Molecular regulation of vertebrate early endoderm development

Early immune evasion in colorectal cancer: interplay between stem cells and the tumor microenvironment

Most colorectal cancers (CRCs) are characterized by a low mutational burden and an immune-cold microenvironment, limiting the efficacy of immune checkpoint blockade (ICB) therapies. While advanced tumors exhibit diverse immune evasion mechanisms, emerging evidence suggests that aspects of immune escape arise much earlier, within precancerous lesions. In this review, we discuss how early driver mutations and epigenetic alterations contribute to the establishment of an immunosuppressive microenvironment in CRC. We also highlight the dynamic crosstalk between cancer cells, stromal niche cells, and immune cells driving immune evasion and liver metastasis. A deeper understanding of these early events may guide the development of more effective preventive and therapeutic strategies for CRC.

SOX17 enables immune evasion of early colorectal adenomas and cancers

A hallmark of cancer is the avoidance of immune destruction. This process has been primarily investigated in locally advanced or metastatic cancer1-3; however, much less is known about how pre-malignant or early invasive tumours evade immune detection. Here, to understand this process in early colorectal cancers (CRCs), we investigated how naive colon cancer organoids that were engineered in vitro to harbour Apc-null, KrasG12D and Trp53-null (AKP) mutations adapted to the in vivo native colonic environment. Comprehensive transcriptomic and chromatin analyses revealed that the endoderm-specifying transcription factor SOX17 became strongly upregulated in vivo. Notably, whereas SOX17 loss did not affect AKP organoid propagation in vitro, its loss markedly reduced the ability of AKP tumours to persist in vivo. The small fraction of SOX17-null tumours that grew displayed notable interferon-γ (IFNγ)-producing effector-like CD8+ T cell infiltrates in contrast to the immune-suppressive microenvironment in wild-type counterparts. Mechanistically, in both endogenous Apc-null pre-malignant adenomas and transplanted organoid-derived AKP CRCs, SOX17 suppresses the ability of tumour cells to sense and respond to IFNγ, preventing anti-tumour T cell responses. Finally, SOX17 engages a fetal intestinal programme that drives differentiation away from LGR5+ tumour cells to produce immune-evasive LGR5- tumour cells with lower expression of major histocompatibility complex class I (MHC-I). We propose that SOX17 is a transcription factor that is engaged during the early steps of colon cancer to orchestrate an immune-evasive programme that permits CRC initiation and progression.

Lineage-specific, fast-evolving GATA-like gene regulates zygotic gene activation to promote endoderm specification and pattern formation in the Theridiidae spider

Background

The process of early development varies across the species-rich phylum Arthropoda. Owing to the limited research strategies for dissecting lineage-specific processes of development in arthropods, little is known about the variations in early arthropod development at molecular resolution. The Theridiidae spider, Parasteatoda tepidariorum, has its genome sequenced and could potentially contribute to dissecting early embryonic processes.

Results

We present genome-wide identification of candidate genes that exhibit locally restricted expression in germ disc forming stage embryos of P. tepidariorum, based on comparative transcriptomes of isolated cells from different regions of the embryo. A subsequent pilot screen by parental RNA interference identifies three genes required for body axis formation. One of them is a GATA-like gene that has been fast evolving after duplication and divergence from a canonical GATA family gene. This gene is designated fuchi nashi (fuchi) after its knockdown phenotypes, where the cell movement toward the formation of a germ disc was reversed. fuchi expression occurs in cells outside a forming germ disc and persists in the endoderm. Transcriptome and chromatin accessibility analyses of fuchi pRNAi embryos suggest that early fuchi activity regulates chromatin state and zygotic gene activation to promote endoderm specification and pattern formation. We also show that there are many uncharacterized genes regulated by fuchi.

Conclusions

Our genome-based research using an arthropod phylogenetically distant from Drosophila identifies a lineage-specific, fast-evolving gene with key developmental roles in one of the earliest, genome-wide regulatory events, and allows for molecular exploration of the developmental variations in early arthropod embryos.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01421-0.

Reprint of: Conditional specification of endomesoderm

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

Expression of Piwi, MMP, TIMP, and Sox during Gut Regeneration in Holothurian Eupentacta fraudatrix (Holothuroidea, Dendrochirotida)

Mesodermal cells of holothurian Eupentacta fraudatrix can transdifferentiate into enterocytes during the regeneration of the digestive system. In this study, we investigated the expression of several genes involved in gut regeneration in E. fraudatrix. Moreover, the localization of progenitor cells of coelomocytes, juvenile cells, and their participation in the formation of the luminal epithelium of the digestive tube were studied. It was shown that Piwi-positive cells were not involved in the formation of the luminal epithelium of the digestive tube. Ef-72 kDa type IV collagenase and Ef-MMP16 had an individual expression profile and possibly different functions. The Ef-tensilin3 gene exhibited the highest expression and indicates its potential role in regeneration. Ef-Sox9/10 and Ef-Sox17 in E. fraudatrix may participate in the mechanism of transdifferentiation of coelomic epithelial cells. Their transcripts mark the cells that plunge into the connective tissue of the gut anlage and give rise to enterocytes. Ef-Sox9/10 probably controls the switching of mesodermal cells to the enterocyte phenotype, while Ef-Sox17 may be involved in the regulation of the initial stages of transdifferentiation.

Conditional specification of endomesoderm

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

Molecular Aspects of Regeneration Mechanisms in Holothurians

Holothurians, or sea cucumbers, belong to the phylum Echinodermata. They show good regenerative abilities. The present review provides an analysis of available data on the molecular aspects of regeneration mechanisms in holothurians. The genes and signaling pathways activated during the asexual reproduction and the formation of the anterior and posterior parts of the body, as well as the molecular mechanisms that provide regeneration of the nervous and digestive systems, are considered here. Damage causes a strong stress response, the signs of which are recorded even at late regeneration stages. In holothurian tissues, the concentrations of reactive oxygen species and antioxidant enzymes increase. Furthermore, the cellular and humoral components of the immune system are activated. Extracellular matrix remodeling and Wnt signaling play a major role in the regeneration in holothurians. All available morphological and molecular data show that the dedifferentiation of specialized cells in the remnant of the organ and the epithelial morphogenesis constitute the basis of regeneration in holothurians. However, depending on the type of damage, the mechanisms of regeneration may differ significantly in the spatial organization of regeneration process, the involvement of different cell types, and the depth of reprogramming of their genome (dedifferentiation or transdifferentiation).

