FAST-1 is a key maternal effector of mesoderm inducers in the early Xenopus embryo

Nodal and churchill1 position the expression of a notch ligand during Xenopus germ layer segregation

Churchill and Nodal signaling, which participate in vertebrates’ germ layer induction, position a domain of Delta/Notch activity, which refines germ layer boundaries during frog gastrulation.

In vertebrates, Nodal signaling plays a major role in endomesoderm induction, but germ layer delimitation is poorly understood. In avian embryos, the neural/mesoderm boundary is controlled by the transcription factor CHURCHILL1, presumably through the repressor ZEB2, but there is scarce knowledge about its role in other vertebrates. During amphibian gastrulation, Delta/Notch signaling refines germ layer boundaries in the marginal zone, but it is unknown the place this pathway occupies in the network comprising Churchill1 and Nodal. Here, we show that Xenopus churchill1 is expressed in the presumptive neuroectoderm at mid-blastula transition and during gastrulation, upregulates zeb2, prevents dll1 expression in the neuroectoderm, and favors neuroectoderm over endomesoderm development. Nodal signaling prevents dll1 expression in the endoderm but induces it in the presumptive mesoderm, from where it activates Notch1 and its target gene hes4 in the non-involuting marginal zone. We propose a model where Nodal and Churchill1 position Dll1/Notch1/Hes4 domains in the marginal zone, ensuring the delimitation between mesoderm and neuroectoderm.

Appropriate Amounts and Activity of the Wilms' Tumor Suppressor Gene, wt1, Are Required for Normal Pronephros Development of Xenopus Embryos

The Wilms' tumor suppressor gene, wt1, encodes a zinc finger-containing transcription factor that binds to a GC-rich motif and regulates the transcription of target genes. wt1 was first identified as a tumor suppressor gene in Wilms' tumor, a pediatric kidney tumor, and has been implicated in normal kidney development. The WT1 protein has transcriptional activation and repression domains and acts as a transcriptional activator or repressor, depending on the target gene and context. In Xenopus, an ortholog of wt1 has been isolated and shown to be expressed in the developing embryonic pronephros. To investigate the role of wt1 in pronephros development in Xenopus embryos, we mutated wt1 by CRISPR/Cas9 and found that the expression of pronephros marker genes was reduced. In reporter assays in which known WT1 binding sequences were placed upstream of the luciferase gene, WT1 activated transcription of the luciferase gene. The injection of wild-type or artificially altered transcriptional activity of wt1 mRNA disrupted the expression of pronephros marker genes in the embryos. These results suggest that the appropriate amounts and activity of WT1 protein are required for normal pronephros development in Xenopus embryos.

Quantitative analysis of transcriptome dynamics provides novel insights into developmental state transitions

BACKGROUND

During embryogenesis, the developmental potential of initially pluripotent cells becomes progressively restricted as they transit to lineage restricted states. The pluripotent cells of Xenopus blastula-stage embryos are an ideal system in which to study cell state transitions during developmental decision-making, as gene expression dynamics can be followed at high temporal resolution.

RESULTS

Here we use transcriptomics to interrogate the process by which pluripotent cells transit to four different lineage-restricted states: neural progenitors, epidermis, endoderm and ventral mesoderm, providing quantitative insights into the dynamics of Waddington's landscape. Our findings provide novel insights into why the neural progenitor state is the default lineage state for pluripotent cells and uncover novel components of lineage-specific gene regulation. These data reveal an unexpected overlap in the transcriptional responses to BMP4/7 and Activin signaling and provide mechanistic insight into how the timing of signaling inputs such as BMP are temporally controlled to ensure correct lineage decisions.

CONCLUSIONS

Together these analyses provide quantitative insights into the logic and dynamics of developmental decision making in early embryos. They also provide valuable lineage-specific time series data following the acquisition of specific lineage states during development.

ERK1/2 is an ancestral organising signal in spiral cleavage

Animal development is classified as conditional or autonomous based on whether cell fates are specified through inductive signals or maternal determinants, respectively. Yet how these two major developmental modes evolved remains unclear. During spiral cleavage-a stereotypic embryogenesis ancestral to 15 invertebrate groups, including molluscs and annelids-most lineages specify cell fates conditionally, while some define the primary axial fates autonomously. To identify the mechanisms driving this change, we study Owenia fusiformis, an early-branching, conditional annelid. In Owenia, ERK1/2-mediated FGF receptor signalling specifies the endomesodermal progenitor. This cell likely acts as an organiser, inducing mesodermal and posterodorsal fates in neighbouring cells and repressing anteriorising signals. The organising role of ERK1/2 in Owenia is shared with molluscs, but not with autonomous annelids. Together, these findings suggest that conditional specification of an ERK1/2+ embryonic organiser is ancestral in spiral cleavage and was repeatedly lost in annelid lineages with autonomous development.

Post-Transcriptional Regulation of Molecular Determinants during Cardiogenesis

Cardiovascular development is initiated soon after gastrulation as bilateral precardiac mesoderm is progressively symmetrically determined at both sides of the developing embryo. The precardiac mesoderm subsequently fused at the embryonic midline constituting an embryonic linear heart tube. As development progress, the embryonic heart displays the first sign of left-right asymmetric morphology by the invariably rightward looping of the initial heart tube and prospective embryonic ventricular and atrial chambers emerged. As cardiac development progresses, the atrial and ventricular chambers enlarged and distinct left and right compartments emerge as consequence of the formation of the interatrial and interventricular septa, respectively. The last steps of cardiac morphogenesis are represented by the completion of atrial and ventricular septation, resulting in the configuration of a double circuitry with distinct systemic and pulmonary chambers, each of them with distinct inlets and outlets connections. Over the last decade, our understanding of the contribution of multiple growth factor signaling cascades such as Tgf-beta, Bmp and Wnt signaling as well as of transcriptional regulators to cardiac morphogenesis have greatly enlarged. Recently, a novel layer of complexity has emerged with the discovery of non-coding RNAs, particularly microRNAs and lncRNAs. Herein, we provide a state-of-the-art review of the contribution of non-coding RNAs during cardiac development. microRNAs and lncRNAs have been reported to functional modulate all stages of cardiac morphogenesis, spanning from lateral plate mesoderm formation to outflow tract septation, by modulating major growth factor signaling pathways as well as those transcriptional regulators involved in cardiac development.

Application of the RNA interference technique to Xenopus embryos: Specific reduction of the β-catenin gene products by short double-stranded RNA produced by recombinant human Dicer

RNA interference (RNAi) is a technique for suppressing the function of specific genes and is widely used in many organisms, including yeast, nematodes, flies, plants, mice, and cultured mammalian cells. As of date, this technique has not been successfully applied to Xenopus laevis embryos. In this study, we applied RNAi to Xenopus embryos using β-catenin as a model gene. Injection of long double-stranded RNA (dsRNA) corresponding to the 3'-untranslated region of β-catenin mRNA into embryos induced embryonic lethality without any specific phenotype. However, injection of short dsRNA, generated from long dsRNA by treatment with recombinant human Dicer, into embryos resulted in decreased expression of endogenous β-catenin mRNA and protein, as well as decreased Wnt signaling activity in the embryos. The decrease in β-catenin mRNA and protein levels was observed only after mid-blastula transition. Embryos injected with short dsRNA showed a characteristic phenotype of enlarged anterior structures and loss of posterior structures. These phenotypes, as well as the increased expression of the anterior gene and decreased expression of the posterior gene, suggest that RNAi against the β-catenin gene suppresses the "late Wnt signaling" involved in proper anterior-posterior patterning of Xenopus embryos. The effect of RNAi on Xenopus embryos was also found to be sensitive to temperature. These results strongly suggest that the RNAi technique can be applied to Xenopus embryos using short dsRNAs, appropriate temperature control, and proper selection of target genes.

