The four R-spondin secreted ligands (RSPO1-RSPO4) act via their cognate LGR4, LGR5 and LGR6 receptors to amplify WNT signalling. Here we report an allelic series of recessive RSPO2 mutations in humans that cause tetra-amelia syndrome, which is characterized by lung aplasia and a total absence of the four limbs. Functional studies revealed impaired binding to the LGR4/5/6 receptors and the RNF43 and ZNRF3 transmembrane ligases, and reduced WNT potentiation, which correlated with allele severity. Unexpectedly, however, the triple and ubiquitous knockout of Lgr4, Lgr5 and Lgr6 in mice did not recapitulate the known Rspo2 or Rspo3 loss-of-function phenotypes. Moreover, endogenous depletion or addition of exogenous RSPO2 or RSPO3 in triple-knockout Lgr4/5/6 cells could still affect WNT responsiveness. Instead, we found that the concurrent deletion of rnf43 and znrf3 in Xenopus embryos was sufficient to trigger the outgrowth of supernumerary limbs. Our results establish that RSPO2, without the LGR4/5/6 receptors, serves as a direct antagonistic ligand to RNF43 and ZNRF3, which together constitute a master switch that governs limb specification. These findings have direct implications for regenerative medicine and WNT-associated cancers.

The formation of primitive (embryonic) blood in vertebrates is mediated by spatio-temporally restricted signaling between different tissue layers. In Xenopus , in which primitive blood originates in the ventral blood island, this involves the secretion of bone morphogenetic protein (BMP) ligands by the ectoderm that signal to the underlying mesoderm during gastrulation. Using novel transgenic reporter lines, we report that the canonical Wnt/β-catenin pathway is also activated in the blood islands in Xenopus . Furthermore, Wnt-reporter activity was also detected in the blood islands of the mouse yolk sac. By using morpholino-mediated depletion in Xenopus , we identified Wnt4 as the ligand that is expressed in the mesoderm of the ventral blood island and is essential for the expression of hematopoietic and erythroid marker genes. Injection of an inducible Wnt-interfering construct further showed that, during gastrulation, Wnt/β-catenin signaling is required both in the mesoderm and in the overlying ectoderm for the formation of the ventral blood island. Using recombination assays with embryonic explants, we document that ectodermal BMP4 expression is dependent on Wnt4 signals from the mesoderm. Our results thus reveal a unique role for Wnt4-mediated canonical signaling in the formation and maintenance of the ventral blood island in Xenopus .

The Zic transcription factors play critical roles during embryonic development. Mutations in the ZIC2 gene are associated with human holoprosencephaly, but the etiology is still unclear. Here, we report a novel function for ZIC2 as a regulator of ␤-catenin⅐TCF4-mediated transcription. We show that ZIC2 can bind directly to the DNA-binding high mobility group box of TCF4 via its zinc finger domain and inhibit the transcriptional activity of the ␤-catenin⅐TCF4 complex. However, the binding of TCF4 to DNA was not affected by ZIC2. Zic2 RNA injection completely inhibited ␤-catenin-induced axis duplication in Xenopus embryos and strongly blocked the ability of ␤-catenin to induce expression of known Wnt targets in animal caps. Moreover, Zic2 knockdown in transgenic Xenopus Wnt reporter embryos led to ectopic Wnt signaling activity mainly at the midbrain-hindbrain boundary. Together, our results demonstrate a previously unknown role for ZIC2 as a transcriptional regulator of the ␤-catenin⅐TCF4 complex.

