DNA-binding specificity and embryological function of Xom (Xvent-2)

Ventx Family and Its Functional Similarities with Nanog: Involvement in Embryonic Development and Cancer Progression

The Ventx family is one of the subfamilies of the ANTP (antennapedia) superfamily and belongs to the NK-like (NKL) subclass. Ventx is a homeobox transcription factor and has a DNA-interacting domain that is evolutionarily conserved throughout vertebrates. It has been extensively studied in Xenopus, zebrafish, and humans. The Ventx family contains transcriptional repressors widely involved in embryonic development and tumorigenesis in vertebrates. Several studies have documented that the Ventx family inhibited dorsal mesodermal formation, neural induction, and head formation in Xenopus and zebrafish. Moreover, Ventx2.2 showed functional similarities to Nanog and Barx1, leading to pluripotency and neural-crest migration in vertebrates. Among them, Ventx protein is an orthologue of the Ventx family in humans. Studies have demonstrated that human Ventx was strongly associated with myeloid-cell differentiation and acute myeloid leukemia. The therapeutic potential of Ventx family inhibition in combating cancer progression in humans is discussed. Additionally, we briefly discuss genome evolution, gene duplication, pseudo-allotetraploidy, and the homeobox family in Xenopus.

Evolution of cis-regulatory modules for the head organizer gene goosecoid in chordates: comparisons between Branchiostoma and Xenopus

BACKGROUND

In cephalochordates (amphioxus), the notochord runs along the dorsal to the anterior tip of the body. In contrast, the vertebrate head is formed anterior to the notochord, as a result of head organizer formation in anterior mesoderm during early development. A key gene for the vertebrate head organizer, goosecoid (gsc), is broadly expressed in the dorsal mesoderm of amphioxus gastrula. Amphioxus gsc expression subsequently becomes restricted to the posterior notochord from the early neurula. This has prompted the hypothesis that a change in expression patterns of gsc led to development of the vertebrate head during chordate evolution. However, molecular mechanisms of head organizer evolution involving gsc have never been elucidated.

RESULTS

To address this question, we compared cis-regulatory modules of vertebrate organizer genes between amphioxus, Branchiostoma japonicum, and frogs, Xenopus laevis and Xenopus tropicalis. Here we show conservation and diversification of gene regulatory mechanisms through cis-regulatory modules for gsc, lim1/lhx1, and chordin in Branchiostoma and Xenopus. Reporter analysis using Xenopus embryos demonstrates that activation of gsc by Nodal/FoxH1 signal through the 5' upstream region, that of lim1 by Nodal/FoxH1 signal through the first intron, and that of chordin by Lim1 through the second intron, are conserved between amphioxus and Xenopus. However, activation of gsc by Lim1 and Otx through the 5' upstream region in Xenopus are not conserved in amphioxus. Furthermore, the 5' region of amphioxus gsc recapitulated the amphioxus-like posterior mesoderm expression of the reporter gene in transgenic Xenopus embryos.

CONCLUSIONS

On the basis of this study, we propose a model, in which the gsc gene acquired the cis-regulatory module bound with Lim1 and Otx at its 5' upstream region to be activated persistently in anterior mesoderm, in the vertebrate lineage. Because Gsc globally represses trunk (notochord) genes in the vertebrate head organizer, this cooption of gsc in vertebrates appears to have resulted in inhibition of trunk genes and acquisition of the head organizer and its derivative prechordal plate.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Xom induces proteolysis of β-catenin through GSK3β-mediated pathway

The dorsal cell fate determination factor β-catenin and its antagonist, the ventral cell fate determination factor Xom, are expressed and distributed in a polarized fashion during early vertebrate embryogenesis. Ubiquitin-mediated proteolysis has been shown to control the abundance of both β-catenin and Xom. However, the mechanism of ubiquitin-mediated proteolysis in regulating dorsoventral patterning remains largely unclear. Our current study shows that Xom induces proteolysis of β-catenin through GSK3-mediated phosphorylation of Ser33/37 of β-catenin. Our findings reveal a novel pathway that regulates β-catenin stability, and suggest, for the first time, a critical function of ubiquitin-mediated proteolysis in balancing the integration of dorsal-ventral signals and the polarized distribution of β-catenin and Xom during dorsoventral axis formation.

A gene regulatory program controlling early Xenopus mesendoderm formation: Network conservation and motifs

Germ layer formation is among the earliest differentiation events in metazoan embryos. In triploblasts, three germ layers are formed, among which the endoderm gives rise to the epithelial lining of the gut tube and associated organs including the liver, pancreas and lungs. In frogs (Xenopus), where early germ layer formation has been studied extensively, the process of endoderm specification involves the interplay of dozens of transcription factors. Here, we review the interactions between these factors, summarized in a transcriptional gene regulatory network (GRN). We highlight regulatory connections conserved between frog, fish, mouse, and human endodermal lineages. Especially prominent is the conserved role and regulatory targets of the Nodal signaling pathway and the T-box transcription factors, Vegt and Eomes. Additionally, we highlight network topologies and motifs, and speculate on their possible roles in development.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Temporally coordinated signals progressively pattern the anteroposterior and dorsoventral body axes

The vertebrate body plan is established through the precise spatiotemporal coordination of morphogen signaling pathways that pattern the anteroposterior (AP) and dorsoventral (DV) axes. Patterning along the AP axis is directed by posteriorizing signals Wnt, fibroblast growth factor (FGF), Nodal, and retinoic acid (RA), while patterning along the DV axis is directed by bone morphogenetic proteins (BMP) ventralizing signals. This review addresses the current understanding of how Wnt, FGF, RA and BMP pattern distinct AP and DV cell fates during early development and how their signaling mechanisms are coordinated to concomitantly pattern AP and DV tissues.

