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Christof Niehrs

Publications and source records attributed to Christof Niehrs.

At least 19 recordsLinked to original sources
PubMed2006

An ancient Wnt-Dickkopf antagonism in Hydra.

The dickkopf (dkk) gene family encodes secreted antagonists of Wnt signalling proteins, which have important functions in the control of cell fate, proliferation, and cell polarity during development. Here, we report the isolation, from a regeneration-specific signal peptide screen, of a novel dickkopf gene from the fresh water cnidarian Hydra. Comparative sequence analysis demonstrates that the Wnt antagonistic subfamily Dkk1/Dkk2/Dkk4 and the non-modulating subfamily Dkk3 separated prior to the divergence of cnidarians and bilaterians. In steady-state Hydra, hydkk1/2/4-expression is inversely related to that of hywnt3a. hydkk1/2/4 is an early injury and regeneration responsive gene, and hydkk1/2/4-expressing gland cells are essential for head regeneration in Hydra, although once the head has regenerated they are excluded from it. Activation of Wnt/beta-Catenin signalling leads to the complete downregulation of hydkk1/2/4 transcripts. When overexpressed in Xenopus, HyDkk1/2/4 has similar Wnt-antagonizing activity to the Xenopus gene Dkk1. Based on the corresponding expression patterns of hydkk1/2/4 and neuronal genes, we suggest that the body column of Hydra is a neurogenic environment suppressing Wnt signalling and facilitating neurogenesis.

Amino Acid Sequence↗
PubMed2006

A role for fibronectin-leucine-rich transmembrane cell-surface proteins in homotypic cell adhesion.

The fibronectin-leucine-rich transmembrane (FLRT) family of leucine-rich repeat (LRR) proteins is implicated in fibroblast growth factor (FGF) signalling, early embryonic development and neurite outgrowth. Here, we have analysed whether FLRTs may also function in cell adhesion. We find that FLRT proteins can physically interact and that FLRT-transfected cultured cells sort out from non-transfected cells, suggesting a change in adhesive properties. A similar sorting effect is also observed in Xenopus embryos and tissue aggregates. FLRT-mediated cell sorting is calcium dependent and substrate independent. Deletion analysis indicates that cell sorting requires the LRR domains, which are dispensable for FLRT-mediated FGF signalling. Conversely, sorting is independent of the cytoplasmic domain, which is essential for FLRT-induced signalling. Therefore, FLRT-mediated FGF signal transduction and homotypic cell sorting can be molecularly uncoupled. The results indicate that FLRT proteins have a dual role, promoting FGF signalling and modulating homotypic cell adhesion.

Animals↗
PubMed2005

Casein kinase 1 gamma couples Wnt receptor activation to cytoplasmic signal transduction.

Signalling by Wnt proteins (Wingless in Drosophila) has diverse roles during embryonic development and in adults, and is implicated in human diseases, including cancer. LDL-receptor-related proteins 5 and 6 (LRP5 and LRP6; Arrow in Drosophila) are key receptors required for transmission of Wnt/beta-catenin signalling in metazoa. Although the role of these receptors in Wnt signalling is well established, their coupling with the cytoplasmic signalling apparatus remains poorly defined. Using a protein modification screen for regulators of LRP6, we describe the identification of Xenopus Casein kinase 1 gamma (CK1gamma), a membrane-bound member of the CK1 family. Gain-of-function and loss-of-function experiments show that CK1gamma is both necessary and sufficient to transduce LRP6 signalling in vertebrates and Drosophila cells. In Xenopus embryos, CK1gamma is required during anterio-posterior patterning to promote posteriorizing Wnt/beta-catenin signalling. CK1gamma is associated with LRP6, which has multiple, modular CK1 phosphorylation sites. Wnt treatment induces the rapid CK1gamma-mediated phosphorylation of these sites within LRP6, which, in turn, promotes the recruitment of the scaffold protein Axin. Our results reveal an evolutionarily conserved mechanism that couples Wnt receptor activation to the cytoplasmic signal transduction apparatus.

Amino Acid Motifs↗
PubMed2005

Axis formation: redundancy rules.

The role of BMP antagonists in the Spemann-Mangold organizer of vertebrate embryos is a controversial issue. A study using combined knock down of multiple antagonists finally reveals dramatic effects.

