Wnt signaling is a key mediator of Cdx1 expression in vivo

Molecular landscape of congenital vertebral malformations: recent discoveries and future directions

Vertebral malformations (VMs) pose a significant global health problem, causing chronic pain and disability. Vertebral defects occur as isolated conditions or within the spectrum of various congenital disorders, such as Klippel-Feil syndrome, congenital scoliosis, spondylocostal dysostosis, sacral agenesis, and neural tube defects. Although both genetic abnormalities and environmental factors can contribute to abnormal vertebral development, our knowledge on molecular mechanisms of numerous VMs is still limited. Furthermore, there is a lack of resource that consolidates the current knowledge in this field. In this pioneering review, we provide a comprehensive analysis of the latest research on the molecular basis of VMs and the association of the VMs-related causative genes with bone developmental signaling pathways. Our study identifies 118 genes linked to VMs, with 98 genes involved in biological pathways crucial for the formation of the vertebral column. Overall, the review summarizes the current knowledge on VM genetics, and provides new insights into potential involvement of biological pathways in VM pathogenesis. We also present an overview of available data regarding the role of epigenetic and environmental factors in VMs. We identify areas where knowledge is lacking, such as precise molecular mechanisms in which specific genes contribute to the development of VMs. Finally, we propose future research avenues that could address knowledge gaps.

Wnt target enhancer regulation by a CDX/TCF transcription factor collective and a novel DNA motif

Transcriptional regulation by Wnt signalling is primarily thought to be accomplished by a complex of β-catenin and TCF family transcription factors (TFs). Although numerous studies have suggested that additional TFs play roles in regulating Wnt target genes, their mechanisms of action have not been investigated in detail. We characterised a Wnt-responsive element (WRE) downstream of the Wnt target gene Axin2 and found that TCFs and Caudal type homeobox (CDX) proteins were required for its activation. Using a new separation-of-function TCF mutant, we found that WRE activity requires the formation of a TCF/CDX complex. Our systematic mutagenesis of this enhancer identified other sequences essential for activation by Wnt signalling, including several copies of a novel CAG DNA motif. Computational and experimental evidence indicates that the TCF/CDX/CAG mode of regulation is prevalent in multiple WREs. Put together, our results demonstrate the complex nature of cis- and trans- interactions required for signal-dependent enhancer activity.

The hemogenic endothelium: a critical source for the generation of PSC-derived hematopoietic stem and progenitor cells

In vitro generation of hematopoietic cells and especially hematopoietic stem cells (HSCs) from human pluripotent stem cells (PSCs) are subject to intensive research in recent decades, as these cells hold great potential for regenerative medicine and autologous cell replacement therapies. Despite many attempts, in vitro, de novo generation of bona fide HSCs remains challenging, and we are still far away from their clinical use, due to insufficient functionality and quantity of the produced HSCs. The challenges of generating PSC-derived HSCs are already apparent in early stages of hemato-endothelial specification with the limitation of recapitulating complex, dynamic processes of embryonic hematopoietic ontogeny in vitro. Further, these current shortcomings imply the incompleteness of our understanding of human ontogenetic processes from embryonic mesoderm over an intermediate, specialized hemogenic endothelium (HE) to their immediate progeny, the HSCs. In this review, we examine the recent investigations of hemato-endothelial ontogeny and recently reported progress for the conversion of PSCs and other promising somatic cell types towards HSCs with the focus on the crucial and inevitable role of the HE to achieve the long-standing goal—to generate therapeutically applicable PSC-derived HSCs in vitro.

Casein kinase 2 interacting protein 1 positively regulates caudal-related homeobox 1 in intestinal-type gastric cancer

BACKGROUND

Gastric cancer (GC) is one of the most common malignancies, and intestinal-type GC is the main histopathologic type of GC in China. We previously reported that casein kinase 2 interacting protein 1 (CKIP-1) acts as a candidate tumor suppressor in intestinal-type GC. CKIP-1 participates in the regulation of multiple signaling pathways, including the Wnt/β-catenin pathway, of which caudal-related homeobox 1 (CDX1) may be a downstream target gene. The purpose of this study was to investigate the relationship between CKIP-1 and CDX1 in intestinal-type GC.

METHODS

Sixty-seven gastroscopy biopsy specimens and surgically resected gastric specimens were divided into four groups: gastric mucosa group, intestinal metaplasia (IM) group, dysplasia group, and intestinal-type GC group. The expression levels of CKIP-1 and CDX1 were detected in these groups and GC cell lines, and the correlations between these expression levels were analyzed. SGC7901 and BGC823 cells were divided into CKIP-1 shRNA groups and CKIP-1 over-expression groups, and CDX1 expression was detected. β-Catenin expression was detected in intestinal-type GC tissue samples and CKIP-1 shRNA and CKIP-1 over-expression SGC7901 cells, and its correlation with CKIP-1 expression in intestinal-type GC tissue was analyzed. The Wnt/β-catenin pathway inhibitor DKK-1 and activator LiCl were incubated with SGC7901 cells, BGC823 cells, and CKIP-1 shRNA and CKIP-1 over-expression SGC7901 and BGC823 cells, following which CDX1 and Ki-67 expression were detected.

RESULTS

The expression levels of CKIP-1 and CDX1 were lower in patients with intestinal-type GC than in patients with IM and dysplasia (both P < 0.05). CKIP-1 and CDX1 expression levels were positively correlated in IM, dysplasia, and intestinal-type GC tissue and cell lines (r = 0.771, P < 0.01; r = 0.597, P < 0.01; r = 0.654, P < 0.01; r = 0.811, P < 0.01, respectively). CDX1 expression was decreased in the CKIP-1 shRNA groups and increased in the CKIP-1 over-expression groups of SGC7901 and BGC823 cells compared to that in the corresponding control groups (both P < 0.05). CKIP-1 expression was negatively correlated with β-catenin expression in intestinal-type GC patients (r = -0.458, P < 0.01). Compared to the control group, β-catenin expression was increased in the CKIP-1 shRNA SGC7901 cell group and decreased in the CKIP-1 over-expression SGC7901 cell group (P < 0.05). CDX1 expression was increased in SGC7901 and BGC823 cells treated with DKK-1, DKK-1 increased CDX1 expression and decreased Ki-67 expression in the CKIP-1 shRNA group; the opposite result was observed in SGC7901 and BGC823 cells treated with LiCl, and LiCl decreased CDX1 expression and increased Ki-67 expression in the CKIP-1 over-expression group (both P < 0.05).

CONCLUSIONS

Through the Wnt/β-catenin signaling pathway, CKIP-1 may positively regulate CDX1 in intestinal-type GC.

Activin A and BMP4 Signaling Expands Potency of Mouse Embryonic Stem Cells in Serum-Free Media

Inhibitors of Mek1/2 and Gsk3β, known as 2i, and, together with leukemia inhibitory factor, enhance the derivation of embryonic stem cells (ESCs) and promote ground-state pluripotency (2i/L-ESCs). However, recent reports show that prolonged Mek1/2 suppression impairs developmental potential of ESCs, and is rescued by serum (S/L-ESCs). Here, we show that culturing ESCs in Activin A and BMP4, and in the absence of MEK1/2 inhibitor (ABC/L medium), establishes advanced stem cells derived from ESCs (esASCs). We demonstrate that esASCs contributed to germline lineages, full-term chimeras and generated esASC-derived mice by tetraploid complementation. We show that, in contrast to 2i/L-ESCs, esASCs display distinct molecular signatures and a stable hypermethylated epigenome, which is reversible and similar to serum-cultured ESCs. Importantly, we also derived novel ASCs (blASCs) from blastocysts in ABC/L medium. Our results provide insights into the derivation of novel ESCs with DNA hypermethylation from blastocysts in chemically defined medium.