The role of targeted protein degradation in early neural development

As neural stem cells differentiate into neurons during neurogenesis, the proteome of the cells is restructured by de novo expression and selective removal of regulatory proteins. The control of neurogenesis at the level of gene regulation is well documented and the regulation of protein abundance through protein degradation via the Ubiquitin/26S proteasome pathway is a rapidly developing field. This review describes our current understanding of the role of the proteasome pathway in neurogenesis. Collectively, the studies show that targeted protein degradation is an important regulatory mechanism in the generation of new neurons.

The integrator complex subunit 6 (Ints6) confines the dorsal organizer in vertebrate embryogenesis

Dorsoventral patterning of the embryonic axis relies upon the mutual antagonism of competing signaling pathways to establish a balance between ventralizing BMP signaling and dorsal cell fate specification mediated by the organizer. In zebrafish, the initial embryo-wide domain of BMP signaling is refined into a morphogenetic gradient following activation dorsally of a maternal Wnt pathway. The accumulation of β-catenin in nuclei on the dorsal side of the embryo then leads to repression of BMP signaling dorsally and the induction of dorsal cell fates mediated by Nodal and FGF signaling. A separate Wnt pathway operates zygotically via Wnt8a to limit dorsal cell fate specification and maintain the expression of ventralizing genes in ventrolateral domains. We have isolated a recessive dorsalizing maternal-effect mutation disrupting the gene encoding Integrator Complex Subunit 6 (Ints6). Due to widespread de-repression of dorsal organizer genes, embryos from mutant mothers fail to maintain expression of BMP ligands, fail to fully express vox and ved, two mediators of Wnt8a, display delayed cell movements during gastrulation, and severe dorsalization. Consistent with radial dorsalization, affected embryos display multiple independent axial domains along with ectopic dorsal forerunner cells. Limiting Nodal signaling or restoring BMP signaling restores wild-type patterning to affected embryos. Our results are consistent with a novel role for Ints6 in restricting the vertebrate organizer to a dorsal domain in embryonic patterning.

Microarray-based identification of Pitx3 targets during Xenopus embryogenesis

BACKGROUND

Unexpected phenotypes resulting from morpholino-mediated translational knockdown of Pitx3 in Xenopus laevis required further investigation regarding the genetic networks in which the gene might play a role. Microarray analysis was, therefore, used to assess global transcriptional changes downstream of Pitx3.

RESULTS

From the large data set generated, selected candidate genes were confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization.

CONCLUSIONS

We have identified four genes as likely direct targets of Pitx3 action: Pax6, β Crystallin-b1 (Crybb1), Hes7.1, and Hes4. Four others show equivocal promise worthy of consideration: Vent2, and Ripply2 (aka Ledgerline or Stripy), eFGF and RXRα. We also describe the expression pattern of additional and novel genes that are Pitx3-sensitive but that are unlikely to be direct targets.

Characterization of Danio rerio Nanog and functional comparison to Xenopus Vents

Nanog is a homeodomain transcription factor associated with the acquisition of pluripotency. Genome analyses of lower and higher vertebrates revealed that the existence of Nanog is restricted to gnathostomata but absent from agnatha and invertebrates. To elucidate the function of Nanog in nonmammalia, we identified the Danio rerio ortholog of Nanog and characterized its role in gain and loss of function experiments. We found Nanog to be crucial for survival of early zebrafish embryos, because depletion of Nanog led to gastrulation defects with subsequent lethality. Mouse Nanog overexpression could rescue these defects. Vice versa, zebrafish Nanog was found to promote proliferation and to inhibit differentiation of mouse embryonic stem cells in the absence of leukemia inhibitory factor. These findings indicate functional conservation of Nanog from teleost fishes to mammals. However, Nanog was lost in the genome of the anurans Xenopus laevis and Xenopus tropicalis. Phylogenetic analysis revealed that deletion probably occurred in a common anuran ancestor along with chromosomal translocations. The closest homologs of Nanog in Xenopus are the Vent proteins. We, therefore, investigated whether the Xvent genes might substitute for Nanog function in Xenopus. Although we found some similarities in phenotypes after overexpression and in the regulation of several marker genes, Xvent1/2 and Nanog cannot substitute each other. Depletion of Nanog in zebrafish cannot be rescued by ectopic expression of Xvent, and Xvent depletion in Xenopus cannot be overcome by ectopic expression of zebrafish Nanog.

Direct response elements of BMP within the PV.1A promoter are essential for its transcriptional regulation during early Xenopus development

Xvent homeobox genes encode transcription factors that repress organizer genes and are essential for dorsoventral specification during early embryogenesis in Xenopus. In contrast to the Xvent-2 gene subfamily, Xvent-1 subfamily members, including PV.1A, have been proposed as indirect targets of Bone Morphogenetic Protein-4 (BMP-4) signaling. Because PV.1A is a critical downstream mediator of, and tightly regulated by, BMP-4 signaling, we hypothesized that its promoter contains a direct BMP-4 response element to effect this transcriptional regulation. We demonstrate that direct regulation by BMP-4 is necessary for transcription of PV.1A: its proximal promoter contains cis-acting binding elements for Smads and Oaz crucial to induction in response to BMP-4 signaling. In addition to these direct cis-acting BMP-4 responsive elements, an indirect Xvent-2 response element and several repressive elements exist in the PV.1A promoter to regulate its transcription. In summary, PV.1A undergoes combinatorial regulation during early Xenopus development as both the direct target of BMP-4 signaling and as the direct and indirect target of positive and negative regulatory factors.

