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

Nuclear localization of beta-catenin in vegetal pole cells during early embryogenesis of the starfish Asterina pectinifera

Recently, beta-catenin has been reported to control the expression of morphogenetic genes through the Wnt signaling pathway in invertebrate embryogenesis. In this study, the distribution pattern of beta-catenin during starfish embryogenesis was investigated using immunohistochemistry. In 16-cell stage embryos, beta-catenin began to accumulate in some nuclei at the vegetal pole. During the early cleavage stage, the cells expressing nuclear beta-catenin increased in number in the vegetal pole region of the embryos, and the beta-catenin signal increased in intensity in each nucleus. At the blastula stage, signal for beta-catenin was also found in the cytoplasm of the cells with nuclear beta-catenin. At the vegetal plate stage, almost all vegetal plate cells expressed beta-catenin in both the nucleus and cytoplasm. When the embryos developed to early gastrulae, cells with nuclear beta-catenin were restricted to the archenteron tip, and the signal gradually faded in later stages. The localization and temporal change of beta-catenin expression suggests that beta-catenin has a pivotal role in archenteron formation in starfish embryos.

Wg/Wnt signal can be transmitted through arrow/LRP5,6 and Axin independently of Zw3/Gsk3beta activity

Activation of the Wnt signaling cascade provides key signals during development and in disease. Here we provide evidence, by designing a Wnt receptor with ligand-independent signaling activity, that physical proximity of Arrow (LRP) to the Wnt receptor Frizzled-2 triggers the intracellular signaling cascade. We have uncovered a branch of the Wnt pathway in which Armadillo activity is regulated concomitantly with the levels of Axin protein. The intracellular pathway bypasses Gsk3beta/Zw3, the kinase normally required for controlling beta-catenin/Armadillo levels, suggesting that modulated degradation of Armadillo is not required for Wnt signaling. We propose that Arrow (LRP) recruits Axin to the membrane, and that this interaction leads to Axin degradation. As a consequence, Armadillo is no longer bound by Axin, resulting in nuclear signaling by Armadillo.

Armadillo/beta-catenin signals in the nucleus--proof beyond a reasonable doubt?

Wnt signalling results in transcriptional stimulation of genes controlling normal and malignant development. A key effector of the canonical Wnt pathway is beta-catenin (also known as Drosophila melanogaster Armadillo (Arm)), thought to function as a nuclear co-activator of TCF transcription factors. This has been challenged by unexpected observations of membrane-bound Arm/beta-catenin signalling activity. Plausible explanations allow these observations to be reconciled with the large body of evidence supporting a nuclear function of Arm/beta-catenin.

Parallel microtubules and other conserved elements of dorsal axial specification in the direct developing frog, Eleutherodactylus coqui

Specification of the dorsal axis in commonly studied frogs, such as Xenopus laevis and Rana pipiens, depends on a microtubule-mediated movement of cytoplasm in the fertilized egg. The Puerto Rican tree frog, Eleutherodactylus coqui, has an egg that is twenty times the volume of that of X. laevis, raising the question as to whether the mechanism of dorsal axial specification is conserved in these large eggs. Fertilized eggs of E. coqui develop a transient array of parallel microtubules, similar to other frogs, but proportionately larger. The array persists after first cleavage, longer than in other frogs, and is gone by the third cleavage. Correlated with the longer life of the parallel microtubules, both 2- and 8-cell E. coqui embryos remain sensitive to gravity-mediated axial specification, a sensitivity lost in X. laevis before the 2-cell stage. Activation of the Wnt/beta-catenin pathway by injected Xwnt8 RNA causes axial formation as in X. laevis. The results indicate that elements of dorsal axial specification are conserved in E. coqui, but they occur later compared to in X. laevis.

Chordin is required for the Spemann organizer transplantation phenomenon in Xenopus embryos

We analyzed the Chordin requirement in Xenopus development. Targeting of both chordin Xenopus laevis pseudoalleles with morpholino antisense oligomers (Chd-MO) markedly decreased Chordin production. Embryos developed with moderately reduced dorsoanterior structures and expanded ventroposterior tissues, phenocopying the zebrafish chordino mutant. A strong requirement for Chordin in dorsal development was revealed by experimental manipulations. First, dorsalization by lithium chloride treatment was completely blocked by Chd-MO. Second, Chd-MO inhibited elongation and muscle differentiation in Activin-treated animal caps. Third, Chd-MO completely blocked the induction of the central nervous system (CNS), somites, and notochord by organizer tissue transplanted to the ventral side of host embryos. Unexpectedly, transplantations into the dorsal side revealed a cell-autonomous requirement of Chordin for neural plate differentiation.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

Analysis of gene function in the zebrafish retina

Mutagenesis screens in zebrafish have uncovered several hundred mutant alleles affecting the development of the retina and established the zebrafish as one of the leading models of vertebrate eye development. In addition to forward genetic mutagenesis approaches, gene function in the zebrafish embryo is being studied using several reverse genetic techniques. Some of these rely on the overexpression of a gene product, others take advantage of antisense oligonucleotides to block function of selected loci. Here we describe these methods in the context of the developing eye.

beta-Catenin in early development of the lancelet embryo indicates specific determination of embryonic polarity

The lancelet (amphioxus) embryo develops from a miolecithal egg and starts gastrulation when it is approximately 400 cells in size, in a fashion similar to that of some non-chordate deuterostomes. Throughout this type of gastrulation, the embryo develops characteristics such as the notochord and hollow nerve cord that commonly appear in chordates. beta-Catenin is an important factor in initiating body patterning. The behavior and developmental pattern of this protein in early lancelet development was examined in this study. Cytoplasmic beta-catenin was localized to the animal pole after fertilization and then was incorporated asymmetrically into the blastomeres during the first cleavage. Asymmetric distribution was observed at least until the 32-cell stage. The first nuclear localization was at the 64-cell stage, and involved all of the cells. At the initial gastrula stage, however, concentrated beta-catenin was found on the dorsal side. LiCl treatment affected the asymmetric pattern of beta-catenin during the first cleavage. LiCl also changed distribution of nuclear beta-catenin at the initial gastrula stage: distribution extended to cells on the animal side. Apparently associated with this change, expression domains of goosecoid, lhx3 and otx also changed to a radially symmetric pattern centered at the animal pole. However, LiCl-treated embryos were able to establish embryonic polarity. The present study suggests that in the lancelet embryo, polarity determination is independent of dorsal morphogenesis.