Segregation of brain and organizer precursors is differentially regulated by Nodal signaling at blastula stage

The blastula Chordin- and Noggin-expressing (BCNE) center comprises animal-dorsal and marginal-dorsal cells of the amphibian blastula and contains the precursors of the brain and the gastrula organizer. Previous findings suggested that the BCNE behaves as a homogeneous cell population that only depends on nuclear β-catenin activity but does not require Nodal and later segregates into its descendants during gastrulation. In contrast to previous findings, in this work, we show that the BCNE does not behave as a homogeneous cell population in response to Nodal antagonists. In fact, we found that chordin.1 expression in a marginal subpopulation of notochordal precursors indeed requires Nodal input. We also establish that an animal BCNE subpopulation of cells that express both, chordin.1 and sox2 (a marker of pluripotent neuroectodermal cells), and gives rise to most of the brain, persisted at blastula stage after blocking Nodal. Therefore, Nodal signaling is required to define a population of chordin.1+ cells and to restrict the recruitment of brain precursors within the BCNE as early as at blastula stage. We discuss our findings in Xenopus in comparison to other vertebrate models, uncovering similitudes in early brain induction and delimitation through Nodal signaling.

This article has an associated First Person interview with the first author of the paper.

Summary: Nodal signaling is involved in the delimitation of the blastula cell populations that give rise to the brain and axial mesoderm in Xenopus.

Generation of a FOXH1 homozygous knockout human embryonic stem cell line by CRISPR/Cas9 system

Human FOXH1 (Forkhead Box H1) gene encodes a human homolog of Xenopus forkhead activing signal transducer-1 and has been shown to play an important role in mesendoderm formation in X. tropicalis and mice. However, little is known about the function of FOXH1 in human development. Here we generated a FOXH1 homozygous knockout human embryonic stem cell (hESC), WAe009-A-42 by CRISPR/Cas9 mediated gene targeting. The WAe009-A-42 retained a typical undifferentiated morphology and normal karyotype, pluripotenty, and trilineage differentiation potential in vivo and vitro.

Determination of Genetic Effects of LIPK and LIPJ Genes on Milk Fatty Acids in Dairy Cattle

In our previous genome-wide association study (GWAS) on milk fatty acids (FAs) in Chinese Holstein, we discovered 83 genome-wide significant single nucleotide polymorphisms (SNPs) associated with milk FAs. Two of them were close to lipase family member K (LIPK) and lipase family member J (LIPJ), respectively. Hence, this study is a follow-up to verify whether the LIPK and LIPJ have significant genetic effects on milk FAs in dairy cattle. By re-sequencing the entire exons, and 3 kb of 5′ and 3′ flanking regions, two and seven SNPs were identified in LIPK and LIPJ, respectively, including a novel SNP, ss158213049726. With the Haploview 4.1 software, we found that five of the SNPs in LIPJ formed a haplotype block (D′ = 0.96 ~ 1.00). Single-locus association analyses revealed that each SNP in LIPK and LIPJ was significantly associated with at least one milk FA (p = < 1.00 × 10⁻⁴ ~ 4.88 × 10⁻²), and the haplotype-based association analyses showed significant genetic effects on nine milk FAs (p = < 1.00 × 10⁻⁴ ~ 3.98 × 10⁻²). Out of these SNPs, the missense mutation in LIPK gene, rs42774527, could change the protein secondary structure and function predicted by SOPMA, SIFT, and PROVEAN softwares. With the Genomatix software, we predicted that two SNPs, rs110322221 in LIPK and rs211373799 in LIPJ, altered the transcription factors binding sites (TFBSs), indicating their potential regulation on promoter activity of the genes. Furthermore, we found that both LIPK and LIPJ had relatively high expressions in the mammary gland. In conclusion, our research is the first to demonstrate that LIPK and LIPJ genes have significant associations with milk FAs, and the identified SNPs might be served as genetic markers to optimize breeding programs for milk FAs in dairy cattle. This research deserves in-depth verification.

Anatomical integration of the sacral-hindlimb unit coordinated by GDF11 underlies variation in hindlimb positioning in tetrapods

Elucidating how body parts from different primordia are integrated during development is essential for understanding the nature of morphological evolution. In tetrapod evolution, while the position of the hindlimb has diversified along with the vertebral formula, the mechanism responsible for this coordination has not been well understood. However, this synchronization suggests the presence of an evolutionarily conserved developmental mechanism that coordinates the positioning of the hindlimb skeleton derived from the lateral plate mesoderm with that of the sacral vertebrae derived from the somites. Here we show that GDF11 secreted from the posterior axial mesoderm is a key factor in the integration of sacral vertebrae and hindlimb positioning by inducing Hox gene expression in two different primordia. Manipulating the onset of GDF11 activity altered the position of the hindlimb in chicken embryos, indicating that the onset of Gdf11 expression is responsible for the coordinated positioning of the sacral vertebrae and hindlimbs. Through comparative analysis with other vertebrate embryos, we also show that each tetrapod species has a unique onset timing of Gdf11 expression, which is tightly correlated with the anteroposterior levels of the hindlimb bud. We conclude that the evolutionary diversity of hindlimb positioning resulted from heterochronic shifts in Gdf11 expression, which led to coordinated shifts in the sacral-hindlimb unit along the anteroposterior axis.

Expression and distribution of forkhead activin signal transducer 2 (FAST2) during follicle development in mouse ovaries and pre-implantation embryos

Xenopus forkhead activin signal transducer 1 (xFAST 1) was first characterized in Xenopus as the transcriptional partner for Smad proteins. FAST2, which is the xFAST 1 homologues in mouse, is expressed during early developmental stages of the organism. However, the function of FAST2 in mouse ovaries and pre-implantation embryos is unclear. Therefore, we investigated its expression during these processes. In postnatal mice, FAST2 was expressed in oocytes and thecal cells from postnatal day (PD) 1 to PD 21. In gonadotropin-induced immature mice, FAST2 was expressed in oocytes, thecal cells and newly formed corpora lutea (CLs), but was expressed at a lower level in degenerated CLs. Similar results were observed upon western blot analyses. In meloxicam-treated immature mice, ovulation was inhibited and FAST2 was expressed in thecal cells, luteinized granulosa cells and entrapped oocytes. Immunofluorescence results showed that FAST2 was expressed in the cytoplasm and nucleus but not the nucleolus from the zygote to 8-cell embryo stage, after which it was localized to the cytoplasm of the morulae and inner cell mass of the blastocysts. Taken together, these observations suggest that FAST2 is expressed in a cell-specific manner during ovarian follicle development, ovulation, luteinization and early embryonic development, and that FAST2 might play important roles in these physiological processes.

Forkhead box transcription factors in embryonic heart development and congenital heart disease

Embryonic heart development is a very complicated process regulated precisely by a network composed of many genes and signaling pathways in time and space. Forkhead box (Fox, FOX) proteins are a family of transcription factors characterized by the presence of an evolutionary conserved "forkhead"or "winged-helix" DNA-binding domain and able to organize temporal and spatial gene expression during development. They are involved in a wide variety of cellular processes, such as cell cycle progression, proliferation, differentiation, migration, metabolism and DNA damage response. An abundance of studies in model organisms and systems has established that Foxa2, Foxc1/c2, Foxh1 and Foxm1, Foxos and Foxps are important components of the signaling pathways that instruct cardiogenesis and embryonic heart development, playing paramount roles in heart development. The previous studies also have demonstrated that mutations in some of the forkhead box genes and the aberrant expression of forkhead box gene are heavily implicated in the congenital heart disease (CHD) of humans. This review primarily focuses on the current understanding of heart development regulated by forkhead box transcription factors and molecular genetic mechanisms by which forkhead box factors modulate heart development during embryogenesis and organogenesis. This review also summarizes human CHD related mutations in forkhead box genes as well as the abnormal expression of forkhead box gene, and discusses additional possible regulatory mechanisms of the forkhead box genes during embryonic heart development that warrant further investigation.

