Xwnt11 is a target of Xenopus Brachyury: regulation of gastrulation movements via Dishevelled, but not through the canonical Wnt pathway

Antagonistic regulation of convergent extension movements in Xenopus by Wnt/beta-catenin and Wnt/Ca2+ signaling

Convergent extension movements are the main driving force of Xenopus gastrulation. A fine-tuned regulation of cadherin-mediated cell-cell adhesion is thought to be required for this process. Members of the Wnt family of extracellular glycoproteins have been shown to modulate cadherin-mediated cell-cell adhesion, convergent extension movements, and cell differentiation. Here we show that endogenous Wnt/beta-catenin signaling activity is essential for convergent extension movements due to its effect on gene expression rather than on cadherins. Our data also suggest that XLEF-1 rather than XTCF-3 is required for convergent extension movements and that XLEF-1 functions in this context in the Wnt/beta-catenin pathway to regulate Xnr-3. In contrast, activation of the Wnt/Ca2+ pathway blocks convergent extension movements, with potential regulation of the Wnt/beta-catenin pathway at two different levels. PKC, activated by the Wnt/Ca2+ pathway, blocks the Wnt/beta-catenin pathway upstream of beta-catenin and phosphorylates Dishevelled. CamKII, also activated by the Wnt/Ca2+ pathway, inhibits the Wnt/beta-catenin signaling cascade downstream of beta-catenin. Thus, an opposing cross-talk of two distinct Wnt signaling cascades regulates convergent extension movements in Xenopus.

The zinc finger protein DIE-1 is required for late events during epithelial cell rearrangement in C. elegans

The mechanism by which epithelial cells undergo directed rearrangement is central to morphogenesis, yet the regulation of these movements remains poorly understood. We have investigated epithelial cell rearrangement (intercalation) in the dorsal hypodermis, or embryonic epidermis, of the C. elegans embryo by analyzing the die-1(w34) mutant, which fails to undergo normal intercalation. Dorsal hypodermal cells of die-1(w34) homozygous embryos initiate but fail to complete the process of intercalation. Multiphoton microscopy reveals that intercalating cells extend monopolar, basolateral protrusions in their direction of migration; posterior dorsal hypodermal cells in die-1(w34) mutants appear to extend protrusions normally, but fail to translocate their cell bodies to complete rearrangement. Despite abnormal intercalation, the subsequent morphogenetic movements that enclose the embryo with epithelial cells and the process of dorsal cell fusion still occur. However, elongation of the embryo into a wormlike shape is disrupted in die-1(w34) embryos, suggesting that intercalation may be necessary for subsequent elongation of the embryo. Actin filaments are not properly organized within the dorsal hypodermis of die-1(w34) embryos, consistent with intercalation's being a necessary prerequisite for elongation. The die-1 gene encodes a C2H2 zinc finger protein containing four fingers, which likely acts as a transcriptional regulator. DIE-1 is present in the nuclei of hypodermal, muscle, gut, and pharyngeal cells; its distribution suggests that DIE-1 acts in each of these tissues to regulate morphogenetic movements. die-1(w34) mutants display morphogenetic defects in the pharynx, gut, and muscle quadrants, in addition to the defects in the dorsal hypodermis, consistent with the DIE-1 expression pattern. Mosaic analysis indicates that DIE-1 is autonomously required in the posterior dorsal hypodermis for intercalation. Our analysis documents for the first time the dynamics of protrusive activity during epithelial cell rearrangement. Moreover, our analysis of die-1 shows that the events of epithelial cell rearrangement are under transcriptional control, and that early and later phases of epithelial cell rearrangement are genetically distinguishable.

The casein kinase I family: roles in morphogenesis

Wnt signals play important roles in development and oncogenesis and are transduced through at least two pathways: a canonical beta-catenin-dependent and a beta-catenin-independent cascade. Casein kinase I (CKI) is required in both invertebrates and vertebrates to transduce canonical Wnt signals. However, its role in the beta-catenin-independent pathway was unknown. During vertebrate embryogenesis, the beta-catenin-independent cascade is thought to control cell movements and has been postulated to be analogous to the Drosophila planar cell polarity pathway, which signals through the JNK cascade. Here, we report that blocking CKI function inhibits embryonic morphogenesis and activates JNK in cell lines. These studies suggest that CKI might also act in the beta-catenin-independent pathway and indicate a role for CKI during convergence extension in early vertebrate development.

