The neural plate specifies somite size in the Xenopus laevis gastrula.

The organizer has traditionally been considered the major source of somite-inducing signals. We show here that signaling from the neural plate specifies somite tissue and regulates somite size in the Xenopus gastrula. Ectopic undifferentiated neural tissue induces massive somite expansion at the expense of intermediate and lateral plate mesoderm. Although the early expanded somite expresses muscle-specific markers, only a portion terminally differentiates, suggesting that myotome development requires additional signals. Explant assays demonstrate that neural tissue induces somite-specific marker expression even in the absence of the organizer. Finally, we demonstrate that neural tissue is required for proper somite development because elimination of neural precursors results in pronounced somite reduction. Thus, an important reciprocal interaction exists between somite and neural tissue that is mutually reinforcing and critical for normal embryonic patterning.

Human disease-causing NOG missense mutations: effects on noggin secretion, dimer formation, and bone morphogenetic protein binding.

Secreted noggin protein regulates bone morphogenetic protein activity during development. In mice, a complete loss of noggin protein leads to multiple malformations including joint fusion, whereas mice heterozygous for Nog loss-of-function mutations are normal. In humans, heterozygous NOG missense mutations have been found in patients with two autosomal dominant disorders of joint development, multiple synostosis syndrome (SYNS1) and a milder disorder proximal symphalangism (SYM1). This study investigated the effect of one SYNS1 and two SYM1 disease-causing missense mutations on the structure and function of noggin. The SYNS1 mutation abolished, and the SYM1 mutations reduced, the secretion of functional noggin dimers in transiently transfected COS-7 cells. Coexpression of mutant noggin with wild-type noggin, to resemble the heterozygous state, did not interfere with wild-type noggin secretion. These data indicate that the human disease-causing mutations are hypomorphic alleles that reduce secretion of functional dimeric noggin. Therefore, we conclude that noggin has both species-specific and joint-specific dosage-dependent roles during joint formation. Surprisingly, in contrast to the COS-7 cell studies, the SYNS1 mutant was able to form dimers in Xenopus laevis oocytes. This finding indicates that there also exist species-specific differences in the ability to process mutant noggin polypeptides.

Xenopus Dishevelled signaling regulates both neural and mesodermal convergent extension: parallel forces elongating the body axis.

During amphibian development, non-canonical Wnt signals regulate the polarity of intercalating dorsal mesoderm cells during convergent extension. Cells of the overlying posterior neural ectoderm engage in similar morphogenetic cell movements. Important differences have been discerned in the cell behaviors associated with neural and mesodermal cell intercalation, raising the possibility that different mechanisms may control intercalations in these two tissues. In this report, targeted expression of mutants of Xenopus Dishevelled (Xdsh) to neural or mesodermal tissues elicited different defects that were consistent with inhibition of either neural or mesodermal convergent extension. Expression of mutant Xdsh also inhibited elongation of neural tissues in vitro in Keller sandwich explants and in vivo in neural plate grafts. Targeted expression of other Wnt signaling antagonists also inhibited neural convergent extension in whole embryos. In situ hybridization indicated that these defects were not due to changes in cell fate. Examination of embryonic phenotypes after inhibition of convergent extension in different tissues reveals a primary role for mesodermal convergent extension in axial elongation, and a role for neural convergent extension as an equalizing force to produce a straight axis. This study demonstrates that non-canonical Wnt signaling is a common mechanism controlling convergent extension in two very different tissues in the Xenopus embryo and may reflect a general conservation of control mechanisms in vertebrate convergent extension.

Xenopus Dan, a member of the Dan gene family of BMP antagonists, is expressed in derivatives of the cranial and trunk neural crest.