Beta-catenin/Tcf-regulated transcription prior to the midblastula transition

Following fertilization, the zygotic genome in many organisms is quiescent until the midblastula transition (MBT), when large-scale transcription begins. In Xenopus embryos, for example, transcription is believed to be repressed until the twelfth cell division. Thus, although dorsal-ventral patterning begins during the first cell cycle, little attention has been given to transcriptional regulation in pre-MBT development. We present evidence that regulated transcription begins during early cleavage stages and that the beta-catenin-Tcf complex is required for the transcription of the Xenopus nodal genes Xnr5 and Xnr6 as early as the 256-cell stage. Moreover, inhibition of beta-catenin/Tcf function can block dorsal development, but only if the inhibition begins early and is maintained throughout pre-MBT stages. Dorsal development can be rescued in ventralized embryos if Tcf-dependent transcription is activated prior to MBT, but activation of Tcf after MBT cannot rescue ventralized embryos, suggesting that beta-catenin/Tcf-dependent transcription is required prior to MBT for dorsal-ventral patterning in Xenopus.

Lef-1 and Tcf-3 transcription factors mediate tissue-specific Wnt signaling during Xenopus development

Wnt signaling functions repeatedly during embryonic development to induce different but specific responses. What molecular mechanisms ensure that Wnt signaling triggers the correct tissue-specific response in different tissues? Early Xenopus development is an ideal model for addressing this fundamental question, since there is a dramatic change in the response to Wnt signaling at the onset of zygotic gene transcription: Wnt signaling components encoded by maternal mRNA establish the dorsal embryonic axis; zygotically expressed Xwnt-8 causes almost the opposite, by promoting ventral and lateral and restricting dorsal mesodermal development. Although Wnt signaling can function through different signal transduction cascades, the same beta-catenin-dependent, canonical Wnt signal transduction pathway mediates Wnt signaling at both stages of Xenopus development. Here we show that, while the function of the transcription factor XTcf-3 is required for early Wnt signaling to establish the dorsal embryonic axis, closely related XLef-1 is required for Wnt signaling to pattern the mesoderm after the onset of zygotic transcription. Our results show for the first time that different transcription factors of the Lef/Tcf family function in different tissues to bring about tissue-specific responses downstream of canonical Wnt signaling.

Evidence that Armadillo transduces wingless by mediating nuclear export or cytosolic activation of Pangolin

Secreted proteins of the Wnt family have profound organizing roles during animal development and are transduced via the activities of the Frizzled (Fz) class of transmembrane receptors and the TCF/LEF/Pangolin class of transcription factors. beta-catenins, including Drosophila Armadillo (Arm), link activation of Fz at the cell surface to transcriptional regulation by TCF in the nucleus. The consensus view is that Wnt signaling induces beta-catenin to enter the nucleus and combine with TCF to form a transcription factor complex in which TCF binds DNA and the C-terminal domain of beta-catenin activates transcription. Here, we present findings, which challenge this view and suggest instead that beta-catenin may transduce Wnt signals by exporting TCF from the nucleus or activating it in the cytoplasm.

A yeast model system for functional analysis of beta-catenin signaling

We have developed a novel Saccharomyces cerevisiae model system to dissect the molecular events of beta-catenin (beta-cat) signaling. Coexpression of mammalian beta-cat with TCF4 or LEF1 results in nuclear accumulation of these proteins and a functional complex that activates reporter gene transcription from constructs containing leukocyte enhancer factor (LEF)/T cell factor (TCF) response elements. Reporter transcription is constitutive, requires expression of both beta-cat and TCF4 or LEF1, and is not supported by mutated LEF/TCF binding elements or by TCF4 or LEF1 mutants. A cytoplasmic domain of E-cadherin or a functional fragment of adenomatous polyposis coli (APC) protein (APC-25) complexes with beta-cat, reduces beta-cat binding to TCF4, and leads to increased cytoplasmic localization of beta-cat and a reduction in reporter activation. Systematic mutation of putative nuclear export signal sequences in APC-25 decreases APC-25 binding to beta-cat and restores reporter gene transcription. Additional beta-cat signaling components, Axin and glycogen synthase kinase 3beta, form a multisubunit complex similar to that found in mammalian cells. Coexpression of the F-box protein beta-transducin repeat-containing protein reduces the stability of beta-cat and decreases reporter activation. Thus, we have reconstituted a functional beta-cat signal transduction pathway in yeast and show that beta-cat signaling can be regulated at multiple levels, including protein subcellular localization, protein complex formation, and protein stability.

The ins and outs of APC and beta-catenin nuclear transport

Adenomatous polyposis coli (APC) and beta-catenin, two key interacting proteins implicated in development and cancer, were recently found to traffic into and out of the nucleus in response to internal and external signals. The two proteins can enter and exit the nucleus independently, a discovery that has prompted debate about the previously proposed role of APC as a beta-catenin chaperone. Here, we review the regulation of APC and beta-catenin subcellular localization, in particular in cancer cells. We speculate that, in non-stimulated cells, APC actively exports beta-catenin from the nucleus to the cytoplasm where its levels are regulated by degradation; and, conversely, that, in cancer cells or those stimulated by Wnt signaling, beta-catenin degradation is inhibited and the accruing protein is capable of moving between the nucleus and cytoplasm independently of APC. Models that link APC and beta-catenin transport to function are discussed.

The competence of marginal zone cells to become Spemann's organizer is controlled by Xcad2

The organizer in vertebrate embryos is responsible for the formation of the primary body axis. In amphibian embryos, the organizer forms in the dorsal marginal zone (prospective dorsal mesoderm) at a location determined by the point of sperm entry. Using inducible versions of axis-inducing proteins, it has been shown that, irrespective of the mode of secondary axis induction, organizer formation in the ventral marginal zone is temporally restricted from the midblastula transition to the onset of gastrulation. Here, we show that the competence of marginal zone cells to respond to organizer-inducing signals is under temporal control, one of the regulators being the homeobox transcription factor Xcad2. Overexpression of Xcad2 restricts the temporal competence for axis induction, whereas partial loss of function expands this competence, supporting our suggestion. We propose that Xcad2 competes with putative axis-inducing signals within the marginal zone to prevent expression of organizer-specific genes. Elimination of endogenous Xcad2 allows for the activation of organizer genes beyond the normal competence window during early/mid-gastrulation. We conclude that Xcad2, through its early expression in the ventrolateral marginal zone, terminates the competence of this embryonic region to respond to organizer-inducing signals by preventing the activation of organizer-specific genes.

