Negative regulation of Armadillo, a Wingless effector in Drosophila

Wnt target gene activation requires β-catenin separation into biomolecular condensates

The Wnt/β-catenin signaling pathway plays numerous essential roles in animal development and tissue/stem cell maintenance. The activation of genes regulated by Wnt/β-catenin signaling requires the nuclear accumulation of β-catenin, a transcriptional co-activator. β-catenin is recruited to many Wnt-regulated enhancers through direct binding to T-cell factor/lymphoid enhancer factor (TCF/LEF) family transcription factors. β-catenin has previously been reported to form phase-separated biomolecular condensates (BMCs), which was implicated as a component of β-catenin's mechanism of action. This function required aromatic amino acid residues in the intrinsically disordered regions (IDRs) at the N- and C-termini of the protein. In this report, we further explore a role for β-catenin BMCs in Wnt target gene regulation. We find that β-catenin BMCs are miscible with LEF1 BMCs in vitro and in cultured cells. We characterized a panel of β-catenin mutants with different combinations of aromatic residue mutations in human cell culture and Drosophila melanogaster. Our data support a model in which aromatic residues across both IDRs contribute to BMC formation and signaling activity. Although different Wnt targets have different sensitivities to loss of β-catenin's aromatic residues, the activation of every target examined was compromised by aromatic substitution. These mutants are not defective in nuclear import or co-immunoprecipitation with several β-catenin binding partners. In addition, residues in the N-terminal IDR with no previously known role in signaling are clearly required for the activation of various Wnt readouts. Consistent with this, deletion of the N-terminal IDR results in a loss of signaling activity, which can be rescued by the addition of heterologous IDRs enriched in aromatic residues. Overall, our work supports a model in which the ability of β-catenin to form biomolecular condensates in the nucleus is tightly linked to its function as a transcriptional co-regulator.

Wg/Wnt-signaling-induced nuclear translocation of β-catenin is attenuated by a β-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 β-catenin/Armadillo (Arm in Drosophila) transducing signals to the nucleus, with IFT-A/Kinesin 2 complexes promoting nuclear translocation of β-catenin/Arm. Here, we demonstrate that a conserved small N-terminal Arm34-87/β-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/β-catenin, and this can be modulated by levels of wild-type β-catenin or IFT140, with the Arm34-87 effect being enhanced or suppressed. Importantly, this mechanism is conserved in mammals with the equivalent β-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 β-catenin peptide and thus might serve as an entry point for therapeutic applications to attenuate Wnt/β-catenin signaling.

Baf-mediated transcriptional regulation of teashirt is essential for the development of neural progenitor cell lineages

Accumulating evidence hints heterochromatin anchoring to the inner nuclear membrane as an upstream regulatory process of gene expression. Given that the formation of neural progenitor cell lineages and the subsequent maintenance of postmitotic neuronal cell identity critically rely on transcriptional regulation, it seems possible that the development of neuronal cells is influenced by cell type-specific and/or context-dependent programmed regulation of heterochromatin anchoring. Here, we explored this possibility by genetically disrupting the evolutionarily conserved barrier-to-autointegration factor (Baf) in the Drosophila nervous system. Through single-cell RNA sequencing, we demonstrated that Baf knockdown induces prominent transcriptomic changes, particularly in type I neuroblasts. Among the differentially expressed genes, our genetic analyses identified teashirt (tsh), a transcription factor that interacts with beta-catenin, to be closely associated with Baf knockdown-induced phenotypes that were suppressed by the overexpression of tsh or beta-catenin. We also found that Baf and tsh colocalized in a region adjacent to heterochromatin in type I NBs. Notably, the subnuclear localization pattern remained unchanged when one of these two proteins was knocked down, indicating that both proteins contribute to the anchoring of heterochromatin to the inner nuclear membrane. Overall, this study reveals that the Baf-mediated transcriptional regulation of teashirt is a novel molecular mechanism that regulates the development of neural progenitor cell lineages.

Wg/Wnt-signaling induced nuclear translocation of β-catenin is attenuated by a β-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 β-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 β-catenin/Arm. Here, we define a small conserved N-terminal Arm/β-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/β-catenin. Importantly, this mechanism is conserved in mammals with the equivalent β-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/β-catenin, and thus this might serve as an entry point for potential therapeutic applications to attenuate Wnt/β-catenin signaling.

Specification of the endocrine primordia controlling insect moulting and metamorphosis by the JAK/STAT signalling pathway

The corpora allata and the prothoracic glands control moulting and metamorphosis in insects. These endocrine glands are specified in the maxillary and labial segments at positions homologous to those forming the trachea in more posterior segments. Glands and trachea can be homeotically transformed into each other suggesting that all three evolved from a metamerically repeated organ that diverged to form glands in the head and respiratory organs in the trunk. While much is known about tracheal specification, there is limited information about corpora allata and prothorathic gland specification. Here we show that the expression of a key regulator of early gland development, the snail gene, is controlled by the Dfd and Scr Hox genes and by the Hedgehog and Wnt signalling pathways that induce localised transcription of upd, the ligand of the JAK/STAT signalling pathway, which lies at the heart of gland specification. Our results show that the same upstream regulators are required for the early gland and tracheal primordia specification, reinforcing the hypothesis that they originated from a segmentally repeated organ present in an ancient arthropod.

Early patterning followed by tissue growth establishes distal identity in Drosophila Malpighian tubules

Specification and elaboration of proximo-distal (P-D) axes for structures or tissues within a body occurs secondarily from that of the main axes of the body. Our understanding of the mechanism(s) that pattern P-D axes is limited to a few examples such as vertebrate and invertebrate limbs. Drosophila Malpighian/renal tubules (MpTs) are simple epithelial tubules, with a defined P-D axis. How this axis is patterned is not known, and provides an ideal context to understand patterning mechanisms of a secondary axis. Furthermore, epithelial tubules are widespread, and their patterning is not well understood. Here, we describe the mechanism that establishes distal tubule and show this is a radically different mechanism to that patterning the proximal MpT. The distal domain is patterned in two steps: distal identity is specified in a small group of cells very early in MpT development through Wingless/Wnt signalling. Subsequently, this population is expanded by proliferation to generate the distal MpT domain. This mechanism enables distal identity to be established in the tubule in a domain of cells much greater than the effective range of Wingless.

ARMADILLO REGULATES NOCICEPTIVE SENSITIVITY IN THE ABSENCE OF INJURY

Abnormal pain has recently been estimated to affect ∼50 million adults each year within the United States. With many treatment options for abnormal pain, such as opioid analgesics, carrying numerous deleterious side effects, research into safer and more effective treatment options is crucial. To help elucidate the mechanisms controlling nociceptive sensitivity, the Drosophila melanogaster larval nociception model has been used to characterize well-conserved pathways through the use of genetic modification and/or injury to alter the sensitivity of experimental animals. Mammalian models have provided evidence of β-catenin signaling involvement in neuropathic pain development. By capitalizing on the conserved nature of β-catenin functions in the fruit fly, here we describe a role for Armadillo, the fly homolog to mammalian β-catenin, in regulating baseline sensitivity in the primary nociceptor of the fly, in the absence of injury, using under- and over-expression of Armadillo in a cell-specific manner. Underexpression of Armadillo resulted in hyposensitivity, while overexpression of wild-type Armadillo or expression of a degradation-resistant Armadillo resulted in hypersensitivity. Neither underexpression nor overexpression of Armadillo resulted in observed dendritic morphological changes that could contribute to behavioral phenotypes observed. These results showed that focused manipulation of Armadillo expression within the nociceptors is sufficient to modulate baseline response in the nociceptors to a noxious stimulus and that these changes are not shown to be associated with a morphogenetic effect.

