Molecular regulation of vertebrate early endoderm development

SOX7 is an immediate-early target of VegT and regulates Nodal-related gene expression in Xenopus

In zebrafish, the divergent F-type SOX casanova acts downstream of Nodal signaling to specify endoderm. While no casanova orthologs have been identified in tetrapods, the F-type SOX, SOX7, is supplied maternally in Xenopus (Fawcett and Klymkowsky, 2004. GER 4, 29). Subsequent RT-PCR and section-based in situ hybridization analyses indicate that SOX7 mRNA is localized to the vegetal region of the blastula-stage embryo. Overexpression and maternal depletion studies reveal that the T-box transcription factor VegT, which initiates mesoendodermal differentiation, directly regulates SOX7 expression. SOX7, but not SOX17 (another F-type SOX), binds to sites within the Xnr5 promoter and SOX7, but not SOX17, induces expression of the Nodal-related genes Xnr1, Xnr2, Xnr4, Xnr5, and Xnr6, the homeodomain transcription factor Mixer, and the endodermal marker SOX17beta; both SOX7 and SOX17 induce expression of the pan-endodermal marker endodermin. SOX7's induction of Xnr expression in animal caps is independent of Mixer and Nodal signaling. In animal caps, VegT's ability to induce Mixer and Edd appears to depend upon SOX7 activity. Whole embryo experiments suggests that vegetal factors partially compensate for the absence of SOX7. Based on the antagonistic effects of SOX7 and SOX3 (Zhang et al., 2004. Dev. Biol. 273, 23) and their common binding sites in the Xnr5 promoter, we propose a model in which competitive interactions between these two proteins are involved in refining the domain of endodermal differentiation.

An endoderm-specific GATA factor gene, dGATAe, is required for the terminal differentiation of the Drosophila endoderm

GATA factors play an essential role in endodermal specification in both protostomes and deuterostomes. In Drosophila, the GATA factor gene serpent (srp) is critical for differentiation of the endoderm. However, the expression of srp disappears around stage 11, which is much earlier than overt differentiation occurs in the midgut, an entirely endodermal organ. We have identified another endoderm-specific Drosophila GATA factor gene, dGATAe. Expression of dGATAe is first detected at stage 8 in the endoderm, and its expression continues in the endodermal midgut throughout the life cycle. srp is required for expression of dGATAe, and misexpression of srp resulted in ectopic dGATAe expression. Embryos that either lacked dGATAe or were injected with double-stranded RNA (dsRNA) corresponding to dGATAe failed to express marker genes that are characteristic of differentiated midgut. Conversely, overexpression of dGATAe induced ectopic expression of endodermal markers even in the absence of srp activity. Transfection of the dGATAe cDNA also induced endodermal markers in Drosophila S2 cells. These studies provide an outline of the genetic pathway that establishes the endoderm in Drosophila. This pathway is triggered by sequential signaling through the maternal torso gene, a terminal gap gene, huckebein (hkb), and finally, two GATA factor genes, srp and dGATAe.

High-throughput functional screen of mouse gastrula cDNA libraries reveals new components of endoderm and mesoderm specification

This study describes a cross-species functional screen of mouse gastrula cDNA libraries for components of endoderm and mesoderm specification. Pools of 96 cDNAs from arrayed mouse gastrula cDNA libraries were transcribed into mRNA and injected into either the presumptive mesoderm or the ectoderm of one-cell Xenopus laevis embryos. Injected embryos were examined at gastrula stage by in situ hybridization with endoderm or mesoderm markers. Using this approach, we screened over 700 pools or approximately 60,000 cDNAs. We identified 17 unique cDNAs that function during mesoderm and/or endoderm specification and 16 that cause general morphology changes. Identified molecules fall into eight general functional groups as follows: cell cycle components (seven), transcription factors (four), extracellular secreted molecules (seven), transmembrane receptors (one), intracellular signaling components (five), microtubule components (two), metabolism molecules (three), and unknown (four). Several of the genes we identified would not have been predicted to be involved in endoderm or mesoderm specification, highlighting the usefulness of nonbiased screening approaches. This includes Otx2, which we show is a downstream target of Xsox17beta. The speed, low cost, and high efficiency of this cross-species screen makes it an ideal method for examining cDNAs from difficult-to-obtain sources. Therefore, this approach complements the current mouse molecular genetics systems and provides a powerful means for the genome-wide examination of mammalian gene function.