Mowat-Wilson syndrome: neurological and molecular study in seven patients

OBJECTIVE

To present a seven-cases serie of Mowat-Wilson syndrome (MWS).

METHOD

All patients with positive mutation for the ZEB2 were evaluated by a geneticist and a neurologist, with clinical and laboratorial characterization.

RESULTS

A peculiar facies and mental retardation were present in all patients. The Denver II scale showed intense delay in all aspects, especially fine motor and adaptive. Acquired microcephaly was observed in five patients. Only one patient did not present epilepsy. Epilepsy was focal and predominating in sleep, with status epilepticus in three patients. The initial seizure was associated with fever in most patients (4/6). The EEG showed epileptic focal activity (5/7). The imaging studies revealed total agenesis (4/7) and partial agenesis of the corpus callosum (1/7).

CONCLUSION

Physicians who care for patients with mental retardation and epilepsy should be aware of SMW.

Genome-wide view of TGFβ/Foxh1 regulation of the early mesendoderm program

Nodal/TGFβ signaling regulates diverse biological responses. By combining RNA-seq on Foxh1 and Nodal signaling loss-of-function embryos with ChIP-seq of Foxh1 and Smad2/3, we report a comprehensive genome-wide interaction between Foxh1 and Smad2/3 in mediating Nodal signaling during vertebrate mesendoderm development. This study significantly increases the total number of Nodal target genes regulated by Foxh1 and Smad2/3, and reinforces the notion that Foxh1-Smad2/3-mediated Nodal signaling directly coordinates the expression of a cohort of genes involved in the control of gene transcription, signaling pathway modulation and tissue morphogenesis during gastrulation. We also show that Foxh1 may function independently of Nodal signaling, in addition to its role as a transcription factor mediating Nodal signaling via Smad2/3. Finally, we propose an evolutionarily conserved interaction between Foxh1 and PouV, a mechanism observed in Pou5f1-mediated regulation of pluripotency in human embryonic stem and epiblast cells.

Developmental enhancers are marked independently of zygotic Nodal signals in Xenopus

To determine the hierarchy of transcriptional regulation within the in vivo vertebrate embryo, we examined whether developmental enhancers were influenced by Nodal signaling during early embryogenesis in Xenopus tropicalis. We find that developmental enhancers, defined by the active enhancer chromatin marks H3K4me1 and H3K27ac, are established as early as blastula stage and that Smad2/3 only strongly associates with these regions at gastrula stages. Significantly, when we perturb Nodal signaling using the drug SB431542, most enhancers remain marked, including at genes known to be sensitive to Nodal signaling. Overall, as enhancers are in an active conformation prior to Nodal signaling and are established independently of Nodal signaling, we suggest that many developmental enhancers are marked maternally, prior to exposure to extrinsic signals.

A potential molecular pathogenesis of cardiac/laterality defects in Oculo-Facio-Cardio-Dental syndrome

Pitx2 is the last effector of the left-right (LR) cascade known to date and plays a crucial role in the patterning of LR asymmetry. In Xenopus embryos, the expression of Pitx2 gene in the left lateral plate mesoderm (LPM) is directly regulated by Xnr1 signaling, which is mediated by Smads and FoxH1. Previous studies suggest that the suppression of Pitx2 gene in the left LPM is a potential cause of cardiac/laterality defects in Oculo-Facio-Cardio-Dental (OFCD) syndrome, which is known to be caused by mutations in BCL6 co-repressor (BCOR) gene. Recently, our work has revealed that the BCL6/BCOR complex blocks Notch-dependent transcriptional activity to protect the expression of Pitx2 in the left LPM from the inhibitory activity of Notch signaling. These studies indicated that uncontrolled Notch activity in the left LPM caused by dysfunction of BCOR may result in cardiac/laterality defects of OFCD syndrome. However, this Notch-dependent inhibitory mechanism of Pitx2 gene transcription still remains unknown. Here we report that transcriptional repressor ESR1, which acts downstream of Notch signaling, inhibits the expression of Pitx2 gene by binding to a left side-specific enhancer (ASE) region in Pitx2 gene and recruiting histone deacetylase 1 (HDAC1) to this region. Once HDAC1 is tethered, histone acetyltransferase p300 is no longer recruited to the Xnr1-dependent transcriptional complex on the ASE region, leading to the suppression of Pitx2 gene in the left LPM. The study presented here uncovers the regulatory mechanism of Pitx2 gene transcription which may contribute to an understanding of pathogenesis of OFCD syndrome.

In vivo T-box transcription factor profiling reveals joint regulation of embryonic neuromesodermal bipotency

The design of effective cell replacement therapies requires detailed knowledge of how embryonic stem cells form primary tissues, such as mesoderm or neurectoderm that later become skeletal muscle or nervous system. Members of the T-box transcription factor family are key in the formation of these primary tissues, but their underlying molecular activities are poorly understood. Here, we define in vivo genome-wide regulatory inputs of the T-box proteins Brachyury, Eomesodermin, and VegT, which together maintain neuromesodermal stem cells and determine their bipotential fates in frog embryos. These T-box proteins are all recruited to the same genomic recognition sites, from where they activate genes involved in stem cell maintenance and mesoderm formation while repressing neurogenic genes. Consequently, their loss causes embryos to form an oversized neural tube with no mesodermal derivatives. This collaboration between T-box family members thus ensures the continuous formation of correctly proportioned neural and mesodermal tissues in vertebrate embryos during axial elongation.

Mouse Tenm4 is required for mesoderm induction

Background

Tenm4 is a mouse homolog of the Drosophila gene Tenascin-m (Ten-m (Odd oz)), which functions in motor neuron routing. Recently, a genome-wide association analysis for bipolar disorder identified a new susceptibility locus at TENM4 increasing the importance of understanding Tenm4. A series of Tenm4 mouse alleles showing a broad range of phenotypes were isolated after ENU mutagenesis. Here, we examine the timing and features of gastrulation failure in a loss of function allele.

Results

Embryonic mesoderm did not form in loss of function Tenm4^m1/m1 mutant embryos. Genes normally expressed in embryonic mesoderm were not expressed in the mutant, the primitive streak did not form, and markers of the anteroposterior axis were not expressed or were mislocalized. The lack of embryonic mesoderm could not be attributed to poor proliferation of the epiblast, as normal numbers of dividing cells were observed. Epiblast cells maintained expression of Pou5f1 suggesting that they remain pluripotent, but they did not have the capacity to form any germ layer derivatives in teratomas, showing that the inability to induce mesoderm is cell autonomous. Misexpression of E-cadherin and N-cadherin suggest that the embryos did not undergo an epithelial-to-mesenchymal transition. In addition, Wnt signaling did not occur in the mutants, as assessed by the TOPGAL reporter assay, while a GSK3β inhibitor partially rescued the mutant embryos, and rescued TOPGAL reporter expression.

Conclusions

These data demonstrate that Tenm4 mutants fail to form a primitive streak and to induce embryonic mesoderm. Markers of anterior posterior patterning fail to be expressed or are mislocalized. Further, Tenm4 mutants lack the ability to differentiate in a cell autonomous manner. Together, our data suggest that embryos become impaired prior to E6.5 and as a result, Wnt signaling fails to occur; however, the involvement of other signaling pathways remains to be examined.

