Distinct pathways for autocrine and paracrine Wingless signalling in Drosophila embryos

The hidden hedgehog of the pituitary: hedgehog signaling in development, adulthood and disease of the hypothalamic-pituitary axis

Hedgehog signaling plays pivotal roles in embryonic development, adult homeostasis and tumorigenesis. However, its engagement in the pituitary gland has been long underestimated although Hedgehog signaling and pituitary embryogenic development are closely linked. Thus, deregulation of this signaling pathway during pituitary development results in malformation of the gland. Research of the last years further implicates a regulatory role of Hedgehog signaling in the function of the adult pituitary, because its activity is also interlinked with homeostasis, hormone production, and most likely also formation of neoplasms of the gland. The fact that this pathway can be efficiently targeted by validated therapeutic strategies makes it a promising candidate for treating pituitary diseases. We here summarize the current knowledge about the importance of Hedgehog signaling during pituitary development and review recent data that highlight the impact of Hedgehog signaling in the healthy and the diseased adult pituitary gland.

Hedgehog signaling

Hedgehog (Hh) proteins constitute one family of a small number of secreted signaling proteins that together regulate multiple aspects of animal development, tissue homeostasis and regeneration. Originally uncovered through genetic analyses in Drosophila, their subsequent discovery in vertebrates has provided a paradigm for the role of morphogens in positional specification. Most strikingly, the Sonic hedgehog protein was shown to mediate the activity of two classic embryonic organizing centers in vertebrates and subsequent studies have implicated it and its paralogs in a myriad of processes. Moreover, dysfunction of the signaling pathway has been shown to underlie numerous human congenital abnormalities and diseases, especially certain types of cancer. This review focusses on the genetic studies that uncovered the key components of the Hh signaling system and the subsequent, biochemical, cell and structural biology analyses of their functions. These studies have revealed several novel processes and principles, shedding new light on the cellular and molecular mechanisms underlying cell-cell communication. Notable amongst these are the involvement of cholesterol both in modifying the Hh proteins and in activating its transduction pathway, the role of cytonemes, filipodia-like extensions, in conveying Hh signals between cells; and the central importance of the Primary Cilium as a cellular compartment within which the components of the signaling pathway are sequestered and interact.

From Drosophila segmentation to human cancer therapy

First described in Drosophila, Hedgehog signalling is a key regulator of embryonic development and tissue homeostasis and its dysfunction underlies a variety of human congenital anomalies and diseases. Although now recognised as a major target for cancer therapy as well as a mediator of directed stem cell differentiation, the unveiling of the function and mechanisms of Hedgehog signalling was driven largely by an interest in basic developmental biology rather than clinical need. Here, I describe how curiosity about embryonic patterning led to the identification of the family of Hedgehog signalling proteins and the pathway that transduces their activity, and ultimately to the development of drugs that block this pathway.

Molecular mechanisms of suppressor of fused in regulating the hedgehog signalling pathway

Highly conserved throughout evolution, the hedgehog (Hh) signalling pathway has been demonstrated to be involved in embryonic development, stem cell maintenance and tissue homeostasis in animals ranging from invertebrates to vertebrates. In the human body, a variety of cancer types are associated with the aberrantly activated Hh signalling pathway. Multiple studies have revealed suppressor of fused (Sufu) as a key negative regulator of this signalling pathway. In vertebrates, Sufu primarily functions as a tumor suppressor factor by interacting with and inhibiting glioma-associated oncogene homologues (GLIs), which are the terminal transcription factors of the Hh signalling pathway and belong to the Kruppel family of zinc finger proteins; by contrast, the regulation of Sufu itself remains relatively unclear. In the present review article, we focus on the effects of Sufu on the Hh signalling pathway in tumourigenesis and the molecular mechanisms underlying the regulation of GLI by Sufu. In addition, the factors modulating the activity of Sufu at post-transcriptional levels are also discussed.

A novel oncolytic adenovirus targeting Wnt signaling effectively inhibits cancer-stem like cell growth via metastasis, apoptosis and autophagy in HCC models

Cancer stem cells (CSCs), which are highly differentiated and self-renewing, play an important role in the occurrence, therapeutic resistant and metastasis of hepatacellular carcinoma (HCC). Oncolytic adenoviruses have targeted killing effect on tumor cells, and are invoked as candidate drugs for cancer treatment. We designed a dual-regulated oncolytic adenovirus Ad.wnt-E1A(△24bp)-TSLC1 that targets Wnt and Rb signaling pathways respectively, and carries the tumor suppressor gene, TSLC1. Previous studies have demonstrated that oncolytic adenovirus mediated TSLC1can target liver cancer and exhibit significant cytotoxicity. However, whether Ad.wnt-E1A(△24bp)-TSLC1 can effectively eliminate liver CSCs remains to be explored. We first used the spheroid culture to enrich the liver CSCs-like cells, and detected the self-renewal capacity, differentiation, drug resistance and tumorigenicity. The results showed that Ad-wnt-E1A(△24bp)-TSLC1 could effectively lead to autophagic death. In addition, recombinant adenovirus effectively induced the apoptosis, inhibit metastasis of hepatic CSCs-like cells in vivo. Further animal experiments indicated that Ad-wnt-E1A(△24bp)-TSLC1could effectively inhibit the growth of transplanted tumor of hepatic CSCs and prolong the survival time of mice. Therefore, the novel oncolytic adenovirus Ad.wnt-E1A(△24bp)-TSLC1 has potential application as a therapeutic target for HCC stem cells.

Molecular-Level Tuning of Cellular Autonomy Controls the Collective Behaviors of Cell Populations

A rigorous understanding of how multicellular behaviors arise from the actions of single cells requires quantitative frameworks that bridge the gap between genetic circuits, the arrangement of cells in space, and population-level behaviors. Here, we provide such a framework for a ubiquitous class of multicellular systems-namely, "secrete-and-sense cells" that communicate by secreting and sensing a signaling molecule. By using formal, mathematical arguments and introducing the concept of a phenotype diagram, we show how these cells tune their degrees of autonomous and collective behavior to realize distinct single-cell and population-level phenotypes; these phenomena have biological analogs, such as quorum sensing or paracrine signaling. We also define the "entropy of population," a measurement of the number of arrangements that a population of cells can assume, and demonstrate how a decrease in the entropy of population accompanies the formation of ordered spatial patterns. Our conceptual framework ties together diverse systems, including tissues and microbes, with common principles.