An integrated regulatory network controlling survival and migration in thyroid organogenesis

The thyroid gland originates from the ventral floor of the foregut as a thickening of the endodermal cell layer. The molecular mechanisms underlying the early steps of thyroid morphogenesis are not known. Gene targeting experiments have contributed to the identification of several transcription factors, in general playing a role in the proliferation, survival, and migration of the thyroid cell precursors. The experiments reported here analyze the expression of the transcription factors Titf1, Hhex, Pax8, and Foxe1 in the thyroid primordium of null mutants of each of them. We found that most of these transcription factors are linked in an integrated regulatory network, each of them controlling the presence of other members of the network. The expression of Foxe1 is regulated in an intriguing fashion as it is strongly dependent on the presence of Pax8 in thyroid precursor cells, while the expression of the same gene in the pharyngeal endoderm surrounding the primordium is dependent on Sonic hedgehog (Shh)-derived signaling. Moreover, by the generation of mouse mutants expressing Foxe1 exclusively in the thyroid primordium, we provide a better understanding of the role of Foxe1 in these cells in order to acquire the competence to migrate into the underlying mesenchyme. In conclusion, we provide the first evidence of gene expression programs, controlled by a hierarchy of transcription factors expressed in the thyroid presumptive gut domain and directing the progression of thyroid morphogenesis.

GATA4, 5 and 6 mediate TGFbeta maintenance of endodermal gene expression in Xenopus embryos

The individual contributions of the three vertebrate GATA factors to endoderm formation have been unclear. Here we detail the early expression of GATA4, 5 and 6 in presumptive endoderm in Xenopus embryos and their induction of endodermal markers in presumptive ectoderm. Induction of HNF3beta by all three GATA factors was abolished when protein synthesis was inhibited, showing that these inductions are indirect. In contrast, whereas induction of Sox17alpha and HNF1beta by GATA4 and 5 was substantially reduced when protein synthesis was inhibited, induction by GATA6 was minimally affected, suggesting that GATA6 is a direct activator of these early endodermal genes. GATA4 induced GATA6 expression in the same assay and antisense morpholino oligonucleotides (MOs), designed to knock down translation of GATA6, blocked induction of Sox17alpha and HNF1beta by GATA4, suggesting that GATA4 induces these genes via GATA6 in this assay. All three GATA factors were induced by activin, although GATA4 and 6 required lower concentrations. GATA MOs inhibited Sox17alpha and HNF1beta induction by activin at low and high concentrations in the order: GATA6>GATA4>GATA5. Together with the timing of their expression and the effects of GATA MOs in vivo, these observations identify GATA6 as the predominant GATA factor in the maintenance of endodermal gene expression by TGFbeta signaling in gastrulating embryos. In addition, examination of gene expression and morphology in later embryos, revealed GATA5 and 6 as the most critical for the development of the gut and the liver.

Sox7 plays crucial roles in parietal endoderm differentiation in F9 embryonal carcinoma cells through regulating Gata-4 and Gata-6 expression

During early rodent development, the parietal endoderm appears from an inner cell mass and produces large amounts of basement membrane components, such as laminin-1 and collagen IV. To elucidate the regulatory network for gene expression during these procedures, we constructed a series of short interfering RNA expression vectors targeted to various transcription factors, transfected them into F9 embryonal carcinoma cells, and evaluated the effects of the gene silencing on the induction of parietal endoderm differentiation and basement membrane component production by treating F9 cells with all trans-retinoic acid and dibutyryl cyclic AMP. Among the transcription factors tested, silencing of Sox7 or combined silencing of Gata-4 and Gata-6 resulted in suppression of cell shape changes and laminin-1 production, which are the hallmarks of parietal endoderm differentiation. In cells silenced for Sox7, induction of Gata-4 and Gata-6 by retinoic acid and cyclic AMP treatment was inhibited, while induction of Sox7 was not affected in cells silenced for Gata-4 and Gata-6, indicating that Sox7 is an upstream regulatory factor for these Gata factors. Nevertheless, silencing of Sox7 did not totally cancel the action of retinoic acid, since upregulation of coup-tf2, keratin 19, and retinoic acid receptor beta2 was not abolished in Sox7-silenced F9 cells. Although overexpression of Sox7 alone was insufficient to induce parietal endoderm differentiation, overexpression of Gata-4 or Gata-6 in Sox7-silenced F9 cells restored the differentiation into parietal endoderm. Sox7 is therefore required for the induction of Gata-4 and Gata-6, and the interplay among these transcription factors plays a crucial role in parietal endoderm differentiation.

