Interplay between FGF, one-eyed pinhead, and T-box transcription factors during zebrafish posterior development

The control of transitions along the main body axis

Although vertebrates display a large variety of forms and sizes, the mechanisms controlling the layout of the basic body plan are substantially conserved throughout the clade. Following gastrulation, head, trunk, and tail are sequentially generated through the continuous addition of tissue at the caudal embryonic end. Development of each of these major embryonic regions is regulated by a distinct genetic network. The transitions from head-to-trunk and from trunk-to-tail development thus involve major changes in regulatory mechanisms, requiring proper coordination to guarantee smooth progression of embryonic development. In this review, we will discuss the key cellular and embryological events associated with those transitions giving particular attention to their regulation, aiming to provide a cohesive outlook of this important component of vertebrate development.

Of Necks, Trunks and Tails: Axial Skeletal Diversity among Vertebrates

The axial skeleton of all vertebrates is composed of individual units known as vertebrae. Each vertebra has individual anatomical attributes, yet they can be classified in five different groups, namely cervical, thoracic, lumbar, sacral and caudal, according to shared characteristics and their association with specific body areas. Variations in vertebral number, size, morphological features and their distribution amongst the different regions of the vertebral column are a major source of the anatomical diversity observed among vertebrates. In this review I will discuss the impact of those variations on the anatomy of different vertebrate species and provide insights into the genetic origin of some remarkable morphological traits that often serve to classify phylogenetic branches or individual species, like the long trunks of snakes or the long necks of giraffes.

The maternal coordinate system: Molecular-genetics of embryonic axis formation and patterning in the zebrafish

Axis specification of the zebrafish embryo begins during oogenesis and relies on proper formation of well-defined cytoplasmic domains within the oocyte. Upon fertilization, maternally-regulated cytoplasmic flow and repositioning of dorsal determinants establish the coordinate system that will build the structure and developmental body plan of the embryo. Failure of specific genes that regulate the embryonic coordinate system leads to catastrophic loss of body structures. Here, we review the genetic principles of axis formation and discuss how maternal factors orchestrate axis patterning during zebrafish early embryogenesis. We focus on the molecular identity and functional contribution of genes controlling critical aspects of oogenesis, egg activation, blastula, and gastrula stages. We examine how polarized cytoplasmic domains form in the oocyte, which set off downstream events such as animal-vegetal polarity and germ line development. After gametes interact and form the zygote, cytoplasmic segregation drives the animal-directed reorganization of maternal determinants through calcium- and cell cycle-dependent signals. We also summarize how maternal genes control dorsoventral, anterior-posterior, mesendodermal, and left-right cell fate specification and how signaling pathways pattern these axes and tissues during early development to instruct the three-dimensional body plan. Advances in reverse genetics and phenotyping approaches in the zebrafish model are revealing positional patterning signatures at the single-cell level, thus enhancing our understanding of genotype-phenotype interactions in axis formation. Our emphasis is on the genetic interrogation of novel and specific maternal regulatory mechanisms of axis specification in the zebrafish.

Fgf-driven Tbx protein activities directly induce myf5 and myod to initiate zebrafish myogenesis

Skeletal muscle derives from dorsal mesoderm formed during vertebrate gastrulation. Fibroblast growth factor (Fgf) signalling cooperates with Tbx transcription factors to promote dorsal mesoderm formation, but their role in myogenesis has been unclear. Using zebrafish, we show that dorsally derived Fgf signals act through Tbx16 and Tbxta to induce slow and fast trunk muscle precursors at distinct dorsoventral positions. Tbx16 binds to and directly activates the myf5 and myod genes, which are required for commitment to myogenesis. Tbx16 activity depends on Fgf signalling from the organiser. In contrast, Tbxta is not required for myf5 expression, but binds a specific site upstream of myod that is not bound by Tbx16 and drives (dependent on Fgf signals) myod expression in adaxial slow precursors, thereby initiating trunk myogenesis. After gastrulation, when similar muscle cell populations in the post-anal tail are generated from tailbud, declining Fgf signalling is less effective at initiating adaxial myogenesis, which is instead initiated by Hedgehog signalling from the notochord. Our findings suggest a hypothesis for ancestral vertebrate trunk myogenic patterning and how it was co-opted during tail evolution to generate similar muscle by new mechanisms.

This article has an associated ‘The people behind the papers’ interview.

Highlighted Article: Tbx16 and Tbxta activate myf5 and myod directly during the earliest myogenesis in zebrafish, and Fgf signalling acts through Tbx16 to drive myogenesis in trunk but not tail.

Small-Molecule Screening in Zebrafish Embryos Identifies Signaling Pathways Regulating Early Thyroid Development

Background: Defects in embryonic development of the thyroid gland are a major cause for congenital hypothyroidism in human newborns, but the underlying molecular mechanisms are still poorly understood. Organ development relies on a tightly regulated interplay between extrinsic signaling cues and cell intrinsic factors. At present, however, there is limited knowledge about the specific extrinsic signaling cues that regulate foregut endoderm patterning, thyroid cell specification, and subsequent morphogenetic processes in thyroid development. Methods: To begin to address this problem in a systematic way, we used zebrafish embryos to perform a series of in vivo phenotype-driven chemical genetic screens to identify signaling cues regulating early thyroid development. For this purpose, we treated zebrafish embryos during different developmental periods with a panel of small-molecule compounds known to manipulate the activity of major signaling pathways and scored phenotypic deviations in thyroid, endoderm, and cardiovascular development using whole-mount in situ hybridization and transgenic fluorescent reporter models. Results: Systematic assessment of drugged embryos recovered a range of thyroid phenotypes including expansion, reduction or lack of the early thyroid anlage, defective thyroid budding, as well as hypoplastic, enlarged, or overtly disorganized presentation of the thyroid primordium after budding. Our pharmacological screening identified bone morphogenetic protein and fibroblast growth factor signaling as key factors for thyroid specification and early thyroid organogenesis, highlighted the importance of low Wnt activities during early development for thyroid specification, and implicated drug-induced cardiac and vascular anomalies as likely indirect mechanisms causing various forms of thyroid dysgenesis. Conclusions: By integrating the outcome of our screening efforts with previously available information from other model organisms including Xenopus, chicken, and mouse, we conclude that signaling cues regulating thyroid development appear broadly conserved across vertebrates. We therefore expect that observations made in zebrafish can inform mammalian models of thyroid organogenesis to further our understanding of the molecular mechanisms of congenital thyroid diseases.