Gene- and tissue-level interactions in normal gastrointestinal development and Hirschsprung disease

The development of the gut from endodermal tissue to an organ with multiple distinct structures and functions occurs over a prolonged time during embryonic days E10.5-E14.5 in the mouse. During this process, one major event is innervation of the gut by enteric neural crest cells (ENCCs) to establish the enteric nervous system (ENS). To understand the molecular processes underpinning gut and ENS development, we generated RNA-sequencing profiles from wild-type mouse guts at E10.5, E12.5, and E14.5 from both sexes. We also generated these profiles from homozygous Ret null embryos, a model for Hirschsprung disease (HSCR), in which the ENS is absent. These data reveal 4 major features: 1) between E10.5 and E14.5 the developmental genetic programs change from expression of major transcription factors and its modifiers to genes controlling tissue (epithelium, muscle, endothelium) specialization; 2) the major effect of Ret is not only on ENCC differentiation to enteric neurons but also on the enteric mesenchyme and epithelium; 3) a muscle genetic program exerts significant effects on ENS development; and 4) sex differences in gut development profiles are minor. The genetic programs identified, and their changes across development, suggest that both cell autonomous and nonautonomous factors, and interactions between the different developing gut tissues, are important for normal ENS development and its disorders.

Nodal signaling has dual roles in fate specification and directed migration during germ layer segregation in zebrafish

During gastrulation, endodermal cells actively migrate to the interior of the embryo, but the signals that initiate and coordinate this migration are poorly understood. By transplanting ectopically induced endodermal cells far from the normal location of endoderm specification, we identified the inputs that drive internalization without the confounding influences of fate specification and global morphogenic movements. We find that Nodal signaling triggers an autocrine circuit for initiating endodermal internalization. Activation of the Nodal receptor directs endodermal specification through sox32 and also induces expression of more Nodal ligands. These ligands act in an autocrine fashion to initiate endodermal cell sorting. Our work defines an 'AND' gate consisting of sox32-dependent endodermal specification and Nodal ligand reception controlling endodermal cell sorting to the inner layer of the embryo at the onset of gastrulation.

Molecular mechanisms of fission in echinoderms: Transcriptome analysis

Echinoderms are capable of asexual reproduction by fission. An individual divides into parts due to changes in the strength of connective tissue of the body wall. The structure of connective tissue and the mechanisms of variations in its strength in echinoderms remain poorly studied. An analysis of transcriptomes of individuals during the process of fission provides a new opportunity to understand the mechanisms of connective tissue mutability. In the holothurian Cladolabes schmeltzii, we have found a rather complex organization of connective tissue. Transcripts of genes encoding a wide range of structural proteins of extracellular matrix, as well as various proteases and their inhibitors, have been discovered. All these molecules may constitute a part of the mechanism of connective tissue mutability. According to our data, the extracellular matrix of echinoderms is substantially distinguished from that of vertebrates by the lack of elastin, fibronectins, and tenascins. In case of fission, a large number of genes of transcription factors and components of different signaling pathways are expressed. Products of these genes are probably involved in regulation of asexual reproduction, connective tissue mutability, and preparation of tissues for subsequent regeneration. It has been shown that holothurian tensilins are a special group of tissue inhibitors of metalloproteinases, which has formed within the class Holothuroidea and is absent from other echinoderms. Our data can serve a basis for the further study of the mechanisms of extracellular matrix mutability, as well as the mechanisms responsible for asexual reproduction in echinoderms.

Role of GATA factors in development, differentiation, and homeostasis of the small intestinal epithelium

The small intestinal epithelium develops from embryonic endoderm into a highly specialized layer of cells perfectly suited for the digestion and absorption of nutrients. The development, differentiation, and regeneration of the small intestinal epithelium require complex gene regulatory networks involving multiple context-specific transcription factors. The evolutionarily conserved GATA family of transcription factors, well known for its role in hematopoiesis, is essential for the development of endoderm during embryogenesis and the renewal of the differentiated epithelium in the mature gut. We review the role of GATA factors in the evolution and development of endoderm and summarize our current understanding of the function of GATA factors in the mature small intestine. We offer perspective on the application of epigenetics approaches to define the mechanisms underlying context-specific GATA gene regulation during intestinal development.

Klf4 is required for germ-layer differentiation and body axis patterning during Xenopus embryogenesis

Klf4 is a transcription factor of the family of Kruppel-like factors and plays important roles in stem cell biology; however, its function during embryogenesis is unknown. Here, we report the characterization of a Klf4 homologue in Xenopus laevis during embryogenesis. Klf4 is transcribed both maternally and zygotically and the transcript is ubiquitous in embryos during germ-layer formation. Klf4 promotes endoderm differentiation in both Nodal/Activin-dependent and -independent manners. Moreover, Klf4 regulates anteroposterior body axis patterning via activation of a subset of genes in the Spemann organizer, such as Noggin, Dkk1 and Cerberus, which encode Nodal, Wnt and BMP antagonists. Loss of Klf4 function leads to the failure of germ-layer differentiation, the loss of responsiveness of early embryonic cells to inducing signals, e.g. Nodal/Activin, and the loss of transcription of genes involved in axis patterning. We conclude that Klf4 is required for germ-layer differentiation and body axis patterning by means of rendering early embryonic cells competent to differentiation signals.

