RESEARCH COMMUNICATION

Default neural induction:neuralization of dissociatedXenopus cells is mediatedby Ras/MAPK activation
Hiroki Kuroda,1 Luis Fuentealba, Atsushi Ikeda,
Bruno Reversade, and E.M. De Robertis2
Howard Hughes Medical Institute and Departmentof Biological Chemistry, University of California,
Los Angeles, California 90095-1662, USA

Xenopus embryonic ectodermal cells dissociated forthree or more hours differentiate into neural tissue instead of adopting their normal epidermal fate. This default type of neural induction occurs in the absence of
Spemann’s organizer signals and is thought to be causedby the dilution of endogenous BMPs into the culture
medium. Unexpectedly, we observed that BMP ligands
continue to signal in dissociated cells. Instead, cell dissociation induces a sustained activation of the Ras/
MAPK pathway, which causes the phosphorylation of
Smad1 at sites that inhibit the activity of this transcription factor. It is this activation of Ras/MAPK that isrequired for neuralization in dissociated ectoderm.
Supplemental material is available at http://www.genesdev.org.
Received February 15, 2005; revised version accepted March24, 2005.

Neural induction in amphibians requires secreted signals, in particular BMP antagonists, produced in the dorsal organizer region of the embryo (Spemann 1938; Harland 2000; De Robertis and Kuroda 2004). However, early
work also showed that brain tissue could be obtained in
the absence of the organizer, for example, by simply culturing newt ectodermal explants in sub-optimal saline
solutions (Barth 1941; Holtfreter 1944). In Xenopus, neural differentiation without the organizer can be induced
by dissociating ectodermal cells, which would otherwise
give rise to epidermis, for 3 h or more (Grunz and Tacke
1989; Sato and Sargent 1989). Addition of Bone Morphogenetic Protein 4 (BMP4) to dispersed Xenopus cells canrestore epidermal differentiation (Wilson and HemmatiBrivanlou 1995), leading to the generally accepted proposal that during cell dissociation endogenous BMPs diffuse into the culture medium (Muñoz-Sanjuán andBrivanlou 2002). In this view, the dilution of BMP protein would be the cause of neural differentiation in dissociated cells.
A large body of work also indicates that neural differentiation in vertebrate embryos can be induced by posi[Keywords: Neural induction; Smad1; MAPK; FGF; IGF; BMP]
1
Present address: Shizuoka University, Faculty of Education (Biology),
836 Ohya, Shizuoka, 422-8529 Japan.
2
Corresponding author.
E-MAIL ederobertis@mednet.ucla.edu; FAX (310) 206-2008.
Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/
gad.1306605.

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tive signals that act through Receptor Tyrosine kinases(RTKs). Growth factors that signal via RTKs, such as
Fibroblast Growth Factor (FGF), Insulin-like Growth
Factor (IGF), and Hepatocyte Growth Factor (HGF), arepotent neural inducers in vertebrates (Wilson and Edlund
2001; De Robertis and Kuroda 2004; Stern 2004). RTK
signaling activates the mitogen-activated protein kinase(MAPK) known as extracellular signal-regulated protein
kinase (ERK) via the Ras pathway, and in this way causes
neural induction.
Two disparate views dominate the neural induction
field at present. Work in the chick embryo has stressed
the importance of FGF signaling, whereas work in Xenopus has tended to emphasize the requirement for antiBMPs in neural induction (Harland 2000; Stern 2004).
We have argued that these apparently conflicting observations can be reconciled through a molecular mechanism in which Ras/MAPK phosphorylation regulates the
BMP transducers Smad1/5/8 (De Robertis and Kuroda
2004). It has been reported that both FGF and IGF canpromote neural induction through the phosphorylation,
via MAPK, of inhibitory sites located in the linker regionof the Smad1 transcription factor (Pera et al. 2003).
Linker phosphorylation of BMP-sensitive Smads by
MAPK opposes the effect of C-terminal Smad phosphorylation by the BMP receptor serine–threonine protein kinase (Fig. 1A), causing inhibition of the nuclear
function of Smad1 and promoting neural development(low BMP/Smad) at the expense of epidermal (high BMP/
Smad) fates (Massagué 2003).
It is known that activation of ERK can be triggered bya large number of external stimuli, including cell stress(Wetzker and Böhmer 2003). In Xenopus embryos, thesimple extirpation of ectodermal explants can activate
ERK, causing this kinase to become diphosphorylated(dpERK) (LaBonne and Whitman 1997; Christen andSlack 1999). However, this ERK activation by microsurgery is transient, lasting <30 min. Transient ERK activation after animal cap dissection is without phenotypic
effect, and the explants go on to differentiate according
to their normal epidermal fate. Since BMP and MAPK
signaling are integrated during neural induction, we decided to investigate whether cell dissociation in conditions that cause neural differentiation might induce astronger, or more prolonged, activation of the Ras/MAPK
pathway.
In the present paper we report two findings: first, that,
unexpectedly, endogenous BMPs continue to signal in an
autocrine fashion in dissociated animal cap ectodermal
cells, at the same levels as in undissociated cells; second,that default neural differentiation is mediated by a sustained activation of the Ras/MAPK pathway induced bycell dissociation. The results indicate that the choice
between epidermal and neural cell fates is determined by
the integration of the opposing activities of BMP receptor and Ras/MAPK at the level of Smad1 phosphorylation.
Results and Discussion
BMPs continue to signal in dissociated cells
Cells from Xenopus animal cap explants develop as epidermis when cultured in saline solution, but when cells