Beta-catenin-dependent Wnt signaling in apical ectodermal ridge induction and FGF8 expression in normal and limbless mutant chick limbs

The fibroblast growth factor (FGF) and beta-catenin-dependent Wnt signaling pathways are key regulators of vertebrate limb development. FGF10 induces expression of Wnt3a, which regulates the formation and FGF8 expression of the apical ectodermal ridge (AER). In amelic limbless limbs, an AER fails to form and FGF8 is not expressed, despite expression of FGF10. It has been found that Wnt3a is initially expressed in limbless ectoderm, although subsequently is drastically reduced. In addition, changes in the expression pattern or level of several Frizzled receptors, Axin, Lef1/Tcf1 and beta-catenin have been found in limbless limbs. Notably, while normal wing buds respond to LiCl-stimulated activation of beta-catenin-dependent signaling by forming ectopic, FGF8-expressing AER, LiCl was unable to induce an AER in limbless wing buds. The results of this study suggest that the limbless gene is required for beta-catenin-dependent Wnt signaling in limb ectoderm leading to FGF8 expression and AER formation.

Patterning the marginal zone of early ascidian embryos: localized maternal mRNA and inductive interactions

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho-1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32-cell stage. FGF-Ras-MAPK signaling is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates.

Effects of heterodimerization and proteolytic processing on Derrière and Nodal activity: implications for mesoderm induction in Xenopus

Derrière is a recently discovered member of the TGFβ superfamily that can induce mesoderm in explant assays and is expressed at the right time and location to mediate mesoderm induction in response to VegT during Xenopus embryogenesis. We show that the ability of Derrière to induce dorsal or ventral mesoderm depends strictly on the location of expression and that a dominant-negative Derrière cleavage mutant completely blocks all mesoderm formation when ectopically expressed. This differs from the activity of similar Xnr2 cleavage mutant constructs, which are secreted and retain signaling activity. Additional analysis of mesoderm induction by Derrière and members of the Nodal family indicates that these molecules are involved in a mutual positive-feedback loop and antagonism of either one of the signals can reduce the other. Interaction between Derrière and members of the Nodal family is also shown to occur through the formation of heterodimeric ligands. Using an oocyte expression system we show direct interaction between the mature Derrière ligand and members of both the Nodal and BMP families. Taken together, these findings indicate that Derrière and Nodal proteins probably work cooperatively to induce mesoderm throughout the marginal zone during early Xenopus development.

Non-canonical Wnt signaling in Xenopus: regulation of axis formation and gastrulation

Wnt proteins form a family of secreted glycoproteins that are involved in different developmental processes such as differentiation, proliferation, cell migration and cell polarity. To exert its function, Wnt proteins activate different intracellular signaling cascades. Whereas the canonical, Wnt/beta-catenin pathway is well characterized, less is known about the function of non-canonical Wnt pathways in vertebrates. I here summarize recent findings implicating important roles for Wnt/Ca(2+) and Wnt/JNK signaling during different aspects of early Xenopus laevis development, namely axis formation and gastrulation movements.

Symplekin, a constitutive protein of karyo- and cytoplasmic particles involved in mRNA biogenesis in Xenopus laevis oocytes

Symplekin is a dual location protein that has been localized to the cytoplasmic plaques of tight junctions but also occurs in the form of interchromatin particles in the karyoplasm. Here we report the identification of two novel and major symplekin-containing protein complexes in both the karyo- and the cytoplasm of Xenopus laevis oocytes. Buffer-extractable fractions from the karyoplasm of stage IV-VI oocytes contain an 11S particle, prepared by immunoselection and sucrose gradient centrifugation, in which symplekin is associated with the subunits of the cleavage and polyadenylation specificity factor (CPSF). Moreover, in immunofluorescence microscopy nuclear symplekin colocalizes with protein CPSF-100 in the "Cajal bodies." However, symplekin is also found in cytoplasmic extracts of enucleated oocytes and egg extracts, where it occurs in 11S as well as in ca. 65S particles, again in association with CPSF-100. This suggests that, in X. laevis oocytes, symplekin is possibly involved in both processes, 3'-end processing of pre-mRNA in the nucleus and regulated polyadenylation in the cytoplasm. We discuss the possible occurrence of similar symplekin-containing particles involved in mRNA metabolism in the nucleus and cytoplasm of other kinds of cells, also in comparison with the nuclear forms of other dual location proteins in nuclei and cell junctions.

Functional diversity of Xenopus lymphoid enhancer factor/T-cell factor transcription factors relies on combinations of activating and repressing elements

Lymphoid enhancer factor/T-cell factor (LEF/TCF) high mobility group box transcription factors are the nuclear transducers of the Wnt/beta-catenin signaling cascade. In Xenopus, three members of the LEF/TCF family, XLEF-1, XTCF-3, and XTCF-4, with distinct but partially overlapping expression patterns have been identified. The individual Xenopus LEF/TCF family members differ extremely in their properties of target gene regulation. We observed that in contrast to LEF-1, neither XTCF-3 nor XTCF-4 can induce secondary axis formation upon ventral overexpression in Xenopus embryos. To identify functional motifs within the LEF/TCF transcription factors responsible for target gene activation or repression, we created various mutants and a set of XLEF-1/XTCF-3 chimeras. In overexpression studies, we asked whether these constructs can mimic an activated Wnt/beta-catenin pathway and lead to the formation of a secondary body axis. In addition, we examined their capacity to rescue a loss-of-function phenotype given by dominant negative LEF-1 expression. We further analyzed their ability to directly activate target genes in reporter gene assays using the LEF/TCF target promoters, siamois and fibronectin. We found that a region homologous to exon IVa of human TCF-1 is an activating element. This is flanked by two small repressing motifs, LVPQ and SXXSS. Our findings implicate that the motifs identified here play an essential role in determining cell type-specific activity of LEF/TCF transcription factors.

Heads or tails? Amphioxus and the evolution of anterior-posterior patterning in deuterostomes

In Xenopus, the canonical Wnt-signaling pathway acting through beta-catenin functions both in establishing the dorso-ventral axis and in patterning the anterior-posterior axis. This pathway also acts in patterning the animal-vegetal axis in sea urchins. However, because sea urchin development is typically indirect, and adult sea urchins have pentamerous symmetry and lack a longitudinal nerve cord, it has not been clear how the roles of the canonical Wnt-signaling pathway in axial patterning in sea urchins and vertebrates are evolutionarily related. The developmental expression patterns of Notch, brachyury, caudal, and eight Wnt genes have now been determined for the invertebrate chordate Amphioxus, which, like sea urchins, has an early embryo that gastrulates by invagination, but like vertebrates, has a later embryo with a dorsal hollow nerve cord that elongates posteriorly from a tail bud. Comparisons of Amphioxus with other deuterostomes suggest that patterning of the ancestral deuterostome embryo along its anterior-posterior axis during the late blastula and subsequent stages involved a posterior signaling center including Wnts, Notch, and transcription factors such as brachyury and caudal. In tunicate embryos, in which cell numbers are reduced and cell fates largely determined during cleavage stages, only vestiges of this signaling center are still apparent; these include localization of Wnt-5 mRNA to the posterior cytoplasm shortly after fertilization and localization of beta-catenin to vegetal nuclei during cleavage stages. Neither in tunicates nor in Amphioxus is there any evidence that the canonical Wnt-signaling pathway functions in establishment of the dorso-ventral axis. Thus, roles for Wnt-signaling in dorso-ventral patterning of embryos may be a vertebrate innovation that arose in connection with the evolution of yolky eggs and gastrulation by extensive involution.

