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At least 1,711 records · Page 95Linked to original sources
PubMed2001

Head inducer Dickkopf-1 is a ligand for Wnt coreceptor LRP6.

BACKGROUND: Dickkopf-1 (Dkk-1) is a head inducer secreted from the vertebrate head organizer and induces anterior development by antagonizing Wnt signaling. Although several families of secreted antagonists have been shown to inhibit Wnt signal transduction by binding to Wnt, the molecular mechanism of Dkk-1 action is unknown. The Wnt family of secreted growth factors initiates signaling via the Frizzled (Fz) receptor and its candidate coreceptor, LDL receptor-related protein 6 (LRP6), presumably through Fz-LRP6 complex formation induced by Wnt. The significance of the Fz-LRP6 complex in signal transduction remains to be established. RESULTS: We report that Dkk-1 is a high-affinity ligand for LRP6 and inhibits Wnt signaling by preventing Fz-LRP6 complex formation induced by Wnt. Dkk-1 binds neither Wnt nor Fz, nor does it affect Wnt-Fz interaction. Dkk-1 function in head induction and Wnt signaling inhibition strictly correlates with its ability to bind LRP6 and to disrupt the Fz-LRP6 association. LRP6 function and Dkk-1 inhibition appear to be specific for the Wnt/Fz beta-catenin pathway. CONCLUSIONS: Our results demonstrate that Dkk-1 is an LRP6 ligand and inhibits Wnt signaling by blocking Wnt-induced Fz-LRP6 complex formation. Our findings thus reveal a novel mechanism for Wnt signal modulation. LRP6 is a Wnt coreceptor that appears to specify Wnt/Fz signaling to the beta-catenin pathway, and Dkk-1, distinct from Wnt binding antagonists, may be a specific inhibitor for Wnt/beta-catenin signaling. Our findings suggest that Wnt-Fz-LRP6 complex formation, but not Wnt-Fz interaction, triggers Wnt/beta-catenin signaling.

Animals↗
PubMed2000

Control of beta-catenin stability: reconstitution of the cytoplasmic steps of the wnt pathway in Xenopus egg extracts.

Regulation of beta-catenin degradation by intracellular components of the wnt pathway was reconstituted in cytoplasmic extracts of Xenopus eggs and embryos. The ubiquitin-dependent beta-catenin degradation in extracts displays a biochemical requirement for axin, GSK3, and APC. Axin dramatically accelerates while dishevelled inhibits beta-catenin turnover. Through another domain, dishevelled recruits GBP/Frat1 to the APC-axin-GSK3 complex. Our results confirm and extend models in which inhibition of GSK3 has two synergistic effects: (1) reduction of APC phosphorylation and loss of affinity for beta-catenin and (2) reduction of beta-catenin phosphorylation and consequent loss of its affinity for the SCF ubiquitin ligase complex. Dishevelled thus stabilizes beta-catenin, which can dissociate from the APC/axin complex and participate in transcriptional activation.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Peroxisome-proliferator-activated receptor gamma suppresses Wnt/beta-catenin signalling during adipogenesis.

The Wnt/beta-catenin signalling pathway appears to operate to maintain the undifferentiated state of preadipocytes by inhibiting adipogenic gene expression. To define the mechanisms regulating suppression of Wnt/beta-catenin signalling, we analysed the beta-catenin expression in response to activation of transcription factors that regulate adipogenesis. The results show an extensive down-regulation of nuclear beta-catenin that occurs during the first few days of differentiation of 3T3-L1 preadipocytes and coincides with the induction of the adipogenic transcription factors, C/EBPbeta (CCAAT-enhancer-binding protein) and PPARgamma (peroxisome-proliferator-activated receptor). To assess the role of each of these factors in this process, we conditionally overexpressed C/EBPbeta in Swiss mouse fibroblasts using the TET-off system. Abundant expression of C/EBPbeta alone had minimal effect on beta-catenin expression, whereas expression of C/EBPbeta, in the presence of dexamethasone, induced PPARgamma expression and caused a measurable decrease in beta-catenin. In addition, exposure of cells expressing both C/EBPbeta and PPARgamma to a potent PPARgamma ligand resulted in an even greater decrease in beta-catenin by mechanisms that involve the proteasome. Our studies also suggest a reciprocal relationship between PPARgamma activity and beta-catenin expression, since ectopic production of Wnt-1 in preadipocytes blocked the induction of PPARgamma gene expression. Moreover, by suppressing beta-catenin expression, ectopic expression of PPARgamma in Wnt-1-expressing preadipocytes rescued the block in adipogenesis after their exposure to the PPARgamma ligand, troglitazone.