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Activin A and BMP4 expand potency of mouse ESCs

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ASCs are hypermethylated and with stable genomic imprints

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ASCs developmentally closed to E4.5–E6.5 in vivo epiblast

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Hypermethylated ASCs directly derived from blastocyst by ABC/L medium

A novel group of secretory cells regulates development of the immature intestinal stem cell niche through repression of the main signaling pathways driving proliferation

The intestinal epithelium has constant turnover throughout the life of the organ, with apoptosis of cells at the tips of folds or villi releasing cells into the lumen. Due to constant turnover, epithelial cells need to be constantly replaced. Epithelial cells are supplied by stem cell niches that form at the base of the interfold space (zebrafish) and crypts (birds and mammals). Within the adult stem cell niche of mammals, secretory cells such as Paneth and goblet cells play a role in modulation of proliferation and stem cell activity, producing asymmetric divisions. Progeny of asymmetric divisions move up the fold or villi, giving rise to all of the epithelial cell types. Although much is known about function and organization of the adult intestinal stem cell niche, less is understood about regulation within the immature stem cell compartment. Following smooth muscle formation, the intestinal epithelium folds and proliferation becomes restricted to the interfold base. Symmetric divisions continue in the developing interfold niche until stem cell progeny begin asymmetric divisions, producing progeny that migrate up the developing folds. Proliferative progeny from the developing stem cell niche begin migrating out of the niche during the third week post-embryogenesis (zebrafish) or during the postnatal period (mammals). Regulation and organization of epithelial proliferation in the immature stem cell niche may be regulated by signals comparable to the adult niche. Here we identify a novel subset of secretory cells associated with the developing stem cell niche that receive Notch signaling (referred to as NRSCs). Inhibition of the embryonic NRSCs between 74 hpf to 120 hpf increases epithelial proliferation as well as EGF and IGF signaling. Inhibition of post-embryonic NRSCs (6 hpf to 12 dpf) also increases epithelial proliferation and expression level of Wnt target genes. We conclude that NRSCs play a role in modulation of epithelial proliferation through repression of signaling pathways that drive proliferation during both embryogenesis and the post embryonic period.

Wnt/β-catenin signaling during early vertebrate neural development

The vertebrate central nervous system (CNS) is comprised of vast number of distinct cell types arranged in a highly organized manner. This high degree of complexity is achieved by cellular communication, including direct cell-cell contact, cell-matrix interactions, and cell-growth factor signaling. Among the several developmental signals controlling the development of the CNS, Wnt proteins have emerged as particularly critical and, hence, have captivated the attention of many researchers. With Wnts' evolutionarily conserved function as primordial symmetry breaking signals, these proteins and their downstream effects are responsible for simultaneously establishing cellular diversity and tissue organization. With their expansive repertoire of secreted agonists and antagonists, cell surface receptors, signaling cascades and downstream biological effects, Wnts are ideally suited to control the complex processes underlying vertebrate neural development. In this review, we will describe the mechanisms by which Wnts exert their potent effects on cells and tissues and highlight the many roles of Wnt signaling during neural development, starting from the initial induction of the neural plate, the subsequent patterning along the embryonic axes, to the intricately organized structure of the CNS. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1239-1259, 2017.

Status of Epstein-Barr Virus Coinfection with Helicobacter pylori in Gastric Cancer

Epstein-Barr virus is a ubiquitous human herpesvirus whose primary infection causes mononucleosis, Burkett's lymphoma, nasopharyngeal carcinoma, autoimmune diseases, and gastric cancer (GC). The persistent infection causes malignancies in lymph and epithelial cells. Helicobacter pylori causes gastritis in human with chronic inflammation. This chronic inflammation is thought to be the cause of genomic instability. About 45%-word population have a probability of having both pathogens, namely, H. pylori and EBV. Approximately 180 per hundred thousand population is developing GC along with many gastric abnormalities. This makes GC the third leading cause of cancer-related death worldwide. Although lots of research are carried out individually for EBV and H. pylori, still there are very few reports available on coinfection of both pathogens. Recent studies suggested that EBV and H. pylori coinfection increases the occurrence of GC as well as the early age of GC detection comparing to individual infection. The aim of this review is to present status on coinfection of both pathogens and their association with GC.

Essential roles for Cdx in murine primitive hematopoiesis

The Cdx transcription factors play essential roles in primitive hematopoiesis in the zebrafish where they exert their effects, in part, through regulation of hox genes. Defects in hematopoiesis have also been reported in Cdx mutant murine embryonic stem cell models, however, to date no mouse model reflecting the zebrafish Cdx mutant hematopoietic phenotype has been described. This is likely due, in part, to functional redundancy among Cdx members and the early lethality of Cdx2 null mutants. To circumvent these limitations, we used Cre-mediated conditional deletion to assess the impact of concomitant loss of Cdx1 and Cdx2 on murine primitive hematopoiesis. We found that Cdx1/Cdx2 double mutants exhibited defects in primitive hematopoiesis and yolk sac vasculature concomitant with reduced expression of several genes encoding hematopoietic transcription factors including Scl/Tal1. Chromatin immunoprecipitation analysis revealed that Scl was occupied by Cdx2 in vivo, and Cdx mutant hematopoietic yolk sac differentiation defects could be rescued by expression of exogenous Scl. These findings demonstrate critical roles for Cdx members in murine primitive hematopoiesis upstream of Scl.

Early molecular events during retinoic acid induced differentiation of neuromesodermal progenitors

Bipotent neuromesodermal progenitors (NMPs) residing in the caudal epiblast drive coordinated body axis extension by generating both posterior neuroectoderm and presomitic mesoderm. Retinoic acid (RA) is required for body axis extension, however the early molecular response to RA signaling is poorly defined, as is its relationship to NMP biology. As endogenous RA is first seen near the time when NMPs appear, we used WNT/FGF agonists to differentiate embryonic stem cells to NMPs which were then treated with a short 2-h pulse of 25 nM RA or 1 µM RA followed by RNA-seq transcriptome analysis. Differential expression analysis of this dataset indicated that treatment with 25 nM RA, but not 1 µM RA, provided physiologically relevant findings. The 25 nM RA dataset yielded a cohort of previously known caudal RA target genes including Fgf8 (repressed) and Sox2 (activated), plus novel early RA signaling targets with nearby conserved RA response elements. Importantly, validation of top-ranked genes in vivo using RA-deficient Raldh2^-/- embryos identified novel examples of RA activation (Nkx1-2, Zfp503, Zfp703, Gbx2, Fgf15, Nt5e) or RA repression (Id1) of genes expressed in the NMP niche or progeny. These findings provide evidence for early instructive and permissive roles of RA in controlling differentiation of NMPs to neural and mesodermal lineages.

OCT4 Acts as an Integrator of Pluripotency and Signal-Induced Differentiation

Cell type specification relies on the capacity of undifferentiated cells to properly respond to specific differentiation-inducing signals. Using genomic approaches along with loss- and gain-of-function genetic models, we identified OCT4-dependent mechanisms that provide embryonic stem cells with the means to customize their response to external cues. OCT4 binds a large set of low-accessible genomic regions. At these sites, OCT4 is required for proper enhancer and gene activation by recruiting co-regulators and RAR:RXR or β-catenin, suggesting an unexpected collaboration between the lineage-determining transcription factor and these differentiation-initiating, signal-dependent transcription factors. As a proof of concept, we demonstrate that overexpression of OCT4 in a kidney cell line is sufficient for signal-dependent activation of otherwise unresponsive genes in these cells. Our results uncover OCT4 as an integral and necessary component of signal-regulated transcriptional processes required for tissue-specific responses.

