Increased Cdx protein dose effects upon axial patterning in transgenic lines of mice

Cdx regulates gene expression through PRC2-mediated epigenetic mechanisms

The extra-embryonic yolk sac contains adjacent layers of mesoderm and visceral endoderm. The mesodermal layer serves as the first site of embryonic hematopoiesis, while the visceral endoderm provides a means of exchanging nutrients and waste until the development of the chorioallantoic placenta. While defects in chorioallantoic fusion and yolk sac hematopoiesis have been described in Cdx mutant mouse models, little is known about the gene targets and molecular mechanisms through which Cdx members regulate these processes. To this end, we used RNA-seq to examine Cdx-dependent gene expression changes in the yolk sac. We find that loss of Cdx function impacts the expression of genes involved in yolk sac hematopoiesis, as previously described, as well as novel Cdx2 target genes. In addition, we observed Cdx-dependent changes in PRC2 subunit expression accompanied by altered H3K27me3 deposition at a subset of Cdx target genes as early as E7.5 in the embryo proper. This study identifies additional Cdx target genes and provides further evidence for Cdx-dependent epigenetic regulation of gene expression in the early embryo, and that this regulation is required to maintain gene expression programs in the extra-embryonic yolk sac at later developmental stages.

Charting human development using a multi-endodermal organ atlas and organoid models

Organs are composed of diverse cell types that traverse transient states during organogenesis. To interrogate this diversity during human development, we generate a single-cell transcriptome atlas from multiple developing endodermal organs of the respiratory and gastrointestinal tract. We illuminate cell states, transcription factors, and organ-specific epithelial stem cell and mesenchyme interactions across lineages. We implement the atlas as a high-dimensional search space to benchmark human pluripotent stem cell (hPSC)-derived intestinal organoids (HIOs) under multiple culture conditions. We show that HIOs recapitulate reference cell states and use HIOs to reconstruct the molecular dynamics of intestinal epithelium and mesenchyme emergence. We show that the mesenchyme-derived niche cue NRG1 enhances intestinal stem cell maturation in vitro and that the homeobox transcription factor CDX2 is required for regionalization of intestinal epithelium and mesenchyme in humans. This work combines cell atlases and organoid technologies to understand how human organ development is orchestrated.

The Cdx transcription factors and retinoic acid play parallel roles in antero-posterior position of the pectoral fin field during gastrulation

The molecular regulators that determine the precise position of the vertebrate limb along the anterio-posterior axis have not been identified. One model suggests that a combination of hox genes in the lateral plate mesoderm (LPM) promotes formation of the limb field, however redundancy among duplicated paralogs has made this model difficult to confirm. In this study, we identify an optimal window during mid-gastrulation stages when transient mis-regulation of retinoic acid signaling or the caudal related transcription factor, Cdx4, both known regulators of hox genes, can alter the position of the pectoral fin field. We show that increased levels of either RA or Cdx4 during mid-gastrulation are sufficient to rostrally shift the position of the pectoral fin field at the expense of surrounding gene expression in the anterior lateral plate mesoderm (aLPM). Alternatively, embryos deficient for both Cdx4 and Cdx1a (Cdx-deficient) form pectoral fins that are shifted towards the posterior and reveal an additional effect on size of the pectoral fin buds. Prior to formation of the pectoral fin buds, the fin field in Cdx-deficient embryos is visibly expanded into the posterior LPM (pLPM) region at the expense of surrounding gene expression. The effects on gene expression immediately post-gastrulation and during somitogenesis support a model where RA and Cdx4 act in parallel to regulate the position of the pectoral fin. Our transient method is a potentially useful model for studying the mechanisms of limb positioning along the AP axis.

