In pursuit of the functions of the Wnt family of developmental regulators: insights from Xenopus laevis

Disruption of mesectodermal lineages by temporal misexpression of the Drosophila POU-domain transcription factor, drifter

Among the first cells to differentiate in the Drosophila ventral nerve cord, the mesectodermal (midline) lineage gives rise to a discrete set of neurons and glia previously demonstrated to play an important role in the organization of the developing nervous system. The relative simplicity of the midline has allowed the elucidation of many aspects of initial lineage commitment and subsequent differentiation. Based upon its mesectodermal expression pattern and loss-of-function phenotype, we have proposed a key role for the Drosophila POU-domain transcription factor, drifter (dfr), in mesectodermal lineage development. In this study, we have examined the developmental consequences of dfr misexpression using transgenic lines expressing wild-type Drifter protein under control of the heat-inducible hsp70 promoter. Induction of ubiquitous DFR protein during a restricted period of embryogenesis causes a defective axonal phenotype characterized by failure of commissure formation. Based on examination of cell-specific markers for mesectodermal cells, these defects appear to be the result of a suppression of single-minded expression resulting in the disruption of mesectodermal lineage designation and differentiation. The observed temporally restricted sensitivity to DFR expression suggests possible interactions between DFR protein and other stage-specific mesectodermal regulatory factors present before or after a defined mesectodermal developmental event.

Characterization of the complete genomic structure of the human WNT-5A gene, functional analysis of its promoter, chromosomal mapping, and expression in early human embryogenesis

We report the complete genomic organization of the human WNT-5A gene, which encodes a cysteine-rich growth factor involved in cell-cell signaling during growth and differentiation. The gene comprises five exons with the terminal exon coding for a large 3'-untranslated region of approximately 6.5 kilobase pairs and utilizes multiple polyadenylation signals to generate at least four discrete transcripts. We discovered a new leader exon interrupted by a 411-base pair intron that was retained in our original cDNA cloning. The promoter region was located in a GpC-rich island and harbored numerous cis-acting elements including several GC boxes and Sp1, AP1, and AP2 binding motifs. It lacked TATA or CAAT boxes typical of housekeeping and growth factor genes. In support of this, primer extension revealed extension two transcription start sites. Transient cell transfection assays showed functional promoter activity for the 3.9-kilobase pair 5'-flanking region. Interestingly, internal and 5' deletions revealed tha the distal promoter was not required for full transcriptional activity and that the first 631 base pairs of WNT-5A harbored the strongest promoter activity. Using a panel of rodent-human hybrid DNAs carrying portions of chromosome 3p, we mapped the gene to 3p14.2-p21.1, between a constitutional and a familial renal cell carcinoma-associated translocation. In situ hybridization analyses of early human embryos at 28-42 days of gestation revealed that WNT-5A transcripts were not restricted to the developing brain and limbs but were also observed in the mesenchyme bordering the pharyngeal clefts and pouches and in the developing gonads and kidneys. The relatively high expression in the celomic epithelium and in the precursors of follicles and seminiferous tubules suggest a novel role for WNT-5A in germ-cell differentiation. This study provides the molecular basis for discerning the regulation of the WNT-5A gene and offers the opportunity to investigate genetic disorders linked to this important gene.

Induction of anteroposterior neural pattern in Xenopus: evidence for a quantitative mechanism

The developing vertebrate nervous system arises from ectoderm in response to inductive signals from the dorsal mesoderm, or Spemann organizer. It displays pronounced anteroposterior (AP) pattern, but the mechanism that generates this pattern is poorly understood. We demonstrate that the inducing ability of dorsal mesoderm is regionalized along the AP axis at the early gastrula stage, using the homeodomain-encoding genes Xanf-2 and en-2 as markers of anterior and mid-neural pattern, respectively. In addition, we show that changing the size ratio of posterior dorsal mesoderm to responding ectoderm affects the type of AP pattern induced. A low ratio leads to induction of anterior neural pattern, while a high ratio leads to expression of only mid-neural pattern. These and other results indicate that a quantitative mechanism specifies AP neural pattern.

Induction of a secondary embryonic axis in zebrafish occurs following the overexpression of beta-catenin

Formation of the vertebrate axis may involve a Wnt signaling cascade similar to the Drosophila wingless pathway. Zebrafish wnt8 is a candidate for involvement in axis specification insofar as it is expressed maternally and when overexpressed it can induce goosecoid, a transcription factor normally expressed in the embryonic shield. In this study we demonstrate that beta-catenin, a cadherin associated protein in the Wnt pathway, is expressed maternally in zebrafish and is widely distributed in the early embryo. Overexpressing beta-catenin in early zebrafish embryos induces goosecoid and ntl, ultimately leading to a duplication of a complete secondary axis. These data are consistent with the involvement of beta-catenin in a Wnt signaling pathway which is involved in mesoderm induction in zebrafish.