Antagonism of Nodal signaling by BMP/Smad5 prevents ectopic primitive streak formation in the mouse amnion

The strength and spatiotemporal activity of Nodal signaling is tightly controlled in early implantation mouse embryos, including by autoregulation and feedback loops, and involves secreted and intracellular antagonists. These control mechanisms, which are established at the extra-embryonic/embryonic interfaces, are essential for anterior-posterior patterning of the epiblast and correct positioning of the primitive streak. Formation of an ectopic primitive streak, or streak expansion, has previously been reported in mutants lacking antagonists that target Nodal signaling. Here, we demonstrate that loss-of-function of a major bone morphogenetic protein (BMP) effector, Smad5, results in formation of an ectopic primitive streak-like structure in mutant amnion accompanied by ectopic Nodal expression. This suggests that BMP/Smad5 signaling contributes to negative regulation of Nodal. In cultured cells, we find that BMP-activated Smad5 antagonizes Nodal signaling by interfering with the Nodal-Smad2/4-Foxh1 autoregulatory pathway through the formation of an unusual BMP4-induced Smad complex containing Smad2 and Smad5. Quantitative expression analysis supports that ectopic Nodal expression in the Smad5 mutant amnion is induced by the Nodal autoregulatory loop and a slow positive-feedback loop. The latter involves BMP4 signaling and also induction of ectopic Wnt3. Ectopic activation of these Nodal feedback loops in the Smad5 mutant amnion results in the eventual formation of an ectopic primitive streak-like structure. We conclude that antagonism of Nodal signaling by BMP/Smad5 signaling prevents primitive streak formation in the amnion of normal mouse embryos.

Transcriptional integration of Wnt and Nodal pathways in establishment of the Spemann organizer

Signaling inputs from multiple pathways are essential for the establishment of distinct cell and tissue types in the embryo. Therefore, multiple signals must be integrated to activate gene expression and confer cell fate, but little is known about how this occurs at the level of target gene promoters. During early embryogenesis, Wnt and Nodal signals are required for formation of the Spemann organizer, which is essential for germ layer patterning and axis formation. Signaling by both Wnt and Nodal pathways is required for the expression of multiple organizer genes, suggesting that integration of these signals is required for organizer formation. Here, we demonstrate transcriptional cooperation between the Wnt and Nodal pathways in the activation of the organizer genes Goosecoid (Gsc), Cerberus (Cer), and Chordin (Chd). Combined Wnt and Nodal signaling synergistically activates transcription of these organizer genes. Effectors of both pathways occupy the Gsc, Cer and Chd promoters and effector occupancy is enhanced with active Wnt and Nodal signaling. This suggests that, at organizer gene promoters, a stable transcriptional complex containing effectors of both pathways forms in response to combined Wnt and Nodal signaling. Consistent with this idea, the histone acetyltransferase p300 is recruited to organizer promoters in a Wnt and Nodal effector-dependent manner. Taken together, these results offer a mechanism for spatial and temporal restriction of organizer gene transcription by the integration of two major signaling pathways, thus establishing the Spemann organizer domain.

Small ubiquitin-like modifier (SUMO) modification mediates function of the inhibitory domains of developmental regulators FOXC1 and FOXC2

FOXC1 and FOXC2 are forkhead transcription factors that play essential roles during development and physiology. Despite their critical role, the mechanisms that regulate the function of these factors remain poorly understood. We have identified conserved motifs within a previously defined N-terminal negative regulatory region of FOXC1/C2 that conforms to the definition of synergy control or SC motifs. Because such motifs inhibit the activity of transcription factors by serving as sites of post-translational modification by small ubiquitin-like modifier (SUMO), we have examined whether FOXC1/C2 are targets of SUMOylation and probed the functional significance of this modification. We find that endogenous FOXC1 forms modified by SUMO2/3 can be detected. Moreover, in cell culture, all three SUMO isoforms are readily conjugated to FOXC1 and FOXC2. The modification can be reconstituted in vitro with purified components and can be reversed in vitro by treatment with the SUMO protease SENP2. SUMOylation of FOXC1 and FOXC2 occurs primarily on one consensus synergy control motif with minor contributions of a second, more degenerate site. Notably, although FOXC1 is also phosphorylated at multiple sites, disruption of sites immediately downstream of the SC motifs does not influence SUMOylation. Consistent with a negative functional role, SUMOylation-deficient mutants displayed higher transcriptional activity when compared with wild type forms despite comparable protein levels and subcellular localization. Thus, the findings demonstrate that SC motifs mediate the inhibitory function of this region by serving as sites for SUMOylation and reveal a novel mechanism for acute and reversible regulation of FOXC1/C2 function.

Cell type-specific target selection by combinatorial binding of Smad2/3 proteins and hepatocyte nuclear factor 4alpha in HepG2 cells

Specific regulation of target genes by transforming growth factor-β (TGF-β) in a given cellular context is determined in part by transcription factors and cofactors that interact with the Smad complex. In this study, we determined Smad2 and Smad3 (Smad2/3) binding regions in the promoters of known genes in HepG2 hepatoblastoma cells, and we compared them with those in HaCaT epidermal keratinocytes to elucidate the mechanisms of cell type- and context-dependent regulation of transcription induced by TGF-β. Our results show that 81% of the Smad2/3 binding regions in HepG2 cells were not shared with those found in HaCaT cells. Hepatocyte nuclear factor 4α (HNF4α) is expressed in HepG2 cells but not in HaCaT cells, and the HNF4α-binding motif was identified as an enriched motif in the HepG2-specific Smad2/3 binding regions. Chromatin immunoprecipitation sequencing analysis of HNF4α binding regions under TGF-β stimulation revealed that 32.5% of the Smad2/3 binding regions overlapped HNF4α bindings. MIXL1 was identified as a new combinatorial target of HNF4α and Smad2/3, and both the HNF4α protein and its binding motif were required for the induction of MIXL1 by TGF-β in HepG2 cells. These findings generalize the importance of binding of HNF4α on Smad2/3 binding genomic regions for HepG2-specific regulation of transcription by TGF-β and suggest that certain transcription factors expressed in a cell type-specific manner play important roles in the transcription regulated by the TGF-β-Smad signaling pathway.

The zebrafish maternal-effect gene mission impossible encodes the DEAH-box helicase Dhx16 and is essential for the expression of downstream endodermal genes

Early animal embryonic development requires maternal products that drive developmental processes prior to the activation of the zygotic genome at the mid-blastula transition. During and after this transition, maternal products may continue to act within incipient zygotic developmental programs. Mechanisms that control maternally-inherited products to spatially and temporally restrict developmental responses remain poorly understood, but necessarily depend on posttranscriptional regulation. We report the functional analysis and molecular identification of the zebrafish maternal-effect gene mission impossible (mis). Our studies suggest requirements for maternally-derived mis function in events that occur during gastrulation, including cell movement and the activation of some endodermal target genes. Cell transplantation experiments show that the cell movement defect is cell autonomous. Within the endoderm induction pathway, mis is not required for the activation of early zygotic genes, but is essential to implement nodal activity downstream of casanova/sox 32 but upstream of sox17 expression. Activation of nodal signaling in blastoderm explants shows that the requirement for mis function in endoderm gene induction is independent of the underlying yolk cell. Positional cloning of mis, including genetic rescue and complementation analysis, shows that it encodes the DEAH-box RNA helicase Dhx16, shown in other systems to act in RNA regulatory processes such as splicing and translational control. Analysis of a previously identified insertional dhx16 mutation shows that the zygotic component of this gene is also essential for embryonic viability. Our studies provide a striking example of the interweaving of maternal and zygotic genetic functions during the egg-to-embryo transition. Maternal RNA helicases have long been known to be involved in the development of the animal germ line, but our findings add to growing evidence that these factors may also control specific gene expression programs in somatic tissues.