Celf1 regulation of dmrt2a is required for somite symmetry and left-right patterning during zebrafish development

RNA-binding proteins (RBPs) bind to numerous and diverse mRNAs to control gene expression post-transcriptionally, although the in vivo functions of specific RBP-mRNA interactions remain largely unknown. Here, we show that an RBP named Cugbp, Elav-like family member 1 (Celf1) controls expression of a gene named doublesex and mab-3 related transcription factor 2a (dmrt2a), which is essential for somite symmetry and left-right patterning during zebrafish development. Celf1 promotes dmrt2a mRNA decay by binding to UGU repeats in the 3'UTR of dmrt2a mRNA such that celf1 overexpression reduces the amount of dmrt2a mRNA, leading to asymmetric somitogenesis and laterality defects. Furthermore, blocking the Celf1-dmrt2a mRNA interaction by a target protector morpholino alleviates failures in somite symmetry and left-right patterning that are caused by celf1 overexpression. Our results therefore demonstrate that Celf1-dependent fine-tuning of dmrt2a expression is essential for generating bilateral symmetry of somites and left-right asymmetric patterning during zebrafish development.

Targeting SOX17 in human embryonic stem cells creates unique strategies for isolating and analyzing developing endoderm

Human embryonic stem cells (hESCs) can provide insights into development of inaccessible human tissues such as embryonic endoderm. Progress in this area has been hindered by a lack of methods for isolating endodermal cells and tracing fates of their differentiated progeny. By using homologous recombination in human ESCs, we inserted an enhanced green fluorescent protein (eGFP) transgene into the SOX17 locus, a postulated marker of human endoderm. FACS purification and gene expression profiling confirmed that SOX17(+)-hESC progeny expressed endodermal markers and unveiled specific cell surface protein combinations that permitted FACS-based isolation of primitive gut tube endodermal cells produced from unmodified human ESCs and from induced pluripotent stem cells (iPSC). Differentiating SOX17(+) endodermal cells expressed markers of liver, pancreas, and intestinal epithelium in vitro and gave rise to endodermal progeny in vivo. Thus, prospective isolation, lineage tracing, and developmental studies of SOX17(+) hESC progeny have revealed fundamental aspects of human endodermal biology.

Regulation of endoderm formation and left-right asymmetry by miR-92 during early zebrafish development

microRNAs (miRNAs) are a family of 21-23 nucleotide endogenous non-coding RNAs that post-transcriptionally regulate gene expression in a sequence-specific manner. Typically, miRNAs downregulate target genes by recognizing and recruiting protein complexes to 3'UTRs, followed by translation repression or mRNA degradation. miR-92 is a well-studied oncogene in mammalian systems. Here, using zebrafish as a model system, we uncovered a novel tissue-inductive role for miR-92 during early vertebrate development. Overexpression resulted in reduced endoderm formation during gastrulation with consequent cardia and viscera bifida. By contrast, depletion of miR-92 increased endoderm formation, which led to abnormal Kupffer's vesicle development and left-right patterning defects. Using target prediction algorithms and reporter constructs, we show that gata5 is a target of miR-92. Alteration of gata5 levels reciprocally mirrored the effects of gain and loss of function of miR-92. Moreover, genetic epistasis experiments showed that miR-92-mediated defects could be substantially suppressed by modulating gata5 levels. We propose that miR-92 is a critical regulator of endoderm formation and left-right asymmetry during early zebrafish development and provide the first evidence for a regulatory function for gata5 in the formation of Kupffer's vesicle and left-right patterning.

Loss of intestinal GATA4 prevents diet-induced obesity and promotes insulin sensitivity in mice

Transcriptional regulation of small intestinal gene expression controls plasma total cholesterol (TC) and triglyceride (TG) levels, which are major determinants of metabolic diseases. GATA4, a zinc finger domain transcription factor, is critical for jejunal identity, and intestinal GATA4 deficiency leads to a jejunoileal transition. Although intestinal GATA4 ablation is known to misregulate jejunal gene expression, its pathophysiological impact on various components of metabolic syndrome remains unknown. Here, we used intestine-specific GATA4 knockout (GATA4iKO) mice to dissect the contribution of GATA4 on obesity development. We challenged adult GATA4iKO mice and control littermates with a Western-type diet (WTD) for 20 wk. Our findings show that WTD-fed GATA4iKO mice are resistant to diet-induced obesity. Accordingly, plasma TG and TC levels are markedly decreased. Intestinal lipid absorption in GATA4iKO mice was strongly reduced, whereas luminal lipolysis was unaffected. GATA4iKO mice displayed a greater glucagon-like peptide-1 (GLP-1) release on normal chow and even after long-term challenge with WTD remained glucose sensitive. In summary, our findings show that the absence of intestinal GATA4 has a beneficial effect on decreasing intestinal lipid absorption causing resistance to hyperlipidemia and obesity. In addition, we show that increased GLP-1 release in GATA4iKO mice decreases the risk for development of insulin resistance.

NO points to epigenetics in vascular development

Our understanding of epigenetic mechanisms important for embryonic vascular development and cardiovascular differentiation is still in its infancy. Although molecular analyses, including massive genome sequencing and/or in vitro/in vivo targeting of specific gene sets, has led to the identification of multiple factors involved in stemness maintenance or in the early processes of embryonic layers specification, very little is known about the epigenetic commitment to cardiovascular lineages. The object of this review will be to outline the state of the art in this field and trace the perspective therapeutic consequences of studies aimed at elucidating fundamental epigenetic networks. Special attention will be paid to the emerging role of nitric oxide in this field.

Endoderm development in Caenorhabditis elegans: the synergistic action of ELT-2 and -7 mediates the specification→differentiation transition