Wnt4 affects morphogenesis when misexpressed in the zebrafish embryo

The Wnts are a family of secreted glycoproteins involved in cell-cell signalling and pattern formation during development, although the extent to which various Wnts are functionally equivalent remains unclear. We have cloned zebrafish (Danio rerio) wnt4, characterized its expression, and compared its activity relative to other Wnts. The wnt4 transcript is first detected early in somitogenesis, in the dorsocaudal region of the forebrain, and then appears in the dorsal and lateral regions of the caudal hindbrain and neural keel. During somitogenesis, wnt4 appears in the floor plate, and this expression is absent in cyclops mutants, which lack floor plate. wnt4 is also expressed in the developing pronephros and gill slit. To characterize the biological activity of wnt4, synthetic zebrafish wnt4 mRNA was injected into embryos of zebrafish and Xenopus laevis. The phenotypic effects of misexpression in the zebrafish include cyclopia, misfolding in the brain, and an anteriorly forking notochord. Comparison of the phenotypes arising from misexpression of wnt4 and Xwnt-5A in both organisms suggests close parallels in the response to these Wnts. Our data suggest that wnt4, like Xwnt-5A, inhibits cell movements, and that these Wnts define a functional class distinct from the class which includes Wnt-1, Xwnt-3A and Xwnt-8.

A soluble form of Wnt-1 protein with mitogenic activity on mammary epithelial cells

The proto-oncogene Wnt-1 plays an essential role in fetal brain development and causes hyperplasia and tumorigenesis when activated ectopically in the mouse mammary gland. When expressed in certain mammary epithelial cell lines, the gene causes morphological transformation and excess cell proliferation at confluence. Like other members of the mammalian Wnt family, Wnt-1 encodes secretory glycoproteins which have been detected in association with the extracellular matrix or cell surface but which have not previously been found in a soluble or biologically active form. We show here that conditioned medium harvested from a mammary cell line expressing Wnt-1 contains soluble Wnt-1 protein and induces mitogenesis and transformation of mammary target cells. By immunodepletion of medium containing epitope-tagged Wnt-1, we show that at least 60% of this activity is specifically dependent on Wnt-1 protein. These results provide the first demonstration that a mammalian Wnt protein can act as a diffusible extracellular signaling factor.

Use of an oocyte expression assay to reconstitute inductive signaling

We have developed a paracrine signaling assay capable of mimicking inductive events in the early vertebrate embryo. RNA encoding one or more secreted proteins is microinjected into a Xenopus laevis oocyte. After a brief incubation to allow translation, a piece of embryonic tissue competent to respond to the signaling protein is grafted onto the oocyte. The secreted protein's effect on the grafted explant is then scored by assaying expression of tissue-specific markers. Explants of ectodermal tissue from blastula or gastrula stage embryos were grafted onto oocytes that had been injected with RNA encoding activin or noggin. We found that the paracrine assay faithfully reconstitutes mesoderm induction by activin and neural induction by noggin. Blastula-stage explants grafted onto activin-expressing oocytes expressed the mesodermal marker genes brachyury, goosecoid, and muscle actin. Gastrula-stage explants grafted onto noggin-expressing oocytes expressed neural cell adhesion molecule (NCAM) and formed cement gland. By injecting pools of RNA synthesized from a cDNA expression library into the oocyte, we also used the assay to screen for secreted neural-inducing proteins. We assayed 20,000 independent transformants of a library constructed from LiCl-dorsalized Xenopus laevis embryos, and we identified two cDNAs that induced neural tissue in ectodermal explants from gastrula-stage embryos. Both cDNAs encode noggin. These results suggest that the paracrine assay will be useful for the cloning of novel signaling proteins as well as for the analysis of known factors.

Cell adhesion and signal transduction: the Armadillo connection

The products of the Drosophila segment polarity gene armadillo and its vertebrate homologue beta-catenin are components of the signal transduction pathway for Wingless/Wnt-1; this signal regulates cell-fate choices in embryos of the fruit fly Drosophila and vertebrates. Armadillo/beta-catenin is also a component of cell-cell adherens junctions in epithelia. How can these two seemingly distinct roles be reconciled? Evidence suggests that Armadillo has distinct functions: one in the adherens junction and one or more in the cytoplasm. The biochemical role of Armadillo may be to serve as a scaffold upon which different multiprotein complexes are assembled.

