Bone morphogenetic proteins (BMPs) participate in the development of nearly all organs and tissues. BMP signaling is mediated by specific Smad proteins, Smad1 and/or Smad5, which undergo serine phosphorylation in response to BMP-receptor activation and are then translocated to the nucleus where they modulate transcription of target genes. We have identified a distantly related member of the Xenopus Smad family, Smad8, which lacks the C-terminal SSXS phosphorylation motif present in other Smads, and which appears to function in the BMP signaling pathway. During embryonic development, the spatial pattern of expression of Smad8 mirrors that of BMP-4. We show that an intact BMP signaling pathway is required for its expression. Overexpression of Smad8 in Xenopus embryos phenocopies the effect of blocking BMP-4 signaling, leading to induction of a secondary axis on the ventral side of intact embryos and to direct neural induction in ectodermal explants. Furthermore, Smad8 can block BMP-4-mediated induction of ventral mesoderm-specific gene expression in ectodermal explants. Overexpression of Smad8 within dorsal cells, however, causes patterning defects that are distinct from those reported in BMP-4-deficient embryos, suggesting that Smad8 may interact with additional signaling pathways. Indeed, overexpression of Smad8 blocks expression of Xbra in whole animals, and partially blocks activin signaling in animal caps. In addition, Smad8 inhibits involution of mesodermal cells during gastrulation, a phenotype that is not observed following blockade of activin or BMPs in Xenopus. Together, these results are consistent with the hypothesis that Smad8 participates in a negative feedback loop in which BMP signaling induces the expression of Smad8, which then functions to negatively modulate the amplitude or duration of signaling downstream of BMPs and, possibly, downstream of other transforming growth factor-beta (TGF-beta) family ligands.

REFERENCES

Bolce
M. E.
,
Hemmati-Brivanlou
A.
,
Kushner
P. D.
,
Harland
R. M.
(
1992
)
Ventral ectoderm of Xenopus forms neural tissue, including hindbrain, in response to activin.
Development
115
,
681
688
Christian
J. L.
,
Moon
R. T.
(
1993
)
Interactions between Xwnt −8 and Spemann organizer signaling pathways generate dorsoventral pattern in the embryonic mesoderm of Xenopus.
Genes Dev
7
,
13
28
Cui
Y.
,
Brown
J. D.
,
Moon
R. T.
,
Christian
J. L.
(
1995
)
Xwnt −8b: a maternally expressed XenopusWnt gene with a potential role in establishing the dorsoventral axis.
Development
121
,
2177
2186
Cui
Y.
,
Tian
Q.
,
Christian
J. L.
(
1996
)
Synergistic effects of Vg1 and Wnt signals in the specification of dorsal mesoderm and endoderm.
Dev. Biol
180
,
22
34
Dale
L.
,
Howes
G.
,
Price
B. M. J.
,
Smith
J. C.
(
1992
)
Bone morphogenetic protein 4: a ventralizing factor in early Xenopus development.
Development
115
,
573
585
Dudley
A. T.
,
Lyons
K. M.
,
Robertson
E. J.
(
1995
)
A requirement for bone morphogenetic protein-7 during development of the mammalian kidney and eye.
Genes Dev
9
,
2795
2807
Dyson
S.
,
Gurdon
J. B.
(
1997
)
Activin signalling has a necessary function in Xenopus early development.
Curr. Biol
7
,
81
84
Endo
T. A.
,
Masuhara
M.
,
Yokouchi
M.
,
Suzuki
R.
,
Sakamoto
H.
,
Mitsui
K.
,
Matsumoto
A.
,
Tanimura
S.
,
Ohtsubo
M.
,
Misawa
H.
,
Miyazaki
T.
,
Leonor
N.
,
Taniguchi
T.
,
Fujita
T.
,
Kanakura
Y.
,
Komiya
S.
,
Yoshimura
A.
(
1997
)
A new protein containing an SH2 domain that inhibits JAK kinases.
Nature
387
,
921
924
Fainsod
A.
,
Steinbeisser
H.
,
De Robertis
E. M.
(
1994
)
On the function of BMP-4 in patterning the marginal zone of the Xenopus embryo.
EMBO J
13
,
5015
5025
Ferguson
E. L.
(
1996
)
Conservation of dorsal-ventral patterning in arthropods and chordates.
Curr. Opin. Genet. Dev
6
,
424
431
Graff
J. M.
(
1997
)
Embryonic patterning: to BMP or not to BMP, that is the question.
Cell
89
,
171
174
Graff
J. M.
,
Bansal
A.
,
Melton
D. A.
(
1996
)
Xenopus mad proteins transduce distinct subsets of signals for the TGFsuperfamily.