Characterization of Cer-1 cis-regulatory region during early Xenopus development

Cerberus-related molecules are well-known Wnt, Nodal, and BMP inhibitors that have been implicated in different processes including anterior–posterior patterning and left–right asymmetry. In both mouse and frog, two Cerberus-related genes have been isolated, mCer-1 and mCer-2, and Xcer and Xcoco, respectively. Until now, little is known about the mechanisms involved in their transcriptional regulation. Here, we report a heterologous analysis of the mouse Cerberus-1 gene upstream regulatory regions, responsible for its expression in the visceral endodermal cells. Our analysis showed that the consensus sequences for a TATA, CAAT, or GC boxes were absent but a TGTGG sequence was present at position -172 to -168 bp, relative to the ATG. Using a series of deletion constructs and transient expression in Xenopus embryos, we found that a fragment of 1.4 kb of Cer-1 promoter sequence could reproduce the endogenous expression pattern of Xenopus cerberus. A 0.7-kb mcer-1 upstream region was able to drive reporter expression to the involuting mesendodermal cells, while further deletions abolished reporter gene expression. Our results suggest that although no sequence similarity was found between mouse and Xenopus cerberus cis-regulatory regions, the signaling cascades regulating cerberus expression, during gastrulation, is conserved.

Conservation and diversification of an ancestral chordate gene regulatory network for dorsoventral patterning

Formation of a dorsoventral axis is a key event in the early development of most animal embryos. It is well established that bone morphogenetic proteins (Bmps) and Wnts are key mediators of dorsoventral patterning in vertebrates. In the cephalochordate amphioxus, genes encoding Bmps and transcription factors downstream of Bmp signaling such as Vent are expressed in patterns reminiscent of those of their vertebrate orthologues. However, the key question is whether the conservation of expression patterns of network constituents implies conservation of functional network interactions, and if so, how an increased functional complexity can evolve. Using heterologous systems, namely by reporter gene assays in mammalian cell lines and by transgenesis in medaka fish, we have compared the gene regulatory network implicated in dorsoventral patterning of the basal chordate amphioxus and vertebrates. We found that Bmp but not canonical Wnt signaling regulates promoters of genes encoding homeodomain proteins AmphiVent1 and AmphiVent2. Furthermore, AmphiVent1 and AmphiVent2 promoters appear to be correctly regulated in the context of a vertebrate embryo. Finally, we show that AmphiVent1 is able to directly repress promoters of AmphiGoosecoid and AmphiChordin genes. Repression of genes encoding dorsal-specific signaling molecule Chordin and transcription factor Goosecoid by Xenopus and zebrafish Vent genes represents a key regulatory interaction during vertebrate axis formation. Our data indicate high evolutionary conservation of a core Bmp-triggered gene regulatory network for dorsoventral patterning in chordates and suggest that co-option of the canonical Wnt signaling pathway for dorsoventral patterning in vertebrates represents one of the innovations through which an increased morphological complexity of vertebrate embryo is achieved.

A gene regulatory network controlling hhex transcription in the anterior endoderm of the organizer

The homeobox gene hhex is one of the earliest markers of the anterior endoderm, which gives rise to foregut organs such as the liver, ventral pancreas, thyroid, and lungs. The regulatory networks controlling hhex transcription are poorly understood. In an extensive cis-regulatory analysis of the Xenopus hhex promoter, we determined how the Nodal, Wnt, and BMP pathways and their downstream transcription factors regulate hhex expression in the gastrula organizer. We show that Nodal signaling, present throughout the endoderm, directly activates hhex transcription via FoxH1/Smad2 binding sites in the proximal -0.44 Kb promoter. This positive action of Nodal is suppressed in the ventral-posterior endoderm by Vent 1 and Vent2, homeodomain repressors that are induced by BMP signaling. Maternal Wnt/β-catenin on the dorsal side of the embryo cooperates with Nodal and indirectly activates hhex expression via the homeodomain activators Siamois and Twin. Siamois/Twin stimulate hhex transcription through two mechanisms: (1) they induce the expression of Otx2 and Lim1 and together Siamois, Twin, Otx2, and Lim1 appear to promote hhex transcription through homeobox sites in a Wnt-responsive element located between -0.65 to -0.55 Kb of the hhex promoter. (2) Siamois/Twin also induce the expression of the BMP-antagonists Chordin and Noggin, which are required to exclude Vents from the organizer allowing hhex transcription. This study reveals a complex network regulating anterior endoderm transcription in the early embryo.

Organizer restriction through modulation of Bozozok stability by the E3 ubiquitin ligase Lnx-like

The organizer anchors the primary embryonic axis, and balance between dorsal (organizer) and ventral domains is fundamental to body patterning. LNX (ligand of Numb protein-X) is a RING finger and four PDZ domain-containing E3 ubiquitin ligase. LNX serves as a binding platform and may have a role in cell fate determination, but its in vivo functions are unknown. Here we show that Lnx-l (Lnx-like) functions as a critical regulator of dorso-ventral axis formation in zebrafish. Depletion of Lnx-l using specific antisense morpholinos (MOs) caused strong embryonic dorsalization. We identified Bozozok (Boz, also known as Dharma or Nieuwkoid) as a binding partner and substrate of Lnx-l. Boz is a homeodomain-containing transcriptional repressor induced by canonical Wnt signalling that is critical for dorsal organizer formation. Lnx-l induced K48-linked polyubiquitylation of Boz, leading to its proteasomal degradation in human 293T cells and in zebrafish embryos. Dorsalization induced by Boz overexpression was suppressed by raising the level of Lnx-l, but Lnx-l failed to counteract dorsalization caused by mutant Boz lacking a critical motif for Lnx-l binding. Furthermore, dorsalization induced by depletion of Lnx-l was alleviated by attenuation of Boz expression. We conclude that Lnx-l modulates Boz activity to prevent the invasion of ventral regions of the embryo by organizer tissue. These studies introduce a ubiquitin ligase, Lnx-l, as a balancing modulator of axial patterning in the zebrafish embryo.