Adipocytes↗
PubMed2003

Histology-based screen for zebrafish mutants with abnormal cell differentiation.

The power of histology to define states of cell differentiation was used as the basis of a mutagenesis screen in zebrafish. In this screen, 7-day-old parthenogenetic half-tetrad larvae from potential carrier females were screened for mutations affecting cell differentiation in hematoxylin and eosin-stained tissue sections. Seven, noncomplementing, recessive mutations were found. Two mutations affect only the retina: segmented photoreceptors (spr) show a discontinuous photoreceptor cell layer; vestigial outer segments (vos) has fewer photoreceptor cells and degenerated outer segments within this cell layer. Three mutants have gut-specific defects: the epithelial cells of kirby (kby) are replaced by ballooned cells; the intestines of stuffy (sfy) and stuffed (sfd) contain increased luminal mucus. Two mutations affect multiple organs: disordered neural retina (dnr) has disrupted retinal layering and mild nuclear abnormalities in the gut and liver; and in huli hutu (hht), the retinal cell layers are disorganized and multiple organs have mild to severe nuclear abnormalities that are reminiscent of the atypia of human neoplasia. Each mutation appears to be homozygous lethal. This screen is proof of principle for the feasibility of histologic screens to yield novel mutations, including potential models of human disease. The throughput for this type of screen may be enhanced by automation.

Animals↗
PubMed2006

Somite compartments in anamniotes.

Somites are a common feature of the phylotypic stage of embryos of all higher chordates. In amniote species like mouse and chick, somite development has been the subject of intense research over many decades, giving insight into the morphological and molecular processes leading to somite compartmentalization and subsequent differentiation. In anamniotes, somite development is much less understood. Except for recent data from zebrafish, and morphological studies in Xenopus, very little is known about the formation of somite compartments and the differentiation of somite derivatives in anamniotes. Here, we give a brief overview on the development of myotome, sclerotome and dermomyotome in various anamniote organisms, and point out the different mechanisms of somite development between anamniotes and the established amniote model systems.

Amphibians↗
PubMed2001

Wnt signaling and mammary tumorigenesis.

Wnt expression patterns during mammary development support a role for Wnts in breast development and in mammary epithelial responses to systemic hormones. The deregulation of Wnt signaling also plays a role in breast cancer. Activation of the Wnt signaling pathway is a major feature of several human neoplasias and appears to lead to the cytosolic stabilization of a transcriptional co-factor, beta-catenin. This co-activator can then regulate transcription from a number of target genes including the cellular oncogenes cyclin D1 and c-myc. This review will summarize the current state of knowledge of Wnt signal transduction in a range of model systems and will then address the role of Wnts and Wnt signaling in mammary development and cancer.

Animals↗
PubMed2007

Identification of aromatase-positive radial glial cells as progenitor cells in the ventricular layer of the forebrain in zebrafish.

Compared with other vertebrates, the brain of adult teleost fish exhibits two unique features: it exhibits unusually high neurogenic activity and strongly expresses aromatase, a key enzyme that converts aromatizable androgens into estrogens. Until now, these two features, high neurogenic and aromatase activities, have never been related to each other. Recently, it was shown that aromatase is expressed in radial glial cells of the forebrain and not in neurons. Here, we further document that Aromatase B is never detected in cells expressing the markers of postmitotic neurons, Hu and acetylated tubulin. By using a combination of bromodeoxyuridine (BrdU) treatment and immunohistochemical techniques, we demonstrate for the first time to our knowledge that aromatase-positive radial cells actively divide to generate newborn cells in many forebrain regions. Such newborn cells can further divide, as shown by BrdU-proliferating cell nuclear antigen double staining. We also demonstrate that, over time, newborn cells move away from the ventricles, most likely by migrating along the radial processes. Finally, by using antisera to Hu and acetylated tubulin, we further document that some of the newborn cells derived from radial glia differentiate into neurons. These data provide new evidence for the mechanism of neurogenesis in the brain of adult fish. In addition, given that estrogens are well-known neurotrophic and neuroprotective factors affecting proliferation, apoptosis, migration, and differentiation, the expression of aromatase in the neural stem cells of the adult strongly demonstrates that the fish brain is an outstanding model for studying the effects of estrogens on adult neurogenesis and brain repair.