γ-secretase promotes Drosophila postsynaptic development through the cleavage of a Wnt receptor

Developing synapses mature through the recruitment of specific proteins that stabilize presynaptic and postsynaptic structure and function. Wnt ligands signaling via Frizzled (Fz) receptors play many crucial roles in neuronal and synaptic development, but whether and how Wnt and Fz influence synaptic maturation is incompletely understood. Here, we show that Fz2 receptor cleavage via the γ-secretase complex is required for postsynaptic development and maturation. In the absence of γ-secretase, Drosophila neuromuscular synapses fail to recruit postsynaptic scaffolding and cytoskeletal proteins, leading to behavioral deficits. Introducing presenilin mutations linked to familial early-onset Alzheimer's disease into flies leads to synaptic maturation phenotypes that are identical to those seen in null alleles. This conserved role for γ-secretase in synaptic maturation and postsynaptic development highlights the importance of Fz2 cleavage and suggests that receptor processing by proteins linked to neurodegeneration may be a shared mechanism with aspects of synaptic development.

Canonical Wnt Signaling Promotes Formation of Somatic Permeability Barrier for Proper Germ Cell Differentiation

Morphogen-mediated signaling is critical for proper organ development and stem cell function, and well-characterized mechanisms spatiotemporally limit the expression of ligands, receptors, and ligand-binding cell-surface glypicans. Here, we show that in the developing Drosophila ovary, canonical Wnt signaling promotes the formation of somatic escort cells (ECs) and their protrusions, which establish a physical permeability barrier to define morphogen territories for proper germ cell differentiation. The protrusions shield germ cells from Dpp and Wingless morphogens produced by the germline stem cell (GSC) niche and normally only received by GSCs. Genetic disruption of EC protrusions allows GSC progeny to also receive Dpp and Wingless, which subsequently disrupt germ cell differentiation. Our results reveal a role for canonical Wnt signaling in specifying the ovarian somatic cells necessary for germ cell differentiation. Additionally, we demonstrate the morphogen-limiting function of this physical permeability barrier, which may be a common mechanism in other organs across species.

Arf6 is necessary for senseless expression in response to wingless signalling during Drosophila wing development

Wnt signalling is a core pathway involved in a wide range of developmental processes throughout the metazoa. In vitro studies have suggested that the small GTP binding protein Arf6 regulates upstream steps of Wnt transduction, by promoting the phosphorylation of the Wnt co-receptor, LRP6, and the release of β-catenin from the adherens junctions. To assess the relevance of these previous findings in vivo, we analysed the consequence of the absence of Arf6 activity on Drosophila wing patterning, a developmental model of Wnt/Wingless signalling. We observed a dominant loss of wing margin bristles and Senseless expression in Arf6 mutant flies, phenotypes characteristic of a defect in high level Wingless signalling. In contrast to previous findings, we show that Arf6 is required downstream of Armadillo/β-catenin stabilisation in Wingless signal transduction. Our data suggest that Arf6 modulates the activity of a downstream nuclear regulator of Pangolin activity in order to control the induction of high level Wingless signalling. Our findings represent a novel regulatory role for Arf6 in Wingless signalling.

Role of Armadillo repeat 2 and kinesin-II motor subunit Klp64D for wingless signaling in Drosophila

Armadillo (Arm) is crucial for transducing Wingless (Wg) signaling. Previously, we have shown that Klp64D, a motor subunit of Drosophila kinesin-II, interacts with Arm for Wg signaling. Molecular basis for this interaction has remained unknown. Here we identify a critical Arm repeat (AR) required for binding Klp64D and Wg signaling. Arm/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varvec{\beta}}$$\end{document}β-catenin family proteins contain a conserved domain of 12 Arm repeats (ARs). Five of these ARs can interact with Klp64D, but only the second AR (AR2) binds to the cargo/tail domain of Klp64D. Overexpression of AR2 in wing imaginal disc is sufficient to cause notched wing margin. This phenotype by AR2 is enhanced or suppressed by reducing or increasing Klp64D expression, respectively. AR2 overexpression inhibits Wg signaling activity in TopFlash assay, consistent with its dominant-negative effects on Klp64D-dependent Wg signaling. Overexpression of the Klp64D cargo domain also results in dominant-negative wing notching. Genetic rescue data indicate that both AR2 and Klp64D cargo regions are required for the function of Arm and Klp64D, respectively. AR2 overexpression leads to an accumulation of Arm with GM130 Golgi marker in Klp64D knockdown. This study suggests that Wg signaling for wing development is regulated by specific interaction between AR2 and the cargo domain of Klp64D.

Wingless and Archipelago, a fly E3 ubiquitin ligase and a homolog of human tumor suppressor FBW7, show an antagonistic relationship in wing development

BACKGROUND

Archipelago (Ago) is a Drosophila homolog of mammalian F-box and WD repeat domain-containing 7 (FBW7, also known as FBXW7). In previous studies, FBW7 has been addressed as a tumor suppressor mediating ubiquitin-dependent proteolysis of several oncogenic proteins. Ubiquitination is a type of protein modification that directs protein for degradation as well as sorting. The level of beta-catenin (β-cat), an intracellular signal transducer in Wnt signaling pathway, is reduced upon overexpression of FBW7 in human cancer cell lines. Loss of function mutations in FBW7 and overactive Wnt signaling have been reported to be responsible for human cancers.

RESULTS

We found that Ago is important for the formation of shafts in chemosensory bristles at wing margin. This loss of shaft phenotype by knockdown of ago was rescued by knockdown of wingless (wg) whereas wing notching phenotype by knockdown of wg was rescued by knockdown of ago, establishing an antagonistic relationship between ago and wg. In line with this finding, knockdown of ago increased the level of Armadillo (Arm), a homolog of β-cat, in Drosophila tissue. Furthermore, knockdown of ago increased the level of Distal-less (Dll) and extracellular Wg in wing discs. In S2 cells, the amount of secreted Wg was increased by knockdown of Ago but decreased by Ago overexpression. Therefore, Ago plays a previously unidentified role in the inhibition of Wg secretion. Ago-overexpressing clones in wing discs exhibited accumulation of Wg in endoplasmic reticulum (ER), suggesting that Ago prevents Wg protein from moving to Golgi from ER.

CONCLUSIONS

We concluded that Ago plays dual roles in inhibiting Wg signaling. First, Ago decreases the level of Arm, by which Wg signaling is downregulated in Wg-responding cells. Second, Ago decreases the level of extracellular Wg by inhibiting movement of Wg from ER to Golgi in Wg-producing cells.