Navigating the pathway from embryonic stem cells to beta cells

The compelling goal of using in vitro differentiation of stem cells to obtain replacement pancreatic beta cells that are clinically effective in treating diabetes has until now eluded researchers. This difficulty raises the question of whether more effective strategies are available. We propose that the native embryonic pathway leading to the definitive endoderm lineage, and continuing on to the endocrine pancreas, is the one most likely to succeed for the in vitro differentiation of embryonic stem cells. We question however whether gain-of-function approaches involving genes necessary for beta cell development are destined to work effectively, and suggest alternative approaches to identifying conditions sufficient for in vitro beta cell differentiation.

Conserved and divergent paths that regulate self-renewal in mouse and human embryonic stem cells

The past few years have seen remarkable progress in our understanding of embryonic stem cell (ES cell) biology. The necessity of examining human ES cells in culture, coupled with the wealth of genomic data and the multiplicity of cell lines available, has enabled researchers to identify critical conserved pathways regulating self-renewal and identify markers that tightly correlate with the ES cell state. Comparison across species has suggested additional pathways likely to be important in long-term self-renewal of ES cells including heterochronic genes, microRNAs, genes involved in telomeric regulation, and polycomb repressors. In this review, we have discussed information on molecules known to be important in ES cell self-renewal or blastocyst development and highlighted known differences between mouse and human ES cells. We suggest that several additional pathways required for self-renewal remain to be discovered and these likely include genes involved in antisense regulation, microRNAs, as well as additional global repressive pathways and novel genes. We suggest that cross species comparisons using large-scale genomic analysis tools are likely to reveal conserved and divergent paths required for ES cell self-renewal and will allow us to derive ES lines from species and strains where this has been difficult.

Thyroid development and its disorders: genetics and molecular mechanisms

Thyroid gland organogenesis results in an organ the shape, size, and position of which are largely conserved among adult individuals of the same species, thus suggesting that genetic factors must be involved in controlling these parameters. In humans, the organogenesis of the thyroid gland is often disturbed, leading to a variety of conditions, such as agenesis, ectopy, and hypoplasia, which are collectively called thyroid dysgenesis (TD). The molecular mechanisms leading to TD are largely unknown. Studies in murine models and in a few patients with dysgenesis revealed that mutations in regulatory genes expressed in the developing thyroid are responsible for this condition, thus showing that TD can be a genetic and inheritable disease. These studies open the way to a novel working hypothesis on the molecular and genetic basis of this frequent human condition and render the thyroid an important model in the understanding of molecular mechanisms regulating the size, shape, and position of organs.

Repression of nodal expression by maternal B1-type SOXs regulates germ layer formation in Xenopus and zebrafish

B1-type SOXs (SOXs 1, 2, and 3) are the most evolutionarily conserved subgroup of the SOX transcription factor family. To study their maternal functions, we used the affinity-purified antibody antiSOX3c, which inhibits the binding of Xenopus SOX3 to target DNA sequences [Development. 130(2003)5609]. The antibody also cross-reacts with zebrafish embryos. When injected into fertilized Xenopus or zebrafish eggs, antiSOX3c caused a profound gastrulation defect; this defect could be rescued by the injection of RNA encoding SOX3DeltaC-EnR, a SOX3-engrailed repression domain chimera. In antiSOX3c-injected Xenopus embryos, normal animal-vegetal patterning of mesodermal and endodermal markers was disrupted, expression domains were shifted toward the animal pole, and the levels of the endodermal markers SOX17 and endodermin increased. In Xenopus, SOX3 acts as a negative regulator of Xnr5, which encodes a nodal-related TGFbeta-family protein. Two nodal-related proteins are expressed in the early zebrafish embryo, squint and cyclops; antiSOX3c-injection leads to an increase in the level of cyclops expression. In both Xenopus and zebrafish, the antiSOX3c phenotype was rescued by the injection of RNA encoding the nodal inhibitor Cerberus-short (CerS). In Xenopus, antiSOX3c's effects on endodermin expression were suppressed by injection of RNA encoding a dominant negative version of Mixer or a morpholino against SOX17alpha2, both of which act downstream of nodal signaling in the endoderm specification pathway. Based on these data, it appears that maternal B1-type SOX functions together with the VegT/beta-catenin system to regulate nodal expression and to establish the normal pattern of germ layer formation in Xenopus. A mechanistically conserved system appears to act in a similar manner in the zebrafish.