Role of Cdx factors in early mesodermal fate decisions

Murine cardiac and hematopoietic progenitors are derived from Mesp1⁺ mesoderm. Cdx function impacts both yolk sac hematopoiesis and cardiogenesis in zebrafish, suggesting that Cdx family members regulate early mesoderm cell fate decisions. We found that Cdx2 occupies a number of transcription factor loci during embryogenesis, including key regulators of both cardiac and blood development, and that Cdx function is required for normal expression of the cardiogenic transcription factors Nkx2-5 and Tbx5 Furthermore, Cdx and Brg1, an ATPase subunit of the SWI/SNF chromatin remodeling complex, co-occupy a number of loci, suggesting that Cdx family members regulate target gene expression through alterations in chromatin architecture. Consistent with this, we demonstrate loss of Brg1 occupancy and altered chromatin structure at several cardiogenic genes in Cdx-null mutants. Finally, we provide evidence for an onset of Cdx2 expression at E6.5 coinciding with egression of cardiac progenitors from the primitive streak. Together, these findings suggest that Cdx functions in multi-potential mesoderm to direct early cell fate decisions through transcriptional regulation of several novel target genes, and provide further insight into a potential epigenetic mechanism by which Cdx influences target gene expression.

Critical Timing without a Timer for Embryonic Development

Timing of embryonic development is precisely controlled, but the mechanisms underlying biological timers are still unclear. Here, a validated model for timing under control of Sonic Hedgehog is revisited and generalized to an arbitrary number of genes. The developmental dynamics where a temporal sequence of gene expression recapitulates a steady-state spatial pattern can be realized through a simple network close to criticality, controlled by the duration of exposure to a morphogen. Criticality simultaneously accounts for many observed biological properties, such as timing, multistability, and canalization of genetic expression. This process can be parsimoniously generalized in many dimensions with a minimum number of genes, all repressing each other with asymmetrical strengths, which also explains sequential activation of different fates. Separation of timescales allows for a simple analytical interpretation. Finally, it is shown that even in the presence of noise, coupling between cells preserves criticality and robust patterning. The model offers a simple theoretical framework for the study of emergent developmental timers.

Cdx2 represses Oct4 function via inducing its proteasome-dependent degradation in early porcine embryos

Reciprocal repression of inner cell mass specific factor OCT4 and trophectoderm specific factor CDX2 promotes mouse first lineage segregation. Studies in mouse embryonic stem (ES) cells revealed that they bind to each other's regulatory regions to reciprocally suppress transcription, additionally they form protein complex for mutual antagonism. However, so far the molecular interaction of Oct4 and Cdx2 in other mammal's early embryo is not yet investigated. Here, over-expression of Cdx2 in early porcine embryo showed CDX2 represses Oct4 through neither the transcriptional repression nor forming repressive complex, but promoting OCT4 nuclear export and proteasomal degradation. The results showed novel molecular regulation of CDX2 on Oct4, and provided important clues for clarifying the mechanism of interaction between CDX2 and Oct4 in embryo of mammals other than mouse.

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.

Cdx1 and Cdx2 exhibit transcriptional specificity in the intestine

The caudal-related homeodomain transcription factors Cdx1 and Cdx2 are expressed in the developing endoderm with expression persisting into adulthood. Cdx1^−/− mutants are viable and fertile and display no overt intestinal phenotype. Cdx2 null mutants are peri-implantation lethal; however, conditional mutation approaches have revealed that Cdx2 is required for patterning the intestinal epithelium and specification of the colon. Cdx2 is also necessary for homeostasis of the intestinal tract in the adult, where Cdx1 and Cdx2 appear to functionally overlap in the distal colon, but not during intestinal development. Cdx1 and Cdx2 exhibit complete overlap of expression in the intestine, although they differ in their relative levels, with Cdx1 maximal in the distal colon and Cdx2 peaking in the proximal cecum. Moreover, Cdx1 protein is graded along the crypt-villus axis, being abundant in the crypts and diminishing towards the villi. Cdx2 is expressed uniformly along this axis, but is differentially phosphorylated; the functional relevance of these expression domains and phosphorylation is currently unknown. Cdx1 and Cdx2 have been suggested to exhibit functional specificity in the intestinal tract. In the present study, using cell-based models, we found that relative to Cdx1, Cdx2 was significantly less potent at effecting a transcriptional response from the Cdx1 promoter, a known Cdx target gene. We subsequently assessed this relationship in vivo using a “gene swap” approach and found that Cdx2 cannot substitute for Cdx1 in this autoregulatory loop. This is in marked contrast with the ability of Cdx2 to support Cdx1 expression and function in paraxial mesoderm and vertebral patterning, thus providing novel in vivo evidence of context-dependent transcriptional specificity between these transcription factors.