Wingless counteracts epithelial folding by increasing mechanical tension at basal cell edges in Drosophila

The modulation of mechanical tension is important for sculpturing tissues during animal development, yet how mechanical tension is controlled remains poorly understood. In Drosophila wing discs, the local reduction of mechanical tension at basal cell edges results in basal relaxation and the formation of an epithelial fold. Here, we show that Wingless, which is expressed next to this fold, promotes basal cell edge tension to suppress the formation of this fold. Ectopic expression of Wingless blocks fold formation, whereas the depletion of Wingless increases fold depth. Moreover, local depletion of Wingless in a region where Wingless signal transduction is normally high results in ectopic fold formation. The depletion of Wingless also results in decreased basal cell edge tension and basal cell area relaxation. Conversely, the activation of Wingless signal transduction leads to increased basal cell edge tension and basal cell area constriction. Our results identify the Wingless signal transduction pathway as a crucial modulator of mechanical tension that is important for proper wing disc morphogenesis.

The selector genes midline and H15 control ventral leg pattern by both inhibiting Dpp signaling and specifying ventral fate

mid and H15 encode Tbx20 transcription factors that specify ventral pattern in the Drosophila leg. We find that there are at least two pathways for mid and H15 specification of ventral fate. In the first pathway, mid and H15 negatively regulate Dpp, the dorsal signal in leg development. mid and H15 block the dorsalizing effects of Dpp signaling in the ventral leg. In loss- and gain-of-function experiments in imaginal discs, we show that mid and H15 block the accumulation of phospho-Mad, the activated form of the Drosophila pSmad1/5 homolog. In a second pathway, we find mid and H15 must also directly promote ventral fate because simultaneously blocking Dpp signaling in mid H15 mutants does not rescue the ventral to dorsal transformation in most ventral leg structures. We show that mid and H15 act as transcriptional repressors in ventral leg development. The two genes repress the Dpp target gene Dad, the laterally expressed gene Upd, and the mid VLE enhancer. This repression depends on the eh1 domain, a binding site for the Groucho co-repressor, and is likely direct because Mid localizes to target gene enhancers in PCR-ChIP assays. A mid allele mutant for the repressing domain (eh1), mid^eh1, was found to be compromised in gain-of-function assays and in rescue of mid H15 loss-of-function. We propose that mid and H15 specify ventral fate through inhibition of Dpp signaling and through coordinating the repression of genes in the ventral leg.

Somatic ERK activation during transit amplification is essential for maintaining the synchrony of germline divisions in Drosophila testis

Transit amplification (TA) of progenitor cells maintains tissue homeostasis by balancing proliferation and differentiation. In Drosophila testis, the germline proliferation is tightly regulated by factors present in both the germline and the neighbouring somatic cyst cells (SCCs). Although the exact mechanism is unclear, the epidermal growth factor receptor (EGFR) activation in SCCs has been reported to control spermatogonial divisions within a cyst, through downstream activations of Rac1-dependent pathways. Here, we report that somatic activation of the mitogen-activated protein kinase (Rolled/ERK) downstream of EGFR is required to synchronize the mitotic divisions and regulate the transition to meiosis. The process operates independently of the Bag-of-marble activity in the germline. Also, the integrity of the somatic cyst enclosure is inessential for this purpose. Together, these results suggest that synchronization of germ-cell divisions through somatic activation of distinct ERK-downstream targets independently regulates TA and subsequent differentiation of neighbouring germline cells.

Kinesin-2 and IFT-A act as a complex promoting nuclear localization of β-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 β-catenin, the mediator of Wnt/Wg-signalling on microtubules. Following pathway activation, Kinesin-2/IFT-A mutant cells exhibit high cytoplasmic β-catenin levels, yet fail to activate Wg-targets. In mutant tissues in both, Drosophila and mouse/MEFs, nuclear localization of β-catenin is markedly reduced. We demonstrate a conserved, motor-domain dependent function of the Kinesin-2/IFT-A complex in promoting nuclear translocation of β-catenin. We show that this is mediated by protecting β-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.

IFT-A proteins and Kinesin-2 modulate canonical Wnt/Wg-signalling independent of their ciliary role, but how is unclear. Here, the authors show that Kinesin-2 and IFT-A act as a complex to promote nuclear translocation of β-catenin in Drosophila and mouse MEF Wnt signalling independent of its ciliary role.

Actomyosin contractility modulates Wnt signaling through adherens junction stability

Actomyosin contractility can influence the canonical Wnt signaling pathway in processes like mesoderm differentiation and tissue stiffness during tumorigenesis. We identified that increased nonmuscle myosin II activation and cellular contraction inhibited Wnt target gene transcription in developing Drosophila imaginal disks. Genetic interactions studies were used to show that this effect was due to myosin-induced accumulation of cortical F-actin resulting in clustering and accumulation of E-cadherin to the adherens junctions. This results in E-cadherin titrating any available β-catenin, the Wnt pathway transcriptional coactivator, to the adherens junctions in order to maintain cell-cell adhesion under contraction. We show that decreased levels of cytoplasmic β-catenin result in insufficient nuclear translocation for full Wnt target gene transcription. Previous studies have identified some of these interactions, but we present a thorough analysis using the wing disk epithelium to show the consequences of modulating myosin phosphatase. Our work elucidates a mechanism in which the dynamic promotion of actomyosin contractility refines patterning of Wnt transcription during development and maintenance of epithelial tissue in organisms.

β-Catenin is a pH sensor with decreased stability at higher intracellular pH

White et al. find that intracellular pH regulates the stability of β-catenin, the Wnt signaling molecule that controls cell polarity, adhesion, and differentiation. A conserved histidine residue in β-catenin mediates pH-dependent binding to the E3 ligase β-TrCP for degradation, and a cancer-associated mutation that bypasses this pH-sensitive regulation induces ectopic tumors in the Drosophila eye.

β-Catenin functions as an adherens junction protein for cell–cell adhesion and as a signaling protein. β-catenin function is dependent on its stability, which is regulated by protein–protein interactions that stabilize β-catenin or target it for proteasome-mediated degradation. In this study, we show that β-catenin stability is regulated by intracellular pH (pHi) dynamics, with decreased stability at higher pHi in both mammalian cells and Drosophila melanogaster. β-Catenin degradation requires phosphorylation of N-terminal residues for recognition by the E3 ligase β-TrCP. While β-catenin phosphorylation was pH independent, higher pHi induced increased β-TrCP binding and decreased β-catenin stability. An evolutionarily conserved histidine in β-catenin (found in the β-TrCP DSGIHS destruction motif) is required for pH-dependent binding to β-TrCP. Expressing a cancer-associated H36R–β-catenin mutant in the Drosophila eye was sufficient to induce Wnt signaling and produced pronounced tumors not seen with other oncogenic β-catenin alleles. We identify pHi dynamics as a previously unrecognized regulator of β-catenin stability, functioning in coincidence with phosphorylation.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

Release and spread of Wingless is required to pattern the proximo-distal axis of Drosophila renal tubules

Wingless/Wnts are signalling molecules, traditionally considered to pattern tissues as long-range morphogens. However, more recently the spread of Wingless was shown to be dispensable in diverse developmental contexts in Drosophila and vertebrates. Here we demonstrate that release and spread of Wingless is required to pattern the proximo-distal (P-D) axis of Drosophila Malpighian tubules. Wingless signalling, emanating from the midgut, directly activates odd skipped expression several cells distant in the proximal tubule. Replacing Wingless with a membrane-tethered version that is unable to diffuse from the Wingless producing cells results in aberrant patterning of the Malpighian tubule P-D axis and development of short, deformed ureters. This work directly demonstrates a patterning role for a released Wingless signal. As well as extending our understanding about the functional modes by which Wnts shape animal development, we anticipate this mechanism to be relevant to patterning epithelial tubes in other organs, such as the vertebrate kidney.

