by Msx1 has been observed in multiple developmental systems
and two mechanisms have been proposed to account for this activity. Forced
expression of Msx1 efficiently blocks terminal differentiation of
multiple mesenchymal and epithelial progenitor cell types in culture and
differentiation of the mammary epithelium in transgenic mice, and this
activity is associated with the upregulation of cyclin D1 expression and an
increase of Cdk4 activity (Hu et al.,
2001
). Msx1 has also been shown to inhibit muscle differentiation
by repressing the expression of myogenic bHLH gene MyoD
(Bendall et al., 1999
). We
observed a similar repression of early differentiation genes by Msx1 in
nervous system development including expression of chick neural bHLH genes
Cath1, Cash1, Ngn1 and Ngn2, and the paired-homeobox gene
Pax7. Taken together, the data in the different systems suggest that
Msx1 blocks terminal differentiation by repressing the expression of
differentiation genes and by modulating cell cycle exit.
The third activity specific to Msx1 and not shared by Msx3 is that Msx1
induces apoptosis in developing neural tube. For proliferating cells, death
represents an alternative pathway to differentiation. Therefore, the increased
apoptosis could result from the inability of the progenitor cells
overexpressing Msx1 to properly leave the cell cycle and undergo
differentiation. It is interesting that increased apoptosis, decreased
proliferation and repression of neuronal differentiation have all been
described as properties of the dorsal midline of the developing telencephalon
(Furuta et al., 1997;
Hebert et al., 2002
;
Monuki et al., 2001
).
Therefore, it is possible that these activities we observed when
ca-Bmpr1 or Msx1 were electroporated into early neural tubes
may be connected to their ability to promote roof-plate development.
In contrast to the activities of Msx1 detailed above, the primary activity
of Msx3 in these assays seems to be the specification of dorsal interneurons.
In mouse, Msx3 is expressed specifically in the dorsal region of the
neural tube in early embryos prior to the onset of dorsal neuronal
differentiation (Wang et al.,
1996). We show that overexpression of Msx3 in the
developing neural tube induces Cath1-expressing progenitor cells and the
ventral expansion of dI1 and dI3 neurons. This activity is distinct from that
of Msx1 as induction of dorsal neuron differentiation was never observed in
Msx1-electroporated embryos.
It is important to note that in these studies we are overexpressing mouse
Msx1 and Msx3 in chick neural tubes. Although two chick Msx
genes have been cloned (Msx1/GHox7 and Msx2/GHox8), which
show high sequence homology and similar tissue distribution to the murine
Msx1 and Msx2 genes
(Coelho et al., 1991;
Yokouchi et al., 1991
), the
chick Msx3 gene has not been identified. Blast searches of two chick
EST databases
(http://www.chickest.udel.edu/
and
http://chick.umist.ac.uk/)
using the mouse Msx3 sequence failed to reveal a Msx3 ortholog. As
the complete sequence of the chick genome is not available, it is not yet
clear if a chick Msx3 gene exists. In the chick electroporation
assay, both chick Msx1 and Msx2 behaved like the mouse
Msx1, causing a reduction in the size of the neural tube and
repressing neuronal differentiation (Y.L. and J.E.J., unpublished). It is
possible that the differences seen in the activities of Msx1 and Msx3 in the
chick neural tube are due to evolutionary differences in the factors that they
interact with in chick, and thus, caution should be taken in assigning
specific functions for the mouse genes. Assignment of specific functions will
require additional loss-of-function analysis. Nevertheless, previous work in
both mouse and chick suggest that Msx3 and Msx1/2 may have opposing functions.
Whereas Msx2 acts to mediate an apoptotic response induced by Bmp signaling in
rhombomeres 3 and 5 of the hindbrain
(Graham et al., 1994
), the
expression of Msx3 in these rhombomeres is selectively repressed
(Shimeld et al., 1996
).
Furthermore, targeted disruption of Smad4, a downstream mediator of TGFß
signaling pathway, results in a reduction of Msx2 expression and an
activation of Msx3 expression in fibroblasts and differentiating ES
cells (Sirard et al., 2000
).
The difference in activities revealed in chick neural tube in our study
demonstrates that Msx1 and Msx3 have distinct activities and likely interact
with different co-factors.
Transcriptional regulation of neural differentiation genes by Msx
Two lines of evidence suggest that the neural bHLH genes that are crucial
for neuronal differentiation might be direct transcriptional targets of Msx1.