Notch and Fgf signaling during electrosensory versus mechanosensory lateral line organ development in a non-teleost ray-finned fish

The lateral line system is a useful model for studying the embryonic and evolutionary diversification of different organs and cell types. In jawed vertebrates, this ancestrally comprises lines of mechanosensory neuromasts over the head and trunk, flanked on the head by fields of electrosensory ampullary organs, all innervated by lateral line neurons in cranial lateral line ganglia. Both types of sense organs, and their afferent neurons, develop from cranial lateral line placodes. Current research primarily focuses on the posterior lateral line primordium in zebrafish, which migrates as a cell collective along the trunk; epithelial rosettes form in the trailing zone and are deposited as a line of neuromasts, within which hair cells and supporting cells differentiate. However, in at least some other teleosts (e.g. catfishes) and all non-teleosts, lines of cranial neuromasts are formed by placodes that elongate to form a sensory ridge, which subsequently fragments, with neuromasts differentiating in a line along the crest of the ridge. Furthermore, in many non-teleost species, electrosensory ampullary organs develop from the flanks of the sensory ridge. It is unknown to what extent the molecular mechanisms underlying neuromast formation from the zebrafish migrating posterior lateral line primordium are conserved with the as-yet unexplored molecular mechanisms underlying neuromast and ampullary organ formation from elongating lateral line placodes. Here, we report experiments in an electroreceptive non-teleost ray-finned fish, the Mississippi paddlefish Polyodon spathula, that suggest a conserved role for Notch signaling in regulating lateral line organ receptor cell number, but potentially divergent roles for the fibroblast growth factor signaling pathway, both between neuromasts and ampullary organs, and between paddlefish and zebrafish.

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Notch and Fgf pathway genes are expressed during paddlefish lateral line development.

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Fgf ligand genes are differentially expressed in neuromasts and ampullary organs.

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DAPT treatment results in irregular organ spacing and supernumerary receptor cells.

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SU5402 treatment yields fewer neuromasts, but ampullary organs form precociously.

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SU5402 treatment also results in supernumerary receptor cells.

γ2 and γ1AP-1 complexes: Different essential functions and regulatory mechanisms in clathrin-dependent protein sorting

γ2 adaptin is homologous to γ1, but is only expressed in vertebrates while γ1 is found in all eukaryotes. We know little about γ2 functions and their relation to γ1. γ1 is an adaptin of the heterotetrameric AP-1 complexes, which sort proteins in and do form clathrin-coated transport vesicles and they also regulate maturation of early endosomes. γ1 knockout mice develop only to blastocysts and thus γ2 does not compensate γ1-deficiency in development. γ2 has not been classified as a clathrin-coated vesicle adaptor protein in proteome analyses and functions for monomeric γ2 in endosomal protein sorting have been proposed, but adaptin interaction studies suggested formation of heterotetrameric AP-1/γ2 complexes. We detected γ2 at the trans-Golgi network, on peripheral vesicles and identified γ2 clathrin-coated vesicles in mice. Ubiquitous σ1A and tissue-specific σ1B adaptins bind γ2 and γ1. σ1B knockout in mice does not effect γ1/σ1A AP-1 levels, but γ2/σ1A AP-1 levels are increased in brain and adipocytes. Also γ2 is essential in development. In zebrafish AP-1/γ2 and AP-1/γ1 fulfill different, essential functions in brain and the vascular system.

Tbx16 and Msgn1 are required to establish directional cell migration of zebrafish mesodermal progenitors

The epithelial to mesenchymal transition (EMT) is an essential process that occurs repeatedly during embryogenesis whereby stably adherent cells convert to an actively migrating state. While much is known about the factors and events that initiate the EMT, the steps that cells undergo to become directionally migratory are far less well understood. Zebrafish embryos lacking the transcription factors Tbx16/Spadetail and Mesogenin1 (Msgn1) are a valuable system for investigating the EMT. Mesodermal cells in these embryos are unable to perform the EMT necessary to leave the most posterior end of the body (the tailbud) and join the pre-somitic mesoderm, a process that is conserved in all vertebrates. It has previously been very difficult to study this EMT in vertebrates because of the multiple cell types in the tailbud and the morphogenetic changes the whole embryo undergoes. Here, we describe a novel tissue explant system for imaging the mesodermal cell EMT in vivo that allows us to investigate the requirements for cells to acquire migratory properties during the EMT with high spatio-temporal resolution. This method revealed that, despite the inability of tbx16;msgn1-deficient cells to leave the tailbud, actin-based protrusions form surprisingly normally in these cells and they become highly motile. However, tbx16;msgn1-deficient cells have specific cell-autonomous defects in the persistence and anterior direction of migration because the lamellipodia they form are not productive in driving anteriorward migration. Additionally, we show that mesoderm morphogenesis and differentiation are separable and that there is a migratory cue that directs mesodermal cell migration that is independent of Tbx16 and Msgn1. This work defines changes that cells undergo as they complete the EMT and provides new insight into the mechanisms required in vivo for cells to become mesenchymal.

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.

Nitroreductase-activatable morpholino oligonucleotides for in vivo gene silencing

Phosphorodiamidate morpholino oligonucleotides are widely used to interrogate gene function in whole organisms, and light-activatable derivatives can reveal spatial and temporal differences in gene activity. We describe here a new class of caged morpholino oligonucleotides that can be activated by the bacterial nitroreductase NfsB. We characterize the activation kinetics of these reagents in vitro and demonstrate their efficacy in zebrafish embryos that express NfsB either ubiquitously or in defined cell populations. In combination with transgenic organisms, such enzyme-actuated antisense tools will enable gene silencing in specific cell types, including tissues that are not amenable to optical targeting.

