Gut reaction to Wnt signaling in worms

Response of canonical Wnt/β-catenin signaling pathway in the intestine to microgravity stress in Caenorhabditis elegans

Considering the short life-cycle property, Caenorhabditis elegans is a suitable animal model to evaluate the long-term effects of microgravity stress on organisms. Canonical Wnt/β-catenin signaling is evolutionarily conserved in various organisms. We here investigated the response of canonical Wnt/β-catenin signaling pathway to microgravity stress in nematodes. We observed the noticeable response of canonical Wnt/β-catenin signaling to microgravity stress. In contrast, we did not detect the obvious response of non-canonical Wnt/β-catenin signaling to microgravity stress. The canonical β-catenin BAR-1 acted in the intestine to regulate the response to simulated microgravity. Moreover, in the intestine, we identified a signaling cascade of canonical Wnt/β-catenin signaling pathway in response to simulated microgravity, and this signaling cascade contained Frizzled receptor MIG-1, Disheveled protein DSH-2, GSK3A/GSK-3, and β-catenin transcriptional factor BAR-1. Our data suggests an important protective response of canonical Wnt/β-catenin signaling to simulated microgravity in nematodes.

Powerful Drosophila screens that paved the wingless pathway

The Wnt/Wingless (Wg) signaling cascade controls a number of biological processes in animal development and adult life; aberrant Wnt/Wg signaling can cause diseases. In the 1980s genes were discovered that encode core Wnt/Wg pathway components: their mutant phenotypes were similar and an outline of a signaling cascade emerged. Over the years our knowledge of this important signaling system increased and more components were uncovered that are instrumental for Wnt/Wg secretion and transduction. Here we provide an overview of these discoveries, the technologies involved, with a particular focus on the important role Drosophila screens played in this process.

β-catenin-dependent Wnt signaling in C. elegans: teaching an old dog a new trick

Wnt signaling is an evolutionarily ancient pathway used to regulate many events during metazoan development. Genetic results from Caenorhabditis elegans more than a dozen years ago suggested that Wnt signaling in this nematode worm might be different than in vertebrates and Drosophila: the worm had a small number of Wnts, too many β-catenins, and some Wnt pathway components functioned in an opposite manner than in other species. Work over the ensuing years has clarified that C. elegans does possess a canonical Wnt/β-catenin signaling pathway similar to that in other metazoans, but that the majority of Wnt signaling in this species may proceed via a variant Wnt/β-catenin signaling pathway that uses some new components (mitogen-activated protein kinase signaling enzymes), and in which some conserved pathway components (β-catenin, T-cell factor [TCF]) are used in new and interesting ways. This review summarizes our current understanding of the canonical and novel TCF/β-catenin-dependent signaling pathways in C. elegans.

Cancer models in Caenorhabditis elegans

Although now dogma, the idea that nonvertebrate organisms such as yeast, worms, and flies could inform, and in some cases even revolutionize, our understanding of oncogenesis in humans was not immediately obvious. Aided by the conservative nature of evolution and the persistence of a cohort of devoted researchers, the role of model organisms as a key tool in solving the cancer problem has, however, become widely accepted. In this review, we focus on the nematode Caenorhabditis elegans and its diverse and sometimes surprising contributions to our understanding of the tumorigenic process. Specifically, we discuss findings in the worm that address a well-defined set of processes known to be deregulated in cancer cells including cell cycle progression, growth factor signaling, terminal differentiation, apoptosis, the maintenance of genome stability, and developmental mechanisms relevant to invasion and metastasis.

Wnt to build a tube: contributions of Wnt signaling to epithelial tubulogenesis

Epithelial tubes are crucial to the function of organ systems including the cardiovascular system, pulmonary system, gastrointestinal tract, reproductive organ systems, excretory system, and auditory system. Using a variety of animal model systems, recent studies have substantiated the role of Wnt signaling via the canonical/beta-catenin-mediated trajectory, the non-canonical Wnt trajectories, or both, in forming epithelial tubular tissues. This review focuses on the involvement of the Wnt pathways in the induction, specification, proliferation, and morphogenesis involved in tubulogenesis within tissues including the lungs, kidneys, ears, mammary glands, gut, and heart. The ultimate goal is to describe the developmental processes forming the various tubulogenic organ systems to determine the relationships between these processes.