GENES & DEVELOPMENT 19:1022–1027 © 2005 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/05; www.genesdev.org

Ras/MAPK and default neural induction

BMP signals are down-regulated in dissociatedanimal cap cells, we examined the level of endogenous BMP receptor (BMPR) signaling (Fig.
1A) on the transcription factor Smad1 using a
C-terminal phospho-specific antibody (Persson et al. 1998). Surprisingly, the same levelsof Smad1 C-terminal phosphorylation weredetected in intact or dissociated animal caps
(Fig. 1C, cf. lanes 3 and 4).
A second line of evidence indicating that
BMPs continue to signal in dissociated animal
caps was provided by knock-down of endogenous BMP ligands. In the course of studies on
the requirement of anti-BMP signals in neuralinduction, we designed and characterized antisense morpholino oligos (BMP-MOs) that inhibit specifically the activity of BMP2, BMP4,
and BMP7 (Fig. 1C, lanes 2,5; B. Reversade, H.
Kuroda, H. Lee, A. Mays, and E.M. De Robertis, in prep.). RT–PCR analyses using BMPMOs showed that endogenous BMP ligands
continue to signal in dissociated ectoderm(Fig. 1D, cf. lanes 3 and 4). As expected, uninjected animal cap explants expressed the epidermal-specific marker Cytokeratin in the absence of the mesoderm marker Myf5. Afterdissociation, these cells gained expression of
the pan-neural marker NCAM and the anterior
gene Otx2, and lost Cytokeratin expression(Fig. 1D, lanes 2,3). Remarkably, coinjection ofanti-BMP morpholinos caused a large increasein neural differentiation in dissociated cells(Fig. 1D, lane 4). This indicates that althoughdissociated cells lost Cytokeratin expression,
they had not yet reached their full neural potential. We conclude that, contrary to current
belief, autocrine signaling by endogenous BMP
proteins continues to take place, at the samelevels as in intact animal cap explants, in dissociated Xenopus ectoderm undergoing neuralization.
Cell dissociation causes sustained
MAPK activation
We next investigated whether activation of
Ras/MAPK, through the molecular mechaFigure 1. Cell dissociation does not cause BMP depletion by dilution, but rather
nism shown in Figure 1A, might provide an
triggers sustained ERK/MAPK activation in Xenopus ectodermal cells. (A) Signaling
explanation for how lowering of Smad1 activpathway integration at the level of Smad1. (B) Default neural induction in dissociity could be achieved in the presence of BMPR
ated animal caps (AC) cells. (C) Western blot assay using anti-C-terminal-phosphosignals in dissociated cells. We used a diphosSmad1 (C-ter-P-Smad1) antibody in animal cap cells microinjected with Smad1
pho-specific ERK antibody (dpERK) that only
mRNA; note that BMPR signaling is not affected by cell dissociation. (WE) Whole
embryo; (diss. AC) dissociated animal caps. (D) RT–PCR of neural (NCAM, Otx2),
recognizes the active form of the enzyme, tomesodermal (Myf5), and loading control (ODC) markers, showing a great increase of
ask whether endogenous ERK signaling wasneural differentiation when Bmp2, Bmp4, and Bmp7 are inhibited with specificactivated by cell dissociation. Surgical exciMOs. (E) Sustained ERK activation revealed by anti-diphospho-ERK (dpERK) antision of the animal cap was sufficient to phosbody in dissociated AC cells. A general ERK antibody serves as loading control. (F,G)
phorylate ERK after 15 min, but this activaActivation of dpERK in a control animal explant 15 min after excision and its
tion was transient and undetectable in whole
inhibition by addition of the MEK inhibitor U0126 at 40 µM. (H,I) Activation of
dpERK in all cells 75 min after dissociation, and its dependence on MEK.
caps after 45 min in culture (Fig. 1E, lanes 1–4).
Remarkably, cell dissociation caused sustained activation of ERK for at least 3 h inculture (Fig. 1E, lanes 5–8). Immunocytochemical stainare dispersed by removing Ca2+ and Mg2+ for three oring showed that ERK was phosphorylated in cells neighmore hours neural differentiation ensues (Fig. 1B). Thisboring the healing animal cap wound 15 min after excidefault neural induction is believed to be caused by thesion (Fig. 1F). We note that this is the same region prediffusion and dilution of endogenous BMPs, but thisviously found to give rise to neural tissue in newt animal
proposition has not been tested directly. To test whether

