The organizer of the mouse gastrula is composed of a dynamic population of progenitor cells for the axial mesoderm

Genetic and biochemical evidence that gastrulation defects in Pofut2 mutants result from defects in ADAMTS9 secretion

Protein O-fucosyltransferase 2 (POFUT2) adds O-linked fucose to Thrombospondin Type 1 Repeats (TSR) in 49 potential target proteins. Nearly half the POFUT2 targets belong to the A Disintegrin and Metalloprotease with ThromboSpondin type-1 motifs (ADAMTS) or ADAMTS-like family of proteins. Both the mouse Pofut2 RST434 gene trap allele and the Adamts9 knockout were reported to result in early embryonic lethality, suggesting that defects in Pofut2 mutant embryos could result from loss of O-fucosylation on ADAMTS9. To address this question, we compared the Pofut2 and Adamts9 knockout phenotypes and used Cre-mediated deletion of Pofut2 and Adamts9 to dissect the tissue-specific role of O-fucosylated ADAMTS9 during gastrulation. Disruption of Pofut2 using the knockout (LoxP) or gene trap (RST434) allele, as well as deletion of Adamts9, resulted in disorganized epithelia (epiblast, extraembryonic ectoderm, and visceral endoderm) and blocked mesoderm formation during gastrulation. The similarity between Pofut2 and Adamts9 mutants suggested that disruption of ADAMTS9 function could be responsible for the gastrulation defects observed in Pofut2 mutants. Consistent with this prediction, CRISPR/Cas9 knockout of POFUT2 in HEK293T cells blocked secretion of ADAMTS9. We determined that Adamts9 was dynamically expressed during mouse gastrulation by trophoblast giant cells, parietal endoderm, the most proximal visceral endoderm adjacent to the ectoplacental cone, extraembryonic mesoderm, and anterior primitive streak. Conditional deletion of either Pofut2 or Adamts9 in the epiblast rescues the gastrulation defects, and identified a new role for O-fucosylated ADAMTS9 during morphogenesis of the amnion and axial mesendoderm. Combined, these results suggested that loss of ADAMTS9 function in the extra embryonic tissue is responsible for gastrulation defects in the Pofut2 knockout. We hypothesize that loss of ADAMTS9 function in the most proximal visceral endoderm leads to slippage of the visceral endoderm and altered characteristics of the extraembryonic ectoderm. Consequently, loss of input from the extraembryonic ectoderm and/or compression of the epiblast by Reichert's membrane blocks gastrulation. In the future, the Pofut2 and Adamts9 knockouts will be valuable tools for understanding how local changes in the properties of the extracellular matrix influence the organization of tissues during mammalian development.

Analysis of extraembryonic mesodermal structure formation in the absence of morphological primitive streak

During mouse gastrulation, the primitive streak is formed on the posterior side of the embryo. Cells migrate out of the primitive streak to form the future mesoderm and endoderm. Fate mapping studies revealed a group of cell migrate through the proximal end of the primitive streak and give rise to the extraembryonic mesoderm tissues such as the yolk sac blood islands and allantois. However, it is not clear whether the formation of a morphological primitive streak is required for the development of these extraembryonic mesodermal tissues. Loss of the Cripto gene in mice dramatically reduces, but does not completely abolish, Nodal activity leading to the absence of a morphological primitive streak. However, embryonic erythrocytes are still formed and assembled into the blood islands. In addition, Cripto mutant embryos form allantoic buds. However, Drap1 mutant embryos have excessive Nodal activity in the epiblast cells before gastrulation and form an expanded primitive streak, but no yolk sac blood islands or allantoic bud formation. Lefty2 embryos also have elevated levels of Nodal activity in the primitive streak during gastrulation, and undergo normal blood island and allantois formation. We therefore speculate that low level of Nodal activity disrupts the formation of morphological primitive streak on the posterior side, but still allows the formation of primitive streak cells on the proximal side, which give rise to the extraembryonic mesodermal tissues formation. Excessive Nodal activity in the epiblast at pre-gastrulation stage, but not in the primitive streak cells during gastrulation, disrupts extraembryonic mesoderm development.

Notochord morphogenesis in mice: Current understanding & open questions

The notochord is a structure common to all chordates, and the feature that the phylum Chordata has been named after. It is a rod-like mesodermal structure that runs the anterior-posterior length of the embryo, adjacent to the ventral neural tube. The notochord plays a critical role in embryonic tissue patterning, for example the dorsal-ventral patterning of the neural tube. The cells that will come to form the notochord are specified at gastrulation. Axial mesodermal cells arising at the anterior primitive streak migrate anteriorly as the precursors of the notochord and populate the notochordal plate. Yet, even though a lot of interest has centered on investigating the functional and structural roles of the notochord, we still have a very rudimentary understanding of notochord morphogenesis. The events driving the formation of the notochord are rapid, taking place over the period of approximately a day in mice. In this commentary, we provide an overview of our current understanding of mouse notochord morphogenesis, from the initial specification of axial mesendodermal cells at the primitive streak, the emergence of these cells at the midline on the surface of the embryo, to their submergence and organization of the stereotypically positioned notochord. We will also discuss some key open questions. Developmental Dynamics 245:547-557, 2016. © 2016 Wiley Periodicals, Inc.

NOTCH activation interferes with cell fate specification in the gastrulating mouse embryo

NOTCH signalling is an evolutionarily conserved pathway involved in intercellular communication essential for cell fate choices during development. Although dispensable for early aspects of mouse development, canonical RBPJ-dependent NOTCH signalling has been shown to influence lineage commitment during embryonic stem cell (ESC) differentiation. NOTCH activation in ESCs promotes the acquisition of a neural fate, whereas its suppression favours their differentiation into cardiomyocytes. This suggests that NOTCH signalling is implicated in the acquisition of distinct embryonic fates at early stages of mammalian development. In order to investigate in vivo such a role for NOTCH signalling in shaping cell fate specification, we use genetic approaches to constitutively activate the NOTCH pathway in the mouse embryo. Early embryonic development, including the establishment of anterior-posterior polarity, is not perturbed by forced NOTCH activation. By contrast, widespread NOTCH activity in the epiblast triggers dramatic gastrulation defects. These are fully rescued in a RBPJ-deficient background. Epiblast-specific NOTCH activation induces acquisition of neurectoderm identity and disrupts the formation of specific mesodermal precursors including the derivatives of the anterior primitive streak, the mouse organiser. In addition, we show that forced NOTCH activation results in misregulation of NODAL signalling, a major determinant of early embryonic patterning. Our study reveals a previously unidentified role for canonical NOTCH signalling during mammalian gastrulation. It also exemplifies how in vivo studies can shed light on the mechanisms underlying cell fate specification during in vitro directed differentiation.

