Par3/bazooka binds NICD and promotes notch signaling during Drosophila development.

The conserved bazooka (baz/par3) gene acts as a key regulator of asymmetrical cell divisions across the animal kingdom. Associated Par3/Baz-Par6-aPKC protein complexes are also well known for their role in the establishment of apical/basal cell polarity in epithelial cells. Here we define a novel, positive function of Baz/Par3 in the Notch pathway. Using Drosophila wing and eye development, we demonstrate that Baz is required for Notch signaling activity and optimal transcriptional activation of Notch target genes. Baz appears to act independently of aPKC in these contexts, as knockdown of aPKC does not cause Notch loss-of-function phenotypes. Using transgenic Notch constructs, our data positions Baz activity downstream of activating Notch cleavage steps and upstream of Su(H)/CSL transcription factor complex activity on Notch target genes. We demonstrate a biochemical interaction between NICD and Baz, suggesting that Baz is required for NICD activity before NICD binds to Su(H). Taken together, our data define a novel role of the polarity protein Baz/Par3, as a positive and direct regulator of Notch signaling through its interaction with NICD.

Wg/Wnt-signaling-induced nuclear translocation of ß-catenin is attenuated by a ß-catenin peptide through its interference with the IFT-A complex.

Wnt/Wingless (Wg) signaling is critical in development and disease, including cancer. Canonical Wnt signaling is mediated by ß-catenin/Armadillo (Arm in Drosophila) transducing signals to the nucleus, with IFT-A/Kinesin 2 complexes promoting nuclear translocation of ß-catenin/Arm. Here, we demonstrate that a conserved small N-terminal Arm34-87/ß-catenin peptide binds to IFT140, acting as a dominant interference tool to attenuate Wg/Wnt signaling in vivo. Arm34-87 expression antagonizes endogenous Wnt/Wg signaling, resulting in the reduction of its target expression. Arm34-87 inhibits Wg/Wnt signaling by interfering with nuclear translocation of endogenous Arm/ß-catenin, and this can be modulated by levels of wild-type ß-catenin or IFT140, with the Arm34-87 effect being enhanced or suppressed. Importantly, this mechanism is conserved in mammals with the equivalent ß-catenin24-79 peptide blocking nuclear translocation and pathway activation, including in cancer cells. Our work indicates that Wnt signaling can be regulated by a defined N-terminal ß-catenin peptide and thus might serve as an entry point for therapeutic applications to attenuate Wnt/ß-catenin signaling.

The complex relationship of Wnt-signaling pathways and cilia.

Wnt family proteins are secreted glycolipoproteins that signal through multitude of signal transduction pathways. The Wnt-pathways are conserved and critical in all metazoans. They are essential for embryonic development, organogenesis and homeostasis, and associated with many diseases when defective or deregulated. Wnt signaling pathways comprise the canonical Wnt pathway, best known for its stabilization of ß-catenin and associated nuclear ß-catenin activity in gene regulation, and several non-canonical signaling branches. Wnt-Planar Cell Polarity (PCP) signaling has received the most attention among the non-canonical Wnt pathways. The relationship of cilia to Wnt-signaling is complex. While it was suggested that canonical Wnt signaling requires cilia this notion was always challenged by results suggesting the opposite. Recent developments provide insight and clarification to the relationship of Wnt signaling pathways and cilia. First, it has been now demonstrated that while ciliary proteins, in particular the IFT-A complex, are required for canonical Wnt/ß-catenin signaling, the cilium as a structure is not. In contrast, recent work has defined a diverged canonical signaling branch (not affecting ß-catenin) to be required for ciliary biogenesis and cilia function. Furthermore, the non-canonical Wnt-PCP pathway does not affect cilia biogenesis per se, but it regulates the position of cilia within cells in many cell types, possibly in all cells where it is active, with cilia being placed near the side of the cell that has the Frizzled-Dishevelled complex. This Wnt/PCP feature is conserved with both centrioles and basal bodies/cilia being positioned accordingly, and it is also used to align mitotic spindles within the Wnt-PCP polarization axis. It also coordinates the alignment of cilia in multiciliated cells. This article addresses these new insights and different links and relationships between cilia and Wnt signaling.