Zfp703 Is a Wnt/β-Catenin Feedback Suppressor Targeting the β-Catenin/Tcf1 Complex

The Wnt/β-catenin signaling pathway controls embryonic development and adult stem cell maintenance through the regulation of transcription. Failure to downregulate Wnt signaling can result in embryonic malformations and cancer, highlighting the important role of negative regulators of the pathway. The Wnt pathway activates several negative feedback targets, including axin2 and Dkk1, that function at different levels of the signaling cascade; however, none have been identified that directly target active β-catenin/Tcf1 transcriptional complexes. We show that Zfp703 is a Wnt target gene that inhibits Wnt/β-catenin activity in Wnt reporter assays and in Wnt-dependent mesoderm differentiation in embryonic stem cells. Zfp703 binds directly to Tcf1 to inhibit β-catenin/Tcf1 complex formation and does so independently of the Groucho/Tle transcriptional corepressor. We propose that Zfp703 is a novel feedback suppressor of Wnt/β-catenin signaling that functions by inhibiting the association of β-catenin with Tcf1 on Wnt response elements in target gene enhancers.

A direct role for murine Cdx proteins in the trunk neural crest gene regulatory network

Numerous studies in chordates and arthropods currently indicate that Cdx proteins have a major ancestral role in the organization of post-head tissues. In urochordate embryos, Cdx loss-of-function has been shown to impair axial elongation, neural tube (NT) closure and pigment cell development. Intriguingly, in contrast to axial elongation and NT closure, a Cdx role in neural crest (NC)-derived melanocyte/pigment cell development has not been reported in any other chordate species. To address this, we generated a new conditional pan-Cdx functional knockdown mouse model that circumvents Cdx functional redundancy as well as the early embryonic lethality of Cdx mutants. Through directed inhibition in the neuroectoderm, we provide in vivo evidence that murine Cdx proteins impact melanocyte and enteric nervous system development by, at least in part, directly controlling the expression of the key early regulators of NC ontogenesis Pax3,Msx1 and Foxd3 Our work thus reveals a novel role for Cdx proteins at the top of the trunk NC gene regulatory network in the mouse, which appears to have been inherited from their ancestral ortholog.

TCF/Lef regulates the Gsx ParaHox gene in central nervous system development in chordates

BACKGROUND

The ParaHox genes play an integral role in the anterior-posterior (A-P) patterning of the nervous system and gut of most animals. The ParaHox cluster is an ideal system in which to study the evolution and regulation of developmental genes and gene clusters, as it displays similar regulatory phenomena to its sister cluster, the Hox cluster, but offers a much simpler system with only three genes.

RESULTS

Using Ciona intestinalis transgenics, we isolated a regulatory element upstream of Branchiostoma floridae Gsx that drives expression within the central nervous system of Ciona embryos. The minimal amphioxus enhancer region required to drive CNS expression has been identified, along with surrounding sequence that increases the efficiency of reporter expression throughout the Ciona CNS. TCF/Lef binding sites were identified and mutagenized and found to be required to drive the CNS expression. Also, individual contributions of TCF/Lef sites varied across the regulatory region, revealing a partial division of function across the Bf-Gsx-Up regulatory element. Finally, when all TCF/Lef binding sites are mutated CNS expression is not only abolished, but a latent repressive function is also unmasked.

CONCLUSIONS

We have identified a B. floridae Gsx upstream regulatory element that drives CNS expression within transgenic Ciona intestinalis, and have shown that this CNS expression is dependent upon TCF/Lef binding sites. We examine the evolutionary and developmental implications of these results, and discuss the possibility of TCF/Lef not only as a regulator of chordate Gsx, but as a deeply conserved regulatory factor controlling all three ParaHox genes across the Metazoa.

Inhibition of β-catenin-TCF1 interaction delays differentiation of mouse embryonic stem cells

Blocking β-catenin/TCF1–mediated transcriptional activation with a specific small molecule or by TCF1 knockdown delays the mouse embryonic stem cell differentiation program and enhances pluripotency.

The ability of mouse embryonic stem cells (mESCs) to self-renew or differentiate into various cell lineages is regulated by signaling pathways and a core pluripotency transcriptional network (PTN) comprising Nanog, Oct4, and Sox2. The Wnt/β-catenin pathway promotes pluripotency by alleviating T cell factor TCF3-mediated repression of the PTN. However, it has remained unclear how β-catenin’s function as a transcriptional activator with TCF1 influences mESC fate. Here, we show that TCF1-mediated transcription is up-regulated in differentiating mESCs and that chemical inhibition of β-catenin/TCF1 interaction improves long-term self-renewal and enhances functional pluripotency. Genetic loss of TCF1 inhibited differentiation by delaying exit from pluripotency and conferred a transcriptional profile strikingly reminiscent of self-renewing mESCs with high Nanog expression. Together, our data suggest that β-catenin’s function in regulating mESCs is highly context specific and that its interaction with TCF1 promotes differentiation, further highlighting the need for understanding how its individual protein–protein interactions drive stem cell fate.

Lineage-Specific Profiling Delineates the Emergence and Progression of Naive Pluripotency in Mammalian Embryogenesis

Naive pluripotency is manifest in the preimplantation mammalian embryo. Here we determine transcriptome dynamics of mouse development from the eight-cell stage to postimplantation using lineage-specific RNA sequencing. This method combines high sensitivity and reporter-based fate assignment to acquire the full spectrum of gene expression from discrete embryonic cell types. We define expression modules indicative of developmental state and temporal regulatory patterns marking the establishment and dissolution of naive pluripotency in vivo. Analysis of embryonic stem cells and diapaused embryos reveals near-complete conservation of the core transcriptional circuitry operative in the preimplantation epiblast. Comparison to inner cell masses of marmoset primate blastocysts identifies a similar complement of pluripotency factors but use of alternative signaling pathways. Embryo culture experiments further indicate that marmoset embryos utilize WNT signaling during early lineage segregation, unlike rodents. These findings support a conserved transcription factor foundation for naive pluripotency while revealing species-specific regulatory features of lineage segregation.

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.

Structure-Function Analysis of the Drosophila melanogaster Caudal Transcription Factor Provides Insights into Core Promoter-preferential Activation

Regulation of RNA polymerase II transcription is critical for the proper development, differentiation, and growth of an organism. The RNA polymerase II core promoter is the ultimate target of a multitude of transcription factors that control transcription initiation. Core promoters encompass the RNA start site and consist of functional elements such as the TATA box, initiator, and downstream core promoter element (DPE), which confer specific properties to the core promoter. We have previously discovered that Drosophila Caudal, which is a master regulator of genes involved in development and differentiation, is a DPE-specific transcriptional activator. Here, we show that the mouse Caudal-related homeobox (Cdx) proteins (mCdx1, mCdx2, and mCdx4) are also preferential core promoter transcriptional activators. To elucidate the mechanism that enables Caudal to preferentially activate DPE transcription, we performed structure-function analysis. Using a systematic series of deletion mutants (all containing the intact DNA-binding homeodomain) we discovered that the C-terminal region of Caudal contributes to the preferential activation of the fushi tarazu (ftz) Caudal target gene. Furthermore, the region containing both the homeodomain and the C terminus of Caudal was sufficient to confer core promoter-preferential activation to the heterologous GAL4 DNA-binding domain. Importantly, we discovered that Drosophila CREB-binding protein (dCBP) is a co-activator for Caudal-regulated activation of ftz. Strikingly, dCBP conferred the ability to preferentially activate the DPE-dependent ftz reporter to mini-Caudal proteins that were unable to preferentially activate ftz transcription themselves. Taken together, it is the unique combination of dCBP and Caudal that enables the co-activation of ftz in a core promoter-preferential manner.