A transgenic Lef1/beta-catenin-dependent reporter is expressed in spatially restricted domains throughout zebrafish development

The Wnt/beta-catenin signaling pathway plays multiple roles during embryonic development, only a few of which have been extensively characterized. Although domains of Wnt expression have been identified throughout embryogenesis, anatomical and molecular characterization of responding cells has been mostly unexplored. We have generated a transgenic zebrafish line that expresses a destabilized green fluorescent protein (GFP) variant under the control of a beta-catenin responsive promoter. Early zygotic expression of this transgene (TOPdGFP) mirrors known domains of Wnt signaling in the embryo. Loss of Lef1 activity results in decreased reporter expression and posterior defects, while loss of Tcf3 (Headless, Hdl) activity does not alter reporter expression, even though it results in loss of forebrain structures. In addition, ectopic Wnt1 expression can activate the reporter. In older embryos, we identify a number of transgene-expressing cell populations as novel sites of beta-catenin signaling. We conclude that our TOP-dGFP reporter line faithfully illustrates domains of beta-catenin activity and enables the identification of responsive cell populations.

β-catenin, MAPK and Smad signaling during earlyXenopusdevelopment

Knowledge of when and where signaling pathways are activated is crucial for understanding embryonic development. In this study, we have systematically analyzed and compared the signaling pattern of four major pathways by localization of the activated key components β-catenin (Wnt proteins), MAPK (tyrosine kinase receptors/FGF), Smad1 (BMP proteins) and Smad2 (Nodal/activin/Vg1). We have determined semi-quantitatively the distribution of these components at 18 consecutive stages in Xenopus development, from early blastula to tailbud stages, by immunofluorescence on serial cryosections. The image obtained is that of very dynamic and widespread activities, with very few inactive regions. Signaling fields can vary from large gradients to restricted areas with sharp borders. They do not respect tissue boundaries. This direct visualization of active signaling verifies several predictions inferred from previous functional data. It also reveals unexpected signal patterns, pointing to some poorly understood aspects of early development. In several instances, the patterns strikingly overlap, suggesting extensive interplay between the various pathways. To test this possibility, we have manipulated maternal β-catenin signaling and determined the effect on the other pathways in the blastula embryo. We found that the patterns of P-MAPK, P-Smad1 and P-Smad2 are indeed strongly dependent on β-catenin at this stage. supplementary material: Supplementary Information

Development in frogs with large eggs and the origin of amniotes

The origin of the amniote egg is one of the most significant events in the evolution of terrestrial vertebrates. This innovation was probably driven by increased egg size, and to find potential parallels, we can examine the derived development of extant amphibians with large eggs. The embryo of the Puerto Rican tree frog, Eleutherodactylus coqui, exhibits an alteration of its fate map and a secondary coverage of its yolky cells, reflecting the large 3.5 mm egg. Comparable changes may have occurred with the derivation of an amniote pattern of development. Future investigations should focus on the molecular organization of the egg. In the model amphibian for development, Xenopus laevis, information for embryonic germ layers, the dorsal axis, and germ cells is stored mainly as localized RNAs at the vegetal pole of the egg. These localizations would likely be changed with increased egg size. A review of the orthologues of the key X. laevis genes raises the possibility that their activities are not conserved in other vertebrates.

Cooperative action of ADMP- and BMP-mediated pathways in regulating cell fates in the zebrafish gastrula

It was shown in Xenopus and chick that Spemann's organizer activity is regulated through the negative action of Anti-Dorsalizing Morphogenetic Protein (ADMP). We report the characterization and functional properties of admp in zebrafish. admp expression profile is consistent with a role in the organizer, including the tail organizer. We studied admp function through overexpression experiments, with the use of a dominant-negative form (TR-ADMP) and of an antisense morpholino-modified oligonucleotide. Our results indicate that the ADMP pathway causes the restriction of anterior and axial fates and that ADMP, BMP2b, and BMP7 pathways have distinct actions but cooperate in establishing proper dorso-ventral regionalization. This is shown by partial rescue of the dorsalized mutant snailhouse and of the ventralized mutant chordino, upon admp and tr-admp RNA injection, respectively. Moreover, ADMP and BMP7 probably form heterodimers as shown by the ability of TR-ADMP and BMP7 to antagonize each other. We observed that a MYC-tagged ADMP was secreted and detected in the extracellular space, suggesting that admp could act at a distance. Simultaneous local inhibition of bmp function at the blastoderm margin and impairment of ADMP secretion led to the induction of secondary head structures, confirming that the two pathways cooperatively regulate organizer formation and activity.

Morphogenetic domains in the yolk syncytial layer of axiating zebrafish embryos

The yolk syncytial layer (YSL) of the teleostean yolk cell is known to play important roles in the induction of cellular mesendoderm, as well as the patterning of dorsal tissues. To determine how this extraembryonic endodermal compartment is subdivided and morphologically transformed during early development, we have examined collective movements of vitally stained YSL nuclei in axiating zebrafish embryos by using four-dimensional confocal microscopy. During blastulation, gastrulation, and early segmentation, zebrafish YSL nuclei display several highly patterned movements, which are organized into spatially distinct morphogenetic domains along the anterior-posterior and dorsal-ventral axes. During the late blastula period, with the onset of epiboly, nuclei throughout the YSL initiate longitudinal movements that are directed along the animal-vegetal axis. As epiboly progresses, nuclei progressively recede from the advancing margin of the epibolic YSL. However, a small group of nuclei is retained at the YSL margin to form a constricting blastoporal ring. During mid-gastrulation, YSL nuclei undergo convergent-extension behavior toward the dorsal midline, with a subset of nuclei forming an axial domain that underlies the notochord. These highly patterned movements of YSL nuclei share remarkable similarities to the morphogenetic movements of deep cells in the overlying zebrafish blastoderm. The macroscopic shape changes of the zebrafish yolk cell, as well as the morphogenetic movements of its YSL nuclei, are homologous to several morphogenetic behaviors that are regionally expressed within the vegetal endodermal cell mass of gastrulating Xenopus embryos. In contrast to the cellular endoderm of Xenopus, the dynamics of zebrafish YSL show that a syncytial endodermal germ layer can express a temporal sequence of morphogenetic domains without undergoing progressive steps of cell fate restriction.