Siamois and Twin are redundant and essential in formation of the Spemann organizer

The Spemann organizer is an essential signaling center in Xenopus germ layer patterning and axis formation. Organizer formation occurs in dorsal blastomeres receiving both maternal Wnt and zygotic Nodal signals. In response to stabilized βcatenin, dorsal blastomeres express the closely related transcriptional activators, Siamois (Sia) and Twin (Twn), members of the paired homeobox family. Sia and Twn induce organizer formation and expression of organizer-specific genes, including Goosecoid (Gsc). In spite of the similarity of Sia and Twn sequence and expression pattern, it is unclear whether these factors function equivalently in promoter binding and subsequent transcriptional activation, or if Sia and Twn are required for all aspects of organizer function. Here we report that Sia and Twn activate Gsc transcription by directly binding to a conserved P3 site within the Wnt-responsive proximal element of the Gsc promoter. Sia and Twn form homodimers and heterodimers by direct homeodomain interaction and dimer forms are indistinguishable in both DNA-binding and activation functions. Sequential chromatin immunoprecipitation reveals that the endogenous Gsc promoter can be occupied by either Sia or Twn homodimers or Sia-Twn heterodimers. Knockdown of Sia and Twn together, but not individually, results in a failure of organizer gene expression and a disruption of axis formation, consistent with a redundant role for Sia and Twn in organizer formation. Furthermore, simultaneous knockdown of Sia and Twn blocks axis induction in response to ectopic Wnt signaling, demonstrating an essential role for Sia and Twn in mediating the transcriptional response to the maternal Wnt pathway. The results demonstrate the functional redundancy of Sia and Twn and their essential role in direct transcriptional responses necessary for Spemann organizer formation.

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

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

Geminin cooperates with Polycomb to restrain multi-lineage commitment in the early embryo

Transient maintenance of a pluripotent embryonic cell population followed by the onset of multi-lineage commitment is a fundamental aspect of development. However, molecular regulation of this transition is not well characterized in vivo. Here, we demonstrate that the nuclear protein Geminin is required to restrain commitment and spatially restrict mesoderm, endoderm and non-neural ectoderm to their proper locations in the Xenopus embryo. We used microarray analyses to demonstrate that Geminin overexpression represses many genes associated with cell commitment and differentiation, while elevating expression levels of genes that maintain pluripotent early and immature neurectodermal cell states. We characterized the relationship of Geminin to cell signaling and found that Geminin broadly represses Activin-, FGF- and BMP-mediated cell commitment. Conversely, Geminin knockdown enhances commitment responses to growth factor signaling and causes ectopic mesodermal, endodermal and epidermal fate commitment in the embryo. We also characterized the functional relationship of Geminin with transcription factors that had similar activities and found that Geminin represses commitment independent of Oct 4 ortholog (Oct25/60) activities, but depends upon intact Polycomb repressor function. Consistent with this, chromatin immunoprecipitation assays directed at mesodermal genes demonstrate that Geminin promotes Polycomb binding and Polycomb-mediated repressive histone modifications, while inhibiting modifications associated with gene activation. This work defines Geminin as an essential regulator of the embryonic transition from pluripotency through early multi-lineage commitment, and demonstrates that functional cooperativity between Geminin and Polycomb contributes to this process.

The role and regulation of GDF11 in Smad2 activation during tailbud formation in the Xenopus embryo

A key role for phosphorylation of Smad2 by TGFβ superfamily ligands in the axial patterning of early embryos is well established. The regulation and role of Smad2 signaling in post-neurula embryonic patterning, however, is less well understood. While a variety of TGFβ superfamily ligands are implicated in various stages of anterior-posterior patterning, the ligand GDF11 has been shown to have a particular role in post-gastrula patterning in the mouse. Mouse GDF11 is specifically localized to the developing tail and is essential for normal posterior axial patterning. Mature GDF11 ligand is inhibited by its own prodomain, and extracellular proteolysis of this prodomain is thought to be necessary for GDF11 activity. The contribution of this proteolytic regulatory mechanism to Smad activation during embryogenesis in vivo, and to the development of posterior pattern, has not been characterized. We investigate here the role of Xenopus GDF11 in the activation of Smad2 during the development of tailbud-stage embryos, and the role of this activation in larval development. We also demonstrate that the activity of BMP-1/Tolloid-like proteases is necessary for the normal GDF11-dependent activation of Smad2 phosphorylation during post-gastrula development. These data demonstrate that GDF11 has a central role in the activation of Smad2 phosphorylation in tailbud stage Xenopus embryos, and provide the first evidence that BMP-1/Tolloid-mediated prodomain cleavage is important for activation of GDF11 in vivo.

PKA-mediated stabilization of FoxH1 negatively regulates ERalpha activity

Estrogen receptor alpha (ERalpha) mediates the mitogenic effects of estrogen. ERalpha signaling regulates the normal growth and differentiation of mammary tissue, but uncontrolled ERalpha activation increases the risk to breast cancer. Estrogen binding induces ligand-dependent ERalpha activation, thereby facilitating ERalpha dimerization, promoter binding and coactivator recruitment. ERalpha can also be activated in a ligand-independent manner by many signaling pathways, including protein kinase A (PKA) signaling. However, in several ERalpha-positive breast cancer cells, PKA inhibits estrogen-dependent cell growth. FoxH1 represses the transcriptional activities of estrogen receptors and androgen receptors (AR). Interestingly, FoxH1 has been found to inhibit the PKA-induced and ligand-induced activation of AR. In the present study, we examined the effects of PKA activation on the ability of FoxH1 to represses ERalpha transcriptional activity. We found that PKA increases the protein stability of FoxH1, and that FoxH1 inhibits PKA-induced and estradiol-induced activation of an estrogen response element (ERE). Furthermore, in MCF7 cells, FoxH1 knockdown increased the PKA-induced and estradiol-induced activation of the ERE. These results suggest that PKA can negatively regulate ERalpha, at least in part, through FoxH1.

A crucial role of a high mobility group protein HMGA2 in cardiogenesis

The high mobility group (HMG) of nuclear proteins regulates expression of many genes through architectural remodelling of the chromatin structure, and formation of multiprotein complexes on promoter/enhancer regions. This leads to the active transcription of their target genes. Here we show that HMGA2, a member of the HMGA sub-family of HMG proteins, has a critical function in cardiogenesis. Overexpression of HMGA2 enhanced, whereas siRNA-mediated knockdown of HMGA2 blocked, cardiomyocyte differentiation of the embryonal carcinoma cell line P19CL6. Moreover, overexpression of a dominant-negative HMGA2 or morpholino-mediated knockdown of HMGA2 expression blocked normal heart formation in Xenopus laevis embryos, suggesting that HMGA2 has an important role in cardiogenesis both in vitro and in vivo. Mechanistically, HMGA2 associated with Smad1/4 and showed synergistic trans-activation of the gene for a cardiac transcription factor Nkx2.5; a conserved HMGA2 binding site was required for the promoter activity of Nkx2.5 gene, both in P19CL6 cells and in transgenic Xenopus embryos. Thus, HMGA2 is a positive regulator of Nkx2.5 gene expression and is essential for normal cardiac development.

Churchill regulates cell movement and mesoderm specification by repressing Nodal signaling

Background

Cell movements are essential to the determination of cell fates during development. The zinc-finger transcription factor, Churchill (ChCh) has been proposed to regulate cell fate by regulating cell movements during gastrulation in the chick. However, the mechanism of action of ChCh is not understood.

Results

We demonstrate that ChCh acts to repress the response to Nodal-related signals in zebrafish. When ChCh function is abrogated the expression of mesodermal markers is enhanced while ectodermal markers are expressed at decreased levels. In cell transplant assays, we observed that ChCh-deficient cells are more motile than wild-type cells. When placed in wild-type hosts, ChCh-deficient cells often leave the epiblast, migrate to the germ ring and are later found in mesodermal structures. We demonstrate that both movement of ChCh-compromised cells to the germ ring and acquisition of mesodermal character depend on the ability of the donor cells to respond to Nodal signals. Blocking Nodal signaling in the donor cells at the levels of Oep, Alk receptors or Fast1 inhibited migration to the germ ring and mesodermal fate change in the donor cells. We also detect additional unusual movements of transplanted ChCh-deficient cells which suggests that movement and acquisition of mesodermal character can be uncoupled. Finally, we demonstrate that ChCh is required to limit the transcriptional response to Nodal.