The Hedgehog signalling pathway in bone formation

The Hedgehog (Hh) signalling pathway plays many important roles in development, homeostasis and tumorigenesis. The critical function of Hh signalling in bone formation has been identified in the past two decades. Here, we review the evolutionarily conserved Hh signalling mechanisms with an emphasis on the functions of the Hh signalling pathway in bone development, homeostasis and diseases. In the early stages of embryonic limb development, Sonic Hedgehog (Shh) acts as a major morphogen in patterning the limb buds. Indian Hedgehog (Ihh) has an essential function in endochondral ossification and induces osteoblast differentiation in the perichondrium. Hh signalling is also involved intramembrane ossification. Interactions between Hh and Wnt signalling regulate cartilage development, endochondral bone formation and synovial joint formation. Hh also plays an important role in bone homeostasis, and reducing Hh signalling protects against age-related bone loss. Disruption of Hh signalling regulation leads to multiple bone diseases, such as progressive osseous heteroplasia. Therefore, understanding the signalling mechanisms and functions of Hh signalling in bone development, homeostasis and diseases will provide important insights into bone disease prevention, diagnoses and therapeutics.

Testing models of the APC tumor suppressor/β-catenin interaction reshapes our view of the destruction complex in Wnt signaling

The Wnt pathway is a conserved signal transduction pathway that contributes to normal development and adult homeostasis, but is also misregulated in human diseases such as cancer. The tumor suppressor adenomatous polyposis coli (APC) is an essential negative regulator of Wnt signaling inactivated in >80% of colorectal cancers. APC participates in a multiprotein "destruction complex" that targets the proto-oncogene β-catenin for ubiquitin-mediated proteolysis; however, the mechanistic role of APC in the destruction complex remains unknown. Several models of APC function have recently been proposed, many of which have emphasized the importance of phosphorylation of high-affinity β-catenin-binding sites [20-amino-acid repeats (20Rs)] on APC. Here we test these models by generating a Drosophila APC2 mutant lacking all β-catenin-binding 20Rs and performing functional studies in human colon cancer cell lines and Drosophila embryos. Our results are inconsistent with current models, as we find that β-catenin binding to the 20Rs of APC is not required for destruction complex activity. In addition, we generate an APC2 mutant lacking all β-catenin-binding sites (including the 15Rs) and find that a direct β-catenin/APC interaction is also not essential for β-catenin destruction, although it increases destruction complex efficiency in certain developmental contexts. Overall, our findings support a model whereby β-catenin-binding sites on APC do not provide a critical mechanistic function per se, but rather dock β-catenin in the destruction complex to increase the efficiency of β-catenin destruction. Furthermore, in Drosophila embryos expressing some APC2 mutant transgenes we observe a separation of β-catenin destruction and Wg/Wnt signaling outputs and suggest that cytoplasmic retention of β-catenin likely accounts for this difference.

Wingless/Wnt signaling in Drosophila: the pattern and the pathway

Wnt signaling generates pattern in all animal embryos, from flies and worms to humans, and promotes the undifferentiated, proliferative state critical for stem cells in adult tissues. Inappropriate Wnt pathway activation is the major cause of colorectal cancers, a leading cause of cancer death in humans. Although this pathway has been studied extensively for years, large gaps remain in our understanding of how it switches on and off, and how its activation changes cellular behaviors. Much of what is known about the pathway comes from genetic studies in Drosophila, where a single Wnt molecule, encoded by wingless (wg), directs an array of cell-fate decisions similar to those made by the combined activities of all 19 Wnt family members in vertebrates. Although Wg specifies fate in many tissues, including the brain, limbs, and major organs, the fly embryonic epidermis has proven to be a very powerful system for dissecting pathway activity. It is a simple, accessible tissue, with a pattern that is highly sensitive to small changes in Wg pathway activity. This review discusses what we have learned about Wnt signaling from studying mutations that disrupt epidermal pattern in the fly embryo, highlights recent advances and controversies in the field, and sets these issues in the context of questions that remain about how this essential signaling pathway functions.

Autonomous Boolean modelling of developmental gene regulatory networks

During early embryonic development, a network of regulatory interactions among genes dynamically determines a pattern of differentiated tissues. We show that important timing information associated with the interactions can be faithfully represented in autonomous Boolean models in which binary variables representing expression levels are updated in continuous time, and that such models can provide a direct insight into features that are difficult to extract from ordinary differential equation (ODE) models. As an application, we model the experimentally well-studied network controlling fly body segmentation. The Boolean model successfully generates the patterns formed in normal and genetically perturbed fly embryos, permits the derivation of constraints on the time delay parameters, clarifies the logic associated with different ODE parameter sets and provides a platform for studying connectivity and robustness in parameter space. By elucidating the role of regulatory time delays in pattern formation, the results suggest new types of experimental measurements in early embryonic development.

Recent advances in hedgehog signalling

Hedgehog (HH) proteins are an important class of secreted intercellular signals. The HH signal-transduction pathway is not fully understood, but a number of novel features have been elucidated recently. It is now clear that, during processing to generate an active signal, Drosophila HH proteins become covalently linked to cholesterol and are thereby largely tethered to the cell surface. HH signalling could therefore be affected by cholesterol metabolism. In addition, the pathway downstream of receptor binding involves a unique signalling complex containing the transcription factor CUBITUS INTERRUPTUS (CI), which becomes dissociated from microtubules in response to HH. This review discusses these new findings and their implications for HH signalling.

Hedgehog-producing cancer cells respond to and require autocrine Hedgehog activity

A number of Smoothened (SMO) pathway antagonists are currently undergoing clinical trials as anticancer agents. These drugs are proposed to attenuate tumor growth solely through inhibition of Hedgehog (HH), which is produced in tumor cells but acts on tumor stromal cells. The pivotal argument underlying this model is that the growth-inhibitory properties of SMO antagonists on HH-producing cancer cells are due to their off-target effects. Here, we show that the tumorigenic properties of such lung cancer cells depend on their intrinsic level of HH activity. Notably, reducing HH signaling in these tumor cells decreases HH target gene expression. Taken together, these results question the dogma that autocrine HH signaling plays no role in HH-dependent cancers, and does so without using SMO antagonists.

Novel therapeutic strategies for targeting liver cancer stem cells

The cancer stem cell (CSC) hypothesis was first proposed over 40 years ago. Advances in CSC isolation were first achieved in hematological malignancies, with the first CSC demonstrated in acute myeloid leukemia. However, using similar strategies and technologies, and taking advantage of available surface markers, CSCs have been more recently demonstrated in a growing range of epithelial and other solid organ malignancies, suggesting that the majority of malignancies are dependent on such a compartment.

Primary liver cancer consists predominantly of hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC). It is believed that hepatic progenitor cells (HPCs) could be the origin of some HCCs and ICCs. Furthermore, stem cell activators such as Wnt/β-catenin, TGF-β, Notch and Hedgehog signaling pathways also expedite tumorigenesis, and these pathways could serve as molecular targets to assist in designing cancer prevention strategies. Recent studies indicate that additional factors such as EpCAM, Lin28 or miR-181 may also contribute to HCC progression by targeting HCC CSCs. Various therapeutic drugs that directly modulate CSCs have been examined in vivo and in vitro. However, CSCs clearly have a complex pathogenesis, with a considerable crosstalk and redundancy in signaling pathways, and hence targeting single molecules or pathways may have a limited benefit for treatment. Many of the key signaling molecules are shared by both CSCs and normal stem cells, which add further challenges for designing molecularly targeted strategies specific to CSCs but sparing normal stem cells to avoid side effects. In addition to the direct control of CSCs, many other factors that are needed for the maintenance of CSCs, such as angiogenesis, vasculogenesis, invasion and migration, hypoxia, immune evasion, multiple drug resistance, and radioresistance, should be taken into consideration when designing therapeutic strategies for HCC.