Wnt signals are targets and mediators of Gli function

There is growing evidence that Gli proteins participate in the mediation of Hedgehog and FGF signaling in neural and mesodermal development. However, little is known about which genes act downstream of Gli proteins. Here we show the regulation of members of the Wnt family by Gli proteins in different contexts. Our findings indicate that Gli2 regulates Wnt8 expression in the ventral marginal zone of the early frog embryo: activating Gli2 constructs induce ectopic Wnt8 expression in animal cap explants, whereas repressor forms inhibit its endogenous expression in the marginal zone. Using truncated Frizzled and dominant-negative Wnt constructs, we then show the requirement of at least two Wnt proteins, Wnt8 and Wnt11, for Gli2/3-induced posterior mesodermal development. Blocking Wnt signals, however, inhibits Gli2/3-induced morphogenesis, but not mesodermal specification. Gli2/3 may therefore normally coordinate the action of these two Wnt proteins, which regulate distinct downstream pathways. In addition, the finding that Gli1 consistently induces a distinct set of Wnt genes in animal cap explants and in skin tumors suggests that Wnt regulation by Gli proteins is general. Such a mechanism may link signals that induce Gli activity, such as FGFs and Hedgehogs, with Wnt function.

Nitric oxide synthase is induced in sporulation of Physarum polycephalum

The myxomycete Physarum polycephalum expresses a calcium-independent nitric oxide (NO) synthase (NOS) resembling the inducible NOS isoenzyme in mammals. We have now cloned and sequenced this, the first nonanimal NOS to be identified, showing that it shares < 39% amino acid identity with known NOSs but contains conserved binding motifs for all NOS cofactors. It lacks the sequence insert responsible for calcium dependence in the calcium-dependent NOS isoenzymes. NOS expression was strongly up-regulated in Physarum macroplasmodia during the 5-day starvation period needed to induce sporulation competence. Induction of both NOS and sporulation competence were inhibited by glucose, a growth signal and known repressor of sporulation, and by L-N6-(1-iminoethyl)-lysine (NIL), an inhibitor of inducible NOS. Sporulation, which is triggered after the starvation period by light exposure, was also prevented by 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of NO-sensitive guanylate cyclase. In addition, also expression of lig1, a sporulation-specific gene, was strongly attenuated by NIL or ODQ. 8-Bromo-cGMP, added 2 h before the light exposure, restored the capacity of NIL-treated macroplasmodia to express lig1 and to sporulate. This indicates that the second messenger used for NO signaling in sporulation of Physarum is cGMP and links this signaling pathway to expression of lig1.

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.

Morphogenesis of prechordal plate and notochord requires intact Eph/ephrin B signaling

Eph receptors and their ligands, the ephrins, mediate cell-to-cell signals implicated in the regulation of cell migration processes during development. We report the molecular cloning and tissue distribution of zebrafish transmembrane ephrins that represent all known members of the mammalian class B ephrin family. The degree of homology among predicted ephrin B sequences suggests that, similar to their mammalian counterparts, zebrafish B-ephrins can also bind promiscuously to several Eph receptors. The dynamic expression patterns for each zebrafish B-ephrin support the idea that these ligands are confined to interact with their receptors at the borders of their complementary expression domains. Zebrafish B-ephrins are expressed as early as 30% epiboly and during gastrula stages: in the germ ring, shield, prechordal plate, and notochord. Ectopic overexpression of dominant-negative soluble ephrin B constructs yields reproducible defects in the morphology of the notochord and prechordal plate by the end of gastrulation. Notably disruption of Eph/ephrin B signaling does not completely destroy structures examined, suggesting that cell fate specification is not altered. Thus abnormal morphogenesis of the prechordal plate and the notochord is likely a consequence of a cell movement defect. Our observations suggest Eph/ephrin B signaling plays an essential role in regulating cell movements during gastrulation.