Nodal signaling in early vertebrate embryos: themes and variations

The nodal family of TGFbeta-related ligands have emerged as critical regulators of early vertebrate embryogenesis. Recent studies in mice, fish, and frogs of nodals and their intracellular transducers allow a comparison of how this signaling pathway is used in the patterning of early embryos of these different vertebrates.

A morphogen gradient of Wnt/β-catenin signalling regulates anteroposterior neural patterning in Xenopus

Anteroposterior (AP) patterning of the vertebrate neural plate is initiated during gastrulation and is regulated by Spemann’s organizer and its derivatives. The prevailing model for AP patterning predicts a caudally increasing gradient of a ‘transformer’ which posteriorizes anteriorly specified neural cells. However, the molecular identity of the transforming gradient has remained elusive. We show that in Xenopus embryos (1) dose-dependent Wnt signalling is both necessary and sufficient for AP patterning of the neuraxis, (2) Wnt/β-catenin signalling occurs in a direct and long-range fashion within the ectoderm, and (3) that there is an endogenous AP gradient of Wnt/β-catenin signalling in the presumptive neural plate of the Xenopus gastrula. Our results indicate that an activity gradient of Wnt/β-catenin signalling acts as transforming morphogen to pattern the Xenopus central nervous system.

Positional cloning of heart and soul reveals multiple roles for PKC lambda in zebrafish organogenesis

BACKGROUND

The Par-3/Par-6/aPKC complex is a key regulator of cell polarity in a number of systems. In Drosophila, this complex acts at the zonula adherens (adherens junctions) to establish epithelial polarity and helps to orient the mitotic spindle during asymmetric neuroblast divisions. In MDCKII cells, this complex localizes to the zonula occludens (tight junctions) and appears to regulate epithelial polarity. However, the in vivo role of this complex during vertebrate embryogenesis is not known, due to the lack of relevant mutations.

RESULTS

We have positionally cloned the zebrafish heart and soul (has) mutation, which affects the morphogenesis of several embryonic tissues, and show that it encodes atypical protein kinase C lambda (aPKC lambda). We find that loss of aPKC lambda affects the formation and maintenance of the zonula adherens in the polarized epithelia of the retina, neural tube, and digestive tract, leading to novel phenotypes, such as the formation of multiple lumens in the developing intestine. In addition, has mutants display defects in gut looping and endodermal organ morphogenesis that appear to be independent of the defects in epithelial polarity. Finally, we show that loss of aPKC lambda leads to defects in spindle orientation during progenitor cell divisions in the neural retina.

CONCLUSIONS

Our results show that aPKC lambda is required for the formation and maintenance of the zonula adherens during early epithelial development in vertebrates and demonstrate a previously undescribed yet critical role for this protein in organ morphogenesis. Furthermore, our studies identify the first genetic locus regulating the orientation of cell division in vertebrates.

New aspects of Wnt signaling pathways in higher vertebrates

The development of tissues and organs in embryos is controlled by an interplay of several signaling pathways that cross-talk to provide positional information and induce cell fate specification. One of the major signaling systems is the Wnt pathway which was recently shown to split into several intracellular branches which regulate multiple cellular functions. In the present review, we discuss novel members and their role in the diversification of the Wnt pathway. Many of these components were studied in model organisms such as C.elegans, Drosophila and Xenopus. Here we focus on recent studies of mutant phenotypes in Mouse and Zebrafish which implicate members of the Wnt pathway in processes such as axis and mesoderm formation, initiation of organ development and stem cell differentiation.

Wnt8 is required in lateral mesendodermal precursors for neural posteriorization in vivo

The dorsal ectoderm of the vertebrate gastrula was proposed by Nieuwkoop to be specified towards an anterior neural fate by an activation signal, with its subsequent regionalization along the anteroposterior (AP) axis regulated by a graded transforming activity, leading to a properly patterned forebrain, midbrain, hindbrain and spinal cord. The activation phase involves inhibition of BMP signals by dorsal antagonists, but the later caudalization process is much more poorly characterized. Explant and overexpression studies in chick, Xenopus, mouse and zebrafish implicate lateral/paraxial mesoderm in supplying the transforming influence, which is largely speculated to be a Wnt family member.

We have analyzed the requirement for the specific ventrolaterally expressed Wnt8 ligand in the posteriorization of neural tissue in zebrafish wild-type and Nodal-deficient embryos (Antivin overexpressing or cyclops;squint double mutants), which show extensive AP brain patterning in the absence of dorsal mesoderm. In different genetic situations that vary the extent of mesodermal precursor formation, the presence of lateral wnt8-expressing cells correlates with the establishment of AP brain pattern. Cell tracing experiments show that the neuroectoderm of Nodal-deficient embryos undergoes a rapid anterior-to-posterior transformation in vivo during a short period at the end of the gastrula stage. Moreover, in both wild-type and Nodal-deficient embryos, inactivation of Wnt8 function by morpholino (MOwnt8) translational interference dose-dependently abrogates formation of spinal cord and posterior brain fates, without blocking ventrolateral mesoderm formation. MOwnt8 also suppresses the forebrain deficiency in bozozok mutants, in which inactivation of a homeobox gene causes ectopic wnt8 expression. In addition, the bozozok forebrain reduction is suppressed in bozozok;squint;cyclops triple mutants, and is associated with reduced wnt8 expression, as seen in cyclops;squint mutants. Hence, whereas boz and Nodal signaling largely cooperate in gastrula organizer formation, they have opposing roles in regulating wnt8 expression and forebrain specification. Our findings provide strong support for a model of neural transformation in which a planar gastrula-stage Wnt8 signal, promoted by Nodal signaling and dorsally limited by Bozozok, acts on anterior neuroectoderm from the lateral mesoderm to produce the AP regional patterning of the CNS.