Here we provide a brief review of molecular signaling in liver CSCs and present insights into new therapeutic strategies for targeting liver CSCs.

Self-induction of a/a or alpha/alpha biofilms in Candida albicans is a pheromone-based paracrine system requiring switching

Like MTL-heterozygous (a/α) cells, white MTL-homozygous (a/a or α/α) cells of Candida albicans, to which a minority of opaque cells of opposite mating type have been added, form thick, robust biofilms. The latter biofilms are uniquely stimulated by the pheromone released by opaque cells and are regulated by the mitogen-activated protein kinase signal transduction pathway. However, white MTL-homozygous cells, to which opaque cells of opposite mating type have not been added, form thinner biofilms. Mutant analyses reveal that these latter biofilms are self-induced. Self-induction of a/a biofilms requires expression of the α-receptor gene STE2 and the α-pheromone gene MFα, and self-induction of α/α biofilms requires expression of the a-receptor gene STE3 and the a-pheromone gene MFa. In both cases, deletion of WOR1, the master switch gene, blocks cells in the white phenotype and biofilm formation, indicating that self-induction depends upon low frequency switching from the white to opaque phenotype. These results suggest a self-induction scenario in which minority opaque a/a cells formed by switching secrete, in a mating-type-nonspecific fashion, α-pheromone, which stimulates biofilm formation through activation of the α-pheromone receptor of majority white a/a cells. A similar scenario is suggested for a white α/α cell population, in which minority opaque α/α cells secrete a-pheromone. This represents a paracrine system in which one cell type (opaque) signals a second highly related cell type (white) to undergo a complex response, in this case the formation of a unisexual white cell biofilm.

Regeneration and transdetermination: the role of wingless and its regulation

Imaginal discs of Drosophila have the remarkable ability to regenerate. After fragmentation wound healing occurs, ectopic wg is induced and a blastema is formed. In some, but not all fragments, the blastema will replace missing structures and a few cells can become more plastic and transdetermine to structures of other discs. A series of systematic cuts through the first leg disc revealed that a cut must transect the dorsal-proximal disc area and that the fragment must also include wg-competent cells. Fragments that fail to both transdetermine and regenerate missing structures will do both when provided with exogenous Wg, demonstrating the necessity of Wg in regenerative processes. In intact leg discs ubiquitously expressed low levels of Wg also leads to blastema formation, regeneration and transdetermination. Two days after exogenous wg induction the endogenous gene is activated, leading to elevated levels of Wg in the dorsal aspect of the leg disc. We identified a wg enhancer that regulates ectopic wg expression. Deletion of this enhancer increases transdetermination, but lowers the amount of ectopic Wg. We speculate that this lessens repression of dpp dorsally, and thus creates a permissive condition under which the balance of ectopic Wg and Dpp is favorable for transdetermination.

Self-induced patched receptor down-regulation modulates cell sensitivity to the hedgehog morphogen gradient

Morphogens form signaling gradients that control patterning processes during development. Responding cells must perceive and interpret the concentration-dependent information provided by the morphogen to generate precise patterns of gene expression and cell differentiation in developing tissues. Generally, the absolute number of activated, ligand-bound receptors determines cell perception of the morphogen. In contrast, cells interpret the morphogen Hedgehog (Hh) by measuring the ratio of bound to unbound molecules of its receptor Patched (Ptc). This ratio depends on both the Hh concentration and the absolute number of Ptc molecules. Here, I describe a posttranscriptional process that controls the absolute amount of Ptc present in a cell, which regulates gradient interpretation, wherein self-induced receptor down-regulation that is independent of ligand binding dictates the cell response to a morphogen gradient.

Evaluating Smoothened as a G-protein-coupled receptor for Hedgehog signalling

The Hedgehog signalling pathway controls numerous developmental processes. In response to Hedgehog, Smoothened (Smo), a seven-pass transmembrane protein, orchestrates pathway signalling and controls transcription factor activation. In the absence of Hedgehog, the receptor Patched indirectly inhibits Smo in a catalytic manner. Many questions surrounding Smo activation and signalling remain. Recent findings in Drosophila and vertebrate systems have provided strong evidence that Smo acts as a G-protein-coupled receptor. We discuss the role and regulation of Smo and reassess similarities between Smo and G-protein-coupled receptors. We also examine recently identified members of the invertebrate and vertebrate Smo signalling cascades that are typical components of G-protein-coupled receptor pathways. Greater understanding of the mechanisms of Smo activation and its signalling pathways will allow implementation of novel strategies to target disorders related to disruption of Hh signalling.

WNTs tune up the neuromuscular junction

Although WNTs have been long thought of as regulators of cell fate, recent studies highlight their involvement in crucial aspects of synaptic development in the nervous system. Particularly compelling are recent studies of the neuromuscular junction in nematodes, insects, fish and mammals. These studies place WNTs as major determinants of synapse differentiation and neurotransmitter receptor clustering.

A screen for modifiers of hedgehog signaling in Drosophila melanogaster identifies swm and mts

Signaling by Hedgehog (Hh) proteins shapes most tissues and organs in both vertebrates and invertebrates, and its misregulation has been implicated in many human diseases. Although components of the signaling pathway have been identified, key aspects of the signaling mechanism and downstream targets remain to be elucidated. We performed an enhancer/suppressor screen in Drosophila to identify novel components of the pathway and identified 26 autosomal regions that modify a phenotypic readout of Hh signaling. Three of the regions include genes that contribute constituents to the pathway-patched, engrailed, and hh. One of the other regions includes the gene microtubule star (mts) that encodes a subunit of protein phosphatase 2A. We show that mts is necessary for full activation of Hh signaling. A second region includes the gene second mitotic wave missing (swm). swm is recessive lethal and is predicted to encode an evolutionarily conserved protein with RNA binding and Zn(+) finger domains. Characterization of newly isolated alleles indicates that swm is a negative regulator of Hh signaling and is essential for cell polarity.