Axis Induction by Wnt Signaling: Target Promoter Responsiveness Regulates Competence

The modulation of inductive competence is a major theme in embryonic development, but, in most cases, the underlying mechanisms are not well understood. In principle, the capacity of extracellular signals to elicit particular responses could be regulated by changes in cell surface receptors, in intracellular signaling pathways, or in the responsiveness of individual target gene promoters. As an example of regulated competence, we have examined dorsal axis induction in Xenopus embryos by Wnt signaling. Competence of Wnt proteins such as Xwnt-8 to induce an ectopic axis or the dorsal early response genes siamois and Xnr3 is lost by the onset of gastrulation, when these same ligands now produce a distinct set of "late" effects, including anterior truncation and induction of the midbrain/hindbrain marker engrailed-2. Although other Wnts apparently make use of alternative signaling mechanisms, we demonstrate that late-expressed Xwnt-8 continues to employ the canonical Wnt signaling pathway used earlier in dorsal axis induction, stabilizing cytosolic beta-catenin, and activating gene expression through Tcf/Lef transcription factors. Moreover, an activated, hormone-inducible version of XTcf-3 (TVGR) that can reproduce both early and late Wnt responses when activated at appropriate stages becomes unable to induce siamois and secondary axes at the same time as Wnt ligands themselves. Finally, we show that TVGR also loses the ability to induce expression of a reporter construct containing a small fragment of the siamois promoter, implying that this fragment contains sequences governing the loss of Wnt responsiveness before gastrulation. Together, these results argue that the competence of Wnts to induce a dorsal axis is lost in the nucleus, as a result of changes in the responsiveness of target promoters.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

Calcium signaling during convergent extension in Xenopus

BACKGROUND

During Xenopus gastrulation, cell intercalation drives convergent extension of dorsal tissues. This process requires the coordination of motility throughout a large population of cells. The signaling mechanisms that regulate these movements in space and time remain poorly understood.

RESULTS

To investigate the potential contribution of calcium signaling to the control of morphogenetic movements, we visualized calcium dynamics during convergent extension using a calcium-sensitive fluorescent dye and a novel confocal microscopy system. We found that dramatic intercellular waves of calcium mobilization occurred in cells undergoing convergent extension in explants of gastrulating Xenopus embryos. These waves arose stochastically with respect to timing and position within the dorsal tissues. Waves propagated quickly and were often accompanied by a wave of contraction within the tissue. Calcium waves were not observed in explants of the ventral marginal zone or prospective epidermis. Pharmacological depletion of intracellular calcium stores abolished the calcium dynamics and also inhibited convergent extension without affecting cell fate. These data indicate that calcium signaling plays a direct role in the coordination of convergent extension cell movements.

CONCLUSIONS

The data presented here indicate that intercellular calcium signaling plays an important role in vertebrate convergent extension. We suggest that calcium waves may represent a widely used mechanism by which large groups of cells can coordinate complex cell movements.

Xenopus Sprouty2 inhibits FGF-mediated gastrulation movements but does not affect mesoderm induction and patterning

Signal transduction through the FGF receptor is essential for the specification of the vertebrate body plan. Blocking the FGF pathway in early Xenopus embryos inhibits mesoderm induction and results in truncation of the anterior-posterior axis. The Drosophila gene sprouty encodes an antagonist of FGF signaling, which is transcriptionally induced by the pathway, but whose molecular functions are poorly characterized. We have cloned Xenopus sprouty2 and show that it is expressed in a similar pattern to known FGFs and is dependent on the FGF/Ras/MAPK pathway for its expression. Overexpression of Xsprouty2 in both embryos and explant assays results in the inhibition of the cell movements of convergent extension. Although blocking FGF/Ras/MAPK signaling leads to an inhibition of mesodermal gene expression, these markers are unaffected by Xsprouty2, indicating that mesoderm induction and patterning occurs normally in these embryos. Finally, using Xenopus oocytes we show that Xsprouty2 is an intracellular antagonist of FGF-dependent calcium signaling. These results provide evidence for at least two distinct FGF-dependent signal transduction pathways: a Sprouty-insensitive Ras/MAPK pathway required for the transcription of most mesodermal genes, and a Sprouty-sensitive pathway required for coordination of cellular morphogenesis.

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.