Conclusion

These data establish a broad role for ChCh in regulating both cell movement and Nodal signaling during early zebrafish development. We show that chch is required to limit mesodermal gene expression, inhibit Nodal-dependant movement of presumptive ectodermal cells and repress the transcriptional response to Nodal signaling. These findings reveal a dynamic role for chch in regulating cell movement and fate during early development.

An early requirement for maternal FoxH1 during zebrafish gastrulation

The Forkhead Box H1 (FoxH1) protein is a co-transcription factor recruited by phosphorylated Smad2 downstream of several TGFbetas, including Nodal-related proteins. We have reassessed the function of zebrafish FoxH1 using antisense morpholino oligonucleotides (MOs). MOs targeting translation of foxH1 disrupt embryonic epiboly movements during gastrulation and cause death on the first day of development. The FoxH1 morphant phenotype is much more severe than that of zebrafish carrying foxh1/schmalspur (sur) DNA-binding domain mutations, FoxH1 splice-blocking morphants or other Nodal pathway mutants, and it cannot be altered by concomitant perturbations in Nodal signaling. Apart from disrupting epiboly, FoxH1 MO treatment disrupts convergence and internalization movements. Late gastrula-stage FoxH1 morphants exhibit delayed mesoderm and endoderm marker gene expression and failed patterning of the central nervous system. Probing FoxH1 morphant RNA by microarray, we identified a cohort of five keratin genes--cyt1, cyt2, krt4, krt8 and krt18--that are normally transcribed in the embryo's enveloping layer (EVL) and which have significantly reduced expression in FoxH1-depleted embryos. Simultaneously disrupting these keratins with a mixture of MOs reproduces the FoxH1 morphant phenotype. Our studies thus point to an essential role for maternal FoxH1 and downstream keratins during gastrulation that is epistatic to Nodal signaling.

Regulation of the response to Nodal-mediated mesoderm induction by Xrel3

The Xenopus egg has a yolk-laden vegetal hemisphere juxtaposed to a darkly pigmented animal hemisphere. Mesoderm is derived from the marginal zone, located at the interface between the two hemispheres. The vegetal-most cells become endoderm and release TGF-beta-related factors, including the Xenopus Nodal related (Xnr) proteins, which diffuse to induce the marginal zone to form mesoderm. The remaining animal cells become ectoderm, but our understanding of the mechanisms that limit the response to induction is incomplete. In this study, we provide evidence to suggest that Xrel3, a member of the Rel/NF-kappaB family, plays a role in defining the boundary separating induced from uninduced cells by regulating Xnr-responsive gene transcription. Ectopic Xrel3 expressed in prospective mesoderm caused repression of mesoderm-specific genes resulting in loss-of-function phenotypes that were rescued by co-expression of Xnr2. Depletion of Xrel3 from embryos with antisense morpholinos increased Xnr-dependent transcription, broadened expression of the pan-mesoderm marker Xbra and sensitized animal cells to mesoderm induction by Xnr2. We propose that an additional component to the mechanism that differentiates the ectoderm from the mesoderm involves regulation of nodal-dependent gene transcription by Xrel3.

Endoglin increases eNOS expression by modulating Smad2 protein levels and Smad2-dependent TGF-beta signaling

The endothelial nitric oxide synthase (eNOS) is a critical regulator of cardiovascular homeostasis, whose dysregulation leads to different vascular pathologies. Endoglin is a component of the transforming growth factor beta (TGF-beta) receptor complex present in endothelial cells that is involved in angiogenesis, cardiovascular development, and vascular homeostasis. Haploinsufficient expression of endoglin has been shown to downregulate endothelium-derived nitric oxide in endoglin(+/-) (Eng(+/-)) mice and cultured endothelial cells. Here, we find that TGF-beta1 leads to an increased vasodilatation in Eng(+/+) mice that is severely impaired in Eng(+/-) mice, suggesting the involvement of endoglin in the TGF-beta regulated vascular homeostasis. The endoglin-dependent induction of eNOS occurs at the transcriptional level and is mediated by the type I TGF-beta receptor ALK5 and its downstream substrate Smad2. In addition, Smad2-specific signaling is upregulated in endoglin-induced endothelial cells, whereas it is downregulated upon endoglin gene suppression with small interference RNA (siRNA). The endoglin-dependent upregulation of Smad2 was confirmed using eNOS and pARE promoters, whose activities are known to be Smad2 dependent, as well as with the interference of Smad2 with siRNA, Smurf2, or a dominant negative form of Smad2. Furthermore, increased expression of endoglin in endoglin-inducible endothelial cells or in transfectants resulted in increased levels of Smad2 protein without affecting the levels of Smad2 mRNA. The increased levels of Smad2 appear to be due to a decreased ubiquitination and proteasome-dependent degradation leading to stabilization of Smad2. These results suggest that endoglin enhances Smad2 protein levels potentiating TGF-beta signaling, and leading to an increased eNOS expression in endothelial cells.

FoxD3 and Grg4 physically interact to repress transcription and induce mesoderm in Xenopus

FoxD3 is a forkhead-related transcriptional regulator that is essential for multiple developmental processes in the vertebrate embryo, including neural crest development and maintenance of mammalian stem cell lineages. Recent results demonstrate a requirement for FoxD3 in Xenopus mesodermal development. In the gastrula, FoxD3 functions as a transcriptional repressor in the Spemann organizer to maintain the expression of Nodal-related members of the transforming growth factor-beta superfamily that induce dorsal mesoderm formation. Here we report that the function of FoxD3 in mesoderm induction is dependent on the recruitment of transcriptional corepressors of the TLE/Groucho family. Structure-function analyses indicate that the transcriptional repression and mesoderm induction activities of FoxD3 are dependent on a C-terminal domain, as well as specific DNA-binding activity conferred by the forkhead domain. The C-terminal domain contains a heptapeptide similar to the eh1/GEH Groucho interaction motif. Deletion and point mutagenesis demonstrated that the FoxD3 eh1/GEH motif is required for both repression of transcription and induction of mesoderm, as well as the direct physical interaction of FoxD3 and Grg4 (Groucho-related gene-4). Consistent with a functional interaction of FoxD3 and Grg4, the transcriptional repression activity of FoxD3 is enhanced by Grg4, and reduced by Grg5, a dominant inhibitory Groucho protein. The results indicate that FoxD3 recruitment of Groucho corepressors is essential for the transcriptional repression of target genes and induction of mesoderm in Xenopus.

FoxH1 negatively modulates flk1 gene expression and vascular formation in zebrafish

Flk1 is the major receptor for VEGF on endothelial cells. During embryogenesis, flk1 is required for both vasculogenesis and angiogenesis and abnormally elevated flk1 expression is often associated with pathological conditions in adults. While the biological function of flk1 has been studied extensively, very little is known about how the flk1 gene is regulated at the transcriptional level. Our transgenic study led to the identification of a flk1 endothelial enhancer positioned approximately 5 kb upstream of the flk1 translation initiation site. Binding sites for FoxH1, scl, ets and gata factors are found in the zebrafish flk1 endothelial enhancer, as well as in upstream sequences of mouse flk1 and human kdr genes, suggesting that the regulatory machinery for flk1/kdr is conserved from fish to mammals. The roles of scl, ets and gata factors in hemangioblasts have been well defined, but the significance of FoxH1 in vessel formation has not been explored previously. Here we show that FoxH1 binds to the flk1 endothelial enhancer in vitro and functions as a repressor for flk1 transcription in cultured cells. Consistent with these findings, the expression level of flk1 is elevated in embryos lacking both maternal and zygotic FoxH1. We further show that overexpression of FoxH1 has a negative effect on vascular formation that can be counteracted by the down-regulation of smad2 activity in zebrafish embryos. Taken together, our data provide the first evidence that flk1 is a direct target of FoxH1 and that FoxH1 is involved in vessel formation in zebrafish.