Spatio-temporal relays control layer identity of direction-selective neuron subtypes in Drosophila

Visual motion detection in sighted animals is essential to guide behavioral actions ensuring their survival. In Drosophila, motion direction is first detected by T4/T5 neurons. Their axons innervate one of the four lobula plate layers. How T4/T5 neurons with layer-specific representation of motion-direction preferences are specified during development is unknown. We show that diffusible Wingless (Wg) between adjacent neuroepithelia induces its own expression to form secondary signaling centers. These activate Decapentaplegic (Dpp) signaling in adjacent lateral tertiary neuroepithelial domains dedicated to producing layer 3/4-specific T4/T5 neurons. T4/T5 neurons derived from the core domain devoid of Dpp signaling adopt the default layer 1/2 fate. Dpp signaling induces the expression of the T-box transcription factor Optomotor-blind (Omb), serving as a relay to postmitotic neurons. Omb-mediated repression of Dachshund transforms layer 1/2- into layer 3/4-specific neurons. Hence, spatio-temporal relay mechanisms, bridging the distances between neuroepithelial domains and their postmitotic progeny, implement T4/T5 neuron-subtype identity.

Comprehensive analysis of CTNNB1 in adrenocortical carcinomas: Identification of novel mutations and correlation to survival

The Wnt/β-Catenin signaling pathway is one of the most frequently altered pathways in adrenocortical carcinomas (ACCs). The aim of this study was to investigate the status of Wnt/β-Catenin signaling pathway by analyzing the expression level of β-Catenin and the mutational status of APC, AXIN2, CTNNB1, and ZNRF3 in ACCs. Mutations in APC, CTNNB1, ZNRF3 and homozygous deletions in ZNRF3 were observed in 3.8% (2/52), 11.5% (6/52), 1.9% (1/52) and 17.3% (9/52) of the cohort respectively. Novel interstitial deletions in CTNNB1 spanning intron 1 to exon 3/intron 3 were also found in 7.7% (4/52) of the tumours. All the observed alterations were mutually exclusive. Nuclear accumulation of β-Catenin, increased expression of Cyclin D1 and significantly higher expression of AXIN2 (p = 0.0039), ZNRF3 (p = 0.0032) and LEF1(p = 0.0090) observed in the tumours harbouring the deletion in comparison to tumours without CTNNB1 mutation demonstrates that the truncated β-Catenin is functionally active and erroneously activates the downstream targets. Significantly lower overall survival rate in patients with tumours harbouring alterations in APC/CTNNB1/ZNRF3 in comparison to those without mutation was observed. In conclusion, the discovery of novel large deletions in addition to the point mutations in CTNNB1 infers that activation of Wnt/β-Catenin pathway via alterations in CTNNB1 occurs frequently in ACCs. We also confirm that alterations in Wnt/β-Catenin signaling pathway members have a negative effect on overall survival of patients.

Temporospatial induction of homeodomain gene cut dictates natural lineage reprogramming

Understanding how cellular identity naturally interconverts with high efficiency and temporospatial precision is crucial for regenerative medicine. Here, we revealed a natural midgut-to-renal lineage conversion event during Drosophila metamorphosis and identified the evolutionarily-conserved homeodomain protein Cut as a master switch in this process. A steep Wnt/Wingless morphogen gradient intersects with a pulse of steroid hormone ecdysone to induce cut expression in a subset of midgut progenitors and reprogram them into renal progenitors. Molecularly, ecdysone-induced temporal factor Broad physically interacts with cut enhancer-bound Wnt pathway effector TCF/β-catenin and likely bridges the distant enhancer and promoter region of cut through its self-association. Such long-range enhancer-promoter looping could subsequently trigger timely cut transcription. Our results therefore led us to propose an unexpected poising-and-bridging mechanism whereby spatial and temporal cues intersect, likely via chromatin looping, to turn on a master transcription factor and dictate efficient and precise lineage reprogramming.

As an embryo develops, an organism transforms from a single cell into an organized collection of different cells, tissues and organs. Regulated by genes and messenger molecules, non-specialized cells known as precursor cells, move, divide and adapt to produce the different cells in the adult body.

However, sometimes already-specialized adult cells can acquire a new role in a process known as lineage reprogramming. Finding ways to artificially induce and control lineage reprogramming could be useful in regenerative medicine. This would allow cells to be reprogrammed to replace those that are lost or damaged. So far, scientists have been unable to develop a clear view of how lineage reprogramming happens naturally.

Here, Xu et al. identified a cell-conversion event in the developing fruit fly. As the fly larva develops into an adult, a group of cells in the midgut reprogramme to become renal cells – the equivalent to human kidney cells. The experiments revealed that a combination of signals from a cell messenger system important for cell specialization (called Wnt) and the hormone that controls molting in insects, activate a gene called cut, which controls the midgut-to-renal lineage reprogramming.

Together, Wnt and the hormone ensure that cut is activated only in a small, specific group of midgut precursor cells at a precise time. The reprogrammed cells then move into the excretory organs, the renal tubes, where they give rise to renal cells. Midgut precursor cells in which cut had been experimentally removed, still traveled into the renal tubes. However, they failed to switch their identity and gave rise to midgut cells instead.

Further examination revealed that both Wnt and the ecdysone hormone are needed to activate the cut gene. This is probably achieved by creating loops in the DNA to bring together the two distantly located key regulatory elements of cut gene expression. If this mechanism can be seen in other contexts it may be possible to adapt it for medical purposes. The ability to reprogramme groups of cells with high specificity could transform medicine. It would make it easier for our bodies to regenerate and repair.

Supramolecular assembly of the beta-catenin destruction complex and the effect of Wnt signaling on its localization, molecular size, and activity in vivo

Wnt signaling provides a paradigm for cell-cell signals that regulate embryonic development and stem cell homeostasis and are inappropriately activated in cancers. The tumor suppressors APC and Axin form the core of the multiprotein destruction complex, which targets the Wnt-effector beta-catenin for phosphorylation, ubiquitination and destruction. Based on earlier work, we hypothesize that the destruction complex is a supramolecular entity that self-assembles by Axin and APC polymerization, and that regulating assembly and stability of the destruction complex underlie its function. We tested this hypothesis in Drosophila embryos, a premier model of Wnt signaling. Combining biochemistry, genetic tools to manipulate Axin and APC2 levels, advanced imaging and molecule counting, we defined destruction complex assembly, stoichiometry, and localization in vivo, and its downregulation in response to Wnt signaling. Our findings challenge and revise current models of destruction complex function. Endogenous Axin and APC2 proteins and their antagonist Dishevelled accumulate at roughly similar levels, suggesting competition for binding may be critical. By expressing Axin:GFP at near endogenous levels we found that in the absence of Wnt signals, Axin and APC2 co-assemble into large cytoplasmic complexes containing tens to hundreds of Axin proteins. Wnt signals trigger recruitment of these to the membrane, while cytoplasmic Axin levels increase, suggesting altered assembly/disassembly. Glycogen synthase kinase3 regulates destruction complex recruitment to the membrane and release of Armadillo/beta-catenin from the destruction complex. Manipulating Axin or APC2 levels had no effect on destruction complex activity when Wnt signals were absent, but, surprisingly, had opposite effects on the destruction complex when Wnt signals were present. Elevating Axin made the complex more resistant to inactivation, while elevating APC2 levels enhanced inactivation. Our data suggest both absolute levels and the ratio of these two core components affect destruction complex function, supporting models in which competition among Axin partners determines destruction complex activity.