Animals↗
PubMed2002

Structure, developmental expression, and physiological regulation of zebrafish IGF binding protein-1.

The biological activity and availability of IGFs are regulated by a group of secreted proteins that belong to the IGF-binding protein (IGFBP) gene family. Although six IGFBPs have been identified and studied in mammals, their nonmammalian orthologs remain poorly defined. In this study, we cloned and characterized the full-length zebrafish IGFBP-1. Sequence analysis indicated that its structure is homologous to mammalian IGFBP-1. Using in situ RNA hybridization and RT-PCR, we discovered that IGFBP-1 mRNA was present in all early embryonic stages albeit at very low levels. IGFBP-1 mRNA was initially expressed in multiple embryonic tissues but became restricted to the liver shortly after hatching. In the adult stage, IGFBP-1 mRNA was found only in the liver at low levels. Prolonged food deprivation caused a significant increase in the hepatic IGFBP-1 mRNA levels, and refeeding restored the IGFBP-1 mRNA to the basal levels. When adult fish or embryos were subjected to hypoxic conditions, the IGFBP-1 mRNA expression increased dramatically. Intriguingly, the hypoxia-induced IGFBP-1 expression operated in different embryonic tissues in a developmental-stage-dependent manner. In early embryos, hypoxia-stimulated IGFBP-1 mRNA expression in the pharyngeal arches, ventricle, atrium, and brain. After hatching, the hypoxia-induced IGFBP-1 expression became liver specific. These results not only provide new information about the structural conservation, developmental expression, and physiological regulation of the IGFBP-1 gene but also present the opportunity to elucidate the developmental role of IGFBP-1 using a unique vertebrate model organism.

Amino Acid Sequence↗
PubMed2006

Developmental toxicity of 4-ring polycyclic aromatic hydrocarbons in zebrafish is differentially dependent on AH receptor isoforms and hepatic cytochrome P4501A metabolism.

Polycyclic aromatic hydrocarbons (PAHs) derived from fossil fuels are ubiquitous contaminants and occur in aquatic habitats as highly variable and complex mixtures of compounds containing 2 to 6 rings. For aquatic species, PAHs are generally accepted as acting through either of two modes of action: (1) "dioxin-like" toxicity mediated by activation of the aryl hydrocarbon receptor (AHR), which controls a battery of genes involved in PAH metabolism, such as cytochrome P4501A (CYP1A) and (2) "nonpolar narcosis", in which tissue uptake is dependent solely on hydrophobicity and toxicity is mediated through non-specific partitioning into lipid bilayers. As part of a systematic analysis of mechanisms of PAH developmental toxicity in zebrafish, we show here that three tetracyclic PAHs (pyrene, chrysene, and benz[a]anthracene) activate the AHR pathway tissue-specifically to induce distinct patterns of CYP1A expression. Using morpholino knockdown of ahr1a, ahr2, and cyp1a, we show that distinct embryolarval syndromes induced by exposure to two of these compounds are differentially dependent on tissue-specific activation of AHR isoforms or metabolism by CYP1A. Exposure of embryos with and without circulation (silent heart morphants) resulted in dramatically different patterns of CYP1A induction, with circulation required to deliver some compounds to internal tissues. Therefore, biological effects of PAHs cannot be predicted simply by quantitative measures of AHR activity or a compound's hydrophobicity. These results indicate that current models of PAH toxicity in fish are greatly oversimplified and that individual PAHs are pharmacologically active compounds with distinct and specific cellular targets.