Wg/Wnt-signaling induced nuclear translocation of ß-catenin is attenuated by a ß-catenin peptide through its interaction with IFT-A in development and cancer cells.

Wnt/Wingless (Wg) signaling is critical for many developmental patterning processes and linked to diseases, including cancer. Canonical Wnt-signaling is mediated by ß-catenin, Armadillo/Arm in Drosophila transducing signal activation to a nuclear response. The IFT-A/Kinesin-2 complex is required to promote the nuclear translocation of ß-catenin/Arm. Here, we define a small conserved N-terminal Arm/ß-catenin (Arm 34-87 ) peptide, which binds IFT140, as a dominant interference tool to attenuate Wg/Wnt-signaling in vivo . Expression of Arm 34-87 is sufficient to antagonize endogenous Wnt/Wg-signaling activation resulting in marked reduction of Wg-signaling target gene expression. This effect is modulated by endogenous levels of Arm and IFT140, with the Arm 34-87 effect being enhanced or suppressed, respectively. Arm 34-87 thus inhibits Wg/Wnt-signaling by interfering with the nuclear translocation of endogenous Arm/ß-catenin. Importantly, this mechanism is conserved in mammals with the equivalent ß-catenin 34-87 peptide blocking nuclear translocation and pathway activation, including in cancer cells. Our work indicates that Wnt-signaling can be regulated by a defined N-terminal peptide of Arm/ß-catenin, and thus this might serve as an entry point for potential therapeutic applications to attenuate Wnt/ß-catenin signaling.

A Van Gogh/Vangl tyrosine phosphorylation switch regulates its interaction with core Planar Cell Polarity factors Prickle and Dishevelled.

Epithelial tissues can be polarized along two axes: in addition to apical-basal polarity they are often also polarized within the plane of the epithelium, known as planar cell polarity (PCP). PCP depends upon the conserved Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl in mammals). Here, taking advantage of the complementary features of Drosophila wing and mouse skin PCP establishment, we dissect how Vang/Vangl phosphorylation on a specific conserved tyrosine residue affects its interaction with two cytoplasmic core PCP factors, Dishevelled (Dsh/Dvl1-3 in mammals) and Prickle (Pk/Pk1-3). We demonstrate that Pk and Dsh/Dvl bind to Vang/Vangl in an overlapping region centered around this tyrosine. Strikingly, Vang/Vangl phosphorylation promotes its binding to Prickle, a key effector of the Vang/Vangl complex, and inhibits its interaction with Dishevelled. Thus phosphorylation of this tyrosine appears to promote the formation of the mature Vang/Vangl-Pk complex during PCP establishment and conversely it inhibits the Vang interaction with the antagonistic effector Dishevelled. Intriguingly, the phosphorylation state of this tyrosine might thus serve as a switch between transient interactions with Dishevelled and stable formation of Vang-Pk complexes during PCP establishment.

The beauty of seeing the real thing: BMP heterodimer detection in vivo reveals dimer composition and activity.

BMP family ligands can direct cells to divide, differentiate, or die, depending on cell context and specific hetero- or homodimer combinations. In this issue of Developmental Cell, Bauer et al. detect endogenous Drosophila ligand dimers in situ and show that BMP dimer composition affects both signaling range and activity.

Notch-dependent Abl signaling regulates cell motility during ommatidial rotation in Drosophila.

A collective cell motility event that occurs during Drosophila eye development, ommatidial rotation (OR), serves as a paradigm for signaling-pathway-regulated directed movement of cell clusters. OR is instructed by the EGFR and Notch pathways and Frizzled/planar cell polarity (Fz/PCP) signaling, all of which are associated with photoreceptor R3 and R4 specification. Here, we show that Abl kinase negatively regulates OR through its activity in the R3/R4 pair. Abl is localized to apical junctional regions in R4, but not in R3, during OR, and this apical localization requires Notch signaling. We demonstrate that Abl and Notch interact genetically during OR, and Abl co-immunoprecipitates in complexes with Notch in eye discs. Perturbations of Abl interfere with adherens junctional organization of ommatidial preclusters, which mediate the OR process. Together, our data suggest that Abl kinase acts directly downstream of Notch in R4 to fine-tune OR via its effect on adherens junctions.