Brain Tumor promotes axon growth across the midline through interactions with the microtubule stabilizing protein Apc2

Commissural axons must cross the midline to establish reciprocal connections between the two sides of the body. This process is highly conserved between invertebrates and vertebrates and depends on guidance cues and their receptors to instruct axon trajectories. The DCC family receptor Frazzled (Fra) signals chemoattraction and promotes midline crossing in response to its ligand Netrin. However, in Netrin or fra mutants, the loss of crossing is incomplete, suggesting the existence of additional pathways. Here, we identify Brain Tumor (Brat), a tripartite motif protein, as a new regulator of midline crossing in the Drosophila CNS. Genetic analysis indicates that Brat acts independently of the Netrin/Fra pathway. In addition, we show that through its B-Box domains, Brat acts cell autonomously to regulate the expression and localization of Adenomatous polyposis coli-2 (Apc2), a key component of the Wnt canonical signaling pathway, to promote axon growth across the midline. Genetic evidence indicates that the role of Brat and Apc2 to promote axon growth across the midline is independent of Wnt and Beta-catenin-mediated transcriptional regulation. Instead, we propose that Brat promotes midline crossing through directing the localization or stability of Apc2 at the plus ends of microtubules in navigating commissural axons. These findings define a new mechanism in the coordination of axon growth and guidance at the midline.

The establishment of neuronal connections that cross the midline of the animal is essential to generate neural circuits that coordinate the left and right sides of the body. Axons that cross the midline to form these connections are called commissural axons and the molecules and mechanisms that control midline axon crossing are remarkably conserved across animal evolution. In this study we have used a genetic screen in the fruit fly in an attempt to uncover additional players in this key developmental process, and have identified a novel role for the Brain Tumor (Brat) protein in promoting commissural axon growth across the midline. Unlike its previous described functions, in the context of midline axon guidance Brat cooperates with the microtubule stabilizing protein Apc2 to coordinate axon growth and guidance. Molecular and genetic analyses point to the conserved B box motifs of the Brat protein as key in promoting the association of Apc2 with the plus ends of microtubules. Brat is highly conserved and future studies will determine whether homologous genes play analogous roles in mammalian neural development.

Beta-catenin cleavage enhances transcriptional activation

Nuclear activation of Wnt/β-catenin signaling is required for cell proliferation in inflammation and cancer. Studies from our group indicate that β-catenin activation in colitis and colorectal cancer (CRC) correlates with increased nuclear levels of β-catenin phosphorylated at serine 552 (pβ-Cat⁵⁵²). Biochemical analysis of nuclear extracts from cancer biopsies revealed the existence of low molecular weight (LMW) pβ-Cat⁵⁵², increased to the exclusion of full size (FS) forms of β-catenin. LMW β-catenin lacks both termini, leaving residues in the armadillo repeat intact. Further experiments showed that TCF4 predominantly binds LMW pβ-Cat⁵⁵² in the nucleus of inflamed and cancerous cells. Nuclear chromatin bound localization of LMW pβ-Cat⁵⁵² was blocked in cells by inhibition of proteasomal chymotrypsin-like activity but not by other protease inhibitors. K48 polyubiquitinated FS and LMW β-catenin were increased by treatment with bortezomib. Overexpressed in vitro double truncated β-catenin increased transcriptional activity, cell proliferation and growth of tumor xenografts compared to FS β-catenin. Serine 552-> alanin substitution abrogated K48 polyubiquitination,  β-catenin nuclear translocation and tumor xenograft growth. These data suggest that a novel proteasome-dependent posttranslational modification of β-catenin enhances transcriptional activation. Discovery of this pathway may be helpful in the development of diagnostic and therapeutic tools in colitis and cancer.

Wnt-Dependent Inactivation of the Groucho/TLE Co-repressor by the HECT E3 Ubiquitin Ligase Hyd/UBR5

Extracellular signals are transduced to the cell nucleus by effectors that bind to enhancer complexes to operate transcriptional switches. For example, the Wnt enhanceosome is a multiprotein complex associated with Wnt-responsive enhancers through T cell factors (TCF) and kept silent by Groucho/TLE co-repressors. Wnt-activated β-catenin binds to TCF to overcome this repression, but how it achieves this is unknown. Here, we discover that this process depends on the HECT E3 ubiquitin ligase Hyd/UBR5, which is required for Wnt signal responses in Drosophila and human cell lines downstream of activated Armadillo/β-catenin. We identify Groucho/TLE as a functionally relevant substrate, whose ubiquitylation by UBR5 is induced by Wnt signaling and conferred by β-catenin. Inactivation of TLE by UBR5-dependent ubiquitylation also involves VCP/p97, an AAA ATPase regulating the folding of various cellular substrates including ubiquitylated chromatin proteins. Thus, Groucho/TLE ubiquitylation by Hyd/UBR5 is a key prerequisite that enables Armadillo/β-catenin to activate transcription.

SoxNeuro and Shavenbaby act cooperatively to shape denticles in the embryonic epidermis of Drosophila

During development, extracellular signals are integrated by cells to induce the transcriptional circuitry that controls morphogenesis. In the fly epidermis, Wingless (Wg)/Wnt signaling directs cells to produce either a distinctly shaped denticle or no denticle, resulting in a segmental pattern of denticle belts separated by smooth, or 'naked', cuticle. Naked cuticle results from Wg repression of shavenbaby (svb), which encodes a transcription factor required for denticle construction. We have discovered that although the svb promoter responds differentially to altered Wg levels, Svb alone cannot produce the morphological diversity of denticles found in wild-type belts. Instead, a second Wg-responsive transcription factor, SoxNeuro (SoxN), cooperates with Svb to shape the denticles. Co-expressing ectopic SoxN with svb rescued diverse denticle morphologies. Conversely, removing SoxN activity eliminated the residual denticles found in svb mutant embryos. Furthermore, several known Svb target genes are also activated by SoxN, and we have discovered two novel target genes of SoxN that are expressed in denticle-producing cells and that are regulated independently of Svb. We conclude that proper denticle morphogenesis requires transcriptional regulation by both SoxN and Svb.