The zebrafish tailbud contains two independent populations of midline progenitor cells that maintain long-term germ layer plasticity and differentiate in response to local signaling cues

Vertebrate body axis formation depends on a population of bipotential neuromesodermal cells along the posterior wall of the tailbud that make a germ layer decision after gastrulation to form spinal cord and mesoderm. Despite exhibiting germ layer plasticity, these cells never give rise to midline tissues of the notochord, floor plate and dorsal endoderm, raising the question of whether midline tissues also arise from basal posterior progenitors after gastrulation. We show in zebrafish that local posterior signals specify germ layer fate in two basal tailbud midline progenitor populations. Wnt signaling induces notochord within a population of notochord/floor plate bipotential cells through negative transcriptional regulation of sox2. Notch signaling, required for hypochord induction during gastrulation, continues to act in the tailbud to specify hypochord from a notochord/hypochord bipotential cell population. Our results lend strong support to a continuous allocation model of midline tissue formation in zebrafish, and provide an embryological basis for zebrafish and mouse bifurcated notochord phenotypes as well as the rare human congenital split notochord syndrome. We demonstrate developmental equivalency between the tailbud progenitor cell populations. Midline progenitors can be transfated from notochord to somite fate after gastrulation by ectopic expression of msgn1, a master regulator of paraxial mesoderm fate, or if transplanted into the bipotential progenitors that normally give rise to somites. Our results indicate that the entire non-epidermal posterior body is derived from discrete, basal tailbud cell populations. These cells remain receptive to extracellular cues after gastrulation and continue to make basic germ layer decisions.

GSK3β inhibition activates the CDX/HOX pathway and promotes hemogenic endothelial progenitor differentiation from human pluripotent stem cells

WNT/β-CATENIN signaling promotes the hematopoietic/endothelial differentiation of human embryonic stem cells and human induced pluripotent stem cells (hiPSCs). The transient addition of a GSK3β inhibitor (GSKi) has been found to facilitate in vitro endothelial cell differentiation from hESCs/hiPSCs. Because hematopoietic and endothelial cells are derived from common progenitors (hemogenic endothelial progenitors [HEPs]), we examined the effect of transient GSKi treatment on hematopoietic cell differentiation from hiPSCs. We found that transient GSKi treatment at the start of hiPSC differentiation induction altered the gene expression profile of the cells. Multiple CDX/HOX genes, which are expressed in the posterior mesoderm of developing embryos, were significantly upregulated by GSKi treatment. Further, inclusion of the GSKi in a serum- and stroma-free culture with chemically defined medium efficiently induced HEPs, and the HEPs gave rise to various lineages of hematopoietic and endothelial cells. Therefore, transient WNT/β-CATENIN signaling triggers activation of the CDX/HOX pathway, which in turn confers hemogenic posterior mesoderm identity to differentiating hiPSCs. These data enhance our understanding of human embryonic hematopoietic/endothelial cell development and provide a novel in vitro system for inducing the differentiation of hematopoietic cells from hiPSCs.

The origin of the mammalian kidney: implications for recreating the kidney in vitro

The mammalian kidney, the metanephros, is a mesodermal organ classically regarded as arising from the intermediate mesoderm (IM). Indeed, both the ureteric bud (UB), which gives rise to the ureter and the collecting ducts, and the metanephric mesenchyme (MM), which forms the rest of the kidney, derive from the IM. Based on an understanding of the signalling molecules crucial for IM patterning and kidney morphogenesis, several studies have now generated UB or MM, or both, in vitro via the directed differentiation of human pluripotent stem cells. Although these results support the IM origin of the UB and the MM, they challenge the simplistic view of a common progenitor for these two populations, prompting a reanalysis of early patterning events within the IM. Here, we review our understanding of the origin of the UB and the MM in mouse, and discuss how this impacts on kidney regeneration strategies and furthers our understanding of human development.

Differential response of epiblast stem cells to Nodal and Activin signalling: a paradigm of early endoderm development in the embryo

Mouse epiblast stem cells (EpiSCs) display temporal differences in the upregulation of Mixl1 expression during the initial steps of in vitro differentiation, which can be correlated with their propensity for endoderm differentiation. EpiSCs that upregulated Mixl1 rapidly during differentiation responded robustly to both Activin A and Nodal in generating foregut endoderm and precursors of pancreatic and hepatic tissues. By contrast, EpiSCs that delayed Mixl1 upregulation responded less effectively to Nodal and showed an overall suboptimal outcome of directed differentiation. The enhancement in endoderm potency in Mixl1-early cells may be accounted for by a rapid exit from the progenitor state and the efficient response to the induction of differentiation by Nodal. EpiSCs that readily differentiate into the endoderm cells are marked by a distinctive expression fingerprint of transforming growth factor (TGF)-β signalling pathway genes and genes related to the endoderm lineage. Nodal appears to elicit responses that are associated with transition to a mesenchymal phenotype, whereas Activin A promotes gene expression associated with maintenance of an epithelial phenotype. We postulate that the formation of definitive endoderm (DE) in embryoid bodies follows a similar process to germ layer formation from the epiblast, requiring an initial de-epithelialization event and subsequent re-epithelialization. Our results show that priming EpiSCs with the appropriate form of TGF-β signalling at the formative phase of endoderm differentiation impacts on the further progression into mature DE-derived lineages, and that this is influenced by the initial characteristics of the cell population. Our study also highlights that Activin A, which is commonly used as an in vitro surrogate for Nodal in differentiation protocols, does not elicit the same downstream effects as Nodal, and therefore may not effectively mimic events that take place in the mouse embryo.