Cell autonomous regulation of multiple Dishevelled-dependent pathways by mammalian Nkd

Genetic studies have identified Drosophila Naked Cuticle (Nkd) as an antagonist of the canonical Wnt/beta-catenin signaling pathway, but its mechanism of action remains obscure [Zeng, W., Wharton, K. A., Jr., Mack, J. A., Wang, K., Gadbaw, M., et al. (2000) Nature (London) 403, 789--795]. Here we have cloned a cDNA encoding a mammalian homolog of Drosophila Nkd, mNkd, and demonstrated that mNkd interacts directly with Dishevelled. Dishevelled is an intracellular mediator of both the canonical Wnt pathway and planar cell polarity (PCP) pathway. Activation of the c-Jun-N-terminal kinase has been implicated in the PCP pathway. We showed that mNkd acts in a cell-autonomous manner not only to inhibit the canonical Wnt pathway but also to stimulate c-Jun-N-terminal kinase activity. Expression of mNkd disrupted convergent extension in Xenopus, consistent with a role for mNkd in the PCP pathway. These data suggest that mNkd may act as a switch to direct Dishevelled activity toward the PCP pathway, and away from the canonical Wnt pathway.

Regulation of eye development by frizzled signaling in Xenopus

Eye development in both invertebrates and vertebrates is regulated by a network of highly conserved transcription factors. However, it is not known what controls the expression of these factors to regulate early eye formation and whether transmembrane signaling events are involved. Here we establish a role for signaling via a member of the frizzled family of receptors in regulating early eye development. We show that overexpression of Xenopus frizzled 3 (Xfz3), a receptor expressed during normal eye development, functions cell autonomously to promote ectopic eye formation and can perturb endogenous eye development. Ectopic eyes obtained with Xfz3 overexpression have a laminar organization similar to that of endogenous eyes and contain differentiated retinal cell types. Ectopic eye formation is preceded by ectopic expression of transcription factors involved in early eye development, including Pax6, Rx, and Otx2. Conversely, targeted overexpression of a dominant-negative form of Xfz3 (Nxfz3), consisting of the soluble extracellular domain of the receptor, results in suppression of endogenous Pax6, Rx, and Otx2 expression and suppression of endogenous eye development. This effect can be rescued by coexpression of Xfz3. Finally, overexpression of Kermit, a protein that interacts with the C-terminal intracellular domain of Xfz3, also blocks endogenous eye development, suggesting that signaling through Xfz3 or a related receptor is required for normal eye development. In summary, we show that frizzled signaling is both necessary and sufficient to regulate eye development in Xenopus.

Asymmetric localization of frizzled and the establishment of cell polarity in the Drosophila wing

The frizzled gene of Drosophila encodes a transmembrane receptor molecule required for cell polarity decisions in the adult cuticle. In the wing, a single trichome is produced by each cell, which normally points distally. In the absence of frizzled function, the trichomes no longer point uniformly distalward. We report that during cell polarization, the Frizzled receptor is localized to the distal cell edge, probably resulting in asymmetric Frizzled activity across the axis of the cell. Furthermore, Frizzled localization correlates with subsequent trichome polarity, suggesting that it may be an instructive cue in the determination of cell polarity. This differential receptor distribution may represent a novel mechanism for amplifying small differences in signaling activity across the axis of a cell.

T-targets: clues to understanding the functions of T-box proteins

Members of the T-box gene family have been identified in both vertebrates and invertebrates, where they play key roles in the regulation of embryonic development, and particularly in morphogenesis and the assignment of cell fate. T-box proteins act as transcription factors which regulate the expression of downstream effector genes. This review focuses on the identification of T-box target genes and the basis of T-box functional specificity.

T-box genes in development: from hydra to humans

The T-box gene family was uncovered less than a decade ago but has been recognized as important in controlling many and varied aspects of development in metazoans from hydra to humans. Extensive screening and database searching has revealed several subfamilies of genes with orthologs in species as diverse as Caenorhabditis elegans and humans. The defining feature of the family is a conserved sequence coding for a DNA-binding motif known as the T-box, named after the first-discovered T-box gene, T or Brachyury. Although several T-box proteins have been shown to function as transcriptional regulators, to date only a handful of downstream target genes have been discovered. Similarly, little is known about regulation of the T-box genes themselves. Although not limited to the embryo, expression of T-box genes is characteristically seen in dynamic and highly specific patterns in many tissues and organs during embryogenesis and organogenesis. The essential role of several T-box genes has been demonstrated by the developmental phenotypes of mutant animals.

