A molecular mechanism for the effect of lithium on development

Lithium, one of the most effective drugs for the treatment of bipolar (manic-depressive) disorder, also has dramatic effects on morphogenesis in the early development of numerous organisms. How lithium exerts these diverse effects is unclear, but the favored hypothesis is that lithium acts through inhibition of inositol monophosphatase (IMPase). We show here that complete inhibition of IMPase has no effect on the morphogenesis of Xenopus embryos and present a different hypothesis to explain the broad action of lithium. Our results suggest that lithium acts through inhibition of glycogen synthase kinase-3 beta (GSK-3 beta), which regulates cell fate determination in diverse organisms including Dictyostelium, Drosophila, and Xenopus. Lithium potently inhibits GSK-3 beta activity (Ki = 2 mM), but is not a general inhibitor of other protein kinases. In support of this hypothesis, lithium treatment phenocopies loss of GSK-3 beta function in Xenopus and Dictyostelium. These observations help explain the effect of lithium on cell-fate determination and could provide insights into the pathogenesis and treatment of bipolar disorder.

The Spemann organizer signal noggin binds and inactivates bone morphogenetic protein 4

Signals released by the Spemann organizer of the amphibian gastrula can directly induce neural tissue from ectoderm and can dorsalize ventral mesoderm to form muscle. The secreted polypeptide noggin mimics these activities and is expressed at the appropriate time and place to participate in the organizer signal. Neural induction and mesoderm dorsalization are antagonized by bone morphogenetic proteins (BMPs), which induce epidermis and ventral mesoderm instead. Here we report that noggin protein binds BMP4 with high affinity and can abolish BMP4 activity by blocking binding to cognate cell-surface receptors. These data suggest that noggin secreted by the organizer patterns the embryo by interrupting BMP signaling.

A major developmental transition in early Xenopus embryos: I. characterization and timing of cellular changes at the midblastula stage

The Xenopus embryo undergoes 12 rapid synchronous cleavages followed by a period of slower asynchronous divisions more typical of somatic cells. This change in cell cleavage has been termed the midblastula transition (MBT). We show that at the MBT the blastomeres become motile and transcriptionally active for the first time. We have investigated the timing of the MBT and found that it does not depend on cell division, on time since fertilization or on a counting mechanism involving the sequential modification of DNA. Rather, the timing of the MBT depends on reaching a critical ratio of nucleus to cytoplasm. We view the MBT as a consequence of the titration of some substance, originally present in the egg, by the exponentially increasing nuclear material. When this substance is exhausted a new cell program is engaged, leading to the acquisition of several new cell properties.

Expression cloning of noggin, a new dorsalizing factor localized to the Spemann organizer in Xenopus embryos

We have cloned a cDNA encoding a novel polypeptide capable of inducing dorsal development in Xenopus embryos. RNA transcripts from this clone rescue normal development when injected into ventralized embryos and result in excessive head development at high doses. Therefore, we have named the cDNA noggin, noggin cDNA contains a single reading frame encoding a 26 kd protein with a hydrophobic amino-terminal sequence, suggesting that it is secreted. In Northern blot analysis this cDNA hybridizes to two mRNAs that are expressed both maternally and zygotically. Although noggin transcript is not localized in the oocyte and cleavage stage embryo, zygotic transcripts are initially restricted to the presumptive dorsal mesoderm and reach their highest levels at the gastrula stage in the dorsal lip of the blastopore (Spemann organizer). In the neurula, noggin is transcribed in the notochord and prechordal mesoderm. The activity of exogenous noggin RNA in embryonic axis induction and the localized expression of endogenous noggin transcripts suggest that noggin plays a role in normal dorsal development.