Loss of FGF-dependent mesoderm identity and rise of endogenous retinoid signalling determine cessation of body axis elongation

By analyzing cellular and molecular changes in key cell populations in the tailbud during embryogenesis, this work uncovers critical signaling events that determine vertebrate body length.

The endogenous mechanism that determines vertebrate body length is unknown but must involve loss of chordo-neural-hinge (CNH)/axial stem cells and mesoderm progenitors in the tailbud. In early embryos, Fibroblast growth factor (FGF) maintains a cell pool that progressively generates the body and differentiation onset is driven by retinoid repression of FGF signalling. This raises the possibility that FGF maintains key tailbud cell populations and that rising retinoid activity underlies cessation of body axis elongation. Here we show that sudden loss of the mesodermal gene (Brachyury) from CNH and the mesoderm progenitor domain correlates with FGF signalling decline in the late chick tailbud. This is accompanied by expansion of neural gene expression and a similar change in cell fate markers is apparent in the human tailbud. Fate mapping of chick tailbud further revealed that spread of neural gene expression results from continued ingression of CNH-derived cells into the position of the mesoderm progenitor domain. Using gain and loss of function approaches in vitro and in vivo, we then show that attenuation of FGF/Erk signalling mediates this loss of Brachyury upstream of Wnt signalling, while high-level FGF maintains Brachyury and can induce ectopic CNH-like cell foci. We further demonstrate a rise in endogenous retinoid signalling in the tailbud and show that here FGF no longer opposes retinoid synthesis and activity. Furthermore, reduction of retinoid signalling at late stages elevated FGF activity and ectopically maintained mesodermal gene expression, implicating endogenous retinoid signalling in loss of mesoderm identity. Finally, axis termination is concluded by local cell death, which is reduced by blocking retinoid signalling, but involves an FGFR-independent mechanism. We propose that cessation of body elongation involves loss of FGF-dependent mesoderm identity in late stage tailbud and provide evidence that rising endogenous retinoid activity mediates this step and ultimately promotes cell death in chick tailbud.

The mechanism that determines body length is unknown but likely operates at the elongating tail end of vertebrate embryos. In the early embryo, fibroblast growth factor (FGF) signalling maintains a proliferative pool of cells in the tailbud that progressively generates the body. It also protects these cells from the differentiating influence of retinoic acid, which is produced by the maturing mesoderm tissues of the extending body. We show here, in the chick embryo, that the “endgame”—that is, the termination of body axis elongation—comes when the mesodermal gene brachyury is suddenly lost from axial stem cell population and presumptive mesoderm cells in the tailbud late in development. Using gain- and loss-of-function approaches, we demonstrate that this step is mediated by loss of FGF signalling. We present evidence that this is due to rising retinoid signalling in the tailbud and that FGF signalling in the tailbud no longer opposes retinoid synthesis and activity. Finally, we reveal that these events are followed by local cell death in the tailbud, which can be reduced by the attenuation of retinoid signalling but involves a mechanism that is independent of FGF signalling via its usual receptor. We propose that cessation of body elongation involves loss of FGF-dependent mesoderm identity in the late tailbud and that this is mediated by rising endogenous retinoid activity, which ultimately promotes cell death in the chick tailbud.

Changes in Cis-regulatory Elements during Morphological Evolution

How have animals evolved new body designs (morphological evolution)? This requires explanations both for simple morphological changes, such as differences in pigmentation and hair patterns between different Drosophila populations and species, and also for more complex changes, such as differences in the forelimbs of mice and bats, and the necks of amphibians and reptiles. The genetic changes and pathways involved in these evolutionary steps require identification. Many, though not all, of these events occur by changes in cis-regulatory (enhancer) elements within developmental genes. Enhancers are modular, each affecting expression in only one or a few tissues. Therefore it is possible to add, remove or alter an enhancer without producing changes in multiple tissues, and thereby avoid widespread (pleiotropic) deleterious effects. Ideally, for a given step in morphological evolution it is necessary to identify (i) the change in phenotype, (ii) the changes in gene expression, (iii) the DNA region, enhancer or otherwise, affected, (iv) the mutation involved, (v) the nature of the transcription or other factors that bind to this site. In practice these data are incomplete for most of the published studies upon morphological evolution. Here, the investigations are categorized according to how far these analyses have proceeded.