Dan is the founding member of the Dan family of secreted cytokines. All members of this family--which includes Gremlin, Cerberus, Dante, PRDC, and several genes identified as expressed sequence tags in the mouse--characterized to date have been shown to antagonize signaling by the bone morphogenetic protein (BMP) family. During mouse embryogenesis, Dan is expressed in a restricted and dynamic pattern. Major sites of transcription include the somites, the myotome and the cranial, and facial and limb mesenchyme. Xenopus Dan (XDan) shares over 76% amino acid identity with mouse Dan (mDan). Here we report that in Xenopus embryos, XDan is expressed both as a maternal transcript and at later stages in populations of cells associated with the cranial and trunk neural crest. The conservation of Dan expression in cells of the head mesenchyme between Xenopus and mouse embryos suggests an important role for BMP antagonists in these tissues.

Distinct enhancer elements control Hex expression during gastrulation and early organogenesis.

In the mouse, embryological and genetic studies have indicated that two spatially distinct signalling centres, the anterior visceral endoderm and the node and its derivatives, are required for the correct patterning of the anterior neural ectoderm. The divergent homeobox gene Hex is expressed in the anterior visceral endoderm, in the node (transiently), and in the anterior definitive endoderm. Other sites of Hex expression include the liver and thyroid primordia and the endothelial cell precursors. We have used transgenic analysis to map the cis-acting regulatory elements controlling Hex expression during early mouse development. A 4.2-kb upstream region is important for Hex expression in the endothelial cell precursors, liver, and thyroid, and a 633-bp intronic fragment is both necessary and sufficient for Hex expression in the anterior visceral endoderm and the anterior definitive endoderm. These same regions drive expression in homologous structures in Xenopus laevis, indicating conservation of these regulatory regions in vertebrates. Analysis of the anterior visceral endoderm/anterior definitive endoderm enhancer identifies a repressor region that is required to downregulate Hex expression in the node once the anterior definitive endoderm has formed. This analysis also reveals that the initiation of Hex expression in the anterior visceral endoderm and axial mesendoderm requires common elements, but maintenance of expression is regulated independently in these tissues.

Calcium signaling during convergent extension in Xenopus.

BACKGROUND: During Xenopus gastrulation, cell intercalation drives convergent extension of dorsal tissues. This process requires the coordination of motility throughout a large population of cells. The signaling mechanisms that regulate these movements in space and time remain poorly understood. RESULTS: To investigate the potential contribution of calcium signaling to the control of morphogenetic movements, we visualized calcium dynamics during convergent extension using a calcium-sensitive fluorescent dye and a novel confocal microscopy system. We found that dramatic intercellular waves of calcium mobilization occurred in cells undergoing convergent extension in explants of gastrulating Xenopus embryos. These waves arose stochastically with respect to timing and position within the dorsal tissues. Waves propagated quickly and were often accompanied by a wave of contraction within the tissue. Calcium waves were not observed in explants of the ventral marginal zone or prospective epidermis. Pharmacological depletion of intracellular calcium stores abolished the calcium dynamics and also inhibited convergent extension without affecting cell fate. These data indicate that calcium signaling plays a direct role in the coordination of convergent extension cell movements. CONCLUSIONS: The data presented here indicate that intercellular calcium signaling plays an important role in vertebrate convergent extension. We suggest that calcium waves may represent a widely used mechanism by which large groups of cells can coordinate complex cell movements.

Regulation of convergent extension in Xenopus by Wnt5a and Frizzled-8 is independent of the canonical Wnt pathway.

The Wnt signaling pathway is increasingly recognized as a highly branched signaling network. Experimental uncoupling of the different branches of this pathway has proven difficult, as many single components are shared downstream by multiple, distinct pathways. In this report, we demonstrate that the upstream Wnt antagonists Xwnt5a and Nxfz-8, which inhibit normal morphogenetic movements during Xenopus gastrulation, act independently of the canonical Wnt signaling pathway. This finding is important, as it highlights the promiscuity of upstream Wnt signaling components and further establishes an important role for non-canonical Wnt signaling in Xenopus morphogenesis.

Cell autonomous regulation of multiple Dishevelled-dependent pathways by mammalian Nkd.

