Beta-catenin translocation into nuclei demarcates the dorsalizing centers in frog and fish embryos

Xwnt-8 and lithium can act upon either dorsal mesodermal or neurectodermal cells to cause a loss of forebrain in Xenopus embryos

When Xenopus gastrulae are made to misexpress Xwnt-8, or are exposed to lithium ions, they develop with a loss of anterior structures. In the current study, we have characterized the neural defects produced by either Xwnt-8 or lithium and have examined potential cellular mechanisms underlying this anterior truncation. We find that the primary defect in embryos exposed to lithium at successively earlier stages during gastrulation is a progressive rostral to caudal deletion of the forebrain, while hindbrain and spinal regions of the CNS remain intact. Misexpression of Xwnt-8 during gastrulation produces an identical loss of forebrain. Our results demonstrate that lithium and Wnts can act upon either prospective neural ectodermal cells, or upon dorsal mesodermal cells, to cause a loss of anterior pattern. Specifically, ectodermal cells isolated from lithium- or Wnt-exposed embryos are unable to form anterior neural tissue in response to inductive signals from normal dorsal mesoderm. In addition, although dorsal mesodermal cells from lithium- or Wnt-exposed embryos are specified properly, and produce normal levels of the anterior neural inducing molecules noggin and chordin, they show a greatly reduced capacity to induce anterior neural tissue in conjugated ectoderm. Taken together, our results are consistent with a model in which Wnt- or lithium-mediated signals can induce either mesodermal or ectodermal cells to produce a dominant posteriorizing morphogen which respecifies anterior neural tissue as posterior.

Cytoplasmically anchored plakoglobin induces a WNT-like phenotype in Xenopus

Plakoglobin is one of two vertebrate proteins closely related to the Drosophila segment polarity gene product armadillo. Overexpression of plakoglobin induces neural axis duplication in Xenopus and the exogenous plakoglobin is localized to nuclei (Karnovsky, A., and Klymkowsky, M. W., Proc. Natl. Acad. Sci. USA 92, 4255, 1995; Rubenstein, A., et al., Dev. Genet., 1997, in press). We have carried out a series of experiments to test whether the nuclear localization of plakoglobin is required for its inductive effects. Prior to the midblastula transition exogenous plakoglobin is cytoplasmic and concentrated in the cortical regions of blastomeres; after the midblastula transition exogenous plakoglobin accumulates in embryonic nuclei. The addition of a "nuclear localization sequence" does not change the timing of plakoglobin's nuclear localization, suggesting that it is anchored in the cytoplasm prior to the midblastula transition. Next, we constructed two "membrane-anchored" forms of plakoglobin. These are exclusively cytoplasmic; yet both were as effective at producing a "Wnt-like" axis duplication as were "free," unfettered forms of plakoglobin. Moreover, expression of anchored plakoglobins had no apparent effect on the cytoplasmic or nuclear levels of beta-catenin. These data indicate that plakoglobin can act cytoplasmically to generate a WNT-like phenotype. Taken together with the ventralizing effects of a mutant from of the XTcf-3 transcription factor, described by Molenaar et al. Cell 86, 391, 1996, we speculate that in the early Xenopus embryo, activation of plakoglobin (or beta-catenin) inhibits the activity of XTcf-3 or a XTcf-3-like factor.

Activation of the Wnt signaling pathway: a molecular mechanism for lithium action

Glycogen synthase kinase-3 beta (GSK-3 beta/zeste-white-3/shaggy) is a negative regulator of the wnt signaling pathway which plays a central role in the development of invertebrates and vertebrates; loss of function and dominant negative mutations in GSK-3 beta lead to activation of the wnt pathway in Drosophila and Xenopus. We now provide evidence that lithium activates downstream components of the wnt signaling pathway in vivo, leading to accumulation of beta-catenin protein. Our data indicate that this activation of the wnt pathway is a consequence of inhibition of GSK-3 beta by lithium. Using a novel assay for GSK-3 beta in oocytes, we show that lithium inhibits GSK-3 beta from species as diverse as Dictyostelium discoideum and Xenopus laevis, providing a biochemical mechanism for the action of lithium on the development of these organisms. Lithium treatment also leads to activation of an AP-1-luciferase reporter in Xenopus embryos, consistent with previous observations that GSK-3 beta inhibits c-jun activity. Activation of the wnt pathway with a dominant negative form of GSK-3 beta is inhibited by myo-inositol, similar to the previously described effect of coinjecting myo-inositol with lithium. The mechanism by which myo-inositol inhibits both dominant negative GSK-3 beta and lithium remains uncertain.