Protein evolution: structure-function relationships of the oncogene beta-catenin in the evolution of multicellular animals

Beta-catenin functions as a cytoskeletal linker protein in cadherin-mediated adhesion and as a signal mediator in wnt-signal transduction pathways. We use a novel integrative approach, combining evolutionary, genomic, and three-dimensional structural data to analyze and trace the structural and functional evolution of beta-catenin genes. This approach also enabled us to examine the effects of gene duplication on the structure and function of beta-catenin genes in Drosophila, C. elegans, and vertebrates. By sampling a large number of different taxa, we identified both ancestral and derived motifs and residues within the different regions of the beta-catenin proteins. Projecting amino acid substitutions onto the three- dimensional structure established for mouse beta-catenin, we identified specific domains that exhibit loss and gain of selective constraints during beta catenin evolution. Structural changes, changes in the amino acid substitution rate, and the appearance of novel functional domains in beta-catenin can be mapped to specific branches on the metazoan tree. Together, our analyses suggest that a single, beta-catenin gene fulfilled both adhesion and signaling functions in the last common ancestor of metazoans some 700 million years ago. In addition, gene duplications facilitated the evolution of beta-catenins with novel functions and allowed the evolution of multiple, single-function proteins (cell adhesion or wnt-signaling) from the ancestral, dual-function protein. Integrative methods such as those we have applied here, utilizing the 'natural experiments' present in animal diversity, can be employed to identify novel and shared functional motifs and residues in virtually any protein among the proteomes of model systems and humans.

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

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

Expression patterns of Wnts, Frizzleds, sFRPs, and misexpression in transgenic mice suggesting a role for Wnts in pancreas and foregut pattern formation

It is well established that gut and pancreas development depend on epithelial-mesenchymal interactions. We show here that several Wnt, Frizzled, and secreted frizzled-related protein (sFRP) encoding mRNAs are present during mouse pancreatic morphogenesis. Wnt5a and 7b mRNA is broadly expressed in foregut mesenchyme starting around embryonic day 10 in mice. Other members expressed are Wnt2b, Wnt5b, and Wnt11. In addition, genes for the Wnt receptors, Frizzled2, 3, 4, 5, 6, 7, 8, and 9 are expressed. To understand potential Wnt functions in pancreas and foregut development in vivo, we analyzed transgenic F0 mouse fetuses expressing Wnt1 and 5a cDNAs under control of the PDX-1 gene promoter. In PDX-Wnt1 fetuses, the foregut region normally comprising the proximal duodenum instead resembles a posterior extension of the stomach, often associated with complete pancreatic and splenic agenesis. Furthermore, the boundary between expression domains of gastric and duodenal markers is shifted in a posterior direction. In PDX-Wnt5a fetuses, several structures derived from the proximal foregut are reduced in size, including the pancreas, spleen, and stomach, without any apparent shift in the stomach to duodenum transition. In these fetuses, overall pancreatic morphology is changed and the pancreatic epithelium is dense and compact, consistent with Wnt5A effects on cell movements and/or attachment. Taken together, these results suggest that Wnt genes participate in epithelial-mesenchymal signaling and may specify region identity in the anterior foregut.

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

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

Oncogenic potential of a C.elegans cdc25 gene is demonstrated by a gain-of-function allele

In multicellular organisms, developmental programmes must integrate with central cell cycle regulation to co-ordinate developmental decisions with cell proliferation. Hyperplasia caused by deregulated proliferation without significant change to other aspects of developmental behaviour is a probable step towards full oncogenesis in many malignancies. CDC25 phosphatase promotes progression through the eukaryotic cell cycle by dephosphorylation of cyclin-dependent kinase and, in humans, different cdc25 family members have been implicated as potential oncogenes. Demonstrating the direct oncogenic potential of a cdc25 gene, we identify a gain-of-function mutant allele of the Caenorhabditis elegans gene cdc-25.1 that causes a deregulated proliferation of intestinal cells resulting in hyperplasia, while other aspects of intestinal cell function are retained. Using RNA-mediated interference, we demonstrate modulation of the oncogenic behaviour of this mutant, and show that a reduction of the wild-type cdc-25.1 activity can cause a failure of proliferation of intestinal and other cell types. That gain and loss of CDC-25.1 activity has opposite effects on cellular proliferation indicates its critical role in controlling C.elegans cell number.