The conserved Pelado/ZSWIM8 protein regulates actin dynamics by promoting linear actin filament polymerization.

Actin filament polymerization can be branched or linear, which depends on the associated regulatory proteins. Competition for actin monomers occurs between proteins that induce branched or linear actin polymerization. Cell specialization requires the regulation of actin filaments to allow the formation of cell type-specific structures, like cuticular hairs in <i>Drosophila</i>, formed by linear actin filaments. Here, we report the functional analysis of CG34401/<i>pelado</i>, a gene encoding a SWIM domain-containing protein, conserved throughout the animal kingdom, called ZSWIM8 in mammals. Mutant <i>pelado</i> epithelial cells display actin hair elongation defects. This phenotype is reversed by increasing actin monomer levels or by either pushing linear actin polymerization or reducing branched actin polymerization. Similarly, in hemocytes, Pelado is essential to induce filopodia, a linear actin-based structure. We further show that this function of Pelado/ZSWIM8 is conserved in human cells, where Pelado inhibits branched actin polymerization in a cell migration context. In summary, our data indicate that the function of Pelado/ZSWIM8 in regulating actin cytoskeletal dynamics is conserved, favoring linear actin polymerization at the expense of branched filaments.

Wnt-frizzled planar cell polarity signaling in the regulation of cell motility.

The molecular complexes underlying planar cell polarity (PCP) were first identified in Drosophila through analysis of mutant phenotypes in the adult cuticle and the orientation of associated polarized protrusions such as wing hairs and sensory bristles. The same molecules are conserved in vertebrates and are required for the localization of polarized protrusions such as primary or sensory cilia and the orientation of hair follicles. Not only is PCP signaling required to align cellular structures across a tissue, it is also required to coordinate movement during embryonic development and adult homeostasis. PCP signaling allows cells to interpret positional cues within a tissue to move in the appropriate direction and to coordinate this movement with their neighbors. In this review we outline the molecular basis of the core Wnt-Frizzled/PCP pathway, and describe how this signaling orchestrates collective motility in Drosophila and vertebrates. Here we cover the paradigms of ommatidial rotation and border cell migration in Drosophila, and convergent extension in vertebrates. The downstream cell biological processes that underlie polarized motility include cytoskeletal reorganization, and adherens junctional and extracellular matrix remodeling. We discuss the contributions of these processes in the respective cell motility contexts. Finally, we address examples of individual cell motility guided by PCP factors during nervous system development and in cancer disease contexts.

Different strategies by distinct Wnt-signaling pathways in activating a nuclear transcriptional response.

The Wnt family of secreted glycolipo-proteins signals through multiple signal transduction pathways and is essential for embryonic development and organ development and homeostasis. The Wnt-pathways are conserved and critical in all metazoans. Wnt signaling pathways comprise the canonical Wnt/ß-catenin pathway and several non-canonical signaling branches, of which Wnt-Planar Cell Polarity (PCP) signaling and the Wnt/Calcium pathway have received the most attention and are best understood. nterestingly, all Wnt-pathways have a nuclear signaling branch and also can affect many cellular processes independent of its nuclear transcriptional regulation. Canonical Wnt/ß-catenin signaling is the most critical for a nuclear transcriptional response, in both development and disease, yet the mechanism(s) on how the "business end" of the pathway, ß-catenin, translocates to the nucleus to act as co-activator to the TCF/Lef transcription factor family still remains obscure. Here we discuss and compare the very different strategies on how the respective Wnt signaling pathways activate a nuclear transcriptional response. We also highlight some recent new insights into how ß-catenin is translocated to the nucleus via an IFT-A, Kinesin-2, and microtubule dependent mechanism and how this aspect of canonical Wnt-signaling uses ciliary proteins in a cilium independent manner, conserved between Drosophila and mammalian cells.

When cells get in the flow.