Neural induction and factors that stabilize a neural fate

The neural ectoderm of vertebrates forms when the bone morphogenetic protein (BMP) signaling pathway is suppressed. Herein, we review the molecules that directly antagonize extracellular BMP and the signaling pathways that further contribute to reduce BMP activity in the neural ectoderm. Downstream of neural induction, a large number of "neural fate stabilizing" (NFS) transcription factors are expressed in the presumptive neural ectoderm, developing neural tube and ultimately in neural stem cells. Herein, we review what is known about their activities during normal development to maintain a neural fate and regulate neural differentiation. Further elucidation of how the NFS genes interact to regulate neural specification and differentiation should ultimately prove useful for regulating the expansion and differentiation of neural stem and progenitor cells.

Sox3 expression is maintained by FGF signaling and restricted to the neural plate by Vent proteins in the Xenopus embryo

The formation of the nervous system is initiated when ectodermal cells adopt the neural fate. Studies in Xenopus demonstrate that inhibition of BMP results in the formation of neural tissue. However, the molecular mechanism driving the expression of early neural genes in response to this inhibition is unknown. Moreover, controversy remains regarding the sufficiency of BMP inhibition for neural induction. To address these questions, we performed a detailed analysis of the regulation of the soxB1 gene, sox3, one of the earliest genes expressed in the neuroectoderm. Using ectodermal explant assays, we analyzed the role of BMP, Wnt and FGF signaling in the regulation of sox3 and the closely related soxB1 gene, sox2. Our results demonstrate that both sox3 and sox2 are induced in response to BMP antagonism, but by distinct mechanisms and that the activation of both genes is independent of FGF signaling. However, both require FGF for the maintenance of their expression. Finally, sox3 genomic elements were identified and characterized and an element required for BMP-mediated repression via Vent proteins was identified through the use of transgenesis and computational analysis. Interestingly, none of the elements required for sox3 expression were identified in the sox2 locus. Together our data indicate that two closely related genes have unique mechanisms of gene regulation at the onset of neural development.

TGFbeta-SMAD signal transduction: molecular specificity and functional flexibility

Ligands of the transforming growth factor-beta (TGFbeta) superfamily of growth factors initiate signal transduction through a bewildering complexity of ligand-receptor interactions. Signalling then converges to nuclear accumulation of transcriptionally active SMAD complexes and gives rise to a plethora of specific functional responses in both embryos and adult organisms. Current research is focused on the mechanisms that regulate SMAD activity to evoke cell-type-specific and context-dependent transcriptional programmes. An equally important challenge is understanding the functional role of signal strength and duration. How are these quantitative aspects of the extracellular signal regulated? How are they then sensed and interpreted, and how do they affect responses?

Xom interacts with and stimulates transcriptional activity of LEF1/TCFs: implications for ventral cell fate determination during vertebrate embryogenesis

LEF1/TCFs are high mobility group box-containing transcriptional factors mediating canonical Wnt/beta-catenin signaling during early embryogenesis and tumorigenesis. Beta-catenin forms a complex with LEF1/TCFs and transactivates LEF1/TCF-mediated transcriptions during dorsalization. Although LEF-mediated transcription is also implicated in ventralization, the underlying molecular mechanism is not well understood. Using the vertebrate Xenopus laevis model system, we found that Xom, which is a ventralizing homeobox protein with dual roles of transcriptional activation and repression, forms a complex with LEF1/TCF through its homeodomain and transactivates LEF1/TCF-mediated transcription through its N-terminal transactivation domain (TAD). Our data show that Xom lacking the N-terminal TAD fails to transactivate ventral genes, such as BMP4 and Xom itself, but retains the ability to suppress transcriptional activation of dorsal gene promoters, such as the Goosecoid promoter, indicating that transactivation and repression are separable functions of Xom. It has been postulated that Xom forms a positive re-enforcement loop with BMP4 to promote ventralization and to suppress dorsal gene expression. Consistent with an essential role of Xom transactivation of LEF1/TCFs during early embryogenesis, we found that expression of the dominant-negative Xom mutant that lacks the TAD fails to re-enforce the ventral signaling of BMP4 and causes a catastrophic effect during gastrulation. Our data suggest that the functional interaction of Xom and LEF1/TCF-factors is essential for ventral cell fate determination and that LEF1/TCF factors may function as a point of convergence to mediate the combined signaling of Wnt/beta-catenin and BMP4/Xom pathways during early embryogenesis.

Chordin expression, mediated by Nodal and FGF signaling, is restricted by redundant function of two beta-catenins in the zebrafish embryo

Using embryos transgenic for the TOP-GFP reporter, we show that the two zebrafish beta-catenins have different roles in the organizer and germ-ring regions of the embryo. beta-Catenin-activated transcription in the prospective organizer region specifically requires beta-catenin-2, whereas the ventrolateral domain of activated transcription is abolished only when both beta-catenins are inhibited. chordin expression during zebrafish gastrulation has been previously shown in both axial and paraxial domains, but is excluded from ventrolateral domains. We show that this gene is expressed in paraxial territories adjacent to the domain of ventrolateral beta-catenin-activated transcription, with only slight overlap, consistent with the now well-known inhibitory effects of Wnt8 on dorsal gene expression. Eliminating both Wnt8/beta-catenin signaling and organizer activity by inhibition of expression of the two beta-catenins results in massive ectopic circumferential expression of chordin and later, by formation of a distinctive embryonic phenotype ('ciuffo') that expresses trunk and anterior neural markers with correct relative anteroposterior patterning. We show that chordin expression is required for this neural gene expression. The Nodal gene squint has been shown to be necessary for optimal expression of chordin and is sufficient in some contexts for its expression. However, chordin is not normally expressed in the ventrolateral germ-ring despite robust expression of squint in this domain. We show the ectopic circumferential expression of chordin and other dorsal genes to be completely dependent on Nodal and FGF signaling, and to be independent of a functional organizer. We propose that whereas the axial domain of chordin expression is formed by cells that are derived from the organizer, the paraxial domain is the result of axial-derived anti-Wnt signals, which relieve the repression that otherwise is set by the Wnt8/beta-catenin/vox,vent pathway on latent germ-ring Nodal/FGF-activated expression.

