Foxh1 and Foxa2 are not required for formation of the midgut and hindgut definitive endoderm

Evolutionarily conserved Wnt/Sp5 signaling is critical for anterior-posterior axis patterning in sea urchin embryos

Studies across a diverse group of metazoan embryos indicate that Wnt signaling often activates the transcription factor Sp5, forming a signaling 'cassette' that plays critical roles in many developmental processes. This study explores the role of Wnt/Sp5 signaling during the specification and patterning of the primary germ layers during early anterior-posterior axis formation in the deuterostome sea urchin embryo. Our functional analyses show that Sp5 is critical for endomesoderm specification downstream of Wnt/β-catenin in posterior cells as well as anterior neuroectoderm patterning downstream of non-canonical Wnt/JNK signaling in anterior cells. Interestingly, expression and functional data comparisons show that Wnt/Sp5 signaling often plays similar roles in posterior endomesoderm as well as neuroectoderm patterning along the AP axis of several deuterostome embryos, including vertebrates. Thus, our findings provide strong support for the idea that Wnt-Sp5 signaling cassettes were critical for the establishment of early germ layers in the common deuterostome ancestor.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Single-Cell RNA-Sequencing-Based CRISPRi Screening Resolves Molecular Drivers of Early Human Endoderm Development

Studies in vertebrates have outlined conserved molecular control of definitive endoderm (END) development. However, recent work also shows that key molecular aspects of human END regulation differ even from rodents. Differentiation of human embryonic stem cells (ESCs) to END offers a tractable system to study the molecular basis of normal and defective human-specific END development. Here, we interrogated dynamics in chromatin accessibility during differentiation of ESCs to END, predicting DNA-binding proteins that may drive this cell fate transition. We then combined single-cell RNA-seq with parallel CRISPR perturbations to comprehensively define the loss-of-function phenotype of those factors in END development. Following a few candidates, we revealed distinct impairments in the differentiation trajectories for mediators of TGFβ signaling and expose a role for the FOXA2 transcription factor in priming human END competence for human foregut and hepatic END specification. Together, this single-cell functional genomics study provides high-resolution insight on human END development.

Gutsy moves in mice: cellular and molecular dynamics of endoderm morphogenesis

Despite the importance of the gut and its accessory organs, our understanding of early endoderm development is still incomplete. Traditionally, endoderm has been difficult to study because of its small size and relative fragility. However, recent advances in live cell imaging technologies have dramatically expanded our understanding of this tissue, adding a new appreciation for the complex molecular and morphogenetic processes that mediate gut formation. Several spatially and molecularly distinct subpopulations have been shown to exist within the endoderm before the onset of gastrulation. Here, we review findings that have uncovered complex cell movements within the endodermal layer, before and during gastrulation, leading to the conclusion that cells from primitive endoderm contribute descendants directly to gut.

SOX17 links gut endoderm morphogenesis and germ layer segregation

Gastrulation leads to three germ layers--ectoderm, mesoderm and endoderm--that are separated by two basement membranes. In the mouse embryo, the emergent gut endoderm results from the widespread intercalation of cells of two distinct origins: pluripotent epiblast-derived definitive endoderm (DE) and extra-embryonic visceral endoderm (VE). Here we image the trajectory of prospective DE cells before intercalating into the VE epithelium. We show that the transcription factor SOX17, which is activated in prospective DE cells before intercalation, is necessary for gut endoderm morphogenesis and the assembly of the basement membrane that separates gut endoderm from mesoderm. Our results mechanistically link gut endoderm morphogenesis and germ layer segregation, two central and conserved features of gastrulation.