Cullin4B/E3-ubiquitin ligase negatively regulates beta-catenin

Beta-catenin is the key transducer of Wingless-type MMTV integration site family member (Wnt) signalling, upregulation of which is the cause of cancer of the colon and other tissues. In the absence of Wnt signals, beta-catenin is targeted to ubiquitin-proteasome-mediated degradation. Here we present the functional characterization of E3-ubiquitin ligase encoded by cul4B. RNAi-mediated knock-down of Cul4B in a mouse cell line C3H T10 (1/2) results in an increase in beta-catenin levels. Loss-of-function mutation in Drosophila cul4 also shows increased beta-catenin/Armadillo levels in developing embryos and displays a characteristic naked-cuticle phenotype. Immunoprecipitation experiments suggest that Cul4B and beta-catenin are part of a signal complex in Drosophila, mouse and human. These preliminary results suggest a conserved role for Cul4B in the regulation of beta-catenin levels.

The contributions of protein kinase A and smoothened phosphorylation to hedgehog signal transduction in Drosophila melanogaster

Protein kinase A (PKA) silences the Hedgehog (Hh) pathway in Drosophila in the absence of ligand by phosphorylating the pathway's transcriptional effector, Cubitus interruptus (Ci). Smoothened (Smo) is essential for Hh signal transduction but loses activity if three specific PKA sites or adjacent PKA-primed casein kinase 1 (CK1) sites are replaced by alanine residues. Conversely, Smo becomes constitutively active if acidic residues replace those phosphorylation sites. These observations suggest an essential positive role for PKA in responding to Hh. However, direct manipulation of PKA activity has not provided strong evidence for positive effects of PKA, with the notable exception of a robust induction of Hh target genes by PKA hyperactivity in embryos. Here we show that the latter response is mediated principally by regulatory elements other than Ci binding sites and not by altered Smo phosphorylation. Also, the failure of PKA hyperactivity to induce Hh target genes strongly through Smo phosphorylation cannot be attributed to the coincident phosphorylation of PKA sites on Ci. Finally, we show that Smo containing acidic residues at PKA and CK1 sites can be stimulated further by Hh and acts through Hh pathways that both stabilize Ci-155 and use Fused kinase activity to increase the specific activity of Ci-155.

Mechanisms of Hedgehog gradient formation and interpretation

Morphogens are molecules that spread from localized sites of production, specifying distinct cell outcomes at different concentrations. Members of the Hedgehog (Hh) family of signaling molecules act as morphogens in different developmental systems. If we are to understand how Hh elicits multiple responses in a temporally and spatially specific manner, the molecular mechanism of Hh gradient formation needs to be established. Moreover, understanding the mechanisms of Hh signaling is a central issue in biology, not only because of the role of Hh in morphogenesis, but also because of its involvement in a wide range of human diseases. Here, we review the mechanisms affecting the dynamics of Hh gradient formation, mostly in the context of Drosophila wing development, although parallel findings in vertebrate systems are also discussed.

Pointed regulates an eye-specific transcriptional enhancer in the Drosophila hedgehog gene, which is required for the movement of the morphogenetic furrow

Drosophila development depends on stable boundaries between cellular territories, such as the embryonic parasegment boundaries and the compartment boundaries in the imaginal discs. Patterning in the compound eye is fundamentally different: the boundary is not stable, but moves (the morphogenetic furrow). Paradoxically, Hedgehog signaling is essential to both: Hedgehog is expressed in the posterior compartments in the embryo and in imaginal discs, and posterior to the morphogenetic furrow in the eye. Therefore, uniquely in the eye, cells receiving a Hedgehog signal will eventually produce the same protein. We report that the mechanism that underlies this difference is the special regulation of hedgehog (hh) transcription through the dual regulation of an eye specific enhancer. We show that this enhancer requires the Egfr/Ras pathway transcription factor Pointed. Recently, others have shown that this same enhancer also requires the eye determining transcription factor Sine oculis (So). We discuss these data in terms of a model for a combinatorial code of furrow movement.

Functional analyses in the milkweed bug Oncopeltus fasciatus (Hemiptera) support a role for Wnt signaling in body segmentation but not appendage development

Specification of the proximal-distal (PD) axis of insect appendages is best understood in Drosophila melanogaster, where conserved signaling molecules encoded by the genes decapentaplegic (dpp) and wingless (wg) play key roles. However, the development of appendages from imaginal discs as in Drosophila is a derived state, while more basal insects produce appendages from embryonic limb buds. Therefore, the universality of the Drosophila limb PD axis specification mechanism has been debated since dpp expression in more basal insect species differs dramatically from Drosophila. Here, we test the function of Wnt signaling in the development of the milkweed bug Oncopeltus fasciatus, a species with the basal state of appendage development from limb buds. RNA interference of wg and pangolin (pan) produce defects in the germband and eyes, but not in the appendages. Distal-less and dachshund, two genes regulated by Wg signaling in Drosophila and expressed in specific PD domains along the limbs of both species, are expressed normally in the limbs of pan-depleted Oncopeltus embryos. Despite these apparently paradoxical results, Armadillo protein, the transducer of Wnt signaling, does not accumulate properly in the nuclei of cells in the legs of pan-depleted embryos. In contrast, engrailed RNAi in Oncopeltus produces cuticular and appendage defects similar to Drosophila. Therefore, our data suggest that Wg signaling is functionally conserved in the development of the germband, while it is not essential in the specification of the limb PD axis in Oncopeltus and perhaps basal insects.

A genetic screen in Drosophila for identifying novel components of the hedgehog signaling pathway

The Hedgehog signaling pathway plays an essential role in the pattern formation and development of metazoan animals. Misregulation of Hedgehog signaling has also been associated with the formation of multiple types of cancer. For these reasons, the Hedgehog pathway has attracted considerable interest. Many proteins required in the Hedgehog pathway have been identified, and while much has been learned about their function in signal transduction, it is clear that this complement of proteins does not comprise the full set necessary for Hedgehog signal transduction. Because significant gaps remain in our knowledge of the molecules required for Hedgehog signaling, we performed an enhancer/suppressor screen in Drosophila melanogaster to identify novel components of the pathway. In addition to the isolation of new alleles of the known pathway components patched and smoothened, this screen identified 14 novel complementation groups and a larger number of loci represented by single alleles. These groups include mutations in the genes encoding the translation factors eRF1 and eIF1A and the kinesin-like protein Pavarotti. It also identified mutations in a gene whose product is necessary for the movement of Hedgehog protein through tissues.

A genetic screen for suppressors of Drosophila NSF2 neuromuscular junction overgrowth

The Drosophila larval neuromuscular system serves as a valuable model for studying the genes required for synaptic development and function. N-Ethylmaleimide sensitive factor (NSF) is a molecule known to be important in vesicular trafficking but neural expression of a dominant negative form of NSF2 induces an unexpected overgrowth of the Drosophila larval neuromuscular synapse. We have taken a genetic approach to understanding this novel phenotype by conducting a gain-of-function modifier screen to isolate genes that interact with the overgrowth phenotype. Our approach was to directly visualize the neuromuscular junction (NMJ) using a GFP transgene and screen for suppressors of NMJ overgrowth using the Gene Search collection of P-element insertions. Of the 3000 lines screened, we identified 99 lines that can partially restore the normal phenotype. Analysis of the GS element insertion sites by inverse PCR and comparison of the flanking DNA sequence to the Drosophila genome sequence revealed nearby genes for all but 10 of the 99 lines. The recovered genes, both known and predicted, include transcription factors, cytoskeletal elements, components of the ubiquitin pathway, and several signaling molecules. This collection of genes that suppress the NSF2 neuromuscular junction overgrowth phenotype is a valuable resource in our efforts to further understand the role of NSF at the synapse.

