Article

Lefty acts as an essential modulator of Nodal activity during sea urchin oral–aboral axis formation

UMR 7009 CNRS, Université de Paris VI, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France.
Developmental Biology (Impact Factor: 3.64). 05/2008; 320(1):49-59. DOI: 10.1016/j.ydbio.2008.04.012
Source: PubMed

ABSTRACT Nodal is a key player in the process regulating oral-aboral axis formation in the sea urchin embryo. Expressed early within an oral organizing centre, it is required to specify both the oral and aboral ectoderm territories by driving an oral-aboral gene regulatory network. A model for oral-aboral axis specification has been proposed relying on the self activation of Nodal and the diffusion of the long-range antagonist Lefty resulting in a sharp restriction of Nodal activity within the oral field. Here, we describe the expression pattern of lefty and analyse its function in the process of secondary axis formation. lefty expression starts at the 128-cell stage immediately after that of nodal, is rapidly restricted to the presumptive oral ectoderm then shifted toward the right side after gastrulation. Consistently with previous work, neither the oral nor the aboral ectoderm are specified in embryos in which Lefty is overexpressed. Conversely, when Lefty's function is blocked, most of the ectoderm is converted into oral ectoderm through ectopic expression of nodal. Reintroducing lefty mRNA in a restricted territory of Lefty depleted embryos caused a dose-dependent effect on nodal expression. Remarkably, injection of lefty mRNA into one blastomere at the 8-cell stage in Lefty depleted embryos blocked nodal expression in the whole ectoderm consistent with the highly diffusible character of Lefty in other models. Taken together, these results demonstrate that Lefty is essential for oral-aboral axis formation and suggest that Lefty acts as a long-range inhibitor of Nodal signalling in the sea urchin embryo.