Abnormalities, Drug-Induced↗
PubMed2002

Dynamics of a developmental switch: recursive intracellular and intranuclear redistribution of Caenorhabditis elegans POP-1 parallels Wnt-inhibited transcriptional repression.

POP-1, a Tcf/Lef factor, functions throughout Caenorhabditis elegans development as a Wnt-dependent reiterative switch to generate nonequivalent sister cells that are born by anterior-posterior cell divisions. We have observed the interaction between POP-1 and a target gene that it represses as it responds to Wnt signaling. Dynamic observations in living embryos reveal that POP-1 undergoes Wnt-dependent nucleocytoplasmic redistribution immediately following cytokinesis, explaining the differential nuclear POP-1 levels in nonequivalent sister cells. In unsignaled (anterior) but not Wnt-signaled (posterior) sister cells, POP-1 progressively coalesces into subnuclear domains during interphase, coincident with its action as a repressor. While the asymmetric distribution of POP-1 in nonequivalent sisters apparently requires a 124-amino-acid internal domain, neither the HMG box nor beta-catenin interaction domains are required. We find that a transcriptional activator, MED-1, associates in vivo with the end-1 and end-3 target genes in the mesoderm (anterior sister) and in the endoderm (posterior sister) following the asymmetric cell division that subdivides the mesendoderm. However, in the anterior sister, binding of POP-1 to the end-1 and end-3 genes blocks their expression. In vivo, binding of POP-1 to the end-1 and end-3 targets (in the posterior sister) is blocked by Wnt/MAPK signaling. Thus, a Tcf/Lef factor represses transactivation of genes in an unsignaled daughter cell by abrogating the function of a bound activator.

Animals↗
PubMed2002

A loss-of-function mutation in the CFC domain of TDGF1 is associated with human forebrain defects.

TDGF1 (CRIPTO) is an EGF-CFC family member and an obligate co-receptor involved in NODAL signaling, a developmental program implicated in midline, forebrain, and left-right axis development in model organisms. Previous studies of CFC1 (CRYPTIC), another member of the EGF-CFC family, demonstrated that normal function of this protein is required for proper laterality development in humans. Here we identify a mutation in the conserved CFC domain of TDGF1 in a patient with midline anomalies of the forebrain. The mutant protein is inactive in a zebrafish rescue assay, indicating a role for TDGF1 in human midline and forebrain development.

Amino Acid Motifs↗
PubMed2001

Neuroepithelial co-expression of Gbx2 and Otx2 precedes Fgf8 expression in the isthmic organizer.

The most studied secondary neural organizer is the isthmic organizer, which is localized at the mid-hindbrain transition of the neural tube and controls the anterior hindbrain and midbrain regionalization. Otx2 and Gbx2 expressions are fundamental for positioning the organizer and the establishment of molecular interactions that induce Fgf8. We present here evidences demonstrating that Otx2 and Gbx2 have an overlapping expression in the isthmic region. This area is the transversal domain where expression of Fgf8 is induced. The Fgf8 protein produced in the isthmus stabilizes and up-regulates Gbx2 expression, which, in turn, down-regulates Otx2 expression. The inductive effect of the Gbx2/Otx2 limit keeps Fgf8 expression stable and thus maintains its positive role in the expression of Pax2, En1,2 and Wnt1.

Animals↗
PubMed2000

Patterning the cranial neural crest: hindbrain segmentation and Hox gene plasticity.

Understanding the patterning mechanisms that control head development--particularly the neural crest and its contribution to bones, nerves and connective tissue--is an important problem, as craniofacial anomalies account for one-third of all human congenital defects. Classical models for craniofacial patterning argue that the morphogenic program and Hox gene identity of the neural crest is pre-patterned, carrying positional information acquired in the hindbrain to the peripheral nervous system and the branchial arches. Recently, however, plasticity of Hox gene expression has been observed in the hindbrain and cranial neural crest of chick, mouse and zebrafish embryos. Hence, craniofacial development is not dependent on neural crest prepatterning, but is regulated by a more complex integration of cell and tissue interactions.