Novel gene ashwin functions in Xenopus cell survival and anteroposterior patterning

The novel gene ashwin was isolated in a differential display screen for genes activated or up-regulated early in neural specification. ashwin is expressed maternally and zygotically, and it is up-regulated in the neural ectoderm after the midgastrula stage. It is expressed in the neural plate and later in the embryonic brain, eyes, and spinal cord. Overexpression of ashwin in whole embryos leads to anterior truncations and other defects. However, a second Organizer does not form, and the secondary axial structures may result from splitting of the Organizer, rather than axis duplication. Morpholino oligonucleotide-mediated reduction in ashwin expression leads to lethality or abnormalities in gastrulation, as well as significant apoptosis in midgastrula embryos. Apoptosis is also observed in midgastrula embryos overexpressing ashwin. Coexpression of ashwin with the bone morphogenetic protein-4 antagonist noggin has a synergistic effect on neural-specific gene expression in isolated animal cap ectoderm. Ashwin has no previously characterized domains, although two nuclear localization signals can be identified. Orthologues have been identified in the human, mouse, chicken, and pufferfish genomes. Our results suggest that ashwin regulates cell survival and anteroposterior patterning.

Inhibitor-resistant type I receptors reveal specific requirements for TGF-beta signaling in vivo

Activin/nodal-like TGF-beta superfamily ligands signal through the type I receptors Alk4, Alk5, and Alk7, and are responsible for mediating a number of essential processes in development. SB-431542, a chemical inhibitor of activin/nodal signaling, acts by specifically interfering with type I receptors. Here, we use inhibitor-resistant mutant receptors to examine the efficacy and specificity of SB-431542 in Xenopus and zebrafish embryos. Treatment with SB-431542 eliminates Smad2 phosphorylation in vivo and generates a phenotype very similar to those observed in genetic mutants in the nodal signaling pathway. Inhibitor-resistant Alk4 efficiently rescues Smad2 signaling, developmental phenotype, and marker gene expression after inhibitor treatment. This system was used to examine type I receptor specificity for several activin/nodal ligands. We find that Alk4 can efficiently rescue signaling by a wide range of ligands, while Alk7 can only weakly rescue signaling by the same ligands. In whole embryos, nodal signaling during gastrulation can be rescued with Alk4, but not Alk7, while Alk5 can only mediate signaling by ligands expressed later in development. The combination of the ALK inhibitor SB-431542 with inhibitor-resistant ALKs provides a powerful set of tools for examining nodal/activin signaling during embryogenesis.

A Serpin family gene, protease nexin-1 has an activity distinct from protease inhibition in early Xenopus embryos

Protease nexin-1 (PN-1)/glia-derived nexin (GDN) is a member of the Serpin (serine proteinase inhibitor) family, and can inhibit thrombin, plasmin, and plasminogen activators. PN-1 has been shown to be a neuroprotective factor in a number of assay systems, and this activity has been assumed to be a function of its protease inhibitory function. Here, we report cloning and characterization of a Xenopus orthologue of PN-1 (xPN-1). xPN-1 was isolated in a functional screen of an egg cDNA library for factors that modify early axial patterning. xPN-1 is expressed maternally through late tadpole stages, and is expressed preferentially in the notochord, the pharyngeal endoderm, the otic vesicle, and the ventral region of the brain in tailbud embryos. Over-expression of xPN-1 causes defective gastrulation, inhibits convergent extension movements in activin induced animal caps, and inhibits expression of a distinct subset of activin induced mesendodermal markers. Interestingly, expression of point or deletion mutation of the Reactive Center Loop of xPN1,which is essential for the protease inhibitory activity of all serpins, had effects on Xenopus development indistinguishable from those of wild type xPN-1. These observations suggest the possibility that xPN-1 has a novel activity in addition to its established function as an inhibitor of serine proteases.

Identification and analysis of the promoter region of the human methionine sulphoxide reductase A gene

MsrA (methionine sulphoxide reductase A) is an antioxidant repair enzyme that reduces oxidized methionine to methionine. Moreover, the oxidation of methionine residues in proteins is considered to be an important consequence of oxidative damage to cells. To understand mechanisms of human msrA gene expression and regulation, we cloned and characterized the 5' promoter region of the human msrA gene. Using 5'-RACE (rapid amplification of cDNA ends) analysis of purified mRNA from human cells, we located the transcription initiation site 59 nt upstream of the reference MsrA mRNA sequence, GenBank accession number BC 054033. The 1.3 kb of sequence located upstream of the first exon of msrA gene was placed upstream of the luciferase reporter gene in a pGL3-Basic vector and transfected into different cell lines. Sequentially smaller fragments of the msrA promoter region were generated by PCR, and expression levels were monitored from these constructs within HEK-293 and MCF7 human cell lines. Analysis of deletion constructs revealed differences in promoter activity in these cell lines. In HEK-293 cells, the promoter activity was constant from the minimal promoter region to the longest fragment obtained. On the other hand, in MCF7 cells we detected a down-regulation in the longest fragment. Mutation of a putative negative regulatory region that is located between -209 and -212 bp (the CCAA box) restored promoter activity in MCF7 cells. The location of the msrA promoter will facilitate analysis of the transcriptional regulation of this gene in a variety of pathological contexts.

Transcriptional regulation of mesendoderm formation in Xenopus

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

A Transforming Growth Factor-β Receptor–Interacting Protein Frequently Mutated in Human Ovarian Cancer

Ovarian carcinomas, particularly recurrent forms, are frequently resistant to transforming growth factor-beta (TGF-beta)-mediated growth inhibition. However, mutations in the TGF-beta receptor I and receptor II (TbetaR-I and TbetaR-II) genes have only been reported in a minority of ovarian carcinomas, suggesting that alterations in TGF-beta-signaling components may play an important role in the loss of TGF-beta responsiveness. Using laser-capture microdissection and nested reverse-transcription-PCR, we found that km23, which interacts with the TGF-beta receptor complex, is altered at a high frequency in human ovarian cancer patients. A novel form of km23, missing exon 3 (Deltaexon3-km23), was found in 2 of 19 tumor tissues from patients with ovarian cancer. In addition to this alteration, a stop codon mutation (TAA --> CAC) was detected in two patients. This alteration results in an elongated protein, encoding 107-amino-acid residues (Delta107km23), instead of the wild-type 96-amino-acid form of km23. Furthermore, five missense mutations (T38I, S55G, T56S, I89V, and V90A) were detected in four patients, providing a total alteration rate of 42.1% (8 of 19 cases) in ovarian cancer. No km23 alterations were detected in 15 normal tissues. Such a high alteration rate in ovarian cancer suggests that km23 may play an important role in either TGF-beta resistance or tumor progression in this disease. In keeping with these findings, the functional studies described herein indicate that both the Deltaexon3-km23 and S55G/I89V-km23 mutants displayed a disruption in binding to the dynein intermediate chain in vivo, suggesting a defect in cargo recruitment to the dynein motor complex. In addition, the Deltaexon3-km23 resulted in an inhibition of TGF-beta-dependent transcriptional activation of both the p3TP-lux and activin responsive element reporters. Collectively, our results suggest that km23 alterations found in ovarian cancer patients result in altered dynein motor complex formation and/or aberrant transcriptional regulation by TGF-beta.