Race: a Drosophila homologue of the angiotensin converting enzyme

We report the isolation and characterization of a putative angiotensin converting enzyme (ACE) in Drosophila, called Race. General interest in mammalian ACE stems from its association with high blood pressure; ACE has also been implicated in a variety of other physiological processes including the processing of neuropeptides and gut peristalsis. Mammalian ACE is a membrane associated zinc binding protease that converts angiotensin I (A I) into angiotensin II (A II). A II functions as a potent vasoconstrictor by triggering a G-coupled receptor system in the smooth muscles that line blood vessels. Drosophila Race is composed of 615 amino acid residues, and shares extensive sequence identity with mammalian ACE over its entire length (over 42% overall identity and greater than 60% similarity). Evidence is presented that Race might correspond to a target of the homeobox regulatory gene, zerknullt (zen). Soon after zen expression is restricted to the dorsal-most regions of the embryonic ectoderm, Race is activated in a coincident pattern and becomes associated with the amnioserosa during germ band elongation, shortening and heart morphogenesis. After germ band elongation, Race is also expressed in both the anterior and posterior midgut, where it persists throughout embryogenesis. Race expression is lost from the dorsal ectoderm in either zen- or dpp- mutants, although gut expression is unaffected. P-transformation assays and genetic complementation tests suggest that Race corresponds to a previously characterized lethal complementation group, 1(2)34Eb. Mutants die during larval/pupal development, and transheterozygotes for two different lethal alleles exhibit male sterility. We propose that Race might play a role in the contractions of the heart, gut, or testes and also suggest that Hox genes might be important for coordinating both developmental and physiological processes.

Effect of activin and lithium on isolated Xenopus animal blastomeres and response alteration at the midblastula transition

Dorsoventral mesoderm patterning in the amphibian embryo involves a series of interactions mediated by several peptide growth factors. Animal blastomeres isolated at the 8-cell stage are useful for studying mesoderm patterning, since they contain the prospective (uninduced) mesoderm region and allow examination of the default state of animal cells. When activin is applied to these dorsal and ventral animal half explants, a competence prepattern for responding to activin is observed. In order to investigate the characteristics of prepatterning, we treated animal blastomeres with the embryo dorsalizing agent LiCl. Treatment with lithium alone did not induce normal trunk mesoderm in either blastomere. Lithium did, however, alter the competence of animal blastomeres to activin. Dorsal mesoderm was formed in the ventral blastomeres, as well as in the dorsal blastomeres. This result reveals that the early dorsoventral polarity in the animal hemisphere is not fixed. Using goosecoid(gsc) and Xwnt-8 genes as dorsal and ventral mesoderm markers, it was verified that lithium modifies the competence to activin. Unexpectedly, lithium treatment on its own resulted in gsc expression in the animal half explants. This suggests that embryo goosecoid expression may be induced by the effect of dorsal determination activity, but not by mesoderm induction. However, lithium induced also the expression of brachyury (Xbra) gene at very low levels. This would indicate the formation of dorsal-anterior mesoderm, which was not identified by the tissue observations. Expression of Xwnt-8, a ventral mesoderm marker usually induced in blastula animal caps by activin, was hardly induced in the blastomere explants. We isolated whole animal half explants at the 8-cell stage and exposed to activin at different stages. It was found that the same concentration of activin induces gsc before the midblastula stage, and induces Xwnt-8 at later stages. This suggests that the response of animal blastomeres alters depending on the stage of activin signaling.

Zebrafish wnt8 and wnt8b share a common activity but are involved in distinct developmental pathways

The specification of the vertebrate body plan is dependent on numerous signaling molecules, including members of the Wnt family. We have identified two zebrafish wnt8 paralogs related to Xwnt-8B and Xwnt-8, respectively. A RT-PCR assay demonstrated that wnt8 is expressed maternally, with transcripts detected throughout embryogenesis, whereas wnt8b transcripts were first detected during late gastrulation. The wnt8 transcripts at 50% epiboly are spatially restricted to those cells at the blastoderm margin, overlying gsc-expressing cells in the axial hypoblast. During late gastrulation, wnt8 was no longer detected in the marginal cells at the dorsal midline and by mid-segmentation, transcripts were found in the presumptive tail bud. In contrast, wnt8b expression is spatially restricted to prospective neuroepithelium, and later to neural-specific structures. Overexpression of both wnts results in two major phenotypes: radialized embryos and embryos with anterior defects. These phenotypes were preceded by significant changes in the spatial expression patterns of gsc and ntl transcripts, reminiscent of activities of Xwnt-8 in Xenopus, and consistent with a role for wnt8 in the specification or patterning of mesoderm.

