Department of Developmental Biology, Faculty of Biology, Utrecht University, Padualaan 8, NL-3584CH Utrecht, The Netherlands
Author for correspondence (e-mail:
r.dono{at}bio.uu.nl)
Accepted 4 July 2003
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SUMMARY |
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Key words: Antisense morpholino oligo, Cell survival, Emx2, ERK, Gastrulation, Neurulation, Xenopus
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Introduction |
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Different types of signalling molecules, their antagonists and receptors
regulate regionalization of the anterior neural plate
(Rubenstein et al., 1998;
Wilson and Rubenstein, 2000
).
For example, antagonism of WNT signalling is necessary for correct subdivision
of the anterior neural plate into telencephalon, diencephalon and eye
territories (Wilson and Rubenstein,
2000
; Houart et al.,
2002
). BMP7 and SHH signalling by the prechordal mesoderm induces
Nkx2.1 and directs neural plate cells towards hypothalamic fate
(Wilson and Rubenstein, 2000
).
Other BMP family members are produced by the non-neural ectoderm adjacent to
the anterior neural plate and regulate expression of anterior neural markers
and dorsal forebrain development in a dose-dependent manner
(Wilson and Rubenstein, 2000
;
Hartley et al., 2001
).
Accordingly, inactivation of the BMP antagonists chordin and
noggin in mouse embryos causes defects in forebrain patterning
(Wilson and Rubenstein, 2000
).
The role of FGFs during forebrain morphogenesis appears widespread as several
FGFs, such as Fgf8, Fgf2 and Fgf9, are expressed by the
anterior neural plate and forebrain primordia (reviewed by
Dono, 2003
). For example,
embryological and genetic studies have shown that FGF8, produced by the
anterior neural ridge, participates in inducing the telencephalon and in
differentiation of anterior midline cells
(Rubenstein et al., 1998
;
Eagleson and Dempewolf, 2002
).
Moreover, FGF8 acts in a dose-dependent manner to control cell survival in the
developing forebrain in the mouse (Storm
et al., 2003
). In zebrafish embryos, FGFs also regulate
dorsoventral forebrain patterning, as evidenced by genetic analysis
(Shanmugalingam et al., 2000
)
and transient inhibition of FGF signal transduction by the chemical inhibitor
SU5402 (Shinya et al., 2001
).
These latter studies showed that FGF8 and FGF3 cooperate to promote
Nkx2.1 expression and morphogenesis of the ventral telencephalon. In
addition, FGF8 and FGF2 can induce dorsal forebrain genes, such as
Emx1, in neuralized Xenopus animal cap explants
(Lupo et al., 2002
).
Cell-cell signalling interactions are modulated by cell surface proteins,
including glypicans. Glypicans, like other heparan sulphate proteoglycans
(HSPG), bind FGFs, WNTs and BMPs through their heparan sulphate
glycosaminoglycan (HS-GAG) side-chains
(Hagihara et al., 2000;
Nybakken and Perrimon, 2002
).
It has been proposed that glypicans regulate cell signalling by either
promoting or stabilizing the interactions of ligands with their cognate high
affinity receptors (Nybakken and Perrimon,
2002
). For example, vertebrate glypican 1 binds FGFs, thereby
favouring assembly of the ligand-receptor complex
(Steinfeld et al., 1996
).
Alternatively, glypicans such as Drosophila Dally-like may shape
ligand gradients by restricting their diffusion within the extracellular
matrix (Baeg et al., 2001
).
Dally, another Drosophila glypican regulates imaginal disc patterning
and morphogenesis by positive and differential modulation of wingless
(wg) and decapentaplegic (dpp) signalling
(Nybakken and Perrimon, 2002
).
Genetic analysis of the zebrafish Knypek shows that this glypican functions to
potentiate non-canonical WNT signalling. By modulating WNT11 activity, Knypek
regulates the convergent-extension movements during zebrafish gastrulation
(Topczewski et al., 2001
). In
mice, glypican 3 is required for the cellular response to BMP and FGF
signalling during organogenesis (Grisaru
et al., 2001
). Furthermore, several glypican family members are
expressed in the developing central nervous system (CNS) (reviewed by
Song and Filmus, 2002
). One of
them, glypican 4 (Gpc4) is predominantly expressed in the presumptive
forebrain territory during head-fold stages in mouse embryos (A.G. and R.D.,
unpublished). Subsequently, its expression persists in neuronal progenitors of
the developing forebrain (Hagihara et al.,
2000
).