Dorsoventral patterning in Xenopus: inhibition of ventral signals by direct binding of chordin to BMP-4

Chordin (Chd) is an abundant protein secreted by Spemann organizer tissue during gastrulation. Chd antagonizes signaling by mature bone morphogenetic proteins (BMPs) by blocking binding to their receptors. Recombinant Xenopus Chd binds to BMP-4 with high affinity (KD, 3 x 10(-10) M), binding specifically to BMPs but not to activin or TGF-beta1. Chd protein is able to dorsalize mesoderm and to neuralize ectoderm in Xenopus gastrula explants at 1 nM. We propose that the noncell-autonomous effects of Spemann's organizer on dorsoventral patterning are executed in part by diffusible signals that directly bind to and neutralize ventral BMPs during gastrulation.

LDL-receptor-related protein 6 is a receptor for Dickkopf proteins

Wnt glycoproteins have been implicated in diverse processes during embryonic patterning in metazoa. They signal through frizzled-type seven-transmembrane-domain receptors to stabilize beta-catenin. Wnt signalling is antagonized by the extracellular Wnt inhibitor dickkopf1 (dkk1), which is a member of a multigene family. dkk1 was initially identified as a head inducer in Xenopus embryos but the mechanism by which it blocks Wnt signalling is unknown. LDL-receptor-related protein 6 (LRP6) is required during Wnt/beta-catenin signalling in Drosophila, Xenopus and mouse, possibly acting as a co-receptor for Wnt. Here we show that LRP6 (ref. 7) is a specific, high-affinity receptor for Dkk1 and Dkk2. Dkk1 blocks LRP6-mediated Wnt/beta-catenin signalling by interacting with domains that are distinct from those required for Wnt/Frizzled interaction. dkk1 and LRP6 interact antagonistically during embryonic head induction in Xenopus where LRP6 promotes the posteriorizing role of Wnt/beta-catenin signalling. Thus, DKKs inhibit Wnt co-receptor function, exemplifying the modulation of LRP signalling by antagonists.

A major developmental transition in early Xenopus embryos: II. Control of the onset of transcription

We have shown in the accompanying paper that a developmental transition occurs at the midblastula stage (cleavage 12) in Xenopus embryos, and that this midblastula transition (MBT) is apparently initiated when the ratio of nucleus to cytoplasm reaches a critical value. One manifestation of this transition is the onset of transcription. We show here that a plasmid containing a cloned gene coding for a yeast leucine tRNA comes under developmental control when injected into cleaving eggs. In pre-MBT eggs this plasmid is transiently transcribed and then becomes inactive; however, it becomes transcriptionally active again at the MBT. This pre-MBT suppression of transcription can be reversed by addition of competing DNA. The amount of DNA needed to induce premature transcription is equal to the amount of nuclear DNA present after 12 cleavages (24 ng), suggesting that the MBT is triggered by the DNA through titration of suppressor components present in the egg.

Activation of cardiac gene expression by myocardin, a transcriptional cofactor for serum response factor

Serum response factor (SRF) regulates transcription of numerous muscle and growth factor-inducible genes. Because SRF is not muscle specific, it has been postulated to activate muscle genes by recruiting myogenic accessory factors. Using a bioinformatics-based screen for unknown cardiac-specific genes, we identified a novel and highly potent transcription factor, named myocardin, that is expressed in cardiac and smooth muscle cells. Myocardin belongs to the SAP domain family of nuclear proteins and activates cardiac muscle promoters by associating with SRF. Expression of a dominant negative mutant of myocardin in Xenopus embryos interferes with myocardial cell differentiation. Myocardin is the founding member of a class of muscle transcription factors and provides a mechanism whereby SRF can convey myogenic activity to cardiac muscle genes.

Mesoderm induction in early Xenopus embryos by heparin-binding growth factors

In early embryonic development the basic body plan arises because cells in different regions become programmed to follow different developmental pathways. We have proposed that in the early amphibian embryo this process of regional specification arises from the action of three different inducing factors, or morphogens, but we have not until now had any idea of their chemical nature. In this paper we report that pure basic fibroblast growth factor (bFGF), at very low concentrations and with high specificity, closely mimics the effect of the ventrovegetal (VV) signal and that the transmission of the natural VV signal can be blocked by heparin, suggesting that it may be a heparin-binding factor such as bFGF.