Animals↗
PubMedDate not supplied

Comparative expression analysis of Adh3 during arthropod, urochordate, cephalochordate, and vertebrate development challenges its predicted housekeeping role.

Gene and genome duplications in the vertebrate lineage explain the complexity of extant gene families. Among these, the medium-chain alcohol dehydrogenase (ADH), which expanded by tandem duplications after the cephalochordate-vertebrate split, is a good model with which to analyze the evolution of gene function. Although the ancestral member of this family, ADH3, has been strictly conserved throughout animal evolution, its physiological role is still controversial. Previous evidence indicates that it contributes to formaldehyde cytoprotection, retinoic acid metabolism, and nitric oxide homeostasis. We performed in situ hybridization during Drosophila, ascidian (Ciona intestinalis), and zebrafish (Danio rerio) development. We showed that Adh3 expression was restricted to the fat body in Drosophila embryos at stage 17 and to the anterior endoderm in C. intestinalis tail bud, whereas in the zebrafish 2.5-day larvae the signal appeared widespread. A more comprehensive expression analysis including amphioxus and mice revealed that ancestral Adh3 was tissue specific, whereas a widespread expression was later attained in vertebrates. These variations occurred concomitantly with the expansion of the ADH family and the acquisition of new functions but were unlinked to the genomic changes that led to the transition from fractional to global methylation in vertebrates. Our data challenge the housekeeping role of ADH3 and question its involvement in the prevertebrate retinoic acid pathway.

Alcohol Dehydrogenase↗
PubMed2004

Tight junction protein Par6 interacts with an evolutionarily conserved region in the amino terminus of PALS1/stardust.

Tight junctions are the structures in mammalian epithelial cells that separate the apical and basolateral membranes and may also be important in the establishment of cell polarity. Two evolutionarily conserved multiprotein complexes, Crumbs-PALS1 (Stardust)-PATJ and Cdc42-Par6-Par3-atypical protein kinase C, have been implicated in the assembly of tight junctions and in polarization of Drosophila melanogaster epithelia. These two complexes have been linked physically and functionally by an interaction between PALS1 and Par6. Here we identify an evolutionarily conserved region in the amino terminus of PALS1 as the Par6 binding site and identify valine and aspartic acid residues in this region as essential for interacting with the PDZ domain of Par6. We have also characterized, in more detail, the amino terminus of Drosophila Stardust and demonstrate that the interaction mechanism between Stardust and Drosophila Par6 is evolutionarily conserved. Par6 interferes with PATJ in binding PALS1, and these two interactions do not appear to function synergistically. Taken together, these results define the molecular mechanisms linking two conserved polarity complexes.

Amino Acid Sequence↗
PubMed2004

Differential oxidation of protein-tyrosine phosphatases.

Oxidation is emerging as an important regulatory mechanism of protein-tyrosine phosphatases (PTPs). Here we report that PTPs are differentially oxidized, and we provide evidence for the underlying mechanism. The membrane-proximal RPTPalpha-D1 was catalytically active but not readily oxidized as assessed by immunoprobing with an antibody that recognized oxidized catalytic site cysteines in PTPs (oxPTPs). In contrast, the membrane-distal RPTPalpha-D2, a poor PTP, was readily oxidized. Oxidized catalytic site cysteines in PTP immunoprobing and mass spectrometry demonstrated that mutation of two residues in the Tyr(P) loop and the WPD loop that reverse catalytic activity of RPTPalpha-D1 and RPTPalpha-D2 also reversed oxidizability, suggesting that oxidizability and catalytic activity are coupled. However, catalytically active PTP1B and LAR-D1 were readily oxidized. Oxidizability was strongly dependent on pH, indicating that the microenvironment of the catalytic cysteine has an important role. Crystal structures of PTP domains demonstrated that the orientation of the absolutely conserved PTP loop arginine correlates with oxidizability of PTPs, and consistently, RPTPmu-D1, with a similar conformation as RPTPalpha-D1, was not readily oxidized. In conclusion, PTPs are differentially oxidized at physiological pH and H(2)O(2) concentrations, and the PTP loop arginine is an important determinant for susceptibility to oxidation.

Animals↗