In the present study, we functionally analyse the Xenopus Gpc4
gene by interfering with protein translation through specific antisense
morpholino oligonucleotides. Such depletion of GPC4 in developing embryos
results in gastrulation and axis elongation defects similar to those caused by
the zebrafish knypek mutation. Furthermore, we identify GPC4 as a key
regulator of dorsoventral forebrain patterning. In particular, loss of GPC4
activity results in downregulation of dorsal forebrain identity genes from
early neural plate stages onwards, and massive cell death in the anterior CNS
during neural tube closure. We show that GPC4 binds FGF2 and that inhibition
of FGF signalling by SU5402 (Mohammadi et
al., 1997) results in dorsal forebrain phenotypes similar to those
of GPC4-depleted embryos. We conclude that establishment and patterning of the
dorsal forebrain territory requires modulation of FGF signalling by GPC4.
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Materials and methods |
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Embryo manipulations
Xenopus laevis eggs were fertilized and cultured following
standard protocols (Sive et al.,
2000). For the functional analysis of GPC4, two-cell stage embryos
were injected with 30-40 ng antisense morpholino oligo per blastomere at the
animal pole. To test the efficiency of the Gpc4Mo in vivo, 600 pg capped
Gpc4GFP mRNA was injected into two-cell stage embryos. Subsequently,
a total of 100 ng Gpc4Mo or CoMo was injected in either one or both
blastomeres. To rescue the molecular and morphological defects of
Gpc4Mo-injected embryos, a total of 60 ng Gpc4Mo (or CoMo) was injected into
both blastomeres of two-cell stage embryos. After completion of the second
division, a total of 800 pg mouse Gpc4 capped mRNA was injected into
the two dorsal blastomeres. For the inhibition of FGF signalling by SU5402
(Calbiochem) treatment of embryos, embryos were cultured in normal medium
(MBS) (Sive et al., 2000
)
until the onset of neurulation (stage 13). From stage 13 onwards, embryos were
cultured in MBS supplemented with SU5402 (0.1 mg/ml final concentration;
dissolved in DMSO) or DMSO (same final concentration) until harvesting them
between stages 15 and 21-22 for analysis.
Whole-mount in situ hybridisation and detection of apoptotic
cells
Whole-mount in situ hybridisation was performed as previously described
(Sive et al., 2000), and
pigment granules were bleached as described
(Song and Slack, 1994
).
Apoptotic cells were detected by using the in situ cell death detection kit
(sections, fluorescein; whole mounts, POD, Roche) according to the
manufacturer instructions with only minor modifications.
Proteins binding assays and immunoblot analysis
For binding assays, NIH3T3 cells were transfected with 10 µg mouse
Gpc4-Myc plasmid. Cells were lysed 36 hours after transfection in PBS
containing 0.5% NP40. After sonication, GST-FGF2 binding assays were performed
as described (Fumagalli et al.,
1994). Proteins were separated by 8% SDS-PAGE and Myc
epitope-tagged GPC4 was detected by anti-Myc antibodies. For analysis of ERK
and SMAD1 phosphorylation levels embryos were lysed and proteins separated on
a 15% gel. Proteins were immunoblotted using anti-pSMAD1
(Persson et al., 1998
),
anti-pERK (Cell Signalling) and anti-
tubulin antibodies (Sigma).