The orphan GPCR, Gpr161, regulates the retinoic acid and canonical Wnt pathways during neurulation

The vacuolated lens (vl) mouse mutation arose on the C3H/HeSnJ background and results in lethality, neural tube defects (NTDs) and cataracts. The vl phenotypes are due to a deletion/frameshift mutation in the orphan GPCR, Gpr161. A recent study using a null allele demonstrated that Gpr161 functions in primary cilia and represses the Shh pathway. We show the hypomorphic Gpr161(vl) allele does not severely affect the Shh pathway. To identify additional pathways regulated by Gpr161 during neurulation, we took advantage of naturally occurring genetic variation in the mouse. Previously Gpr161(vl-C3H) was crossed to different inbred backgrounds including MOLF/EiJ and the Gpr161(vl) mutant phenotypes were rescued. Five modifiers were mapped (Modvl: Modifier of vl) including Modvl5(MOLF). In this study we demonstrate the Modvl5(MOLF) congenic rescues the Gpr161(vl)-associated lethality and NTDs but not cataracts. Bioinformatics determined the transcription factor, Cdx1, is the only annotated gene within the Modvl5 95% CI co-expressed with Gpr161 during neurulation and not expressed in the eye. Using Cdx1 as an entry point, we identified the retinoid acid (RA) and canonical Wnt pathways as downstream targets of Gpr161. QRT-PCR, ISH and IHC determined that expression of RA and Wnt genes are down-regulated in Gpr161(vl/vl) but rescued by the Modvl5(MOLF) congenic during neurulation. Intraperitoneal RA injection restores expression of canonical Wnt markers and rescues Gpr161(vl/vl) NTDs. These results establish the RA and canonical Wnt as pathways downstream of Gpr161 during neurulation, and suggest that Modvl5(MOLF) bypasses the Gpr161(vl) mutation by restoring the activity of these pathways.

Caudal regulates the spatiotemporal dynamics of pair-rule waves in Tribolium

In the short-germ beetle Tribolium castaneum, waves of pair-rule gene expression propagate from the posterior end of the embryo towards the anterior and eventually freeze into stable stripes, partitioning the anterior-posterior axis into segments. Similar waves in vertebrates are assumed to arise due to the modulation of a molecular clock by a posterior-to-anterior frequency gradient. However, neither a molecular candidate nor a functional role has been identified to date for such a frequency gradient, either in vertebrates or elsewhere. Here we provide evidence that the posterior gradient of Tc-caudal expression regulates the oscillation frequency of pair-rule gene expression in Tribolium. We show this by analyzing the spatiotemporal dynamics of Tc-even-skipped expression in strong and mild knockdown of Tc-caudal, and by correlating the extension, level and slope of the Tc-caudal expression gradient to the spatiotemporal dynamics of Tc-even-skipped expression in wild type as well as in different RNAi knockdowns of Tc-caudal regulators. Further, we show that besides its absolute importance for stripe generation in the static phase of the Tribolium blastoderm, a frequency gradient might serve as a buffer against noise during axis elongation phase in Tribolium as well as vertebrates. Our results highlight the role of frequency gradients in pattern formation.

One of the most popular problems in development is how the anterior-posterior axis of vertebrates, arthropods and annelids is partitioned into segments. In vertebrates, and recently shown in the beetle Tribolium castaneum, segments are demarcated by means of gene expression waves that propagate from posterior to anterior as the embryo elongates. These waves are assumed to arise due to the regulation of a molecular clock by a frequency gradient. However, to date, neither a candidate nor a functional role has been identified for such a frequency gradient. Here we provide evidence that a static expression gradient of caudal regulates pair-rule oscillations during blastoderm stage in Tribolium. In such a static setup, a frequency gradient is essential to convert clock oscillations into a striped pattern. We further show that a frequency gradient might be essential even in the presence of axis elongation as a buffer against noise. Our work also provides the best evidence to date that Caudal acts as a type of morphogen gradient in the blastoderm of short-germ arthropods; however, Caudal seems to convey positional information through frequency regulation of pair-rule oscillations, rather than through threshold concentration levels in the gradient.

RAGE, receptor of advanced glycation endoproducts, negatively regulates chondrocytes differentiation

RAGE, receptor for advanced glycation endoproducts (AGE), has been characterized as an activator of osteoclastgenesis. However, whether RAGE directly regulates chondrocyte proliferation and differentiation is unclear. Here, we show that RAGE has an inhibitory role in chondrocyte differentiation. RAGE expression was observed in chondrocytes from the prehypertrophic to hypertrophic regions. In cultured cells, overexpression of RAGE or dominant-negative-RAGE (DN-RAGE) demonstrated that RAGE inhibited cartilaginous matrix production, while DN-RAGE promoted production. Additionally, RAGE regulated Ihh and Col10a1 negatively but upregulated PTHrP receptor. Ihh promoter analysis and real-time PCR analysis suggested that downregulation of Cdxs was the key for RAGE-induced inhibition of chondrocyte differentiation. Overexpression of the NF-κB inhibitor I-κB-SR inhibited RAGE-induced NF-κB activation, but did not influence inhibition of cartilaginous matrix production by RAGE. The inhibitory action of RAGE was restored by the Rho family GTPases inhibitor Toxin B. Furthermore, inhibitory action on Ihh, Col10a1 and Cdxs was reproduced by constitutively active forms, L63RhoA, L61Rac, and L61Cdc42, but not by I-κB-SR. Cdx1 induced Ihh and Col10a1 expressions and directly interacted with Ihh promoter. Retinoic acid (RA) partially rescued the inhibitory action of RAGE. These data combined suggests that RAGE negatively regulates chondrocyte differentiation at the prehypertrophic stage by modulating NF-κB-independent and Rho family GTPases-dependent mechanisms.

Spalt-like 4 promotes posterior neural fates via repression of pou5f3 family members in Xenopus

Amphibian neural development occurs as a two-step process: (1) induction specifies a neural fate in undifferentiated ectoderm; and (2) transformation induces posterior spinal cord and hindbrain. Signaling through the Fgf, retinoic acid (RA) and Wnt/β-catenin pathways is necessary and sufficient to induce posterior fates in the neural plate, yet a mechanistic understanding of the process is lacking. Here, we screened for factors enriched in posterior neural tissue and identify spalt-like 4 (sall4), which is induced by Fgf. Knockdown of Sall4 results in loss of spinal cord marker expression and increased expression of pou5f3.2 (oct25), pou5f3.3 (oct60) and pou5f3.1 (oct91) (collectively, pou5f3 genes), the closest Xenopus homologs of mammalian stem cell factor Pou5f1 (Oct4). Overexpression of the pou5f3 genes results in the loss of spinal cord identity and knockdown of pou5f3 function restores spinal cord marker expression in Sall4 morphants. Finally, knockdown of Sall4 blocks the posteriorizing effects of Fgf and RA signaling in the neurectoderm. These results suggest that Sall4, activated by posteriorizing signals, represses the pou5f3 genes to provide a permissive environment allowing for additional Wnt/Fgf/RA signals to posteriorize the neural plate.

Induction and dorsal restriction of Paired-box 3 (Pax3) gene expression in the caudal neuroectoderm is mediated by integration of multiple pathways on a short neural crest enhancer

Pax3 encodes a paired-box transcription factor with key roles in neural crest and neural tube ontogenesis. Robust control of Pax3 neural expression is ensured by two redundant sets of cis-regulatory modules (CRMs) that integrate anterior-posterior (such as Wnt-βCatenin signaling) as well as dorsal-ventral (such as Shh-Gli signaling) instructive cues. In previous work, we sought to characterize the Wnt-mediated regulation of Pax3 expression and identified the Cdx transcription factors (Cdx1/2/4) as critical intermediates in this process. We identified the neural crest enhancer-2 (NCE2) from the 5'-flanking region of Pax3 as a Cdx-dependent CRM that recapitulates the restricted expression of Pax3 in the mouse caudal neuroectoderm. While this is consistent with a key role in relaying the inductive signal from posteriorizing Wnt ligands, the broad expression of Cdx proteins in the tailbud region is not consistent with the restricted activity of NCE2. This implies that other positive and/or negative inputs are required and, here, we report a novel role for the transcription factor Zic2 in this regulation. Our data strongly suggests that Zic2 is involved in the induction (as a direct Pax3NCE2 activator and Cdx neural cofactor) as well as the maintenance of Pax3 dorsal restriction (as a target of the ventral Shh repressive input). We also provide evidence that the inductive Cdx-Zic2 interaction is integrated on NCE2 with a positive input from the neural-specific transcription factor Sox2. Altogether, our data provide important mechanistic insights into the coordinated integration of different signaling pathways on a short Pax3 CRM.