First, in our pursuit of factors that regulate the expression of the neural
bHLH genes by yeast one-hybrid screening, Msx1 was identified to be potential
regulator for both Math1 and Mash1 and several consensus
sites for Msx1 binding are present in the enhancer regions of
Math1/Cath1 and Mash1/Cash1 (S. Verma-Kurvari, P. J. Ebert,
and J.E.J., unpublished). Furthermore, both Msx1 and Msx3 can bind to these
consensus sites in vitro (Y.L. and J.E.J., unpublished). However, because in
vivo Msx1 represses the bHLH factor expression and Msx3 induces Cath1
expression, additional in vivo co-factors or chromatin properties that
modulate these activities must be invoked.
The analysis of the mutants Msx1a and Msx3a underscores the importance of
the homeodomains of Msx proteins for the activities seen in the overexpression
assays. Both mutants have three amino acids in the N-terminal arm of the
homeodomain replaced by arginine. Msx1a has previously been shown to be unable
to bind DNA (Zhang et al.,
1996; Zhang et al.,
1997
). Unfortunately, the N-terminal arm is not only responsible
for DNA-binding, it is also a major contact point for protein-protein
interactions between Msx1 and basal transcription factors such as TBP or other
homeodomain proteins such as Dlx (Zhang et
al., 1996
; Zhang et al.,
1997
). Therefore, these data alone are not sufficient to
distinguish whether the regulation of neural differentiation genes by Msx1
depends upon its ability to bind DNA. However, Msx1a was not inert when
electroporated into the neural tube but rather appeared to function as a
dominant negative, suggesting that the mutant protein could form a
non-functional complex with co-factor(s) important for Msx1 activity. The fact
that Msx1a and Msx3a did not show similar dominant-negative activity
(Table 1) implies that these
two related factors use different mechanisms for activity, and suggest that
they do not share protein interaction surfaces. Consistent with this notion,
sequences outside the homeodomains in Msx1 and Msx3 are not related.
Concluding statements
The data presented here clearly demonstrate distinct activities for the
related transcription factors Msx1 and Msx3. The activities shown for Msx1 and
Msx3 are not necessarily indicative of their in vivo functions under normal
expression conditions, as this would require additional loss-of-function
experiments. However, in the dorsal neural tube, Msx genes are likely to be
effectors of the Bmp pathway because they are induced upon activation of the
pathway, and we show that the two Msx factors tested can mimic multiple
distinct phenotypes seen with Bmp signaling activation. It is important to
note that the activities of the Msx factors alone do not account for all the
phenotypes seen with activation of the Bmp pathway. Important directions for
the future will be to determine other effectors of this signaling pathway, how
they interact with the Msx factors and the mechanisms controlling the temporal
competence of the developing neural tube to respond to these signals.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bach, A., Lallemand, Y., Nicola, M. A., Ramos, C., Mathis, L.,
Maufras, M. and Robert, B. (2003). Msx1 is required for
dorsal diencephalon patterning. Development
130,4025
-4036.
Barlow, A. J. and Francis-West, P. H. (1997).
Ectopic application of recombinant BMP-2 and BMP-4 can change patterning of
developing chick facial primordia. Development
124,391
-398.
Barth, K. A., Kishimoto, Y., Rohr, K. B., Seydler, C.,
Schulte-Merker, S. and Wilson, S. W. (1999). Bmp activity
establishes a gradient of positional information throughout the entire neural
plate. Development 126,4977
-4987.
Bei, M. and Maas, R. (1998). FGFs and BMP4
induce both Msx1-independent and Msx1-dependent signaling pathways in early
tooth development. Development
125,4325
-4333.
Bendall, A. J., Ding, J., Hu, G., Shen, M. M. and Abate-Shen,
C. (1999). Msx1 antagonizes the myogenic activity of Pax3 in
migrating limb muscle precursors. Development
126,4965
-4976.
Birren, S. J., Lo, L. and Anderson, D. J.
(1993). Sympathetic neuroblasts undergo a developmental switch in
trophic dependence. Development
119,597
-610.
Briscoe, J. and Ericson, J. (2001). Specification of neuronal fates in the ventral neural tube. Curr. Opin. Neurobiol. 11,43 -49.[CrossRef][Medline]
Catron, K. M., Zhang, H., Marshall, S. C., Inostroza, J. A., Wilson, J. M. and Abate, C. (1995). Transcriptional repression by Msx-1 does not require homeodomain DNA-binding sites. Mol. Cell Biol. 15,861 -871.[Abstract]
Catron, K. M., Wang, H., Hu, G., Shen, M. M. and Abate-Shen, C. (1996). Comparison of MSX-1 and MSX-2 suggests a molecular basis for functional redundancy. Mech. Dev. 55,185 -199.[CrossRef][Medline]
Chen, Y., Bei, M., Woo, I., Satokata, I. and Maas, R.