Cell
85
,
479
487
Graff
J. M.
,
Thies
R. S.
,
Song
J. J.
,
Celeste
A. J.
,
Melton
D. A.
(
1994
)
Studies with a Xenopus BMP receptor suggest that ventral mesoderm-inducing signals override dorsal signals in vivo.
Cell
79
,
169
179
Hammerschmidt
M.
,
Brook
A.
,
McMahon
A. P.
(
1997
)
The world according to hedgehog.
Trends Genet
13
,
14
21
Hansen
C. S.
,
Marion
C. D.
,
Steele
K.
,
George
S.
,
Smith
W. C.
(
1997
)
Direct neural induction and selective inhibition of mesoderm and epidermis inducers by Xnr3.
Development
124
,
483
492
Harland
R. M.
(
1991
)
In situ hybridization: an improved whole-mount method for Xenopus embryos.
Methods Cell Biol
36
,
685
695
Hawley
S. H. B.
,
Wunnenberg-Stapleton
K.
,
Hashimoto
C.
,
Laurent
M. N.
,
Watabe
T.
,
Blumberg
B. W.
,
Cho
K. W. Y.
(
1995
)
Disruption of BMP signals in embryonic Xenopus ectoderm leads to direct neural induction.
Genes Dev
9
,
2923
2935
Hayashi
H.
,
Abdollah
S.
,
Qiu
Y.
,
Cai
J.
,
Xu
Y.-Y.
,
Grinnell
B. W.
,
Richardson
M. A.
,
Topper
J. N.
,
Gimbrone
M. A.
Jr.
,
Wrana
J. L.
,
Falb
D.
(
1997
)
The MAD-related protein Smad7 associates with the TGFreceptor and functions as an antagonist of TGF signaling.
Cell
89
,
1165
1173
Hemmati-Brivanlou
A.
,
Melton
D. A.
(
1992
)
A truncated activin receptor inhibits mesoderm induction and formation of axial structures in Xenopus embryos.
Nature
359
,
609
614
Hemmati-Brivanlou
A.
,
Melton
D. A.
(
1994
)
Inhibition of activin receptor signaling promotes neuralization in Xenopus.
Cell
77
,
273
281
Hemmati-Brivanlou
A.
,
Thomsen
G. H.
(
1995
)
Ventral mesodermal patterning in Xenopus embryos: expression patterns and activities of BMP-2 and BMP-4.
Dev. Genet
17
,
78
89
Hogan
B. L. M.
(
1996
)
Bone morphogenetic proteins: multifunctional regulators of veretebrate development.
Genes Dev
10
,
1580
1594
Horb
M. E.
,
Thomsen
G. H.
(
1997
)
A vegetally localized T-box transcription factor in Xenopus eggs specifies mesoderm and endoderm and is essential for embryonic mesoderm formation.
Development
124
,
1689
1698
Imamura
T.
,
Takase
M.
,
Nishihara
A.
,
Oeda
E.
,
Hanai
J.-i.
,
Kawabata
M.
,
Miyazono
K.
(
1997
)
Smad6 inhibits signalling by the TGF-superfamily.
Nature
389
,
622
626
Ishikawa
T.
,
Yoshioka
H.
,
Ohuchi
H.
,
Noji
S.
,
Nohno
T.
(
1995
)
Truncated type II receptor for BMP-4 induces secondary axial structures in Xenopus embryos.
Biochem. Biophys. Res. Comm
216
,
26
33
Keller
R. E.
(
1984
)
The cellular basis of gastrulation in Xenopus laevis: Active postinvolution convergence and extension by mediolateral interdigitation.
Amer. Zool
24
,
589
603
Köster
M.
,
Plessow
S.
,
Clement
J. H.
,
Lorenz
A.
,
Tiedemann
H.
,
Knöchel
W.
(
1991
)
Bone morphogenetic protein 4 (BMP-4), a member of the TGF-family, in early embryos of Xenopus laevis: analysis of mesoderm inducing activity.
Mech. Dev
33
,
191
200
Kretzschmar
M.
,
Liu
F.
,
Hata
A.
,
Doody
J.
,
Massague
J.
(
1997
)
The TGF-beta family mediator Smad1 is phosphorylated directly and activated functionally by the BMP receptor kinase.
Genes Dev
11
,
984
995
Lee
C.-H.
,
Gumbiner
B. M.
(
1995
)
Disruption of gastrulation movements in Xenopus by a dominant-negative mutant for C-cadherin.
Dev. Biol
171
,
363
373
Luo
G.
,
Hofmann
C.
,
Bronckers
A. L.
,
Sohocki
M.
,
Bradley
A.
,
Karsenty
G.
(
1995
)
BMP-7 is an inducer of nephrogenesis, and is also required for eye development and skeletal patterning.