YY1 acts as a transcriptional activator of Hoxa5 gene expression in mouse organogenesis

The Hox gene family encodes homeodomain-containing transcriptional regulators that confer positional information to axial and paraxial tissues in the developing embryo. The dynamic Hox gene expression pattern requires mechanisms that differentially control Hox transcription in a precise spatio-temporal fashion. This implies an integrated regulation of neighbouring Hox genes achieved through the sharing and the selective use of defined enhancer sequences. The Hoxa5 gene plays a crucial role in lung and gut organogenesis. To position Hoxa5 in the regulatory hierarchy that drives organ morphogenesis, we searched for cis-acting regulatory sequences and associated trans-acting factors required for Hoxa5 expression in the developing lung and gut. Using mouse transgenesis, we identified two DNA regions included in a 1.5-kb XbaI-XbaI fragment located in the Hoxa4-Hoxa5 intergenic domain and known to control Hoxa4 organ expression. The multifunctional YY1 transcription factor binds the two regulatory sequences in vitro and in vivo. Moreover, the mesenchymal deletion of the Yy1 gene function in mice results in a Hoxa5-like lung phenotype with decreased Hoxa5 and Hoxa4 gene expression. Thus, YY1 acts as a positive regulator of Hoxa5 expression in the developing lung and gut. Our data also support a role for YY1 in the coordinated expression of Hox genes for correct organogenesis.

1,25-dihydroxyvitamin D in the pathogenesis of Barrett's esophagus and esophageal adenocarcinoma

The incidence of reflux-related esophageal disease - Barrett's esophagus and esophageal adenocarcinoma - is rising, and the prognosis remains poor. Evidence exists that 1,25-dihydroxyvitamin D may augment the course of colon, breast and prostate cancer but little knowledge exists regarding its impact on disease of the esophagus. Important immune cells involved in reflux-related esophageal disease include CD4(+) T cells, macrophages and dendritic cells, and key signaling pathways include Wnt, Hedgehog, NFκ-B and IL-6-JAK-STAT. There is an inter-relationship between these entities and 1,25-dihydroxyvitamin D, which has been described in animal models and some human tissue. Despite this, there is an incomplete understanding of how the immune cell population and signaling pathways contribute to the course and prognosis of Barrett's esophagus and esophageal adenocarcinoma. More investigation with a focus on the clinical outcomes of patients with Barrett's esophagus and esophageal adenocarcinoma and the immune cell population and cell signaling activity in the diseased esophagus is necessary to determine the immunomodulatory role of 1,25-dihydroxyvitamin D in the pathogenesis of esophageal diseases.

Characterization of a novel transgenic mouse line expressing Cre recombinase under the control of the Cdx2 neural specific enhancer

Several genetically modified mouse models have been generated in order to drive expression of the Cre recombinase in the neuroectoderm. However, none of them specifically targets the posterior neural plate during neurulation. To fill this gap, we have generated a new transgenic mouse line in which Cre expression is controlled by a neural specific enhancer (NSE) from the Caudal-related homeobox 2 (Cdx2) locus. Analyses of Cre activity via breeding with R26R-YFP reporter mice have indicated that the Cdx2NSE-Cre mouse line allows for recombination of LoxP sites in most cells of the posterior neural plate as soon as from the head fold stage. Detailed examination of double-transgenic embryos has revealed that this novel Cre-driver line allows targeting the entire posterior neural tube with an anterior limit in the caudal hindbrain. Of note, the Cdx2NSE regulatory sequences direct Cre expression along the whole dorso-ventral axis (including pre-migratory neural crest cells) and, accordingly, YFP fluorescence has been also observed in multiple non-cranial neural crest derivatives of double-transgenic embryos. Therefore, we believe that the Cdx2NSE-Cre mouse line represents an important novel genetic tool for the study of early events occurring in the caudal neuroectoderm during the formation of both the central and the peripheral nervous systems.

Caudal genes in blood development and leukemia

Members of the caudal gene family (in mice and humans: Cdx1, Cdx2, and Cdx4) have been studied during early development as regulators of axial elongation and anteroposterior patterning. In the adult, Cdx1 and Cdx2, but not Cdx4, have been intensively explored for their function in intestinal tissue homeostasis and the pathogenesis of gastrointestinal cancers. Involvement in embryonic hematopoiesis was first demonstrated in zebrafish, where cdx genes render posterior lateral plate mesoderm competent to respond to genes specifying hematopoietic fate, and compound mutations in cdx genes thus result in a bloodless phenotype. Parallel studies performed in zebrafish embryos and murine embryonic stem cells (ESCs) delineate conserved pathways between fish and mammals, corroborating a BMP/Wnt-Cdx-Hox axis during blood development that can be employed to augment derivation of blood progenitors from pluripotent stem cells in vitro. The molecular regulation of Cdx genes appears complex, as more recent data suggest involvement of non-Hox-related mechanisms and the existence of auto- and cross-regulatory loops governed by morphogens. Here, we will review the role of Cdx genes during hematopoietic development by comparing effects in zebrafish and mice and discuss their participation in malignant blood diseases.

Caudal-related homeobox (Cdx) protein-dependent integration of canonical Wnt signaling on paired-box 3 (Pax3) neural crest enhancer

One of the earliest events in neural crest development takes place at the neural plate border and consists in the induction of Pax3 expression by posteriorizing Wnt·β-catenin signaling. The molecular mechanism of this regulation is not well understood, but several observations suggest a role for posteriorizing Cdx transcription factors (Cdx1/2/4) in this process. Cdx genes are known as integrators of posteriorizing signals from Wnt, retinoic acid, and FGF pathways. In this work, we report that Wnt-mediated regulation of murine Pax3 expression is indirect and involves Cdx proteins as intermediates. We show that Pax3 transcripts co-localize with Cdx proteins in the posterior neurectoderm and that neural Pax3 expression is reduced in Cdx1-null embryos. Using Wnt3a-treated P19 cells and neural crest-derived Neuro2a cells, we demonstrate that Pax3 expression is induced by the Wnt-Cdx pathway. Co-transfection analyses, electrophoretic mobility shift assays, chromatin immunoprecipitation, and transgenic studies further indicate that Cdx proteins operate via direct binding to an evolutionarily conserved neural crest enhancer of the Pax3 proximal promoter. Taken together, these results suggest a novel neural function for Cdx proteins within the gene regulatory network controlling neural crest development.

An Ebox element in the proximal Gata4 promoter is required for Gata4 expression in vivo

GATA4 is an essential transcription factor required for the development and function of multiple tissues, including a major role in gonadogenesis. Despite its crucial role, the molecular mechanisms that regulate Gata4 expression in vivo remain poorly understood. We recently found that the Gata4 gene is expressed as multiple transcripts with distinct 5′ origins. These co-expressed alternative transcripts are generated by different non-coding first exons with transcripts E1a and E1b being the most prominent. Moreover, we previously showed that an Ebox element, located in Gata4 5′ flanking sequences upstream of exon 1a, is important for the promoter activity of these sequences in cell lines. To confirm the importance of this element in vivo, we generated and characterized Gata4 Ebox knockout mice. Quantitative PCR analyses realized on gonads, heart and liver at three developmental stages (embryonic, pre-pubertal and adult) revealed that the Ebox mutation leads to a robust and specific decrease (up to 89%) of Gata4 E1a transcript expression in all tissues and stages examined. However, a detailed characterization of the gonads revealed normal morphology and GATA4 protein levels in these mutants. Our qPCR data further indicate that this outcome is most likely due to the presence of Gata4 E1b mRNA, whose expression levels were not decreased by the Ebox mutation. In conclusion, our work clearly confirms the importance of the proximal Ebox element and suggests that adequate GATA4 protein expression is likely protected by a compensation mechanism between Gata4 E1a and E1b transcripts operating at the translational level.