Morphological and Gene Expression Changes in Cattle Embryos from Hatched Blastocyst to Early Gastrulation Stages after Transfer of In Vitro Produced Embryos

A detailed morphological staging system for cattle embryos at stages following blastocyst hatching and preceding gastrulation is presented here together with spatiotemporal mapping of gene expression for BMP4, BRACHYURY, CERBERUS1 (CER1), CRIPTO, EOMESODERMIN, FURIN and NODAL. Five stages are defined based on distinct developmental events. The first of these is the differentiation of the visceral hypoblast underlying the epiblast, from the parietal hypoblast underlying the mural trophoblast. The second concerns the formation of an asymmetrically positioned, morphologically recognisable region within the visceral hypoblast that is marked by the presence of CER1 and absence of BMP4 expression. We have termed this the anterior visceral hypoblast or AVH. Intra-epiblast cavity formation and the disappearance of the polar trophoblast overlying the epiblast (Rauber’s layer) have been mapped in relation to AVH formation. The third chronological event involves the transition of the epiblast into the embryonic ectoderm with concomitant onset of posterior NODAL, EOMES and BRACHYURY expression. Lastly, gastrulation commences as the posterior medial embryonic ectoderm layer thickens to form the primitive streak and cells ingress between the embryonic ectoderm and hypoblast. At this stage a novel domain of CER1 expression is seen whereas the AVH disappears. Comparison with the mouse reveals that while gene expression patterns at the onset of gastrulation are well conserved, asymmetry establishment, which relies on extraembryonic tissues such as the hypoblast and trophoblast, has diverged in terms of both gene expression and morphology.

Coming together is a beginning: the making of an intervertebral disc

The intervertebral disc (IVD) is a complex fibrocartilaginous structure located between the vertebral bodies that allows for movement and acts as a shock absorber in our spine for daily activities. It is composed of three components: the nucleus pulposus (NP), annulus fibrosus, and cartilaginous endplate. The characteristics of these cells are different, as they produce specific extracellular matrix (ECM) for tissue function and the niche in supporting the differentiation status of the cells in the IVD. Furthermore, cell heterogeneities exist in each compartment. The cells and the supporting ECM change as we age, leading to degenerative outcomes that often lead to pathological symptoms such as back pain and sciatica. There are speculations as to the potential of cell therapy or the use of tissue engineering as treatments. However, the nature of the cells present in the IVD that support tissue function is not clear. This review looks at the origin of cells in the making of an IVD, from the earliest stages of embryogenesis in the formation of the notochord, and its role as a signaling center, guiding the formation of spine, and in its journey to become the NP at the center of the IVD. While our current understanding of the molecular signatures of IVD cells is still limited, the field is moving fast and the potential is enormous as we begin to understand the progenitor and differentiated cells present, their molecular signatures, and signals that we could harness in directing the appropriate in vitro and in vivo cellular responses in our quest to regain or maintain a healthy IVD as we age.

Expression of the embryonic morphogen Nodal in differentiated thyroid carcinomas: Immunohistochemistry assay in tissue microarray and The Cancer Genome Atlas data analysis

BACKGROUND

Nodal, an embryonic morphogen, plays a role in tumorigenesis of melanoma, breast, and prostate cancer; however, its role in thyroid carcinoma is unknown. We examined Nodal expression in thyroid tumors by immunohistochemistry assay and The Cancer Genome Atlas (TCGA) analysis.

METHODS

An immunohistochemistry assay was performed in a tissue microarray comprising 128 classic papillary thyroid carcinomas (PTC), 58 follicular thyroid carcinomas (FTC), 19 follicular variants of PTC (FVPTC), 57 follicular adenomas (FA), 54 adenomatous goiters (AG), and 5 normal thyroid tissues. The TCGA database was examined to evaluate the expression of Nodal mRNA in normal thyroid and PTC.

RESULTS

The proportion of tumors showing negative Nodal expression in PTC, FTC, FVPTC, FA, and AG was 0%, 1.7%, 0%, 14%, and 41%, respectively. For the diagnosis of malignant tumors, the sensitivity, specificity, positive predictive value, and negative predictive value of positive Nodal staining was 99%, 27%, 72%, and 97%, respectively. High Nodal expression was associated with older age and BRAF mutation in PTC. TCGA analysis revealed PTC had greater Nodal mRNA expression than normal thyroid (P = .012).

CONCLUSION

Nodal staining might be useful "rule-out test" for the diagnosis of malignant thyroid tumor. Nodal may be associated with the tumorigenesis of thyroid malignancy.

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.

Nature and extent of left/right axis defects in T(Wis) /T(Wis) mutant mouse embryos

BACKGROUND

Mutations in the T-box gene Brachyury have well known effects on invagination of the endomesodermal layer during gastrulation, but the gene also plays a role in the determination of left/right axis determination that is less well studied. Previous work has implicated node morphology in this effect. We use the T(Wis) allele of Brachyury to investigate the molecular and morphological effects of the T locus on axis determination in the mouse.

RESULTS

Similar to embryos mutant for the T allele, T(Wis) /T(Wis) embryos have a high incidence of ventral and/or reversed heart looping. In addition, heterotaxia between the direction of heart looping and the direction of embryo turning is common. Scanning electron microscopy reveals defects in node morphology including irregularity, smaller size, and a decreased number of cilia, although the cilia appear morphologically normal. Molecular analysis shows a loss of perinodal expression of genes involved in Nodal signaling, namely Cer2, Gdf1, and Nodal itself. There is also loss of Dll1 expression, a key component of the Notch signaling pathway, in the presomitic mesoderm.

CONCLUSIONS

Morphological abnormalities of the node as well as disruptions of the molecular cascade of left/right axis determination characterize T(Wis) /T(Wis) mutants. Decreased Notch signaling may account for both the morphological defects and the absence of expression of genes in the Nodal signaling pathway.

Follow your gut: relaying information from the site of left-right symmetry breaking in the mouse

A central unresolved question in the molecular cascade that drives establishment of left-right (LR) asymmetry in vertebrates are the mechanisms deployed to relay information between the midline site of symmetry-breaking and the tissues which will execute a program of asymmetric morphogenesis. The cells located between these two distant locations must provide the medium for signal relay. Of these, the gut endoderm is an attractive candidate tissue for signal transmission since it comprises the epithelium that lies between the node, where asymmetry originates, and the lateral plate, where asymmetry can first be detected. Here, focusing on the mouse as a model, we review our current understanding and entertain open questions concerning the relay of LR information from its origin.