Zebrafish Tbx16 regulates intermediate mesoderm cell fate by attenuating Fgf activity

Progenitors of the zebrafish pronephros, red blood and trunk endothelium all originate from the ventral mesoderm and often share lineage with one another, suggesting that their initial patterning is linked. Previous studies have shown that spadetail (spt) mutant embryos, defective in tbx16 gene function, fail to produce red blood cells, but retain the normal number of endothelial and pronephric cells. We report here that spt mutants are deficient in all the types of early blood, have fewer endothelial cells as well as far more pronephric cells compared to wildtype. In vivo cell tracing experiments reveal that blood and endothelium originate in spt mutants almost exclusive from the dorsal mesoderm whereas, pronephros and tail originate from both dorsal and ventral mesoderm. Together these findings suggest possible defects in posterior patterning. In accord with this, gene expression analysis shows that mesodermal derivatives within the trunk and tail of spt mutants have acquired more posterior identity. Secreted signaling molecules belonging to the Fgf, Wnt and Bmp families have been implicated as patterning factors of the posterior mesoderm. Further investigation demonstrates that Fgf and Wnt signaling are elevated throughout the nonaxial region of the spt gastrula. By manipulating Fgf signaling we show that Fgfs both promote pronephric fate and repress blood and endothelial fate. We conclude that Tbx16 plays an important role in regulating the balance of intermediate mesoderm fates by attenuating Fgf activity.

Mutations in DSTYK and dominant urinary tract malformations

BACKGROUND

Congenital abnormalities of the kidney and the urinary tract are the most common cause of pediatric kidney failure. These disorders are highly heterogeneous, and the etiologic factors are poorly understood.

METHODS

We performed genomewide linkage analysis and whole-exome sequencing in a family with an autosomal dominant form of congenital abnormalities of the kidney or urinary tract (seven affected family members). We also performed a sequence analysis in 311 unrelated patients, as well as histologic and functional studies.

RESULTS

Linkage analysis identified five regions of the genome that were shared among all affected family members. Exome sequencing identified a single, rare, deleterious variant within these linkage intervals, a heterozygous splice-site mutation in the dual serine-threonine and tyrosine protein kinase gene (DSTYK). This variant, which resulted in aberrant splicing of messenger RNA, was present in all affected family members. Additional, independent DSTYK mutations, including nonsense and splice-site mutations, were detected in 7 of 311 unrelated patients. DSTYK is highly expressed in the maturing epithelia of all major organs, localizing to cell membranes. Knockdown in zebrafish resulted in developmental defects in multiple organs, which suggested loss of fibroblast growth factor (FGF) signaling. Consistent with this finding is the observation that DSTYK colocalizes with FGF receptors in the ureteric bud and metanephric mesenchyme. DSTYK knockdown in human embryonic kidney cells inhibited FGF-stimulated phosphorylation of extracellular-signal-regulated kinase (ERK), the principal signal downstream of receptor tyrosine kinases.

CONCLUSIONS

We detected independent DSTYK mutations in 2.3% of patients with congenital abnormalities of the kidney or urinary tract, a finding that suggests that DSTYK is a major determinant of human urinary tract development, downstream of FGF signaling. (Funded by the National Institutes of Health and others.).

Notochord vacuoles are lysosome-related organelles that function in axis and spine morphogenesis

The zebrafish notochord vacuole, which has long been known to be important for vertebrate development but poorly classified at a cell biological level, is identified as a specialized lysosome-related organelle that is necessary both early, for embryonic axis elongation, and late, for spine morphogenesis.

The notochord plays critical structural and signaling roles during vertebrate development. At the center of the vertebrate notochord is a large fluid-filled organelle, the notochord vacuole. Although these highly conserved intracellular structures have been described for decades, little is known about the molecular mechanisms involved in their biogenesis and maintenance. Here we show that zebrafish notochord vacuoles are specialized lysosome-related organelles whose formation and maintenance requires late endosomal trafficking regulated by the vacuole-specific Rab32a and H⁺-ATPase–dependent acidification. We establish that notochord vacuoles are required for body axis elongation during embryonic development and identify a novel role in spine morphogenesis. Thus, the vertebrate notochord plays important structural roles beyond early development.

From notochord formation to hereditary chordoma: the many roles of Brachyury

Chordoma is a rare, but often malignant, bone cancer that preferentially affects the axial skeleton and the skull base. These tumors are both sporadic and hereditary and appear to occur more frequently after the fourth decade of life; however, modern technologies have increased the detection of pediatric chordomas. Chordomas originate from remnants of the notochord, the main embryonic axial structure that precedes the backbone, and share with notochord cells both histological features and the expression of characteristic genes. One such gene is Brachyury, which encodes for a sequence-specific transcription factor. Known for decades as a main regulator of notochord formation, Brachyury has recently gained interest as a biomarker and causative agent of chordoma, and therefore as a promising therapeutic target. Here, we review the main characteristics of chordoma, the molecular markers, and the clinical approaches currently available for the early detection and possible treatment of this cancer. In particular, we report on the current knowledge of the role of Brachyury and of its possible mechanisms of action in both notochord formation and chordoma etiogenesis.

Regulated tissue fluidity steers zebrafish body elongation

The tailbud is the posterior leading edge of the growing vertebrate embryo and consists of motile progenitors of the axial skeleton, musculature and spinal cord. We measure the 3D cell flow field of the zebrafish tailbud and identify changes in tissue fluidity revealed by reductions in the coherence of cell motion without alteration of cell velocities. We find a directed posterior flow wherein the polarization between individual cell motion is high, reflecting ordered collective migration. At the posterior tip of the tailbud, this flow makes sharp bilateral turns facilitated by extensive cell mixing due to increased directional variability of individual cell motions. Inhibition of Wnt or Fgf signaling or cadherin 2 function reduces the coherence of the flow but has different consequences for trunk and tail extension. Modeling and additional data analyses suggest that the balance between the coherence and rate of cell flow determines whether body elongation is linear or whether congestion forms within the flow and the body axis becomes contorted.

Crosstalk between Fgf and Wnt signaling in the zebrafish tailbud

Fibroblast growth factor (Fgf) and Wnt signaling are necessary for the intertwined processes of tail elongation, mesodermal development and somitogenesis. Here, we use pharmacological modifiers and time-resolved quantitative analysis of both nascent transcription and protein phosphorylation in the tailbud, to distinguish early effects of signal perturbation from later consequences related to cell fate changes. We demonstrate that Fgf activity elevates Wnt signaling by inhibiting transcription of the Wnt antagonists dkk1 and notum1a. PI3 kinase signaling also increases Wnt signaling via phosphorylation of Gsk3β. Conversely, Wnt can increase signaling within the Mapk branch of the Fgf pathway as Gsk3β phosphorylation elevates phosphorylation levels of Erk. Despite the reciprocal positive regulation between Fgf and Wnt, the two pathways generally have opposing effects on the transcription of co-regulated genes. This opposing regulation of target genes may represent a rudimentary relationship that manifests as out-of-phase oscillation of Fgf and Wnt target genes in the mouse and chick tailbud. In summary, these data suggest that Fgf and Wnt signaling are tightly integrated to maintain proportional levels of activity in the zebrafish tailbud, and this balance is important for axis elongation, cell fate specification and somitogenesis.