A regulatory role of Wnt signaling pathway in the hematopoietic differentiation of murine embryonic stem cells

One of the most important issues in stem cell research is to understand the regulatory mechanisms responsible for their differentiation. An extensive understanding of mechanism underlying the process of differentiation is crucial in order to prompt stem cells to perform a particular function after differentiation. To elucidate the molecular mechanisms responsible for the hematopoietic differentiation of embryonic stem cells (ESCs), we investigated murine ES cells for the presence of hematopoietic lineage markers as well as Wnt signaling pathway during treatments with different cytokines alone or in combination with another. Here we report that Wnt/beta-catenin signaling is down-regulated in hematopoietic differentiation of murine ES cells. We also found that differentiation induced by the interleukin-3, interleukin-6, and erythropoietin combinations resulted in high expression of CD3e, CD11b, CD45R/B220, Ly-6G, and TER-119 in differentiated ES cells. A high expression of beta-catenin was observed in two undifferentiated ES cell lines. Gene and protein expression analysis revealed that the members downstream of Wnt in this signaling pathway including beta-catenin, GSK-3beta, Axin, and TCF4 were significantly down-regulated as ES cells differentiated into hematopoietic progenitors. Our results show that the Wnt/beta-catenin signaling pathway plays a role in the hematopoietic differentiation of murine ESCs and also may support beta-catenin as a crucial factor in the maintenance of ES cells in their undifferentiated state.

Nemo is an inducible antagonist of Wingless signaling during Drosophila wing development

The cellular events that govern patterning during animal development must be precisely regulated. This is achieved by extrinsic factors and through the action of both positive and negative feedback loops. Wnt/Wg signals are crucial across species in many developmental patterning events. We report that Drosophila nemo (nmo) acts as an intracellular feedback inhibitor of Wingless (Wg) and that it is a novel Wg target gene. Nemo antagonizes the activity of the Wg signal, as evidenced by the finding that reduction of nmo rescues the phenotypic defects induced by misexpression of various Wg pathway components. In addition, the activation of Wg-dependent gene expression is suppressed in wing discs ectopically expressing nmo and enhanced cell autonomously in nmo mutant clones. We find that nmo itself is a target of Wg signaling in the imaginal wing disc. nmo expression is induced upon high levels of Wg signaling and can be inhibited by interfering with Wg signaling. Finally, we observe alterations in Arm stabilization upon modulation of Nemo. These observations suggest that the patterning mechanism governed by Wg involves a negative feedback circuit in which Wg induces expression of its own antagonist Nemo.

Reading the Hedgehog morphogen gradient by measuring the ratio of bound to unbound Patched protein

Morphogens are 'form-generating' substances that spread from localized sites of production and specify distinct cellular outcomes at different concentrations. A cell's perception of morphogen concentration is thought to be determined by the number of active receptors, with inactive receptors making little if any contribution. Patched (Ptc), the receptor for the morphogen Hedgehog (Hh), is active in the absence of ligand and blocks the expression of target genes by inhibiting Smoothened (Smo), an essential transducer of the Hh signal. Hh binding to Ptc abrogates the ability of Ptc to inhibit Smo, thereby unleashing Smo activity and inducing target gene expression. Here, we show that a cell's measure of ambient Hh concentration is not determined solely by the number of active (unliganded) Ptc molecules. Instead, we find that Hh-bound Ptc can titrate the inhibitory action of unbound Ptc. Furthermore, we demonstrate that this effect is sufficient to allow normal reading of the Hh gradient in the presence of a form of Ptc that cannot bind the ligand but retains its ability to inhibit Smo. These results support a model in which the ratio of bound to unbound Ptc molecules determines the cellular response to Hh.

Hedgehog Signaling in the Drosophila Eye and Head: An Analysis of the Effects of Differentpatched Trans-heterozygotes

Characterization of different alleles of the Hedgehog receptor patched (ptc) indicates that they can be grouped into several classes. Most mutations result in complete loss of Ptc function. However, missense mutations located within the putative sterol-sensing domain (SSD) or C terminus of ptc encode antimorphic proteins that are unable to repress Smo activity and inhibit wild-type Ptc from doing so, but retain the ability to bind and sequester Hh. Analysis of the eye and head phenotypes of Drosophila melanogaster in various ptc/ptctuf1 heteroallelic combinations shows that these two classes of ptc allele can be easily distinguished by their eye phenotype, but not by their head phenotype. Adult eye size is inversely correlated with head vertex size, suggesting an alteration of cell fate within the eye-antennal disc. A balance between excess cell division and cell death in the mutant eye discs may also contribute to final eye size. In addition, contrary to results reported recently, the role of Hh signaling in the Drosophila head vertex appears to be primarily in patterning rather than in proliferation, with Ptc and Smo having opposing effects on formation of medial structures.

A role for Wnt signalling in self-renewal of haematopoietic stem cells

Haematopoietic stem cells (HSCs) have the ability to renew themselves and to give rise to all lineages of the blood; however, the signals that regulate HSC self-renewal remain unclear. Here we show that the Wnt signalling pathway has an important role in this process. Overexpression of activated beta-catenin expands the pool of HSCs in long-term cultures by both phenotype and function. Furthermore, HSCs in their normal microenvironment activate a LEF-1/TCF reporter, which indicates that HCSs respond to Wnt signalling in vivo. To demonstrate the physiological significance of this pathway for HSC proliferation we show that the ectopic expression of axin or a frizzled ligand-binding domain, inhibitors of the Wnt signalling pathway, leads to inhibition of HSC growth in vitro and reduced reconstitution in vivo. Furthermore, activation of Wnt signalling in HSCs induces increased expression of HoxB4 and Notch1, genes previously implicated in self-renewal of HSCs. We conclude that the Wnt signalling pathway is critical for normal HSC homeostasis in vitro and in vivo, and provide insight into a potential molecular hierarchy of regulation of HSC development.