The transition from specification of cell identity to the differentiation of cells into an appropriate and enduring state is critical to the development of embryos. Transcriptional profiling in Caenorhabditis elegans has revealed a large number of genes that are expressed in the fully differentiated intestine; however, no regulatory factor has been found to be essential to initiate their expression once the endoderm has been specified. These gut-expressed genes possess a preponderance of GATA factor binding sites and one GATA factor, ELT-2, fulfills the expected characteristics of a key regulator of these genes based on its persistent expression exclusively in the developing and differentiated intestine and its ability to bind these regulatory sites. However, a striking characteristic of elt-2(0) knockout mutants is that while they die shortly after hatching owing to an obstructed gut passage, they nevertheless contain a gut that has undergone complete morphological differentiation. We have discovered a second gut-specific GATA factor, ELT-7, that profoundly synergizes with ELT-2 to create a transcriptional switch essential for gut cell differentiation. ELT-7 is first expressed in the early endoderm lineage and, when expressed ectopically, is sufficient to activate gut differentiation in nonendodermal progenitors. elt-7 is transcriptionally activated by the redundant endoderm-specifying factors END-1 and -3, and its product in turn activates both its own expression and that of elt-2, constituting an apparent positive feedback system. While elt-7 loss-of-function mutants lack a discernible phenotype, simultaneous loss of both elt-7 and elt-2 results in a striking all-or-none block to morphological differentiation of groups of gut cells with a region-specific bias, as well as reduced or abolished gut-specific expression of a number of terminal differentiation genes. ELT-2 and -7 synergize not only in activation of gene expression but also in repression of a gene that is normally expressed in the valve cells, which immediately flank the termini of the gut tube. Our results point to a developmental strategy whereby positive feedback and cross-regulatory interactions between two synergistically acting regulatory factors promote a decisive and persistent transition of specified endoderm progenitors into the program of intestinal differentiation.

Delta-Notch signaling is involved in the segregation of the three germ layers in Xenopus laevis

In vertebrates, the induction of the three germ layers (ectoderm, mesoderm and endoderm) has been extensively studied, but less is known about how they segregate. Here, we investigated whether Delta-Notch signaling is involved in this process. Activating the pathway in the marginal zone with Notch(ICD) resulted in an expansion of endodermal and neural ectoderm precursors, leaving a thinner mesodermal ring around the blastopore at gastrula stage, when germ layers are segregated. On the other hand, when the pathway was blocked with Delta-1(STU) or with an antisense morpholino oligonucleotide against Notch, the pan-mesodermal brachyury (bra) domain was expanded and the neural border was moved animalwards. Strikingly, the suprablastoporal endoderm was either expanded when Delta-1 signaling was blocked, or reduced after the general knock-down of Notch. In addition, either activating or blocking the pathway delays the blastopore closure. We conclude that the process of delimiting the three germ layers requires Notch signaling, which may be finely regulated by ligands and/or involve non-canonical components of the pathway. Moreover, Notch activity must be modulated at appropriate levels during this process in order to keep normal morphogenetic movements during gastrulation.

CDK9/cyclin complexes modulate endoderm induction by direct interaction with Mix.3/mixer

Mix-related homeodomain proteins are involved in endoderm formation in the early vertebrate embryo. We used a yeast two-hybrid screen to identify proteins that interact with Mix.3/mixer to regulate endoderm induction. We demonstrate that cyclin-dependent kinase 9 (CDK9) interacts with the carboxyl terminal domain of Mix.3. CDK9 is the catalytic subunit of the PTEF-b transcription elongation complex that phosphorylates the C-terminal domain of RNA polymerase II to promote efficient elongation of nascent transcripts. Using whole embryo transcription reporter and animal pole explant assays, we show that Mix.3 activity is regulated by CDK9/cyclin complexes. Co-expression of cyclin T2 and cyclin K had different effects on Mix.3 transcriptional activity and endoderm induction. Our data suggest that binding of CDK9, and the recruitment of different cyclin partners, can modulate the endoderm-inducing activity of Mix.3 during embryonic development. Developmental Dynamics 238:1346-1357, 2009. (c) 2009 Wiley-Liss, Inc.

Sox17 facilitates the differentiation of mouse embryonic stem cells into primitive and definitive endoderm in vitro

The Sox family of HMG (high mobility group)-box transcription factors are highly conserved in vertebrates. Sox members are involved in various developmental processes. Among them Sox17 has been demonstrated to function as an endoderm determinant in zebrafish and Xenopus, respectively. However, little is known about the role of Sox17 in mouse embryonic stem cell (ESC) differentiation. In our research, we investigated the effect of Sox17 on mouse ESC and embryoid body (EB) differentiation. The results demonstrated that Sox17 overexpression upregulated a set of endoderm-specific gene markers, suggesting that Sox17 overexpression induced an ESC differentiation program towards both primitive and definitive endoderm. We believe this finding brings new insights into the understanding of ESC differentiation and the organogenesis of endodermal derivatives.

Bmp signaling is necessary and sufficient for ventrolateral endoderm specification in Xenopus

Here we show that Bmp signaling is necessary and sufficient for the specification of ventral endoderm in Xenopus embryos. Overexpression of Bmp4 in ectoderm induces markers of endoderm, including Sox17beta, Mixer, and VegT, but cannot induce the expression of the dorsoanterior markers, Xhex and Cerberus. Furthermore, knockdown approaches using overexpression of Bmp antagonists and morpholinos designed against Bmp4, Bmp2, and Bmp7 demonstrate that Bmp signaling is critical for ventral, but not dorsoanterior endoderm formation. This activity is not simply a result of embryonic dorsalization as markers for dorsal endoderm are not expanded. We further show that endodermal cells of either ventral or dorsal character do not form when both Wnt and Bmp signals are abolished. Overall, this report strongly suggests that Bmp plays an essential role in ventral endoderm specification.

SOX17 directly activatesZfp202transcription during in vitro endoderm differentiation

SOX17 is a SRY-related high-mobility group (HMG) box transcription factor that is necessary for endoderm formation in multiple species. Despite its essential function during endoderm formation and differentiation, few direct targets of SOX17 are known. To identify targets of SOX17, we isolated SOX17 binding sites with a chromatin immunoprecipitation (ChIP)-cloning screen. SOX17-ChIP identified zinc finger protein 202 ( Zfp202) as a direct target of SOX17 during endoderm differentiation of F9 embryonal carcinoma cells. A sequence in the first intron of Zfp202 activated transcription in differentiated F9 cells, and overexpression of Sox17 increased the transcriptional activity of this sequence. SOX17 binds to a site within this sequence in electrophoretic mobility shift assays, and mutation of this site decreases the transcriptional activation. Zfp202 is induced concomitantly with Sox17 during endoderm differentiation of F9 cells. We also show that ZFP202 represses Hnf4a, which has been reported for the human ortholog ZNF202. Identifying targets of SOX17 will help to elucidate the molecular basis of endoderm differentiation and may provide a better understanding of the role of endoderm in patterning the other germ layers.