Molecular nature of Spemann's organizer: the role of the Xenopus homeobox gene goosecoid

This study analyzes the function of the homeobox gene goosecoid in Xenopus development. First, we find that goosecoid mRNA distribution closely mimics the expected localization of organizer tissue in normal embryos as well as in those treated with LiCl and UV light. Second, goosecoid mRNA accumulation is induced by activin, even in the absence of protein synthesis. It is not affected by bFGF and is repressed by retinoic acid. Lastly, microinjection of goosecoid mRNA into the ventral side of Xenopus embryos, where goosecoid is normally absent, leads to the formation of an additional complete body axis, including head structures and abundant notochordal tissue. The results suggest that the goosecoid homeodomain protein plays a central role in executing Spemann's organizer phenomenon.

WDR5 associates with histone H3 methylated at K4 and is essential for H3 K4 methylation and vertebrate development

Histone H3 lysine 4 (K4) methylation has been linked to the transcriptional activation in a variety of eukaryotic species. Here we show that a common component of MLL1, MLL2, and hSet1 H3 K4 methyltransferase complexes, the WD40-repeat protein WDR5, directly associates with histone H3 di- and trimethylated at K4 and with H3-K4-dimethylated nucleosomes. WDR5 is required for binding of the methyltransferase complex to the K4-dimethylated H3 tail as well as for global H3 K4 trimethylation and HOX gene activation in human cells. WDR5 is essential for vertebrate development, in that WDR5-depleted X. laevis tadpoles exhibit a variety of developmental defects and abnormal spatial Hox gene expression. Our results are the first demonstration that a WD40-repeat protein acts as a module for recognition of a specific histone modification and suggest a mechanism for reading and writing an epigenetic mark for gene activation.

Synergistic induction of mesoderm by FGF and TGF-beta and the identification of an mRNA coding for FGF in the early Xenopus embryo

The primary patterning event in early vertebrate development is the formation of the mesoderm and its subsequent induction of the neural tube. Classic experiments suggest that the vegetal region signals the animal hemisphere to diverge from the pathway of forming ectoderm to form mesoderm such as muscle. Here we show that bovine basic FGF has a limited capacity to induce muscle actin expression in animal hemisphere cells. This level of expression can be raised to levels normally induced in the embryo by another mammalian growth factor, TGF-beta, which by itself will not induce actin expression. We show that the Xenopus embryo contains an mRNA encoding a protein highly homologous to basic FGF. These results together with the identification of a maternal mRNA with strong homology to TGF-beta, suggest that molecules closely related to FGF and TGF-beta are the natural inducers of mesoderm in vertebrate development.

A dual-kinase mechanism for Wnt co-receptor phosphorylation and activation

Signalling by the Wnt family of secreted lipoproteins has essential functions in development and disease. The canonical Wnt/beta-catenin pathway requires a single-span transmembrane receptor, low-density lipoprotein (LDL)-receptor-related protein 6 (LRP6), whose phosphorylation at multiple PPPSP motifs is induced upon stimulation by Wnt and is critical for signal transduction. The kinase responsible for LRP6 phosphorylation has not been identified. Here we provide biochemical and genetic evidence for a 'dual-kinase' mechanism for LRP6 phosphorylation and activation. Glycogen synthase kinase 3 (GSK3), which is known for its inhibitory role in Wnt signalling through the promotion of beta-catenin phosphorylation and degradation, mediates the phosphorylation and activation of LRP6. We show that Wnt induces sequential phosphorylation of LRP6 by GSK3 and casein kinase 1, and this dual phosphorylation promotes the engagement of LRP6 with the scaffolding protein Axin. We show further that a membrane-associated form of GSK3, in contrast with cytosolic GSK3, stimulates Wnt signalling and Xenopus axis duplication. Our results identify two key kinases mediating Wnt co-receptor activation, reveal an unexpected and intricate logic of Wnt/beta-catenin signalling, and illustrate GSK3 as a genuine switch that dictates both on and off states of a pivotal regulatory pathway.