Mesogenin causes embryonic mesoderm progenitors to differentiate during development of zebrafish tail somites

The molecular mechanism underlying somite development differs along the embryonic antero-posterior axis. In zebrafish, cell lineage tracing and genetic analysis have revealed a difference in somite development between the trunk and tail. For instance, spadetail/tbx16 (spt) mutant embryos lack trunk somites but not tail ones. Trunk and tail somites are developed from mesodermal progenitor cells (MPCs) located in the tailbud. While the undifferentiated state of MPCs is maintained by mutual activation between Wnt and Brachyury/Ntl, the mechanism by which the MPCs differentiate into presomitic mesoderm (PSM) cells remains largely unclear. Especially, the molecules that promote PSM differentiation during tail development should be clarified. Here, we show that zebrafish embryos defective in mesogenin1 (msgn1) and spt failed to differentiate into PSM cells in tail development and show increased expression of wnt8 and ntl. Msgn1 acted in a cell-autonomous manner and as a transcriptional activator in PSM differentiation. The expression of msgn1 initially overlapped with that of ntl in the ventral tailbud, as previously reported; and its mis-expression caused ectopic expression of tbx24, a PSM marker gene, only in the tailbud and posterior notochord, both of which expressed ntl in zebrafish embryos. Furthermore, the PSM-inducing activity of misexpressed msgn1 was enhanced by co-expression with ntl. Thus, Msgn1 exercised its PSM-inducing activity in cells expressing ntl. Based on these results, we speculate that msgn1 expression in association with that of ntl may allow the differentiation of progenitor cells to proceed during development of somites in the tail.

RIPPLY3 is a retinoic acid-inducible repressor required for setting the borders of the pre-placodal ectoderm

Retinoic acid signaling is a major component of the neural posteriorizing process in vertebrate development. Here, we identify a new role for the retinoic acid receptor (RAR) in the anterior of the embryo, where RAR regulates Fgf8 expression and formation of the pre-placodal ectoderm (PPE). RARα2 signaling induces key pre-placodal genes and establishes the posterolateral borders of the PPE. RAR signaling upregulates two important genes, Tbx1 and Ripply3, during early PPE development. In the absence of RIPPLY3, TBX1 is required for the expression of Fgf8 and hence, PPE formation. In the presence of RIPPLY3, TBX1 acts as a transcriptional repressor, and functions to restrict the positional expression of Fgf8, a key regulator of PPE gene expression. These results establish a novel role for RAR as a regulator of spatial patterning of the PPE through Tbx1 and RIPPLY3. Moreover, we demonstrate that Ripply3, acting downstream of RAR signaling, is a key player in establishing boundaries in the PPE.

Genomic targets of Brachyury (T) in differentiating mouse embryonic stem cells

Background

The T-box transcription factor Brachyury (T) is essential for formation of the posterior mesoderm and the notochord in vertebrate embryos. Work in the frog and the zebrafish has identified some direct genomic targets of Brachyury, but little is known about Brachyury targets in the mouse.

Methodology/Principal Findings

Here we use chromatin immunoprecipitation and mouse promoter microarrays to identify targets of Brachyury in embryoid bodies formed from differentiating mouse ES cells. The targets we identify are enriched for sequence-specific DNA binding proteins and include components of signal transduction pathways that direct cell fate in the primitive streak and tailbud of the early embryo. Expression of some of these targets, such as Axin2, Fgf8 and Wnt3a, is down regulated in Brachyury mutant embryos and we demonstrate that they are also Brachyury targets in the human. Surprisingly, we do not observe enrichment of the canonical T-domain DNA binding sequence 5′-TCACACCT-3′ in the vicinity of most Brachyury target genes. Rather, we have identified an (AC)_n repeat sequence, which is conserved in the rat but not in human, zebrafish or Xenopus. We do not understand the significance of this sequence, but speculate that it enhances transcription factor binding in the regulatory regions of Brachyury target genes in rodents.

Conclusions/Significance

Our work identifies the genomic targets of a key regulator of mesoderm formation in the early mouse embryo, thereby providing insights into the Brachyury-driven genetic regulatory network and allowing us to compare the function of Brachyury in different species.

Fgf differentially controls cross-antagonism between cardiac and haemangioblast regulators

Fibroblast growth factor (Fgf) has been implicated in the control of heart size during development, although whether this is by controlling cell fate, survival or proliferation has not been clear. Here, we show that Fgf, without affecting survival or proliferation, acts during gastrulation to drive cardiac fate and restrict anterior haemangioblast fate in zebrafish embryos. The haemangioblast programme was thought to be activated before the cardiac programme and is repressive towards it, suggesting that activation by Fgf of the cardiac programme might be via suppression of the haemangioblast programme. However, we show that the cardiac regulator nkx2.5 can also repress the haemangioblast programme and, furthermore, that cardiac specification still requires Fgf signalling even when haemangioblast regulators are independently suppressed. We further show that nkx2.5 and the cloche candidate gene lycat are expressed during gastrulation and regulated by Fgf, and that nkx2.5 overexpression, together with loss of the lycat targets etsrp and scl can stably induce expansion of the heart. We conclude that Fgf controls cardiac and haemangioblast fates by the simultaneous regulation of haemangioblast and cardiac regulators. We propose that elevation of Fgf signalling in the anterior haemangioblast territory could have led to its recruitment into the heart field during evolution, increasing the size of the heart.

Dissecting the role of Fgf signaling during gastrulation and left-right axis formation in mouse embryos using chemical inhibitors

Fgf signaling plays pivotal roles in mouse gastrulation and left-right axis formation. However, although genetic analyses have revealed important aspects of Fgf signaling in these processes, the temporal resolution of genetic studies is low. Here, we combined whole-embryo culture with application of chemical compounds to inhibit Fgf signaling at specific time points. We found that sodium chlorate and PD173074 are potent inhibitors of Fgf signaling in early mouse embryos. Fgf signaling is required for the epithelial-to-mesenchymal transition of the primitive streak before the onset of gastrulation. Once gastrulation begins, Fgf signaling specifies mesodermal fates via the Ras/MAPK downstream cascade. Finally, Fgf signaling on the posterior side of the embryo during gastrulation induces Nodal expression in the node via Tbx6-Dll1, the initial event required for Nodal expression in the left lateral plate mesoderm.