Genetic studies have identified Drosophila Naked Cuticle (Nkd) as an antagonist of the canonical Wnt/beta-catenin signaling pathway, but its mechanism of action remains obscure [Zeng, W., Wharton, K. A., Jr., Mack, J. A., Wang, K., Gadbaw, M., et al. (2000) Nature (London) 403, 789--795]. Here we have cloned a cDNA encoding a mammalian homolog of Drosophila Nkd, mNkd, and demonstrated that mNkd interacts directly with Dishevelled. Dishevelled is an intracellular mediator of both the canonical Wnt pathway and planar cell polarity (PCP) pathway. Activation of the c-Jun-N-terminal kinase has been implicated in the PCP pathway. We showed that mNkd acts in a cell-autonomous manner not only to inhibit the canonical Wnt pathway but also to stimulate c-Jun-N-terminal kinase activity. Expression of mNkd disrupted convergent extension in Xenopus, consistent with a role for mNkd in the PCP pathway. These data suggest that mNkd may act as a switch to direct Dishevelled activity toward the PCP pathway, and away from the canonical Wnt pathway.

Mutation and analysis of Dan, the founding member of the Dan family of transforming growth factor beta antagonists.

The Dan family of transforming growth factor beta antagonists is a large, evolutionarily conserved family of proteins. Little is known about either the specificity of these antagonists or the biological roles of these proteins. We have characterized Dan, the founding member of this family, with regard to both its biochemical specificity and its biological roles. Although DAN is not an efficient antagonist of BMP-2/4 class signals, we found that DAN was able to interact with GDF-5 in a frog embryo assay, suggesting that DAN may regulate signaling by the GDF-5/6/7 class of BMPs in vivo. Intriguingly, in developing neurons, Dan mRNA was localized to axons, suggesting a potential role for the DAN protein in axonal outgrowth or guidance. Mice lacking Dan activity were generated by gene targeting and displayed subtle, background-dependent defects.

Hypaxial muscle migration during primary myogenesis in Xenopus laevis.

In contrast to many vertebrates, the ventral body wall muscles and limb muscles of Xenopus develop at different times. The ventral body wall forms in the tadpole, while limb (appendicular) muscles form during metamorphosis to the adult frog. In organisms that have been examined thus far, a conserved mechanism has been shown to control migratory muscle precursor specification, migration, and differentiation. Here, we show that the process of ventral body wall formation in Xenopus laevis is similar to hypaxial muscle development in chickens and mice. Cells specified for the migratory lineage display an upregulation of pax3 in the ventro-lateral region of the somite. These pax3-positive cells migrate ventrally, away from the somite, and undergo terminal differentiation with the expression of myf-5, followed by myoD. Several other genes are selectively expressed in the migrating muscle precursor population, including neural cell adhesion molecule (NCAM), Xenopus kit related kinase (Xkrk1), and Xenopus SRY box 5 (sox5). We have also found that muscle precursor migration is highly coordinated with the migration of neural crest-derived melanophores. However, by extirpating neural crest at an early stage and allowing embryos to develop, we determined that muscle precursor migration is not dependent on physical or genetic interaction with melanophores.

Ski represses bone morphogenic protein signaling in Xenopus and mammalian cells.

The bone morphogenic proteins (BMPs) play important roles in vertebrate development. In Xenopus, BMPs act as epidermal inducers and also as negative regulators of neurogenesis. Antagonism of BMP signaling results in neuralization. BMPs signal through the cell-surface receptors and downstream Smad molecules. Upon stimulation with BMP, Smad1, Smad5, and Smad8 are phosphorylated by the activated BMP receptors, form a complex with Smad4, and translocate into the nucleus, where they regulate the expression of BMP target genes. Here, we show that the Ski oncoprotein can block BMP signaling and the expression of BMP-responsive genes in both Xenopus and mammalian cells by directly interacting with and repressing the activity of BMP-specific Smad complexes. This ability to antagonize BMP signaling results in neuralization by Ski in the Xenopus embryo and blocking of osteoblast differentiation of murine W-20-17 cells. Thus, Ski is able to repress the activity of all receptor-associated Smads and may regulate vertebrate development by modulating the signaling activity of transforming growth factor-beta family members.