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Results |
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Xenopus Gpc4 is a maternally expressed gene as transcripts are
detected in the animal hemisphere from the two-cell stage up to blastula
stages (Fig. 1A; data not
shown). At the onset of gastrulation, expression expands to the marginal zone
(Fig. 1B). During progression
of gastrulation (Fig. 1C,D),
Gpc4 transcripts become progressively localized to the dorsal side of
the embryo. In particular, high levels of Gpc4 transcripts are
detected in the area of Spemann's organizer during gastrulation
(Fig. 1C,D). At this stage, the
Gpc4 transcript domain encompasses those of Noggin (compare
Fig. 1D and E) (Smith and Harland, 1992) and
Chordin (data not shown), which indicates that Gpc4 is
expressed by the prechordal endomesoderm and chordamesoderm (see also
Ohkawara et al., 2003
). In
addition, the Gpc4 expression domain also encompasses that of
Sox2 (Mizuseki et al.,
1998
), an early marker for neural fates (compare
Fig. 1D and F). This latter
result shows that presumptive neuroectodermal cells express Gpc4
during neural cell fate specification.
|
By mid-neurulation (Fig.
1J), the anterior Gpc4 expression resolves into two
distinct domains. The posterior domain overlaps with that of Emx2
(compare Fig. 1J and K), one of
the earliest genes expressed in presumptive dorsal forebrain territories
(Pannese et al., 1998). In the
developing dorsal forebrain, Gpc4 transcripts persist up to early
neural tube stages (Fig.1L;
data not shown). From tailbud stages onwards, other predominant sites of
Gpc4 expression include the developing branchial arches, somites and
pronephric ducts (data not shown).
GPC4 is required for gastrulation and nervous system patterning in
Xenopus embryos
An antisense morpholino oligonucleotide directed against the 5'
leader of the Xenopus Gpc4 mRNA was used to block GPC4 protein
translation. Initially, we assessed the efficiency of two candidate oligos
(see Materials and methods). One of these, Gpc4Mo, blocks translation of
Gpc4 mRNA very efficiently both in vitro
(Fig. 2A, upper panel) and in
vivo (Fig. 2C,D). Therefore,
Gpc4Mo and an unrelated control antisense morpholino oligo (CoMo;
Fig. 2A lower panel,
Fig. 2B) were used for all
studies shown.
|
To investigate the molecular and cellular defects underlying the
gross-morphological alterations of GPC4-depleted embryos
(Fig. 2), we analysed the
expression of genes regulating gastrulation and neurulation. The expression of
Goosecoid (Gsc) (Cho et
al., 1991) appears initially normal, indicating that GPC4 does not
affect establishment of Spemann's organizer (compare
Fig. 3A and B; n=3/3).
During gastrulation, Gsc-expressing cells ingress and move toward the
anterior of the embryo. Because of this anterior expansion, the Gsc
expression domain narrows and elongates in control embryos
(Fig. 3C), whereas it remains
broad in Gpc4Mo-injected embryos (Fig.
3D; n=8/8). Changes in the spatial distribution of
mesodermal and neuroectodermal genes become more apparent towards the end of
gastrulation. For example, Xenopus Brachyury (Xbra)
(Smith et al., 1991
) is
detected in the developing mesoderm around the blastopore and in the
presumptive notochord in control embryos
(Fig. 3E). In Gpc4Mo-injected
embryos, the length of the presumptive notochord is very much reduced (arrow
in Fig. 3F; n=9/10)
and Xbra expression remains predominantly around the enlarged
blastopore. Accordingly, analysis of Noggin expression in the
prospective notochord (Smith and Harland,
1992
) shows that the posterior extension of its expression domain
is shorter and remains wider in comparison with control embryos (compare
Fig. 3G and H; n=13/17). By contrast, the anterior Noggin (asterisks in
Fig. 3G,H; n=13/17)
and Dkk1 expression domains (data not shown), which mark the anterior
endoderm and prechordal endomesoderm, seem normal. Neural induction is also
not affected, as expression levels of the pan-neural marker Sox2
(Mizuseki et al., 1998
) are
normal (compare Fig. 3I and J).
However, the posterior neuroectoderm lacks the characteristic neural plate
morphology (asterisk in Fig.
3J; n=9/10) apparent in control embryos (asterisk,
Fig. 3I), which is in agreement
with the altered Xbra and Noggin expression in the notochord
(compare Fig. 3F and H).