SUMMARYContinuous neurogenesis in the adult nervous system requires a delicate balance between proliferation and differentiation. Although Wnt/b-catenin and Hedgehog signalling pathways are thought to share a mitogenic function in adult neural stem/progenitor cells, it remains unclear how they interact in this process. Adult amphibians produce retinal neurons from a pool of neural stem cells localised in the ciliary marginal zone (CMZ). Surprisingly, we found that perturbations of the Wnt and Hedgehog pathways result in opposite proliferative outcomes of neural stem/progenitor cells in the CMZ. Additionally, our study revealed that Wnt and Hedgehog morphogens are produced in mutually exclusive territories of the post-embryonic retina. Using genetic and pharmacological tools, we found that the Wnt and Hedgehog pathways exhibit reciprocal inhibition. Our data suggest that Sfrp-1 and Gli3 contribute to this negative cross-regulation. Altogether, our results reveal an unexpected antagonistic interplay of Wnt and Hedgehog signals that may tightly regulate the extent of neural stem/progenitor cell proliferation in the Xenopus retina. and a transgenic male. The latter was selected beforehand as having a single transgene insertion site (as inferred by mendelian ratios in its progeny) in order to ensure homogeneous levels of GFP expression in the offspring. Construction of the LEF1-VP16 and LEF1-EnR transgenesis vectors has been described previously (Denayer et al., 2008) and transgenic X. tropicalis lines (LEF1-VP16Tg and LEF1-EnRTg) were generated as described (Sekkali et al., 2008). These constructs are fused with the dexamethasone-responsive hormone-binding domain of the human glucocorticoid receptor (GR). Expression constructs and morpholinospCS2- TCF3-VP16GR and pCS2-dnTCF3-GR (de Croze et al., 2011), pCS2-Ihh-CD2 [previously called Bhh (Locker et al., 2006)], pCS2-Smo-M2 , pCS2-cyclinA2 and pCS2-cdk2 (Decembrini et al., 2006) and pCS2-GFP (a gift from David Turner, University of Michigan, Ann Arbor, USA) were described previously. pCS2-Shh-CD2 and pCS2-Dhh-CD2 (previously called Chh) were generated by subcloning the N-terminal coding regions (devoid of the C-terminal cleavage product) of Shh and Dhh cDNAs (Ekker et al., 1995) Microinjection and in vivo DNA lipofectionCapped mRNAs encoding TCF3-VP16GR and GFP were transcribed from pCS2 plasmids after NotI digestion using the mMessage mMachine SP6 Kit (Ambion). Then, 400 pg of each mRNA was injected into two blastomeres of four-cell stage embryos. Gli3, Sfrp-1 or standard control morpholino oligonucleotides (Gene Tools) were injected into one blastomere at the one-cell stage (30 ng). Their efficacy was tested by analysing in vivo GFP fluorescence following co-injection of a chimeric GFP construct fused downstream of the morpholino-complementary sequence (supplementary material Fig. S1).Lipofection experiments were performed by cotransfecting the indicated pCS2 constructs together with pCS2-GFP at stage 18 into the presumptive region of the retina, as previously...

SUMMARY Mucociliary epithelia provide a first line of defense against pathogens. Impaired regeneration and remodeling of mucociliary epithelia are associated with dysregulated Wnt/β-catenin signaling in chronic airway diseases, but underlying mechanisms remain elusive, and studies yield seemingly contradicting results. Employing the Xenopus mucociliary epidermis, the mouse airway, and human airway Basal cells, we characterize the evolutionarily conserved roles of Wnt/β-catenin signaling in vertebrates. In multiciliated cells, Wnt is required for cilia formation during differentiation. In Basal cells, Wnt prevents specification of epithelial cell types by activating ΔN-TP63, a master transcription factor, which is necessary and sufficient to mediate the Wnt-induced inhibition of specification and is required to retain Basal cells during development. Chronic Wnt activation leads to remodeling and Basal cell hyperplasia, which are reversible in vivo and in vitro, suggesting Wnt inhibition as a treatment option in chronic lung diseases. Our work provides important insights into mucociliary signaling, development, and disease.