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Kuroda et al.

cap explants in which wound healing was delayed bysub-optimal saline solutions (Holtfreter 1944). ERK diphosphorylation is mediated by MEK (for MAPK/ERK
kinase), which can be inhibited by the specific chemical
inhibitor U0126 (Fig. 1A) in the micromolar range (Fig.
1E, lane 9). U0126 blocked both the transient ERK activation (Fig. 1F,G) in animal caps, and the sustained
dpERK signal seen in dissociated cells (Fig. 1H,I). Interestingly, immunocytochemistry (see Materials andMethods) showed that after dispersal all ectodermal cells
were strongly dpERK positive (Fig. 1H). We conclude
that cell dissociation causes a sustained activation of the
MEK/ERK pathway in Xenopus ectoderm.
Ras/MAPK mediates neuralization
We next asked whether MEK/ERK activation was required for default neural induction. As shown in Figure 2A,
control animal cap cells dissociated for 5 h and thenallowed to reaggregate by addition of Ca2+ and Mg2+ differentiated into histotypic brain after 2 d in culture. Addition of the MEK inhibitor U0126 to the culture medium during dissociation blocked neural differentiation,
causing the histological and molecular differentiation of
epidermal tissue instead (Fig. 2B,D,E). In RT–PCR analyses carried out at early neurula (stage 13), cell dissociation eliminated Cytokeratin expression and induced expression of the pan-neural markers Sox2 and NCAM (Fig.
2F, lanes 2,3). Addition of U0126 inhibited neural differentiation in dissociated cells and restored epidermal fate(Fig. 2F, lane 4). The MEK inhibitor was effective even if
added 75 min after the initial dissociation (Supplementary Fig. 1A), indicating that its effect on neural differentiation was not dependent on the transient ERK activation caused by animal cap excision, but rather on thesustained activation described here for dissociated cells(Fig. 1E). The effect of U0126 on neural differentiation
was specific for the ERK pathway (Supplementary Fig.
1B). The results indicate that MEK/ERK activation is required for default neural induction in dissociated Xenopus ectodermal cells; when ERK activation is prevented,
dissociated cells develop as epidermis instead.
MAPK-resistant Smad1 inhibits neuralization

Figure 2. Histological sections, whole-mount in situ hybridizations, and RT–PCR analyses showing that inhibition of ERK blocks
neural induction in dissociated animal cap cells. (A) Reaggregate
from control (Co) untreated cells dissociated for 5 h, showing brain
structures. (B) Reaggregate from U0126-treated cells differentiatedinto epidermis. (C) Head region of sibling intact stage 32 embryo(Sive et al. 2000). (ey) Eye-like structure; (br) brain; (ep) epidermis.
(D) Control reaggregates with low-Cytokeratin (CytoK) by in situ
hybridization. (E) Strong expression of CytoK in reaggregate treatedwith U0126. (F) RT–PCR analysis of dissociated AC cells at stage 13.
Pan-neural marker (Sox2, NCAM) expression was inhibited and epidermal CytoK was induced by U0126 treatment, Smad-LM (but not
Smad1-WT) mRNA injection, and 50 ng/mL (2 nM) BMP4 protein.