Reiterative roles for FGF signaling in the establishment of size and proportion of the zebrafish heart

Development of a functional organ requires the establishment of its proper size as well as the establishment of the relative proportions of its individual components. In the zebrafish heart, organ size and proportion depend heavily on the number of cells in each of its two major chambers, the ventricle and the atrium. Heart size and chamber proportionality are both affected in zebrafish fgf8 mutants. To determine when and how FGF signaling influences these characteristics, we examined the effect of temporally controlled pathway inhibition. During cardiac specification, reduction of FGF signaling inhibits formation of both ventricular and atrial cardiomyocytes, with a stronger impact on ventricular cells. After cardiomyocyte differentiation begins, reduction of FGF signaling can still result in a deficiency of ventricular cardiomyocytes. Consistent with two temporally distinct roles for FGF, we find that increased FGF signaling induces a cardiomyocyte surplus only before cardiac differentiation begins. Thus, FGF signaling first regulates heart size and chamber proportionality during cardiac specification and later refines ventricular proportion by regulating cell number after the onset of differentiation. Together, our data demonstrate that a single signaling pathway can act reiteratively to coordinate organ size and proportion.

In vivo time-lapse imaging delineates the zebrafish pituitary proopiomelanocortin lineage boundary regulated by FGF3 signal

The anterior pituitary gland (adenohypophysis) comprises anterior and intermediate lobes (the pars distalis and pars intermedia) arising from placodal ectoderm at the anterior neural ridge. Signaling molecules including SHH, FGF, WNT, BMP and Notch are involved in regulating primordial pituitary proliferation and lineage determination. However, morphogenic events and molecular mechanisms governing anterior and intermediate lobe specification are not clear. Pituitary expression of proopiomelanocortin (POMC), the common precursor for adrenocorticotropin (ACTH) of pars distalis corticotropes and alpha-melanocyte-stimulating hormone (alpha-MSH) of pars intermedia melanotropes, provides a unique marker for anterior and intermediate lobe morphogenesis. We performed time-lapse confocal microscopy lineage tracing in live zebrafish embryos expressing GFP driven by the pomc promoter and show distinct migration pathways of POMC cells destined to the anterior and intermediate lobes. Using morpholino oligonucleotides, we show that hypomorphic FGF3 down-regulation induces specific defects of pars intermedia POMC cells while pomc, growth hormone and prolactin expression remain intact in the pars distalis. This lineage-specific process is independent of the FGF3 effect on early pituitary specifying transcription factors as indicated by normal Lim3 and Pit1 expression in hypomorphic FGF3 morphants. These findings suggest that the FGF3 signal, in addition to its previously described role of regulating progenitor proliferation and survival, delineates the melanotrope and corticotrope lineage boundary, contributing to establishment of the pituitary pars distalis and pars intermedia.

Midkine-b regulates cell specification at the neural plate border in zebrafish

Midkines compose a family of secreted, heparin-binding growth factors with neurotrophic activity in vitro, but largely unknown functions in mammals in vivo. Here we show that one member of this family, Midkine-b (Mdkb), is responsible for establishment of the neural plate border in zebrafish. We propose that MdkB acts downstream of several signaling factors, most notably retinoic acid, implicated in neural crest cell (ncc) induction and refines a zone of competence for ncc and Rohon-Beard (RB) sensory neuron formation. Overexpression of Mdkb expands the cell numbers of various ncc precursor subtypes and results in a significant increase in the number of RB neurons at the neural plate border. On the other hand, Morpholino-mediated knockdown of Mdkb leads to a dramatic reduction of ncc and a loss of sensory neurons. Our results imply that Mdkb is required for the earliest steps of cell specification at the neural plate border in zebrafish.

ERK1 and ERK2 MAPK are key regulators of distinct gene sets in zebrafish embryogenesis

Background

The MAPK signaling proteins are involved in many eukaryotic cellular processes and signaling networks. However, specific functions of most of these proteins in vertebrate development remain elusive because of potential redundancies. For instance, the upstream activation pathways for ERK1 and ERK2 are highly similar, and also many of their known downstream targets are common. In contrast, mice and zebrafish studies indicate distinct roles for both ERKs in cellular proliferation, oncogenic transformation and development. A major bottleneck for further studies is that relatively little is known of in vivo downstream signaling specific for these kinases.

Results

Microarray based gene expression profiling of ERK1 and ERK2 knockdown zebrafish embryos at various stages of early embryogenesis resulted in specific gene expression signature sets that showed pronounced differences in gene ontology analyses. In order to predict functions of these genes, zebrafish specific in silico signaling pathways involved in early embryogenesis were constructed using the GenMAPP program. The obtained transcriptome signatures were analyzed in the BMP, FGF, Nodal and Wnt pathways. Predicted downstream effects of ERK1 and ERK2 knockdown treatments on key pathways responsible for mesendoderm development were confirmed by whole mount in situ hybridization experiments.

Conclusion

The gene ontology analyses showed that ERK1 and ERK2 target common and distinct gene sets, confirming the difference in knockdown phenotypes and diverse roles for these kinases during embryogenesis. For ERK1 we identified specific genes involved in dorsal-ventral patterning and subsequent embryonic cell migration. For ERK2 we identified genes involved in cell-migration, mesendoderm differentiation and patterning.

The specific function of ERK2 in the initiation, maintenance and patterning of mesoderm and endoderm formation was biologically confirmed.

The small molecule Mek1/2 inhibitor U0126 disrupts the chordamesoderm to notochord transition in zebrafish

Background

Key molecules involved in notochord differentiation and function have been identified through genetic analysis in zebrafish and mice, but MEK1 and 2 have so far not been implicated in this process due to early lethality (Mek1-/-) and functional redundancy (Mek2-/-) in the knockout animals.