The Wnt pathway limits BMP signaling outside of the germline stem cell niche in Drosophila ovaries

The mechanisms that modulate and limit the signaling output of adult stem cell niches remain poorly understood. To gain further insights into how these microenvironments are regulated in vivo, we performed a candidate gene screen designed to identify factors that restrict BMP signal production to the cap cells that comprise the germline stem cell (GSC) niche of Drosophila ovaries. Through these efforts, we found that disruption of Wnt4 and components of the canonical Wnt pathway results in a complex germ cell phenotype marked by an expansion of GSC-like cells, pre-cystoblasts and cystoblasts in young females. This phenotype correlates with an increase of decapentaplegic (dpp) mRNA levels within escort cells and varying levels of BMP responsiveness in the germline. Further genetic experiments show that Wnt4, which exhibits graded expression in somatic cells of germaria, activates the Wnt pathway in posteriorly positioned escort cells. The activation of the Wnt pathway appears to be limited by the BMP pathway itself, as loss of Mad in escort cells results in the expansion of Wnt pathway activation. Wnt pathway activity changes within germaria during the course of aging, coincident with changes in dpp production. These data suggest that mutual antagonism between the BMP and Wnt pathways in somatic cells helps to regulate germ cell differentiation.

Proteomic analysis reveals APC-dependent post-translational modifications and identifies a novel regulator of β-catenin

Wnt signaling generates patterns in all embryos, from flies to humans, and controls cell fate, proliferation and metabolic homeostasis. Inappropriate Wnt pathway activation results in diseases, including colorectal cancer. The adenomatous polyposis coli (APC) tumor suppressor gene encodes a multifunctional protein that is an essential regulator of Wnt signaling and cytoskeletal organization. Although progress has been made in defining the role of APC in a normal cellular context, there are still significant gaps in our understanding of APC-dependent cellular function and dysfunction. We expanded the APC-associated protein network using a combination of genetics and a proteomic technique called two-dimensional difference gel electrophoresis (2D-DIGE). We show that loss of Drosophila Apc2 causes protein isoform changes reflecting misregulation of post-translational modifications (PTMs), which are not dependent on β-catenin transcriptional activity. Mass spectrometry revealed that proteins involved in metabolic and biosynthetic pathways, protein synthesis and degradation, and cell signaling are affected by Apc2 loss. We demonstrate that changes in phosphorylation partially account for the altered PTMs in APC mutants, suggesting that APC mutants affect other types of PTM. Finally, through this approach Aminopeptidase P was identified as a new regulator of β-catenin abundance in Drosophila embryos. This study provides new perspectives on the cellular effects of APC that might lead to a deeper understanding of its role in development.

Dlg5 maintains apical polarity by promoting membrane localization of Crumbs during Drosophila oogenesis

Apical-basal polarity plays critical roles in the functions of epithelial tissues. However, the mechanisms of epithelial polarity establishment and maintenance remain to be fully elucidated. Here we show that the membrane-associated guanylate kinase (MAGUK) family protein Dlg5 is required for the maintenance of apical polarity of follicle epithelium during Drosophila oogenesis. Dlg5 localizes at the apical membrane and adherens junction (AJ) of follicle epithelium in early stage egg chambers. Specifically, we demonstrate that the major function of Dlg5 is to promote apical membrane localization of Crumbs, since overexpression of Crumbs but not other major apical or AJ components could rescue epithelial polarity defects resulted from loss of Dlg5. Furthermore, we performed a structure-function analysis of Dlg5 and found that the C-terminal PDZ3 and PDZ4 domains are required for all Dlg5’s functions as well as its ability to localize to apical membrane. The N-terminal coiled-coil motif could be individually targeted to the apical membrane, while the central linker region could be targeted to AJ. Lastly, the MAGUK core domains of PDZ4-SH3-GUK could be individually targeted to apical, AJ and basolateral membranes.

The expression of the T-box selector gene midline in the leg imaginal disc is controlled by both transcriptional regulation and cell lineage

The Drosophila Tbx20 homologs midline and H15 act as selector genes for ventral fate in Drosophila legs. midline and H15 expression defines the ventral domain of the leg and the two genes are necessary and sufficient for the development of ventral fate. Ventral-specific expression of midline and H15 is activated by Wingless (Wg) and repressed by Decapentaplegic (Dpp). Here we identify VLE, a 5 kb enhancer that drives ventral specific expression in the leg disc that is very similar to midline expression. Subdivision of VLE identifies two regions that mediate both activation and repression and third region that only mediates repression. Loss- and gain-of-function genetic mosaic analysis shows that the activating and repressing regions respond to Wg and Dpp signaling respectively. All three repression regions depend on the activity of Mothers-against-decapentaplegic, a Drosophila r-Smad that mediates Dpp signaling, and respond to ectopic expression of the Dpp target genes optomoter-blind and Dorsocross 3. However, only one repression region is responsive to loss of schnurri, a co-repressor required for direct repression by Dpp-signaling. Thus, Dpp signaling restricts midline expression through both direct repression and through the activation of downstream repressors. We also find that midline and H15 expression are both subject to cross-repression and feedback inhibition. Finally, a lineage analysis indicates that ventral midline-expressing cells and dorsal omb-expressing cells do not mix during development. Together this data indicates that the ventral-specific expression of midline results from both transcriptional regulation and from a lack of cell-mixing between dorsal and ventral cells.

Wnt signaling-mediated redox regulation maintains the germ line stem cell differentiation niche

Adult stem cells continuously undergo self-renewal and generate differentiated cells. In the Drosophila ovary, two separate niches control germ line stem cell (GSC) self-renewal and differentiation processes. Compared to the self-renewing niche, relatively little is known about the maintenance and function of the differentiation niche. In this study, we show that the cellular redox state regulated by Wnt signaling is critical for the maintenance and function of the differentiation niche to promote GSC progeny differentiation. Defective Wnt signaling causes the loss of the differentiation niche and the upregulated BMP signaling in differentiated GSC progeny, thereby disrupting germ cell differentiation. Mechanistically, Wnt signaling controls the expression of multiple glutathione-S-transferase family genes and the cellular redox state. Finally, Wnt2 and Wnt4 function redundantly to maintain active Wnt signaling in the differentiation niche. Therefore, this study has revealed a novel strategy for Wnt signaling in regulating the cellular redox state and maintaining the differentiation niche.

DOI:http://dx.doi.org/10.7554/eLife.08174.001

An animal or plant has many different types of cells that have specific roles in the life of the organism. These cells are organized into tissues. In most tissues in adult animals, small groups of cells called stem cells are responsible for replacing the other cells that have been lost due to disease, injury, or as part of normal body maintenance.

The ‘germ line’ stem cells of female fruit flies—which produce female sex cells (or eggs)—are an effective system for studying how stem cells are regulated. These cells live in an area of the ovary called a stem cell niche. Each time a stem cell divides, it produces one stem cell and one other daughter cell. This daughter cell then moves into another niche called the ‘differentiation’ niche and undergoes a series of divisions that produce the egg cells. The differentiation niche is formed by escort cells and is crucial for producing the egg cells, but it is not clear how the escort cells promote this process, or how the niche is maintained.

Wang et al. have now studied the differentiation niche in more detail. The experiments show that a cell communication system called Wnt signaling maintains the differentiation niche by controlling the ability of the escort cells to grow and divide. If Wnt signaling is defective, the differentiation niche is lost, which disrupts the formation of egg cells.

Further experiments show that two proteins called Wnt2 and Wnt4 in the differentiation niche—which activate Wnt signaling—act as signals to regulate the niche, mainly by controlling the expression of four particular genes. These four genes encode enzymes that remove ‘reactive oxygen species’ from cells. Wang et al.'s findings have revealed an important role for Wnt signaling in maintaining the differentiation niche. The next step is to figure out the details of how this works.