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Available from: Veronique Duboc, Jan 13, 2015
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    • "Nodal signaling pathway antagonist, which diffuses farther than the Nodal ligand (Duboc et al., 2008), establishing lateral inhibition that results in a sharp restriction of Nodal to the ventral side of the embryo (Fig. 7C). The presence of a potential Oct1/2 binding site in the Nodal promoter and the absence of early nodal expression in the Oct1/2 morphants strongly suggest that Oct1/2 regulates nodal expression directly by binding to its cis-regulatory region. "
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    ABSTRACT: The TGFβ family member Nodal is expressed early in the presumptive ventral ectoderm of the early sea urchin embryo and its activity is crucial for dorsal-ventral (D/V) axis specification. Analysis of the nodal promoter identified a number of critical binding sites for transcription factors of different families including Sox, Oct, TCF and bZIP, but in most cases the specific factors that regulate nodal expression are not known. In this study, we report that the maternal factor Oct1/2 functions as a positive regulator of nodal and that its activity is essential for the initiation of nodal expression. Inhibition of Oct1/2 mRNA translation produced embryos with severe axial defects similar to those observed following inhibition of Nodal function. We show that perturbing Oct1/2 function specifically disrupted specification of the ventral and dorsal ectodermal regions and that these effects were caused by the failure of nodal to be expressed early in development. Furthermore, we identified the key gene vg1/univin, which is also necessary for nodal expression, as an additional factor that was completely dependent on Oct1/2 for its zygotic expression. These data demonstrate that the maternal Oct1/2 protein plays an early and essential role in D/V axis specification by initiating the expression of nodal and vg1/univin, two genes that act at the top of the D/V ectoderm gene regulatory network.
    Developmental Biology 07/2011; 357(2):440-9. DOI:10.1016/j.ydbio.2011.07.005 · 3.64 Impact Factor
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    • "Nodal proteins are TGFβ-family signaling ligands that mediate epigenetic symmetry breaking processes in animal development, including mesoderm induction in vertebrates, axial patterning of neuroectoderm in vertebrates and echinoderms, and left-right axis specification in chordates, echinoderms, and gastropods (Schier and Shen, 2000; Duboc et al., 2004; Chea et al., 2005; Duboc et al., 2005; Lupo et al., 2006; Shen, 2007; Grande and Patel, 2009). In deuterostomes nodal is part of a feedback circuit wherein autocrine and paracrine Nodal signaling locally activates the nodal gene, as well as expression of its competitive inhibitor Lefty, which is more diffusible than Nodal and hence restricts the domain of nodal activity (Juan and Hamada, 2001; Chen and Schier, 2002; Duboc et al., 2008). Thus, nodal and lefty, together with their encoded proteins, embody a reactiondiffusion circuit that works by local (short-range) activation and lateral (long-range) inhibition (Turing, 1952; Gierer and Meinhardt, 1972). "
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    ABSTRACT: Nodal proteins are diffusible morphogens that drive pattern formation via short-range feedback activation coupled to long-range Lefty-mediated inhibition. In the sea urchin embryo, specification of the secondary (oral-aboral) axis occurs via zygotic expression of nodal, which is localized to the prospective oral ectoderm at early blastula stage. In mid-blastula stage embryos treated with low micromolar nickel or zinc, nodal expression expands progressively beyond the confines of this localized domain to encompass the entire equatorial circumference of the embryo, producing radialized embryos lacking an oral-aboral axis. RNAseq analysis of embryos treated with nickel, zinc, or cadmium (which does not radialize embryos) showed that several genes involved in endocytosis were similarly perturbed by nickel and zinc but not cadmium. Inhibiting dynamin, a GTPase required for receptor-mediated endocytosis, phenocopies the effects of nickel and zinc, suggesting that dynamin-mediated endocytosis is required as a sink to limit the range of Nodal signaling.
    Developmental Dynamics 03/2011; 240(3):704-11. DOI:10.1002/dvdy.22557 · 2.67 Impact Factor
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    • "Models of ectodermal specification suggest that Nodal signaling is limited to the oral ectoderm by Lefty, which depends on Nodal and has long-range inhibitory functions (Duboc et al., 2008). A reaction-diffusion model in which Lefty acts as a feedback inhibitor has been proposed to explain how it restricts Nodal signaling to oral ectoderm (Duboc et al., 2008; Bolouri and Davidson, 2009). BMP2/4, which also acts downstream of Nodal, is transcribed in the oral ectoderm (Angerer et al., 2000; Duboc et al., 2004), yet acts outside of oral ectoderm to induce aboral ectoderm (Lapraz et al., 2009). "
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    ABSTRACT: The ciliary band is a distinct region of embryonic ectoderm that is specified between oral and aboral ectoderm. Flask-shaped ciliary cells and neurons differentiate in this region and they are patterned to form an integrated tissue that functions as the principal swimming and feeding organ of the larva. TGFβ signaling, which is known to mediate oral and aboral patterning of the ectoderm, has been implicated in ciliary band formation. We have used morpholino knockdown and ectopic expression of RNA to alter TGFβ signaling at the level of ligands, receptors, and signal transduction components and assessed the differentiation and patterning of the ciliary band cells and associated neurons. We propose that the primary effects of these signals are to position the ciliary cells, which in turn support neural differentiation. We show that Nodal signaling, which is known to be localized by Lefty, positions the oral margin of the ciliary band. Signaling from BMP through Alk3/6, affects the position of the oral and aboral margins of the ciliary band. Since both Nodal and BMP signaling produce ectoderm that does not support neurogenesis, we propose that formation of a ciliary band requires protection from these signals. Expression of BMP2/4 and Nodal suppress neural differentiation. However, the response to receptor knockdown or dominant-negative forms of signal transduction components indicate signaling is not acting directly on unspecified ectoderm cells to prevent their differentiation as neurons. Instead, it produces a restricted field of ciliary band cells that supports neurogenesis. We propose a model that incorporates spatially regulated control of Nodal and BMP signaling to determine the position and differentiation of the ciliary band, and subsequent neural patterning.
    Developmental Biology 11/2010; 347(1):71-81. DOI:10.1016/j.ydbio.2010.08.009 · 3.64 Impact Factor
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