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

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

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

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

Temporally and spatially regulated expression of a candidate G-protein-coupled receptor during cerebral cortical development

Genes expressed in layer-specific patterns in the mammalian cerebral cortex may play a role in specifying the identity of different cortical layers. Using PCR-differential display, we identified a cDNA that encodes rCNL3, a gene cloned previously by sequence homology to G-protein-coupled receptors. rCNL3 is expressed predominantly in layers 2-4 of the young rat cortex and in the developing and adult striatum. Cortical expression of rCNL3 begins postnatally at P3 and continues at high levels until around P15, while striatal expression begins at E20 and continues through adulthood. rCNL3 expression is not detectable in the ventricular zone precursors that generate the neurons of layers 2-4. The expression pattern of rCNL3 in the developing cortex suggests that rCNL3 is not involved in the initial specification of laminar fate, but rather may be involved with later differentiation events within the superficial cortical layers.

Getting into shape: epidermal morphogenesis in Caenorhabditis elegans embryos

The change in shape of the C. elegans embryo from an ovoid ball of cells into a worm-shaped larva is driven by three events within the cells of the hypodermis (epidermis): (1) intercalation of two rows of dorsal cells, (2) enclosure of the ventral surface by hypodermis, and (3) elongation of the embryo. While the behavior of the hypodermal cells involved in each of these processes differs dramatically, it is clear that F-actin and microtubules have essential functions in each of these processes, whereas contraction of actomyosin structures appears to be involved specifically in elongation. Molecular analysis of these processes is revealing components specific to C. elegans as well as components found in other systems. Since C. elegans hypodermal cells demonstrate dramatically different behaviors during intercalation, enclosure and elongation, the study of cytoskeletal dynamics in these processes may reveal both unique and conserved activities during distinct epithelial morphogenetic movements. BioEssays 23:12-23, 2001.

GSK3, a master switch regulating cell-fate specification and tumorigenesis

Until recently, protein kinase GSK3 (glycogen synthase kinase 3), an essential component for cell-fate specification, had been considered a constitutively activated enzyme subject to developmentally regulated inhibition through hierarchical, linear signaling paths. Data from various systems now indicate more complex scenarios involving activating as well as inhibiting circuits, and the differential formation of multi-protein complexes that antagonistically affect GSK3 function.

When cells tell their neighbors which direction to divide

Many cells must divide in specific orientations, yet for only a handful of cases do we have some understanding of how cells choose division orientations. We know of only a few cases where division orientations are controlled by specific cell-cell interactions. These cases are of interest, because they tell us something new and seemingly fundamental about how cells can function during development. Here, the evidence that interactions control division orientation in some cells of the early C. elegans embryo is presented, and what is known about how contact can regulate division orientation is discussed. Whether contact-mediated division orientation is a peculiarity of C. elegans or whether it may be more widespread is addressed.

Cross-regulation of the Wnt signalling pathway: a role of MAP kinases

The Wnt signal transduction pathway regulates various aspects of embryonal development and is involved in cancer formation. Wnts induce the stabilisation of cytosolic (beta)-catenin, which then associates with TCF transcription factors to regulate expression of Wnt-target genes. At various levels the Wnt pathway is subject to cross-regulation by other components. Recent evidence suggests that a specific MAP kinase pathway involving the MAP kinase kinase kinase TAK1 and the MAP kinase NLK counteract Wnt signalling. In particular, homologues of TAK1 and NLK, MOM-4 and LIT-1, negatively regulate Wnt-controlled cell fate decision in the early Caenorhabditis elegans embryo. Moreover, TAK1 activates NLK, which phosphorylates TCFs bound to (beta)-catenin. This blocks nuclear localization and DNA binding of TCFs. Since TAK1 is activated by TGF-(beta) and various cytokines, it might provide an entry point for regulation of the Wnt system by other pathways. In addition, alterations in TAK1-NLK might play a role in cancer.