DOI:http://dx.doi.org/10.7554/eLife.08174.002

Components of Intraflagellar Transport Complex A Function Independently of the Cilium to Regulate Canonical Wnt Signaling in Drosophila

The development of multicellular organisms requires the precisely coordinated regulation of an evolutionarily conserved group of signaling pathways. Temporal and spatial control of these signaling cascades is achieved through networks of regulatory proteins, segregation of pathway components in specific subcellular compartments, or both. In vertebrates, dysregulation of primary cilia function has been strongly linked to developmental signaling defects, yet it remains unclear whether cilia sequester pathway components to regulate their activation or cilia-associated proteins directly modulate developmental signaling events. To elucidate this question, we conducted an RNAi-based screen in Drosophila non-ciliated cells to test for cilium-independent loss-of-function phenotypes of ciliary proteins in developmental signaling pathways. Our results show no effect on Hedgehog signaling. In contrast, our screen identified several cilia-associated proteins as functioning in canonical Wnt signaling. Further characterization of specific components of Intraflagellar Transport complex A uncovered a cilia-independent function in potentiating Wnt signals by promoting β-catenin/Armadillo activity.

A novel GSK3-regulated APC:Axin interaction regulates Wnt signaling by driving a catalytic cycle of efficient βcatenin destruction

APC, a key negative regulator of Wnt signaling in development and oncogenesis, acts in the destruction complex with the scaffold Axin and the kinases GSK3 and CK1 to target βcatenin for destruction. Despite 20 years of research, APC's mechanistic function remains mysterious. We used FRAP, super-resolution microscopy, functional tests in mammalian cells and flies, and other approaches to define APC's mechanistic role in the active destruction complex when Wnt signaling is off. Our data suggest APC plays two roles: (1) APC promotes efficient Axin multimerization through one known and one novel APC:Axin interaction site, and (2) GSK3 acts through APC motifs R2 and B to regulate APC:Axin interactions, promoting high-throughput of βcatenin to destruction. We propose a new dynamic model of how the destruction complex regulates Wnt signaling and how this goes wrong in cancer, providing insights into how this multiprotein signaling complex is assembled and functions via multivalent interactions.

DOI:http://dx.doi.org/10.7554/eLife.08022.001

An embryo starts off as a small ball of stem cells, each of which has the potential to become any type of cell in the body. Adult organs and tissues also contain small numbers of stem cells that can replace old or damaged cells. In both of these processes, stem cells need to ‘decide’ when they should start to change into a more specialized cell type, and which cell fate to choose (e.g., liver cell vs kidney cell). A signaling pathway involving Wnt proteins helps to direct many of these decisions. But if the ‘Wnt signaling pathway’ becomes activated at the wrong time, it can lead to cancer. For example, the first step in development of colon cancer is the inappropriate activation of Wnt signaling, and is most often caused by mutations in the gene that encodes a protein called APC. The APC protein is a tumor suppressor and normally inhibits Wnt signaling. However, even after over 20 years of effort, it remains largely mysterious how APC does this.

APC is known to work with another protein called Axin as part of a large protein machine. This protein complex performs one of the first steps in a process that ultimately marks a key component of the Wnt signaling pathway for destruction. Pronobis et al. have now used a range of techniques to define APC's role in this so-called ‘destruction complex’. This analysis revealed the internal structure of a complex made from APC and Axin, and showed that cable- and sheet-like assemblies of Axin were intertwined with APC cables. Further experiments then revealed how APC and Axin proteins are added into or leave these complexes, and showed that this is critical for this protein machine to work.

Pronobis et al.'s data also suggest that APC plays two roles, which make the destruction complex more efficient. Firstly, it can interact with Axin via two separate interaction sites that help to assemble the destruction complex. Secondly, specific features in APC allow it to interact with a third protein (called GSK3), which can then regulate how APC interacts with Axin. One of the next challenges will be to uncover how APC helps to transfer the components of Wnt signaling to the next step of their destruction, and to clear up the role played by GSK3.

DOI:http://dx.doi.org/10.7554/eLife.08022.002

Antero-posterior patterning of Drosophila ocelli requires an anti-repressor mechanism within the hh-pathway mediated by the Six3 gene Optix

In addition to the compound eyes, most insects possess a set of three dorsal ocelli that develop at the vertices of a triangular cuticle patch, forming the ocellar complex. The wingless and hedgehog signaling pathways, together with the transcription factor encoded by orthodenticle, are known to play major roles in the specification and patterning of the ocellar complex. Specifically, hedgehog is responsible for the choice between ocellus and cuticle fates within the ocellar complex primordium. However, the interaction between signals and transcription factors known to date do not fully explain how this choice is controlled. We show that this binary choice depends on dynamic changes in the domains of hedgehog signaling. In this dynamics, the restricted expression of engrailed, a hedgehog-signaling target, is key because it defines a domain within the complex where hh transcription is maintained while the pathway activity is blocked. We show that the Drosophila Six3, Optix, is expressed in and required for the development of the anterior ocellus specifically. Optix would not act as an ocellar selector, but rather as a patterning gene, limiting the en expression domain. Our results indicate that, despite their genetic and structural similarity, anterior and posterior ocelli are under different genetic control.

N-cadherin negatively regulates collective Drosophila glial migration via actin cytoskeleton remodeling

Cell migration is an essential and highly regulated process. During development, glia and neurons migrate over long distances, in most cases collectively, to reach their final destination and build the sophisticated architecture of the nervous system, the most complex tissue of the body. Collective migration is highly stereotyped and efficient, defects in the process leading to severe human diseases that include mental retardation. This dynamic process entails extensive cell communication and coordination, hence the real challenge is to analyze it in the whole organism and at cellular resolution. We here investigate the impact of the N-cadherin adhesion molecule on collective glial migration using the Drosophila developing wing and cell-type specific manipulation of gene expression. We show that N-cadherin timely accumulates in glial cells and that its levels affect migration efficiency. N-cadherin works as a molecular brake in a dosage dependent manner by negatively controlling actin nucleation and cytoskeleton remodeling through α/β catenins. This is the first in vivo evidence for N-cadherin negatively and cell autonomously controlling collective migration.

Protection of armadillo/β-Catenin by armless, a novel positive regulator of wingless signaling

This study uses an RNAi screen in Drosophila to identify a UBX protein, Armless, as a novel positive regulator of the important Wingless/Wnt signaling pathway, acting to stabilize Armadillo/?-Catenin by antagonizing its turnover.

The Wingless (Wg/Wnt) signaling pathway is essential for metazoan development, where it is central to tissue growth and cellular differentiation. Deregulated Wg pathway activation underlies severe developmental abnormalities, as well as carcinogenesis. Armadillo/β-Catenin plays a key role in the Wg transduction cascade; its cytoplasmic and nuclear levels directly determine the output activity of Wg signaling and are thus tightly controlled. In all current models, once Arm is targeted for degradation by the Arm/β-Catenin destruction complex, its fate is viewed as set. We identified a novel Wg/Wnt pathway component, Armless (Als), which is required for Wg target gene expression in a cell-autonomous manner. We found by genetic and biochemical analyses that Als functions downstream of the destruction complex, at the level of the SCF/Slimb/βTRCP E3 Ub ligase. In the absence of Als, Arm levels are severely reduced. We show by biochemical and in vivo studies that Als interacts directly with Ter94, an AAA ATPase known to associate with E3 ligases and to drive protein turnover. We suggest that Als antagonizes Ter94's positive effect on E3 ligase function and propose that Als promotes Wg signaling by rescuing Arm from proteolytic degradation, spotlighting an unexpected step where the Wg pathway signal is modulated.