New imaging approaches question a long-standing model for how the eyes of fruit flies acquire their geometric patterning.

Tissue fluidity mediated by adherens junction dynamics promotes planar cell polarity-driven ommatidial rotation.

The phenomenon of tissue fluidity-cells' ability to rearrange relative to each other in confluent tissues-has been linked to several morphogenetic processes and diseases, yet few molecular regulators of tissue fluidity are known. Ommatidial rotation (OR), directed by planar cell polarity signaling, occurs during Drosophila eye morphogenesis and shares many features with polarized cellular migration in vertebrates. We utilize in vivo live imaging analysis tools to quantify dynamic cellular morphologies during OR, revealing that OR is driven autonomously by ommatidial cell clusters rotating in successive pulses within a permissive substrate. Through analysis of a rotation-specific nemo mutant, we demonstrate that precise regulation of junctional E-cadherin levels is critical for modulating the mechanical properties of the tissue to allow rotation to progress. Our study defines Nemo as a molecular tool to induce a transition from solid-like tissues to more viscoelastic tissues broadening our molecular understanding of tissue fluidity.

Phosphatidic acid increases Notch signalling by affecting Sanpodo trafficking during Drosophila sensory organ development.

Organ cell diversity depends on binary cell-fate decisions mediated by the Notch signalling pathway during development and tissue homeostasis. A clear example is the series of binary cell-fate decisions that take place during asymmetric cell divisions that give rise to the sensory organs of Drosophila melanogaster. The regulated trafficking of Sanpodo, a transmembrane protein that potentiates receptor activity, plays a pivotal role in this process. Membrane lipids can regulate many signalling pathways by affecting receptor and ligand trafficking. It remains unknown, however, whether phosphatidic acid regulates Notch-mediated binary cell-fate decisions during asymmetric cell divisions, and what are the cellular mechanisms involved. Here we show that increased phosphatidic acid derived from Phospholipase D leads to defects in binary cell-fate decisions that are compatible with ectopic Notch activation in precursor cells, where it is normally inactive. Null mutants of numb or the α-subunit of Adaptor Protein complex-2 enhance dominantly this phenotype while removing a copy of Notch or sanpodo suppresses it. In vivo analyses show that Sanpodo localization decreases at acidic compartments, associated with increased internalization of Notch. We propose that Phospholipase D-derived phosphatidic acid promotes ectopic Notch signalling by increasing receptor endocytosis and inhibiting Sanpodo trafficking towards acidic endosomes.

Planar cell polarity: moving from single cells to tissue-scale biology.

Planar cell polarity (PCP) reflects cellular orientation within the plane of an epithelium. PCP is crucial during many biological patterning processes and for organ function. It is omnipresent, from convergent-extension mechanisms during early development through to terminal organogenesis, and it regulates many aspects of cell positioning and orientation during tissue morphogenesis, organ development and homeostasis. Suzanne Eaton used the power of Drosophila as a model system to study PCP, but her vision of, and impact on, PCP studies in flies translates to all animal models. As I highlight here, Suzanne's incorporation of quantitative biophysical studies of whole tissues, integrated with the detailed cell biology of PCP phenomena, completely changed how the field studies this intriguing feature. Moreover, Suzanne's impact on ongoing and future PCP studies is fundamental, long-lasting and transformative.

Mutations associated with human neural tube defects display disrupted planar cell polarity in Drosophila.

Planar cell polarity (PCP) and neural tube defects (NTDs) are linked, with a subset of NTD patients found to harbor mutations in PCP genes, but there is limited data on whether these mutations disrupt PCP signaling in vivo. The core PCP gene Van Gogh (Vang), Vangl1/2 in mammals, is the most specific for PCP. We thus addressed potential causality of NTD-associated Vangl1/2 mutations, from either mouse or human patients, in Drosophila allowing intricate analysis of the PCP pathway. Introducing the respective mammalian mutations into Drosophila Vang revealed defective phenotypic and functional behaviors, with changes to Vang localization, post-translational modification, and mechanistic function, such as its ability to interact with PCP effectors. Our findings provide mechanistic insight into how different mammalian mutations contribute to developmental disorders and strengthen the link between PCP and NTD. Importantly, analyses of the human mutations revealed that each is a causative factor for the associated NTD.