Interaction among GSK-3, GBP, axin, and APC in Xenopus axis specification

Glycogen synthase kinase 3 (GSK-3) is a constitutively active kinase that negatively regulates its substrates, one of which is beta-catenin, a downstream effector of the Wnt signaling pathway that is required for dorsal-ventral axis specification in the Xenopus embryo. GSK-3 activity is regulated through the opposing activities of multiple proteins. Axin, GSK-3, and beta-catenin form a complex that promotes the GSK-3-mediated phosphorylation and subsequent degradation of beta-catenin. Adenomatous polyposis coli (APC) joins the complex and downregulates beta-catenin in mammalian cells, but its role in Xenopus is less clear. In contrast, GBP, which is required for axis formation in Xenopus, binds and inhibits GSK-3. We show here that GSK-3 binding protein (GBP) inhibits GSK-3, in part, by preventing Axin from binding GSK-3. Similarly, we present evidence that a dominant-negative GSK-3 mutant, which causes the same effects as GBP, keeps endogenous GSK-3 from binding to Axin. We show that GBP also functions by preventing the GSK-3-mediated phosphorylation of a protein substrate without eliminating its catalytic activity. Finally, we show that the previously demonstrated axis-inducing property of overexpressed APC is attributable to its ability to stabilize cytoplasmic beta-catenin levels, demonstrating that APC is impinging upon the canonical Wnt pathway in this model system. These results contribute to our growing understanding of how GSK-3 regulation in the early embryo leads to regional differences in beta-catenin levels and establishment of the dorsal axis.

Biochemical interactions in the wnt pathway

The wnt signal transduction pathway is involved in many differentiation events during embryonic development and can lead to tumor formation after aberrant activation of its components. The cytoplasmic component beta-catenin is central to the transmission of wnt signals to the nucleus: in the absence of wnts beta-catenin is constitutively degraded in proteasomes, whereas in the presence of wnts beta-catenin is stabilized and associates with HMG box transcription factors of the LEF/TCF family. In tumors, beta-catenin degradation is blocked by mutations of the tumor suppressor gene APC (adenomatous polyposis coli), or of beta-catenin itself. As a consequence, constitutive TCF/beta-catenin complexes are formed and activate oncogenic target genes. This review discusses the mechanisms that silence the pathway in cells that do not receive a wnt signal and goes on to describe the regulatory steps involved in the activation of the pathway.

TCF transcription factors: molecular switches in carcinogenesis

Although originally cloned as lymphoid transcription factors, members of the T-cell factor (Tcf) family are now well recognized as key activators/repressors in many developmental processes. Transcriptionally inert Tcf factors become potent transactivators upon interaction with the Wnt signaling product beta-catenin or its Drosophila counterpart Armadillo. In contrast, Tcf proteins mediate repression when bound to members of the Groucho family of transcriptional repressors, CBP and CtBP. Recently, Tcf factors have been reported as tumor inducers, aberrantly activating their target genes as a result of elevated beta-catenin levels in many types of cancer. These abnormal beta-catenin levels are usually caused by stabilizing mutations in beta-catenin itself or truncating mutations in the adenomatous polyposis coli (APC) tumor suppressor gene. In this review, we will give a chronological overview of the Tcf factors and the phenotypes of Tcf mutant mice, as well as Tcf-binding partners. We will discuss Tcf signaling upon interaction with different partners, resulting in activator and repressor roles of Tcf factors in the light of carcinogenic events.

Drosophila APC2 is a cytoskeletally-associated protein that regulates wingless signaling in the embryonic epidermis

The tumor suppressor adenomatous polyposis coli (APC) negatively regulates Wingless (Wg)/Wnt signal transduction by helping target the Wnt effector beta-catenin or its Drosophila homologue Armadillo (Arm) for destruction. In cultured mammalian cells, APC localizes to the cell cortex near the ends of microtubules. Drosophila APC (dAPC) negatively regulates Arm signaling, but only in a limited set of tissues. We describe a second fly APC, dAPC2, which binds Arm and is expressed in a broad spectrum of tissues. dAPC2's subcellular localization revealed colocalization with actin in many but not all cellular contexts, and also suggested a possible interaction with astral microtubules. For example, dAPC2 has a striking asymmetric distribution in neuroblasts, and dAPC2 colocalizes with assembling actin filaments at the base of developing larval denticles. We identified a dAPC2 mutation, revealing that dAPC2 is a negative regulator of Wg signaling in the embryonic epidermis. This allele acts genetically downstream of wg, and upstream of arm, dTCF, and, surprisingly, dishevelled. We discuss the implications of our results for Wg signaling, and suggest a role for dAPC2 as a mediator of Wg effects on the cytoskeleton. We also speculate on more general roles that APCs may play in cytoskeletal dynamics.