Finally, analysis of Et expression
(Li et al., 1997
) in
GPC4-depleted embryos shows that two retinal and eye primordia develop
(compare Fig. 3K and L). These
findings are in agreement with normal Shh expression in the ventral
midline (data not shown). Taken together, these results show that inhibition
of GPC4 function during gastrulation affects anteroposterior axis elongation,
whereas the head organizer, specification of the anterior neuroectoderm and
ventral midline formation seem normal.
|
|
|
Rescue of forebrain patterning defects by co-injection of mouse
Gpc4 mRNA
The following rescue experiment was performed to assess whether the
molecular and morphological defects in forebrain patterning are specifically
caused by the interference of Gpc4Mo with GPC4 function. Xenopus
embryos were co-injected with Gpc4Mo and mouse Gpc4 mRNA, which lacks
the Gpc4Mo target sequence (data not shown). Such co-injection, rescues
Emx2 expression in 69% of all embryos
(Fig. 5N;
Table 1). Furthermore, forehead
morphology and Emx2 distribution in the dorsal forebrain of rescued
tailbud embryos (Fig. 5R) are
similar to control embryos (Fig.
5O). By contrast, mouse Gpc4 mRNA does not significantly
alter Emx2 expression and dorsal forebrain patterning upon
co-injection with CoMo (Fig.
5M,P). Taken together, these results demonstrate that GPC4
function is required to regulate expression of dorsal forebrain identity
genes.
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Discussion |
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GPC4 is required for forebrain patterning in Xenopus
embryos
It is unlikely that GPC4 acts during head and anterior neural plate
induction, as the cement gland, ventral forebrain, two eye primordia and
olfactory placodes form. The latter two structures derive from the most
anterior neural plate (Rubenstein et al.,
1998), which indicates that the most anterior brain structures are
present in GPC4-depleted Xenopus embryos. In agreement with this,
Otx2, the earliest anterior neural plate marker
(Rubenstein et al., 1998
), is
expressed during gastrulation and is only downregulated during neurulation. In
contrast to abrogation of GPC4, inhibition of Dkk1 and Igf,
which regulate head- and anterior neural plate induction, results in severe
microcephaly and a complete loss of the cement gland and eyes
(Glinka et al., 1998
;
Pera et al., 2001
). Moreover,
abrogation of Tlc and Axin, two inhibitors of WNT
signalling, disrupts anteroposterior regionalization of the forebrain, causing
loss of both ventral and dorsal forebrain and eye fields
(Wilson and Rubenstein, 2000
;
Houart et al., 2002
). These
phenotypes are much more severe, and their appearance significantly precedes
the ones observed in GPC4-depleted Xenopus embryos.
Subsequently, inductive signals emanating from the prechordal plate (e.g.
SHH) and anterior neural ridge (e.g. FGF8) act on anterior neural plate cells
to establish regional differences, such as specification of dorsal and ventral
forebrain identities (Rubenstein et al.,
1998). Gpc4 is expressed by the prechordal endomesoderm
during gastrulation and by the anterior neural plate at the time when these
signalling centers are active. However, the Shh and Fgf8
expression domains are established correctly in Gpc4Mo-injected
Xenopus embryos. Inactivation of Shh and Fgf8
causes ventral forebrain defects
(Rubenstein et al., 1998
) in
contrast to interfering with GPC4 activity (this study). Therefore, the dorsal
forebrain defects observed in Gpc4Mo-injected embryos most likely arise by
altering the reception of signals targeted to dorsal neuroectodermal cells
prior to closure of the anterior neural tube (see below).