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We next investigated whether phosphorylation of thelinker Smad1 MAPK sites (Fig. 1A) was involved in the
choice between epidermis and neural tissue, by comparing the effects of wild-type (WT) and linker mutant (LM)human Smad1 mRNA. The LM-Smad1 mutant has four
serine-to-alanine substitutions at conserved PXSP sitesthat eliminate MAPK phosphorylation (Kretzschmar etal. 1997; Pera et al. 2003). Microinjection of this MAPKinsensitive LM-Smad1 mRNA strongly inhibited Sox2
and NCAM expression and restored epidermal (Cytokeratin-positive) differentiation of dissociated ectodermal cells (Fig. 2F, lane 5), whereas WT-Smad1 mRNA at
the same concentration failed to block neuralization orcause epidermal differentiation (Fig. 2F, lane 6). Additionof recombinant BMP4 protein to dissociated ectoderm(Wilson and Hemmati-Brivanlou 1995) blocked neuralinduction and restored epidermal differentiation (Fig. 2F,
lane 7). Exogenous BMP4 addition did not affect ERK
activation, but caused a large increase in reserve Smadtranscriptional activity detected by a BMP response element (BRE) reporter gene in animal cap (Supplementary
Fig. 2). Thus, although endogenous levels of Smad1 activity in Xenopus animal caps are sufficient to cause epidermal differentiation, a large reserve pool of inactive
BMP Smads exists. This spare capacity for additional
BMP response explains why the addition of excess BMP4
protein restores epidermal differentiation to dissociatedanimal cap cells. The antineural effects of Smad1 linker
mutants indicate that the inhibitory linker phosphorylation of Smad1 by ERK/MAPK plays a critical role inneural induction by cell dissociation.
Activation of the Ras/MAPK pathway
ERK activation is a late step in the Ras/MAPK signaltransduction pathway regulated by RTKs (Fig. 3A). To
investigate at which level cell dissociation activates theRas/MAPK pathway we used, in addition to the MEK
chemical inhibitor U0126, a dominant-negative form ofRas (Ribisi et al. 2000) and Xenopus Sprouty2 mRNA.

Ras/MAPK and default neural induction

man Smad1; the antibody targeted the most C-terminalsite in the linker region. Western blots of cell lysates
showed that sustained linker phosphorylation of endogenous Xenopus Smad1 was, indeed, elicited by cell dispersal (Fig. 4A, lanes 1–8). Smad1 linker phosphorylation
correlated with the time course of ERK activation (Fig.
1E) and could be blocked by inhibiting MEK/ERK activity with U0126 (Fig. 4A, lane 9). These experiments on
Smad1 linker phosphorylation reflect in vivo protein levels and did not involve overexpression of Smad1. We
conclude that an inhibitory site on the endogenous
Smad1 transcription factor becomes phosphorylated
when Xenopus animal cap cells are dissociated.
Choosing between neural and epidermal fates
The default neural induction model has provided a very
important paradigm for understanding how neural tissue
is induced in Xenopus (Muñoz-Sanjuán and Brivanlou
2002). In this model, BMP signaling in the ectoderm
plays a critical role in the choice between two alternative fates, epidermis or neural tissue (Harland 2000; De
Robertis and Kuroda 2004). Dissociation of ectodermal
cells induces anterior neural tissue and was believed toact through the dilution of endogenous BMPs, which
would diffuse into the culture medium. Here we tested
this proposal and found that autocrine BMP signals continue to function in dissociated cells at the same levels
as those of animal cap explants that develop into epidermis.
In the dissociated animal cap system, anterior neural
Figure 3. Dissociation activates much of the Ras/MAPK pathway.
(A) Diagram of ERK pathway components. Dimerized FGF receptors(FGFR) become phosphorylated and recruit the Grb2–SOS complex via FGFR substrate protein 2 (FRS2). SOS activates Ras by exchanging GDP for GTP, allowing Ras to bind to and activate Raf(MAPKKK), which in turn activates MEK (MAPKK), and finally ERK(MAPK). (B) RT–PCR analysis of dissociated AC cells treated with
U0126 or derived from dn-Ras, Xsprouty2, or dnFGFR4a mRNAinjected embryos. Inhibitors of the Ras/MAPK pathway, except for
dnFGFR4a, blocked neural induction by cell dissociation, causing
epidermal differentiation. The amount of dnFGFR4a mRNA (50 pg/
blastomere at the four-cell stage) was sixfold less than that used byother authors (Hongo et al. 1999), as higher concentrations were
toxic to the embryos.