MOM-4, a MAP kinase kinase kinase-related protein, activates WRM-1/LIT-1 kinase to transduce anterior/posterior polarity signals in C. elegans

In C. elegans, a Wnt/WG-like signaling pathway down-regulates the TCF/LEF-related protein, POP-1, to specify posterior cell fates. Effectors of this signaling pathway include a beta-catenin homolog, WRM-1, and a conserved protein kinase, LIT-1. WRM-1 and LIT-1 form a kinase complex that can directly phosphorylate POP-1, but how signaling activates WRM-1/LIT-1 kinase is not yet known. Here we show that mom-4, a genetically defined effector of polarity signaling, encodes a MAP kinase kinase kinase-related protein that stimulates the WRM-1/LIT-1-dependent phosphorylation of POP-1. LIT-1 kinase activity requires a conserved residue analogous to an activating phosphorylation site in other kinases, including MAP kinases. These findings suggest that anterior/posterior polarity signaling in C. elegans may involve a MAP kinase-like signaling mechanism.

WRM-1 activates the LIT-1 protein kinase to transduce anterior/posterior polarity signals in C. elegans

During C. elegans development, Wnt/WG signaling is required for differences in cell fate between sister cells born from anterior/posterior divisions. A beta-catenin-related gene, wrm-1, and the lit-1 gene are effectors of this signaling pathway and appear to downregulate the activity of POP-1, a TCF/LEF-related protein, in posterior daughter cells. We show here that lit-1 encodes a serine/threonine protein kinase homolog related to the Drosophila tissue polarity protein Nemo. We demonstrate that the WRM-1 protein binds to LIT-1 in vivo and that WRM-1 can activate the LIT-1 protein kinase when coexpressed in vertebrate tissue culture cells. This activation leads to phosphorylation of POP-1 and to apparent changes in its subcellular localization. Our findings provide evidence for novel regulatory avenues for an evolutionarily conserved Wnt/WG signaling pathway.

ASYMMETRIC CELL DIVISION IN PLANTS

Asymmetric cell divisions generate cells with different fates. In plants, where cells do not move relative to another cell, the specification and orientation of these divisions is an important mechanism to generate the overall cellular pattern during development. This review summarizes our knowledge of selected cases of asymmetric cell division in plants, in the context of recent insights into mechanisms underlying this process in bacteria, algae, yeast, and animals.

Mechanisms of Wnt signaling in development

Wnt genes encode a large family of secreted, cysteine-rich proteins that play key roles as intercellular signaling molecules in development. Genetic studies in Drosophila and Caenorhabditis elegans, ectopic gene expression in Xenopus, and gene knockouts in the mouse have demonstrated the involvement of Wnts in processes as diverse as segmentation, CNS patterning, and control of asymmetric cell divisions. The transduction of Wnt signals between cells proceeds in a complex series of events including post-translational modification and secretion of Wnts, binding to transmembrane receptors, activation of cytoplasmic effectors, and, finally, transcriptional regulation of target genes. Over the past two years our understanding of Wnt signaling has been substantially improved by the identification of Frizzled proteins as cell surface receptors for Wnts and by the finding that beta-catenin, a component downstream of the receptor, can translocate to the nucleus and function as a transcriptional activator. Here we review recent data that have started to unravel the mechanisms of Wnt signaling.

Cellular mechanisms of wingless/Wnt signal transduction

Wg/Wnt signaling regulates cell proliferation and differentiation in species as divergent as nematodes, flies, frogs, and humans. Many components of this highly conserved process have been characterized and work from a number of laboratories is beginning to elucidate the mechanism by which this class of secreted growth factor triggers cellular decisions. The Wg/Wnt ligand apparently binds to Frizzled family receptor molecules to initiate a signal transduction cascade involving the novel cytosolic protein Dishevelled and the serine/threonine kinase Zeste-white 3/GSK3. Antagonism of Zw3 activity leads to stabilization of Armadillo/beta-catenin, which provides a transactivation domain when complexed with the HMG box transcription factor dTCF/LEF-1 and thereby activates expression of Wg/Wnt-responsive genes. The Wg/Wnt ligands pass through the secretory pathway and associate with extracellular matrix components; recent work shows that sulfated glycosaminoglycans are essential for proper transduction of the signal. Mutant forms of Wg in Drosophila reveal separable aspects of Wg function and suggest that proper transport of the protein across cells is essential for cell fate specification. Complex interactions with the Notch and EGF/Ras signaling pathways also play a role in cell fate decisions during different phases of Drosophila development. These many facets of Wg/Wnt signaling have been elucidated through studies in a variety of species, each with powerful and unique experimental approaches. The remarkable conservation of this pathway suggests that Wg/Wnt signal transduction represents a fundamental mechanism for the generation of diverse cell fates in animal embryos.