Early embryonic expression of a LIM-homeobox geneCs-lhx3is downstream of β-catenin and responsible for the endoderm differentiation inCiona savignyiembryos

In early Ciona embryos, nuclear accumulation of β-catenin is most probably the first step of endodermal cell specification. If β-catenin is mis- and/or overexpressed, presumptive notochord cells and epidermal cells change their fates into endodermal cells, whereas if β-catenin nuclear localization is downregulated by the overexpression of cadherin, the endoderm differentiation is suppressed, accompanied with the differentiation of extra epidermal cells (Imai, K., Takada, N., Satoh, N. and Satou, Y. (2000) Development127, 3009-3020). Subtractive hybridization screens of mRNAs between β-catenin overexpressed embryos and cadherin overexpressed embryos were conducted to identify potential β-catenin target genes that are responsible for endoderm differentiation in Ciona savignyi embryos. We found that a LIM-homeobox gene (Cs-lhx3), an otx homolog (Cs-otx) and an NK-2 class gene (Cs-ttf1) were among β-catenin downstream genes. In situ hybridization signals for early zygotic expression of Cs-lhx3 were evident only in the presumptive endodermal cells as early as the 32-cell stage, those of Cs-otx in the mesoendodermal cells at the 32-cell stage and those of Cs-ttf1 in the endodermal cells at the 64-cell stage. Later, Cs-lhx3 was expressed again in a set of neuronal cells in the tailbud embryo, while Cs-otx was expressed in the anterior nervous system of the embryo. Expression of all three genes was upregulated in β-catenin overexpressed embryos and downregulated in cadherin overexpressed embryos. Injection of morpholino oligonucleotides against Cs-otx did not affect the embryonic endoderm differentiation, although the formation of the central nervous system was suppressed. Injection of Cs-ttf1 morpholino oligonucleotides also failed to suppress the endoderm differentiation, although injection of its synthetic mRNAs resulted in ectopic development of endoderm differentiation marker alkaline phosphatase. By contrast, injection of Cs-lhx3 morpholino oligo suppressed the endodermal cell differentiation and this suppression was rescued by injection of Cs-lhx3 mRNA into eggs. In addition, although injection of delE-Ci-cadherin mRNA into eggs resulted in the suppression of alkaline phosphatase development, injection of delE-Ci-cadherin mRNA with Cs-lhx3 mRNA rescued the alkaline phosphatase development. These results strongly suggest that a LIM-homeobox gene Cs-lhx3 is one of the β-catenin downstream genes and that its early expression in embryonic endodermal cells is responsible for their differentiation.

A possible role for the WNT-1 pathway in oral carcinogenesis

Reductions in cell-cell adhesion and stromal and vascular invasion are essential steps in the progression from localized malignancy to metastatic disease for all cancers. Proteins involved in intercellular adhesion, such as E-cadherin and catenin, probably play an important role in metastatic processes and cellular differentiation. While E-cadherin and beta-catenin expression has been extensively studied in many forms of human cancers, less is known about the role of the Wingless-Type-1 (WNT-1) pathway in human tumors. A large body of genetic and biochemical evidence has identified beta-catenin as a key downstream component of the WNT signaling pathway, and recent studies of colorectal tumors have shown a functional link among beta-catenin, adenomatous polyposis coli gene product (APC), and other components of the WNT-1 pathway. WNT-1 pathway signaling is thought to be mediated via interactions between beta-catenin and members of the LEF-1/TCF family of transcription factors. The WNT signal stabilizes beta-catenin protein and promotes its accumulation in the cytoplasm and nucleus. In the nucleus, beta-catenin associates with TCF to form a functional transcription factor which mediates the transactivation of target genes involved in the promotion of tumor progression, invasion, and metastasis, such as C-Myc, cyclin D1, c-jun, fra-1, and u-PAR. There is a strong correlation between the ability of the WNT-1 gene to induce beta-catenin accumulation and its transforming potential in vivo, suggesting that the WNT-1 gene activates an intracellular signaling pathway that can induce the morphological transformation of cells. For these reasons, data obtained from the study of the WNT-1 pathway could be important in our understanding of the mechanisms of epithelial tumors, in general, and probably also of oral squamous cell carcinoma, in particular.

The role of maternal axin in patterning the Xenopus embryo

Regulation of the stability of beta catenin protein is a critical role of Wnt signaling cascades. In early Xenopus development, dorsal axis specification depends on regulation of beta catenin by both cytoplasmic and nuclear mechanisms. While the cytoplasmic protein axin is known as a key component of the cytoplasmic beta catenin degradation complex, loss-of-function studies are needed to establish whether it is required for dorso-ventral patterning in the embryo, and to test where in the embryo it carries out its function. Here, we show that embryos lacking maternal axin protein have increased levels of soluble beta catenin protein and increased nuclear localization of beta catenin in ventral nuclei at the blastula stage. These embryos gastrulate abnormally and develop with excessive notochord and head structures, and reduced tail and ventral components. They show increased expression of dorsal markers, including siamois, Xnr3, chordin, gsc, Xhex, and Otx2, decreased expression of Xwnt 8 and Xbra, and little alteration of BMP4 and Xvent1 and -2 mRNA levels. The ventral halves of axin-depleted embryos at the gastrula stage have dramatically increased levels of chordin expression, and severely decreased levels of Xwnt 8 mRNA expression, while BMP4 transcript levels are only slightly reduced. This dorso-anterior phenotype is rescued by axin mRNA injected into the vegetal pole of axin-depleted oocytes before fertilization. Interestingly, the phenotype was rescued by ventral but not dorsal injection of axin mRNA, at the 4-cell stage, although dorsal injection into wild-type embryos does cause ventralization. These results show directly that the localized ventral activity of maternal axin is critical for the correct patterning of the early Xenopus embryo.

Timing of endogenous activin-like signals and regional specification of theXenopusembryo

Signaling by activin-like ligands is important for induction and patterning of mesoderm and endoderm. We have used an antibody that specifically recognizes the phosphorylated and activated form of Smad2, an intracellular transducer of activin-like ligands, to examine how this signaling pathway patterns the early mesendoderm. In contrast to the simple expectation that activin-like signaling should be highest on the dorsal side of the gastrula stage embryo, we have found that while Smad2 phosphorylation is highest dorsally before gastrulation, signaling is attenuated dorsally and is highest on the ventral side by mid-gastrulation. Early dorsal initiation of Smad2 phosphorylation results from cooperation between the vegetally localized maternal transcription factor VegT and dorsally localized β-catenin. The subsequent ventral appearance of Smad2 phosphorylation is dependent on VegT, but not on signaling from the dorsal side. Dorsal attenuation of Smad2 phosphorylation during gastrulation is mediated by early dorsal expression of feedback inhibitors of activin-like signals.

In addition to regulation of Smad2 phosphorylation by the expression of activin-like ligands and their antagonists, the responsiveness of embryonic cells to activin-like ligands is also temporally regulated. Ectopic Vg1, Xnr1 and derrière all fail to activate Smad2 phosphorylation until after the midblastula transition, and the onset of responsiveness to these ligands is independent of transcription. Furthermore, the timing of cellular responsiveness differs for Xnr1 and derrière, and these distinct temporal patterns of responsiveness can be correlated with their distinctive phenotypic effects. These observations suggest that the timing of endogenous activin-like signaling is a determinant of patterning in the early Xenopus embryo.

