TGF-β signalling pathways in early Xenopus development

doi:10.1016/S0959-437X(00)00229-X

Current Opinion in Genetics & Development

Volume 11, Issue 5, 1 October 2001, Pages 533-540

https://doi.org/10.1016/S0959-437X(00)00229-X Get rights and content

Abstract

Many different ligands of the TGF-β superfamily signal in the early Xenopus embryo and are required for the specification and patterning of the three germ layers as well as for gastrulation. Recent advances in the field are helping us understand how ligand activity is regulated both spatially and temporally, the mechanism by which the signals are transduced to the nucleus and how essentially the same signalling pathway can activate completely different sets of genes in different regions of the embryo.

Introduction

The members of the TGF-β superfamily that are important in early Xenopus embryos belong to two subfamilies: those functionally related to Activin, which include members of the Nodal family (Xnr1–6), Derrière, Vg-1, and Activin itself and members of the bone morphogenetic protein (BMP) subfamily, in particular BMP2, 4 and 7. The Activin-related ligands are required for mesoderm and endoderm specification and patterning, for promoting gastrulation movements and for establishing left–right asymmetry 1••, 2, 3. The BMPs control the fundamental decision between formation of neural versus other ectodermal cell fates and are also involved in patterning ventral and lateral mesoderm (reviewed in 4). Moreover, TGF-β superfamily members have the ability to act as morphogens, with different doses of ligand turning on particular combinations of downstream genes, and thus instructing different groups of cells to adopt distinct fates 5, 6, 7. Recent advances have now provided some insights into how gradients of active ligand might be established in the embryo. Furthermore, many components of the signalling pathways have now been identified in Xenopus, allowing us to trace the pathways from receptors to nucleus. With this information, it is becoming clear how the same signalling pathway can be involved in distinct biological responses.

Section snippets

Regulation of ligand activity or how to makegradients

Gradients of active ligands have long been thought to underlie the mechanism that establishes position in the developing embryo. Conceptually, gradients of active ligands can be established by a variety of mechanisms. The simplest model involves the production of the ligand at a local source from which it diffuses away to a more distant sink: high ligand concentrations are achieved close to the source and low concentrations at a distance. Alternatively, the ligand may be produced at an equal

Transducing the signals to the nucleus

TGF-β superfamily members function by signalling to the nucleus and regulating the transcription of target genes. A major breakthrough has been the identification of many components of the TGF-β signalling pathways, making it now possible to trace the signalling pathways from receptors to the nucleus, and to identify the critical points at which the pathway can be regulated.

Specificity of the responses

How can the same signalling pathway activate different genes in different regions of the embryo? Recent work 68••, 69, 70••, 71• has identified important determinants of specificity in the form of transcription factors that are required to recruit activated Smad complexes to specific promoter elements.

Smads bind DNA either very weakly (Smad1, 3, 4) or not at all (Smad2), and have a low level of sequence specificity, recognizing a 4 base-pair motif (GTCT) which will occur on average once every

Conclusions and future perspectives

I have focused here on the molecular details of the TGF-β signalling pathways in Xenopus embryos. Many components are known, although given the complexity of other signalling pathways, undoubtedly there are more to be discovered. The most striking feature of these pathways is that a host of different ligands activate a common short signal transduction pathway to elicit a complex array of transcriptional responses, which ultimately specify and pattern the germ layers of the embryo. The pathways

Update

Since completion of this manuscript, a paper from the Whitman group 75• has reported biochemical evidence for the role of the EGF-CFC family member Cripto as a co-receptor for Nodal. Cripto interacts with the type I receptor ALK4 which permits Nodal binding to the receptor complex, resulting in phosphorylation of Smad2. These experiments were performed in Xenopus embryos, but with the mouse family members. It therefore still remains to be demonstrated that FRL-1 plays this role in Xenopus.

Acknowledgements

I am very grateful to Mike Howell for Fig. 2 and thank him, Karolien De Bosscher, Les Dale, Gareth Inman, Helen McNeill, Francisco Nicolás, Becky Randall, Jim Smith and Astrid Steinmair for useful discussions and helpful comments on the manuscript. I thank Malcolm Whitman for generously sending a manuscript prior to publication.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest
•• of outstanding interest