Distinct Wnt-driven primitive streak-like populations reflect in vivo lineage precursors

During gastrulation, epiblast cells are pluripotent and their fate is thought to be constrained principally by their position. Cell fate is progressively restricted by localised signalling cues from areas including the primitive streak. However, it is unknown whether this restriction accompanies, at the individual cell level, a reduction in potency. Investigation of these early transition events in vitro is possible via the use of epiblast stem cells (EpiSCs), self-renewing pluripotent cell lines equivalent to the postimplantation epiblast. Strikingly, mouse EpiSCs express gastrulation stage regional markers in self-renewing conditions. Here, we examined the differentiation potential of cells expressing such lineage markers. We show that undifferentiated EpiSC cultures contain a major subfraction of cells with reversible early primitive streak characteristics, which is mutually exclusive to a neural-like fraction. Using in vitro differentiation assays and embryo grafting we demonstrate that primitive streak-like EpiSCs are biased towards mesoderm and endoderm fates while retaining pluripotency. The acquisition of primitive streak characteristics by self-renewing EpiSCs is mediated by endogenous Wnt signalling. Elevation of Wnt activity promotes restriction towards primitive streak-associated lineages with mesendodermal and neuromesodermal characteristics. Collectively, our data suggest that EpiSC pluripotency encompasses a range of reversible lineage-biased states reflecting the birth of pioneer lineage precursors from a pool of uncommitted EpiSCs similar to the earliest cell fate restriction events taking place in the gastrula stage epiblast.

Early left-right asymmetries during axial morphogenesis in the chick embryo

The primitive node is the "hub" of early left-right patterning in the chick embryo: (1) it undergoes asymmetrical morphogenesis immediately after its appearance at Stage 4; (2) it is closely linked to the emerging asymmetrical expression of nodal and shh at Stage 5; and (3) its asymmetry is spatiotemporally related to the emerging notochord, the midline barrier maintaining molecular left-right patterning from Stage 6 onward. Here, we study the correlation of node asymmetry to notochord marker expression using high-resolution histology, and we test pharmacological inhibition of shh signaling using cyclopamine at Stages 4 and 5. Just as noggin expression mirrors an intriguing structural continuity between the right node shoulder and the notochord, shh expression in the left node shoulder confirms a similar continuity with the future floor plate. Shh inhibition at Stage 4 or 5 suppressed nodal in both its paraxial or lateral plate mesoderm domains, respectively, and resulted in randomized heart looping. Thus, the "primordial" paraxial nodal asymmetry at Stage 4/5 (1) appears to be dependent on, but not instructed by, shh signaling and (2) may be fixed by asymmetrical roots of the notochord and the floor plate, thereby adding further twists to the node's pivotal role during left-right patterning.

Mixl1 localizes to putative axial stem cell reservoirs and their posterior descendants in the mouse embryo

Mixl1 is thought to play important roles in formation of mesoderm and endoderm. Previously, Mixl1 expression was reported in the posterior primitive streak and allantois, but the precise spatiotemporal whereabouts of Mixl1 protein throughout gastrulation have not been elucidated. To localize Mixl1 protein, immunohistochemistry was carried out at 2-4 h intervals on mouse gastrulae between primitive streak and 16-somite pair (s) stages (~E6.5-9.5). Mixl1 localized to the entire primitive streak early in gastrulation. However, by headfold stages (~E7.75-8.0), Mixl1 diminished within the mid-streak but remained concentrated at either end of the streak, and localized throughout midline posterior visceral endoderm. At the streak's anterior end, Mixl1 was confined to the posterior crown cells of Hensen's node, which contribute to dorsal hindgut endoderm, and the posterior notochord. In the posterior streak, Mixl1 localized to the Allantoic Core Domain (ACD), which is the source of most of the allantois and contributes to the posterior embryonic-extraembryonic interface. In addition, Mix1 co-localized with the early hematopoietic marker, Runx1, in the allantois and visceral yolk sac blood islands. During hindgut invagination (4-16s, ~E8.5-9.5), Mixl1 localized to the hindgut lip, becoming concentrated within the midline anastomosis of the splanchnopleure, which appears to create the ventral component of the hindgut and omphalomesenteric artery. Surrounding the distal hindgut, Mixl1 identified midline cells within tailbud mesoderm. Mixl1 was also found in the posterior notochord. These findings provide a critical systematic, and tissue-level understanding of embryonic Mixl1 localization, and support its role in regulation of crucial posterior axial mesendodermal stem cell niches during embryogenesis.

The formation of proximal and distal definitive endoderm populations in culture requires p38 MAPK activity

Endoderm formation in the mammal is a complex process with two lineages forming during the first weeks of development, the primitive (or extraembryonic) endoderm, which is specified in the blastocyst, and the definitive endoderm that forms later, at gastrulation, as one of the germ layers of the embryo proper. Fate mapping evidence suggests that the definitive endoderm arises as two waves, which potentially reflect two distinct cell populations. Early primitive ectoderm-like (EPL) cell differentiation has been used successfully to identify and characterise mechanisms regulating molecular gastrulation and lineage choice during differentiation. The roles of the p38 MAPK family in the formation of definitive endoderm were investigated using EPL cells and chemical inhibitors of p38 MAPK activity. These approaches define a role for p38 MAPK activity in the formation of the primitive streak and a second role in the formation of the definitive endoderm. Characterisation of the definitive endoderm populations formed from EPL cells demonstrates the formation of two distinct populations, defined by gene expression and ontogeny, that were analogous to the proximal and distal definitive endoderm populations of the embryo. Formation of the proximal definitive endoderm was found to require p38 MAPK activity and is correlated with molecular gastrulation, defined by the expression of brachyury (T). Distal definitive endoderm formation also requires p38 MAPK activity but can form when T expression is inhibited. Understanding lineage complexity will be a prerequisite for the generation of endoderm derivatives for commercial and clinical use.

Efficient and rapid induction of human iPSCs/ESCs into nephrogenic intermediate mesoderm using small molecule-based differentiation methods

The first step in developing regenerative medicine approaches to treat renal diseases using pluripotent stem cells must be the generation of intermediate mesoderm (IM), an embryonic germ layer that gives rise to kidneys. In order to achieve this goal, establishing an efficient, stable and low-cost method for differentiating IM cells using small molecules is required. In this study, we identified two retinoids, AM580 and TTNPB, as potent IM inducers by high-throughput chemical screening, and established rapid (five days) and efficient (80% induction rate) IM differentiation from human iPSCs using only two small molecules: a Wnt pathway activator, CHIR99021, combined with either AM580 or TTNPB. The resulting human IM cells showed the ability to differentiate into multiple cell types that constitute adult kidneys, and to form renal tubule-like structures. These small molecule differentiation methods can bypass the mesendoderm step, directly inducing IM cells by activating Wnt, retinoic acid (RA), and bone morphogenetic protein (BMP) pathways. Such methods are powerful tools for studying kidney development and may potentially provide cell sources to generate renal lineage cells for regenerative therapy.