A Wnt signaling pathway controls hox gene expression and neuroblast migration in C. elegans

The specification of body pattern along the anteroposterior (A/P) body axis is achieved largely by the actions of conserved clusters of Hox genes. Limiting expression of these genes to localized regional domains and controlling the precise patterns of expression within those domains is critically important for normal patterning. Here we report that egl-20, a C. elegans gene required to activate expression of the Hox gene mab-5 in the migratory neuroblast QL, encodes a member of the Wnt family of secreted glycoproteins. We have found that a second Wnt pathway gene, bar-1, which encodes a beta-catenin/Armadillo-like protein, is also required for activation of mab-5 expression in QL. In addition, we describe the gene pry-1, which is required to limit expression of the Hox genes lin-39, mab-5 and egl-5 to their correct local domains. We find that egl-20, pry-1 and bar-1 all function in a linear genetic pathway with conserved Wnt signaling components, suggesting that a conserved Wnt pathway activates expression of mab-5 in the migratory neuroblast QL. Moreover, we find that members of this Wnt signaling system play a major role in both the general and fine-scale control of Hox gene expression in other cell types along the A/P axis.

The Xenopus Wnt effector XTcf-3 interacts with Groucho-related transcriptional repressors

Tcf/Lef transcription factors mediate signalling from Wingless/Wnt proteins by recruiting Armadillo/beta-catenin as a transcriptional co-activator. However, studies of Drosophila, Xenopus and Caenorhabditis elegans have indicated that Tcf factors may also be transcriptional repressors. Here we show that Tcf factors physically interact with members of the Groucho family of transcriptional repressors. In transient transfection assays, the Xenopus Groucho homologue XGrg-4 inhibited activation of transcription of synthetic Tcf reporter genes. In contrast, the naturally truncated Groucho-family member XGrg-5 enhanced transcriptional activation. Injection of XGrg-4 into Xenopus embryos repressed transcription of Siamois and Xnr-3, endogenous targets of beta-catenin-Tcf. Dorsal injection of XGrg-4 had a ventralizing effect on Xenopus embryos. Secondary-axis formation induced by a dominant-positive Armadillo-Tcf fusion protein was inhibited by XGrg-4 and enhanced by XGrg-5. These data indicate that expression of Tcf target genes is regulated by a balance between Armadillo and Groucho.

Differential molecular interactions of beta-catenin and plakoglobin in adhesion, signaling and cancer

Plakoglobin and beta-catenin are homologous proteins functioning in cell adhesion and transactivation. Their activities are controlled by three types of interactions: those with cadherins in adherens junctions, linking them to the actin cytoskeleton; interactions in the nucleus, where they bind to transcription factors and stimulate gene expression; interactions of free cytoplasmic beta-catenin with axin and adenomatous polyposis coli (APC) protein which target it for degradation. Studies in the past year have demonstrated the complex interplay between these three types of interactions and the different behavior of beta-catenin and plakoglobin in their involvement in morphogenesis and tumorigenesis strongly suggesting that catenins play key roles in adhesion-mediated signaling.

Modulation of transcriptional regulation by LEF-1 in response to Wnt-1 signaling and association with beta-catenin

Wnt signaling is thought to be mediated via interactions between beta-catenin and members of the LEF-1/TCF family of transcription factors. Here we study the mechanism of transcriptional regulation by LEF-1 in response to a Wnt-1 signal under conditions of endogenous beta-catenin in NIH 3T3 cells, and we examine whether association with beta-catenin is obligatory for the function of LEF-1. We find that Wnt-1 signaling confers transcriptional activation potential upon LEF-1 by association with beta-catenin in the nucleus. By mutagenesis, we identified specific residues in LEF-1 important for interaction with beta-catenin, and we delineated two transcriptional activation domains in beta-catenin whose function is augmented in specific association with LEF-1. Finally, we show that a Wnt-1 signal and beta-catenin association are not required for the architectural function of LEF-1 in the regulation of the T-cell receptor alpha enhancer, which involves association of LEF-1 with a different cofactor, ALY. Thus, LEF-1 can assume diverse regulatory functions by association with different proteins.