Sprouty2 is a phosphoprotein that binds to the Grb2
adaptor (Nutt et al. 2001; Hanafusa et al. 2002), inhibiting the pathway just downstream of RTKs (Fig. 3A). All
three inhibitors were able to block neural and restore
epidermal markers in dissociated cells (Fig. 3B, lanes
3–6). Dominant-negative FGF receptor-4a (dnFGFR4amRNA) (Hongo et al. 1999) only slightly inhibited neuralization and was unable to cause epidermal differentiation (Fig. 3B, lane 7), suggesting that Ras/MAPK activation in dissociated cells might involve other RTKs or beligand-independent. We conclude that most of the Ras/
MAPK pathway, from Grb2 on down, is activated by cell
dispersal.
Cell dissociation phosphorylates endogenous Smad1
To determine whether Smad1 itself is phosphorylated indissociated Xenopus ectodermal cells, a phospho-specificantibody directed against a Smad1 MAPK site in thelinker region was generated. Four conserved PXSPMAPK phosphorylation sites exist in Xenopus and hu-

Figure 4. Cell dissociation causes linker-specific phosphorylationof Smad1 in Xenopus ectoderm. (A) Western blot time course using
a linker-specific phospho-Smad1 antibody that reacts with endogenous normal levels of Smad1. Cell dissociation induced sustained
linker phosphorylation of Smad1, which was blocked by the MEK
inhibitor U0126. A general Smad1 antibody serves as loading control. (B) Model for neural induction in dissociated AC cells. Ectodermal BMPs signal in autocrine fashion, but in dissociated cells
their effect is counterbalanced by activation of the Ras/MAPK pathway, which causes an inhibitory phosphorylation in Smad1, lowering its activity.

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Kuroda et al.

differentiation is entirely dependent on the activation of
the Ras/MAPK pathway (Fig. 4B). This was shown by
inhibition of MEK/ERK pathway components (bySprouty2, dn-Ras, or U0126) or injection of mutant
forms of Smad1 resistant to phosphorylation by MAPK,
which inhibit neutralization and result in epidermal differentiation. Injection of dnFGFR4a mRNA only slightly
inhibited neural differentiation in our study, althoughstrong inhibitory effects for anterior neural differentiation have been noted by others in Xenopus embryos(Hongo et al. 1999; Delaune et al. 2005). Low levels of
FGF signaling are sufficient for anterior neural induction(Delaune et al. 2005); perhaps the amount of dnFGFR4amRNA injected was insufficient to block FGF signaling.
Another possibility is that many other RTKs for FGF,
IGF, and other growth factors exist on surface of embryonic cells. The molecular mechanism by which a sustained activation of Ras/MAPK is induced by cell dissociation is unknown. It could involve, for example, theactivation of multiple RTKs by increasing the rate ofdimerization of these receptors in the cell membrane.
Transient versus sustained activation of Ras/MAPK is
known to cause distinct phenotypic effects in other systems; for example, in cultured pheochromocytoma PC12
cells EGF causes transient activation of Ras/MAPK andcell proliferation, while NGF causes sustained activationand neuronal differentiation (Marshall 1995).
Cell dissociation during the late blastula stage induces
neuralization of ectodermal cells in Xenopus. However,work in the chick has suggested that BMP inhibition isrequired only as a late step during gastrulation (Linker
and Stern 2004). The situation appears to be different in
Xenopus embryos, in which injection of multiple BMPMOs or dnBMPRs can greatly increase neural marker
expression (Yamamoto et al. 2001; B. Reversade and E.M.
De Robertis, in prep.). In addition, the effects on neuraldifferentiation of manipulating the BMP and FGF pathways are maximal at pre-gastrula stages in Xenopus (Delaune et al. 2005; Wawersik et al. 2005). This timing
correlates with the observation that blastula cells on thedorsal side of the embryo, where the CNS will form,
have high ERK and low BMPR signaling (Schohl and Fagotto 2002). The early BMP inhibition in the Xenopus
blastula is achieved through transcriptional down-regulation of BMP4 and the expression of secreted BMP
antagonists, such as Chordin and Noggin, in the future
CNS by the action of an early ␤-catenin signal (Baker
et al. 1999; Kuroda et al. 2004). The second critical step,which is the principal cause of neural differentiation indissociated cells, is the activation of Ras/MAPK. Because
endogenous BMP signals are still active in animal cap
cells after dissociation (Fig. 1C,D), this system has an
absolute requirement for sustained Ras/MAPK activity
in order to attain the very low levels of endogenous
BMP/Smad1 signals required for neural differentiation.
We conclude that anterior neural differentiation in
Xenopus ectoderm results from the balanced integration of MAPK and BMPR signals at the level of Smad
(Fig. 1A).
Classically, neural induction is considered the primary
embryonic induction (Spemann 1938). As such, it has
been the subject of intense investigation since it was
discovered in 1924 that Spemann’s organizer could induce neural tissue. An unresolved mystery in neural induction research, dating from the 1930s, is why deadorganizer tissue and many heterologous substances such