Regulation of Wingless and Vestigial expression in wing and haltere discs of Drosophila

In the third thoracic segment of Drosophila, wing development is suppressed by the homeotic selector gene Ultrabithorax (Ubx) in order to mediate haltere development. Previously, we have shown that Ubx represses dorsoventral (DV) signaling to specify haltere fate. Here we examine the mechanism of Ubx-mediated downregulation of DV signaling. We show that Wingless (Wg) and Vestigial (Vg) are differentially regulated in wing and haltere discs. In wing discs, although Vg expression in non-DV cells is dependent on DV boundary function of Wg, it maintains its expression by autoregulation. Thus, overexpression of Vg in non-DV cells can bypass the requirement for Wg signaling from the DV boundary. Ubx functions, at least, at two levels to repress Vestigial expression in non-DV cells of haltere discs. At the DV boundary, it functions downstream of Shaggy/GSK3 beta to enhance the degradation of Armadillo (Arm), which causes downregulation of Wg signaling. In non-DV cells, Ubx inhibits event(s) downstream of Arm, but upstream of Vg autoregulation. Repression of Vg at multiple levels appears to be crucial for Ubx-mediated specification of the haltere fate. Overexpression of Vg in haltere discs is enough to override Ubx function and cause haltere-to-wing homeotic transformations.

Design and constraints of the Drosophila segment polarity module: robust spatial patterning emerges from intertwined cell state switches

The Drosophila segment polarity genes constitute the last tier in the segmentation cascade; their job is to maintain the boundaries between parasegments and provide positional "read-outs" within each parasegment for the entire developmental history of the animal. These genes constitute a relatively well-defined network with a relatively well-understood patterning task. In a previous publication (von Dassow et al. 2000. Nature 406:188-192) we showed that a computer model predicts the segment polarity network to be a robust boundary-making device. Here we elaborate those findings. First, we explore the constraints among parameters that govern the network model. Second, we test architectural variants of the core network, and show that the network tolerates a wide variety of adjustments in design. Third, we evaluate several topologically identical models that incorporate more or less molecular detail, finding that more-complex models perform noticeably better than simplified ones. Fourth, we discuss two instances in which the failure of the network model to behave in a life-like fashion highlights mechanistic details that need further experimental investigation. We conclude with an explanation of how the segment polarity network can be understood as an interwoven conspiracy of simple dynamical elements, several bistable switches and a homeostat. The robustness with which the network as a whole maintains a spatial regime of stable cell state emerges from generic dynamical properties of these simple elements.

Different mechanisms initiate and maintain wingless expression in the Drosophila wing hinge

The Drosophila gene wingless encodes a secreted signalling molecule that is required for many patterning events in both embryonic and postembryonic development. In the wing wingless is expressed in a complex and dynamic pattern that is controlled by several different mechanisms. These involve the Hedgehog and Notch pathways and the nuclear proteins Pannier and U-shaped. In this report, we analyse the mechanisms that drive wingless expression in the wing hinge. We present evidence that wingless is initially activated by a secreted signal that requires the genes vestigial, rotund and nubbin. Later in development, wingless expression in the wing hinge is maintained by a different mechanism, which involves an autoregulatory loop and requires the genes homothorax and rotund. We discuss the role of wingless in patterning the wing hinge.

Expression of Wnt ligands and Frizzled receptors in colonic mucosa and in colon carcinoma

AIMS

Signalling through the Wnt pathway is integrally associated with colon carcinogenesis. Although activating mutations in the genes for adenomatous polyposis coli (APC) and beta-catenin are clearly associated with colon cancer, less is understood about the role of the upstream secreted ligands (Wnts) and their receptors (frizzled, Fz) in this process. In other systems, increased Wnt signalling has been shown to alter the expression of components of this pathway. This study was designed to test the hypothesis that colon cancer is characterised by aberrant expression of specific Wnt genes and Fz receptors.

METHODS

The expression of Wnt genes was assessed by in situ, antisense RNA hybridisation in paraffin wax embedded samples of normal and malignant human colon tissues with probes specific for the individual Wnt genes. The expression of Fz1 and Fz2 was determined by immunoperoxidase based antibody staining on human tissues.

RESULTS

Changes in the expression of some ligands and receptors were seen in colon cancer. For example, Wnt2 mRNA was detected in colon cancer but was undetectable in normal colonic mucosa. Differential expression of Wnt5a in normal mucosa was also noted, with increased expression at the base of the crypts compared with the luminal villi and slightly increased expression in colon cancer. Wnt7a exhibited minimal expression in both normal and malignant colon tissues, whereas other Wnt ligands including Wnts 1, 4, 5b, 6, 7b, and 10b were expressed equally and strongly in both normal and malignant colon tissues. In defining cellular responses and phenotype, the type and distribution of Fz receptors may be as important as the pattern of Wnt ligand expression. No expression of Fz receptor 1 and 2 was seen in normal colonic mucosa and in well differentiated tumours. However, poorly differentiated tumours exhibited a high degree of Fz receptor expression, especially at the margin of cellular invasion.

CONCLUSIONS

These data indicate that the expression of members of the Wnt signal transduction pathway, distinct from APC and beta-catenin, is integrally associated with the process of colon carcinogenesis. Wnt2, and possibly Wnt5a, may be involved in the progression from normal mucosa to cancer and the expression of Fz1/2 receptors may be involved in processes associated with tumour invasion. Altered expression of these Wnts and Fz receptors may prove useful as prognostic or diagnostic markers for patients with colon cancer.

The transcriptional repressor Brinker antagonizes Wingless signaling

In the embryonic midgut of Drosophila, Wingless (Wg) signaling elicits threshold-specific transcriptional response, that is, low-signaling levels activate target genes, whereas high-signaling levels repress them. Wg-mediated repression of the HOX gene Ultrabithorax (Ubx) is conferred by a response sequence within the Ubx B midgut enhancer, called WRS-R. It further depends on the Teashirt (Tsh) repressor, which acts through the WRS-R without binding to it. Here, we show that Wg-mediated repression of Ubx B depends on Brinker, which binds to the WRS-R. Furthermore, Brinker blocks transcriptional activation by ubiquitous Wg signaling. Brinker binds to Tsh in vitro, recruits Tsh to the WRS-R, and we find mutual physical interactions between Brinker, Tsh, and the corepressor dCtBP. This suggests that the three proteins may form a ternary repressor complex at the WRS-R to quench the activity of the nearby-bound dTCF/Armadillo transcription complex. Finally, brinker and tsh produce similar mutant phenotypes in the ventral epidermis, and double mutants mimic overactive Wg signaling in this tissue. This suggests that Brinker may have a widespread function in antagonizing Wg signaling.

Pygopus, a nuclear PHD-finger protein required for Wingless signaling inDrosophila

The secreted glycoprotein Wingless (Wg) acts through a conserved signaling pathway to regulate target gene expression. Wg signaling causes nuclear translocation of Armadillo, the fly β-catenin, which then complexes with the DNA-binding protein TCF, enabling it to activate transcription. Though many nuclear factors have been implicated in modulating TCF/Armadillo activity, their importance remains poorly understood. This work describes a ubiquitously expressed protein, called Pygopus, which is required for Wg signaling throughout Drosophila development. Pygopus contains a PHD finger at its C terminus, a motif often found in chromatin remodeling factors. Overexpression of pygopus also blocks the pathway, consistent with the protein acting in a complex. The pygopus mutant phenotype is highly, though not exclusively, specific for Wg signaling. Epistasis experiments indicate that Pygopus acts downstream of Armadillo nuclear import, consistent with the nuclear location of heterologously expressed protein. Our data argue strongly that Pygopus is a new core component of the Wg signaling pathway that acts downstream or at the level of TCF.