As an embryo develops, its cells must work together to build mature tissues and organs. During the formation of the nervous system, for example, a sheet of cells destined to become the brain and spinal cord folds up into a tube spanning the length of the embryo. Normally, this tube ­ known as the 'neural tube' ­ zips up, and the cells that will eventually become skin and other surrounding tissues close in over it. If the neural tube does not close completely, different parts of the spinal cord or brain can remain unprotected. This can cause diseases called neural tube defects, such as spina bifida, which is characterized by holes in the backbone exposing the spinal cord and surrounding membranes. Patients with neural tube defects can have similar genetic mutations, for example, in the genes controlling a process called "planar cell polarity", or PCP for short. Cells arranged in flat sheets use the PCP process to sense direction, and it is this process that allows structures, such as the scales on a fish or the hairs on a mouse, to all point in the same direction. PCP is also important in embryonic development: sheets of cells that can sense direction correctly can therefore move collectively to complete complex tasks (such as closing the neural tube). However, no-one knew whether the specific PCP gene mutations implicated in neural tube defects in humans actually affected the cells' ability to sense direction, or indeed whether they were even involved in causing the diseases. Humphries et al. set out to find out more about these mutations using fruit flies as a model system. The fruit fly is widely used to study the genes and signals involved in direction sensing, especially PCP. Problems with PCP produce easily measurable changes in the wing and eye, showing what went wrong and how badly. Humphries et al. genetically engineered fruit flies to have the same mutations as human patients and revealed that these mutations did indeed alter cells' ability to sense direction. These experiments also showed that each mutation did so in a different way, and with varying severity. This explained why the same mutations caused different levels of neural defects in mice (which are commonly used to study human diseases) and suggests that they might contribute to neural tube disorders in humans. These results show potential connections between neural tube defects and direction sensing in cells. In the future, this study and follow-up work could help researchers to understand what types of mutation have the most impact, which may eventually allow doctors to better predict who is most at risk of being affected by these conditions.

Notch signaling coordinates ommatidial rotation in the Drosophila eye via transcriptional regulation of the EGF-Receptor ligand Argos.

In all metazoans, a small number of evolutionarily conserved signaling pathways are reiteratively used during development to orchestrate critical patterning and morphogenetic processes. Among these, Notch (N) signaling is essential for most aspects of tissue patterning where it mediates the communication between adjacent cells to control cell fate specification. In Drosophila, Notch signaling is required for several features of eye development, including the R3/R4 cell fate choice and R7 specification. Here we show that hypomorphic alleles of Notch, belonging to the Nfacet class, reveal a novel phenotype: while photoreceptor specification in the mutant ommatidia is largely normal, defects are observed in ommatidial rotation (OR), a planar cell polarity (PCP)-mediated cell motility process. We demonstrate that during OR Notch signaling is specifically required in the R4 photoreceptor to upregulate the transcription of argos (aos), an inhibitory ligand to the epidermal growth factor receptor (EGFR), to fine-tune the activity of EGFR signaling. Consistently, the loss-of-function defects of Nfacet alleles and EGFR-signaling pathway mutants are largely indistinguishable. A Notch-regulated aos enhancer confers R4 specific expression arguing that aos is directly regulated by Notch signaling in this context via Su(H)-Mam-dependent transcription.

Regulation of Numb during planar cell polarity establishment in the Drosophila eye.