A novel POZ/zinc finger protein,champignon, interferes with gastrulation movements inXenopus

We have cloned a novel krüppel-like transcription factor of Xenopus that encodes POZ/zinc finger protein by expression cloning. Overexpression of mRNA resulted in interference with gastrulation. Because the injected embryo looks like a mushroom in appearance at the neurula stage, we have named this gene champignon (cpg). In cpg-injected embryos, the blastopore appeared normally, but regressed thereafter. The injected embryos then elongated along the primary dorsoventral axis during the tailbud stage. Histologic sections and reverse transcription-polymerase chain reaction analysis showed that cpg had no effect on the cell differentiation. The animal pole region of cpg-injected embryos was thick during the gastrula stage, and mesodermal cells remained in the marginal zone. Furthermore, neither Keller-sandwich explants nor activin-treated animal cap explants excised from cpg-injected embryos elongated. These results suggest that cpg acts as a potent inhibitor of cell migration and cell intercalation during gastrulation.

Inhibition of the Wnt signaling pathway by Idax, a novel Dvl-binding protein

In attempting to clarify the roles of Dvl in the Wnt signaling pathway, we identified a novel protein which binds to the PDZ domain of Dvl and named it Idax (for inhibition of the Dvl and Axin complex). Idax and Axin competed with each other for the binding to Dvl. Immunocytochemical analyses showed that Idax was localized to the same place as Dvl in cells and that expression of Axin inhibited the colocalization of Dvl and Idax. Further, Wnt-induced accumulation of beta-catenin and activation of T-cell factor in mammalian cells were suppressed by expression of Idax. Expression of Idax in Xenopus embryos induced ventralization with a reduction in the expression of siamois, a Wnt-inducible gene. Idax inhibited Wnt- and Dvl- but not beta-catenin-induced axis duplication. It is known that Dvl is a positive regulator in the Wnt signaling pathway and that the PDZ domain is important for this activity. Therefore, these results suggest that Idax functions as a negative regulator of the Wnt signaling pathway by directly binding to the PDZ domain of Dvl.

Calmodulin differentially modulates Smad1 and Smad2 signaling

The members of the Smad protein family are intracellular mediators of transforming growth factor beta (TGF-beta) signaling. Smad1 transduces bone morphogenetic protein signals, inducing formation of ventral mesoderm in Xenopus embryos, whereas Smad2 transduces activin/TGF-beta signals, generating dorsal mesoderm. Calmodulin directly binds to many Smads and was shown to down-regulate Smad2 activity in a cell culture system (Zimmerman, C. M., Kariapper, M. S. T., and Mathews, L. S. (1997) J. Biol. Chem. 273, 677-680). Here, we extend those data and demonstrate that calmodulin alters Smad signaling in living embryos, increasing Smad1 activity while inhibiting Smad2 function. To characterize this regulation, we undertook a structure-function analysis and found that calmodulin binds to two distinct and conserved regions in both Smad1 and Smad2. Receptor tyrosine kinase signaling also modifies Smad activity (Kretzschmar, M., Doody, J., and Massagué, J. (1997) Nature 389, 618-622; Kretzschmar, M., Doody, J., Timokhina, I., and Massagué, J. (1999) Genes Dev. 13, 804-816; de Caestecker, M. P., Parks, W. T., Frank, C. J., Castagnino, P., Bottaro, D. P., Roberts, A. B., and Lechleider, R. J. (1998) Genes Dev. 12, 1587-1592). We show that calmodulin binding to Smads inhibits subsequent Erk2-dependent phosphorylation of Smads and vice versa. These observations suggest the presence of a cross-talk between three major signaling cascades as follows: Ca(2+)/calmodulin, receptor tyrosine kinase, and TGF-beta pathways.