Activin alters the kinetics of endoderm induction in embryonic stem cells cultured on collagen gels

Embryonic stem cell-derived endoderm is critical for the development of cellular therapies for the treatment of disease such as diabetes, liver cirrhosis, or pulmonary emphysema. Here, we describe a novel approach to induce endoderm from mouse embryonic stem (mES) cells using fibronectin-coated collagen gels. This technique results in a homogeneous endoderm-like cell population, demonstrating endoderm-specific gene and protein expression, which remains committed following in vivo transplantation. In this system, activin, normally an endoderm inducer, caused an 80% decrease in the Foxa2-positive endoderm fraction, whereas follistatin increased the Foxa2-positive endoderm fraction to 78%. Our work suggests that activin delays the induction of endoderm through its transient precursors, the epiblast and mesendoderm. Long-term differentiation displays a twofold reduction in hepatic gene expression and threefold reduction in hepatic protein expression of activin-treated cells compared with follistatin-treated cells. Moreover, subcutaneous transplantation of activin-treated cells in a syngeneic mouse generated a heterogeneous teratoma-like mass, suggesting that these were a more primitive population. In contrast, follistatin-treated cells resulted in an encapsulated epithelial-like mass, suggesting that these cells remained committed to the endoderm lineage. In conclusion, we demonstrate a novel technique to induce the direct differentiation of endoderm from mES cells without cell sorting. In addition, our work suggests a new role for activin in induction of the precursors to endoderm and a new endoderm-enrichment technique using follistatin.

Zebrafish cdx1b regulates expression of downstream factors of Nodal signaling during early endoderm formation

We identified a zebrafish caudal-related homeobox (cdx1b) gene, which shares syntenic conservation with both human and mouse Cdx1. Zebrafish cdx1b transcripts are maternally deposited. cdx1b is uniformly expressed in both epiblast and hypoblast cells from late gastrulation to the 1-2s stages and can be identified in the retinas, brain and somites during 18-22 hpf stages. After 28 hours of development, cdx1b is exclusively expressed in the developing intestine. Both antisense morpholino oligonucleotide-mediated knockdown and overexpression experiments were conducted to analyze cdx1b function. Hypoplastic development of the liver and pancreas and intestinal abnormalities were observed in 96 hpf cdx1b morphants. In 85% epiboly cdx1b morphants, twofold decreases in the respective numbers of gata5-, cas-, foxa2- and sox17-expressing endodermal precursors were identified. Furthermore, ectopic cdx1b expression caused substantial increases in the respective numbers of gata5-, cas-, foxa2- and sox17-expressing endodermal precursors and altered their distribution patterns in 85% epiboly injected embryos. Conserved Cdx1-binding motifs were identified in both gata5 and foxa2 genes by interspecific sequence comparisons. Cdx1b can bind to the Cdx1-binding motif located in intron 1 of the foxa2 gene based on an electrophoretic mobility shift assay. Co-injection of either zebrafish or mouse foxa2 mRNA with the cdx1b MO rescued the expression domains of ceruloplasmin in the liver of 53 hpf injected embryos. These results indicate that zebrafish cdx1b regulates foxa2 expression and may also modulate gata5 expression, thus affecting early endoderm formation. This study underscores a novel role of zebrafish cdx1b in the development of different digestive organs compared with its mammalian homologs.

GATA factors induce mouse embryonic stem cell differentiation toward extraembryonic endoderm

The GATA family of transcription factors are implicated in early embryonic development. There are six factors in this family in vertebrates. GATA4 and GATA6 have been demonstrated to induce mouse embryonic stem (mES) cells differentiation toward extraembryonic endoderm (ExE). We investigated the effect of GATA3 on the differentiation of mES cells both in the ES cell and in the embryoid body (EB) states. The results demonstrate that GATA3 overexpression can initiate the ES cell differentiation program toward ExE. Furthermore, overexpression of GATA1 and GATA2 in ES cells and EBs resulted in similar effects. We believe this finding can augment our understanding of mouse ES cell differentiation.

Specification of germ layer identity in the chick gastrula

BACKGROUND

Chick definitive endoderm is an important source of signals that pattern the early embryo forming a central structure around which the body plan is constructed. Although the origin of definitive endoderm has been mapped in the chick, arising principally from rostral streak at elongating streak stages, it is not known when this layer first becomes fully committed to its germ layer fate, an important issue to resolve in light of its critical role in subsequent patterning of the early embryo.

RESULTS

Through gene expression screening of chick gastrula, we identified molecular markers of definitive endoderm restricted to rostral (Sox17) and caudal (Gata5/6) regions, suggesting that at least two subpopulations of definitive endodermal cells exist during ingression. We show (1) that presumptive mesoderm cells migrate to the middle layer and remain mesenchymal when transplanted to rostral primitive streak, and prospective endoderm cells enter the lower layer and become epithelial when transplanted to caudal primitive streak; and (2) that presumptive endoderm cells and mesoderm cells lose normal gene expression (Sox17 and Wnt8c, respectively) when transplanted outside of their normal position of origin. Moreover, when rostral or caudal primitive streak segments are transplanted into rostral blastoderm isolates (RBIs), both types of transplants express Sox17 4-6 hours later--consistent with their new position, regardless of their presumptive germ layer origin--and prospective mesoderm transplants, which normally express Wnt8c, turn off expression, suggesting that signals within the rostral blastoderm induce endoderm gene expression, and repress mesoderm gene expression, during gastrulation.

CONCLUSION

Our results demonstrate that germ layer identity is fixed at the time populations of endoderm and mesoderm cells ingress through the primitive streak, whereas their gene expression patterns remain labile. In addition, our results show that inductive and repressive signals are present, and that these signals regulate gene expression of both ingressed endoderm and mesoderm cells. Thus, gastrula cells display elements of both pre-patterning and plasticity, with endoderm the first germ layer becoming committed to its fate during early gastrulation stages.