The opposing homeobox genes Goosecoid and Vent1/2 self-regulate Xenopus patterning

We present a loss-of-function study using antisense morpholino (MO) reagents for the organizer-specific gene Goosecoid (Gsc) and the ventral genes Vent1 and Vent2. Unlike in the mouse Gsc is required in Xenopus for mesodermal patterning during gastrulation, causing phenotypes ranging from reduction of head structures-including cyclopia and holoprosencephaly-to expansion of ventral tissues in MO-injected embryos. The overexpression effects of Gsc mRNA require the expression of the BMP antagonist Chordin, a downstream target of Gsc. Combined Vent1 and Vent2 MOs strongly dorsalized the embryo. Unexpectedly, simultaneous depletion of all three genes led to a rescue of almost normal development in a variety of embryological assays. Thus, the phenotypic effects of depleting Gsc or Vent1/2 are caused by the transcriptional upregulation of their opposing counterparts. A principal function of Gsc and Vent1/2 homeobox genes might be to mediate a self-adjusting mechanism that restores the basic body plan when deviations from the norm occur, rather than generating individual cell types. The results may shed light on the molecular mechanisms of genetic redundancy.

Repression of Wnt/beta-catenin signaling in the anterior endoderm is essential for liver and pancreas development

The liver and pancreas are specified from the foregut endoderm through an interaction with the adjacent mesoderm. However, the earlier molecular mechanisms that establish the foregut precursors are largely unknown. In this study, we have identified a molecular pathway linking gastrula-stage endoderm patterning to organ specification. We show that in gastrula and early-somite stage Xenopus embryos, Wnt/beta-catenin activity must be repressed in the anterior endoderm to maintain foregut identity and to allow liver and pancreas development. By contrast, high beta-catenin activity in the posterior endoderm inhibits foregut fate while promoting intestinal development. Experimentally repressing beta-catenin activity in the posterior endoderm was sufficient to induce ectopic organ buds that express early liver and pancreas markers. beta-catenin acts in part by inhibiting expression of the homeobox gene hhex, which is one of the earliest foregut markers and is essential for liver and pancreas development. Promoter analysis indicates that beta-catenin represses hhex transcription indirectly via the homeodomain repressor Vent2. Later in development, beta-catenin activity has the opposite effect and enhances liver development. These results illustrate that turning Wnt signaling off and on in the correct temporal sequence is essential for organ formation, a finding that might directly impact efforts to differentiate liver and pancreas tissue from stem cells.

Germ layer induction in ESC--following the vertebrate roadmap

Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.

A mechanism for the sharp transition of morphogen gradient interpretation in Xenopus

Background

One way in which positional information is established during embryonic development is through the graded distribution of diffusible morphogens. Unfortunately, little is known about how cells interpret different concentrations of morphogen to activate different genes or how thresholds are generated in a morphogen gradient.

Results

Here we show that the concentration-dependent induction of the T-box transcription factor Brachyury (Xbra) and the homeobox-containing gene Goosecoid (Gsc) by activin in Xenopus can be explained by the dynamics of a simple network consisting of three elements with a mutual negative feedback motif that can function to convert a graded signal (activin) into a binary output (Xbra on and Gsc off, or vice versa). Importantly, such a system can display sharp thresholds. Consistent with the predictions of our model, Xenopus ectodermal cells display a binary response at the single cell level after treatment with activin.

Conclusion

This kind of simple network with mutual negative feedback might provide a general mechanism for selective gene activation in response to different levels of a single external signal. It provides a mechanism by which a sharp boundary might be created between domains of different cell types in response to a morphogen gradient.

Transcriptional regulation of mesendoderm formation in Xenopus

Mesoderm and endoderm formation in Xenopus involves the coordinated efforts of maternally and zygotically expressed transcription factors together with growth factor signalling, including members of the TGFbeta and wnt families. In this review we discuss our current state of knowledge of these pathways, and describe in more detail some of the transcription factor-DNA interactions that are involved in mesendoderm formation.

Tcf- and Vent-binding sites regulate neural-specific geminin expression in the gastrula embryo

Vertebrate neural development has been extensively investigated. However, it is unknown for any vertebrate gene how the onset of neural-specific expression in early gastrula embryos is transcriptionally regulated. geminin expression is among the earliest markers of dorsal, prospective neurectoderm at early gastrulation in Xenopus laevis. Here, we identified two 5' sequence domains that are necessary and sufficient to drive neural-specific expression during gastrulation in transgenic Xenopus embryos. Each domain contained putative binding sites for the transcription factor Tcf, which can mediate Wnt signaling and for Vent homeodomain proteins, transcriptional repressors that mediate BMP signaling. Results from embryos transgenic for constructs with mutated Tcf or Vent sites demonstrated that signaling through the Tcf sites was required for dorsal-specific expression at early gastrulation, while signaling through the Vent sites restricted geminin expression to the prospective neurectoderm at mid-gastrulation. Consistent with these results, geminin 5' regulatory sequences and endogenous Xgem responded positively to Wnt signaling and negatively to BMP signaling. The two 5' sequence domains were also conserved among geminin orthologs. Together, these results demonstrate that signaling through Tcf and Vent binding sites regulates transcription of geminin in prospective neurectoderm during gastrulation.