Molecular mechanisms of Barrett's esophagus

Barrett's esophagus (BE) is defined as the metaplastic conversion of esophageal squamous epithelium to intestinalized columnar epithelium. As a premalignant lesion of esophageal adenocarcinoma (EAC), BE develops as a result of chronic gastroesophageal reflux disease (GERD). Many studies have been conducted to understand the molecular mechanisms of this disease. This review summarizes recent results involving squamous and intestinal transcription factors, signaling pathways, stromal factors, microRNAs, and other factors in the development of BE. A conceptual framework is proposed to guide future studies. We expect elucidation of the molecular mechanisms of BE to help in the development of improved management of GERD, BE, and EAC.

Modulation of Tcf3 repressor complex composition regulates cdx4 expression in zebrafish

The caudal homeobox (cdx) gene family is critical for specification of caudal body formation and erythropoiesis. In zebrafish, cdx4 expression is controlled by the Wnt pathway, but the molecular mechanism of this regulation is not fully understood. Here, we provide evidence that Tcf3 suppresses cdx4 expression through direct binding to multiple sites in the cdx4 gene regulatory region. Tcf3 requires corepressor molecules such as Groucho (Gro)/TLE and HDAC1 for activity. Using zebrafish embryos and cultured mammalian cells, we show that the transcription factor E4f1 derepresses cdx4 by dissociating corepressor proteins from Tcf3 without inhibiting its binding to cis-regulatory sites in the DNA. Further, the E3 ubiquitin ligase Lnx2b, acting as a scaffold protein irrespective of its enzymatic activity, counteracts the effects of E4f1. We propose that the modulation of Tcf3 repressor function by E4f1 assures precise and robust regulation of cdx4 expression in the caudal domain of the embryo.

Cdx mediates neural tube closure through transcriptional regulation of the planar cell polarity gene Ptk7

The vertebrate Cdx genes (Cdx1, Cdx2 and Cdx4) encode homeodomain transcription factors with well-established roles in anteroposterior patterning. To circumvent the peri-implantation lethality inherent to Cdx2 loss of function, we previously used the Cre-loxP system to ablate Cdx2 at post-implantation stages and confirmed a crucial role for Cdx2 function in events related to axial extension. As considerable data suggest that the Cdx family members functionally overlap, we extended this analysis to assess the consequence of concomitant loss of both Cdx1 and Cdx2. Here, we report that Cdx1-Cdx2 double mutants exhibit a severely truncated anteroposterior axis. In addition, these double mutants exhibit fused somites, a widened mediolateral axis and craniorachischisis, a severe form of neural tube defect in which early neurulation fails and the neural tube remains open. These defects are typically associated with deficits in planar cell polarity (PCP) signaling in vertebrates. Consistent with this, we found that expression of Ptk7, which encodes a gene involved in PCP, is markedly reduced in Cdx1-Cdx2 double mutants, and is a candidate Cdx target. Genetic interaction between Cdx mutants and a mutant allele of Scrib, a gene involved in PCP signaling, is suggestive of a role for Cdx signaling in the PCP pathway. These findings illustrate a novel and pivotal role for Cdx function upstream of Ptk7 and neural tube closure in vertebrates.

Negative autoregulation of Oct3/4 through Cdx1 promotes the onset of gastrulation

Gastrulation marks the onset of germ layer formation from undifferentiated precursor cells maintained by a network including the Pou5f1 gene, Oct3/4. Negative regulation of the undifferentiated state is a prerequisite for germ layer formation and subsequent development. A novel cross-regulatory network was characterized including the Pou5f1 and Cdx1 genes as part of the signals controlling the onset of gastrulation. Of particular interest was the observation that, preceding gastrulation, the Xenopus Oct3/4 factors, Oct60, Oct25, and Oct91, positively regulate Cdx1 expression through FGF signaling, and during gastrulation the Oct3/4 factors become repressors of Cdx1. Cdx1 negatively regulates the Pou5f1 genes during gastrulation, thus contributing to the repression of the network maintaining the undifferentiated state and promoting the onset of gastrulation. These regulatory interactions suggest that Oct3/4 initiates its own negative autoregulation through Cdx1 up-regulation to begin the repression of pluripotency in preparation for the onset of gastrulation and germ layer differentiation.

Cdx1 refines positional identity of the vertebrate hindbrain by directly repressing Mafb expression

An interplay of transcription factors interprets signalling pathways to define anteroposterior positions along the vertebrate axis. In the hindbrain, these transcription factors prompt the position-appropriate appearance of seven to eight segmental structures, known as rhombomeres (r1-r8). The evolutionarily conserved Cdx caudal-type homeodomain transcription factors help specify the vertebrate trunk and tail but have not been shown to directly regulate hindbrain patterning genes. Mafb (Kreisler, Krml1, valentino), a basic domain leucine zipper transcription factor, is required for development of r5 and r6 and is the first gene to show restricted expression within these two segments. The homeodomain protein vHnf1 (Hnf1b) directly activates Mafb expression. vHnf1 and Mafb share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of Mafb and, therefore, cannot establish the posterior Mafb expression boundary. Upon identifying regulatory sequences responsible for posterior Mafb repression, we have used in situ hybridization, immunofluorescence and chromatin immunoprecipitation (ChIP) analyses to determine that Cdx1 directly inhibits early Mafb expression in the neural tube posterior of the r6/r7 boundary, which is the anteriormost boundary of Cdx1 expression in the hindbrain. Cdx1 dependent repression of Mafb is transient. After the 10-somite stage, another mechanism acts to restrict Mafb expression in its normal r5 and r6 domain, even in the absence of Cdx1. Our findings identify Mafb as one of the earliest direct targets of Cdx1 and show that Cdx1 plays a direct role in early hindbrain patterning. Thus, just as Cdx2 and Cdx4 govern the trunk-to-tail transition, Cdx1 may regulate the hindbrain-to-spinal cord transition.

Cdx4 is a Cdx2 target gene

The products of the Cdx genes, Cdx1, Cdx2 and Cdx4, play multiple roles in early vertebrate development, and have been proposed to serve to relay signaling information from Wnt, RA and FGF pathways to orchestrate events related to anterior-posterior vertebral patterning and axial elongation. In addition, Cdx1 and Cdx2 have been reported to both autoregulate and to be subject to cross regulation by other family members. We have now found that Cdx4 expression is significantly down regulated in Cdx2(-/-) mutants suggesting previously unrecognized cross-regulatory interactions. Moreover, we have previously shown that Cdx4 is a direct target of the canonical Wnt signaling pathway, and that Cdx1 physically interacts with LEF/TCF members in an autoregulatory loop. We therefore investigated the means by which Cdx2 impacted on Cdx4 expression and assessed potential interaction between Cdx2 and canonical Wnt signaling on the Cdx4 promoter. We found that the Cdx4 promoter was regulated by Cdx2 in transient transfection assays. Electrophoretic mobility shift assays showed that Cdx2 bound to predicted Cdx response elements in the Cdx4 promoter which, when mutated, significantly reduced activity. Consistent with these data, chromatin immunoprecipitation assays from embryos demonstrated occupancy of the Cdx4 promoter by Cdx2 in vivo. However, we failed to observe an interaction between Cdx2 and components of the canonical Wnt signaling pathway. These findings suggest that, while both canonical Wnt and Cdx2 can regulate the activity of the Cdx4 promoter, they appear to operate through distinct mechanisms.