The Sox axis, Nodal signaling, and germ layer specification

Asymmetries in the egg, established during oogenesis, set the stage for a cascade of intercellular signaling events leading to differential gene expression and subsequent tissue and organ formation. Maternally supplied Sox-type transcription factors have recently emerged as key components in the patterning of the early embryo and the regulation of embryonic stem cell differentiation. In deuterostomes, B1-type Soxs are asymmetrically localized to the future animal/ectodermal region where they act to suppress mesendodermal, and favor neuroectodermal differentiation, while vegetally localized F-type Soxs are involved in mesendodermal differentiation. Here, we review past observations and present new data from studies on the clawed frog Xenopus laevis. Animally localized Sox3 acts to inhibit Nodal (Xnr5 and Xnr6) expression, and induces the expression of genes (Ectodermin, Xema, and Coco) whose products repress Nodal signaling. Vegetally localized Sox7 positively regulates Nodal (Xnr4, Xnr5, and Xnr6) expression, as well as the expression of genes involved in mesodermal (Xmenf, Slug, and Snail) and endodermal (Endodermin and Sox17beta) differentiation. Given the evolutionary strategy of using common regulatory networks, it seems likely that a homologous Sox-Axis is active during embryonic development in many metazoans.

POU-V factors antagonize maternal VegT activity and beta-Catenin signaling in Xenopus embryos

VegT and beta-Catenin are key players in the hierarchy of factors that are required for induction and patterning of mesendoderm in Xenopus embryogenesis. By descending the genetic cascades, cells lose their pluripotent status and are determined to differentiate into distinct tissues. Mammalian Oct-3/4, a POU factor of subclass V (POU-V), is required for the maintenance of pluripotency of embryonic stem cells. However, its molecular function within the early embryo is yet poorly understood. We here show that the two maternal Xenopus POU-V factors, Oct-60 and Oct-25, inhibit transcription of genes activated by VegT and beta-Catenin. Maternal POU-V factors and maternal VegT show an opposite distribution along the animal/vegetal axis. Oct-25, VegT and Tcf3 interact with each other and form repression complexes on promoters of VegT and beta-Catenin target genes. We suggest that POU-V factors antagonize primary inducers to allow germ layer specification in a temporally and spatially coordinated manner.

Prospective isolation and global gene expression analysis of definitive and visceral endoderm

In spite of the therapeutic importance of endoderm derivatives such as the pancreas, liver, lung, and intestine, there are few molecular markers specific for early endoderm. In order to identify endoderm-specific genes as well as to define transcriptional differences between definitive and visceral endoderm, we performed microarray analysis on E8.25 definitive and visceral endoderm. We have developed an early endoderm gene expression signature, and clarified the transcriptional similarities and differences between definitive and visceral endoderm. Additionally, we have developed methods for flow cytometric isolation of definitive and visceral endoderm. These results shed light on the mechanism of endoderm formation and should facilitate investigation of endoderm formation from embryonic stem cells.

Nutritional endoderm in a direct developing frog: A potential parallel to the evolution of the amniote egg

The egg of the direct-developing frog, Eleutherodactylus coqui, has 20 x the volume as that of the model amphibian, Xenopus laevis. Increased egg size led to the origin of nutritional endoderm, a novel cell type that provides nutrition but does not differentiate into digestive tract tissues. As the E. coqui endoderm develops, a distinct boundary exists between differentiating intestinal cells and large yolky cells, which persists even when yolk platelets are depleted. The yolky cells do not become tissues of the digestive tract and are lost, as shown by histology and lineage tracing. EcSox17, an endodermal transcriptional factor, did not distinguish these two cell types, however. When cleavage of the yolky cells was inhibited, embryogenesis continued, indicating that some degree of incomplete cleavage can be tolerated. The presence of cellularized nutritional endoderm in E. coqui may parallel changes that occurred in the evolution of the amniote egg 360 million years ago.

Xenopus cDNA microarray identification of genes with endodermal organ expression

The endoderm is classically defined as the innermost layer of three Metazoan germ layers. During organogenesis, the endoderm gives rise to the digestive and respiratory tracts as well as associated organs such as the liver, pancreas, and lung. At present, however, how the endoderm forms the variety of cell types of digestive and respiratory tracts as well as the budding organs is not well understood. In order to investigate the molecular basis and mechanism of organogenesis and to identify the endodermal organ-related marker genes, we carried out microarray analysis using Xenopus cDNA chips. To achieve this goal, we isolated the Xenopus gut endoderm from three different stages of Xenopus organogenesis, and separated each stage of gut endoderm into anterior and posterior regions. Competitive hybridization of cDNA between the anterior and posterior endoderm regions, to screen genes that specifically expressed in the major organs, revealed 915 candidates. We then selected 104 clones for in situ hybridization analysis. Here, we report the identification and expression patterns of the 104 Xenopus endodermal genes, which would serve as useful markers for studying endodermal organ development.

The Macrostomum lignano EST database as a molecular resource for studying platyhelminth development and phylogeny

We report the development of an Expressed Sequence Tag (EST) resource for the flatworm Macrostomum lignano. This taxon is of interest due to its basal placement within the flatworms. As such, it provides a useful comparative model for understanding the development of neural and sensory organization. It was anticipated on the basis of previous studies [e.g., Sánchez-Alvarado et al., Development, 129:5659-5665, (2002)] that a wide range of developmental markers would be expressed in later-stage macrostomids, and this proved to be the case, permitting recovery of a range of gene sequences important in development. To this end, an adult Macrostomum cDNA library was generated and 7,680 Macrostomum ESTs were sequenced from the 5' end. In addition, 1,536 of these aforementioned sequences were sequenced from the 3' end. Of the roughly 5,416 non-redundant sequences identified, 68% are similar to previously reported genes of known function. In addition, nearly 100 specific clones were obtained with potential neural and sensory function. From these data, an annotated searchable database of the Macrostomum EST collection has been made available on the web. A major objective was to obtain genes that would allow reconstruction of embryogenesis, and in particular neurogenesis, in a basal platyhelminth. The sequences recovered will serve as probes with which the origin and morphogenesis of lineages and tissues can be followed. To this end, we demonstrate a protocol for combined immunohistochemistry and in situ hybridization labeling in juvenile Macrostomum, employing homologs of lin11/lim1 and six3/optix. Expression of these genes is shown in the context of the neuropile/muscle system.