Results

Here, we reveal a potential role for Mek1/2 during notochord development by using the small molecule Mek1/2 inhibitor U0126 which blocks phosphorylation of the Mek1/2 target gene Erk1/2 in vivo. Applying the inhibitor from early gastrulation until the 18-somite stage produces a specific and consistent phenotype with lack of dark pigmentation, shorter tail and an abnormal, undulated notochord. Using morphological analysis, in situ hybridization, immunhistochemistry, TUNEL staining and electron microscopy, we demonstrate that in treated embryos the chordamesoderm to notochord transition is disrupted and identify disorganization in the medial layer of the perinotochordal basement mebrane as the probable cause of the undulations and bulges in the notochord. We also examined and excluded FGF as the upstream signal during this process.

Conclusion

Using the small chemical U0126, we have established a novel link between MAPK-signaling and notochord differentiation. Our phenotypic analysis suggests a potential connection between the MAPK-pathway, the COPI-mediated intracellular transport and/or the copper-dependent posttranslational regulatory processes during notochord differentiation.

Smarcd3 regulates the timing of zebrafish myogenesis onset

A cascade of signaling events triggers myogenesis in vertebrates. Although studies of zebrafish indicate that fibroblast growth factor (Fgf), Hedgehog (Hh), and the T-box transcription factors, No tail (Ntl) and T-box gene 16 (Tbx16), regulate myogenesis, the hierarchy of these factors has not been determined. Recently, another transcriptional cofactor, Smarcd3, a subunit of the SWI/SNF chromatin-remodeling complex, has been shown to be required for heart muscle formation in mouse. In zebrafish, fgf8 and ntl expression commences during blastula stages, whereas myogenesis, as indicated by myod expression, does not begin until much later during mid-gastrula stages. smarcd3b expression, on the other hand, becomes enriched in the marginal zone just prior to the beginning of myod expression. Overexpression of smarcd3 shifts the onset of myod and myf5 expression earlier, and myod and myf5 expression in adaxial cells, the earliest muscle precursors, requires Smarcd3, indicating that Smarcd3 is the limiting factor that regulates the onset of myogenesis. Smarcd3 physically interacts with Ntl, and Smarcd3 overexpression fails to rescue myod expression in ntl mutants, demonstrating that function of Smarcd3 depends on Ntl activity. We propose a model in which cooperative activity of Fgf, Ntl, and Smarcd3 is required for the onset of myogenesis, with Smarcd3b serving as the primary regulator of the timing of myogenesis onset.

Nodal signaling activates differentiation genes during zebrafish gastrulation

Nodal signals induce mesodermal and endodermal progenitors during vertebrate development. To determine the role of Nodal signaling at a genomic level, we isolated Nodal-regulated genes by expression profiling using macroarrays and gene expression databases. Putative Nodal-regulated genes were validated by in situ hybridization screening in wild type and Nodal signaling mutants. 46 genes were identified, raising the currently known number of Nodal-regulated genes to 72. Based on their expression patterns along the dorsoventral axis, most of these genes can be classified into two groups. One group is expressed in the dorsal margin, whereas the other group is expressed throughout the margin. In addition to transcription factors and signaling components, the screens identified several new functional classes of Nodal-regulated genes, including cytoskeletal components and molecules involved in protein secretion or endoplasmic reticulum stress. We found that x-box binding protein-1 (xbp1) is a direct target of Nodal signaling and required for the terminal differentiation of the hatching gland, a specialized secretory organ whose specification is also dependent on Nodal signaling. These results indicate that Nodal signaling regulates not only specification genes but also differentiation genes.

Depletion of Med10 enhances Wnt and suppresses Nodal signaling during zebrafish embryogenesis

The transcriptional Mediator (MED) is a multiprotein complex that transmits information from transcription factors to RNA polymerase II (PolII) to regulate transcription. At present, the role of distinct MED subunits in general transcription versus transcription stimulated by specific signaling pathways is unclear. By means of positional cloning, we reveal that the zebrafish mutant tennismatch is a hypomorphic allele of Med10, a conserved MED middle domain subunit. Using morpholino antisense oligonucleotides, we further demonstrate that reduction of Med10 levels led to an enhancement of the Wnt signaling pathway, while also suggesting a role for Med10 in mediating the Nodal signaling pathway. In contrast to the dual roles of Med10, reduction of Med12 and Med13 levels, two MED subunits in the regulatory domain, led to an enhancement of the Wnt signaling pathway but not the Nodal pathway, while reduction of Med15 levels, a MED subunit in the tail domain, suppressed the Nodal signaling pathway but not the Wnt signaling pathway. Thus, Med10 appears to be a unique MED subunit that differentially transduces information from distinct signaling pathways during zebrafish embryogenesis.

Fgf signaling negatively regulates Nodal-dependent endoderm induction in zebrafish

In zebrafish development, Nodal signaling is critical for the induction of endoderm and mesoderm. Three transcription factors downstream of Nodal, Bonnie and Clyde (Bon), Faust (Fau)/Gata5 and Casanova (Cas), are required for endoderm induction. However, it is not yet fully understood how the Nodal signaling pathway regulates the decision process of endoderm and mesoderm induction. In this study, we focused on Fgf signaling, downstream of Nodal signaling, during endoderm induction. We found that activation of Fgf signaling decreases the number of cas-expressing endodermal cells. Conversely, inhibition of this signaling increases the number of endodermal cells without affecting the expression of Nodal, Nodal antagonists, bon or fau/gata5. Inhibition of Fgf signaling in endoderm mutants suggests that this signaling negatively regulates cas expression by a pathway parallel to Bon and Fau/Gata5 in the molecular cascade leading to endoderm. Furthermore, activation of Fgf signaling can overcome Cas-mediated abrogation of mesodermal gene expression. Altogether, these results suggest that Fgf signaling negatively regulates endoderm induction, possibly through repression of cas expression and down-regulation of Cas function.

Mesoderm induction: from caps to chips

Vertebrate mesoderm induction is one of the classical problems in developmental biology. Various developmental biology approaches, particularly in Xenopus and zebrafish, have identified many of the key factors that are involved in this process and have provided major insights into how these factors interact as part of a signalling and transcription-factor network. These data are beginning to be refined by high-throughput approaches such as microarray assays. Future challenges include understanding how the prospective mesodermal cells integrate the various signals they receive and how they resolve this information to regulate their morphogenetic behaviours and cell-fate decisions.

