VegT activation of Sox17 at the midblastula transition alters the response to nodal signals in the vegetal endoderm domain

In Xenopus, the prospective endoderm and mesoderm are localized to discrete, adjacent domains at the beginning of gastrulation, and this is made evident by the expression of Sox17 in vegetal blastomeres and Brachyury (Xbra) in marginal blastomeres. Here, we examine the regulation of Sox17alpha expression and the role of Sox17alpha in establishing the vegetal endodermal gene expression domain. Injection of specific inhibitors of VegT or Nodal resulted in a loss of Sox17alpha expression in the gastrula. However, the onset of Sox17alpha expression at the midblastula transition was dependent on VegT, but not on Nodal function, indicating that Sox17alpha expression is initiated by VegT and then maintained by Nodal signals. Consistent with these results, VegT, but not Xenopus Nodal-related-1 (Xnr1), can activate Sox17alpha expression at the midblastula stage in animal explants. In addition, VegT activates Sox17alpha in the presence of cycloheximide or a Nodal antagonist, suggesting that Sox17alpha is an immediate-early target of VegT in vegetal blastomeres. Given that Nodal signals are necessary and sufficient for both mesodermal and endodermal gene expression, we propose that VegT activation of Sox17alpha at the midblastula transition prevents mesodermal gene expression in response to Nodal signals, thus establishing the vegetal endodermal gene expression domain. Supporting this idea, Sox17alpha misexpression in the marginal zone inhibits the expression of multiple mesodermal genes. Furthermore, in animal explants, Sox17alpha prevents the induction of Xbra and MyoD, but not Sox17beta or Mixer, in response to Xnr1. Therefore, VegT activation of Sox17alpha plays an important role in establishing a region of endoderm-specific gene expression in vegetal blastomeres.     

The pitx2 homeobox protein is required early for endoderm formation and nodal signaling.

Nodal and Nodal-related factors play fundamental roles in a number of developmental processes, including mesoderm and endoderm formation, patterning of the anterior neural plate, and determination of bilateral asymmetry in vertebrates. pitx2, a paired-like homeobox gene, has been proposed to act downstream of Nodal in the gene cascade providing left-right cues to the developing organs. Here, we report that pitx2 is required early in the Nodal signaling pathway for specification of the endodermal and mesodermal germ layers. We found that pitx2 is expressed very early during Xenopus and zebrafish development and in many regions where Nodal signaling is required, including the presumptive mesoderm and endoderm at the blastula and gastrula stages and the prechordal mesoderm at later stages. In Xenopus embryos, overexpression of pitx2 caused ectopic expression of goosecoid and sox-17 and interfered with mesoderm formation. Overexpression of pitx2 in Xenopus animal cap explants partially mimics the effects of Nodal overexpression, suggesting that pitx2 is a mediator of Nodal signaling during specification of the endoderm and prechordal plate, but not during mesoderm induction. We further demonstrate that pitx2 is induced by Nodal signaling in Xenopus animal caps and that the early expression of zebrafish pitx2 is absent when the Nodal signaling pathway is inactive. Inhibition of pitx2 function using a chimeric EnR-pitx2 blocked specification of the mesoderm and endoderm and caused severe embryonic defects resembling those seen when Nodal signaling is inhibited. Following inhibition of pitx2 function, the fate of ventral vegetal blastomeres was shifted from an endodermal to a more mesodermal fate, an effect that was reversed by wild-type pitx2. Finally, we show that inhibition of pitx2 function interferes with the response of cells to Nodal signaling. Our results provide direct evidence that pitx2 function is required for normal specification of the endodermal and mesodermal germ layers.     

Anteriorward shifting of asymmetric Xnr1 expression and contralateral communication in left-right specification in Xenopus

Transient asymmetric Nodal signaling in the left lateral plate mesoderm (L LPM) during tailbud/early somitogenesis stages is associated in all vertebrates examined with the development of stereotypical left-right (L-R) organ asymmetry. In Xenopus, asymmetric expression of Nodal-related 1 (Xnr1) begins in the posterior L LPM shortly after the initiation of bilateral perinotochordal expression in the posterior tailbud. The L LPM expression domain rapidly shifts forward to cover much of the flank of the embryo before being progressively downregulated, also in a posterior-to-anterior direction. The mechanisms underlying the initiation and propagation of Nodal/Xnr1 expression in the L LPM, and its transient nature, are not well understood. Removing the posterior tailbud domain prevents Xnr1 expression in the L LPM, consistent with the idea that normal embryos respond to a posteriorly derived asymmetrically acting positive inductive signal. The forward propagation of asymmetric Xnr1 expression occurs LPM-autonomously via planar tissue communication. The shifting is prevented by Nodal signaling inhibitors, implicating an underlying requirement for Xnr1-to-Xnr1 induction. It is also unclear how asymmetric Nodal signals are modulated during L-R patterning. Small LPM grafts overexpressing Xnr1 placed into the R LPM of tailbud embryos induced the expression of the normally L-sided genes Xnr1, Xlefty, and XPitx2, and inverted body situs, demonstrating the late-stage plasticity of the LPM. Orthogonal Xnr1 signaling from the LPM strongly induced Xlefty expression in the midline, consistent with recent findings in the mouse and demonstrating for the first time in another species conservation in the mechanism that induces and maintains the midline barrier. Our findings suggest that there is long-range contralateral communication between L and R LPM, involving Xlefty in the midline, over a substantial period of tailbud embryogenesis, and therefore lend further insight into how, and for how long, the midline maintains a L versus R status in the LPM.     