A genetic regulatory network for Xenopus mesendoderm formation

We have constructed a genetic regulatory network (GRN) summarising the functional relationships between the transcription factors (TFs) and embryonic signals involved in Xenopus mesendoderm formation. It is supported by a relational database containing the experimental evidence and both are available in interactive form via the World Wide Web. This network highlights areas for further study and provides a framework for systematic interrogation of new data. Comparison with the equivalent network for the sea urchin identifies conserved features of the deuterostome ancestral pathway, including positive feedback loops, GATA factors, SoxB, Brachyury and a previously underemphasised role for beta-catenin. In contrast, some features central to one species have not yet been found in the other, for example, Krox and Otx in sea urchin, and Mix and Nodal in Xenopus. Such differences may represent evolved features or may eventually be resolved. For example, in Xenopus, Nodal-related genes are positively regulated by beta-catenin and at least one of them is repressed by Sox3, as is the uncharacterised early signal (ES) inducing endomesoderm in the sea urchin, suggesting that ES may be a Nodal-like TGF-beta. Wider comparisons of such networks will inform our understanding of developmental evolution.

Inhibition of FGF signaling causes expansion of the endoderm in Xenopus

Fibroblast growth factor (FGF) is established as an initiator of signaling events critical for neurogenesis and mesoderm formation during early Xenopus embryogenesis. However, less is known about the role FGF signaling plays in endoderm specification. Here, we show for the first time that endoderm-specific genes are induced when FGF signaling is blocked in animal cap explants. This block of FGF signaling is also responsible for a significant enhancement of endodermal gene expression in animal cap explants that are injected with a dominant-negative BMP-4 receptor (DNBR) RNA or treated with activin, however, neural and mesoderm gene expression is diminished. Consistent with these results, the injection of dominant-negative FGF receptor (DNFR) RNA expands endodermal cell fate boundaries while FGF treatment dramatically reduces endoderm in whole embryos. Taken together, these results indicate that inhibition of FGF signaling promotes endoderm formation, whereas the presence of active FGF signaling is necessary for neurogenesis/mesoderm formation.

brinker and optomotor-blind act coordinately to initiate development of the L5 wing vein primordium in Drosophila

The stereotyped pattern of Drosophila wing veins is determined by the action of two morphogens, Hedgehog (Hh) and Decapentaplegic (Dpp), which act sequentially to organize growth and patterning along the anterior-posterior axis of the wing primordium. An important unresolved question is how positional information established by these morphogen gradients is translated into localized development of morphological structures such as wing veins in precise locations. In the current study, we examine the mechanism by which two broadly expressed Dpp signaling target genes, optomotor-blind (omb) and brinker (brk), collaborate to initiate formation of the fifth longitudinal (L5) wing vein. omb is broadly expressed at the center of the wing disc in a pattern complementary to that of brk, which is expressed in the lateral regions of the disc and represses omb expression. We show that a border between omb and brk expression domains is necessary and sufficient for inducing L5 development in the posterior regions. Mosaic analysis indicates that brk-expressing cells produce a short-range signal that can induce vein formation in adjacent omb-expressing cells. This induction of the L5 primordium is mediated by abrupt, which is expressed in a narrow stripe of cells along the brk/omb border and plays a key role in organizing gene expression in the L5 primordium. Similarly, in the anterior region of the wing, brk helps define the position of the L2 vein in combination with another Dpp target gene, spalt. The similar mechanisms responsible for the induction of L5 and L2 development reveal how boundaries set by dosage-sensitive responses to a long-range morphogen specify distinct vein fates at precise locations.