Zebrafish endoderm formation is regulated by combinatorial Nodal, FGF and BMP signalling

In the zebrafish embryo, the mesoderm and endoderm originate from common precursors and segregate during gastrulation by mechanisms that are largely unknown. Understanding how the signalling pathways that regulate endoderm and mesoderm formation interact is crucial to understanding how the germ layers are established. Here, we have analysed how the FGF and BMP pathways interact with Nodal signalling during the process of endoderm formation. We found that activation of the FGF/ERK pathway disrupts endoderm formation in the embryo and antagonizes the ability of an activated form of Tar/Acvr1b to induce endoderm at the animal pole. By contrast, inhibition of FGF signalling increases the number of endodermal precursors and potentiates the ability of Tar*/Acvr1b to induce endoderm at the animal pole. Using a pharmacological inhibitor of the FGF receptor, we show that reducing FGF signalling partially rescues the deficit of endoderm precursors in bon mutant embryos. Furthermore, we found that overexpression of BMPs compromises endoderm formation, suggesting that formation of endoderm precursors is negatively regulated by BMPs on the ventral side. We show that simultaneous inhibition of the FGF/Ras and BMP pathways results in a dramatic increase in the number of endoderm precursors. Taken together, these data strongly suggest that BMP and FGF-ERK pathways cooperate to restrict the number of endodermal progenitors induced in response to Nodal signalling. Finally, we investigated the molecular basis for the FGF-MAPK-dependent repression of endoderm formation. We found that FGF/ERK signalling causes phosphorylation of Casanova/Sox32, an important regulator of endoderm determination, and provide evidence that this phosphorylation attenuates its ability to induce sox17. These results identify a molecular mechanism whereby FGF attenuates Nodal-induced endodermal transcription factors and highlight a potential mechanism whereby mesoderm and endoderm fates could segregate from each other.

Bidirectional fusion of the heart-forming fields in the developing chick embryo

It is generally thought that the early pre-tubular chick heart is formed by fusion of the anterior or cephalic limits of the paired cardiogenic fields. However, this study shows that the heart fields initially fuse at their midpoint to form a transitory "butterfly"-shaped, cardiogenic structure. Fusion then progresses bi-directionally along the longitudinal axis in both cranial and caudal directions. Using in vivo labeling, we demonstrate that cells along the ventral fusion line are highly motile, crossing future primitive segments. We found that mesoderm cells migrated cephalically from the unfused tips of the anterior/cephalic wings into the head mesenchyme in the region that has been called the secondary heart field. Perturbing the anterior/cranial fusion results in formation of a bi-conal heart. A theoretical role of the ventral fusion line acting as a "heart organizer" and its role in cardia bifida is discussed.

Molecular genetics of axis formation in zebrafish

The basic vertebrate body plan of the zebrafish embryo is established in the first 10 hours of development. This period is characterized by the formation of the anterior-posterior and dorsal-ventral axes, the development of the three germ layers, the specification of organ progenitors, and the complex morphogenetic movements of cells. During the past 10 years a combination of genetic, embryological, and molecular analyses has provided detailed insights into the mechanisms underlying this process. Maternal determinants control the expression of transcription factors and the location of signaling centers that pattern the blastula and gastrula. Bmp, Nodal, FGF, canonical Wnt, and retinoic acid signals generate positional information that leads to the restricted expression of transcription factors that control cell type specification. Noncanonical Wnt signaling is required for the morphogenetic movements during gastrulation. We review how the coordinated interplay of these molecules determines the fate and movement of embryonic cells.

Efficient RNA interference in zebrafish embryos using siRNA synthesized with SP6 RNA polymerase

Double-stranded RNA (dsRNA) has been shown to be a useful tool for silencing genes in zebrafish (Danio rerio), while the blocking specificity of dsRNA is still of major concern for application. It was reported that siRNA (small interfering RNA) prepared by endoribonuclease digestion (esiRNA) could efficiently silence endogenous gene expression in mammalian embryos. To test whether esiRNA could work in zebrafish, we utilized Escherichia coli RNaseIII to digest dsRNA of zebrafish no tail (ntl), a mesoderm determinant in zebrafish and found that esi-ntl could lead to developmental defects, however, the effective dose was so close to the toxic dose that esi-ntl often led to non-specific developmental defects. Consequently, we utilized SP6 RNA polymerase to produce si-ntl, siRNA designed against ntl, by in vitro transcription. By injecting in vitro synthesized si-ntl into zebrafish zygotes, we obtained specific phenocopies of reported mutants of ntl. We achieved up to a 59%no tail phenotype when the injection concentration was as high as 4 microg/microL. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization analysis showed that si-ntl could largely and specifically reduce mRNA levels of the ntl gene. As a result, our data indicate that esiRNA is unable to cause specific developmental defects in zebrafish, while siRNA should be an alternative for downregulation of specific gene expression in zebrafish in cases where RNAi techniques are applied to zebrafish reverse genetics.

Cripto-1: a multifunctional modulator during embryogenesis and oncogenesis

It is increasingly evident that genes known to perform critical roles during early embryogenesis, particularly during stem cell renewal, pluripotentiality and survival, are also expressed during the development of cancer. In this regard, oncogenesis may be considered as the recapitulation of embryogenesis in an inappropriate temporal and spatial manner. The epidermal growth factor-Cripto-1/FRL1/cryptic family of proteins consists of extracellular and cell-associated proteins that have been identified in several vertebrate species. During early embryogenesis, epidermal growth factor-Cripto-1/FRL1/cryptic proteins perform an obligatory role as coreceptors for the transforming growth factor-beta subfamily of proteins, which includes Nodal. Cripto-1 has also been shown to function as a ligand through a Nodal/Alk4-independent signaling pathway that involves binding to glypican-1 and the subsequent activation through src of phosphoinositol-3 kinase/Akt and ras/mitogen-activated protein kinase intracellular pathways. Expression of Cripto-1 is increased in several human cancers and its overexpression is associated with the development of mammary tumors in mice. Here, we review the role of Cripto-1 during embryogenesis, cell migration, invasion and angiogenesis and how these activities may relate to cellular transformation and tumorigenesis. We also briefly discuss evidence suggesting that Cripto-1 may be involved in stem cell maintenance.

The Sp1-related transcription factors sp5 and sp5-like act downstream of Wnt/beta-catenin signaling in mesoderm and neuroectoderm patterning

BACKGROUND

Wnt/beta-catenin signaling regulates many processes during vertebrate development, including patterning of the mesoderm along the dorso-ventral axis and patterning of the neuroectoderm along the anterior-posterior axis during gastrulation. However, relatively little is known about Wnt target genes mediating these effects.