The roles of maternal alpha-catenin and plakoglobin in the early Xenopus embryo

Catenins (alpha-, beta- and gamma- or plakoglobin) are cytoplasmic cadherin-associated proteins. Studies on cultured cells have suggested that both alpha-catenin and plakoglobin are important for the adhesive function of cadherins. alpha-catenin binds to both beta-catenin and plakoglobin and may link the cadherin/catenin complex to actin filaments. Separate domains of plakoglobin bind to cadherin and alpha-catenin, suggesting it may act as a bridge between these molecules. However, plakoglobin may have other activities: it is expressed in both desmosomal junctions in association with desmogleins and the cytoplasm in conjunction with APC, and previous work suggests it may act in a dorsal signalling pathway when overexpressed in Xenopus embryos. Here, we have studied the roles of alpha-catenin and plakoglobin directly, by depleting the maternal mRNAs coding for each of them in developing Xenopus embryos. We find that depletion of maternal alpha-catenin causes the loss of intercellular adhesion at the blastula stage, similar to that reported previously for EP cadherin. Depletion of plakoglobin results in a partial loss of adhesion, and a loss of embryonic shape, but does not affect dorsal signalling.

LEF-1, a nuclear factor coordinating signaling inputs from wingless and decapentaplegic

wingless and decapentaplegic signal during endoderm induction in Drosophila to regulate expression of the homeotic gene Ultrabithorax. Here, we define a minimal wingless response sequence in the midgut enhancer of Ultrabithorax. We show that this sequence is recognized by the murine transcription factor LEF-1 (lymphocyte enhancer binding factor 1) in a ternary complex with armadillo protein, the cytoplasmic target of the wingless signaling pathway. In stable transformants, transcriptional stimulation of the Ultrabithorax enhancer by LEF-1 depends on armadillo. Furthermore, overexpression of LEF-1 bypasses the need for wingless signaling and causes phenotypes in the midgut, notum, and wing that mimic wingless hyperstimulation. Finally, efficient transcriptional stimulation by LEF-1 in the midgut depends also on the decapentaplegic response sequence and is limited spatially by decapentaplegic signaling. Thus, LEF-1 coordinates inputs from multiple positional signals, consistent with its architectural role in regulating the assembly of multiprotein enhancer complexes.

Frzb-1 is a secreted antagonist of Wnt signaling expressed in the Spemann organizer

Frzb-1 is a secreted protein containing a domain similar to the putative Wnt-binding region of the frizzled family of transmembrane receptors. Frzb-1 is widely expressed in adult mammalian tissues. In the Xenopus gastrula, it is expressed and regulated as a typical Spemann organizer component. Injection of frzb-1 mRNA blocks expression of XMyoD mRNA and leads to embryos with enlarged heads and shortened trunks. Frzb-1 antagonizes the effects of Xwnt-8 ectopic expression in a non-cell-autonomous manner. Cultured cells transfected with a membrane-tethered form of Wnt-1 bind epitope-tagged Frzb-1 in the 10(-10) M range. The results strengthen the view that the Spemann organizer is a source of secreted inhibitory factors.

Establishment of the dorso-ventral axis in Xenopus embryos is presaged by early asymmetries in beta-catenin that are modulated by the Wnt signaling pathway

Eggs of Xenopus laevis undergo a postfertilization cortical rotation that specifies the position of the dorso-ventral axis and activates a transplantable dorsal-determining activity in dorsal blastomeres by the 32-cell stage. There have heretofore been no reported dorso-ventral asymmetries in endogenous signaling proteins that may be involved in this dorsal-determining activity during early cleavage stages. We focused on beta-catenin as a candidate for an asymmetrically localized dorsal-determining factor since it is both necessary and sufficient for dorsal axis formation. We report that beta-catenin displays greater cytoplasmic accumulation on the future dorsal side of the Xenopus embryo by the two-cell stage. This asymmetry persists and increases through early cleavage stages, with beta-catenin accumulating in dorsal but not ventral nuclei by the 16- to 32-cell stages. We then investigated which potential signaling factors and pathways are capable of modulating the steady-state levels of endogenous beta-catenin. Steady-state levels and nuclear accumulation of beta-catenin increased in response to ectopic Xenopus Wnt-8 (Xwnt-8) and to the inhibition of glycogen synthase kinase-3, whereas neither Xwnt-5A, BVg1, nor noggin increased beta-catenin levels before the mid-blastula stage. As greater levels and nuclear accumulation of beta-catenin on the future dorsal side of the embryo correlate with the induction of specific dorsal genes, our data suggest that early asymmetries in beta-catenin presage and may specify dorso-ventral differences in gene expression and cell fate. Our data further support the hypothesis that these dorso-ventral differences in beta-catenin arise in response to the postfertilization activation of a signaling pathway that involves Xenopus glycogen synthase kinase-3.

Microtubule-mediated transport of organelles and localization of beta-catenin to the future dorsal side of Xenopus eggs

The dorsal-ventral axis in frog embryos is specified during the first cell cycle, when the cortex rotates relative to the cytoplasmic core along parallel microtubules associated with the core. Cytoplasmic transfer experiments suggest that dorsal determinants are transported 90 degrees from the vegetal pole to the dorsal equator, even though the cortex rotates only 30 degrees. Here we show that, during rotation, small endogenous organelles are rapidly propelled along the subcortical microtubules toward the future dorsal side and that fluorescent carboxylated beads injected into the vegetal pole are transported at least 60 degrees toward the equator. We also show that deuterium oxide, which broadens the zone of dorsalization even though it reduces the extent of rotation and is known to randomize the microtubules, also randomizes the direction of organelle transport. Moreover, beta-catenin, a component of the Wnt signaling pathway that possesses dorsalizing activity in Xenopus, colocalizes with subcortical microtubules at the dorsal side of the egg at the end of rotation. We propose that cortical rotation functions to align subcortical microtubules, which then mediate the transport of dorsal determinants toward their plus ends on one side of the egg.