In Xenopus and mouse embryos, cells of the presumptive forebrain
begin to express Gpc4 during neurulation (this study) (A.G. and R.D.,
unpublished), and in the embryonic mouse brain expression persists in
telencephalic neural precursors (Hagihara
et al., 2000). Mutations in human GPC3 and GPC4
genes, which are next to one another on the X-chromosome, have been linked to
the Simpson-Golabi-Behmel syndrome (SGBS). The SGBS syndrome is characterized
by general pre- and postnatal overgrowth (reviewed by
DeBaun et al., 2001
). A
fraction of SGBS patients also show mental retardation, seizures and a high
risk for neuroblastoma (DeBaun et al.,
2001
). In the present study, we show that abrogation of GPC4
activity in Xenopus embryos disrupts forebrain patterning and cell
survival, and causes microcephaly. Therefore, our findings raise the
possibility that some of the CNS abnormalities affecting SGBS patients may
arise as a consequence of disrupting Gpc4 gene function during
neurulation. In GPC4-depleted Xenopus embryos, the expression of
dorsal forebrain identity genes, such as Emx2 and Emx1, is
disrupted already during neurulation. Previous genetic analysis of Emx genes
in mice has established that they regulate regionalization and expansion of
the dorsal forebrain compartment and subsequent cerebral cortex morphogenesis
(Yoshida et al., 1997
;
Mallamaci et al., 2000
). In
particular, Emx1 and Emx2 compound-mutant embryos have
greatly reduced telencephalic vesicles prior to initiation of cerebral cortex
development (Bishop et al.,
2003
). Therefore, the dorsal forebrain defects observed in
GPC4-depleted Xenopus embryos could be a consequence of mainly
disrupting expression of the EMX genes during neurulation.
GPC4 modulates FGF signalling in the developing dorsal forebrain
Patterning of the vertebrate CNS depends to a large extent on extracellular
regulation of signals (Rubenstein et al.,
1998; Wilson and Rubenstein,
2000
). Glypicans regulate signalling by modulating the formation
of receptor-ligand complexes (Nybakken and
Perrimon, 2002
). In agreement with this, abrogation of GPC4
function in neurulating Xenopus embryos reduces phosphorylation of
ERK protein kinases, which are specific targets of FGF signalling
(Christen and Slack, 1999
).
This result shows that GPC4 participates in enhancing FGF signal transduction
during embryogenesis. Similarly, genetic studies in Drosophila show
that formation of an active FGF receptor-ligand complex depends on the
presence of HSPGs (Lin et al.,
1999
). Inhibition of FGF signalling by SU5402 in Xenopus
embryos phenocopies aspects of depleting GPC4 function, such as loss-of
Emx2 expression and increased apoptosis of forebrain progenitors.
Several FGF ligands and their cognate receptors are expressed during
patterning of the vertebrate CNS (Dono,
2003
). Genetic and functional analysis established that two of
these ligands, FGF8 and FGF3, function during formation of mid-hindbrain and
rhombomere boundaries, respectively, in vertebrate embryos. Moreover, both FGF
ligands participate in patterning of the anterior telencephalic midline and
the anterior and post-optic commissure
(Wilson and Rubenstein, 2000
;
Shinya et al., 2001
). The
present study establishes that FGF signalling also regulates dorsal forebrain
development, but the involved FGF ligand(s) remains to be identified.
Candidates are FGF9 (Song and Slack,
1996
) and, in particular, FGF2, as this FGF ligand is present
throughout the brain during Xenopus neurulation
(Song and Slack, 1994
) and
binds GPC4 (this study). FGF2-deficient mice display defects in dorsal
telencephalon patterning, albeit only much later during cerebral cortex layer
formation (Dono, 2003
).
Therefore, further functional and genetic analysis is necessary to identify
and study the FGF ligands interacting with GPC4 in embryos.
Comparative analysis of GPC4-depleted and SU5402-treated Xenopus
embryos suggests that modulation of BMP and/or WNT signalling does not
significantly contribute to Emx2 regulation in the dorsal forebrain.
By contrast, the similarities in the axis defects between GPC4-depleted
Xenopus (Ohkawara et al.,
2003) and knypek-deficient zebrafish embryos points to
possible effects on non-canonical WNT signalling during gastrulation (see
before). Therefore, glypicans may control the activity of different ligands in
a stage- and/or tissue-specific manner as shown for Drosophila Dally,
which regulates wg during embryonic development and dpp
signalling during post-embryonic development
(Nybakken and Perrimon, 2002
).
Modifications of proteins by HS-GAG side chains are not uniform and changes in
the distribution of sulphate groups affect ligand-binding properties. Enzymes
involved in HSPG biosynthesis modify the HS-GAG side chains of Glypicans and
regulate their ability to bind signal peptides during Drosophila
embryogenesis (Giraldez et al.,
2002
). It will be important to determine if, and to what extent,
alterations of HS-GAG side-chains of GPC4 can confer it with the ability to
bind WNT during gastrulation (Ohkawara et
al., 2003
) and FGF ligands during neurulation (this study). Such
alterations may explain cell-type and developmental-stage specific modulation
of ligand-receptor interactions by glypicans during vertebrate
embryogenesis.