Use of large-scale expression cloning screens in the Xenopus laevis tadpole to identify gene function.

We have conducted an expression cloning screen of approximately 50, 000 cDNAs from a tadpole stage Xenopus laevis cDNA library to functionally identify genes affecting a wide range of cellular and developmental processes. Fifty-seven cDNAs were isolated for their ability to alter gross tadpole morphology or the expression patterns of tissue-specific markers. Thirty-seven of the cDNAs have not been previously described for Xenopus, and 15 of these show little or no similarity to sequences in the NCBI database. The screen and the identified genes are presented in this paper to demonstrate the power, ease, speed, and flexibility of expression cloning in the X. laevis embryo. Future screens such as this one can be done on a larger scale and will complement the sequence-based screens and genome-sequencing projects which are producing a large body of novel genes without ascribed functions.

Ras-mediated FGF signaling is required for the formation of posterior but not anterior neural tissue in Xenopus laevis.

Fibroblast growth factor (FGF) has been proposed to be involved in the specification and patterning of the developing vertebrate nervous system. There is conflicting evidence, however, concerning the requirement for FGF signaling in these processes. To provide insight into the signaling mechanisms that are important for neural induction and anterior-posterior neural patterning, we have employed the dominant negative Ras mutant, N17Ras, in addition to a truncated FGF receptor (XFD). Both N17Ras and XFD, when expressed in Xenopus laevis animal cap ectoderm, inhibit the ability of FGF to generate neural pattern. They also block induction of posterior neural tissue by XBF2 and XMeis3. However, neither XFD nor N17Ras inhibits noggin, neurogenin, or XBF2 induction of anterior neural markers. MAP kinase activation has been proposed to be necessary for neural induction, yet N17Ras inhibits the phosphorylation of MAP kinase that usually follows explantation of explants. In whole embryos, Ras-mediated FGF signaling is critical for the formation of posterior neural tissues but is dispensable for neural induction.

Xotx5b, a new member of the Otx gene family, may be involved in anterior and eye development in Xenopus laevis.

We describe the cloning, expression pattern and functional overexpression analysis of Xotx5b, a new member of the Otx gene family in Xenopus laevis. Early expression of Xotx5b resembles that of Xotx2, being detected in the organizer region at early gastrula stage, and, shortly after, also in anterior neuroectoderm. During neurula stages Xotx5b exhibits a changing and dynamic pattern of expression. After neural tube closure, Xotx5b is expressed in the eye and pineal gland, both involved in photoreception. Overexpression of Xotx5b has a similar effect to that of Xotx2, producing posterior truncations and inducing ectopic cement gland and neural tissue in whole embryos. In animal cap assays, Xotx5b and Xotx2 are both able to activate XAG, to strongly suppress the expression of the epidermal marker XK81, and to reciprocally activate each other. Finally, in einsteck transplantation assays, Xotx5b is able to respecify a tail/trunk organizer to a head organizer.

Dishevelled controls cell polarity during Xenopus gastrulation.

Although cell movements are vital for establishing the normal architecture of embryos, it is unclear how these movements are regulated during development in vertebrates. Inhibition of Xenopus Dishevelled (Xdsh) function disrupts convergent extension movements of cells during gastrulation, but the mechanism of this effect is unclear, as cell fates are not affected. In Drosophila, Dishevelled controls both cell fate and cell polarity, but whether Dishevelled is involved in controlling cell polarity in vertebrate embryos has not been investigated. Here we show, using time-lapse confocal microscopy, that the failure of cells lacking Xdsh function to undergo convergent extension results from defects in cell polarity. Furthermore, Xdsh mutations that inhibit convergent extension correspond to mutations in Drosophila Dishevelled that selectively perturb planar cell polarity. Finally, the localization of Xdsh at the membrane of normal dorsal mesodermal cells is consistent with Xdsh controlling cell polarity. Our results show that polarized cell behaviour is essential for convergent extension and is controlled by vertebrate Dishevelled. Thus, a vertebrate equivalent of the Drosophila planar cell polarity signalling cascade may be required for normal gastrulation.