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.

Breaking evolutionary and pleiotropic constraints in mammals: On sloths, manatees and homeotic mutations

Background

Mammals as a rule have seven cervical vertebrae, except for sloths and manatees. Bateson proposed that the change in the number of cervical vertebrae in sloths is due to homeotic transformations. A recent hypothesis proposes that the number of cervical vertebrae in sloths is unchanged and that instead the derived pattern is due to abnormal primaxial/abaxial patterning.

Results

We test the detailed predictions derived from both hypotheses for the skeletal patterns in sloths and manatees for both hypotheses. We find strong support for Bateson's homeosis hypothesis. The observed vertebral and rib patterns cannot be explained by changes in primaxial/abaxial patterning. Vertebral patterns in sloths and manatees are similar to those in mice and humans with abnormal numbers of cervical vertebrae: incomplete and asymmetric homeotic transformations are common and associated with skeletal abnormalities. In sloths the homeotic vertebral shift involves a large part of the vertebral column. As such, similarity is greatest with mice mutant for genes upstream of Hox.

Conclusions

We found no skeletal abnormalities in specimens of sister taxa with a normal number of cervical vertebrae. However, we always found such abnormalities in conspecifics with an abnormal number, as in many of the investigated dugongs. These findings strongly support the hypothesis that the evolutionary constraints on changes of the number of cervical vertebrae in mammals is due to deleterious pleitropic effects. We hypothesize that in sloths and manatees low metabolic and activity rates severely reduce the usual stabilizing selection, allowing the breaking of the pleiotropic constraints. This probably also applies to dugongs, although to a lesser extent.

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.

Skeletal development in sloths and the evolution of mammalian vertebral patterning

Mammals show a very low level of variation in vertebral count, particularly in the neck. Phenotypes exhibited at various stages during the development of the axial skeleton may play a key role in testing mechanisms recently proposed to explain this conservatism. Here, we provide osteogenetic data that identify developmental criteria with which to recognize cervical vs. noncervical vertebrae in mammals. Except for sloths, all mammals show the late ossification of the caudal-most centra in the neck after other centra and neural arches. In sloths with 8-10 ribless neck vertebrae, the caudal-most neck centra ossify early, matching the pattern observed in cranial thoracic vertebrae of other mammals. Accordingly, we interpret the ribless neck vertebrae of three-toed sloths caudal to V7 as thoracic based on our developmental criterion. Applied to the unusual vertebral phenotype of long-necked sloths, these data support the interpretation that elements of the axial skeleton with origins from distinct mesodermal tissues have repatterned over the course of evolution.

Predicting embryonic patterning using mutual entropy fitness and in silico evolution

During vertebrate embryogenesis, the expression of Hox genes that define anterior-posterior identity follows general rules: temporal colinearity and posterior prevalence. A mathematical measure for the quality or fitness of the embryonic pattern produced by a gene regulatory network is derived. Using this measure and in silico evolution we derive gene interaction networks for anterior-posterior (AP) patterning under two developmental paradigms. For patterning during growth (paradigm I), which is appropriate for vertebrates and short germ-band insects, the algorithm creates gene expression patterns reminiscent of Hox gene expression. The networks operate through a timer gene, the level of which measures developmental progression (a candidate is the widely conserved posterior morphogen Caudal). The timer gene provides a simple mechanism to coordinate patterning with growth rate. The timer, when expressed as a static spatial gradient, functions as a classical morphogen (paradigm II), providing a natural way to derive the AP patterning, as seen in long germ-band insects that express their Hox genes simultaneously, from the ancestral short germ-band system. Although the biochemistry of Hox regulation in higher vertebrates is complex, the actual spatiotemporal expression phenotype is not, and simple activation and repression by Hill functions suffices in our model. In silico evolution provides a quantitative demonstration that continuous positive selection can generate complex phenotypes from simple components by incremental evolution, as Darwin proposed.