Mouse primordial germ cells: a reappraisal

Current dogma is that mouse primordial germ cells (PGCs) segregate within the allantois, or source of the umbilical cord, and translocate to the gonads, differentiating there into sperm and eggs. In light of emerging data on the posterior embryonic-extraembryonic interface, and the poorly studied but vital fetal-umbilical connection, we have reviewed the past century of experiments on mammalian PGCs and their relation to the allantois. We demonstrate that, despite best efforts and valuable data on the pluripotent state, what is and is not a PGC in vivo is obscure. Furthermore, sufficient experimental evidence has yet to be provided either for an extragonadal origin of mammalian PGCs or for their segregation within the posterior region. Rather, most evidence points to an alternative hypothesis that PGCs in the mouse allantois are part of a stem/progenitor cell pool that exhibits all known PGC "markers" and that builds/reinforces the fetal-umbilical interface, common to amniotes. We conclude by suggesting experiments to distinguish the mammalian germ line from the soma.

A regulatory network controls nephrocan expression and midgut patterning

Although many regulatory networks involved in defining definitive endoderm have been identified, the mechanisms through which these networks interact to pattern the endoderm are less well understood. To explore the mechanisms involved in midgut patterning, we dissected the transcriptional regulatory elements of nephrocan (Nepn), the earliest known midgut specific gene in mice. We observed that Nepn expression is dramatically reduced in Sox17(-/-) and Raldh2(-/-) embryos compared with wild-type embryos. We further show that Nepn is directly regulated by Sox17 and the retinoic acid (RA) receptor via two enhancer elements located upstream of the gene. Moreover, Nepn expression is modulated by Activin signaling, with high levels inhibiting and low levels enhancing RA-dependent expression. In Foxh1(-/-) embryos in which Nodal signaling is reduced, the Nepn expression domain is expanded into the anterior gut region, confirming that Nodal signaling can modulate its expression in vivo. Together, Sox17 is required for Nepn expression in the definitive endoderm, while RA signaling restricts expression to the midgut region. A balance of Nodal/Activin signaling regulates the anterior boundary of the midgut expression domain.

Wnt/β-catenin signalling regulates Sox17 expression and is essential for organizer and endoderm formation in the mouse

Several signalling cascades are implicated in the formation and patterning of the three principal germ layers, but their precise temporal-spatial mode of action in progenitor populations remains undefined. We have used conditional gene deletion of mouse β-catenin in Sox17-positive embryonic and extra-embryonic endoderm as well as vascular endothelial progenitors to address the function of canonical Wnt signalling in cell lineage formation and patterning. Conditional mutants fail to form anterior brain structures and exhibit posterior body axis truncations, whereas initial blood vessel formation appears normal. Tetraploid rescue experiments reveal that lack of β-catenin in the anterior visceral endoderm results in defects in head organizer formation. Sox17 lineage tracing in the definitive endoderm (DE) shows a cell-autonomous requirement for β-catenin in midgut and hindgut formation. Surprisingly, wild-type posterior visceral endoderm (PVE) in midgut- and hindgut-deficient tetraploid chimera rescues the posterior body axis truncation, indicating that the PVE is important for tail organizer formation. Upon loss of β-catenin in the visceral endoderm and DE lineages, but not in the vascular endothelial lineage, Sox17 expression is not maintained, suggesting downstream regulation by canonical Wnt signalling. Strikingly, Tcf4/β-catenin transactivation complexes accumulated on Sox17 cis-regulatory elements specifically upon endoderm induction in an embryonic stem cell differentiation system. Together, these results indicate that the Wnt/β-catenin signalling pathway regulates Sox17 expression for visceral endoderm pattering and DE formation and provide the first functional evidence that the PVE is necessary for gastrula organizer gene induction and posterior axis development.