Initiating head development in mouse embryos: integrating signalling and transcriptional activity

The generation of an embryonic body plan is the outcome of inductive interactions between the progenitor tissues that underpin their specification, regionalization and morphogenesis. The intercellular signalling activity driving these processes is deployed in a time- and site-specific manner, and the signal strength must be precisely controlled. Receptor and ligand functions are modulated by secreted antagonists to impose a dynamic pattern of globally controlled and locally graded signals onto the tissues of early post-implantation mouse embryo. In response to the WNT, Nodal and Bone Morphogenetic Protein (BMP) signalling cascades, the embryo acquires its body plan, which manifests as differences in the developmental fate of cells located at different positions in the anterior–posterior body axis. The initial formation of the anterior (head) structures in the mouse embryo is critically dependent on the morphogenetic activity emanating from two signalling centres that are juxtaposed with the progenitor tissues of the head. A common property of these centres is that they are the source of antagonistic factors and the hub of transcriptional activities that negatively modulate the function of WNT, Nodal and BMP signalling cascades. These events generate the scaffold of the embryonic head by the early-somite stage of development. Beyond this, additional tissue interactions continue to support the growth, regionalization, differentiation and morphogenesis required for the elaboration of the structure recognizable as the embryonic head.

Live imaging of whole mouse embryos during gastrulation: migration analyses of epiblast and mesodermal cells

During gastrulation in the mouse embryo, dynamic cell movements including epiblast invagination and mesodermal layer expansion lead to the establishment of the three-layered body plan. The precise details of these movements, however, are sometimes elusive, because of the limitations in live imaging. To overcome this problem, we developed techniques to enable observation of living mouse embryos with digital scanned light sheet microscope (DSLM). The achieved deep and high time-resolution images of GFP-expressing nuclei and following 3D tracking analysis revealed the following findings: (i) Interkinetic nuclear migration (INM) occurs in the epiblast at embryonic day (E)6 and 6.5. (ii) INM-like migration occurs in the E5.5 embryo, when the epiblast is a monolayer and not yet pseudostratified. (iii) Primary driving force for INM at E6.5 is not pressure from neighboring nuclei. (iv) Mesodermal cells migrate not as a sheet but as individual cells without coordination.

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.

Genetic dissection of ventral folding morphogenesis in mouse: embryonic visceral endoderm-supplied BMP2 positions head and heart

Ventral folding morphogenesis (VFM), a vital morphogenetic process in amniotes, mediates gut endoderm internalization, linear heart tube formation, ventral body wall closure and encasement of the fetus in extraembryonic membranes. Aberrant VFM underlies a number of birth defects such as gastroschisis and ectopia cordis in human and misplacement of head and heart in mouse. Recent cell lineage-specific mouse mutant analyses identified the Bone Morphogenetic Protein (BMP) pathway and Anterior Visceral Endoderm (AVE) as key regulators of anterior VFM. Loss of BMP2 expression solely from embryonic visceral endoderm (EmVE) and the AVE blocks formation of foregut invagination, and simultaneously, aberrantly positions the heart anterior/dorsal to the head, suggesting a mechanistic link between foregut and head/heart morphogenesis.

Generation of knock-in mice that express nuclear enhanced green fluorescent protein and tamoxifen-inducible Cre recombinase in the notochord from Foxa2 and T loci

The node and the notochord are important embryonic signaling centers that control embryonic pattern formation. Notochord progenitor cells present in the node and later in the posterior end of the notochord move anteriorly to generate the notochord. To understand the dynamics of cell movement during notochord development and the molecular mechanisms controlling this event, analyses of cell movements using time-lapse imaging and conditional manipulation of gene activities are required. To achieve this goal, we generated two knock-in mouse lines that simultaneously express nuclear enhanced green fluorescent protein (EGFP) and tamoxifen-inducible Cre, CreER(T2) , from two notochord gene loci, Foxa2 and T (Brachury). In Foxa2(nEGFP-CreERT2/+) and T(nEGFP-CreERT2/+) embryos, nuclei of the Foxa2 or T-expressing cells, which include the node, notochord, and endoderm (Foxa2) or wide range of posterior mesoderm (T), were labeled with EGFP at intensities that can be used for live imaging. Cre activity was also induced in cells expressing Foxa2 and T 1 day after tamoxifen administration. These mice are expected to be useful tools for analyzing the mechanisms of notochord development.

Developmental mechanisms directing early anterior forebrain specification in vertebrates

Research from the last 15 years has provided a working model for how the anterior forebrain is induced and specified during the early stages of embryogenesis. This model relies on three basic processes: (1) induction of the neural plate from naive ectoderm requires the inhibition of BMP/TGFβ signaling; (2) induced neural tissue initially acquires an anterior identity (i.e., anterior forebrain); (3) maintenance and expansion of the anterior forebrain depends on the antagonism of posteriorizing signals that would otherwise transform this tissue into posterior neural fates. In this review, we present a historical perspective examining some of the significant experiments that have helped to delineate this molecular model. In addition, we discuss the function of the relevant tissues that act prior to and during gastrulation to ensure proper anterior forebrain formation. Finally, we elaborate data, mainly obtained from the analyses of mouse mutants, supporting a role for transcriptional repressors in the regulation of cell competence within the anterior forebrain. The aim of this review is to provide the reader with a general overview of the signals as well as the signaling centers that control the development of the anterior neural plate.

Foxa1 and Foxa2 are required for formation of the intervertebral discs

The intervertebral disc (IVD) is composed of 3 main structures, the collagenous annulus fibrosus (AF), which surrounds the gel-like nucleus pulposus (NP), and hyaline cartilage endplates, which are attached to the vertebral bodies. An IVD is located between each vertebral body. Degeneration of the IVD is thought to be a major cause of back pain, a potentially chronic condition for which there exist few effective treatments. The NP forms from the embryonic notochord. Foxa1 and Foxa2, transcription factors in the forkhead box family, are expressed early during notochord development. However, embryonic lethality and the absence of the notochord in Foxa2 null mice have precluded the study of potential roles these genes may play during IVD formation. Using a conditional Foxa2 allele in conjunction with a tamoxifen-inducible Cre allele (ShhcreER^T2), we removed Foxa2 from the notochord of E7.5 mice null for Foxa1. Foxa1^−/−;Foxa2^c/c;ShhcreER^T2 double mutant animals had a severely deformed nucleus pulposus, an increase in cell death in the tail, decreased hedgehog signaling, defects in the notochord sheath, and aberrant dorsal-ventral patterning of the neural tube. Embryos lacking only Foxa1 or Foxa2 from the notochord were indistinguishable from control animals, demonstrating a functional redundancy for these genes in IVD formation. In addition, we provide in vivo genetic evidence that Foxa genes are required for activation of Shh in the notochord.