Role of caudal in hindgut specification and gastrulation suggests homology between Drosophila amnioproctodeal invagination and vertebrate blastopore

During early embryogenesis in Drosophila, caudal mRNA is distributed as a gradient with its highest level at the posterior of the embryo. This suggests that the Caudal homeodomain transcription factor might play a role in establishing the posterior domains of the embryo that undergo gastrulation and give rise to the posterior gut. By generating embryos lacking both the maternal and zygotic mRNA contribution, we show that caudal is essential for invagination of the hindgut primordium and for further specification and development of the hindgut. These effects are achieved by the function of caudal in activating different target genes, namely folded gastrulation, which is required for invagination of the posterior gut primordium, and fork head and wingless, which are required to promote development of the internalized hindgut primordium. caudal is not sufficient for hindgut gastrulation and development, however, as it does not play a significant role in activating expression of the genes tailless, huckebein, brachyenteron and bowel. We argue that caudal and other genes expressed at the posterior of the Drosophila embryo (fork head, brachyenteron and wingless) constitute a conserved constellation of genes that plays a required role in gastrulation and gut development.

pha-4, an HNF-3 homolog, specifies pharyngeal organ identity in Caenorhabditis elegans

To build complex organs, embryos have evolved mechanisms that integrate the development of cells unrelated to one another by cell type or ancestry. Here we show that the pha-4 locus establishes organ identity for the Caenorhabditis elegans pharynx. In pha-4 mutants, pharyngeal cells are transformed into ectoderm. Conversely, ectopic pha-4 expression produces excess pharyngeal cells. pha-4 encodes an HNF-3 homolog selectively expressed in the nascent digestive tract, including all pharynx precursors at the time they are restricted to a pharyngeal fate. We suggest that pha-4 is a key component of a transcription-based mechanism to endow cells with pharyngeal organ identity.

Axis determination in Xenopus involves biochemical interactions of axin, glycogen synthase kinase 3 and beta-catenin

Signaling by the Wnt family of extracellular proteins is critical in a variety of developmental processes in which cell and tissue polarity are established [1-5]. Wnt signal transduction has been studied mostly by the genetic approach in Drosophila and Caenorhabditis elegans [1,2,5], but the biochemical mechanisms involved remain to be elucidated. The Wnt pathway also operates during axis determination in vertebrates [3,5]. Frizzled receptors transduce a signal to Dishevelled, leading to inactivation of glycogen synthase kinase 3 (GSK3) and regulation of gene expression by the complex of beta-catenin with LEF/TCF (lymphocyte enhancer factor/T-cell factor) transcription factors [3,5]. Axin is a negative regulator of Wnt signaling and dorsal axial development in vertebrates [6]. Here, we demonstrate that axin is associated with GSK3 in the Xenopus embryo and we localize the GSK3-binding domain to a short region of axin. Binding of GSK3 correlates with the ability of axin to inhibit axial development and with the axis-inducing activity of its dominant-negative form (delta RGS). We also find that wild-type axin, but not delta RGS, forms a complex with beta-catenin. Thus, axin may act as a docking station mediating negative regulation of beta-catenin by GSK3 during dorsoventral axis determination in vertebrate embryos.

Effects of forced expression of an NH2-terminal truncated beta-Catenin on mouse intestinal epithelial homeostasis