An Oct-1 binding site mediates activation of the gata2 promoter by BMP signaling

The gata2 gene encodes a transcription factor implicated in regulating early patterning of ectoderm and mesoderm, and later in numerous cell-specific gene expression programs. Activation of the gata2 gene during embryogenesis is dependent on the bone morphogenetic protein (BMP) signaling pathway, but the mechanism for how signaling controls gene activity has not been defined. We developed an assay in Xenopus embryos to analyze regulatory sequences of the zebrafish gata2 promoter that are necessary to mediate the response to BMP signaling during embryogenesis. We show that activation is Smad dependent, since it is blocked by expression of the inhibitory Smad6. Deletion analysis identified an octamer binding site that is necessary for BMP-mediated induction, and that interacts with the POU homeodomain protein Oct-1. However, this element is not sufficient to transfer a BMP response to a heterologous promoter, requiring an additional more proximal cooperating element. Based on recent studies with other BMP-dependent promoters (Drosophila vestigial and Xenopus Xvent-2), our studies of the gata2 gene suggest that POU-domain proteins comprise a common component of the BMP signaling pathway, cooperating with Smad proteins and other transcriptional activators.

Xenopus as a model system to study transcriptional regulatory networks

Development is controlled by a complex series of events requiring sequential gene activation. Understanding the logic of gene networks during development is necessary for a complete understanding of how genes contribute to phenotype. Pioneering work initiated in the sea urchin and Drosophila has demonstrated that reasonable transcriptional regulatory network diagrams representing early development in multicellular animals can be generated through use of appropriate genomic, genetic, and biochemical tools. Establishment of similar regulatory network diagrams for vertebrate development is a necessary step. The amphibian Xenopus has long been used as a model for vertebrate early development and has contributed greatly to the elucidation of gene regulation. Because the best and most extensively studied transcriptional regulatory network in Xenopus is that underlying the formation and function of Spemann's organizer, we describe the current status of our understanding of this gene regulatory network and its relationship to mesodermal patterning. Seventy-four transcription factors currently known to be expressed in the mesoendoderm of Xenopus gastrula were characterized according to their modes of action, DNA binding consensus sequences, and target genes. Among them, nineteen transcription factors were characterized sufficiently in detail, allowing us to generate a gene regulatory network diagram. Additionally, we discuss recent amphibian work using a combined DNA microarray and bioinformatics approach that promises to accelerate regulatory network studies.

Functional Specificity of the Xenopus T-Domain Protein Brachyury Is Conferred by Its Ability to Interact with Smad1

Members of the T-box gene family play important and diverse roles in development and disease. Here, we study the functional specificities of the Xenopus T-domain proteins Xbra and VegT, which differ in their abilities to induce gene expression in prospective ectodermal tissue. In particular, VegT induces strong expression of goosecoid whereas Xbra cannot. Our results indicate that Xbra is unable to induce goosecoid because it directly activates expression of Xom, a repressor of goosecoid that acts downstream of BMP signaling. We show that the inability of Xbra to induce goosecoid is imposed by an N-terminal domain that interacts with the C-terminal MH2 domain of Smad1, a component of the BMP signal transduction pathway. Interference with this interaction causes ectopic activation of goosecoid and anteriorization of the embryo. These findings suggest a mechanism by which individual T-domain proteins may interact with different partners to elicit a specific response.

Patterning the forebrain: FoxA4a/Pintallavis and Xvent2 determine the posterior limit of Xanf1 expression in the neural plate

During early development of the nervous system in vertebrates, expression of the homeobox gene Anf/Hesx1/Rpx is restricted to the anterior neural plate subdomain corresponding to the presumptive forebrain. This expression is essential for normal forebrain development and ectopic expression of Xenopus Anf, Xanf1 (also known as Xanf-1), results in severe forebrain abnormalities. By use of transgenic embryos and a novel bi-colour reporter technique, we have identified a cis-regulatory element responsible for transcriptional repression of Xanf1 that defines its posterior expression limit within the neural plate. Using this element as the target in a yeast one-hybrid system, we identified two transcription factors, FoxA4a/Pintallavis and Xvent2 (also known as Xvent-2), which are normally expressed posterior to Xanf1. Overexpression of normal and dominant-negative versions of these factors, as well as inhibition of their mRNA translation by antisense morpholinos, show that they actually function as transcriptional repressors of Xanf1 just behind its posterior expression limit. The extremely high similarity of the identified Anf cis-regulatory sequences in Xenopus, chick and human, indicates that the mechanism restricting posterior expression of Anf in Xenopus is shared among vertebrates. Our findings support Nieuwkoop's activation-transformation model for neural patterning, according to which the entire neurectoderm is initially specified towards an anterior fate, which is later suppressed posteriorly as part of the trunk formation process.

Repression of the vertebrate organizer by Wnt8 is mediated by Vent and Vox

Dorsoventral (DV) patterning of vertebrate embryos requires the concerted action of the Bone Morphogenetic Protein (BMP) and Wnt signaling pathways. In contrast to our understanding of the role of BMP in establishing ventral fates, our understanding of the role of Wnts in ventralizing embryos is less complete. Wnt8 is required for ventral patterning in both Xenopus and zebrafish; however, its mechanism of action remains unclear. We have used the zebrafish to address the requirement for Wnt8 in restricting the size of the dorsal organizer. Epistasis experiments suggest that Wnt8 achieves this restriction by regulating the early expression of the transcriptional repressors Vent and Vox. Our data show that vent and vox are direct transcriptional targets of Wnt8/beta-catenin. Additionally, we show that Wnt8 and Bmp2b co-regulate vent and vox in a dynamic fashion. Thus, whereas both Wnt8 and zygotic BMP are ventralizing agents that regulate common target genes, their temporally different modes of action are necessary to pattern the embryo harmoniously along its DV axis.

BMP4: Its Role in Development of the Hematopoietic System and Potential as a Hematopoietic Growth Factor

Blood formation occurs throughout the life of an individual in a process driven by hematopoietic stem cells (HSCs). The ability of bone marrow (BM) and cord blood (CB) HSC to undergo self-renewal and develop into multiple blood lineages has made these cells an important clinical resource. Transplantation with BM- and CB-derived HSCs is now used extensively for treatment of hematological disorders, malignancies, and immunodeficiencies. An understanding of the embryonic origin of HSC and the factors regulating their generation and expansion in vivo will provide important information for the manipulation of these cells ex vivo. This is critical for the further development of CB transplantation, the potential of which is limited by small numbers of HSC in the donor population. Although the origins of HSCs have become clearer and progress has been made in identifying genes that are critical for the formation and maintenance of HSCs, less is known about the signals that commit specific populations of mesodermal precursors to hematopoietic cell fate. Critical signals acting on these precursor cells are likely to be derived from visceral endoderm in yolk sac and from underlying stroma in the aorta-gonad-mesonephros region. Here we summarize briefly the origin of yolk sac and embryonic HSCs before detailing evidence that bone morphogenic protein-4 (BMP4) has a crucial role in Xenopus and mammalian HSC development. We discuss evidence that BMP4 acts as a hematopoietic growth factor and review its potential to modulate HSC in ex vivo expansion cultures from cord blood.

Active repression of organizer genes by C-terminal domain of PV.1

PV.1, a homeotic protein, ventralizes dorsal mesoderm and inhibits neuralization by mediating BMP-4 signaling in Xenopus embryo. In our previous report antimorphic PV.1 causes a secondary axis by inducing the ectopic organizer. We analyzed the structure of this transcription factor through domain level assessment. In a phenotype-inducing test, half of the N-terminus at the N-terminal side was unessential for inducing ventralization of embryos. We examined the transacting activity of several regions of PV.1 utilizing GAL4 hybrid system. The C-terminal region/GAL4DBD (DNA binding domain) exhibited strong repressive activity on a reporter gene (operator/promoter/reporter; Gal4-TK-luc) as much as the whole polypeptide/GAL4DBD, whereas the N-terminal region/GAL4DBD showed only modest repression. The results suggest that PV.1 functions as a transcriptional repressor and this repressive activity is localized mostly to the C-terminal region. Additional characterizations of N- and C-terminus with respect to the effects on the expression of other genes are described.

bozozok directly represses bmp2b transcription and mediates the earliest dorsoventral asymmetry of bmp2b expression in zebrafish

Formation of the gastrula organizer requires suppression of ventralizing signals and, in fish and frog, the need to counteract the effect of ubiquitously present maternal factors that activate the expression of Bmps. How the balance between dorsalizing and ventralizing factors is shifted towards organizer establishment at late blastula stages is not well understood. Mutations in zebrafish bozozok (boz) cause severe defects in axial mesoderm and anterior neurectoderm and affect organizer formation. The boz gene encodes the homeodomain protein Bozozok/Dharma and its expression in the region of the organizer is activated through beta-catenin signaling. Here, we investigate the molecular mechanism by which boz contributes to the establishment of the organizer. We demonstrate that the homeodomain protein Boz acts as a transcriptional repressor in zebrafish: overexpression of an En-Boz fusion protein can rescue the boz phenotype, whereas a VP16-Boz fusion protein acts as an antimorph. Expression analysis of bmp2b indicates that Boz negatively regulates bmp2b in the prospective organizer. We demonstrate that this Boz activity is independent of that of other zygotic genes, because it also occurs when translation of zygotic genes is suppressed by cycloheximide (CHX). We identify two high-affinity binding sites for Boz within the first intron of the bmp2b gene. Deletion of these control elements abolishes Boz-dependent repression of bmp2b in the early blastula. Thus, Boz directly represses bmp2b by binding to control elements in the bmp2b locus. We propose that early transcriptional repression of bmp2b by Boz is one of the first steps toward formation of a stable organizer, whereas the later-acting Bmp antagonists (e.g. Chordin, Noggin) modulate Bmp activity in the gastrula to induce patterning along the dorsoventral axis. Thus, similar to Drosophila Dpp, asymmetry of Bmp expression in zebrafish is initiated at the transcriptional level, and the shape of the gradient and its function as a morphogen are later modulated by post-transcriptional mechanisms.

Direct and indirect regulation of derrière, a Xenopus mesoderm-inducing factor, by VegT

One candidate for an endogenous mesoderm-inducing factor in Xenopus is derrière, a member of the TGFβ family closely related to Vg1. In this paper we first show that derrière is able to exert long-range effects in the early Xenopus embryo, reinforcing the view that it functions as a secreted factor required for proper formation of posterior structures. Analysis of the derrière promoter shows that expression of the gene is controlled through a complex inductive network involving VegT and TGFβ-related molecules and also, perhaps, FGF family members. The work confirms that derrière plays an important role in mesoderm formation and it illustrates the complex regulation to which inducing factors are subject.

Regulated proteolysis of Xom mediates dorsoventral pattern formation during early Xenopus development

To identify a regulatory role for proteolysis during early Xenopus development, we developed a biochemical screen for proteins that are degraded in an embryonic stage-specific manner. We found that Xom, a homeobox transcriptional repressor of dorsal-specific genes, was degraded precipitously during early gastrulation. Xom degradation is regulated by phosphorylation at a GSK3-like consensus site and is most likely mediated by the SCF-beta-TRCP complex. Expression of nondegradable Xom represses transcription of dorsal genes much more effectively than wild-type Xom and results in a more strongly ventralized phenotype. We propose that regulated Xom proteolysis plays an essential role in the establishment of the dorsoventral axis, by converting a gradient in BMP abundance into a sharp dorsoventral pattern.