Sox2 acts through Sox21 to regulate transcription in pluripotent and differentiated cells

Sox2 is an important transcriptional regulator in embryonic and adult stem cells. Recently, Sox2 was identified as an oncogene in many endodermal cancers, including colon cancer. There is great interest in how Sox2 cooperates with other transcription factors to regulate stem cell renewal, differentiation, and reprogramming. However, we still lack a general understanding of Sox2 transcriptional action. To determine transcriptional partners of Sox2 in adult cells, we generated mice where gene expression could be induced by an externally applied stimulus. We analyzed the consequences in the intestine where cell turnover is rapid. Sox2 expression, but not Oct4, specifically increased the numbers of stem cells and repressed Cdx2, a master regulator of endodermal identity. In vivo studies demonstrated that Sox21, another member of the SoxB gene family, was a specific, immediate, and cell-autonomous target of Sox2 in intestinal stem cells. In vitro experiments showed that Sox21 was sufficient to repress Cdx2 in colon cancer cells and in pluripotent stem cells. Sox21 was also specifically induced by Sox2 in fibroblasts and inhibition of Sox21 blocked reprogramming to the pluripotent state. These results show that transcriptional induction of Sox21 is a rapid and general mediator of the effects of Sox2 on cell identity in a wide range of cell types.

GATA6 and FOXA2 regulate Wnt6 expression during extraembryonic endoderm formation

One of the earliest epithelial-to-mesenchymal transitions in mouse embryogenesis involves the differentiation of inner cell mass cells into primitive and then into parietal endoderm. These processes can be recapitulated in vitro using F9 teratocarcinoma cells, which differentiate into primitive endoderm when treated with retinoic acid (RA) and into parietal endoderm with subsequent treatment with dibutyryl cyclic adenosine monophosphate (db-cAMP). Our previous work on how primitive endoderm develops revealed that the Wnt6 gene is upregulated by RA, leading to the activation of the canonical WNT-β-catenin pathway. The mechanism by which Wnt6 is regulated was not determined, but in silico analysis of the human WNT6 promoter region had suggested that the GATA6 and FOXA2 transcription factors might be involved [1]. Subsequent analysis determined that both Gata6 and Foxa2 mRNA are upregulated in F9 cells treated with RA or RA and db-cAMP. More specifically, overexpression of Gata6 or Foxa2 alone induced molecular and morphological markers of primitive endoderm, which occurred concomitantly with the upregulation of the Wnt6 gene. Gata6- or Foxa2-overexpressing cells were also found to have increased levels in T-cell factor (TCF)-dependent transcription, and when these cells were treated with db-cAMP, they developed into parietal endoderm. Chromatin immunoprecipitation analysis revealed that GATA6 and FOXA2 were bound to the Wnt6 promoter, and overexpression studies showed that these transcription factors were sufficient to switch on the gene expression of a Wnt6 reporter construct. Together, these results provide evidence for the direct regulation of Wnt6 that leads to the activation of the canonical WNT-β-catenin pathway and subsequent induction of primitive extraembryonic endoderm.

Vertebrate intestinal endoderm development

The endoderm gives rise to the lining of the esophagus, stomach and intestines, as well as associated organs. To generate a functional intestine, a series of highly orchestrated developmental processes must occur. In this review, we attempt to cover major events during intestinal development from gastrulation to birth, including endoderm formation, gut tube growth and patterning, intestinal morphogenesis, epithelial reorganization, villus emergence, as well as proliferation and cytodifferentiation. Our discussion includes morphological and anatomical changes during intestinal development as well as molecular mechanisms regulating these processes.

Formation of the murine endoderm: lessons from the mouse, frog, fish, and chick

The mammalian definitive endoderm arises as a simple epithelial sheet. This sheet of cells will eventually produce the innermost tube that comprises the entire digestive tract from the esophagus to the colon as well as the epithelial component of the digestive and respiratory organs including the thymus, thyroid, lung, liver, gallbladder, and pancreas. Thus a wide array of tissue types are derived from the early endodermal sheet, and understanding the morphological and molecular mechanisms used to produce this tissue is integral to understanding the development of all these organs. The goal of this chapter is to summarize what is known about the morphological and molecular mechanisms used to produce this embryonic germ layer. Although this chapter mainly focuses on the mechanisms used to generate the murine endoderm, supportive or suggestive data from other species, including chick, frog (Xenopus laevis), and the Zebrafish (Danio rerio) are also examined.