Pinin modulates expression of an intestinal homeobox gene, Cdx2, and plays an essential role for small intestinal morphogenesis

Pinin (Pnn), a nuclear speckle-associated protein, has been shown to function in maintenance of epithelial integrity through altering expression of several key adhesion molecules. Here we demonstrate that Pnn plays a crucial role in small intestinal development by influencing expression of an intestinal homeobox gene, Cdx2. Conditional inactivation of Pnn within intestinal epithelia resulted in significant downregulation of a caudal type homeobox gene, Cdx2, leading to obvious villus dysmorphogenesis and severely disrupted epithelial differentiation. Additionally, in Pnn-deficient small intestine, we observed upregulated Tcf/Lef reporter activity, as well as misregulated expression/distribution of beta-catenin and Tcf4. Since regulation of Cdx gene expression has been closely linked to Wnt/beta-catenin signaling activity, we explored the possibility of Pnn's interaction with beta-catenin, a major effector of the canonical Wnt signaling pathway. Co-immunoprecipitation assays revealed that Pnn, together with its interaction partner CtBP2, a transcriptional co-repressor, was in a complex with beta-catenin. Moreover, both of these proteins were found to be recruited to the proximal promoter area of Cdx2. Taken together, our results suggest that Pnn is essential for tight regulation of Wnt signaling and Cdx2 expression during small intestinal development.

Hox genes and segmentation of the hindbrain and axial skeleton

Segmentation is an important process that is frequently used during development to segregate groups of cells with distinct features. Segmental compartments provide a mechanism for generating and organizing regional properties along an embryonic axis and within tissues. In vertebrates the development of two major systems, the hindbrain and the paraxial mesoderm, displays overt signs of compartmentalization and depends on the process of segmentation for their functional organization. The hindbrain plays a key role in regulating head development, and it is a complex coordination center for motor activity, breathing rhythms, and many unconscious functions. The paraxial mesoderm generates somites, which give rise to the axial skeleton. The cellular processes of segmentation in these two systems depend on ordered patterns of Hox gene expression as a mechanism for generating a combinatorial code that specifies unique identities of the segments and their derivatives. In this review, we compare and contrast the signaling inputs and transcriptional mechanisms by which Hox gene regulatory networks are established during segmentation in these two different systems.

Cdx2 regulates patterning of the intestinal epithelium

Cdx1, Cdx2 and Cdx4 encode homeodomain transcription factors that are involved in vertebral anterior-posterior (AP) patterning. Cdx1 and Cdx2 are also expressed in the intestinal epithelium during development, suggesting a role in this tissue. Intestinal defects have not been reported in Cdx1 null mutants, while Cdx2 null mutants die at embryonic day 3.5 (E3.5), thus precluding assessment of the null phenotype at later stages. To circumvent this latter shortcoming, we have used a conditional Cre-lox strategy to inactivate Cdx2 in the intestinal epithelium. Using this approach, we found that ablation of Cdx2 at E13.5 led to a transformation of the small intestine to a pyloric stomach-like identity, although the molecular nature of the underlying mesenchyme remained unchanged. Further analysis of Cdx1-Cdx2 double mutants suggests that Cdx1 does not play a critical role in the development of the small intestine, at least after E13.5.

Alternative splicing of Tcf7l2 transcripts generates protein variants with differential promoter-binding and transcriptional activation properties at Wnt/beta-catenin targets

Alternative splicing can produce multiple protein products with variable domain composition from a single gene. The mouse Tcf7l2 gene is subject to alternative splicing. It encodes TCF4, a member of the T-cell factor (TCF) family of DNA-binding proteins and a nuclear interaction partner of β-catenin which performs essential functions in Wnt growth factor signalling. Multiple TCF4 isoforms, potentially exhibiting cell-type-specific distribution and differing in gene regulatory properties, could strongly influence tissue-specific Wnt responses. Therefore, we have examined mouse Tcf7l2 splice variants in neonatal tissues, embryonic stem cells and neural progenitors. By polymerase chain reaction amplification, cloning and sequencing, we identify a large number of alternatively spliced transcripts and report a highly flexible combinatorial repertoire of alternative exons. Many, but not all of the variants exhibit a broad tissue distribution. Moreover, two functionally equivalent versions of the C-clamp, thought to represent an auxiliary DNA-binding domain, were identified. Depending upon promoter context and precise domain composition, TCF4 isoforms exhibit strikingly different transactivation potentials at natural Wnt/β-catenin target promoters. However, differences in C-clamp-mediated DNA binding can only partially explain functional differences among TCF4 variants. Still, the cell-type-specific complement of TCF4 isoforms is likely to be a major determinant for the context-dependent transcriptional output of Wnt/β-catenin signalling.

Cdx2 regulation of posterior development through non-Hox targets

The homeodomain transcription factors Cdx1, Cdx2 and Cdx4 play essential roles in anteroposterior vertebral patterning through regulation of Hox gene expression. Cdx2 is also expressed in the trophectoderm commencing at E3.5 and plays an essential role in implantation, thus precluding assessment of the cognate-null phenotype at later stages. Cdx2 homozygous null embryos generated by tetraploid aggregation exhibit an axial truncation indicative of a role for Cdx2 in elaborating the posterior embryo through unknown mechanisms. To better understand such roles, we developed a conditional Cdx2 floxed allele in mice and effected temporal inactivation at post-implantation stages using a tamoxifen-inducible Cre. This approach yielded embryos that were devoid of detectable Cdx2 protein and exhibited the axial truncation phenotype predicted from previous studies. This phenotype was associated with attenuated expression of genes encoding several key players in axial elongation, including Fgf8, T, Wnt3a and Cyp26a1, and we present data suggesting that T, Wnt3a and Cyp26a1 are direct Cdx2 targets. We propose a model wherein Cdx2 functions as an integrator of caudalizing information by coordinating axial elongation and somite patterning through Hox-independent and -dependent pathways, respectively.

Effect of retinoic acid signaling on Wnt/beta-catenin and FGF signaling during body axis extension

Cell-cell signaling regulated by retinoic acid (RA), Wnt/beta-catenin, and fibroblast growth factor (FGF) is important during body axis extension, and interactions between these pathways have been suggested. At early somite stages, Wnt/beta-catenin and FGF signaling domains exist both anterior and posterior to the developing trunk, whereas RA signaling occurs in between in the trunk under the control of the RA-synthesizing enzyme retinaldehyde dehydrogenase-2 (Raldh2). Previous studies demonstrated that vitamin A deficient quail embryos and Raldh2(-/-) mouse embryos lacking RA synthesis exhibit ectopic expression of Fgf8 and Wnt8a in the developing trunk. Here, we demonstrate that Raldh2(-/-) mouse embryos display an expansion of FGF signaling into the trunk monitored by Sprouty2 and Pea3 expression, and an expansion of Wnt/beta-catenin signaling detected by expression of Axin2, Tbx6, Cdx2, and Cdx4. Following loss of RA signaling, the caudal expression domains of Fgf8, Wnt8a, and Wnt3a expand anteriorly into the trunk, but no change is observed in caudal expression of Fgf4 or Fgf17 plus caudal expression of Fgf18 and Cdx1 is reduced. These findings suggest that RA repression of Fgf8, Wnt8a, and Wnt3a in the developing trunk functions to down-regulate FGF signaling and Wnt/beta-catenin signaling as the body axis extends.