A spontaneous mouse mutation, mesenchymal dysplasia (mes), is caused by a deletion of the most C-terminal cytoplasmic domain of patched (ptc)

A recessive mouse mutation, mesenchymal dysplasia (mes), which arose spontaneously on Chromosome 13, causes excess skin, increased body weight, and mild preaxial polydactyly. Fine gene mapping in this study indicated that mes is tightly linked to patched (ptc) that encodes a transmembrane receptor protein for Shh. Molecular characterization of the ptc gene of the mes mutant and an allelism test using a ptc knockout allele (ptc(-)) demonstrated that mes is caused by a deletion of the most C-terminal cytoplasmic domain of the ptc gene. Since mes homozygous embryos exhibit normal spinal cord development as compared with ptc(-) homozygotes, which die around 10 dpc with severe neural tube defects, the C-terminal cytoplasmic domain lost in mes mutation is dispensable for inhibition of Shh signaling in early embryogenesis. However, compound heterozygotes of ptc(-) and mes alleles, which survive up to birth and die neonatally, had increased body weight and exhibited abnormal anteroposterior axis formation of the limb buds. These findings indicate that Ptc is a negative regulator of body weight and ectopic activation of Shh signaling in the anterior mesenchyme of the limb buds, and that the C-terminal cytoplasmic domain of Ptc is involved in its repressive action.

Studies on human colon cancer gene APC by targeted expression in Drosophila

Mutations in human Adenomatous Polyposis Coli (APC) gene are associated with both familial and sporadic colorectal tumors. APC is known to down regulate beta-catenin levels, a transducer of Wnt signaling. The aim of this study is to provide transgenic Drosophila expressing either full-length or truncated forms of human APC (hAPC) protein and methods for using them in functional genomics and drug screening. Consistent with its biochemical properties, targeted expression of either full-length hAPC or its beta-catenin binding domain alone negatively regulated the function of the beta-catenin homologue, Armadillo (Arm) and thereby, inhibited Wnt/Wg signaling during fly development. hAPC inhibited Arm function even in the absence of GSK-3beta activity, although the latter was required to mediate the degradation of Arm. Consistent with this, hAPC suppressed the phenotypes induced by the over-expression of degradation-resistant forms of Arm. Subsequently, using hAPC-induced eye phenotypes as the assay in a suppressor-enhancer screen, we have identified two new loci in Drosophila, which modulate Wnt/Wg signaling. In addition, an anti-colon cancer drug, indomethacin, specifically enhanced hAPC-induced phenotypes.

Successive specification of Drosophila neuroblasts NB 6-4 and NB 7-3 depends on interaction of the segment polarity genes wingless, gooseberry and naked cuticle

The Drosophila central nervous system derives from neural precursor cells, the neuroblasts (NBs), which are born from the neuroectoderm by the process of delamination. Each NB has a unique identity, which is revealed by the production of a characteristic cell lineage and a specific set of molecular markers it expresses. These NBs delaminate at different but reproducible time points during neurogenesis (S1-S5) and it has been shown for early delaminating NBs (S1/S2) that their identities depend on positional information conferred by segment polarity genes and dorsoventral patterning genes. We have studied mechanisms leading to the fate specification of a set of late delaminating neuroblasts, NB 6-4 and NB 7-3, both of which arise from the engrailed (en) expression domain, with NB 6-4 delaminating first. In contrast to former reports, we did not find any evidence for a direct role of hedgehog in the process of NB 7-3 specification. Instead, we present evidence to show that the interplay of the segmentation genes naked cuticle (nkd) and gooseberry (gsb), both of which are targets of wingless (wg) activity, leads to differential commitment to NB 6-4 and NB 7-3 cell fate. In the absence of either nkd or gsb, one NB fate is replaced by the other. However, the temporal sequence of delamination is maintained, suggesting that formation and specification of these two NBs are under independent control.

Apical localization of wingless transcripts is required for wingless signaling

Many developing and adult tissues are comprised of polarized epithelia. Proteins that are asymmetrically distributed in these cells are thought to be localized by protein trafficking. Here we show that the distribution and function of the signaling protein Wingless is predetermined by the subcellular localization of its mRNA. High-resolution in situ hybridization reveals apical transcript localization in the majority of tissues examined. This localization is mediated by two independently acting elements in the 3' UTR. Replacement of these elements with non- or basolaterally localizing elements yields proteins with altered intracellular and extracellular distributions and reduced signaling activities. This novel aspect of the wingless signaling pathway is conserved and may prove to be a mechanism used commonly for establishing epithelial cell polarity.

Wingless effects mesoderm patterning and ectoderm segmentation events via induction of its downstream target sloppy paired

Inactivation of either the secreted protein Wingless (Wg) or the forkhead domain transcription factor Sloppy Paired (Slp) has been shown to produce similar effects in the developing Drosophila embryo. In the ectoderm, both gene products are required for the formation of the segmental portions marked by naked cuticle. In the mesoderm, Wg and Slp activities are crucial for the suppression of bagpipe (bap), and hence visceral mesoderm formation, and the promotion of somatic muscle and heart formation within the anterior portion of each parasegment. In this report, we show that, during these developmental processes, wg and slp act in a common pathway in which slp serves as a direct target of Wg signals that mediates Wg effects in both germ layers. We present evidence that the induction of slp by Wg involves binding of the Wg effector Pangolin (Drosophila Lef-1/TCF) to multiple binding sites within a Wg-responsive enhancer that is located in 5′ flanking regions of the slp1 gene. Based upon our genetic and molecular analysis, we conclude that Wg signaling induces striped expression of Slp in the mesoderm. Mesodermal Slp is then sufficient to abrogate the induction of bagpipe by Dpp/Tinman, which explains the periodic arrangement of trunk visceral mesoderm primordia in wild type embryos. Conversely, mesodermal Slp is positively required, although not sufficient, for the specification of somatic muscle and heart progenitors. We propose that Wg-induced slp provides striped mesodermal domains with the competence to respond to subsequent slp-independent Wg signals that induce somatic muscle and heart progenitors. We also propose that in wg-expressing ectodermal cells, slp is an integral component in an autocrine feedback loop of Wg signaling.