Boundaries and functional domains in the animal/vegetal axis of Xenopus gastrula mesoderm

Patterning of the Xenopus gastrula marginal zone in the axis running equatorially from the Spemann organizer-the so--called "dorsal/ventral axis"--has been extensively studied. It is now evident that patterning in the animal/vegetal axis also needs to be taken into consideration. We have shown that an animal/vegetal pattern is apparent in the marginal zone by midgastrulation in the polarized expression domains of Xenopus brachyury (Xbra) and Xenopus nodal-related factor 2 (Xnr2). In this report, we have followed cells expressing Xbra in the presumptive trunk and tail at the gastrula stage, and find that they fate to presumptive somite, but not to ventrolateral mesoderm of the tailbud embryo. From this, we speculate that the boundary between the Xbra- and Xnr2-expressing cells at gastrula corresponds to a future tissue boundary. In further experiments, we show that the level of mitogen-activated protein kinase (MAPK) activation is polarized along the animal/vegetal axis, with the Xnr2-expressing cells in the vegetal marginal zone having no detectable activated MAPK. We show that inhibition of MAPK activation in Xenopus animal caps results in the conversion of Xnr2 from a dorsal mesoderm inducer to a ventral mesoderm inducer, supporting a role for Xnr2 in induction of ventral mesoderm.     

Specification of ectoderm restricts the size of the animal plate and patterns neurogenesis in sea urchin embryos

The animal plate of the sea urchin embryo becomes the apical organ, a sensory structure of the larva. In the absence of vegetal signaling, an expanded and unpatterned apical organ forms. To investigate the signaling that restricts the size of the animal plate and patterns neurogenesis, we have expressed molecules that regulate specification of ectoderm in embryos and chimeras. Enhancing oral ectoderm suppresses serotonergic neuron differentiation, whereas enhancing aboral or ciliary band ectoderm increases differentiation of serotonergic neurons. In embryos in which vegetal signaling is blocked, Nodal expression does not reduce the size of the thickened animal plate; however, almost no neurons form. Expression of BMP in the absence of vegetal signaling also does not restrict the size of the animal plate, but abundant serotonergic neurons form. In chimeras in which vegetal signaling is blocked in the entire embryo, and one half of the embryo expresses Nodal, serotonergic neuron formation is suppressed in both halves. In similar chimeras in which vegetal signaling is blocked and one half of the embryo expresses Goosecoid (Gsc), serotonergic neurons form only in the half of the embryo not expressing Gsc. We propose that neurogenesis is specified by a maternal program that is restricted to the animal pole by signaling that is dependent on nuclearization of beta-catenin and specifies ciliary band ectoderm. Subsequently, neurogenesis in the animal plate is patterned by suppression of serotonergic neuron formation by Nodal. Like other metazoans, echinoderms appear to have a phase of neural development during which the specification of ectoderm restricts and patterns neurogenesis.     

casanova plays an early and essential role in endoderm formation in zebrafish.

The cellular and molecular mechanisms that regulate endoderm development in vertebrates have only recently begun to be explored. Here we show that the zebrafish locus casanova plays an early and essential role in this process. casanova mutants lack a gut tube and do not express any molecular markers of endoderm differentiation. The early endodermal expression of genes such as axial, gata5, and fkd2 does not initiate in casanova mutants, indicating that the endoderm is defective from the onset of gastrulation. Mosaic analysis demonstrates that casanova functions cell autonomously within the endodermal progenitors. We also report the isolation of a zebrafish homologue of Mixer, a gene important for early endoderm formation in Xenopus. casanova does not encode zebrafish Mixer, and mixer expression is normal in casanova mutants, indicating that casanova acts downstream of, or parallel to, mixer to promote endoderm formation. We further find that the forerunner cells, a specialized group of noninvoluting dorsal mesendodermal cells, do not form in casanova mutants. Studies of casanova mutants do not support an important role for the forerunner cells in either dorsal axis or tail development, as has been previously proposed. In addition, although different populations of mesodermal precursors are generated normally in casanova mutants, morphogenetic defects in the heart, vasculature, blood, and kidney are apparent, suggesting a possible role for the endoderm in morphogenesis of these organs.