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as fatty acids, sterols, methylene blue, and even sandparticles were able to induce neural differentiation in
newt embryonic ectoderm (Spemann 1938; Holtfreterand Hamburger 1955; De Robertis and Kuroda 2004). In
future, it will be interesting to reinvestigate these observations in light of the essential role played by Ras/MAPK
activation in default neural differentiation.
Materials and methods
Embryo manipulations
Xenopus laevis embryos were obtained by in vitro fertilization (Sive et al.
2000). AC cell dissociations were performed in Ca2+, Mg2+ free 1× Steinberg’s solution (CMFSS: 58 mM NaCl, 0.67 mM KCl, 4.6 mM Tris-HCl
at pH 7.4, 100 mg/L kanamycin) containing 0.1% Bovine serum albumin(BSA) at stage 9 (7 h after fertilization at 22°C). The outer layer of cells
was discarded, and the inner layer disaggregated into a single-cell suspension by gentle pipetting. All steps were carried out in 35 × 10-mm
plastic plates (Fisher) coated with 6% Poly (2-hydroxyethyl methacrylate)in ethanol (Polysciences) and allowed to dry for 30 min. For cell reaggregations, cells from 10 explants were transferred 5 h after dissociation into
1× Steinberg’s solution (SS) containing 0.83 mM MgSO4 and 0.34 mM
Ca(NO3)2 in 1.5 mL of NoStick Hydrophobic Microtubes (GENEMATE),
centrifuged at 1000 rpm for 10–15 sec; then fresh SS was added and thecells were cultured until sibling embryos reached stage 14 for in situ
hybridization and stage 32 for histological assays. Hematoxylin solution(Gill’s formulation #3, Fisher) and eosin solution containing 0.2% eosin
Y (Sigma) in 70% ethanol containing 0.02% acetic acid were used forhistological staining. RT–PCR conditions and primers, as well as the
protocol for whole-mount in situ hybridization, are described at http://
www.hhmi.ucla.edu/derobertis/index.html.
RNA and morpholino injections
To generate synthetic mRNAs, plasmids pCS2-smad1-WT, pCS2-smad1LM, pCS2-Xsprouty2, and pCS2-dnFGFR4a were linearized with NotI
and transcribed with SP6 RNA polymerase, and pSP64T-dnRas with
EcoRI and SP6 RNA polymerase (Hongo et al. 1999; Pera et al. 2003).
After initial titration, the doses of mRNA were microinjected into each
blastomere at the four-cell stage: 50 pg for human Smad1-WT, 50 pg forhuman Smad1-LM, 500 pg for Sprouty2, 50 pg for dn-FGFR4a, and 250 pg
for dnRas mRNAs. The 25-bp morpholino antisense oligomers for
Bmp2, Bmp4, and Bmp7 targeted regions lacking mismatches with anyof the allelic forms in the Xenopus EST database were obtained from
Gene Tools, LLC and analyzed for biochemical and biological activity(B. Reversade, H. Kuroda, H. Lee, A. Mays, and E.M. De Robertis, in
prep.). They consist of the following sequences: Bmp2-MO, 5⬘-GATCCCAGCGACCATTGTCAACCTG-3⬘; Bmp4-MO, 5⬘-CAGCATTCGGTTACCAGGAATCATG-3⬘; Bmp7-MO, 5⬘-TTACTGTCAAAGCATTCA
TTTTGTC-3⬘. Control morpholinos of randomized sequence were without effect. Morpholinos were resuspended in sterile water to a concentration of 1 mM each, then further diluted to give a working solution of
0.25 mM (Bmp2-MO:Bmp4-MO:Bmp7-MO:H2O = 1:1:1:1), and 2 nL was
injected into each blastomere at the four-cell stage.
Western blots
Explant or cell lysates were prepared in PhosphoSafe Extraction Buffer(Novagen), lipids were removed by extracting once with an equal volumeof 1,1,2-trichloro-1,2,2-trifluoroethane (Kodak), and proteins were separated by 4%–15% gradient SDS-PAGE gels (Bio-Rad). Western blots were
performed using polyclonal rabbit antibodies against ERK1/2 (1:1000;
Cell Signaling), Smad1 (1:1000; Upstate), and C-terminal phospho-Smad1(1:4000; kind gift of C. Heldin, Ludwig Institute, Uppsala, Sweden) andmonoclonal mouse antibody against diphospho-ERK1/2 (anti-dpERK-YT
antibody, 1:1000; Sigma). To detect Smad1 C-terminal phosphorylation,embryos injected with Smad1-WT mRNA (500 pg) were used. The rabbit
polyclonal antibody against phospho-linker Smad-1 was prepared by Covance Research Products and affinity-purified over a column of
SSDPGS(PO3)PFQMPADT (Princeton BioMolecules Corporation) after
negative selection with the unphosphorylated peptide. The antibody
stains phospho-Smad-1 specifically in Western blots at 1:200,000 dilution (L. Fuentealba, A. Ikeda, and E.M. De Robertis, in prep.). Unphosphorylated peptide (6 nM) was added as a competitor.