Monounsaturated fatty acid modification of Wnt protein: its role in Wnt secretion

The secretion and extracellular transport of Wnt protein are thought to be well-regulated processes. Wnt is known to be acylated with palmitic acid at a conserved cysteine residue (Cys77 in murine Wnt-3a), and this residue appears to be required for the control of extracellular transport. Here, we show that murine Wnt-3a is also acylated at a conserved serine residue (Ser209). Of note, we demonstrated that this residue is modified with a monounsaturated fatty acid, palmitoleic acid. Wnt-3a defective in acylation at Ser209 is not secreted from cells in culture or in Xenopus embryos, but it is retained in the endoplasmic reticulum (ER). Furthermore, Porcupine, a protein with structural similarities to membrane-bound O-acyltransferases, is required for Ser209-dependent acylation, as well as for Wnt-3a transport from the ER for secretion. These results strongly suggest that Wnt protein requires a particular lipid modification for proper intracellular transport during the secretory process.

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.

Transgenic Xenopus embryos from sperm nuclear transplantations reveal FGF signaling requirements during gastrulation

We have developed a simple approach for large-scale transgenesis in Xenopus laevis embryos and have used this method to identify in vivo requirements for FGF signaling during gastrulation. Plasmids are introduced into decondensed sperm nuclei in vitro using restriction enzyme-mediated integration (REMI). Transplantation of these nuclei into unfertilized eggs yields hundreds of normal, diploid embryos per day which develop to advanced stages and express integrated plasmids nonmosaically. Transgenic expression of a dominant negative mutant of the FGF receptor (XFD) after the mid-blastula stage uncouples mesoderm induction, which is normal, from maintenance of mesodermal markers, which is lost during gastrulation. By contrast, embryos expressing XFD contain well-patterned nervous systems despite a putative role for FGF in neural induction.

Overexpression of cadherins and underexpression of beta-catenin inhibit dorsal mesoderm induction in early Xenopus embryos

The cadherin-catenin complex has an important role in cell-cell adhesion and may also function in signaling pathways. We report that overexpression of three cadherin types in Xenopus embryos causes them to develop with reduced dorsal axial structures. The same phenotype is produced in embryos that have been depleted of maternal beta-catenin protein by an antisense oligodeoxynucleotide complementary to beta-catenin mRNA. They show an inhibition in the expression of dorsal mesodermal markers MyoD and goosecoid, but not of ventral and general mesodermal markers. They lack notochords, somites, and neural tubes and are defective in dorsal mesodermal signaling in Nieuwkoop assays. The phenotype can be rescued by the injection of beta-catenin mRNA and not by the injection of Xwnt-8 mRNA. These results show that beta-catenin has an important role in dorsal mesoderm induction. They directly demonstrate the activity of a maternal mRNA in axis specification.

Injected Xwnt-8 RNA acts early in Xenopus embryos to promote formation of a vegetal dorsalizing center

Expression cloning from a pool of gastrula cDNAs identified the Wnt family member Xwnt-8 as having dorsal axis-inducing activity in Xenopus embryos. Microinjected Xwnt-8 mRNA was able to rescue the development of a dorsally complete anterior-posterior axis in embryos ventralized by exposure to UV light. Axis induction was observed in embryos injected in either marginal or vegetal blastomeres at the 32-cell stage. Vegetal blastomeres receiving Xwnt-8 mRNA contributed progeny not to the induced dorsal axis, but to the endoderm, a result consistent with Xwnt-8 causing cells to act as a Nieuwkoop center (the vegetal-inducing component of normal dorsal axis formation), rather than as a Spemann organizer (the induced dorsal marginal zone component that directly forms the dorsal mesoderm). Xwnt-8, which is normally expressed ventrally in midgastrula and neurula embryos, appears to mimic, when injected, maternally encoded dorsal mesoderm-inducing factors that act early in development.