Zic3 is required in the migrating primitive streak for node morphogenesis and left-right patterning

In humans, loss-of-function mutations in ZIC3 cause isolated cardiovascular malformations and X-linked heterotaxy, a disorder with abnormal left-right asymmetry of organs. Zic3 null mice recapitulate the human heterotaxy phenotype but also have early gastrulation defects, axial patterning defects and neural tube defects complicating an assessment of the role of Zic3 in cardiac development. Zic3 is expressed ubiquitously during critical stages of left-right patterning but its later expression in the developing heart remains controversial and the molecular mechanism(s) by which it causes heterotaxy are unknown. To define the temporal and spatial requirements, for Zic3 in left-right patterning, we generated conditional Zic3 mice and Zic3-LacZ-BAC reporter mice. The latter provide compelling evidence that Zic3 is expressed in the mouse node and absent in the heart. Conditional deletion using T-Cre identifies a requirement for Zic3 in the primitive streak and migrating mesoderm for proper left-right patterning and cardiac development. In contrast, Zic3 is not required in heart progenitors or the cardiac compartment. In addition, the data demonstrate abnormal node morphogenesis in Zic3 null mice and identify similar node dysplasia when Zic3 was specifically deleted from the migrating mesoderm and primitive streak. These results define the temporal and spatial requirements for Zic3 in node morphogenesis, left-right patterning and cardiac development and suggest the possibility that a requirement for Zic3 in node ultrastructure underlies its role in heterotaxy and laterality disorders.

Evolution of the Discs large gene family provides new insights into the establishment of apical epithelial polarity and the etiology of mental retardation

Cell polarity is essential to the function of many cell types, such as epithelial cells and neurons. The Discs large (Dlg) scaffolding protein was identified in Drosophila as a major regulator of basolateral epithelial identity. Four Dlg orthologs (Dlg1 through 4) are found in vertebrates, and mutations in the human Dlg3 gene are associated with X-linked mental retardation. We recently found that Dlg3 controls apical epithelial polarity and tight junction formation and contributes to neural induction in mouse development.¹ During evolution, Dlg3 acquired specific PPxY motifs, which bind to the WW domains of the E3 ubiquitin ligases, Nedd4 and Nedd4-2. This interaction results in monoubiquitination of Dlg3, leading to directed microtubule-dependent protein trafficking, via the exocyst complex, in different polarized cell types. Directed trafficking of Dlg3 plays an important role, during both mammalian development and in adulthood, in the establishment and maintenance of specialized apical cell junctions, such as tight junctions in epithelial cells and synapses in neurons.

DUXO, a novel double homeobox transcription factor, is a regulator of the gastrula organizer in human embryonic stem cells

Human embryonic stem cells differentiate into gastrula organizer cells that express typical markers and induce secondary axes when injected into frog embryos. Here, we report that these human organizer cells express DUXO (DUX of the Organizer), a novel member of the double-homeobox (DUX) family of transcription factors, a group of genes unique to placental mammals. Both of DUXO's homeodomains share high similarity with those of Siamois and Twin, the initial inducers of the amphibian gastrula organizer. DUXO overexpression in human embryoid bodies induces organizer related genes, whereas its knock down hampers formation of the organizer and its derivatives. Finally, we show that DUXO regulates GOOSECOID, the canonical organizer marker, in a direct manner, suggesting that DUXO is a major regulator of human organizer formation.

Foxa2 mediates critical functions of prechordal plate in patterning and morphogenesis and is cell autonomously required for early ventral endoderm morphogenesis

Axial mesendoderm is comprised of prechordal plate and notochord. Lack of a suitable Cre driver has hampered the ability to genetically dissect the requirement for each of these components, or genes expressed within them, to anterior patterning. Here, we have utilized Isl1-Cre to investigate roles of the winged helix transcription factor Foxa2 specifically in prechordal plate and ventral endoderm. Foxa2loxP/loxP; Isl1-Cre mutants died at 13.5 dpc, exhibiting aberrations in anterior neural tube and forebrain patterning, and in ventral foregut morphogenesis and cardiac fusion. Molecular analysis of Foxa2loxP/loxP; Isl1-Cre mutants indicated that Foxa2 is required in Isl1 lineages for expression of notochord and dorsal foregut endoderm markers, Shh. Brachyury, and Hlxb9. Our results support a requirement for Foxa2 in prechordal plate for notochord morphogenesis, axial patterning, and patterning of dorsal foregut endoderm. Loss of Foxa2 in ventral endoderm resulted in reduced expression of Sox17, Gata4, and ZO proteins, accounting at least in part for observed lack of foregut fusion, cardia bifida, and increased apoptosis of ventral endoderm.

Foxa2 mediates critical functions of prechordal plate in patterning and morphogenesis and is cell autonomously required for early ventral endoderm morphogenesis

Axial mesendoderm is comprised of prechordal plate and notochord. Lack of a suitable Cre driver has hampered the ability to genetically dissect the requirement for each of these components, or genes expressed within them, to anterior patterning. Here, we have utilized Isl1-Cre to investigate roles of the winged helix transcription factor Foxa2 specifically in prechordal plate and ventral endoderm. Foxa2^loxP/loxP; Isl1-Cre mutants died at 13.5 dpc, exhibiting aberrations in anterior neural tube and forebrain patterning, and in ventral foregut morphogenesis and cardiac fusion. Molecular analysis of Foxa2^loxP/loxP; Isl1-Cre mutants indicated that Foxa2 is required in Isl1 lineages for expression of notochord and dorsal foregut endoderm markers, Shh. Brachyury, and Hlxb9. Our results support a requirement for Foxa2 in prechordal plate for notochord morphogenesis, axial patterning, and patterning of dorsal foregut endoderm. Loss of Foxa2 in ventral endoderm resulted in reduced expression of Sox17, Gata4, and ZO proteins, accounting at least in part for observed lack of foregut fusion, cardia bifida, and increased apoptosis of ventral endoderm.