Animals↗
PubMed2005

Global gene expression profiling and cluster analysis in Xenopus laevis.

We have undertaken a large-scale microarray gene expression analysis using cDNAs corresponding to 21,000 Xenopus laevis ESTs. mRNAs from 37 samples, including embryos and adult organs, were profiled. Cluster analysis of embryos of different stages was carried out and revealed expected affinities between gastrulae and neurulae, as well as between advanced neurulae and tadpoles, while egg and feeding larvae were clearly separated. Cluster analysis of adult organs showed some unexpected tissue-relatedness, e.g. kidney is more related to endodermal than to mesodermal tissues and the brain is separated from other neuroectodermal derivatives. Cluster analysis of genes revealed major phases of co-ordinate gene expression between egg and adult stages. During the maternal-early embryonic phase, genes maintaining a rapidly dividing cell state are predominantly expressed (cell cycle regulators, chromatin proteins). Genes involved in protein biosynthesis are progressively induced from mid-embryogenesis onwards. The larval-adult phase is characterised by expression of genes involved in metabolism and terminal differentiation. Thirteen potential synexpression groups were identified, which encompass components of diverse molecular processes or supra-molecular structures, including chromatin, RNA processing and nucleolar function, cell cycle, respiratory chain/Krebs cycle, protein biosynthesis, endoplasmic reticulum, vesicle transport, synaptic vesicle, microtubule, intermediate filament, epithelial proteins and collagen. Data filtering identified genes with potential stage-, region- and organ-specific expression. The dataset was assembled in the iChip microarray database, , which allows user-defined queries. The study provides insights into the higher order of vertebrate gene expression, identifies synexpression groups and marker genes, and makes predictions for the biological role of numerous uncharacterized genes.

Animals↗
PubMed2005

Fibroblast growth factor signaling during early vertebrate development.

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

Animals↗
PubMed2004

An atlas of differential gene expression during early Xenopus embryogenesis.

We have carried out a large-scale, semi-automated whole-mount in situ hybridization screen of 8369 cDNA clones in Xenopus laevis embryos. We confirm that differential gene expression is prevalent during embryogenesis since 24% of the clones are expressed non-ubiquitously and 8% are organ or cell type specific marker genes. Sequence analysis and clustering yielded 723 unique genes displaying a differential expression pattern. Of these, 18% were already described in Xenopus, 47% have homologs and 35% are lacking significant sequence similarity in databases. Many of them encode known developmental regulators. We classified 363 of the 723 genes for which a Gene Ontology annotation for molecular function could be attributed and found 'DNA binding' and 'enzyme' the most represented terms. The most common protein domains encoded in these embryonic, differentially expressed genes are the homeobox and RNA Recognition Motif (RRM). Fifty-nine putative orthologs of human disease genes, and 254 organ or cell specific marker genes were identified. Markers were found for nasal placode and archenteron roof, organs for which a specific marker was previously unavailable. Markers were also found for novel subdomains of various other organs. The tissues for which most markers were found are muscle and epidermis. Expression of cell cycle regulators fell in two classes, containing proliferation-promoting and anti-proliferative genes, respectively. We identified 66 new members of the BMP4, chromatin, endoplasmic reticulum, and karyopherin synexpression groups, thus providing a first glimpse of their probable cellular roles. Cluster analysis of tissues to measure tissue relatedness yielded some unorthodox affinities besides expectable lineage relationships. In conclusion, this study represents an atlas of gene expression patterns, which reveals embryonic regionalization, provides novel marker genes, and makes predictions about the functional role of unknown genes.

Animals↗
PubMed2004

Nuclear reprogramming of human somatic cells by xenopus egg extract requires BRG1.

Animal cloning by nuclear transplantation in amphibia was demonstrated almost half a century ago and raised the question of the mechanisms and genes involved in nuclear reprogramming. Here, we demonstrate nuclear reprogramming of permeabilized human cells using extracts from Xenopus laevis eggs and early embryos. We show upregulation of pluripotency markers Oct-4 and germ cell alkaline phosphatase (GCAP) in 293T cells and human primary leukocytes. Reprogrammed leukocytes had a limited life span and did not express surface antigens characteristic of pluripotent cells, indicating that reprogramming was incomplete. Reprogramming activity was detected in egg and early embryo extracts until early blastula stage. Late blastula-stage extracts were not only inactive but also inhibitory to reprogramming. Screening for factors required for reprogramming identified the chromatin remodeling ATPase BRG1. Antibody depletion of BRG1 protein or expression of dominant-negative BRG1 abolished the reprogramming ability of amphibian extracts. Conversely, overexpression of BRG1 in Xenopus animal caps extended their competence from blastula to gastrula stage to respond to basic fibroblast growth factor (bFGF) treatment with induction of the mesodermal marker Xbra. Dissection of the molecular machinery using a simplified assay system may aid in achieving complete nuclear reprogramming of somatic cells for regenerative medicine.