RESULTS

Using zebrafish DNA microarrays, we have identified several new targets of Wnt/beta-catenin signaling, including sp5-like (sp5l, previously called spr2), a zinc-finger transcription factor of the Sp1 family. sp5-like is a direct target of Wnt/beta-catenin signaling and acts together with its paralog sp5 (previously called bts1) downstream of wnt8 in patterning of the mesoderm and neuroectoderm because (1) overexpression of sp5-like, like overexpression of wnt8, posteriorizes the neuroectoderm, (2) sp5-like morpholino-mediated knockdown, like wnt8 knockdown, causes anteriorization of the hindbrain, (3) combined knockdown of sp5 and sp5-like, like loss of wnt8, causes expansion of dorsal mesoderm, (4) sp5-like knockdown reduces the defects in mesoderm and neuroectoderm patterning caused by wnt8 overexpression, and (5) inhibition of sp5-like enhances the effects of hypomorphic loss of wnt8. Importantly, (6) overexpression of sp5-like is able to partially restore normal hindbrain patterning in wnt8 morphants.

CONCLUSIONS

sp5-like is a direct target of Wnt/beta-catenin signaling during gastrulation and, together with sp5, acts as a required mediator of the activities of wnt8 in patterning the mesoderm and neuroectoderm. We conclude that sp5 transcription factors mediate the downstream responses to Wnt/beta-catenin signaling in several developmental processes in zebrafish.

Transgenic zebrafish reveal stage-specific roles for Bmp signaling in ventral and posterior mesoderm development

Bone morphogenetic protein (Bmp) signaling is crucial for the formation and patterning of zebrafish ventral and posterior mesoderm. Mutants defective in the Bmp pathway have expanded trunk muscle, abnormal tails and severely impaired development of ventral mesodermal derivatives such as vasculature, blood and pronephros. As Bmps continue to be expressed in the ventral and posterior mesoderm after gastrulation, it is likely that Bmp signaling continues to play an important developmental role during outgrowth of the posterior body. However, because Bmp signaling plays an essential role during the gastrula stages, it has not been possible with mutants or standard disruption techniques to determine the later functions of the Bmp pathway. To study the role of Bmp signaling in the ventral and posterior mesoderm during trunk and tail outgrowth, we generated a transgenic zebrafish line containing a heatshock-inducible dominant-negative Bmp receptor-GFP fusion. Our data show that Bmps are important for tail organizer formation and for patterning the ventral mesoderm during early gastrulation. However, from mid-gastrulation to the early somitogenesis stages, Bmp signaling is important for ventral tail fin development and for preventing secondary tail formation. We conclude that the role of Bmp signaling in the ventral and posterior mesoderm changes as gastrulation proceeds.

Wnt/beta-catenin regulation of the Sp1-related transcription factor sp5l promotes tail development in zebrafish

Tail formation in vertebrates involves the specification of a population of multipotent precursors, the tailbud, which will give rise to all of the posterior structures of the embryo. Wnts are signaling proteins that are candidates for promoting tail outgrowth in zebrafish, although which Wnts are involved, what genes they regulate, and whether Wnts are required for initiation or maintenance steps in tail formation has not been resolved. We show here that both wnt3a and wnt8 are expressed in the zebrafish tailbud and that simultaneous inhibition of both wnt3a and wnt8 using morpholino oligonucleotides can completely block tail formation. In embryos injected with wnt3a and wnt8 morpholinos, expression of genes in undifferentiated presomitic mesoderm is initiated, but not maintained. To identify genes that might function downstream of Wnts in tail formation, a DNA microarray screen was conducted, revealing that sp5l, a member of the Sp1 family of zinc-finger transcription factors, is activated by Wnt signaling. Moreover, we show that sp5l expression in the developing tail is dependent on both wnt3a and wnt8 function. Supporting a role for sp5l in tail formation, we find that inhibition of sp5l strongly enhances the effects of wnt3a inhibition, and overexpression of sp5l RNA is able to completely restore normal tail development in wnt3a morphants. These data place sp5l downstream of wnt3a and wnt8 in a Wnt/beta-catenin signaling pathway that controls tail development in zebrafish.

Zebrafish Polycomb group gene ph2α is required for epiboly and tailbud formation acting downstream of FGF signaling

We analyzed Polycomb group gene ph2alpha functionally in zebrafish embryos by a gene knock-down procedure using morpholino antisense oligos. Inhibition of ph2alpha message translation resulted in abnormal epibolic movements as well as a thick tailbud or incomplete covering of the yolk plug. At the 24hpf stage, morphants had short trunks and tails, phenotypes similar to those with disturbances in FGF signaling. Accordingly, we looked at the effects of ph2alpha expression upstream and downstream of the FGF pathway. Treatment with SU5402, an inhibitor of Fgfrs, or injection of dominant-negative Fgfr1 DNA markedly reduced ph2alpha expression in the tailbud. In addition, cells expressing mRNAs for no tail, spadetail, myoD, and papc, which are involved in FGF-related development of posterior mesoderm, were distributed abnormally. Collectively, the data argue that ph2alpha is required for epiboly and tailbud formation, acting downstream of the FGF signaling pathway.

Fibroblast growth factor signaling during early vertebrate development

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

Combinatorial gene regulation by Bmp and Wnt in zebrafish posterior mesoderm formation

Combinatorial signaling is an important mechanism that allows the embryo to utilize overlapping signaling pathways to specify different territories. In zebrafish, the Wnt and Bmp pathways interact to regulate the formation of the posterior body. In order to understand how this works mechanistically, we have identified tbx6 as a posterior mesodermal gene activated by both of these signaling pathways. We isolated a genomic fragment from the tbx6 gene that recapitulates the endogenous tbx6 expression, and used this to ask how the Bmp and Wnt signaling pathways combine to regulate gene expression. We find that the tbx6 promoter utilizes distinct domains to integrate the signaling inputs from each pathway, including multiple Tcf/LEF sites and a novel Bmp-response element. Surprisingly, we found that overexpression of either signaling pathway can activate the tbx6 promoter and the endogenous gene, whereas inputs from both pathways are required for the normal pattern of expression. These results demonstrate that both Bmp and Wnt are present at submaximal levels, which allows the pathways to function combinatorially. We present a model in which overlapping Wnt and Bmp signals in the ventrolateral region activate the expression of tbx6 and other posterior mesodermal genes, leading to the formation of posterior structures.