Cellular determination in the Xenopus retina is independent of lineage and birth date

Xenopus embryos injected with tritiated thymidine throughout the stages of embryonic retinal neurogenesis showed that more than 95% of the embryonic retinal cells are born within a 25 hr period. While there are shallow central to peripheral, dorsal to ventral, and interlaminar gradients of neurogenesis in these eyes, throughout most of this 25 hr period, postmitotic cells are being added to all sectors and layers. Small clones of differentiated retinal neurons and glia derived from single neuroepithelial cells injected with HRP. These clones were elongated radially. They were also composed of many different combinations of cell types, suggesting a mechanism whereby determination is arbitrarily and independently assigned to postmitotic cells. Such a model, when tested statistically, fits our data very well. We present a scheme for cellular determination in the Xenopus retina in which a coherent group of clonally related cells stretch out radially as lamination begins. This brings different cells into different microenvironments. Local interactions in these microenvironments then lead the cells toward specific fates.

Translational control by CPEB: a means to the end

The regulated translation of messenger RNA is essential for cell-cycle progression, establishment of the body plan during early development, and modulation of key activities in the central nervous system. Cytoplasmic polyadenylation, which is one mechanism of controlling translation, is driven by CPEB--a highly conserved, sequence-specific RNA-binding protein that binds to the cytoplasmic polyadenylation element, and modulates translational repression and mRNA localization. What are the features and functions of this multifaceted protein?

Wnt-11 activation of a non-canonical Wnt signalling pathway is required for cardiogenesis

Formation of the vertebrate heart requires a complex interplay of several temporally regulated signalling cascades. In Xenopus laevis, cardiac specification occurs during gastrulation and requires signals from the dorsal lip and underlying endoderm. Among known Xenopus Wnt genes, only Wnt-11 shows a spatiotemporal pattern of expression that correlates with cardiac specification, which indicates that Wnt-11 may be involved in heart development. Here we show, through loss- and gain-of-function experiments, that XWnt-11 is required for heart formation in Xenopus embryos and is sufficient to induce a contractile phenotype in embryonic explants. Treating the mouse embryonic carcinoma stem cell line P19 with murine Wnt-11 conditioned medium triggers cardiogenesis, which indicates that the function of Wnt-11 in heart development has been conserved in higher vertebrates. XWnt-11 mediates this effect by non-canonical Wnt signalling, which is independent of beta-catenin and involves protein kinase C and Jun amino-terminal kinase. Our results indicate that the cardiac developmental program requires non-canonical Wnt signal transduction.

A beta-catenin/XTcf-3 complex binds to the siamois promoter to regulate dorsal axis specification in Xenopus

The Wnt pathway regulates the early dorsal-ventral axis in Xenopus through a complex of beta-catenin and HMG box transcription factors of the Lef/Tcf family. We show that the promoter of the dorsalizing homeo box gene siamois is a direct target for the beta-catenin/XTcf-3 complex, establishing a link between the Wnt pathway and the activation of genes involved in specifying the dorsal axis. By injecting siamois reporter constructs into the animal pole of Xenopus embryos, we show that a 0.8-kb fragment of the siamois promoter is strongly activated by beta-catenin. The proximal 0.5 kb, which is also activated by beta-catenin, contains three Lef/Tcf-binding sites. Mutations in these sites eliminate the beta-catenin-mediated activation of siamois and show that siamois is regulated by the beta-catenin/XTcf-3 complex, in combination with additional transcriptional activators. When expressed at the equator of the embryo, the siamois promoter is activated to much higher levels on the dorsal side than the ventral side. Ectopic ventral expression of beta-catenin raises the ventral expression of the siamois promoter to the dorsal levels. Conversely, ectopic dorsal expression of dominant-negative XTcf-3 abolishes the dorsal activation of the siamois promoter. Furthermore, elimination of the Lef/Tcf sites elevates the ventral expression of siamois, revealing a repressive role for XTcf-3 in the absence of beta-catenin. Finally, we find that the endogenous siamois activator, although present throughout the dorsal side of the embryo, is most potent in the dorsal vegetal region. We propose that the dorsal activation of siamois by the beta-catenin/XTcf-3 complex combined with the ventral repression of siamois by XTcf-3 results in the restriction of endogenous siamois expression to the dorsal side of Xenopus embryos.