Ras/MAPK and default neural induction

Immunostaining
Dissociated cells were allowed to attach for 1 min to uncoated plastic
tissue culture dishes in CMFSS and fixed with MEMFA for 2 h at room
temperature (Sive et al. 2000), washed with PBS, refixed (Christen andSlack 1999) with 0.1 M K2Cr2O7 in PBS for 1 h, washed three times with
PBS for 5 min, dehydrated into methanol, treated with 5% hydrogen
peroxide in methanol for 1 h, and washed twice with methanol for 5 min;the animal caps were stored at −20°C until used. Samples were rehydrated into PBS, washed three times with PBS for 10 min, incubated with
BSA buffered Triton (BBT: 1% BSA and 0.1% Triton-X in PBS) buffer for
2 h, incubated with 1:10,000 diluted anti-dpERK-YT antibody (Sigma) in
BBT at 4°C overnight, washed four times with BBT at room temperaturefor 1 h, treated with 1:1000 diluted anti-mouse IgG-conjugated to alkaline phosphatase (Novagen) in BBT at 4°C overnight, washed with BBT
for 1 h, washed with PBS-Tween (PBSw: 0.1% Tween-20 in PBS) for 4 h,washed twice with alkaline phosphatase buffer (APB: 0.1 M Tris-HCl at
pH 9.5, 0.1 M NaCl, 50 mM MgCl2) for 5 min, and stained with BM
Purple substrate (Roche) diluted 1:10 in APB at 4°C overnight (Sive et al.
2000). The staining reaction was stopped by washing twice with PBS. All
steps were performed in plastic dishes for dissociated AC cells and in
15 × 45-mm glass vials (Fisher) for AC explants.

Acknowledgments
We thank E. Amaya, H. Okamoto, and C. Heldin for materials; A. Mays
and D. Greenberg for technical assistance; and S. Millard, X. Zhan,
J. Kim, H. Lee, E. Eivers, and R. Nishinakamura for comments and advice
on the manuscript. This work was supported by the NIH (R37 HD2150219). H.K. is an Associate and E.M.D.R. an Investigator of the Howard
Hughes Medical Institute.

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