Differential DNA binding and transcription modulation by three T-box proteins, T, TBX1 and TBX2

T-box genes encode a family of phylogenetically conserved DNA-binding proteins that regulate gene expression during embryogenesis. While the developmental importance of many T-box genes has been well documented, little is known about how family members differ in their DNA binding properties and ability to modulate transcription. Here we show that although TBX1, TBX2 and the Xenopus T protein (Xbra) share only 50-60% identity within their DNA-binding domains they can bind the same DNA sequence in vitro. However, the proteins differ in three important respects. While TBX1 protein binds a palindromic T oligonucleotide as a dimer, as had been previously reported for Xbra, TBX2 appears to bind the same DNA sequence as a monomer. Also, T protein/DNA complexes are stabilized in vitro by the addition of specific antibodies, whereas TBX2/DNA complexes are not stabilized by antibodies. Most importantly, TBX2 represses while Xbra activates transcription of the same chimeric reporter plasmid. TBX1, although capable of binding to the chimeric promoter, has no effect on transcription. Thus, while the DNA binding domains of T-box proteins share substantial homology, these proteins differ in both their DNA binding and transcriptional modulation properties. These results suggest that the various T-box proteins, while highly conserved, likely use different mechanisms to modulate transcription and may have different targets in vivo.

Cadherins in embryonic and neural morphogenesis

Cadherins not only maintain the structural integrity of cells and tissues but also control a wide array of cellular behaviours. They are instrumental for cell and tissue polarization, and they regulate cell movements such as cell sorting, cell migration and cell rearrangements. Cadherins may also contribute to neurite outgrowth and pathfinding, and to synaptic specificity and modulation in the central nervous system.

Dishevelled-1 regulates microtubule stability: a new function mediated by glycogen synthase kinase-3beta

Dishevelled has been implicated in the regulation of cell fate decisions, cell polarity, and neuronal function. However, the mechanism of Dishevelled action remains poorly understood. Here we examine the cellular localization and function of the mouse Dishevelled protein, DVL-1. Endogenous DVL-1 colocalizes with axonal microtubules and sediments with brain microtubules. Expression of DVL-1 protects stable microtubules from depolymerization by nocodazole in both dividing cells and differentiated neuroblastoma cells. Deletion analyses reveal that the PDZ domain, but not the DEP domain, of DVL-1 is required for microtubule stabilization. The microtubule stabilizing function of DVL-1 is mimicked by lithium-mediated inhibition of glycogen synthase kinase-3beta (GSK-3beta) and blocked by expression of GSK-3beta. These findings suggest that DVL-1, through GSK-3beta, can regulate microtubule dynamics. This new function of DVL-1 in controlling microtubule stability may have important implications for Dishevelled proteins in regulating cell polarity.

Origin of the precerebellar system

The precerebellar system provides the principal input to the cerebellum and is essential for coordinated motor activity. Using a FLP recombinase-based fate mapping approach, we provide direct evidence in the mouse that this ventral brainstem system derives from dorsally located rhombic neuroepithelium. Moreover, by fate mapping at the resolution of a gene expression pattern, we have uncovered an unexpected subdivision within the precerebellar primordium: embryonic expression of Wnt1 appears to identify the class of precerebellar progenitors that will later project mossy fibers from the brainstem to the cerebellum, as opposed to the class of precerebellar neurons that project climbing fibers. Differential gene expression therefore appears to demarcate two populations within the precerebellar primordium, grouping progenitors by their future type of axonal projection and synaptic partner rather than by final topographical position.

A direct screen for secreted proteins in Xenopus embryos identifies distinct activities for the Wnt antagonists Crescent and Frzb-1

To determine the spectrum of secreted proteins that are present in the extracellular space of early Xenopus embryos, a direct secretion screen was performed. Surprisingly, 24% of previously identified bona fide secretory proteins corresponded to four secreted Wnt antagonists of the same family: frzb-1, sizzled, sfrp-2 and crescent. sfrp-2 and crescent are novel components of the growing cocktail of growth factor antagonists secreted by Spemann organizer cells in Xenopus. Crescent is first expressed at blastula, defining a deep endodermal region that may be homologous to the avian hypoblast. Unlike other members of this family of inhibitors, microinjection of crescent mRNA causes the development of cyclopic embryos, even though the amount of anterior neural tissue is normal. In crescent-injected embryos, studies with specific markers indicate that morphogenetic movements of the anterior midline are abnormal, resulting in a more posterior location of prechordal plate and ventral forebrain markers with respect to the developing eye field. The results are discussed in light of recent findings in zebrafish and Xenopus that suggest that Wnt signaling through non-canonical (non-beta-catenin dependent) pathways plays a pivotal role in the regulation of morphogenetic movements.