Cdx and Hox genes differentially regulate posterior axial growth in mammalian embryos

Hox and Cdx transcription factors regulate embryonic positional identities. Cdx mutant mice display posterior body truncations of the axial skeleton, neuraxis, and caudal urorectal structures. We show that trunk Hox genes stimulate axial extension, as they can largely rescue these Cdx mutant phenotypes. Conversely, posterior (paralog group 13) Hox genes can prematurely arrest posterior axial growth when precociously expressed. Our data suggest that the transition from trunk to tail Hox gene expression successively regulates the construction and termination of axial structures in the mouse embryo. Thus, Hox genes seem to differentially orchestrate posterior expansion of embryonic tissues during axial morphogenesis as an integral part of their function in specifying head-to-tail identity. In addition, we present evidence that Cdx and Hox transcription factors exert these effects by controlling Wnt signaling. Concomitant regulation of Cyp26a1 expression, restraining retinoic acid signaling away from the posterior growth zone, may likewise play a role in timing the trunk-tail transition.

Down-regulation of Cdx2 in colorectal carcinoma cells by the Raf-MEK-ERK 1/2 pathway

Cdx2 is a homeodomain transcription factor that regulates normal intestinal cell differentiation. Cdx2 is frequently lost during progression of colorectal cancer (CRC) and is widely viewed as a colorectal tumour suppressor. A previous study suggested that activation of protein kinase C (PKC) may be responsible for Cdx2 down-regulation in CRC cells. Here we show that activation of PKC does indeed promote down-regulation of Cdx2 at both the mRNA and protein levels. However, PKC-dependent loss of Cdx2 is dependent upon activation of the Raf-MEK-ERK1/2 pathway. Indeed, specific activation of the ERK1/2 pathway using the conditional kinase DeltaRaf-1:ER is sufficient to inhibit Cdx2 transcription. The Raf-MEK-ERK1/2 pathway is hyper-activated in a large fraction of colorectal cancers due to mutations in K-Ras and we show that treatment of CRC cell lines with MEK inhibitors causes an increase in Cdx2 expression. Furthermore, activation of the ERK1/2 pathway promotes the phosphorylation and proteasome-dependent degradation of the Cdx2 protein. The inhibitory effect of ERK1/2 upon Cdx2 in CRC cells is in sharp contrast to its stimulatory effect upon Cdx2 expression in trophectoderm and trophoblast stem cells. These results provide important new insights into the regulation of the Cdx2 tumour suppressor by linking it to ERK1/2, a pathway which is frequently activated in CRC.

Stem cells, signals and vertebrate body axis extension

The progressive generation of chick and mouse axial tissues - the spinal cord, skeleton and musculature of the body - has long been proposed to depend on the activity of multipotent stem cells. Here, we evaluate evidence for the existence and multipotency of axial stem cells. We show that although the data strongly support their existence, there is little definitive information about their multipotency or extent of contribution to the axis. We also review the location and molecular characteristics of these putative stem cells, along with their evolutionary conservation in vertebrates and the signalling mechanisms that regulate and arrest axis extension.

Overlapping functions of Cdx1, Cdx2, and Cdx4 in the development of the amphibian Xenopus tropicalis

Using Xenopus tropicalis, we present the first analysis of the developmental effects that result from knocking down the function of the three Cdx genes present in the typical vertebrate genome. Knockdowns of individual Cdx genes lead to a similar range of posterior defects; compound Cdx knockdowns result in increasingly severe posterior truncations, accompanied by posterior shifts and reduction of 5' Hox gene expression. We provide evidence that Cdx and Wnt3A genes are components of a positive feedback loop operating in the posterior axis. We show that Cdx function is required during later, but not early stages of development, for correct regional specification of the endoderm and morphogenesis of the gut. Our results support the hypothesis that during amphibian development the overall landscape of Cdx activity in the embryo is more important than the specific function of individual Cdx proteins.