Nodal induces ectopic goosecoid and lim1 expression and axis duplication in zebrafish

One of the first intercellular signalling events in the vertebrate embryo leads to mesoderm formation and axis determination. In the mouse, a gene encoding a new member of the TGF-beta superfamily, nodal, is disrupted in a mutant deficient in mesoderm formation (Zhou et al., 1993, Nature 361, 543). nodal mRNA is found in prestreak mouse embryos, consistent with a role in the development of the dorsal axis. To examine the biological activities of nodal, we have studied the action of this factor in eliciting axis determination in the zebrafish, Danio rerio. Injection of nodal mRNA into zebrafish embryos caused the formation of ectopic axes that included notochord and somites. Axis duplication was preceded by the generation of an apparent ectopic shield (organizer equivalent) in nodal-injected embryos, as indicated by the appearance of a region over-expressing gsc and lim1; isolation and expression in the shield of the lim1 gene is reported here. These results suggest a role for a nodal-like factor in pattern formation in zebrafish.

Beta-catenin localization during Xenopus embryogenesis: accumulation at tissue and somite boundaries

beta-catenin is a cytoplasmic protein associated with cadherin adhesion molecules and has been implicated in axis formation in Xenopus (McCrea, P. D., Brieher, W. M. and Gumbiner, B. M. (1993) J. Cell Biol. 127, 477–484). We have studied its distribution in Xenopus embryos by immunofluorescence on frozen sections. Consistent with its function in cell-cell adhesion, beta-catenin is present in every cell. However, high levels are expressed in certain regions and different tissues of the embryo. No simple correlation appears to exist between the levels of beta-catenin with the expected strength of adhesion. High levels of beta-catenin were found in regions undergoing active morphogenetic movements, such as the marginal zone of blastulae and gastrulae. This suggests that high expression of beta-catenin could be involved in dynamic adhesion events. Surprisingly, beta-catenin also accumulates on plasma membranes that probably do not establish direct or strong contacts with other cells. In particular, high amounts of beta-catenin are found transiently at boundaries between tissue anlagen and at the intersomitic boundaries. This unexpected pattern of beta-catenin expression raises the possibility that this molecule participates in developmental processes, perhaps independently of its classical role in cell-cell adhesion.

Molecular biology of embryonic development: how far have we come in the last ten years?

The successes of molecular developmental biology over the last ten years have been particularly impressive in those directions favored by its major paradigms. New technologies have both guided and been guided by the progress of the field. I review briefly some of the major insights into embryonic development that have derived from research in four specific areas: early embryogenesis of various forms; 'pattern formation'; evolutionary conservation of regulatory elements; and spatial mechanisms of gene regulation. There remain many major problem areas, some of which may require new orientations to solve.

Morphological differences in Xenopus embryonic mesodermal cells are specified as an early response to distinct threshold concentrations of activin

The involution of presumptive mesoderm that occurs during amphibian gastrulation is a complex process requiring the coordinated action of a diverse range of cells. We show that cells with distinct morphologies, resembling each of those normally found in the involuting tissue of the Xenopus embryo, are induced in dispersed animal pole cells by different doses of the potent mesoderm-inducing factor activin. Each cell type is induced within a restricted dose range of activin concentrations, the boundaries of which are well demarcated shortly after activin treatment. In contrast, Brachyury and goosecoid, two genes thought to pattern the presumptive mesoderm, and the gene encoding platelet-derived growth factor receptor alpha, which is expressed in the mesoderm of gastrula stage embryos, are induced by broad, overlapping ranges of high activin concentrations at such early times. Similarly, the response of the gene encoding platelet-derived growth factor A, which is expressed normally in ectoderm of gastrula stage embryos, diminishes gradually as the activin concentration increases. Dose windows for the expression of these four genes narrow and become distinct from one another in cell aggregates after several hours in culture, suggesting that activin prompts a dynamic program of gene expression in induced mesoderm.

Evidence for a mitogenic effect of Wnt-1 in the developing mammalian central nervous system

The analysis of mutant alleles at the Wnt-1 locus has demonstrated that Wnt-1-mediated cell signalling plays a critical role in development of distinct regions of the embryonic central nervous system (CNS). To determine how these signals participate in the formation of the CNS, we have ectopically expressed this factor in the spinal cord under the control of the Hoxb-4 Region A enhancer. Ectopic Wnt-1 expression causes a dramatic increase in the number of cells undergoing mitosis in the ventricular region and a concomitant ventricular expansion. Although this leads to consistent changes in the relative proportions of dorsal and ventral regions, Wnt-1 does not appear to act as a primary patterning signal. Rather, our experiments indicate that Wnt-1 can act as a mitogen in the developing CNS.