(1996). Msx1 controls inductive signaling in mammalian tooth
morphogenesis. Development
122,3035
-3044.
Coelho, C. N., Sumoy, L., Rodgers, B. J., Davidson, D. R., Hill, R. E., Upholt, W. B. and Kosher, R. A. (1991). Expression of the chicken homeobox-containing gene GHox-8 during embryonic chick limb development. Mech. Dev. 34,143 -154.[CrossRef][Medline]
Cornell, R. A. and Ohlen, T. V. (2000). Vnd/nkx, ind/gsh, and msh/msx: conserved regulators of dorsoventral neural patterning? Curr. Opin. Neurobiol. 10, 63-71.[CrossRef][Medline]
Davidson, D. (1995). The function and evolution of Msx genes: pointers and paradoxes. Trends Genet. 11,405 -411.[CrossRef][Medline]
Ericson, J., Thor, S., Edlund, T., Jessell, T. M. and Yamada, T. (1992). Early stages of motor neuron differentiation revealed by expression of homeobox gene Islet-1. Science 256,1555 -1560.[Medline]
Ericson, J., Morton, S., Kawakami, A., Roelink, H. and Jessell, T. M. (1996). Two critical periods of Sonic Hedgehog signaling required for the specification of motor neuron identity. Cell 87,661 -673.[Medline]
Furuta, Y., Piston, D. W. and Hogan, B. L.
(1997). Bone morphogenetic proteins (BMPs) as regulators of
dorsal forebrain development. Development
124,2203
-2212.
Ganan, Y., Macias, D., Duterque-Coquillaud, M., Ros, M. A. and
Hurle, J. M. (1996). Role of TGF beta s and BMPs as signals
controlling the position of the digits and the areas of interdigital cell
death in the developing chick limb autopod.
Development 122,2349
-2357.
Garcia-Castro, M. I., Marcelle, C. and Bronner-Fraser, M.
(2002). Ectodermal Wnt function as a neural crest inducer.
Science 297,848
-851.
Gowan, K., Helms, A. W., Hunsaker, T. L., Collisson, T., Ebert, P. J., Odom, R. and Johnson, J. E. (2001). Crossinhibitory activities of ngn1 and math1 allow specification of distinct dorsal interneurons. Neuron 31,219 -232.[Medline]
Graham, A., Francis-West, P., Brickell, P. and Lumsden, A. (1994). The signalling molecule BMP4 mediates apoptosis in the rhombencephalic neural crest. Nature 372,684 -686.[CrossRef][Medline]
Gross, M. K., Dottori, M. and Goulding, M. (2002). Lbx1 specifies somatosensory association interneurons in the dorsal spinal cord. Neuron 34,535 -549.[Medline]
Hamburger, V. and Hamilton, H. L. (1992). A series of normal stages in the development of the chick embryo. 1951. Dev. Dyn. 195,231 -272.[Medline]
Hebert, J. M., Mishina, Y. and McConnell, S. K. (2002). BMP signaling is required locally to pattern the dorsal telencephalic midline. Neuron 35,1029 -1041.[Medline]
Helms, A. W. and Johnson, J. E. (1998).
Progenitors of dorsal commissural interneurons are defined by MATH1
expression. Development
125,919
-928.
Helms, A. W. and Johnson, J. E. (2003). Specification of dorsal spinal cord interneurons. Curr. Opin. Neurobiol. 13,42 -49.[CrossRef][Medline]
Hill, R. E., Jones, P. F., Rees, A. R., Sime, C. M., Justice, M. J., Copeland, N. G., Jenkins, N. A., Graham, E. and Davidson, D. R. (1989). A new family of mouse homeo box-containing genes: molecular structure, chromosomal location, and developmental expression of Hox-7.1. Genes Dev. 3,26 -37.[Abstract]
Hu, G., Lee, H., Price, S., Shen, M. and Abate-Shen, C. (2001). Msx homeobox genes inhibit differentiation through upregulation of cyclin D1. Development 128,2373 -2384.[Medline]
Le Douarin, N. M. (1982). The Neural Crest, Vol. 12. Cambridge, UK: Cambridge University Press.