Alkaline Phosphatase↗
PubMed2004

Reliability of gene expression ratios for cDNA microarrays in multiconditional experiments with a reference design.

In a typical gene expression profiling experiment with multiple conditions, a common reference sample is used for co-hybridization with the samples to yield expression ratios. Differential expression for any other sample pair can then be calculated by assembling the ratios from their hybridizations with the reference. In this study we test the validity of this approach. Differential expression of a sample pair (i, j) was obtained in two ways: directly, by hybridizations of sample i versus j, and indirectly, by multiplying the expression ratios for hybridizations of sample i versus pool and pool versus sample j. We performed gene expression profiling using amphibian embryos (Xenopus laevis). Every sample combination of four different stages and a pool was profiled. Direct and indirect values were compared and used as the quality criterion for the data. Based on this criterion, 82% of all ratios were found to be sufficiently accurate. To increase the reliability of the signals, several widely used filtering techniques were tested. Filtering by differences of repeated hybridizations was found to be the optimal filter. Finally, we compared microarray-based gene expression profiles with the corresponding expression patterns obtained by whole-mount in situ hybridizations, resulting in a 90% correspondence.

Algorithms↗
PubMed2004

Transcriptional regulation of BMP4 synexpression in transgenic Xenopus.

Synexpression groups are genetic modules composed of genes that share both a complex expression pattern and the biological process in which they function. Here we investigate the regulation of BMP4 synexpression by studying the enhancers of bambi, smad7 and vent2 in Xenopus. We find that a BMP4 synexpression promoter module is compact and (i) requires direct BMP responsiveness through Smad and Smad-cofactor binding motifs, (ii) may contain an evolutionary conserved BMP-responsive element, bre7 (TGGCGCC), that is crucial for expression of bambi and smad7 and is highly prognostic for novel BMP-responsive enhancers (BREs); and (iii) requires a narrow window of BMP inducibility, because minor enhancement or reduction of BMP responsiveness abolishes synexpression. Furthermore, we used a bioinformatic model to predict in silico 13 novel BREs, and tested five of them that were found in the id1-4 genes. The results highlight that in vivo analysis is required to reveal the physiological, spatio-temporal regulation of BMP-responsive genes.

Animals↗
PubMed2004

R-Spondin2 is a secreted activator of Wnt/beta-catenin signaling and is required for Xenopus myogenesis.

We have carried out a small pool expression screen for modulators of the Wnt/beta-catenin pathway and identified Xenopus R-spondin2 (Rspo2) as a secreted activator of this cascade. Rspo2 is coexpressed with and positively regulated by Wnt signals and synergizes with Wnts to activate beta-catenin. Analyses of functional interaction with components of the Wnt/beta-catenin pathway suggest that Rspo2 functions extracellularly at the level of receptor ligand interaction. In addition to activating the Wnt/beta-catenin pathway, Rspo2 overexpression blocks Activin, Nodal, and BMP4 signaling in Xenopus, raising the possibility that it may negatively regulate the TGF-beta pathway. Antisense Morpholino experiments in Xenopus embryos and RNAi experiments in HeLa cells reveal that Rspo2 is required for Wnt/beta-catenin signaling. In Xenopus embryos depleted of Rspo2, the muscle markers myoD and myf5 fail to be activated and later muscle development is impaired. Thus, Rspo2 functions in a positive feedback loop to stimulate the Wnt/beta-catenin cascade.

Animals↗
PubMed2004

Norrin and frizzled; a new vein for the eye.

The Wnt signaling network is arguably one of the most complex molecular modules by which cells communicate. Though we become accustomed to novel components, the discovery of an unexpected Wnt receptor ligand, Norrin, is surprising even to the connoisseur.