Receptor-interacting protein kinase 4 (RIPK4)-deficient mice have epidermal defects and fusion of all external orifices. These are similar to Bartsocas-Papas syndrome and popliteal pterygium syndrome (PPS) in humans, for which causative mutations have been documented in the RIPK4 and IRF6 (interferon regulatory factor 6) gene, respectively. Although genetically distinct, these syndromes share the anomalies of marked pterygia, syndactyly, clefting and hypoplastic genitalia. Despite the strong resemblance of these two syndromes, no molecular connection between the transcription factor IRF6 and the kinase RIPK4 was known and the mechanism underlying the phenotype was unclear. Here we describe that RIPK4 deficiency in mice causes epithelial fusions associated with abnormal periderm development and aberrant ectopic localization of E-cadherin on the apical membrane of the outer peridermal cell layers. In Xenopus, RIPK4 depletion causes the absence of ectodermal epiboly and concomitant gastrulation defects that phenocopy ectopic expression of dominant-negative IRF6. We found that IRF6 controls RIPK4 expression and that wild-type, but not kinase-dead, RIPK4 can complement the gastrulation defect in Xenopus caused by IRF6 malfunctioning. In contrast to the mouse, we observed only minor effects on cadherin membrane expression in Xenopus RIPK4 morphants. However, gastrulation defects were associated with a virtual absence of cortical actin in the ectodermal cells that face the blastocoel cavity and this was phenocopied in embryos expressing dominant-negative IRF6. A role for RIPK4 in actin cytoskeleton organization was also revealed in mouse epidermis and in human epithelial HaCaT cells. In conclusion, we showed that in mice RIPK4 is implicated in cortical actin organization and in E-cadherin localization or function, which can explain the characteristic epithelial fusions observed in PPSs. In addition, we provide a novel molecular link between IRF6 and RIPK4 that unifies the different PPSs to a common molecular pathway.

The retina of fish and amphibian contains genuine neural stem cells located at the most peripheral edge of the ciliary marginal zone (CMZ). However, their cell-of-origin as well as the mechanisms that sustain their maintenance during development are presently unknown. We identified Hes4 (previously named XHairy2), a gene encoding a bHLH-O transcriptional repressor, as a stem cell-specific marker of the Xenopus CMZ that is positively regulated by the canonical Wnt pathway and negatively by Hedgehog signaling. We found that during retinogenesis, Hes4 labels a small territory, located first at the pigmented epithelium (RPE)/neural retina (NR) border and later in the retinal margin, that likely gives rise to adult retinal stem cells. We next addressed whether Hes4 might impart this cell subpopulation with retinal stem cell features: inhibited RPE or NR differentiation programs, continuous proliferation, and slow cell cycle speed. We could indeed show that Hes4 overexpression cell autonomously prevents retinal precursor cells from commitment toward retinal fates and maintains them in a proliferative state. Besides, our data highlight for the first time that Hes4 may also constitute a crucial regulator of cell cycle kinetics. Hes4 gain of function indeed significantly slows down cell division, mainly through the lengthening of G1 phase. As a whole, we propose that Hes4 maintains particular stemness features in a cellular cohort dedicated to constitute the adult retinal stem cell pool, by keeping it in an undifferentiated and slowly proliferative state along embryonic retinogenesis. Stem Cells 2012;30:2784–2795

Background Respiratory system development is regulated by a complex series of endoderm – mesoderm interactions that are not fully understood. Recently Xenopus has emerged as an alternative model to investigate early respiratory system development, but the extent to which the morphogenesis and molecular pathways involved are conserved between Xenopus and mammals has not been systematically documented. Results In this study we provide a histological and molecular atlas of Xenopus respiratory system development, focusing on Nkx2.1+ respiratory cell fate specification in the developing foregut. We document the expression patterns of Wnt/β-catenin, FGF, and BMP signaling components in the foregut and show that the molecular mechanisms of respiratory lineage induction are remarkably conserved between Xenopus and mice. Finally, using a number of functional experiments we refine the epistatic relationships between FGF, Wnt and BMP signaling in early Xenopus respiratory system development. Conclusions We demonstrate that Xenopus trachea and lung development, before metamorphosis, is comparable at the cellular and molecular levels to embryonic stages of mouse respiratory system development between E8.5 to E10.5. This molecular atlas provides a fundamental starting point for further studies using Xenopus as a model to define the conserved genetic programs controlling early respiratory system development.