Lee, K. J. and Jessell, T. M. (1999). The specification of dorsal cell fates in the vertebrate central nervous system. Annu. Rev. Neurosci. 22,261 -294.[CrossRef][Medline]
Lee, K. J., Mendelsohn, M. and Jessell, T. M.
(1998). Neuronal patterning by BMPs: a requirement for GDF7 in
the generation of a discrete class of commissural interneurons in the mouse
spinal cord. Genes Dev.
12,3394
-3407.
Lee, K. J., Dietrich, P. and Jessell, T. M. (2000). Genetic ablation reveals that the roof plate is essential for dorsal interneuron specification. Nature 403,734 -740.[CrossRef][Medline]
Lee, M. K., Tuttle, J. B., Rebhun, L. I., Cleveland, D. W. and Frankfurter, A. (1990). The expression and posttranslational modification of a neuron-specific beta-tubulin isotype during chick embryogenesis. Cell Motil. Cytoskel. 17,118 -132.[Medline]
Liem, K. F., Jr, Tremml, G., Roelink, H. and Jessell, T. M. (1995). Dorsal differentiation of neural plate cells induced by BMP-mediated signals from epidermal ectoderm. Cell 82,969 -979.[Medline]
Liem, K. F., Jr, Tremml, G. and Jessell, T. M. (1997). A role for the roof plate and its resident TGFbeta-related proteins in neuronal patterning in the dorsal spinal cord. Cell 91,127 -138.[CrossRef][Medline]
Mansouri, A. and Gruss, P. (1998). Pax3 and Pax7 are expressed in commissural neurons and restrict ventral neuronal identity in the spinal cord. Mech. Dev. 78,171 -178.[CrossRef][Medline]
Mehra-Chaudhary, R., Matsui, H. and Raghow, R. (2001). Msx3 protein recruits histone deacetylase to down-regulate the Msx1 promoter. Biochem. J. 353, 13-22.[CrossRef][Medline]
Millonig, J. H., Millen, K. J. and Hatten, M. E. (2000). The mouse Dreher gene Lmx1a controls formation of the roof plate in the vertebrate CNS. Nature 403,764 -769.[CrossRef][Medline]
Monuki, E. S., Porter, F. D. and Walsh, C. A. (2001). Patterning of the dorsal telencephalon and cerebral cortex by a roof plate-Lhx2 pathway. Neuron 32,591 -604.[Medline]
Muller, T., Brohmann, H., Pierani, A., Heppenstall, P. A., Lewin, G. R., Jessell, T. M. and Birchmeier, C. (2002). The homeodomain factor lbx1 distinguishes two major programs of neuronal differentiation in the dorsal spinal cord. Neuron 34,551 -562.[Medline]
Muramatsu, T., Mizutani, Y., Ohmori, Y. and Okumura, J. (1997). Comparison of three nonviral transfection methods for foreign gene expression in early chicken embryos in ovo. Biochem. Biophys. Res. Commun. 230,376 -380.[CrossRef][Medline]
Newberry, E. P., Latifi, T., Battaile, J. T. and Towler, D. A. (1997). Structure-function analysis of Msx2-mediated transcriptional suppression. Biochemistry 36,10451 -10462.[CrossRef][Medline]
Nguyen, V. H., Trout, J., Connors, S. A., Andermann, P.,
Weinberg, E. and Mullins, M. C. (2000). Dorsal and
intermediate neuronal cell types of the spinal cord are established by a BMP
signaling pathway. Development
127,1209
-1220.
Panchision, D. M., Pickel, J. M., Studer, L., Lee, S. H.,
Turner, P. A., Hazel, T. G. and McKay, R. D. (2001).
Sequential actions of BMP receptors control neural precursor cell production
and fate. Genes Dev. 15,2094
-2110.
Panganiban, G., Sebring, A., Nagy, L. and Carroll, S. (1995). The development of crustacean limbs and the evolution of arthropods. Science 270,1363 -1366.[Abstract]
Parr, B. A., Shea, M. J., Vassileva, G. and McMahon, A. P.
(1993). Mouse Wnt genes exhibit discrete domains of expression in
the early embryonic CNS and limb buds. Development
119,247
-261.