The establishment of planar cell polarity (PCP) in the Drosophila eye requires correct specification of the R3/R4 pair of photoreceptor cells, determined by a Frizzled mediated signaling event that specifies R3 and induces Delta to activate Notch signaling in the neighboring cell, specifying it as R4. Here, we investigated the role of the Notch signaling negative regulator Numb in the specification of R3/R4 fates and PCP establishment in the Drosophila eye. We observed that Numb is transiently upregulated in R3 at the time of R3/R4 specification. This regulation of Numb levels in developing photoreceptors occurs at the post-transcriptional level and is dependent on Dishevelled, an effector of Frizzled signaling, and Lethal Giant Larva. We detected PCP defects in cells homozygous for numb15, but these defects were due to a loss of function mutation in fat (fatQ805⁎) being present in the numb15 chromosome. However, mosaic overexpression of Numb in R4 precursors (only) caused PCP defects and numb loss-of-function alleles had a modifying effect on the defects found in a hypomorphic dishevelled mutation. Our results suggest that Numb levels are upregulated to reinforce the bias of Notch signaling activation in the R3/R4 pair, two post-mitotic cells that are not specified by asymmetric cell division.

Integrins are required for synchronous ommatidial rotation in the Drosophila eye linking planar cell polarity signalling to the extracellular matrix.

Integrins mediate the anchorage between cells and their environment, the extracellular matrix (ECM), and form transmembrane links between the ECM and the cytoskeleton, a conserved feature throughout development and morphogenesis of epithelial organs. Here, we demonstrate that integrins and components of the ECM are required during the planar cell polarity (PCP) signalling-regulated cell movement of ommatidial rotation in the Drosophila eye. The loss-of-function mutations of integrins or ECM components cause defects in rotation, with mutant clusters rotating asynchronously compared to wild-type clusters. Initially, mutant clusters tend to rotate faster, and at later stages they fail to be synchronous with their neighbours, leading to aberrant rotation angles and resulting in a disorganized ommatidial arrangement in adult eyes. We further demonstrate that integrin localization changes dynamically during the rotation process. Our data suggest that core Frizzled/PCP factors, acting through RhoA and Rho kinase, regulate the function/activity of integrins and that integrins thus contribute to the complex interaction network of PCP signalling, cell adhesion and cytoskeletal elements required for a precise and synchronous 90° rotation movement.

Kinesin-2 and IFT-A act as a complex promoting nuclear localization of ß-catenin during Wnt signalling.

Wnt/Wg-signalling is critical signalling in all metazoans. Recent studies suggest that IFT-A proteins and Kinesin-2 modulate canonical Wnt/Wg-signalling independently of their ciliary role. Whether they function together in Wnt-signalling and their mechanistic role in the pathway remained unresolved. Here we demonstrate that Kinesin-2 and IFT-A proteins act as a complex during Drosophila Wg-signalling, affecting pathway activity in the same manner, interacting genetically and physically, and co-localizing with ß-catenin, the mediator of Wnt/Wg-signalling on microtubules. Following pathway activation, Kinesin-2/IFT-A mutant cells exhibit high cytoplasmic ß-catenin levels, yet fail to activate Wg-targets. In mutant tissues in both, Drosophila and mouse/MEFs, nuclear localization of ß-catenin is markedly reduced. We demonstrate a conserved, motor-domain dependent function of the Kinesin-2/IFT-A complex in promoting nuclear translocation of ß-catenin. We show that this is mediated by protecting ß-catenin from a conserved cytoplasmic retention process, thus identifying a mechanism for Kinesin-2/IFT-A in Wnt-signalling that is independent of their ciliary role.

LZTR1 is a regulator of RAS ubiquitination and signaling.

In genetic screens aimed at understanding drug resistance mechanisms in chronic myeloid leukemia cells, inactivation of the cullin 3 adapter protein-encoding leucine zipper-like transcription regulator 1 (LZTR1) gene led to enhanced mitogen-activated protein kinase (MAPK) pathway activity and reduced sensitivity to tyrosine kinase inhibitors. Knockdown of the Drosophila LZTR1 ortholog CG3711 resulted in a Ras-dependent gain-of-function phenotype. Endogenous human LZTR1 associates with the main RAS isoforms. Inactivation of LZTR1 led to decreased ubiquitination and enhanced plasma membrane localization of endogenous KRAS (V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog). We propose that LZTR1 acts as a conserved regulator of RAS ubiquitination and MAPK pathway activation. Because LZTR1 disease mutations failed to revert loss-of-function phenotypes, our findings provide a molecular rationale for LZTR1 involvement in a variety of inherited and acquired human disorders.