Conditional BMP inhibition in Xenopus reveals stage-specific roles for BMPs in neural and neural crest induction

Bone morphogenetic protein (BMP) inhibition has been proposed as the primary determinant of neural cell fate in the developing Xenopus ectoderm. The evidence supporting this hypothesis comes from experiments in explanted "animal cap" ectoderm and in intact embryos using BMP antagonists that are unregulated and active well before gastrulation. While informative, these experiments cannot answer questions regarding the timing of signals and the behavior of cells in the more complex environment of the embryo. To examine the effects of BMP antagonism at defined times in intact embryos, we have generated a novel, two-component system for conditional BMP inhibition. We find that while blocking BMP signals induces ectopic neural tissue both in animal caps and in vivo, in intact embryos, it can only do so prior to late blastula stage (stage 9), well before the onset of gastrulation. Later inhibition does not induce neural identity, but does induce ectopic neural crest, suggesting that BMP antagonists play temporally distinct roles in establishing neural and neural crest identity. By combining BMP inhibition with fibroblast growth factor (FGF) activation, the neural inductive response in whole embryos is greatly enhanced and is no longer limited to pre-gastrula ectoderm. Thus, BMP inhibition during gastrulation is insufficient for neural induction in intact embryos, arguing against a BMP gradient as the sole determinant of ectodermal cell fate in the frog.

Identification of novel genes affecting mesoderm formation and morphogenesis through an enhanced large scale functional screen in Xenopus

The formation of mesoderm is an important developmental process of vertebrate embryos, which can be broken down into several steps; mesoderm induction, patterning, morphogenesis and differentiation. Although mesoderm formation in Xenopus has been intensively studied, much remains to be learned about the molecular events responsible for each of these steps. Furthermore, the interplay between mesoderm induction, patterning and morphogenesis remains obscure. Here, we describe an enhanced functional screen in Xenopus designed for large-scale identification of genes controlling mesoderm formation. In order to improve the efficiency of the screen, we used a Xenopus tropicalis unique set of cDNAs, highly enriched in full-length clones. The screening strategy incorporates two mesodermal markers, Xbra and Xmyf-5, to assay for cell fate specification and patterning, respectively. In addition we looked for phenotypes that would suggest effects in morphogenesis, such as gastrulation defects and shortened anterior-posterior axis. Out of 1728 full-length clones we isolated 82 for their ability to alter the phenotype of tadpoles and/or the expression of Xbra and Xmyf-5. Many of the clones gave rise to similar misexpression phenotypes (synphenotypes) and many of the genes within each synphenotype group appeared to be involved in similar pathways. We determined the expression pattern of the 82 genes and found that most of the genes were regionalized and expressed in mesoderm. We expect that many of the genes identified in this screen will be important in mesoderm formation.

Xenopus p21-activated kinase 5 regulates blastomeres' adhesive properties during convergent extension movements

The p21-activated kinase (PAK) proteins regulate many cellular events including cell cycle progression, cell death and survival, and cytoskeleton rearrangements. We previously identified X-PAK5 that binds the actin and microtubule networks, and could potentially regulate their coordinated dynamics during cell motility. In this study, we investigated the functional importance of this kinase during gastrulation in Xenopus. X-PAK5 is mainly expressed in regions of the embryo that undergo extensive cell movements during gastrula such as the animal hemisphere and the marginal zone. Expression of a kinase-dead mutant inhibits convergent extension movements in whole embryos and in activin-treated animal cap by modifying behavior of cells. This phenotype is rescued in embryo by adding back X-PAK5 catalytic activity. The active kinase decreases cell adhesiveness when expressed in animal hemisphere and inhibits the calcium-dependent reassociation of cells, while dead X-PAK5 kinase localizes to cell-cell junctions and increases cell adhesion. In addition, endogenous X-PAK5 colocalizes with adherens junction proteins and its activity is regulated by extracellular calcium. Taken together, our results suggest that X-PAK5 regulates convergent extension movements in vivo by modulating the calcium-mediated cell-cell adhesion.

Of Fox and Frogs: Fox (fork head/winged helix) transcription factors in Xenopus development

Transcription factors of the Fox (fork head box) family have been found in all metazoan organisms. They are characterised by an evolutionary conserved DNA-binding domain of winged helix structure. In the South African clawed frog, Xenopus laevis, more than 30 Fox genes have been found so far. This review summarises our present knowledge regarding the general structure and common features of the fork head box and will then characterise Fox genes that have been described in Xenopus. Special attention was paid to the temporal and spatial expression patterns during early embryonic development. For some of these genes, the molecular mechanisms leading to their regulation after the onset of zygotic transcription are known. We also report on functional aspects including target gene regulation, cell or tissue specification and interference with the cell cycle. Finally, Fox proteins serve as mediators of signalling pathways and they might function as checkpoint molecules for the cross-regulatory interactions of different intracellular signal transduction chains.

Integration of Smad and Forkhead Pathways in the Control of Neuroepithelial and Glioblastoma Cell Proliferation

FoxO Forkhead transcription factors are shown here to act as signal transducers at the confluence of Smad, PI3K, and FoxG1 pathways. Smad proteins activated by TGF-beta form a complex with FoxO proteins to turn on the growth inhibitory gene p21Cip1. This process is negatively controlled by the PI3K pathway, a known inhibitor of FoxO localization in the nucleus, and by the telencephalic development factor FoxG1, which we show binds to FoxO-Smad complexes and blocks p21Cip1 expression. We suggest that the activity of this network confers resistance to TGF-beta-mediated cytostasis during the development of the telencephalic neuroepithelium and in glioblastoma brain tumor cells.

Identification of a phylogenetically conserved activin-responsive enhancer in the Zic3 gene

Multiple signaling pathways are involved in the induction of the organizer, a major center controlling vertebrate body plan formation. To study these signals, we have focused on the regulation of the Zic3 gene, which codes for a zinc finger transcription factor expressed in the organizer region at the beginning of gastrulation. We searched for DNA regulatory elements in the Zic3 promoter by testing their ability to drive reporter gene expression in early embryos. By this approach, we identified an activin responsive enhancer (Zic3-ARE), which was located in the Zic3 first intron and was essential for dorsal activation of the reporter. The Zic3-ARE was stimulated by activin and Nodal ligands, but not by a dominant negative bone morphogenetic protein (BMP) receptor. The Zic3-ARE contains a repeating consensus homeodomain binding sequence, CTAATTAAA, suggesting involvement of a homeodomain transcription factor(s). Mutations in this motif abolished enhancer activity in dorsal marginal zone and its response to activin in animal pole explants. Inhibition of either Wnt/beta-catenin or activin/Nodal signaling suppressed Zic3-ARE activity in dorsal blastomeres, further illustrating the importance of these pathways in activation of organizer genes.

Molecular link in the sequential induction of the Spemann organizer: direct activation of the cerberus gene by Xlim-1, Xotx2, Mix.1, and Siamois, immediately downstream from Nodal and Wnt signaling

To elucidate the molecular basis of organizer functions in Xenopus, we sought the target genes of the LIM homeodomain protein Xlim-1, which is one of the organizer-specific transcriptional activators. We found that an activated form of Xlim-1, Xlim-1/3m, initiates ectopic expression of the head-inducing organizer factor gene cerberus in animal caps. Thus, we analyzed the cerberus promoter using reporter assays. We show that three consecutive TAAT motifs of the homeodomain-binding sites between positions -141 and -118, collectively designated the "3xTAAT element," are crucial for the response of the cerberus promoter to Xlim-1/3m, and for its activation in the dorsal region of the embryo. Because cooperative activation of the cerberus promoter by Xnr1 and Xwnt8 also requires the 3xTAAT element, we focused on homeodomain transcriptional activators downstream from either Nodal or Wnt signaling. We found that wild-type Xlim-1 synergistically activates the cerberus promoter with Mix.1 and Siamois through the 3xTAAT element, and this synergy requires the LIM domains of Xlim-1. In contrast, Xotx2 acts synergistically with Mix.1 and Siamois through the TAATCT sequence at -95. Electrophoretic mobility shift assays revealed that Xlim-1, Siamois, and Mix.1 are likely to bind as a complex, in a LIM domain-dependent manner, to the region containing the 3xTAAT element. These data suggest that cerberus is a direct target for Xlim-1, Mix.1, Siamois, and Xotx2. Therefore, we propose a model for the molecular link in the inductive sequence from the formation of the organizer to anterior neural induction.