Animals↗
PubMed2003

The transmembrane protein XFLRT3 forms a complex with FGF receptors and promotes FGF signalling.

Fibroblast growth factors (FGFs) signal through high-affinity tyrosine kinase receptors to regulate a diverse range of cellular processes, including cell growth, differentiation and migration, as well as cell death. Here we identify XFLRT3, a member of a leucine-rich-repeat transmembrane protein family, as a novel modulator of FGF signalling. XFLRT3 is co-expressed with FGFs, and its expression is both induced after activation and downregulated after inhibition of FGF signalling. In gain- and loss-of function experiments, FLRT3 and FLRT2 phenocopy FGF signalling in Xenopus laevis. XFLRT3 signalling results in phosphorylation of ERK and is blocked by MAPK phosphatase 1, but not by expression of a dominant-negative phosphatidyl inositol 3-OH kinase (PI(3)K) mutant. XFLRT3 interacts with FGF receptors (FGFRs) in co-immunoprecipitation experiments in vitro and in bioluminescence resonance energy transfer assays in vivo. The results indicate that XFLRT3 is a transmembrane modulator of FGF-MAP kinase signalling in vertebrates.

Animals↗
PubMed2003

Dkk1 and noggin cooperate in mammalian head induction.

Growth factor antagonists play important roles in mediating the inductive effects of the Spemann organizer in amphibian embryos and its equivalents in other vertebrates. Dual inhibition of Wnt and BMP signals has been proposed to confer head organizer activity. We tested the requirement of this coinhibition in Xenopus and mice. In Xenopus, simultaneous reduction of the BMP antagonists chordin and noggin, and the Wnt antagonist dickkopf1 (dkk1) leads to anterior truncations. In mice, compound mutants for dkk1 and noggin display severe head defects, with deletion of all head structures anterior to the mid-hindbrain boundary. These defects arise as a result of a failure in anterior specification at the gastrula stage. The results provide genetic evidence for the dual inhibition model and indicate that dkk1 and noggin functionally cooperate in the head organizer.

Animals↗
PubMed2003

Kremen2 modulates Dickkopf2 activity during Wnt/LRP6 signaling.

Dickkopf1 (Dkk1) is a secreted antagonist of the Wnt/beta-catenin signaling pathway that acts by direct binding to and inhibiting the Wnt co-receptor LRP6. The related Dkk2, however, can function either as LRP6 agonist or antagonist, depending on the cellular context, suggesting that its activity is modulated by unknown co-factors. We have recently identified the transmembrane proteins Kremen1 and -2 as additional Dkk receptors, which bind to both Dkk1 and Dkk2 with high affinity. Here we show that Kremen2 (Krm2) regulates Dkk2 activity during Wnt signaling. In human 293 fibroblasts transfected dkk2 activates LRP6 signaling. However, co-transfection of krm2 blocks the ability of Dkk2 to activate LRP6 and enhances inhibition of Wnt/Frizzled signaling. Krm2 also co-operates with Dkk4 to inhibit Wnt signaling, but not with Dkk3, which has no effect on Wnt signaling. Likewise, in Xenopus embryos, Dkk2 and Krm2 co-operate in Wnt inhibition leading to anteriorized embryos. Finally, we show that interaction with Krm2 is mediated by the second cysteine-rich domain of Dkks. These results suggest that Krm2 can function as a switch that turns Dkk2 from an activator into an inhibitor of Wnt/lRP6 signaling.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Cyclic expression of esr9 gene in Xenopus presomitic mesoderm.

During somitogenesis, the cycling expression of members of the Notch signalling cascade is involved in a segmentation clock that regulates the periodic budding of somites in chicken, mouse, and zebrafish. In frog, genes with cycling expression in the presomitic mesoderm have not been reported. Here, we describe the expression of Xenopus esr9 and esr10, two new members of the Hairy/Enhancer of split related family of bHLH proteins. We show that they are expressed in a highly dynamic fashion, with their mRNA levels oscillating periodically in the presomitic mesoderm during somitogenesis. This dynamic expression is independent of de novo protein synthesis. Thus, expression of esr9 and esr10 is an indicator of the segmentation clock in the amphibian embryo. This confirms the evolutionary conservation of a molecular pathway involved in vertebrate segmentation clock.

Amino Acid Sequence↗