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ACKNOWLEDGMENTS |
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Footnotes |
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REFERENCES |
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---|
Baeg, G. H., Lin, X., Khare, N., Baumgartner, S. and Perrimon,
N. (2001). Heparan sulfate proteoglycans are critical for the
organization of the extracellular distribution of Wingless.
Development 128,87
-94.
Bishop, K. M., Garel, S., Nakagawa, Y., Rubenstein, J. L. and O'Leary, D. D. (2003). Emx1 and Emx2 cooperate to regulate cortical size, lamination, neuronal differentiation, development of cortical efferents, and thalamocortical pathfinding. J. Comp. Neurol. 457,345 -360.[CrossRef][Medline]
Bourguignon, C., Li, J. and Papalopulu, N.
(1998). XBF-1, a winged helix transcription factor with dual
activity, has a role in positioning neurogenesis in Xenopus competent
ectoderm. Development
125,4889
-4900.
Bradley, L. C., Snape, A., Bhatt, S. and Wilkinson, D. G. (1993). The structure and expression of the Xenopus Krox-20 gene: conserved and divergent patterns of expression in rhombomeres and neural crest. Mech. Dev. 40,73 -84.[CrossRef][Medline]
Brunelli, S., Faiella, A., Capra, V., Nigro, V., Simeone, A., Cama, A. and Boncinelli, E. (1996). Germline mutations in the homeobox gene EMX2 in patients with severe schizencephaly. Nat. Genet. 12,94 -96.[Medline]
Cho, K. W., Goetz, J., Wright, C. V., Fritz, A., Hardwicke, J. and De Robertis, E. M. (1988). Differential utilization of the same reading frame in a Xenopus homeobox gene encodes two related proteins sharing the same DNA-binding specificity. EMBO J. 7,2139 -2149.[Abstract]
Cho, K. W., Blumberg, B., Steinbeisser, H. and De Robertis, E. M. (1991). Molecular nature of Spemann's organizer: the role of the Xenopus homeobox gene goosecoid. Cell 67,1111 -1120.[Medline]
Christen, B. and Slack, J. M. (1999). Spatial
response to fibroblast growth factor signalling in Xenopus embryos.
Development 126,119
-125.
DeBaun, M. R., Ess, J. and Saunders, S. (2001). Simpson Golabi Behmel syndrome: progress toward understanding the molecular basis for overgrowth, malformation, and cancer predisposition. Mol. Genet. Metab. 72,279 -286.[CrossRef][Medline]
Dono, R. (2003). Fibroblast growth factors as
regulators of central nervous system development and function. Am.
J. Physiol. Regul. Integr. Comp. Physiol.
284,R867
-R881.