Characterization of Brachyury genes in the dogfish S. canicula and the lamprey L. fluviatilis. Insights into gastrulation in a chondrichthyan

In order to gain insights into the evolution of gastrulation mechanisms among vertebrates, we have characterized a Brachyury-related gene in a lamprey, Lampetra fluviatilis, and in a chondrichthyan, Scyliorhinus canicula. These two genes, respectively termed LfT and ScT, share with their osteichthyan counterparts prominent expression sites in the developing notochord, the tailbud, but also a transient expression in the prechordal plate, which is likely to be ancestral among vertebrates. In addition, the lamprey LfT gene is transcribed in the endoderm of the pharyngeal arches and the epiphysis, two expression sites that have not been reported thus far in gnathostomes, and, as in the chick, in the differentiating nephrotomes. Since Brachyury expression in nascent mesoderm and endoderm is highly conserved among vertebrates as well as cephalochordates, we have used this marker to identify these cell populations during gastrulation in the dogfish. The results suggest that these cells are initially present over the whole margin of the blastoderm and are displaced during gastrulation to its posterior part, which may correspond to the site of mesoderm and endoderm internalization. These data provide the first molecular characterization of gastrulation in a chondrichthyan. They indicate that gastrulation in the dogfish and in some amniotes shares striking similarities despite the phylogenetic distance between these species. This supports the hypothesis that the extensively divergent morphologies of gastrulae among vertebrates largely result from adaptations to the presence of yolk.

Novel gene expression domains reveal early patterning of the Xenopus endoderm

The endoderm gives rise the respiratory and digestive tract epithelia as well as associated organs such as the liver, lungs and pancreas. Investigations examining the molecular basis of embryonic endodermal patterning and organogenesis have been hampered by the lack of regionally expressed molecular markers in the early endoderm. By differentially screening an arrayed cDNA library, combined with an in situ hybridization screen we identified 13 new genes regionally expressed in the early tailbud endoderm of the Xenopus embryo. The putative proteins encoded by these cDNAs include a cell surface transporter, secreted proteins, a protease, a protease inhibitor, an RNA-binding protein, a phosphatase inhibitor and several enzymes. We find that the expression of these genes falls into one of three re-occurring domains in the tailbud embryo; (1). a ventral midgut, (2). posterior to the midgut and (3). in the dorsal endoderm beneath the notochord. Several of these genes are also regionally expressed at gastrula and neurula stages and appear to mark territories that were previously only predicted by the endoderm fate map. This indicates that there is significant positional identity in the early endoderm long before stages 28-32 when regional specification of the endoderm is thought to occur. These new genes provide valuable tools for studying endodermal patterning and organogenesis in Xenopus.

Development and differentiation of the intestinal epithelium

The gastrointestinal tract develops from a simple tube to a complex organ with patterns of differentiation along four axes of asymmetry. The organ is composed of all three germ layers signaling to each other during development to form the adult structure. The gut epithelium is a constitutively developing tissue, constantly differentiating from a stem cell in a progenitor pool throughout the life of the organism. Signals from the adjacent mesoderm and between epithelial cells are required for normal orderly development/differentiation, homeostasis, and apoptosis. Embryonically important patterning factors are used during adult stages for these processes. Such critical pathways as the hedgehog, bone morphogenetic protein, Notch, Sox, and Wnt systems are used both in embryologic and adult times of gut development. We focus on and review the roles of these factors in gut epithelial cell development and differentiation.

Darmin is a novel secreted protein expressed during endoderm development in Xenopus

Endoderm development is an area of intense interest in developmental biology, but progress has been hampered by the lack of specific markers for differentiated endodermal cells. In an unbiased secretion cloning screen of Xenopus gastrula embryos we isolated a novel gene, designated Darmin. Darmin encodes a secreted protein of 56 kDa containing a peptidase M20 domain characteristic of the glutamate carboxypeptidase group of zinc metalloproteases. We also identified homologous Darmin genes in other eukaryotes and in prokaryotes suggesting that Darmin is the founding member of a family of evolutionarily conserved proteins. Xenopus Darmin showed zygotic expression in the early endoderm and later became restricted to the midgut. By secretion cloning of Xenopus cleavage-stage embryos we isolated another novel protein, designated Darmin-related (Darmin-r) due to its sequence similarity with Darmin. Darmin-r was maternally expressed and showed at later stages expression in the lens and pronephric glomus. The endoderm-specific expression of Darmin makes this gene a useful marker for the study of endoderm development.