Differentiation of definitive endoderm from mouse embryonic stem cells

Efficient production of definitive endoderm from embryonic stem (ES) cells opens doors to the possibilities of differentiation of endoderm-derived tissues such as the intestines, pancreas, and liver that could address the needs of people with chronic diseases involving these organs. The lessons learned from developmental biology have contributed significantly to in vitro differentiation of definitive endoderm. Gastrulation, a process that results in the production of all three embryonic germ cell layers, definitive endoderm, mesoderm, and ectoderm, is an important step in embryonic development. Gastrulation occurs as a result of the events that are orchestrated by the signaling pathways involving Nodal, FGF, Wnt, and BMP. Understanding these signaling pathways has led to the introduction of key ingredients such as Activin A, FGF, Wnt, and BMP to the differentiation protocols that have been able to produce definitive endoderm from ES cells. Efficient production of definitive endoderm needs to meet the specific criteria that include (a) increase in the production of markers of definitive endoderm such as Sox 17, FOXA2, GSC, and Mixl1; (b) decrease in the production of markers of primitive/visceral/parietal endoderm, Sox 7 and OCT4; and (c) decrease in the mesoderm markers (Brachyury, MEOX) and ectoderm markers (Sox1 and ZIC1).

Live imaging fluorescent proteins in early mouse embryos

Mouse embryonic development comprises highly dynamic and coordinated events that drive key cell lineage specification and morphogenetic events. These processes involve cellular behaviors including proliferation, migration, apoptosis, and differentiation, each of which is regulated both spatially and temporally. Live imaging of developing embryos provides an essential tool to investigate these coordinated processes in three-dimensional space over time. For this purpose, the development and application of genetically encoded fluorescent protein (FP) reporters has accelerated over the past decade allowing for the high-resolution visualization of developmental progression. Ongoing efforts are aimed at generating improved reporters, where spectrally distinct as well as novel FPs whose optical properties can be photomodulated, are exploited for live imaging of mouse embryos. Moreover, subcellular tags in combination with using FPs allow for the visualization of multiple subcellular characteristics, such as cell position and cell morphology, in living embryos. Here, we review recent advances in the application of FPs for live imaging in the early mouse embryo, as well as some of the methods used for ex utero embryo development that facilitate on-stage time-lapse specimen visualization.

Mouse primitive streak forms in situ by initiation of epithelial to mesenchymal transition without migration of a cell population

BACKGROUND

During gastrulation, an embryo acquires the three primordial germ layers that will give rise to all of the tissues in the body. In amniote embryos, this process occurs via an epithelial to mesenchymal transition (EMT) of epiblast cells at the primitive streak. Although the primitive streak is vital to development, many aspects of how it forms and functions remain poorly understood.

RESULTS

Using live, 4 dimensional imaging and immunohistochemistry, we have shown that the posterior epiblast of the pre-streak murine embryo does not display convergence and extension behavior or large scale migration or rearrangement of a cell population. Instead, the primitive streak develops in situ and elongates by progressive initiation EMT in the posterior epiblast. Loss of basal lamina (BL) is the first step of this EMT, and is strictly correlated with ingression of nascent mesoderm. Once the BL is lost in a given region, cells leave the epiblast by apical constriction in order to enter the primitive streak.

CONCLUSIONS

This is the first description of dynamic cell behavior during primitive streak formation in the mouse embryo, and reveals mechanisms that are quite distinct from those observed in other amniote model systems. Unlike chick and rabbit, the murine primitive streak arises in situ by progressive initiation of EMT beginning in the posterior epiblast, without large-scale movement or convergence and extension of epiblast cells.

Dlg3 trafficking and apical tight junction formation is regulated by nedd4 and nedd4-2 e3 ubiquitin ligases

The Drosophila Discs large (Dlg) scaffolding protein acts as a tumor suppressor regulating basolateral epithelial polarity and proliferation. In mammals, four Dlg homologs have been identified; however, their functions in cell polarity remain poorly understood. Here, we demonstrate that the X-linked mental retardation gene product Dlg3 contributes to apical-basal polarity and epithelial junction formation in mouse organizer tissues, as well as to planar cell polarity in the inner ear. We purified complexes associated with Dlg3 in polarized epithelial cells, including proteins regulating directed trafficking and tight junction formation. Remarkably, of the four Dlg family members, Dlg3 exerts a distinct function by recruiting the ubiquitin ligases Nedd4 and Nedd4-2 through its PPxY motifs. We found that these interactions are required for Dlg3 monoubiquitination, apical membrane recruitment, and tight junction consolidation. Our findings reveal an unexpected evolutionary diversification of the vertebrate Dlg family in basolateral epithelium formation.

Structure and biology of the intervertebral disk in health and disease

The intervertebral disks along the spine provide motion and protection against mechanical loading. The 3 structural components, nucleus pulposus, annulus fibrosus, and cartilage endplate, function as a synergistic unit, though each has its own role. The cells within each of these components have distinct origins in development and morphology, producing specific extracellular matrix proteins that are organized into unique architectures fit for intervertebral disk function. This article focuses on various aspects of intervertebral disk biology and disruptions that could lead to diseases such as intervertebral disk degeneration.

Rac1 mediates morphogenetic responses to intercellular signals in the gastrulating mouse embryo

The establishment of the mammalian body plan depends on signal-regulated cell migration and adhesion, processes that are controlled by the Rho family of GTPases. Here we use a conditional allele of Rac1, the only Rac gene expressed early in development, to define its roles in the gastrulating mouse embryo. Embryos that lack Rac1 in the epiblast (Rac1Δepi) initiate development normally: the signaling pathways required for gastrulation are active, definitive endoderm and all classes of mesoderm are specified, and the neural plate is formed. After the initiation of gastrulation, Rac1Δepi embryos have an enlarged primitive streak, make only a small amount of paraxial mesoderm, and the lateral anlage of the heart do not fuse at the midline. Because these phenotypes are also seen in Nap1 mutants, we conclude that Rac1 acts upstream of the Nap1/WAVE complex to promote migration of the nascent mesoderm. In addition to migration phenotypes, Rac1Δepi cells fail to adhere to matrix, which leads to extensive cell death. Cell death is largely rescued in Rac1Δepi mutants that are heterozygous for a null mutation in Pten, providing evidence that Rac1 is required to link signals from the basement membrane to activation of the PI3K-Akt pathway in vivo. Surprisingly, the frequency of apoptosis is greater in the anterior half of the embryo, suggesting that cell survival can be promoted either by matrix adhesion or by signals from the posterior primitive streak. Rac1 also has essential roles in morphogenesis of the posterior notochordal plate (the node) and the midline.