Pierani, A., Moran-Rivard, L., Sunshine, M. J., Littman, D. R., Goulding, M. and Jessell, T. M. (2001). Control of interneuron fate in the developing spinal cord by the progenitor homeodomain protein Dbx1. Neuron 29,367 -384.[Medline]
Riddle, R. D., Ensini, M., Nelson, C., Tsuchida, T., Jessell, T. M. and Tabin, C. (1995). Induction of the LIM homeobox gene Lmx1 by WNT7a establishes dorsoventral pattern in the vertebrate limb. Cell 83,631 -640.[Medline]
Robert, B., Sassoon, D., Jacq, B., Gehring, W. and Buckingham, M. (1989). Hox-7, a mouse homeobox gene with a novel pattern of expression during embryogenesis. EMBO J. 8, 91-100.[Abstract]
Satokata, I., Ma, L., Ohshima, H., Bei, M., Woo, I., Nishizawa, K., Maeda, T., Takano, Y., Uchiyama, M., Heaney, S. et al. (2000). Msx2 deficiency in mice causes pleiotropic defects in bone growth and ectodermal organ formation. Nat. Genet. 24,391 -395.[CrossRef][Medline]
Satokata, I. and Maas, R. (1994). Msx1 deficient mice exhibit cleft palate and abnormalities of craniofacial and tooth development. Nat. Genet. 6, 348-356.[Medline]
Shimeld, S. M., McKay, I. J. and Sharpe, P. T. (1996). The murine homeobox gene Msx-3 shows highly restricted expression in the developing neural tube. Mech. Dev. 55,201 -210.[CrossRef][Medline]
Sirard, C., Kim, S., Mirtsos, C., Tadich, P., Hoodless, P. A.,
Itie, A., Maxson, R., Wrana, J. L. and Mak, T. W. (2000).
Targeted disruption in murine cells reveals variable requirement for Smad4 in
transforming growth factor beta-related signaling. J. Biol.
Chem. 275,2063
-2070.
Song, K., Wang, Y. and Sassoon, D. (1992). Expression of Hox-7.1 in myoblasts inhibits terminal differentiation and induces cell transformation. Nature 360,477 -481.[CrossRef][Medline]
Suzuki, A., Ueno, N. and Hemmati-Brivanlou, A.
(1997). Xenopus msx1 mediates epidermal induction and neural
inhibition by BMP4. Development
124,3037
-3044.
Tanabe, Y., William, C. and Jessell, T. M. (1998). Specification of motor neuron identity by the MNR2 homeodomain protein. Cell 95, 67-80.[Medline]
Timmer, J., Wang, C. and Niswander, L. (2002). BMP signaling patterns the dorsal and intermediate neural tube via regulation of homeobox and helix-loop-helix transcription factors. Development 129,2459 -2472.[Medline]
Tsuchida, T., Ensini, M., Morton, S. B., Baldassare, M., Edlund, T., Jessell, T. M. and Pfaff, S. L. (1994). Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes. Cell 79,957 -970.[Medline]
Vainio, S., Karavanova, I., Jowett, A. and Thesleff, I. (1993). Identification of BMP-4 as a signal mediating secondary induction between epithelial and mesenchymal tissues during early tooth development. Cell 75,45 -58.[Medline]
Wang, W., Chen, X., Xu, H. and Lufkin, T. (1996). Msx3: a novel murine homologue of the Drosophila msh homeobox gene restricted to the dorsal embryonic central nervous system. Mech. Dev. 58,203 -215.[CrossRef][Medline]
Woloshin, P., Song, K., Degnin, C., Killary, A. M., Goldhamer, D. J., Sassoon, D. and Thayer, M. J. (1995). MSX1 inhibits myoD expression in fibroblast x 10T1/2 cell hybrids. Cell 82,611 -620.[Medline]
Yokouchi, Y., Ohsugi, K., Sasaki, H. and Kuroiwa, A. (1991). Chicken homeobox gene Msx-1: structure, expression in limb buds and effect of retinoic acid. Development 113,431 -444.[Abstract]
Yuan, S. and Schoenwolf, G. C. (1999). The spatial and temporal pattern of C-Lmx1 expression in the neuroectoderm during chick neurulation. Mech. Dev. 88,243 -247.[CrossRef][Medline]
Zhang, H., Catron, K. M. and Abate-Shen, C.
(1996). A role for the Msx-1 homeodomain in transcriptional
regulation: residues in the N-terminal arm mediate TATA binding protein
interaction and transcriptional repression. Proc. Natl. Acad. Sci.
USA 93,1764
-1769.
Zhang, H., Hu, G., Wang, H., Sciavolino, P., Iler, N., Shen, M. M. and Abate-Shen, C. (1997). Heterodimerization of Msx and Dlx homeoproteins results in functional antagonism. Mol. Cell Biol. 17,2920 -2932.[Abstract]
Related articles in Development: