Regulation of dorsal gene expression in Xenopus by the ventralizing homeodomain gene Vox

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.

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 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.

The transcription factor Vox represses endoderm development by interacting with Casanova and Pou2

Endoderm and mesoderm are both formed upon activation of Nodal signaling but how endoderm differentiates from mesoderm is still poorly explored. The sox-related gene casanova (sox32) acts downstream of the Nodal signal, is essential for endoderm development and requires the co-factor Pou2 (Pou5f1, Oct3, Oct4) in this process. Conversely, BMP signals have been shown to inhibit endoderm development by an as yet unexplained mechanism. In a search for Casanova regulators in zebrafish, we identified two of its binding partners as the transcription factors Pou2 and Vox, a member of the Vent group of proteins also involved in the patterning of the gastrula. In overexpression studies we show that vox and/or Vent group genes inhibit the capacity of Casanova to induce endoderm, even in the presence of its co-factor Pou2, and that Vox acts as a repressor in this process. We further show that vox, but not other members of the Vent group, is essential for defining the proper endodermal domain size at gastrulation. In this process, vox acts downstream of BMPs. Cell fate analysis further shows that Vox plays a key role downstream of BMP signals in regulating the capacity of Nodal to induce endoderm versus mesoderm by modulating the activity of the Casanova/Pou2 regulatory system.

Wnt signaling in vertebrate axis specification

The Wnt pathway is a major embryonic signaling pathway that controls cell proliferation, cell fate, and body-axis determination in vertebrate embryos. Soon after egg fertilization, Wnt pathway components play a role in microtubule-dependent dorsoventral axis specification. Later in embryogenesis, another conserved function of the pathway is to specify the anteroposterior axis. The dual role of Wnt signaling in Xenopus and zebrafish embryos is regulated at different developmental stages by distinct sets of Wnt target genes. This review highlights recent progress in the discrimination of different signaling branches and the identification of specific pathway targets during vertebrate axial development.

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.

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.

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.

Zebrafish Naked1 and Naked2 antagonize both canonical and non-canonical Wnt signaling

Wnt signaling controls a wide range of developmental processes and its aberrant regulation can lead to disease. To better understand the regulation of this pathway, we identified zebrafish homologues of Naked Cuticle (Nkd), Nkd1 and Nkd2, which have previously been shown to inhibit canonical Wnt/beta-catenin signaling. Zebrafish nkd1 expression increases substantially after the mid-blastula transition in a pattern mirroring that of activated canonical Wnt/beta-catenin signaling, being expressed in both the ventrolateral blastoderm margin and also in the axial mesendoderm. In contrast, zebrafish nkd2 is maternally and ubiquitously expressed. Overexpression of Nkd1 or Nkd2a suppressed canonical Wnt/beta-catenin signaling at multiple stages of early zebrafish development and also exacerbated the cyclopia and axial mesendoderm convergence and extension (C&E) defect in the non-canonical Wnt/PCP mutant silberblick (slb/wnt11). Thus, Nkds are sufficient to antagonize both canonical and non-canonical Wnt signaling. Reducing Nkd function using antisense morpholino oligonucleotides resulted in increased expression of canonical Wnt/beta-catenin target genes. Finally, reducing Nkd1 function in slb mutants suppressed the axial mesendoderm C&E defect. These data indicate that zebrafish Nkd1 and Nkd2 function to limit both canonical and non-canonical Wnt signaling.

Maternal control of vertebrate dorsoventral axis formation and epiboly by the POU domain protein Spg/Pou2/Oct4

Dorsoventral (DV) axis formation of the vertebrate embryo is controlled by the maternal genome and is subsequently refined zygotically. In the zygote, repression of ventralizing Bmp activity on the dorsal side through chordin and noggin is crucial for establishment of a dorsally located organizer. This interplay generates a zygotic Bmp activity gradient that defines distinct positional values along the DV axis. The maternal processes that control expression of the zygotic genes implicated in DV patterning are largely unknown. spiel-ohne-grenzen (spg/pou2) is a maternally and zygotically expressed zebrafish gene that encodes the POU domain transcription factor Pou2, an ortholog of mammalian Oct4/Pou5f1. We show that embryos that are genetically depleted of both maternal and zygotic pou2 function (MZspg) exhibit extreme DV patterning defects and, independently, a blastoderm-specific arrest of epiboly. Dorsal tissues expand to the ventral side at the expense of ventrolateral tissue in MZspg embryos. Dorsally expressed Bmp-antagonists, such as Chd and Nog1, and Gsc are ectopically activated at ventral levels in MZspg. Lack of ventral specification is apparent very early, suggesting that maternal processes are affected in MZspg. Indeed, maternal pou2 function is necessary to initiate zygotic expression of ventrally expressed genes such as bmp2b and bmp4, and for proper activation of bmp7, vox, vent and eve1. A constitutively active Alk8-TGFbeta-receptor can ectopically induce bmp2b and bmp4 and rescues the dorsalization of MZspg. This indicates that pou2 acts upstream of Alk8, a maternally provided receptor implicated in the activation of zygotic bmp2b and bmp4 transcription. Consistent with this possibility, Bmp gene misexpression can rescue MZspg embryos, indicating that TGFbeta-mediated signal transduction itself is intact in absence of Pou2. Inhibition of Fgf signaling, another pathway with early dorsalizing activity, can also restore and even ventralize MZspg embryos. The requirement for pou2 to initiate bmp2b expression can therefore be bypassed by releasing the repressive function of Fgf signaling upon bmp2b transcription. In transplantation experiments, we find that dorsalized cells from prospective ventrolateral regions of MZspg embryos are non cell-autonomously respecified to a ventral fate within wild-type host embryos. Analysis of pou2 mRNA injected MZspg embryos shows that pou2 is required on the ventral side of cleavage stage embryos. Based on the maternal requirement for pou2 in ventral specification, we propose that ventral specification employs an active, pou2-dependent maternal induction step, rather than a default ventralizing program.

Genomic analysis of Xenopus organizer function

Background

Studies of the Xenopus organizer have laid the foundation for our understanding of the conserved signaling pathways that pattern vertebrate embryos during gastrulation. The two primary activities of the organizer, BMP and Wnt inhibition, can regulate a spectrum of genes that pattern essentially all aspects of the embryo during gastrulation. As our knowledge of organizer signaling grows, it is imperative that we begin knitting together our gene-level knowledge into genome-level signaling models. The goal of this paper was to identify complete lists of genes regulated by different aspects of organizer signaling, thereby providing a deeper understanding of the genomic mechanisms that underlie these complex and fundamental signaling events.

Results

To this end, we ectopically overexpress Noggin and Dkk-1, inhibitors of the BMP and Wnt pathways, respectively, within ventral tissues. After isolating embryonic ventral halves at early and late gastrulation, we analyze the transcriptional response to these molecules within the generated ectopic organizers using oligonucleotide microarrays. An efficient statistical analysis scheme, combined with a new Gene Ontology biological process annotation of the Xenopus genome, allows reliable and faithful clustering of molecules based upon their roles during gastrulation. From this data, we identify new organizer-related expression patterns for 19 genes. Moreover, our data sub-divides organizer genes into separate head and trunk organizing groups, which each show distinct responses to Noggin and Dkk-1 activity during gastrulation.

Conclusion

Our data provides a genomic view of the cohorts of genes that respond to Noggin and Dkk-1 activity, allowing us to separate the role of each in organizer function. These patterns demonstrate a model where BMP inhibition plays a largely inductive role during early developmental stages, thereby initiating the suites of genes needed to pattern dorsal tissues. Meanwhile, Wnt inhibition acts later during gastrulation, and is essential for maintenance of organizer gene expression throughout gastrulation, a role which may depend on its ability to block the expression of a host of ventral, posterior, and lateral fate-specifying factors.

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.

Temporal analysis of the early BMP functions identifies distinct anti-organizer and mesoderm patterning phases

BMP signaling performs multiple important roles during early embryogenesis. Signaling through the BMP pathway is mediated by different BMP ligands expressed in partially overlapping temporal and spatial patterns. Assignment of different BMP-dependent activities to the individual ligands has relied on the patterns of expression of the various BMP genes. Temporal analysis of BMP signaling prior to and during gastrulation was performed using glucocorticoid-controlled Smad proteins. Overexpression of the BMP-specific Smad1 and Smad5 revealed that suppression of Spemann's organizer formation in Xenopus embryos can only take place by activating the BMP pathway prior to the onset of gastrulation. Blocking BMP signaling with the inhibitory Smad, Smad6, results in dorsalized embryos or secondary axis induction, only when activated up to early gastrula stages. BMP2 efficiently represses organizer-specific transcription from the midblastula transition onwards while BMP4 is unable to prevent the early activation of organizer-specific genes. Manipulation of the BMP pathway during mid/late gastrula affects mesodermal patterning with no external phenotypic effects. These observations suggest that the malformations resulting from inhibition or promotion of organizer formation, ventralized or dorsalized, respectively, are the result of a very early BMP function, through its antagonism of organizer formation. This function is apparently fulfilled by BMP2 and only at its latest phase by BMP4. Subsequently, BMP functions in the patterning of the mesoderm with no apparent phenotypic effects.

Functional and hierarchical interactions among zebrafish vox/vent homeobox genes

The vertebrate Vox/Vent family of transcription factors plays a crucial role in the establishment of the dorsoventral (DV) axis, by repressing organizer genes such as bozozok/dharma, goosecoid, and chordino. In Danio rerio (zebrafish), members of the vox/vent gene family (vox/vega1, vent/vega2, and ved) are thought to share expression patterns and functional properties. Bringing novel insights in the differential activity of the zebrafish vox/vent genes, we propose a critical role for the ved gene in DV patterning of vertebrate embryos. ved is not only expressed as a maternal gene, but it also appears to function as a repressor of dorsal factors involved in organizer formation. At early- and mid-gastrula stage, ved appears to be finely controlled by antagonist crosstalks in a complex regulatory network, involving gradients of bone morphogenetic protein (BMP) activity, dorsal factors, and vox/vent family members. We show that ved transcripts are ventrally restricted by BMP factors such as bmp2b, bmp7, smad5, and alk8, and by dorsal factors (chd and gsc). Alteration of ved expression in both vox and vent deletion mutants and vox and vent mRNAs-injected embryos, suggests that vox and vent function downstream of BMP signaling to negatively regulate ved expression. This inhibitory role is emphasized by a vox and vent redundant activity, compared with single gene effects.

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.

Analysis of Spemann organizer formation in Xenopus embryos by cDNA macroarrays

The understanding of vertebrate development has greatly benefited from the study of gastrulation in the Xenopus embryo. Over the years, the molecular dissection of the Spemann organizer has proven to be a very fruitful source for gene discovery. Here, we report a comprehensive screen of gene expression in the Xenopus gastrula using cDNA macroarrays. Nylon filters containing more than 72000 cDNAs from a gastrula stage library were hybridized with differential probes from embryos in which organizer induction had been inhibited by reducing Nodal-related or maternal beta-Catenin signaling. Combining the changes in gene expression levels caused by these two major signaling pathways in a single graph identified both known and novel dorsoventral regulated genes. The most highly enriched organizer-specific genes were the secreted molecules chordin and Xnr-3, followed by the transmembrane protein paraxial protocadherin (PAPC). Ventral-specific abundant cDNAs included S10-40-H5, members of the Hyaluronan synthase family, Xvent-2 and XFD2/FoxI1. A differential probe of dorsal and ventral lips identified many more organizer-specific cDNAs than the screens inhibiting Nodal-related and beta-Catenin signaling, suggesting that additional, as yet uncharacterized signaling pathways, contribute to organizer formation. Finally, extension of this approach to the blastula preorganizer signaling center identified the transcription factor pintallavis/FoxA2 as a new preorganizer component.

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.

The competence of marginal zone cells to become Spemann's organizer is controlled by Xcad2

The organizer in vertebrate embryos is responsible for the formation of the primary body axis. In amphibian embryos, the organizer forms in the dorsal marginal zone (prospective dorsal mesoderm) at a location determined by the point of sperm entry. Using inducible versions of axis-inducing proteins, it has been shown that, irrespective of the mode of secondary axis induction, organizer formation in the ventral marginal zone is temporally restricted from the midblastula transition to the onset of gastrulation. Here, we show that the competence of marginal zone cells to respond to organizer-inducing signals is under temporal control, one of the regulators being the homeobox transcription factor Xcad2. Overexpression of Xcad2 restricts the temporal competence for axis induction, whereas partial loss of function expands this competence, supporting our suggestion. We propose that Xcad2 competes with putative axis-inducing signals within the marginal zone to prevent expression of organizer-specific genes. Elimination of endogenous Xcad2 allows for the activation of organizer genes beyond the normal competence window during early/mid-gastrulation. We conclude that Xcad2, through its early expression in the ventrolateral marginal zone, terminates the competence of this embryonic region to respond to organizer-inducing signals by preventing the activation of organizer-specific genes.

Antimorphic PV.1 causes secondary axis by inducing ectopic organizer

Xenopus homeobox gene, PV.1 ventralizes activin-induced dorsal mesoderm and inhibits neuralization of ectoderm in animal cap when overexpressed. Here we generated PV.1/engrailed fusion construct (N-PV1-EnR) to perform loss-of-function study for this transcription factor. N-PV1-EnR showed an extremely antimorphic effect, causing a partial secondary embryonic axis when expressed at ventral marginal zone of blastula. In ventral marginal zone cells, this chimeric protein induced organizer genes and suppressed ventral markers mimicking those effects reported for dominant negative BMP-4 receptor (DNBR). Moreover, N-PV1-EnR rescued the ventralized embryos caused by the ectopic dorsal expression of PV.1 but not by that of Xvent-2. These results suggested that PV.1 functions at downstream of BMP-4 as a ventralizing effector which acts separately from Xvent-2 and the dominant negative effect gained by this specific mutant is applicable for the further studies of BMP-4 downstream pathway.

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.

Siamois functions in the early blastula to induce Spemann's organiser

In Xenopus, the Spemann organiser is defined as a dorsal territory in the early gastrula that initiates development of the embryonic axis. It has been shown that the early zygotic transcription factor Siamois is essential for Spemann's organiser formation. By the onset of gastrulation, the organiser is patterned into a vegetal head organiser, which induces anterior structures upon transplantation, and a more animal trunk organiser, which induces a posterior neuraxis. However, it is unclear when these distinct organiser domains are initially specified. To shed light on this question, we analysed the temporal activity of Siamois, as this factor induces both head and trunk development, when ectopically expressed via mRNA injection. In this study, we expressed Siamois ectopically at different time points and analysed the extent of axial development. Using a hormone-inducible version of Siamois, we found evidence for a tight window of competence during which ventral cells can respond to Siamois by commencing both the head and the trunk genetic programmes. The competence to form Spemann's organiser was lost 2 h before gastrulation, although partial axis formation could still occur following delayed activation of Siamois. We demonstrate that this late response to Siamois involves a new role for this gene, which can indirectly repress ventral gene expression, in the absence of known organiser genes.

The homeobox genesvoxandventare redundant repressors of dorsal fates in zebrafish

Ventralizing transcriptional repressors in the Vox/Vent family have been proposed to be important regulators of dorsoventral patterning in the early embryo. While the zebrafish genes vox (vega1) and vent (vega2) both have ventralizing activity in overexpression assays, loss-of-function studies are needed to determine whether these genes have distinct or redundant functions in dorsoventral patterning and to provide critical tests of the proposed regulatory interactions among vox, vent and other genes that act to establish the dorsoventral axis. We show that vox and vent are redundant repressors of dorsal fates in zebrafish. Mutants that lack vox function have little or no dorsoventral patterning defect, and inactivation of either vox or vent by injection of antisense morpholino oligonucleotides has little or no effect on the embryo. In contrast, embryos that lack both vox and vent function have a dorsalized phenotype. Expression of dorsal mesodermal genes, including chordin, goosecoid and bozozok, is strongly expanded in embryos that lack vox and vent function, indicating that the redundant action of vox and vent is required to restrict dorsal genes to their appropriate territories. Our genetic analysis indicates that the dorsalizing transcription factor Bozozok promotes dorsal fates indirectly, by antagonizing the expression of vox and vent. In turn, vox and vent repress chordin expression, restricting its function as an antagonist of ventral fates to the dorsal side of the embryo. Our results support a model in which BMP signaling induces the expression of ventral genes, while vox and vent act redundantly to prevent the expression of chordin, goosecoid and other dorsal genes in the lateral and ventral mesendoderm.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

Characterization of Amphioxus AmphiVent, an evolutionarily conserved marker for chordate ventral mesoderm

Structure and developmental expression are described for amphioxus AmphiVent, a homolog of vertebrate Vent genes. In amphioxus, AmphiVent-expressing ventral mesoderm arises at midneurula by outgrowth from the paraxial mesoderm, but in vertebrates, Vent-expressing ventral mesoderm originates earlier, at the gastrula stage. In other embryonic tissues (nascent paraxial mesoderm, neural plate, endoderm, and tailbud), AmphiVent and its vertebrate homologs are expressed in similar spatiotemporal domains, indicating conservation of many Vent gene functions during chordate evolution. The ventral mesoderm evidently develops precociously in vertebrates because their relatively large embryos probably require an early and extensive deployment of the mesoderm-derived circulatory system. The vertebrate ventral mesoderm, in spite of its strikingly early advent, still resembles the nascent ventral mesoderm of amphioxus in expressing Vent homologs. This coincidence may indicate that Vent homologs in vertebrates and amphioxus play comparable roles in ventral mesoderm specification.

Turning mesoderm into blood: the formation of hematopoietic stem cells during embryogenesis

The formation of hematopoietic stem cells during development occurs by a multistep process that begins with the induction of ventral mesoderm. This mesoderm is patterned during gastrulation by a bone morphogenetic protein (BMP) signaling pathway that is mediated, at least in part, by members of the Mix and Vent families of homeobox transcription factors. Following gastrulation, a subset of ventral mesoderm is specified to become hematopoietic stem cells. Key determinants of hematopoietic fate include the product of the zebrafish cloche gene and the basic helix-loop-helix transcription factor SCL. Future studies in Xenopus and zebrafish should reveal other critical factors in this developmental pathway.

The establishment of Spemann's organizer and patterning of the vertebrate embryo

Molecular studies have begun to unravel the sequential cell-cell signalling events that establish the dorsal-ventral, or 'back-to-belly', axis of vertebrate animals. In Xenopus and zebrafish, these events start with the movement of membrane vesicles associated with dorsal determinants. This mediates the induction of mesoderm by generating gradients of growth factors. Dorsal mesoderm then becomes a signalling centre, the Spemann's organizer, which secretes several antagonists of growth-factor signalling. Recent studies have led to new models for the regulation of cell-cell signalling during development, which may also apply to the homeostasis of adult tissues.

The ventralizing activity of Radar, a maternally expressed bone morphogenetic protein, reveals complex bone morphogenetic protein interactions controlling dorso-ventral patterning in zebrafish

In Xenopus and zebrafish, BMP2, 4 and 7 have been implicated, after the onset of zygotic expression, in inducing and maintaining ventro-lateral cell fate during early development. We provide evidence here that a maternally expressed bone morphogenetic protein (BMP), Radar, may control early ventral specification in zebrafish. We show that Radar ventralizes zebrafish embryos and induces the early expression of bmp2b and bmp4. The analysis of Radar overexpression in both swirl/bmp2b mutants and embryos expressing truncated BMP receptors shows that Radar-induced ventralization is dependent on functional BMP2/4 pathways, and may initially rely on an Alk6-related signaling pathway. Finally, we show that while radar-injected swirl embryos still exhibit a strongly dorsalized phenotype, the overexpression of Radar into swirl/bmp2b mutant embryos restores ventral marker expression, including bmp4 expression. Our results suggest that a complex regulation of different BMP pathways controls dorso-ventral (DV) patterning from early cleavage stages until somitogenesis.

The homeodomain transcription factor Xvent-2 mediates autocatalytic regulation of BMP-4 expression in Xenopus embryos

Like other genes of the transforming growth factor-beta family, the BMP-4 gene is regulated by an autocatalytic loop. In Xenopus embryos this loop can be ectopically induced by injection of BMP-2 RNA. However, cycloheximide treatment subsequent to BMP-2 overexpression revealed that BMP signaling is not direct but requires additional factor(s). As putative mediator we have identified Xvent-2 which is activated by BMP-2/4 signaling and, in turn, activates BMP-4 transcription. Using promoter/reporter assays we have delineated Xvent-2 responsive elements within the BMP-4 gene. We further demonstrate that Xvent-2 which has recently been characterized as a transcriptional repressor can also act, context dependent, as an activator binding two copies of a 5'-CTAATT-3' motif in the second intron of the BMP-4 gene. Replacement of Xvent-2 target sites within the goosecoid (gsc) promoter by the BMP-4 enhancer converts Xvent-2 caused repression of gsc to strong activation. This switch is obviously due to adjacent nucleotides probably binding a transcriptional co-activator interacting with Xvent-2. A model is presented describing the mechanism of BMP-4 gene activation in Xenopus embryos at the early gastrula stage.

Patterning the early zebrafish by the opposing actions of bozozok and vox/vent

Fish and frog embryos are patterned along the dorsal-ventral axis during the gastrula stage by opposing gradients of Bmps and Bmp inhibitory proteins. Three transcriptional repressors with partially overlapping expression domains have been proposed to be important mediators of Bmp function in Xenopus. We find that two related factors are expressed in the early zebrafish embryo. Although these factors are considerably divergent from the related Xenopus genes, they are expressed in domains similar to those of their Xenopus relatives throughout embryogenesis. Both of the zebrafish genes, which we have named vox and vent, are potent ventralizing factors in both zebrafish and Xenopus embryos. Using mutants in the Bmp pathway, we find that there are Bmp-dependent and Bmp-independent domains of vox expression, whereas vent is mostly dependent upon Bmp signaling. We show that ectopic vox or vent negatively regulates expression of the early dorsal gene bozozok (boz) and that ectopic boz eliminates vox and vent expression. Moreover, the normal exclusion of vox and vent from the organizer region is lost in boz mutant embryos. Our results show that boz and vox/vent are mutually antagonistic and indicate that the early establishment of the size of the organizer domain is dependent on an interplay between these early expressed transcriptional repressors.

The Xvex-1 antimorph reveals the temporal competence for organizer formation and an early role for ventral homeobox genes

The organizer in vertebrate embryos has been shown to play a central role in their development by antagonizing ventralizing signals and promoting dorsal development. The ventral homeobox gene, Xvex-1, is capable of fulfilling some of the functions of BMP-4. By fusion to activation and repression domains, Xvex-1 was shown to function as a repressor of transcription. The activator version of Xvex-1, the antimorph, was made inducible by fusion to the ligand binding domain of the glucocorticoid receptor. The organizer genes, gsc and Otx-2, were identified as direct targets of Xvex-1. The XVEX-1 antimorph can induce the formation of secondary axes. Temporal analysis of secondary axis induction revealed that the competence to induce a secondary organizer ends with the onset of gastrulation. The same temporal competence window was exhibited by an inducible gsc construct. Partial loss of Xvex-1 activity was able to improve the efficiency of secondary axis induction by the dominant negative BMP receptor or Smad6. These observations together with the early widespread expression of Xvex-1 throughout the embryo prior to gastrulation encoding a homeodomain repressor protein, suggest that elements of the ventral signaling pathway play an important role during late blastula in restricting the formation of Spemann's organizer.

Transcriptional regulation in Xenopus: a bright and froggy future

Xenopus has played a key role in defining the general mechanisms that underlie early vertebrate development. Recent studies reveal how the transcriptional regulation of signaling and transcription factors is used to pattern the early dorsal-ventral axis. With the development of new methods for producing transgenic frogs, Xenopus will become a very attractive system for studying transcriptional regulation at all stages of embryogenesis.