Eagleson, G. W. and Dempewolf, R. D. (2002). The role of the anterior neural ridge and Fgf-8 in early forebrain patterning and regionalization in Xenopus laevis. Comp. Biochem. Physiol. B. Biochem. Mol. Biol. 132,179 -189.[CrossRef][Medline]
Fumagalli, S., Totty, N. F., Hsuan, J. J. and Courtneidge, S. A. (1994). A target for Src in mitosis. Nature 368,871 -874.[CrossRef][Medline]
Giraldez, A. J., Copley, R. R. and Cohen, S. M. (2002). HSPG modification by the secreted enzyme Notum shapes the Wingless morphogen gradient. Dev. Cell 2, 667-676.[Medline]
Glinka, A., Wu, W., Delius, H., Monaghan, A. P., Blumenstock, C. and Niehrs, C. (1998). Dickkopf-1 is a member of a new family of secreted proteins and functions in head induction. Nature 391,357 -362.[CrossRef][Medline]
Grisaru, S., Cano-Gauci, D., Tee, J., Filmus, J. and Rosenblum, N. D. (2001). Glypican-3 modulates BMP- and FGF-mediated effects during renal branching morphogenesis. Dev. Biol. 231,31 -46.[CrossRef][Medline]
Hagihara, K., Watanabe, K., Chun, J. and Yamaguchi, Y. (2000). Glypican-4 is an FGF2-binding heparan sulfate proteoglycan expressed in neural precursor cells. Dev. Dyn. 219,353 -367.[CrossRef][Medline]
Hartley, K. O., Hardcastle, Z., Friday, R. V., Amaya, E. and Papalopulu, N. (2001). Transgenic Xenopus embryos reveal that anterior neural development requires continued suppression of BMP signaling after gastrulation. Dev. Biol. 238,168 -184.[CrossRef][Medline]
Hollemann, T. and Pieler, T. (2000). Xnkx-2.1: a homeobox gene expressed during early forebrain, lung and thyroid development in Xenopus laevis. Dev. Genes Evol. 210,579 -581.[CrossRef][Medline]
Houart, C., Caneparo, L., Heisenberg, C., Barth, K., Take-Uchi, M. and Wilson, S. (2002). Establishment of the telencephalon during gastrulation by local antagonism of Wnt signaling. Neuron 35,255 -265.[Medline]
Li, H., Tierney, C., Wen, L., Wu, J. Y. and Rao, Y.
(1997). A single morphogenetic field gives rise to two retina
primordia under the influence of the prechordal plate.
Development 124,603
-615.
Lin, X., Buff, E. M., Perrimon, N. and Michelson, A. M.
(1999). Heparan sulfate proteoglycans are essential for FGF
receptor signaling during Drosophila embryonic development.
Development 126,3715
-3723.
Lupo, G., Harris, W. A., Barsacchi, G. and Vignali, R.
(2002). Induction and patterning of the telencephalon in
Xenopus laevis. Development
129,5421
-5436.
Mallamaci, A., Muzio, L., Chan, C. H., Parnavelas, J. and Boncinelli, E. (2000). Area identity shifts in the early cerebral cortex of Emx2/ mutant mice. Nat. Neurosci. 3,679 -686.[CrossRef][Medline]
Mizuseki, K., Kishi, M., Matsui, M., Nakanishi, S. and Sasai,
Y. (1998). Xenopus Zic-related-1 and Sox-2, two
factors induced by chordin, have distinct activities in the initiation of
neural induction. Development
125,579
-587.
Mohammadi, M., McMahon, G., Sun, L., Tang, C., Hirth, P., Yeh,
B. K., Hubbard, S. R. and Schlessinger, J. (1997).
Structures of the tyrosine kinase domain of fibroblast growth factor receptor
in complex with inhibitors. Science
276,955
-960.
Nybakken, K. and Perrimon, N. (2002). Heparan sulfate proteoglycan modulation of developmental signaling in Drosophila. Biochim. Biophys. Acta 1573,280 -291.[Medline]
Ohkawara, B., Yamamoto, T. S., Tada, M. and Ueno, N.
(2003). Role of glypican 4 in the regulation of convergent
extension movements during gastrulation in Xenopus laevis.Development 130,2129
-2138.
Pannese, M., Polo, C., Andreazzoli, M., Vignali, R., Kablar, B.,
Barsacchi, G. and Boncinelli, E. (1995). The
Xenopus homologue of Otx2 is a maternal homeobox gene that demarcates
and specifies anterior body regions. Development
121,707
-720.
Pannese, M., Lupo, G., Kablar, B., Boncinelli, E., Barsacchi, G. and Vignali, R. (1998). The Xenopus Emx genes identify presumptive dorsal telencephalon and are induced by head organizer signals. Mech. Dev. 73,73 -83.[CrossRef][Medline]
Pera, E. M., Wessely, O., Li, S. Y. and De Robertis, E. M. (2001). Neural and head induction by insulin-like growth factor signals. Dev. Cell 1,655 -665.[Medline]
Persson, U., Izumi, H., Souchelnytskyi, S., Itoh, S., Grimsby, S., Engstrom, U., Heldin, C. H., Funa, K. and ten Dijke, P. (1998). The L45 loop in type I receptors for TGF-beta family members is a critical determinant in specifying Smad isoform activation. FEBS Lett. 434,83 -87.[CrossRef][Medline]
Rubenstein, J. L., Shimamura, K., Martinez, S. and Puelles, L. (1998). Regionalization of the prosencephalic neural plate. Annu. Rev. Neurosci. 21,445 -477.[CrossRef][Medline]
Shanmugalingam, S., Houart, C., Picker, A., Reifers, F.,
Macdonald, R., Barth, A., Griffin, K., Brand, M. and Wilson, S. W.