Xwnt11 and the regulation of gastrulation in Xenopus

The molecular basis of gastrulation is poorly understood. In this paper we address this problem by taking advantage of the observation that the transcription activator Brachyury is essential for gastrulation movements in Xenopus and mouse embryos. We infer from this observation that amongst the target genes of Brachyury are some that are involved in the regulation of gastrulation. In the course of a screen for Brachyury targets we identified Xwnt11. Use of a dominant-negative Xwntll construct confirms that signalling by this class of Wnts is essential for normal gastrulation movements, and further investigation suggests that Xwntll signals not through the canonical Wnt signalling pathway involving GSK-3 and beta-catenin but through another route, which may require small GTPases such as Rho and Rac. Future work will concentrate on elucidating the Xwnt11 signal transduction pathway and on investigating its influence on cell shape and polarity during Xenopus gastrulation.

Role of frizzled 7 in the regulation of convergent extension movements during gastrulation in Xenopus laevis

Wnt signalling plays a crucial role in the control of morphogenetic movements. We describe the expression and functional analyses of frizzled 7 (Xfz7) during gastrulation in Xenopus. Low levels of Xfz7 transcripts are expressed maternally during cleavage stages; its zygotic expression strongly increases at the beginning of gastrulation and is predominantly localized to the presumptive neuroectoderm and deep cells of the involuting mesoderm. Overexpression of Xfz7 in the dorsal equatorial region affects the movements of convergent extension and delays mesodermal involution. It alters the correct localization, but not the expression, of mesodermal and neural markers. These effects can be rescued by extra-Xfz7, which is a secreted form of the receptor that also weakly inhibits convergent extension when overexpressed. This suggests that the wild-type and truncated receptors have opposing effects when coexpressed and that overexpression of Xfz7 causes an increased signalling activity. Consistent with this, Xfz7 biochemically and functionally interacts with Xwnt11. In addition, Dishevelled, but not (β)-catenin, synergizes with Xfz7 to affect convergent extension. Furthermore, overexpression of Xfz7 and Xwnt11 also affects convergent extension in activin-treated animal caps, and this can be efficiently reversed by coexpression of Cdc42(T17N), a dominant negative mutant of the small GTPase Cdc42 known as a key regulator of actin cytoskeleton. Conversely, Cdc42(G12V), a constitutively active mutant, rescues the effects of extra-Xfz7 on convergent extension in a dose-dependent manner. That both gain-of-function and loss-of-function of both frizzled and dishevelled produce the same phenotype has been well described in Drosophila tissue polarity. Therefore, our results suggest an endogenous role of Xfz7 in the regulation of convergent extension during gastrulation.

A role for Wnts in morpho-genesis and tissue polarity

Recent studies have shown that secreted Wnt proteins control morphogenetic movements in fish and frog embryos. The analysis of Dishevelled, a cytoplasmic mediator of Wnt signalling, reveals unexpected similarity between gastrulation in vertebrates and polarization of cells in Drosophila epithelia.

The Wnt/Ca2+ pathway: a new vertebrate Wnt signaling pathway takes shape

Members of the vertebrate Wnt family have been subdivided into two functional classes according to their biological activities. Some Wnts signal through the canonical Wnt-1/wingless pathway by stabilizing cytoplasmic beta-catenin. By contrast other Wnts stimulate intracellular Ca2+ release and activate two kinases, CamKII and PKC, in a G-protein-dependent manner. Moreover, putative Wnt receptors belonging to the Frizzled gene family have been identified that preferentially couple to the two prospective pathways in the absence of ectopic Wnt ligand and that might account for the signaling specificity of the Wnt pathways. As Ca2+ release was the first described feature of the noncanonical pathway, and as Ca2+ probably plays a key role in the activation of CamKII and PKC, we have named this Wnt pathway the Wnt/Ca2+ pathway.