Retinoic acid and Wnt/beta-catenin have complementary roles in anterior/posterior patterning embryos of the basal chordate amphioxus

A role for Wnt/beta-catenin signaling in axial patterning has been demonstrated in animals as basal as cnidarians, while roles in axial patterning for retinoic acid (RA) probably evolved in the deuterostomes and may be chordate-specific. In vertebrates, these two pathways interact both directly and indirectly. To investigate the evolutionary origins of interactions between these two pathways, we manipulated Wnt/beta-catenin and RA signaling in the basal chordate amphioxus during the gastrula stage, which is the RA-sensitive period for anterior/posterior (A/P) patterning. The results show that Wnt/beta-catenin and RA signaling have distinctly different roles in patterning the A/P axis of the amphioxus gastrula. Wnt/beta-catenin specifies the identity of the ends of the embryo (high Wnt = posterior; low Wnt = anterior) but not intervening positions. Thus, upregulation of Wnt/beta-catenin signaling induces ectopic expression of posterior markers at the anterior tip of the embryo. In contrast, RA specifies position along the A/P axis, but not the identity of the ends of the embryo-increased RA signaling strongly affects the domains of Hox expression along the A/P axis but has little or no effect on the expression of either anterior or posterior markers. Although the two pathways may both influence such things as specification of neuronal identity, interactions between them in A/P patterning appear to be minimal.

The role of the visceral mesoderm in the development of the gastrointestinal tract

The gastrointestinal (GI) tract forms from the endoderm (which gives rise to the epithelium) and the mesoderm (which develops into the smooth muscle layer, the mesenchyme, and numerous other cell types). Much of what is known of GI development has been learned from studies of the endoderm and its derivatives, because of the importance of epithelial biology in understanding and treating human diseases. Although the necessity of epithelial-mesenchymal cross talk for GI development is uncontested, the role of the mesoderm remains comparatively less well understood. The transformation of the visceral mesoderm during development is remarkable; it differentiates from a very thin layer of cells into a complex tissue comprising smooth muscle cells, myofibroblasts, neurons, immune cells, endothelial cells, lymphatics, and extracellular matrix molecules, all contributing to the form and function of the digestive system. Understanding the molecular processes that govern the development of these cell types and elucidating their respective contribution to GI patterning could offer insight into the mechanisms that regulate cell fate decisions in the intestine, which has the unique property of rapid cell renewal for the maintenance of epithelial integrity. In reviewing evidence from both mammalian and nonmammalian models, we reveal the important role of the visceral mesoderm in the ontogeny of the GI tract.

BMP and Wnt specify hematopoietic fate by activation of the Cdx-Hox pathway

The formation of blood in the embryo is dependent on bone morphogenetic protein (BMP), but how BMP signaling intersects with other regulators of hematopoietic development is unclear. Using embryonic stem (ES) cells, we show that BMP4 first induces ventral-posterior (V-P) mesoderm and subsequently directs mesodermal cells toward blood fate by activating Wnt3a and upregulating Cdx and Hox genes. When BMP signaling is blocked during this latter phase, enforced expression of either Cdx1 or Cdx4 rescues hematopoietic development, thereby placing BMP4 signaling upstream of the Cdx-Hox pathway. Wnt signaling cooperates in BMP-induced hemogenesis, and the Wnt effector LEF1 mediates BMP4 activation of Cdx genes. Our data suggest that BMP signaling plays two distinct and sequential roles during blood formation, initially as an inducer of mesoderm, and later to specify blood via activation of Wnt signaling and the Cdx-Hox pathway.

Establishment of Hox vertebral identities in the embryonic spine precursors

The vertebrate spine exhibits two striking characteristics. The first one is the periodic arrangement of its elements-the vertebrae-along the anteroposterior axis. This segmented organization is the result of somitogenesis, which takes place during organogenesis. The segmentation machinery involves a molecular oscillator-the segmentation clock-which delivers a periodic signal controlling somite production. During embryonic axis elongation, this signal is displaced posteriorly by a system of traveling signaling gradients-the wavefront-which depends on the Wnt, FGF, and retinoic acid pathways. The other characteristic feature of the spine is the subdivision of groups of vertebrae into anatomical domains, such as the cervical, thoracic, lumbar, sacral, and caudal regions. This axial regionalization is controlled by a set of transcription factors called Hox genes. Hox genes exhibit nested expression domains in the somites which reflect their linear arrangement along the chromosomes-a property termed colinearity. The colinear disposition of Hox genes expression domains provides a blueprint for the regionalization of the future vertebral territories of the spine. In amniotes, Hox genes are activated in the somite precursors of the epiblast in a temporal colinear sequence and they were proposed to control their progressive ingression into the nascent paraxial mesoderm. Consequently, the positioning of the expression domains of Hox genes along the anteroposterior axis is largely controlled by the timing of Hox activation during gastrulation. Positioning of the somitic Hox domains is subsequently refined through a crosstalk with the segmentation machinery in the presomitic mesoderm. In this review, we focus on our current understanding of the embryonic mechanisms that establish vertebral identities during vertebrate development.

Novel pre- and post-gastrulation expression of Gata4 within cells of the inner cell mass and migratory neural crest cells

GATA4 is a transcription factor known to be important for the development of many organs such as the heart, intestine, and gonads. However, information regarding the control of its expression is only now beginning to emerge. To further understand the regulation of Gata4 expression during mouse embryonic development, we have generated a novel knockin allele allowing expression of the Cre recombinase under the control of Gata4 regulatory sequences. When these Gata4(Cre/+) mice were crossed with the Cre reporter mouse R26R-YFP, we surprisingly found widespread mosaic YFP expression in e10.0 embryos. This particular expression pattern was traced back to the e5.5 stage via a cell lineage study, suggesting activation of transcription at the Gata4 locus around the blastocyst stage. In accordance with this hypothesis, we found that Gata4 is expressed in cultured embryonic stem (ES) cells and within the inner cell mass (ICM) of e4.5 blastocysts. Interestingly, such early Gata4 transcription can be recapitulated in transgenic reporter studies using 5 kb of the proximal rat Gata4 promoter. During mouse development, these 5-kb regulatory sequences were previously reported to direct reporter gene expression to Sertoli cells of the testes [Mazaud Guittot et al. (2007) Biol Reprod 76:85-95]. We now show that these regulatory sequences can also drive robust fluorescent reporter gene expression in migratory neural crest cells. Comparisons to Wnt1-Cre-mediated YFP labelling of neural crest cells suggest that most of the migratory neural crest cells are labelled in e9.5 to e11.5 Gata4p[5kb]-RFP or -GFP embryos. Analysis of GFP transcription via whole-mount in situ hybridization in e10.5 and e11.5 embryos demonstrated that the 5-kb Gata4 promoter is preferentially active in cells of the boundary caps at the dorsal root entry zone and motor exit points flanking the neural tube. RT-PCR gene expression analysis of FACS-purified GFP-positive cells from e9.5 Gata4p[5kb]-GFP embryos revealed co-expression of Gata4 with many neural crest stem cell markers. Together with sphere-forming and differentiation cell culture assays, our results indicate that the Gata4 promoter is active within at least a subset of the neural crest stem cells. Taken altogether, our studies have revealed new Gata4 expression patterns during mouse embryonic development, which are controlled by its 5-kb proximal 5' flanking sequences.

Combinatorial signalling controls Neurogenin2 expression at the onset of spinal neurogenesis

A central issue during embryonic development is to define how different signals cooperate in generating unique cell types. To address this issue, we focused on the function and the regulation of the proneural gene Neurogenin2 (Neurog2) during early mouse spinal neurogenesis. We showed that Neurog2 is first expressed in cells within the neural plate anterior to the node from the 5 somite-stage. The analysis of Neurog2 mutants established a role for this gene in triggering neural differentiation during spinal cord elongation. We identified a 798 base pair enhancer element (Neurog2-798) upstream of the Neurog2 coding sequence that directs the early caudal expression of Neurog2. Embryo culture experiments showed that Retinoic Acid (RA), Sonic hedgehog (Shh) and Fibroblast Growth Factor signals act in concert on this enhancer to control the spatial and temporal induction of Neurog2. We further demonstrated by transgenesis that two RA response elements and a Gli binding site within the Neurog2-798 element are absolutely required for its activity, strongly suggesting that the regulation of Neurog2 early expression by RA and Shh signals is direct. Our data thus support a model where signal integration at the level of a single enhancer constitutes a key mechanism to control the onset of neurogenesis.