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: 3.55). 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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    • "Whole mount in situ hybridization studies in a number of laboratories have identified a handful of genes expressed specifically in oral NSM (among recent studies, Duboc et al., 2010; Poustka et al. 2007; Ransick and Davidson, 2006; Rizzo et al., 2006; R ¨ ottinger et al., 2006; Sharma and Ettensohn, 2011; Walton et al., 2009), but the functional interactions among these genes in general remain to be demonstrated. Almost the only causal relationship established is that specification of the oral NSM is dependent on the Nodal signal emanating from the oral ectoderm (Duboc et al., 2008, 2010). This accounts for the oral placement of this domain. "
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    ABSTRACT: Specification of the non-skeletogenic mesoderm (NSM) in sea urchin embryos depends on Delta signaling. Signal reception leads to expression of regulatory genes that later contribute to the aboral NSM regulatory state. In oral NSM, this is replaced by a distinct oral regulatory state in consequence of Nodal signaling. Through regulome wide analysis we identify the homeobox gene not as an immediate Nodal target. not expression in NSM causes extinction of the aboral regulatory state in the oral NSM, and expression of a new suite of regulatory genes. All NSM specific regulatory genes are henceforth expressed exclusively, in oral or aboral domains, presaging the mesodermal cell types that will emerge. We have analyzed the regulatory linkages within the aboral NSM gene regulatory network. A linchpin of this network is gataE which as we show is a direct Gcm target and part of a feedback loop locking down the aboral regulatory state.
    Preview · Article · Dec 2012 · Developmental Biology
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    • "In sea urchin ectoderm, Nodal patterns both the ventro-dorsal (oral-aboral) and left-right axes [59], but was not found to be transcribed in brittle star gastrula (Table 3); nor was its antagonist Lefty, which limits Nodal to the ventral side during sea urchin development [60]. On the other hand, a number of genes downstream from Nodal and key to specification of different ectodermal regions [61] were found in brittle star (Table 3). "
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    ABSTRACT: Background The gastrula stage represents the point in development at which the three primary germ layers diverge. At this point the gene regulatory networks that specify the germ layers are established and the genes that define the differentiated states of the tissues have begun to be activated. These networks have been well-characterized in sea urchins, but not in other echinoderms. Embryos of the brittle star Ophiocoma wendtii share a number of developmental features with sea urchin embryos, including the ingression of mesenchyme cells that give rise to an embryonic skeleton. Notable differences are that no micromeres are formed during cleavage divisions and no pigment cells are formed during development to the pluteus larval stage. More subtle changes in timing of developmental events also occur. To explore the molecular basis for the similarities and differences between these two echinoderms, we have sequenced and characterized the gastrula transcriptome of O. wendtii. Methods Development of Ophiocoma wendtii embryos was characterized and RNA was isolated from the gastrula stage. A transcriptome data base was generated from this RNA and was analyzed using a variety of methods to identify transcripts expressed and to compare those transcripts to those expressed at the gastrula stage in other organisms. Results Using existing databases, we identified brittle star transcripts that correspond to 3,385 genes, including 1,863 genes shared with the sea urchin Strongylocentrotus purpuratus gastrula transcriptome. We characterized the functional classes of genes present in the transcriptome and compared them to those found in this sea urchin. We then examined those members of the germ-layer specific gene regulatory networks (GRNs) of S. purpuratus that are expressed in the O. wendtii gastrula. Our results indicate that there is a shared ‘genetic toolkit’ central to the echinoderm gastrula, a key stage in embryonic development, though there are also differences that reflect changes in developmental processes. Conclusions The brittle star expresses genes representing all functional classes at the gastrula stage. Brittle stars and sea urchins have comparable numbers of each class of genes and share many of the genes expressed at gastrulation. Examination of the brittle star genes in which sea urchin orthologs are utilized in germ layer specification reveals a relatively higher level of conservation of key regulatory components compared to the overall transcriptome. We also identify genes that were either lost or whose temporal expression has diverged from that of sea urchins.
    Full-text · Article · Sep 2012 · EvoDevo
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    • "Although it is possible that Wnt/-catenin signaling directly affects regulators of nodal transcription, it is also possible that it could work indirectly through other factors known to restrict Nodal expression to the oral ectoderm. These currently include Lefty, which is a direct antagonist of Nodal signaling (Duboc et al., 2008), BMP2/4 (Saudemont et al., 2010;Yaguchi et al., 2010), endocytic activity (Ertl et al., 2011), and the presence of sulfated proteoglygans in the extracellular environment that are thought to control Nodal diffusion (Bergeron et al., 2010). There also could be several intermediate steps, because there is an interval of ~5 hours between mesenchyme blastula and gastrula stages when the Wnt1 effect on nodal expression occurs. "
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    ABSTRACT: Wnt and Nodal signaling pathways are required for initial patterning of cell fates along anterior-posterior (AP) and dorsal-ventral (DV) axes, respectively, of sea urchin embryos during cleavage and early blastula stages. These mechanisms are connected because expression of nodal depends on early Wnt/β-catenin signaling. Here, we show that an important subsequent function of Wnt signaling is to control the shape of the nodal expression domain and maintain correct specification of different cell types along the axes of the embryo. In the absence of Wnt1, the posterior-ventral region of the embryo is severely altered during early gastrulation. Strikingly, at this time, nodal and its downstream target genes gsc and bra are expressed ectopically, extending posteriorly to the blastopore. They override the initial specification of posterior-ventral ectoderm and endoderm fates, eliminating the ventral contribution to the gut and displacing the ciliary band dorsally towards, and occasionally beyond, the blastopore. Consequently, in Wnt1 morphants, the blastopore is located at the border of the re-specified posterior-ventral oral ectoderm and by larval stages it is in the same plane near the stomodeum on the ventral side. In normal embryos, a Nodal-dependent process downregulates wnt1 expression in dorsal posterior cells during early gastrulation, focusing Wnt1 signaling to the posterior-ventral region where it suppresses nodal expression. These subsequent interactions between Wnt and Nodal signaling are thus mutually antagonistic, each limiting the range of the other's activity, in order to maintain and stabilize the body plan initially established by those same signaling pathways in the early embryo.
    Full-text · Article · Mar 2012 · Development
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