Cubitus interruptus-independent transduction of the Hedgehog signal in Drosophila

The Hedgehog (Hh) family of secreted proteins are key factors that control pattern formation in invertebrates and vertebrates. The manner in which Hh molecules regulate a target cell remains poorly understood. In the Drosophila embryo, Hh is produced in identical stripes of cells in the posterior compartment of each segment. From these cells a Hh signal acts in both anterior and posterior directions. In the anterior cells, the target genes wingless and patched are activated whereas posterior cells respond to Hh by expressing rhomboid and patched. Here, we have examined the role of the transcription factor Cubitus interruptus (Ci) in this process. So far, Ci has been thought to be the most downstream component of the Hh pathway capable of activating all Hh functions. However, our current study of a null ci allele, indicates that it is actually not required for all Hh functions. Whereas Hh and Ci are both required for patched expression, the target genes wingless and rhomboid have unequal requirements for Hh and Ci activity. Hh is required for the maintenance of wingless expression before embryonic stage 11 whereas Ci is necessary only later during stage 11. For rhomboid expression Hh is required positively whereas Ci exhibits negative input. These results indicate that factors other than Ci are necessary for Hh target gene regulation. We present evidence that the zinc-finger protein Teashirt is one candidate for this activity. We show that it is required positively for rhomboid expression and that Teashirt and Ci act in a partially redundant manner before stage 11 to maintain wingless expression in the trunk.

Altered HOX and WNT7A expression in human lung cancer

HOX genes encode transcription factors that control patterning and cell fates. Alterations in HOX expression have been clearly implicated in leukemia, but their role in most other malignant diseases remains unknown. By using degenerate reverse transcription-PCR and subsequent real-time quantitative assays, we examined HOX expression in lung cancer cell lines, direct tumor-control pairs, and bronchial epithelial cultures. As in leukemia, genes of the HOX9 paralogous group and HOXA10 were frequently overexpressed. For HOXB9, we confirmed that elevated RNA was associated with protein overexpression. In some cases, marked HOX overexpression was associated with elevated FGF10 and FGF17. During development, the WNT pathway affects cell fate, polarity, and proliferation, and WNT7a has been implicated in the maintenance of HOX expression. In contrast to normal lung and mortal short-term bronchial epithelial cultures, WNT7a was frequently reduced or absent in lung cancers. In immortalized bronchial epithelial cells, WNT7a was lost concomitantly with HOXA1, and a statistically significant correlation between the expression of both genes was observed in lung cancer cell lines. Furthermore, we identified a homozygous deletion of beta-catenin in the mesothelioma, NCI-H28, associated with reduced WNT7a and the lowest overall cell line expression of HOXA1, HOXA7, HOXA9, and HOXA10, whereas HOXB9 levels were unaffected. Of note, both WNT7a and beta-catenin are encoded on chromosome 3p, which undergoes frequent loss of heterozygosity in these tumors. Our results suggest that alterations in regulatory circuits involving HOX, WNT, and possibly fibroblast growth factor pathways occur frequently in lung cancer.

Differential regulation of glycogen synthase kinase 3beta by insulin and Wnt signaling

Glycogen synthase kinase 3beta (GSK3beta) is a key component in many biological processes including insulin and Wnt signaling. Since the activation of each signaling pathway results in a decrease in GSK3beta activity, we examined the specificity of their downstream effects in the same cell type. Insulin induces an increased activity of glycogen synthase but has no influence on the protein level of beta-catenin. In contrast, Wnt increases the cytosolic pool of beta-catenin but not glycogen synthase activity. We found that, unlike insulin, neither the phosphorylation status of the serine9 residue of GSK3beta nor the activity of protein kinase B is regulated by Wnt. Although the decrease in GSK3beta activity is required, GSK3beta may not be the limiting component for Wnt signaling in the cells that we examined. Our results suggest that the axin-conductin complexed GSK3beta may be dedicated to Wnt rather than insulin signaling. Insulin and Wnt pathways regulate GSK3beta through different mechanisms, and therefore lead to distinct downstream events.

Regulation of cyclooxygenase-2 and periostin by Wnt-3 in mouse mammary epithelial cells

Wnt family members are critical in developmental processes and have been shown to promote carcinogenesis when ectopically expressed in the mouse mammary gland. The gene expression pattern mediated by Wnt is pivotal for these diverse responses. The Wnt pathway has been conserved among different species. Genetic studies have shown that Wnt effects are mediated, at least in part, by beta-catenin, which regulates transcription of "downstream genes." Wnt stimulation inactivates glycogen-synthase kinase-3beta (GSK-3) with subsequent stabilization of beta-catenin, which after heterodimerizing with lymphocyte enhancer factor-1/T-cell factor cofactors stimulates transcription. To establish whether Wnt-stimulated transcription is mediated solely by beta-catenin, a comparison was made of gene expression profiles in response to Wnt-3, overexpression of beta-catenin, and inhibition of GSK-3. Infection of cells with Wnt-3 and inhibition of GSK-3 regulate a set of genes that include cyclooxygenase-2 and periostin. Interestingly, overexpression of beta-catenin or reducing beta-catenin levels with antisense oligonucleotide transfection did not have any effect on cyclooxygenase-2 or periostin expression, thereby defining a Wnt pathway, which cannot be mimicked by beta-catenin overexpression.

Origin of the precerebellar system

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

Hedgehog signal transduction in the posterior compartment of the Drosophila wing imaginal disc

Drosophila Hedgehog (Hh) is secreted by Posterior (P) compartment cells and induces Anterior (A) cells to create a developmental organizer at the AP compartment border. Hh signaling converts Fused (Fu) to a hyperphosphorylated form, Fu*. We show that A border cells of wing imaginal discs contain Fu*. Unexpectedly, P cells also produce Fu*, in a Hh-dependent and Ptc-independent manner. Increasing Ptc, the putative Hh receptor expressed specifically by A cells, reduced Fu*. These results are consistent with proposals that Ptc downregulates Hh signaling and suggest that a receptor other than Ptc mediates Hh signaling in P cells of imaginal discs. We conclude that Hh signals in these P cells and that the outputs of the pathway are blocked by transcriptional repression.

Posttranscriptional regulation of smoothened is part of a self-correcting mechanism in the Hedgehog signaling system

Hedgehog signaling, mediated through its Patched-Smoothened receptor complex, is essential for pattern formation in animal development. Activating mutations within Smoothened have been associated with basal cell carcinoma, suggesting that smoothened is a protooncogene. Thus, regulation of Smoothened levels might be critical for normal development. We show that Smoothened protein levels in Drosophila embryos are regulated posttranscriptionally by a mechanism dependent on Hedgehog signaling but not on its nuclear effector Cubitus interruptus. Hedgehog signaling upregulates Smoothened levels, which are otherwise downregulated by Patched. Demonstrating properties of a self-correcting system, the Hedgehog signaling pathway adjusts the concentrations of Smoothened and Patched to each other and to that of the Hedgehog signal, which ensures that activation of Hedgehog target genes by Smoothened signaling becomes strictly dependent on Hedgehog.