Axis formation and patterning in zebrafish

A large collection of mutations affecting zebrafish embryogenesis was described in 1996. The cloning of the affected genes has now provided novel insights into the role and regulation of signaling by BMP, Nodal, Wnt, FGF, Hedgehog, Delta, Slit, retinoic acid and lipids. Detailed analyses have revealed a complex genetic network that patterns the early embryo.

Possible roles of beta-catenin in evagination of the optic primordium in rat embryos

The roles of beta-catenin in evagination of the optic primordium in rat embryos were studied using immunostaining. High levels of beta-catenin appeared transiently in the evaginating optic primordium. Evagination of the optic primordium was suppressed in embryos treated with LiCl. In deficient optic vesicles of these embryos, accumulation of beta-catenin was decreased. Deficient optic vesicles also showed suppression of cyclin D1 accumulation and bromodeoxyuridine incorporation, no break in the deposition of laminin and type IV collagen at the basement membrane (BM) and prevention of the change in distribution of microtubules and microfilaments. These results suggest that beta-catenin regulates cell proliferation, breakdown of BM and changes in cell shape in the evaginating optic primordium to cause optic vesicle formation.

Interactions between Wnt and Vg1 signalling pathways initiate primitive streak formation in the chick embryo

The posterior marginal zone (PMZ) of the chick embryo has Nieuwkoop centre-like properties: when transplanted to another part of the marginal zone, it induces a complete embryonic axis, without making a cellular contribution to the induced structures. However, when the PMZ is removed, the embryo can initiate axis formation from another part of the remaining marginal zone. Chick Vg1 can mimic the axis-inducing ability of the PMZ, but only when misexpressed somewhere within the marginal zone. We have investigated the properties that define the marginal zone as a distinct region. We show that the competence of the marginal zone to initiate ectopic primitive streak formation in response to cVg1 is dependent on Wnt activity. First, within the Wnt family, only Wnt8C is expressed in the marginal zone, in a gradient decreasing from posterior to anterior. Second, misexpression of Wnt1 in the area pellucida enables this region to form a primitive streak in response to cVg1. Third, the Wnt antagonists Crescent and Dkk-1 block the primitive streak-inducing ability of cVg1 in the marginal zone. These findings suggest that Wnt activity defines the marginal zone and allows cVg1 to induce an axis. We also present data suggesting some additional complexity: first, the Vg1 and Wnt pathways appear to regulate the expression of downstream components of each other’s pathway; and second, misexpression of different Wnt antagonists suggests that different classes of Wnts may cooperate with each other to regulate axis formation in the normal embryo.

Nuclear particles containing RNA polymerase III complexes associated with the junctional plaque protein plakophilin 2

Plakophilin 2, a member of the arm-repeat protein family, is a dual location protein that occurs both in the cytoplasmic plaques of desmosomes as an architectural component and in an extractable form in the nucleoplasm. Here we report the existence of two nuclear particles containing plakophilin 2 and the largest subunit of RNA polymerase (pol) III (RPC155), both of which colocalize and are coimmunoselected with other pol III subunits and with the transcription factor TFIIIB. We also show that plakophilin 2 is present in the pol III holoenzyme, but not the core complex, and that it binds specifically to RPC155 in vitro. We propose the existence of diverse nuclear particles in which proteins known as plaque proteins of intercellular junctions are complexed with specific nuclear proteins.

Role of the iroquois3 homeobox gene in organizer formation

In zebrafish, the organizer is thought to consist of two regions, the yolk syncytial layer (YSL) and the shield. The dorsal YSL appears to send signals that affect formation of the shield in the overlying mesendoderm. We show here that a domain of dorsal deep cells located between the YSL and the shield is marked by expression of the iro3 gene. As gastrulation proceeds, the iro3 positive domain involutes and migrates to the animal pole. Iro3 expression is regulated by Nodal and bone morphogenic protein antagonists. Overexpression of iro3 induced ectopic expression of shield-specific genes. This effect was mimicked by an Iro3-Engrailed transcriptional repressor domain fusion, whereas an Iro3-VP16 activator domain fusion behaved as a dominant negative or antimorphic form. These results suggest that Iro3 acts as a transcriptional repressor and further implicate the iro3 gene in regulating organizer formation. We propose that the iro3-expressing dorsal deep cells represent a distinct organizer domain that receives signals from the YSL and in turn sends signals to the forming shield, thereby influencing its expansion and differentiation.

Vertebrate proteins related to Drosophila Naked Cuticle bind Dishevelled and antagonize Wnt signaling

Wnt signals control cell fate decisions and orchestrate cell behavior in metazoan animals. In the fruit fly Drosophila, embryos defective in signaling mediated by the Wnt protein Wingless (Wg) exhibit severe segmentation defects. The Drosophila segment polarity gene naked cuticle (nkd) encodes an EF hand protein that regulates early Wg activity by acting as an inducible antagonist. Nkd antagonizes Wg via a direct interaction with the Wnt signaling component Dishevelled (Dsh). Here we describe two mouse and human proteins, Nkd1 and Nkd2, related to fly Nkd. The most conserved region among the fly and vertebrate proteins, the EFX domain, includes the putative EF hand and flanking sequences. EFX corresponds to a minimal domain required for fly or vertebrate Nkd to interact with the basic/PDZ domains of fly Dsh or vertebrate Dvl proteins in the yeast two-hybrid assay. During mouse development, nkd1 and nkd2 are expressed in multiple tissues in partially overlapping, gradient-like patterns, some of which correlate with known patterns of Wnt activity. Mouse Nkd1 can block Wnt1-mediated, but not beta-catenin-mediated, activation of a Wnt-dependent reporter construct in mammalian cell culture. Misexpression of mouse nkd1 in Drosophila antagonizes Wg function. The data suggest that the vertebrate Nkd-related proteins, similar to their fly counterpart, may act as inducible antagonists of Wnt signals.

Neural induction in the absence of mesoderm: beta-catenin-dependent expression of secreted BMP antagonists at the blastula stage in Xenopus

A growing body of work indicates that neural induction may be initiated prior to the establishment of the gastrula mesodermal organizer. Here, we examine neural induction in Xenopus embryos in which mesoderm induction has been blocked by Cerberus-short, a reagent that specifically inhibits Nodal-related (Xnr) signals. We find that extensive neural structures with cyclopic eyes and brain tissue are formed despite the absence of mesoderm. This neural induction correlates with the expression of chordin and other BMP inhibitors-such as noggin, follistatin, and Xnr3-at the blastula stage, and requires beta-Catenin signaling. Activation of the beta-Catenin pathway by mRNA microinjections or by treatment with LiCl leads to differentiation of neurons, as well as neural crest, in ectodermal explants. Xnr signals are required for the maintenance, but not for the initiation, of BMP antagonist expression. Recent work has demonstrated a role for beta-Catenin signaling in neural induction mediated by the transcriptional down-regulation of BMP-4 expression. The present results suggest an additional function for beta-Catenin, the early activation of expression of secreted BMP antagonists, such as Chordin, in a preorganizer region in the dorsal side of the Xenopus blastula.