Essential roles of fibronectin in the development of the left-right embryonic body plan

Studies in Xenopus laevis suggested that cell-extracellular matrix (ECM) interactions regulate the development of the left-right axis of asymmetry; however, the identities of ECM components and their receptors important for this process have remained unknown. We discovered that FN is required for the establishment of the asymmetric gene expression pattern in early mouse embryos by regulating morphogenesis of the node, while cellular fates of the nodal cells, canonical Wnt and Shh signaling within the node were not perturbed by the absence of FN. FN is also required for the expression of Lefty 1/2 and activation of SMADs 2 and 3 at the floor plate, while cell fate specification of the notochord and the floor plate, as well as signaling within and between these two embryonic organizing centers remained intact in FN-null mutants. Furthermore, our experiments indicate that a major cell surface receptor for FN, integrin α5β1, is also required for the development of the left-right asymmetry, and that this requirement is evolutionarily conserved in fish and mice. Taken together, our studies demonstrate the requisite role for a structural ECM protein and its integrin receptor in the development of the left-right axis of asymmetry in vertebrates.

Loss of procollagen IIA from the anterior mesendoderm disrupts the development of mouse embryonic forebrain

Morphogenesis of the mammalian forebrain is influenced by the patterning activity of signals emanating from the anterior mesendoderm. In this study, we show that procollagen IIA (IIA), an isoform of the cartilage extracellular matrix protein encoded by an alternatively spliced transcript of Col2a1, is expressed in the prechordal plate and the anterior definitive endoderm. In the absence of IIA activity, the null mutants displayed a partially penetrant phenotype of loss of head tissues, holoprosencephaly, and loss of mid-facial structures, which is associated with reduced sonic hedgehog (Shh) expression in the prechordal mesoderm. Genetic interaction studies reveal that IIA function in forebrain and face development does not involve bone morphogenetic protein receptor 1A (BMPR1A)- or NODAL-mediated signaling activity.

Germ layer differentiation during early hindgut and cloaca formation in rabbit and pig embryos

Relative to recent advances in understanding molecular requirements for endoderm differentiation, the dynamics of germ layer morphology and the topographical distribution of molecular factors involved in endoderm formation at the caudal pole of the embryonic disc are still poorly defined. To discover common principles of mammalian germ layer development, pig and rabbit embryos at late gastrulation and early neurulation stages were analysed as species with a human-like embryonic disc morphology, using correlative light and electron microscopy. Close intercellular contact but no direct structural evidence of endoderm formation such as mesenchymal-epithelial transition between posterior primitive streak mesoderm and the emerging posterior endoderm were found. However, a two-step process closely related to posterior germ layer differentiation emerged for the formation of the cloacal membrane: (i) a continuous mesoderm layer and numerous patches of electron-dense flocculent extracellular matrix mark the prospective region of cloacal membrane formation; and (ii) mesoderm cells and all extracellular matrix including the basement membrane are lost locally and close intercellular contact between the endoderm and ectoderm is established. The latter process involves single cells at first and then gradually spreads to form a longitudinally oriented seam-like cloacal membrane. These gradual changes were found from gastrulation to early somite stages in the pig, whereas they were found from early somite to mid-somite stages in the rabbit; in both species cloacal membrane formation is complete prior to secondary neurulation. The results highlight the structural requirements for endoderm formation during development of the hindgut and suggest new mechanisms for the pathogenesis of common urogenital and anorectal malformations.

Left-right patterning in the mouse requires Epb4.1l5-dependent morphogenesis of the node and midline

The mouse node is a transient early embryonic structure that is required for left-right asymmetry and for generation of the axial midline, which patterns neural and mesodermal tissues. The node is a shallow teardrop-shaped pit that sits at the distal tip of the early headfold (e7.75) embryo. The shape of the node is believed to be important for generation of the coherent leftward fluid flow required for initiation of left-right asymmetry, but little is known about the morphogenesis of the node. Here we show that the FERM domain protein Lulu/Epb4.1l5 is required for left-right asymmetry in the early mouse embryo. Unlike other genes previously shown to be required for left-right asymmetry in the mouse, lulu is not required for specification of node cell identity, for Nodal signaling in the node or for ciliogenesis. Instead, lulu is required for proper morphogenesis of the node and midline. The precursors of the wild-type node undergo a series of rapid morphological transitions. First, node precursors arise from an epithelial-to-mesenchymal transition at the anterior primitive streak. While in the mesenchymal layer, the node precursors form several ciliated rosette-like clusters; they then rapidly undergo a mesenchymal-to-epithelial transition to insert into the outer, endodermal layer of the embryo. In lulu mutants, node precursor cells are specified and form clusters, but those clusters fail to coalesce to make a single continuous node epithelium. The data suggest that the assembly of the contiguous node epithelium from mesenchymal clusters requires a rapid reorganization of apical-basal polarity that depends on Lulu/Epb4.1l5.

BMP antagonism protects Nodal signaling in the gastrula to promote the tissue interactions underlying mammalian forebrain and craniofacial patterning

Holoprosencephaly (HPE) is the most common forebrain and craniofacial malformation syndrome in humans. The genetics of HPE suggest that it often stems from a synergistic interaction of mutations in independent loci. In mice, several combinations of mutations in Nodal signaling pathway components can give rise to HPE, but it is not clear whether modest deficits of Nodal signaling along with lesions in other pathways might also cause such defects. We find that HPE results from simultaneous reduction of Nodal signaling and an organizer BMP (bone morphogenetic protein) antagonist, either Chordin or Noggin. These defects result from reduced production of tissues that promote forebrain and craniofacial development. Nodal promotes the expression of genes in the anterior primitive streak that are important for the development of these tissues, whereas BMP inhibits their expression. Pharmacological and transgenic manipulation of these signaling pathways suggests that BMP and Nodal antagonize each other prior to intracellular signal transduction. Biochemical experiments in vitro indicate that secreted Bmp2 and Nodal can form extracellular complexes, potentially interfering with receptor activation. Our results reveal that the patterning of forebrain and medial craniofacial elements requires a fine balance between BMP and Nodal signaling during primitive streak development, and provide a potential mechanistic basis for a new multigenic model of HPE.