Wnt signalling goes nuclear

The Wnt signalling cascade is a highly conserved signalling pathway throughout the animal kingdom. In Xenopus, Wnt signalling functions in mesodermal dorsoventral patterning. Earlier work on deciphering the components of the wnt signalling cascade left a gap between cytosolic beta-catenin, the final member of the cascade, and the nuclear target genes. Several recent papers now reveal how the Wnt signal is transmitted into the nucleus. Surprisingly, beta-catenin directly interacts with the transcription factor LEF-1/XTCF-3, and thereby is not only translocated into the nucleus but also modulates the properties of LEF-1/XTCF-3 as a transcription factor.

Adenomatous polyposis coli tumor suppressor protein has signaling activity in Xenopus laevis embryos resulting in the induction of an ectopic dorsoanterior axis

Mutations in the adenomatous polyposis coli (APC) tumor suppressor gene are linked to both familial and sporadic human colon cancer. So far, a clear biological function for the APC gene product has not been determined. We assayed the activity of APC in the early Xenopus embryo, which has been established as a good model for the analysis of the signaling activity of the APC-associated protein beta-catenin. When expressed in the future ventral side of a four-cell embryo, full-length APC induced a secondary dorsoanterior axis and the induction of the homeobox gene Siamois. This is similar to the phenotype previously observed for ectopic beta-catenin expression. In fact, axis induction by APC required the availability of cytosolic beta-catenin. These results indicate that APC has signaling activity in the early Xenopus embryo. Signaling activity resides in the central domain of the protein, a part of the molecule that is missing in most of the truncating APC mutations in colon cancer. Signaling by APC in Xenopus embryos is not accompanied by detectable changes in expression levels of beta-catenin, indicating that it has direct positive signaling activity in addition to its role in beta-catenin turnover. From these results we propose a model in which APC acts as part of the Wnt/beta-catenin signaling pathway, either upstream of, or in conjunction with, beta-catenin.

Dynamic microtubules and specification of the zebrafish embryonic axis

BACKGROUND

The zebrafish is emerging as an important genetic system for the study of vertebrate development, and many zygotic mutations affecting embryogenesis have been isolated. The early events in development are under the control of maternal genes but are relatively unexplored. Here, the process of axis specification is investigated.

RESULTS

The vegetal pole of the zygote transiently contains a dense array of parallel microtubules, while microtubules near the equator are disorganized. Irradiation of the zygote with ultraviolet light disrupts the formation of the vegetal microtubule array and causes loss of the axis; brief treatment with nocodazole at this stage also causes defects in the axis. During cleavage stages, yolk cortical microtubules reorganize to form arrays that apparently extend from marginal blastomeres. Prolonged exposure to cold (18 degrees C) or incubation in nocodazole prior to the 32-cell stage disrupts cortical microtubules and causes premature formation of the yolk syncytial layer; these treatments also prevent formation of an axis, as indicated by the absence of goosecoid and forkhead2 expression and of translocation of beta-catenin into nuclei. Cortical microtubule arrays are required for the transport of particles from the vegetal hemisphere into marginal blastomeres, as shown by the movement of polystyrene beads; treatments that prevent axis formation also prevent the entry of beads into blastomeres.

CONCLUSIONS

To form an organizer, zebrafish blastomeres appear to require substances which are transported from the vegetal hemisphere of the yolk cell by cortical microtubules. Initial asymmetry appears dependent on an array of parallel microtubules at the vegetal pole.

Cadherins and catenins in development

Cadherins and catenins represent key molecules during development. Recent findings demonstrate the involvement of cadherins and catenins in signaling pathways. In a working hypothesis, signaling via beta-catenin regulates the epithelial-mesenchymal transition in vertebrate development.

Nuclear localization of beta-catenin by interaction with transcription factor LEF-1

Vertebrate beta-catenin and Drosophila Armadillo share structural similarities suggesting that beta-catenin, like Armadillo, has a developmental signaling function. Both proteins are present as components of cell adherens junctions, but accumulate in the cytoplasm upon Wingless/Wnt signaling. beta-Catenin has axis-inducing properties like Wnt when injected into Xenopus blastomeres, providing evidence for participation of beta-catenin in the Wnt-pathway, but until now no downstream targets for beta-catenin have been identified. Here we demonstrate that beta-catenin binds to the HMG-type transcription factor lymphoid enhancer factor-1 (LEF-1), resulting in a nuclear translocation of beta-catenin both in cultured mouse cells and after ectopic expression of LEF-1 in two-cell mouse embryos. LEF-1/beta-catenin complexes bind to the promoter region of the E-cadherin gene in vitro, suggesting that this interaction could regulate E-cadherin transcription. As shown for beta-catenin, ectopic expression of LEF-1 in Xenopus embryos caused duplication of the body axis, indicating a regulatory role for a LEF-1-like molecule in dorsal mesoderm formation.