(2000). Ace/Fgf8 is required for forebrain commissure formation
and patterning of the telencephalon. Development
127,2549
-2561.
Shinya, M., Koshida, S., Sawada, A., Kuroiwa, A. and Takeda,
H. (2001). Fgf signalling through MAPK cascade is required
for development of the subpallial telencephalon in zebrafish embryos.
Development 128,4153
-4164.
Simeone, A., Gulisano, M., Acampora, D., Stornaiuolo, A., Rambaldi, M. and Boncinelli, E. (1992). Two vertebrate homeobox genes related to the Drosophila empty spiracles gene are expressed in the embryonic cerebral cortex. EMBO J. 11,2541 -2550.[Abstract]
Sive, H. L., Grainger, R. M. and Harland, R. M. (2000). Early Development of Xenopus laevis: a Laboratory Manual. New York: Cold Spring Harbor Laboratory Press.
Smith, J. C., Price, B. M., Green, J. B., Weigel, D. and Herrmann, B. G. (1991). Expression of a Xenopus homolog of Brachyury (T) is an immediate-early response to mesoderm induction. Cell 67,79 -87.[Medline]
Smith, W. C. and Harland, R. M. (1992). Expression cloning of noggin, a new dorsalizing factor localized to the Spemann organizer in Xenopus embryos. Cell 70,829 -840.[Medline]
Song, H. H. and Filmus, J. (2002). The role of glypicans in mammalian development. Biochim. Biophys. Acta 1573,241 -246.[Medline]
Song, J. and Slack, J. M. (1994). Spatial and temporal expression of basic fibroblast growth factor (FGF-2) mRNA and protein in early Xenopus development. Mech. Dev. 48,141 -151.[CrossRef][Medline]
Song, J. and Slack, J. M. (1996). XFGF-9: a new fibroblast growth factor from Xenopus embryos. Dev. Dyn. 206,427 -436.[CrossRef][Medline]
Steinfeld, R., Van Den Berghe, H. and David, G. (1996). Stimulation of fibroblast growth factor receptor-1 occupancy and signaling by cell surface-associated syndecans and glypican. J. Cell Biol. 133,405 -416.[Abstract]
Storm, E. E., Rubenstein, J. L. and Martin, G. R.
(2003). Dosage of Fgf8 determines whether cell survival is
positively or negatively regulated in the developing forebrain.
Proc. Natl. Acad. Sci. USA
100,1757
-1762.
Topczewski, J., Sepich, D. S., Myers, D. C., Walker, C., Amores, A., Lele, Z., Hammerschmidt, M., Postlethwait, J. and Solnica-Krezel, L. (2001). The zebrafish glypican knypek controls cell polarity during gastrulation movements of convergent extension. Dev. Cell 1,251 -264.[Medline]
Wallingford, J. B., Fraser, S. E. and Harland, R. M. (2002). Convergent extension: the molecular control of polarized cell movement during embryonic development. Dev. Cell 2, 695-706.[Medline]
Watanabe, K., Yamada, H. and Yamaguchi, Y. (1995). K-glypican: a novel GPI-anchored heparan sulfate proteoglycan that is highly expressed in developing brain and kidney. J. Cell Biol. 130,1207 -1218.[Abstract]
Wilson, S. W. and Rubenstein, J. L. (2000). Induction and dorsoventral patterning of the telencephalon. Neuron 28,641 -651.[Medline]
Yoshida, M., Suda, Y., Matsuo, I., Miyamoto, N., Takeda, N.,
Kuratani, S. and Aizawa, S. (1997). Emx1 and Emx2 functions
in development of dorsal telencephalon. Development
124,101
-111.