Genes Dev
9
,
2808
2820
Lustig
K. D.
,
Kroll
K. L.
,
Sun
E. E.
,
Kirschner
M. W.
(
1996
)
Expression cloning of a Xenopus T-related gene (Xombi) involved in mesodermal patterning and blastopore lip formation.
Development
122
,
4001
4012
Macías-Silva
M.
,
Abdollah
S.
,
Hoodless
P. A.
,
Pirone
R.
,
Attisano
L.
,
Wrana
J. L.
(
1996
)
MADR2 is a substrate of the TGFbeta receptor and its phosphorylation is required for nuclear accumulation and signaling.
Cell
87
,
1215
1224
Naka
T.
,
Narazaki
M.
,
Hirata
M.
,
Matsumotot
T.
,
Minamoto
S.
,
Aono
A.
,
Nishimotot
N.
,
Kajita
T.
,
Taga
T.
,
Yoshizaki
K.
,
Akira
S.
,
Kishimoto
T.
(
1997
)
Structure and function of a new STAT-induced STAT inhibitor.
Nature
387
,
924
929
Nakao
A.
,
Afrakhte
M.
,
Moren
A.
,
Nakayama
T.
,
Christian
J. L.
,
Heuchel
R.
,
Itoh
S.
,
Kawabata
M.
,
Heldin
N.-E.
,
Heldin
C.-H.
,
ten Dijke
P.
(
1997
)
Identification of Smad7, a TGF--inducible antagonist of TGF- signalling.
Nature
389
,
631
635
Rupp
R. A. W.
,
Snider
L.
,
Weintraub
H.
(
1994
)
Xenopus embryos regulate the nuclear localization of XMyoD.
Genes Dev
8
,
1311
1323
Schmidt
J. E.
,
Suzuki
A.
,
Ueno
N.
,
Kimelman
D.
(
1995
)
Localized BMP-4 mediates dorsal/ventral patterning in the early Xenopus embryo.
Dev. Biol
169
,
37
50
Schweitzer
R.
,
Shilo
B.-Z.
(
1997
)
A thousand and one roles for the Drosophila EGF receptor.
Trends Genet
13
,
191
196
Smith
W. C.
,
McKendry
R.
,
Ribisi
S.
,
Harland
R. M.
(
1995
)
A nodal -related gene defines a physical and functional domain within the Spemann organizer.
Cell
82
,
37
46
Starr
R.
,
Willson
T. A.
,
Viney
E. M.
,
Murray
L. J. L.
,
Rayner
J. R.
,
Jenkins
B. J.
,
Gonda
T. J.
,
Alexander
W. S.
,
Metcalf
D.
,
Nicola
N. A.
,
Hilton
D. J.
(
1997
)
A family of cytokine-inducible inhibitors of signalling.
Nature
387
,
917
921
Stennard
F.
,
Carnac
G.
,
Gurdon
J. B.
(
1996
)
The Xenopus T-box gene, Antipodean, encodes a vegetally localised maternal mRNA and can trigger mesoderm formation.
Development
122
,
4179
4188
Suzuki
A.
,
Chang
C.
,
Yingling
J. M.
,
Wang
X. F.
,
Hemmati-Brivanlou
A.
(
1997
)
Smad5 induces ventral fates in Xenopus embryo.
Dev. Biol
184
,
402
405
Thomsen
G. H.
(
1996
)
Xenopus mothers against decapentaplegic is an embryonic ventralizing agent that acts downstream of the BMP-2/4 receptor.
Development
122
,
2359
2366
Thomsen
G. H.
(
1997
)
Antagonism within and around the organizer: BMP inhibitors in vertebrate body patterning.
Trends Genet
13
,
209
211
Tsuneizumi
K.
,
Nakayama
T.
,
Kamoshida
Y.
,
Kornberg
T. B.
,
Christian
J. L.
,
Tabata
T.
(
1997
)
Daughters against dpp modulates dpp organizing activity in Drosophila wing development.
Nature
389
,
627
631
Yamashita
H.
,
ten Dijke
P.
,
Heldin
C.-H.
,
Miyazono
K.
(
1996
)
Bone morphogenetic protein receptors.
Bone
19
,
569
574
Watabe
T.
,
Kim
S.
,
Candia
A.
,
Rothbächer
U.
,
Hashimoto
C.
,
Inoue
K.
,
Cho
K. W. Y.
(
1995
)
Molecular mechanisms of Spemann's organizer formation: conseved growth factor synergy between Xenopus and mouse.
Genes Dev
9
,
3038
3050
Zhang
J.
,
King
M. L.
(
1996
)
Xenopus VegT RNA is localized to the vegetal cortex during oogenesis and encodes a novel T-box transcription factor involved in mesodermal patterning.
Development
122
,
4119
4129
.
This content is only available via PDF.