The entire mesodermal mantle behaves as Spemann's organizer in dorsoanterior enhanced Xenopus laevis embryos

The body plan of Xenopus laevis can be respecified by briefly exposing early cleavage stage embryos to lithium. Such embryos develop exaggerated dorsoanterior structures such as a radial eye and cement gland (K.R. Kao, Y. Masui, and R.P. Elinson, 1986, Nature (London) 322, 371-373). In this paper, we demonstrate that the enhanced dorsoanterior phenotype results from an overcommitment of mesoderm to dorsoanterior mesoderm. Histological and immunohistochemical observations reveal that the embryos have a greatly enlarged notochord with very little muscle tissue. In addition, they develop a radial, beating heart, suggesting that lithium also specifies anterior mesoderm and pharyngeal endoderm. Randomly oriented diametrically opposed marginal zone grafts from lithium-treated embryos, when transplanted into ultraviolet (uv)-irradiated axis-deficient hosts, rescue dorsal axial structures. These transplantation experiments demonstrate that the entire marginal zone of the early gastrula consists of presumptive dorsal mesoderm. Vital dye marking experiments also indicate that the entire marginal zone maps to the prominent proboscis that is composed of chordamesoderm and represents the long axis of the embryo. These results suggest that lithium respecifies the mesoderm of Xenopus laevis embryos so that it differentiates into the Spemann organizer. We suggest that the origin of the dorsoanterior enhanced phenotypes generated by lithium and the dorsoanterior deficient phenotypes generated by uv irradiation are due to relative quantities of organizer. Our evidence demonstrates the existence of a continuum of body plan phenotypes based on this premise.

A Mechanism for Wnt Coreceptor Activation

LDL receptor related proteins 5 and 6 (LRP5/6) and their Drosophila homolog Arrow are single-span transmembrane proteins essential for Wnt/beta-catenin signaling, likely via acting as Wnt coreceptors. How Wnt activates LRP5/6/Arrow to initiate signal transduction is not well defined. Here we show that a PPPSP motif, which is reiterated five times in the LRP5/6/Arrow intracellular domain, is necessary and sufficient to trigger Wnt/beta-catenin signaling. A single PPPSP motif, upon transfer to the LDL receptor, fully activates the Wnt pathway, inducing complete axis duplication in Xenopus and TCF/beta-catenin-responsive transcription in human cells. We further show that Wnt signal-ing stimulates, and requires, phosphorylation of the PPPSP motif, which creates an inducible docking site for Axin, a scaffolding protein controlling beta-catenin stability. Our study identifies a critical signaling module and a key phosphorylation-dependent activation step of the Wnt receptor complex and reveals a unifying logic for transmembrane signaling by Wnts, growth factors, and cytokines.

CtBP, an unconventional transcriptional corepressor in development and oncogenesis

CtBP family proteins are conserved among vertebrates and invertebrates and function as transcriptional corepressors. They repress transcription in a histone deacetylase-dependent or -independent manner. CtBPs play important roles during development and oncogenesis. In this review, their unusual properties, the mechanisms of transcriptional repression, regulation, and their biological functions are discussed.