The organizer factors Chordin and Noggin are required for mouse forebrain development.

In mice, there is evidence suggesting that the development of head and trunk structures is organized by distinctly separated cell populations. The head organizer is located in the anterior visceral endoderm (AVE) and the trunk organizer in the node and anterior primitive streak. In amphibians, Spemann's organizer, which is homologous to the node, partially overlaps with anterior endoderm cells expressing homologues of the AVE markers cerberus, Hex and Hesx1. For mice, this raises the question of whether the AVE and node are independent of each other, as suggested by their anatomical separation, or functionally interdependent as is the case in amphibians. Chordin and Noggin are secreted bone morphogenetic protein (BMP) antagonists expressed in the mouse node, but not in the AVE. Here we show that mice double-homozygous mutants that are for chordin and noggin display severe defects in the development of the prosencephalon. The results show that BMP antagonists in the node and its derivatives are required for head development.

In Xenopus embryos, BMP heterodimers are not required for mesoderm induction, but BMP activity is necessary for dorsal/ventral patterning.

The activity of bone morphogenetic protein (BMP) heterodimers has been shown to be more potent than that of homodimers in a number of contexts, including mesoderm induction. Although BMP-2/7 and -4/7 heterodimers are potent inducers of ventral mesoderm in ectodermal explants, we show that they are not a necessary component of the primary mesoderm-inducing signal in intact Xenopus embryos. The secreted BMP antagonists noggin and gremlin both efficiently block mesoderm induction by BMP homo- and heterodimers in animal caps. When these antagonists are ectopically expressed in the ventral marginal zone of early embryos the initial formation of mesoderm as indicated by panmesodermal markers remains unaffected. Only the subsequent dorsal/ventral patterning of this mesoderm appears to be altered, with expression of a number of organizer-specific transcripts observed in the marginal zone where BMP signaling has been abolished. Thus, we conclude that BMPs do not contribute an essential signal to mesodermal induction or patterning until gastrulation. The activities of noggin and gremlin are strikingly different from that of the multifunctional antagonist cerberus, which completely abolishes mesoderm induction when misexpressed during early development.

Wnt signaling in Xenopus embryos inhibits bmp4 expression and activates neural development.

We report a new role for Wnt signaling in the vertebrate embryo: the induction of neural tissue from ectoderm. Early expression of mouse wnt8, Xwnt8, beta-catenin, or dominant-negative GSK3 induces the expression of neural-specific markers and inhibits the expression of Bmp4 in Xenopus ectoderm. We show that Wnt8, but not the BMP antagonist Noggin, can inhibit Bmp4 expression at early gastrula stages. Furthermore, inhibition of beta-catenin activity in the neural ectoderm of whole embryos by a truncated TCF results in a decrease in neural development. Therefore, we suggest that a cleavage-stage Wnt signal normally contributes to an early repression of Bmp4 on the dorsal side of the embryo and sensitizes the ectoderm to respond to neural inducing signals from the organizer. The Wnt targets Xnr3 and siamois have been shown previously to have neuralizing activity when overexpressed. However, antagonists of Wnt signaling, dnXwnt8 and Nxfrz8, inhibit Wnt-mediated Xnr3 and siamois induction, but not neural induction, suggesting an alternative mechanism for Bmp repression and neuralization. Conversely, dnTCF blocks both Wnt-mediated Xnr3 and neural induction, suggesting that both pathways require this transcription factor.