Molecular targets of lithium action

Lithium is highly effective in the treatment of bipolar disorder and also has multiple effects on embryonic development, glycogen synthesis, hematopoiesis, and other processes. However, the mechanism of lithium action is still unclear. A number of enzymes have been proposed as potential targets of lithium action, including inositol monophosphatase, a family of structurally related phosphomonoesterases, and the protein kinase glycogen synthase kinase-3. These potential targets are widely expressed, require metal ions for catalysis, and are generally inhibited by lithium in an uncompetitive manner, most likely by displacing a divalent cation. Thus, the challenge is to determine which target, if any, is responsible for a given response to lithium in cells. Comparison of lithium effects with genetic disruption of putative target molecules has helped to validate these targets, and the use of alternative inhibitors of a given target can also lend strong support for or against a proposed mechanism of lithium action. In this review, lithium sensitive enzymes are discussed, and a number of criteria are proposed to evaluate which of these enzymes are involved in the response to lithium in a given setting.

Armadillo nuclear import is regulated by cytoplasmic anchor Axin and nuclear anchor dTCF/Pan

Drosophila melanogaster Armadillo plays two distinct roles during development. It is a component of adherens junctions, and functions as a transcriptional activator in response to Wingless signaling. In the current model, Wingless signal causes stabilization of cytoplasmic Armadillo allowing it to enter the nucleus where it can activate transcription. However, the mechanism of nuclear import and export remains to be elucidated. In this study, we show that two gain-of-function alleles of Armadillo activate Wingless signaling by different mechanisms. The S10 allele was previously found to localize to the nucleus, where it activates transcription. In contrast, the ΔArm allele localizes to the plasma membrane, and forces endogenous Arm into the nucleus. Therefore, ΔArm is dependent on the presence of a functional endogenous allele of arm to activate transcription. We show that ΔArm may function by titrating Axin protein to the membrane, suggesting that it acts as a cytoplasmic anchor keeping Arm out of the nucleus. In axin mutants, Arm is localized to the nuclei. We find that nuclear retention is dependent on dTCF/Pangolin. This suggests that cellular distribution of Arm is controlled by an anchoring system, where various nuclear and cytoplasmic binding partners determine its localization.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

Characterization and developmental expression of the amphioxus homolog of Notch (AmphiNotch): evolutionary conservation of multiple expression domains in amphioxus and vertebrates

Notch encodes a transmembrane protein that functions in intercellular signaling. Although there is one Notch gene in Drosophila, vertebrates have three or more with overlapping patterns of embryonic expression. We cloned the entire 7575-bp coding region of an amphioxus Notch gene (AmphiNotch), encoding 2524 amino acids, and obtained the exon/intron organization from a genomic cosmid clone. Southern blot and PCR data indicate that AmphiNotch is the only Notch gene in amphioxus. AmphiNotch, like Drosophila Notch and vertebrate Notch1 and Notch2, has 36 EGF repeats, 3 Notch/lin-12 repeats, a transmembrane region, and 6 ankyrin repeats. Phylogenetic analysis places it at the base of all the vertebrate genes, suggesting it is similar to the ancestral gene from which the vertebrate Notch family genes evolved. AmphiNotch is expressed in all three embryonic germ layers in spatiotemporal patterns strikingly similar to those of all the vertebrate homologs combined. In the developing nerve cord, AmphiNotch is first expressed in the posteriormost part of the neural plate, then it becomes more broadly expressed and later is localized dorsally in the anteriormost part of the nerve cord corresponding to the diencephalon. In late embryos and larvae, AmphiNotch is also expressed in parts of the pharyngeal endoderm, in the anterior gut diverticulum, and, like AmphiPax2/5/8, in the rudiment of Hatschek's kidney. A comparison with Notch1 and Pax5 and Pax8 expression in the embryonic mouse kidney helps support homology of the amphioxus and vertebrate kidneys. AmphiNotch is also an early marker for presumptive mesoderm, transcripts first being detectable at the gastrula stage in a ring of mesendoderm just inside the blastopore and subsequently in the posterior mesoderm, notochord, and somites. As in sea urchins and vertebrates, these domains of AmphiNotch expression overlap with those of several Wnt genes and brachyury. These relationships suggest that amphioxus shares with other deuterostomes a common mechanism for patterning along the anterior/posterior axis involving a posterior signaling center in which the Notch and Wnt pathways and brachyury interact.

foxD5a, a Xenopus winged helix gene, maintains an immature neural ectoderm via transcriptional repression that is dependent on the C-terminal domain

Xenopus foxD5a, the full-length fork head gene previously described as a PCR fragment (XFLIP), is first detectable at stage II of oogenesis. Low-abundance maternal transcripts are localized to the animal hemisphere of the cleavage embryo, and protein can be translocated to the nucleus prior to the onset of zygotic transcription. Zygotic expression is strongest in the presumptive neural ectoderm at gastrula and neural plate stages, but there is minor paraxial mesodermal expression during primary gastrulation that becomes significant in the tail bud during secondary gastrulation. Expression of foxD5a in animal cap explants induces elongation and expression of mesodermal, neural-inducing, and early neural-specifying genes, indicating a role in dorsal axis formation. Zygotic foxD5a expression is induced strongly by siamois, moderately by cerberus, weakly by Wnt8 and noggin, and not by chordin in animal cap explants. Expression of foxD5a in whole embryos has differential dorsal and ventral effects. Ventral mRNA injection induces partial secondary axes composed of expanded mesodermal and epidermal tissues, but does not induce ectopic neural tissues. Dorsal mRNA injection causes hypertrophy of the neural plate and expansion of early neural genes (sox3 and otx2), but this is not the result of increased proliferation or expanded neural-inducing mesoderm. The neural plate appears to be maintained in an immature state because otx2 expression is expanded and expression of en2, Krox20, proneural genes (Xnrgn1, neuroD) and a neural differentiation gene (n-tubulin) is repressed in foxD5a-expressing cells. These results indicate that foxD5a maintains an undifferentiated neural ectoderm after neural induction. Expression of foxD5a constructs fused with the engrailed repressor domain or with the VP16 activation domain demonstrates that FoxD5a acts as a transcriptional repressor in axis formation and neural plate expansion. Deletion constructs indicate that this activity requires the C-terminal domain of the protein.