A dynamic expression survey identifies transcription factors relevant in mouse digestive tract development

Tissue-restricted transcription factors (TFs), which confer specialized cellular properties, are usually identified through sequence homology or cis-element analysis of lineage-specific genes; conventional modes of mRNA profiling often fail to report non-abundant TF transcripts. We evaluated the dynamic expression during mouse gut organogenesis of 1381 transcripts, covering nearly every known and predicted TF, and documented the expression of approximately 1000 TF genes in gastrointestinal development. Despite distinctive structures and functions, the stomach and intestine exhibit limited differences in TF genes. Among differentially expressed transcripts, a few are virtually restricted to the digestive tract, including Nr2e3, previously regarded as a photoreceptor-specific product. TFs that are enriched in digestive organs commonly serve essential tissue-specific functions, hence justifying a search for other tissue-restricted TFs. Computational data mining and experimental investigation focused interest on a novel homeobox TF, Isx, which appears selectively in gut epithelium and mirrors expression of the intestinal TF Cdx2. Isx-deficient mice carry a specific defect in intestinal gene expression: dysregulation of the high density lipoprotein (HDL) receptor and cholesterol transporter scavenger receptor class B, type I (Scarb1). Thus, integration of developmental gene expression with biological assessment, as described here for TFs, represents a powerful tool to investigate control of tissue differentiation.

Fgf signaling negatively regulates Nodal-dependent endoderm induction in zebrafish

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

Development of the endoderm and gut in medaka, Oryzias latipes

We performed an extensive analysis of endodermal development and gut tube morphogenesis in the medaka embryo by histology and in situ hybridization. The markers used in these analyses included sox17, sox32, foxA2, gata-4, -5, -6 and shh. sox17, sox32, foxA2, and gata-5 and -6 are expressed in the early endoderm to the onset of gut tube formation. Sections of medaka embryos hybridized with foxA2, a pan-endodermal marker during gut morphogenesis, demonstrated that gut tube formation is initiated in the anterior portion and that the anterior and mid/posterior gut undergo distinct morphogenetic processes. Tube formation in the anterior endoderm that is fated to the pharynx and esophagus is much delayed and appears to be independent of gut morphogenesis. The overall aspects of medaka gut development are similar to those of zebrafish, except that zebrafish tube formation initiates at both the anterior and posterior portions. Our results therefore describe both molecular and morphological aspects of medaka digestive system development that will be necessary for the characterization of medaka mutants.

Gene regulatory networks and the evolution of animal body plans

Development of the animal body plan is controlled by large gene regulatory networks (GRNs), and hence evolution of body plans must depend upon change in the architecture of developmental GRNs. However, these networks are composed of diverse components that evolve at different rates and in different ways. Because of the hierarchical organization of developmental GRNs, some kinds of change affect terminal properties of the body plan such as occur in speciation, whereas others affect major aspects of body plan morphology. A notable feature of the paleontological record of animal evolution is the establishment by the Early "Cambrian of virtually all phylum-level body plans. We identify a class of GRN component, the kernels" of the network, which, because of their developmental role and their particular internal structure, are most impervious to change. Conservation of phyletic body plans may have been due to the retention since pre-Cambrian time of GRN kernels, which underlie development of major body parts.

Genomic profiling of mixer and Sox17beta targets during Xenopus endoderm development

The transcription factors Mixer and Sox17beta have well-characterized roles in endoderm specification during Xenopus embryogenesis. In order to more thoroughly understand the mechanisms by which these endodermal regulators act, we expressed Mixer and Sox17beta in naïve ectodermal tissue and, using oligonucleotide-based microarrays, compared their genomic transcriptional profile to that of unaffected tissue. Using this approach, we identified 71 transcripts that are upregulated by Mixer or Sox17beta, 63 of which have previously uncharacterized roles in endoderm development. Furthermore, an in situ hybridization screen using antisense probes for several of these clones identified six targets of Mixer and/or Sox17beta that are expressed in the endoderm during gastrula stages, providing new and regional markers of the endoderm. Our results contribute further insight into the functions of Mixer and Sox17beta and bring us closer to understanding at the molecular level the pathways that regulate endoderm development.

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.

Genomic inventory and expression of Sox and Fox genes in the cnidarian Nematostella vectensis

The Sox and Forkhead (Fox) gene families are comprised of transcription factors that play important roles in a variety of developmental processes, including germ layer specification, gastrulation, cell fate determination, and morphogenesis. Both the Sox and Fox gene families are divided into subgroups based on the amino acid sequence of their respective DNA-binding domains, the high-mobility group (HMG) box (Sox genes) or Forkhead domain (Fox genes). Utilizing the draft genome sequence of the cnidarian Nematostella vectensis, we examined the genomic complement of Sox and Fox genes in this organism to gain insight into the nature of these gene families in a basal metazoan. We identified 14 Sox genes and 15 Fox genes in Nematostella and conducted a Bayesian phylogenetic analysis comparing HMG box and Forkhead domain sequences from Nematostella with diverse taxa. We found that the majority of bilaterian Sox groups have clear Nematostella orthologs, while only a minority of Fox groups are represented, suggesting that the evolutionary pressures driving the diversification of these gene families may be distinct from one another. In addition, we examined the expression of a subset of these genes during development in Nematostella and found that some of these genes are expressed in patterns consistent with roles in germ layer specification and the regulation of cellular behaviors important for gastrulation. The diversity of expression patterns among members of these gene families in Nematostella reinforces the notion that despite their relatively simple morphology, cnidarians possess much of the molecular complexity observed in bilaterian taxa.