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Most vertebrate body display bilateral symmetry, however, the visceral organs show left-right (L-R) asymmetric trait, such as heart and liver (Palmer 2004, Bisgrove et al., 2015). It has been reported that the development of endoderm and the establishment of L-R asymmetric pattern was influenced by nodal-related genes, which were expressed in left lateral plate mesoderm (Rebagliati et al., 1998a,b, Hill 2001, Long et al., 2003). There are three homologues of nodal in zebrafish: cyclops (cyc), squint (sqt) and southpaw (spaw) (Erter et al., 1998, Rebagliati et al., 1998a,b, Long et al., 2003).
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Smads are critical intracellular signal transducers for transforming growth factor-β (TGF-β) in mammalian cells. In this study, we have identified WD repeat-containing protein 74 (WDR74) as a novel transcriptional coactivator for Smads in the canonical TGF-β signaling pathway. Through direct interactions with Smad proteins, WDR74 enhances TGF-β-mediated phosphorylation and nuclear accumulation of Smad2 and Smad3. Consequently, WDR74 enables stronger transcriptional responses and more robust TGF-β-induced physiological responses. Our findings have elucidated a critical role of WDR74 in regulating TGF-β signaling.
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Vegt controls the zygotic transcription of the Xenopus Nodal ligands (Kofron et al., 1999; Xanthos et al., 2001), initiating Nodal signaling and mesendoderm development (reviewed in Whitman, 2001). The Nodal ligands activate the phosphorylation of R-Smad2/3, which complexes with co-Smad4 to activate target genes (reviewed in Hill, 2001). Foxh1 has been implicated as a critical Smad2/3 co-factor (Chen et al., 1996, 1997), and has an essential function during mesoderm and endoderm development based on studies in Xenopus (Howell et al., 2002; Kofron et al., 2004; Chiu et al., 2014), zebrafish (Pogoda et al., 2000; Kunwar et al., 2003; Sirotkin et al., 2000; Slagle et al., 2011), mouse (Hoodless et al., 2001; Yamamoto et al., 2001), and differentiated human embryonic stem cells (Yoon et al., 2011).
The interplay between transcription factors and chromatin dictates gene regulatory network activity. Germ layer specification is tightly coupled with zygotic gene activation and, in most metazoans, is dependent upon maternal factors. We explore the dynamic genome-wide interactions of Foxh1, a maternal transcription factor that mediates Nodal/TGF-β signaling, with cis-regulatory modules (CRMs) during mesendodermal specification. Foxh1 marks CRMs during cleavage stages and recruits the co-repressor Tle/Groucho in the early blastula. We highlight a population of CRMs that are continuously occupied by Foxh1 and show that they are marked by H3K4me1, Ep300, and Fox/Sox/Smad motifs, suggesting interplay between these factors in gene regulation. We also propose a molecular “hand-off” between maternal Foxh1 and zygotic Foxa at these CRMs to maintain enhancer activation. Our findings suggest that Foxh1 functions at the top of a hierarchy of interactions by marking developmental genes for activation, beginning with the onset of zygotic gene expression.
Small C-terminal domain phosphatase 3 dephosphorylates the linker sites of receptor-regulated smads (R-Smads) to ensure transforming growth factor β (TGFβ)-mediated germ layer induction in Xenopus embryos
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Citation Excerpt :
As such, shortly after the MBT, a good supply of responsive R-Smad allows for the rapid activation of TGFβ signaling. Spatially, SCP3 is enriched vegetally, consistent with the vegetally enriched active R-Smads at stage 9.5–10, during which Nodal/Activin and BMP signals start to be highly activated (1, 7, 10, 12). Compared with SCP3 in the vegetal part, the lower level of SCP3 in the animal part causes weaker but necessary dephosphorylation effects, possibly contributing to activation of R-Smads not restricted in the vegetal region, such as activated Smad1 in the ventral side across the animal-vegetal axis during gastrulation (10–12).
Germ layer induction is one of the earliest events shortly after fertilization that initiates body formation of vertebrate embryos. In Xenopus, the maternally deposited transcriptional factor VegT promotes the expression of zygotic Nodal/Activin ligands that further form a morphogen gradient along the vegetal-animal axis and trigger the induction of the three germ layers. Here we found that SCP3 (small C-terminal domain phosphatase 3) is maternally expressed and vegetally enriched in Xenopus embryos and is essential for the timely induction of germ layers. SCP3 is required for the full activation of Nodal/Activin and bone morphogenetic protein signals and functions via dephosphorylation in the linker regions of receptor-regulated Smads. Consistently, the linker regions of receptor-regulated Smads are heavily phosphorylated in fertilized eggs, and this phosphorylation is gradually removed when embryos approach the midblastula transition. Knockdown of maternal SCP3 attenuates these dephosphorylation events and the activation of Nodal/Activin and bone morphogenetic protein signals after midblastula transition. This study thus suggested that the maternal SCP3 serves as a vegetally enriched, intrinsic factor to ensure a prepared status of Smads for their activation by the upcoming ligands during germ layer induction of Xenopus embryos.
Background: In Xenopus embryos, zygotically activated TGFβ signaling regulates the formation of three germ layers.
Results: Vegetally enriched SCP3 is required for TGFβ signaling and germ layer induction and functions by removing the inhibitory linker phosphorylation on R-Smads.
Conclusion: SCP3 ensures TGFβ-mediated germ layer formation.
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ReviewTGF-β signalling pathways in early Xenopus development

Abstract

Introduction

Section snippets

Regulation of ligand activity or how to makegradients

Transducing the signals to the nucleus

Specificity of the responses

Conclusions and future perspectives

Update

Acknowledgements

References and recommended reading

Cell

Curr Biol

Cell

Curr Biol

Mech Dev

Mech Dev

Biochem Biophys Res Commun

Cell

Cell

Dev Biol

Cell

Cell

Dev Biol

Trends Genet

Cell

Cell

Cell

Dev Biol

Mech Dev

Cell

Dev Biol

J Biol Chem

Dev Biol

Dev Biol

Mech Dev

Mol Cell

J Biol Chem

Differentiation

Cell

Mol Cell

Activin/Nodal responsiveness and asymmetric expression of a Xenopus nodal-related gene converge on a FAST-regulated module in intron 1

Development

Formation and function of Spemann's organizer

Annu Rev Cell Dev Biol

Concentration-dependent patterning of the Xenopus ectoderm by BMP4 and its signal transducer Smad1

Development

Graded changes in dose of a Xenopus Activin A homologue elicit stepwise transitions in embryonic cell fate

Nature

Activin signalling and response to a morphogen gradient

Nature

Arkadia enhances nodal-related signalling to induce mesendoderm

Nature

Bmp-4 acts as a morphogen in dorsoventral mesoderm patterning in Xenopus

Development

Review
TGF-β signalling pathways in early Xenopus development