beta-Catenin functions as a downstream component of the Wnt/Wingless signal transduction pathway and as an effector of cell-cell adhesion through its association with cadherins. To explore the in vivo effects of beta-catenin on proliferation, cell fate specification, adhesion, and migration in a mammalian epithelium, a human NH2-terminal truncation mutant (DeltaN89 beta-catenin) was expressed in the 129/Sv embryonic stem cell-derived component of the small intestine of adult C57Bl/6-ROSA26 left and right arrow 129/Sv chimeric mice. DeltaN89 beta-Catenin was chosen because mutants of this type are more stable than the wild-type protein, and phenocopy activation of the Wnt/Wingless signaling pathway in Xenopus and Drosophila. DeltaN89 beta-Catenin had several effects. Cell division was stimulated fourfold in undifferentiated cells located in the proliferative compartment of the intestine (crypts of Lieberkühn). The proliferative response was not associated with any discernible changes in cell fate specification but was accompanied by a three- to fourfold increase in crypt apoptosis. There was a marked augmentation of E-cadherin at the adherens junctions and basolateral surfaces of 129/Sv (DeltaN89 beta-catenin) intestinal epithelial cells and an accompanying slowing of cellular migration along crypt-villus units. 1-2% of 129/Sv (DeltaN89 beta-catenin) villi exhibited an abnormal branched architecture. Forced expression of DeltaN89 beta-catenin expression did not perturb the level or intracellular distribution of the tumor suppressor adenomatous polyposis coli (APC). The ability of DeltaN89 beta-catenin to interact with normal cellular pools of APC and/or augmented pools of E-cadherin may have helped prevent the 129/Sv gut epithelium from undergoing neoplastic transformation during the 10-mo period that animals were studied. Together, these in vivo studies emphasize the importance of beta-catenin in regulating normal adhesive and signaling functions within this epithelium.

Wnt signaling: why is everything so negative?

The Wnt proteins constitute a family of secreted glycoproteins the members of which have essential signaling roles during embryogenesis. The recent identification of several new regulators of this signal transduction pathway have revealed unexpectedly intricate levels of constraint on Wnt-dependent gene activation, and studies in developing embryos and in cell culture systems have allowed a more complete understanding of the functional and biochemical interactions between components of this evolutionarily conserved pathway.

Tight junctions

Tight junctions are the most apical intercellular junctions of epithelial and endothelial cells and create a regulatable semipermeable diffusion barrier between individual cells. On a cellular level, they form an intramembrane diffusion fence that restricts the intermixing of apical and basolateral membrane components. In addition to these well defined functions, more recent evidence suggests that tight junctions are also involved in basic cellular processes like the regulation of cell growth and differentiation.

Beta-catenin: a key mediator of Wnt signaling

Beta-catenin is a pivotal player in the signaling pathway initiated by Wnt proteins, mediators of several developmental processes. beta-catenin activity is controlled by a large number of binding partners that affect the stability and the localization of beta-catenin and is thereby able to participate in such varying processes as gene expression and cell adhesion. Activating mutations in beta-catenin and in components regulating its stability can contribute to the formation of certain tumors.

Signal transduction by the Wnt family of ligands

The Wnt genes encode a large family of secreted polypeptides that mediate cell-cell communication in diverse developmental processes. The loss or inappropriate activation of Wnt expression has been shown to alter cell fate, morphogenesis and mitogenesis. Recent progress has identified Wnt receptors and components of an intracellular signalling pathway that mediate Wnt-dependent transcription. This review will highlight this 'core' Wnt signal-transduction pathway, but also aims to reveal the potential diversity of Wnt signalling targets. Particular attention will be paid to the overlap between developmental biology and oncogenesis, since recent progress shows Wnt signalling forms a paradigm for an interdisciplinary approach.

Tissue polarity points from cells that have higher Frizzled levels towards cells that have lower Frizzled levels

BACKGROUND

The frizzled (fz) gene of Drosophila encodes the founding member of the large family of receptors for the Wnt family of signaling molecules. It was originally studied in the adult epidermis, where it plays a key role in the generation of tissue polarity. Mutations in components of the fz signal transduction pathway disrupt tissue polarity; on the wing, hairs normally point distally but their polarity is altered by these mutations.

RESULTS

We devised a method to induce a gradient of fz expression with the highest levels near the distal wing tip. The result was a large area of proximally pointing hairs in this region. This reversal of polarity was seen when fz expression was induced just before the start of hair morphogenesis when polarity is established, suggesting that the gradient of Fz protein acted fairly directly to reverse hair polarity. A similar induction of the dishevelled (dsh) gene, which acts cell autonomously and functions downstream of fz in the generation of tissue polarity, resulted in a distinct tissue polarity phenotype, but no reversal of polarity; this argues that fz signaling was required for polarity reversal. Furthermore, the finding that functional dsh was required for the reversal of polarity argues that the reversal requires normal fz signal transduction.

CONCLUSIONS

The data suggest that cells sense the level of Fz protein on neighboring cells and use this information in order to polarize themselves. A polarizing signal is transmitted from cells with higher Fz levels to cells with lower levels. Our observations enable us to propose a general mechanism to explain how Wnts polarize target cells.