GATA factors as key regulatory molecules in the development of Drosophila endoderm

Essential roles for GATA factors in the development of endoderm have been reported in various animals. A Drosophila GATA factor gene, serpent (srp, dGATAb, ABF), is expressed in the prospective endoderm, and loss of srp activity causes transformation of the prospective endoderm into ectodermal foregut and hindgut, indicating that srp acts as a selector gene to specify the developmental fate of the endoderm. While srp is expressed in the endoderm only during early stages, it activates a subsequent GATA factor gene, dGATAe, and the latter continues to be expressed specifically in the endoderm throughout life. dGATAe activates various functional genes in the differentiated endodermal midgut. An analogous mode of regulation has been reported in Caenorhabditis elegans, in which a pair of GATA genes, end-1/3, specifies endodermal fate, and a downstream pair of GATA genes, elt-2/7, activates genes in the differentiated endoderm. Functional homology of GATA genes in nature is apparently extendable to vertebrates, because endodermal GATA genes of C. elegans and Drosophila induce endoderm development in Xenopus ectoderm. These findings strongly imply evolutionary conservation of the roles of GATA factors in the endoderm across the protostomes and the deuterostomes.

Eomesodermin is a localized maternal determinant required for endoderm induction in zebrafish

In zebrafish, endoderm induction occurs in marginal blastomeres and requires Casanova (Cas), the first endoderm-specific factor expressed in the embryo. Whereas the transcription factors Gata5 and Bon are necessary and sufficient for cas expression in marginal blastomeres, Bon and Gata5 are unable to induce cas in animal pole cells, suggesting that cas expression requires an additional, unidentified factor(s). Here, we show that cas expression depends upon the T box transcription factor Eomesodermin (Eomes), a maternal determinant that is localized to marginal blastomeres. Eomes synergizes potently with Bon and Gata5 to induce cas, even in animal pole blastomeres. We show that Eomes is required for endogenous endoderm induction, acting via an essential binding site in the cas promoter. Direct physical interactions between Eomes, Bon, and Gata5 suggest that Eomes promotes endoderm induction in marginal blastomeres by facilitating the assembly of a transcriptional activating complex on the cas promoter.

Genetic redundancy in endoderm specification within the genus Caenorhabditis

Specification of the endoderm precursor, the E cell, in Caenorhabditis elegans requires a genomic region called the Endoderm Determining Region (EDR). We showed previously that end-1, a gene within the EDR encoding a GATA-type transcription factor, restores endoderm specification to embryos deleted for the EDR and obtained evidence for genetic redundancy in this process. Here, we report molecular identification of end-3, a nearby paralog of end-1 in the EDR, and show that end-1 and end-3 together define the endoderm-specifying properties of the EDR. Both genes are expressed in the early E lineage and each is individually sufficient to specify endodermal fate in the E cell and in non-endodermal precursors when ectopically expressed. The loss of function of both end genes, but not either one alone, eliminates endoderm in nearly all embryos and results in conversion of E into a C-like mesectodermal precursor, similar to deletions of the EDR. While two putative end-1 null mutants display no overt phenotype, a missense mutation that alters a residue in the zinc finger domain of END-3 results in misspecification of E in approximately 9% of mutant embryos. We report that the EDR in C. briggsae, which is estimated to have diverged from C. elegans approximately 50--120 myr ago, contains three end-like genes, resulting from both the ancient duplication that produced end-1 and end-3 in C. elegans, and a more recent duplication of end-3 in the lineage specific to C. briggsae. Transgenes containing the C. briggsae end homologs show E lineage-specific expression and function in C. elegans, demonstrating their functional conservation. Moreover, RNAi experiments indicate that the C. briggsae end genes also function redundantly to specify endoderm. We propose that duplicated end genes have been maintained over long periods of evolution, owing in part to their synergistic function.

Gata4 expression in lateral mesoderm is downstream of BMP4 and is activated directly by Forkhead and GATA transcription factors through a distal enhancer element

The GATA family of zinc-finger transcription factors plays key roles in the specification and differentiation of multiple cell types during development. GATA4 is an early regulator of gene expression during the development of endoderm and mesoderm, and genetic studies in mice have demonstrated that GATA4 is required for embryonic development. Despite the importance of GATA4 in tissue specification and differentiation, the mechanisms by which Gata4 expression is activated and the transcription factor pathways upstream of GATA4 remain largely undefined. To identify transcriptional regulators of Gata4 in the mouse, we screened conserved noncoding sequences from the mouse Gata4 gene for enhancer activity in transgenic embryos. Here, we define the regulation of a distal enhancer element from Gata4 that is sufficient to direct expression throughout the lateral mesoderm, beginning at 7.5 days of mouse embryonic development. The activity of this enhancer is initially broad but eventually becomes restricted to the mesenchyme surrounding the liver. We demonstrate that the function of this enhancer in transgenic embryos is dependent upon highly conserved Forkhead and GATA transcription factor binding sites, which are bound by FOXF1 and GATA4, respectively. Furthermore, the activity of the Gata4 lateral mesoderm enhancer is attenuated by the BMP antagonist Noggin, and the enhancer is not activated in Bmp4-null embryos. Thus, these studies establish that Gata4 is a direct transcriptional target of Forkhead and GATA transcription factors in the lateral mesoderm, and demonstrate that Gata4 lateral mesoderm enhancer activation requires BMP4, supporting a model in which GATA4 serves as a downstream effector of BMP signaling in the lateral mesoderm.

Xema, a foxi-class gene expressed in the gastrula stage Xenopus ectoderm, is required for the suppression of mesendoderm

The molecular basis of vertebrate germ layer formation has been the focus of intense scrutiny for decades, and the inductive interactions underlying this process are well defined. Only recently, however, have studies demonstrated that the regulated inhibition of ectopic germ layer formation is also crucial for patterning the early vertebrate embryo. We report here the characterization of Xema (Xenopus Ectodermally-expressed Mesendoderm Antagonist), a novel member of the Foxi-subclass of winged-helix transcription factors that is involved in the suppression of ectopic germ layer formation in the frog, Xenopus laevis. Xema transcripts are restricted to the animal pole ectoderm during early Xenopus development. Ectopic expression of Xema RNA inhibits mesoderm induction, both by growth factors and in the marginal zone, in vivo. Conversely, introduction of antisense morpholino oligonucleotides directed against the Xema transcript stimulates the expression of a broad range of mesodermal and endodermal marker genes in the animal pole. Our studies demonstrate that Xema is both necessary and sufficient for the inhibition of ectopic mesendoderm in the cells of the presumptive ectoderm, and support a model in which Fox proteins function in part to restrict inappropriate germ layer development throughout the vertebrate embryo.