The wingless signalling pathway and the patterning of the wing margin in Drosophila

The margin of the wing of Drosophila is defined and patterned from a stripe of cells expressing the wingless (wg) gene that is established during the third larval instar in the developing wing blade. The expression of the genes cut and achaete in a small domain in the prospective wing margin region reflects the activity of wg and probably mediate its function. Our results indicate that, in the wing margin, the wingless signal requires the activity of at least three genes: armadillo (arm), dishevelled (dsh) and shaggy (sgg) and that the functional relationship between these genes and wg is the same as that which exist during the patterning of the larval epidermis. These observations indicate that arm, dsh and sgg encode elements of a unique ‘wingless signalling pathway’ that is used several times throughout development.

Wnt-3a regulates somite and tailbud formation in the mouse embryo

Amphibian studies have implicated Wnt signaling in the regulation of mesoderm formation, although direct evidence is lacking. We have characterized the expression of 12 mammalian Wnt-genes, identifying three that are expressed during gastrulation. Only one of these, Wnt-3a, is expressed extensively in cells fated to give rise to embryonic mesoderm, at egg cylinder stages. A likely null allele of Wnt-3a was generated by gene targeting. All Wnt-3a-/Wnt-3a- embryos lack caudal somites, have a disrupted notochord, and fail to form a tailbud. Thus, Wnt-3a may regulate dorsal (somitic) mesoderm fate and is required, by late primitive steak stages, for generation of all new embryonic mesoderm. Wnt-3a is also expressed in the dorsal CNS. Mutant embryos show CNS dysmorphology and ectopic expression of a dorsal CNS marker. We suggest that dysmorphology is secondary to the mesodermal and axial defects and that dorsal patterning of the CNS may be regulated by inductive signals arising from surface ectoderm.

Expression of Wnt-1 in PC12 cells results in modulation of plakoglobin and E-cadherin and increased cellular adhesion

The Wnt-1 gene plays an essential role in fetal brain development and encodes a secreted protein whose signaling mechanism is presently unknown. In this report we have investigated intracellular mechanisms by which the Wnt-1 gene induces morphological changes in PC12 pheochromocytoma cells. PC12 cells expressing Wnt-1 show increased steady-state levels of the adhesive junction protein plakoglobin, and an altered distribution of this protein within the cell. This effect appears similar to a modulation of the plakoglobin homolog, Armadillo, that occurs in Drosophila embryos in response to the Wnt-1 homolog, wingless (Riggleman, B., P. Schedl, and E. Wieschaus. 1990. Cell. 63:549-560). In addition, PC12/Wnt-1 cells show elevated expression of E-cadherin and increased calcium-dependent cell-cell adhesion. These results imply evolutionary conservation of cellular responses to Wnt-1/wingless and indicate that in certain cell types Wnt-1 may act to modulate cell adhesion mechanisms.

Activity of Wnt-1 as a transmembrane protein

The product of the Wnt-1 proto-oncogene is a cysteine-rich glycoprotein that plays a crucial role in the development of the vertebrate central nervous system. Wnt-1 protein is secreted but remains associated with the cell surface and extracellular matrix. The function of Wnt-1 in several different biological settings can be carried out by cells that receive the Wnt signal from adjacent cells. Ectopic expression of Wnt-1 in certain mammary gland cell lines, such as C57MG, causes morphological transformation; C57MG cells can also be transformed by a paracrine mechanism when mixed with other cell types secreting Wnt-1 protein. To ask whether Wnt-1 protein can function while bound to the cell of origin, a variety of cell types were programmed to produce chimeric proteins containing the complete sequence of mature Wnt-1 protein fused to part or all of the transmembrane protein CD4 or CD8. The chimeras were found at the cell surface of transfected cells and did not appear to be proteolytically processed. In autocrine and paracrine transformation assays with C57MG cells and in an axis induction assay in Xenopus laevis embryos, the Wnt-1/CD4 or CD8 fusions retained significant activity, as did a secreted chimera containing the CD8 extracellular domain but lacking the transmembrane domain. However, a chimera lacking a spacer between the Wnt-1 and the transmembrane domains was weakly active and only in autocrine transformation. These results show that tethering Wnt-1 to the cell surface still allows Wnt-1-mediated cell-to-cell signaling.