Cooperative interaction of Xvent-2 and GATA-2 in the activation of the ventral homeobox gene Xvent-1B

The Xvent family of homeobox transcription factors is essential for the establishment of the dorsal-ventral body axis during Xenopus embryogenesis. In contrast to Xvent-2B and other members of the Xvent-2 subfamily, Xvent-1B is not a direct response gene of bone morphogenetic protein-4 signaling. Xvent-1B is activated by Xvent-2, but CHX experiments revealed the requirement of additional factors. In this study, we report on the cooperative effect of Xvent-2 and the zinc finger transcription factor GATA-2 on the promoter of the Xvent-1B gene. We show that GATA-2 is a direct target gene of bone morphogenetic protein-4 and that GATA-2 interacts with Xvent-2 to activate transcription of Xvent-1B. Both transcription factors bind to distinct elements within the Xvent-1B promoter, and GATA-2 physically interacts with the C-terminal domain of Xvent-2. Promoter/reporter studies in Xenopus embryos revealed that full activation of Xvent-1B requires both Xvent-2 and GATA-2. Moreover, the two factors are sufficient to direct transcription of Xvent-1B in the presence of CHX at the ventral side of the embryo. The failure of both factors to activate Xvent-1B on the dorsal side suggests the existence of a dorsal inhibitor. This inhibitor is likely a component of the dorsal Wnt signaling pathway because nuclear translocation of beta-catenin before midblastula transition results in a suppression of Xvent-1B transcription.

A study of mesoderm patterning through the analysis of the regulation of Xmyf-5 expression

Xenopus laevis has been a particularly useful model organism for identifying factors involved in the induction and patterning of the mesoderm, however, much remains to be learned about how these factors interact. The myogenic transcription factor Xmyf-5 is the earliest known gene to be expressed specifically in the dorsolateral mesoderm of the gastrula, a domain that is established by the interaction of dorsal and ventral signals. For this reason, we have begun to investigate how the expression of Xmyf-5 is regulated. We have identified a 7.28 kb Xenopus tropicalis Xmyf-5 (Xtmyf-5) genomic DNA fragment that accurately recapitulates the expression of the endogenous gene. Deletion and mutational analysis has identified HBX2, an essential element, approximately 1.2 kb upstream from the start of transcription, which is necessary for both activation and repression of Xtmyf-5 expression, implying that positional information is integrated at this site. Electrophoretic mobility shift assays demonstrate that HBX2 specifically interacts with gastrula stage embryonic extracts and that in vitro translated Xvent-1 protein binds to one of its functional motifs. Combined with gain- and loss-of-function experiments, the promoter analysis described here suggests that Xvent-1 functions to repress Xmyf-5 expression in the ventral domain of the marginal zone. Furthermore, the identification of HBX2 provides a tool with which to identify other molecules involved in the regulation of Xmyf-5 expression during gastrulation.

Transcription factor AP-2 is an essential and direct regulator of epidermal development in Xenopus

Expression of the Xenopus homolog of the mammalian transcription factor AP-2alpha (XAP-2) is activated throughout the animal hemisphere shortly after the midblastula transition, and becomes restricted to prospective epidermis by the end of gastrulation, under the control of BMP signal modulation. Elevated expression in the future neural crest region begins at this time. Ectopic expression of XAP-2 can restore transcription of epidermal genes in neuralized ectoderm, both in ectodermal explants and in the intact embryo. Likewise, loss of XAP-2 function, accomplished by injection of antisense oligonucleotides or by overexpression of antimorphic XAP-2 derivatives, leads to loss of epidermal and gain of neural gene expression. These treatments also result in gastrulation failure. Thus, AP-2 is a critical regulator of ectodermal determination that is required for normal epidermal development and morphogenesis in the frog embryo.

Targeted gene expression in transgenic Xenopus using the binary Gal4-UAS system

The transgenic technique in Xenopus allows one to misexpress genes in a temporally and spatially controlled manner. However, this system suffers from two experimental limitations. First, the restriction enzyme-mediated integration procedure relies on chromosomal damage, resulting in a percentage of embryos failing to develop normally. Second, every transgenic embryo has unique sites of integration and unique transgene copy number, resulting in variable transgene expression levels and variable phenotypes. For these reasons, we have adapted the Gal4-UAS method for targeted gene expression to Xenopus. This technique relies on the generation of transgenic lines that carry "activator" or "effector" constructs. Activator lines express the yeast transcription factor, Gal4, under the control of a desired promoter, whereas effector lines contain DNA-binding motifs for Gal4-(UAS) linked to the gene of interest. We show that on intercrossing of these lines, the effector gene is transcribed in the temporal and spatial manner of the activator's promoter. Furthermore, we use the Gal4-UAS system to misexpress Xvent-2, a transcriptional target of bone morphogenetic protein 4 (BMP4) signaling during early embryogenesis. Embryos inheriting both the Gal4 activator and Xvent-2 effector transgenes display a consistent microcephalic phenotype. Finally, we exploit this system to characterize the neural and mesodermal defects obtained from early misexpression of Xvent-2. These results emphasize the potential of this system for the controlled analyses of gene function in Xenopus.

Autoregulation of Xvent-2B; direct interaction and functional cooperation of Xvent-2 and Smad1

Members of the Xvent-2 homeodomain transcription factor family are immediate response genes of BMP-4 signaling. The bone morphogenetic protein response element (BRE) of Xvent-2B was previously identified and characterized with respect to Smad1 and Smad4 binding sites. In this study, we further report on the transcriptional regulation of Xvent-2B. We provide evidence that Xvent-2B (Xvent-2) maintains its own expression through autoregulation. This activity was demonstrated for the endogenous gene by reverse transcriptase-PCR analysis and was found to be insensitive to cycloheximide. Localized by DNase I footprinting were several Xvent-2 binding sites within the proximal upstream region including the BRE. In the early Xenopus embryo, the BRE was shown to be sufficient to drive expression of a green fluorescent protein reporter in a similar pattern compared with the endogenous gene. Furthermore, Xvent-2B was able to activate the BRE in luciferase reporter assays, and in co-injection experiments Xvent-2B and Smad1 were found to synergistically activate the BRE. Moreover, glutathione S-transferase pull-down experiments demonstrated that Xvent-2B directly and specifically interacts with Smad1. This association was mediated by the MH1 domain of Smad1 and required the C-terminal domain of Xvent-2. The failure of an Xvent-2 mutant lacking the C terminus to stimulate the BRE underlines the significance of the C-terminal domain in the described autoregulatory loop.