The Wg/Wnt signaling pathway, found in most animals, is essential for regulating tissue growth and the formation of different cell types during development. Defects in the Wg/Wnt signaling relay can have serious consequences, ranging from aberrant organ patterning to malignant tumor formation. A pivotal step in the transmission of the Wg/Wnt signal is the stabilization of the protein Armadillo/β-Catenin, a key component of the pathway. However, the means by which the levels of this protein are regulated remain unclear. Here, we describe a novel control point of Armadillo/β-Catenin levels. Using RNA interference, we performed a screen in the fruit fly Drosophila melanogaster and identified Armless, a protein whose biological function was previously unknown, as a novel regulator of Wg/Wnt signaling, essential for the Wg/Wnt-dependent expression of downstream target genes. Our experiments suggest that Armless interferes with the tagging of Armadillo/β-Catenin with ubiquitin, thereby sparing it from proteasomal degradation. We also show that Armless directly interacts in vivo with Ter94, a ubiquitous ATPase involved in protein turnover. Our results suggest that Armless antagonizes Ter94's function in protein turnover, thereby acting as a positive regulator of Wg/Wnt signaling by promoting the stabilization of Armadillo/β-Catenin.

Slmb antagonises the aPKC/Par-6 complex to control oocyte and epithelial polarity

The Drosophila anterior-posterior axis is specified when the posterior follicle cells signal to polarise the oocyte, leading to the anterior/lateral localisation of the Par-6/aPKC complex and the posterior recruitment of Par-1, which induces a microtubule reorganisation that localises bicoid and oskar mRNAs. Here we show that oocyte polarity requires Slmb, the substrate specificity subunit of the SCF E3 ubiquitin ligase that targets proteins for degradation. The Par-6/aPKC complex is ectopically localised to the posterior of slmb mutant oocytes, and Par-1 and oskar mRNA are mislocalised. Slmb appears to play a related role in epithelial follicle cells, as large slmb mutant clones disrupt epithelial organisation, whereas small clones show an expansion of the apical domain, with increased accumulation of apical polarity factors at the apical cortex. The levels of aPKC and Par-6 are significantly increased in slmb mutants, whereas Baz is slightly reduced. Thus, Slmb may induce the polarisation of the anterior-posterior axis of the oocyte by targeting the Par-6/aPKC complex for degradation at the oocyte posterior. Consistent with this, overexpression of the aPKC antagonist Lgl strongly rescues the polarity defects of slmb mutant germline clones. The role of Slmb in oocyte polarity raises an intriguing parallel with C. elegans axis formation, in which PAR-2 excludes the anterior PAR complex from the posterior cortex to induce polarity, but its function can be substituted by overexpressing Lgl.

Kinesin-II recruits Armadillo and Dishevelled for Wingless signaling in Drosophila

Wingless (Wg)/Wnt signaling is fundamental in metazoan development. Armadillo (Arm)/β-catenin and Dishevelled (Dsh) are key components of Wnt signal transduction. Recent studies suggest that intracellular trafficking of Wnt signaling components is important, but underlying mechanisms are not well known. Here, we show that Klp64D, the Drosophila homolog of Kif3A kinesin II subunit, is required for Wg signaling by regulating Arm during wing development. Mutations in klp64D or RNAi cause wing notching and loss of Wg target gene expression. The wing notching phenotype by Klp64D knockdown is suppressed by activated Arm but not by Dsh, suggesting that Klp64D is required for Arm function. Furthermore, klp64D and arm mutants show synergistic genetic interaction. Consistent with this genetic interaction, Klp64D directly binds to the Arm repeat domain of Arm and can recruit Dsh in the presence of Arm. Overexpression of Klp64D mutated in the motor domain causes dominant wing notching, indicating the importance of the motor activity. Klp64D shows subcellular localization to intracellular vesicles overlapping with Arm and Dsh. In klp64D mutants, Arm is abnormally accumulated in vesicular structures including Golgi, suggesting that intracellular trafficking of Arm is affected. Human KIF3A can also bind β-catenin and rescue klp64D RNAi phenotypes. Taken together, we propose that Klp64D is essential for Wg signaling by trafficking of Arm via the formation of a conserved complex with Arm.

Modulating and measuring Wingless signalling

The main Wnt ligand of Drosophila activates a conserved canonical signalling pathway to regulate a plethora of cellular activities during development, regeneration and nervous system function. Here I first describe experimental means of measuring and modulating Wingless signalling in Drosophila cell culture. Various reporters have been devised by placing TCF-binding sites or DNA fragments from known target genes upstream of luciferase-coding sequences. Signalling can be activated in cells by addition of Wingless conditioned medium, treatment with a chemical inhibitor of Shaggy/GSK3 or transfection with a plasmid encoding activated Armadillo (Drosophila β-catenin). Measuring Wingless signalling in intact tissue is somewhat more challenging than in cell culture. Synthetic transgenic reporters have been devised but further improvements are needed to achieve sensitive responsiveness to Wingless at all times and places. As an alternative, gene traps in frizzled3 and notum/wingful, two context-independent endogenous targets, can be used as reporters. It is hoped that further modification of these loci could lead to more versatile and sensitive means of detecting signalling. Many genetic tools are available to trigger ectopic signalling or prevent endogenous signalling. These mostly rely on RNAi-producing transgenes or the generation of mutant patches by mitotic recombination. New developments in genome engineering are opening further means of manipulating the components of Wingless signalling with exquisite temporal and spatial precision.

Evolutionary adaptation of the fly Pygo PHD finger toward recognizing histone H3 tail methylated at arginine 2

Pygo proteins promote Armadillo- and β-catenin-dependent transcription, by relieving Groucho-dependent repression of Wnt targets. Their PHD fingers bind histone H3 tail methylated at lysine 4, and to the HD1 domain of their Legless/BCL9 cofactors, linking Pygo to Armadillo/β-catenin. Intriguingly, fly Pygo orthologs exhibit a tryptophan > phenylalanine substitution in their histone pocket-divider which reduces their affinity for histones. Here, we use X-ray crystallography and NMR, to discover a conspicuous groove bordering this phenylalanine in the Drosophila PHD-HD1 complex--a semi-aromatic cage recognizing asymmetrically methylated arginine 2 (R2me2a), a chromatin mark of silenced genes. Our structural model of the ternary complex reveals a distinct mode of dimethylarginine recognition, involving a polar interaction between R2me2a and its groove, the structural integrity of which is crucial for normal tissue patterning. Notably, humanized fly Pygo derepresses Notch targets, implying an inherent Notch-related function of classical Pygo orthologs, disabled in fly Pygo, which thus appears dedicated to Wnt signaling.

Brain tumor specifies intermediate progenitor cell identity by attenuating β-catenin/Armadillo activity

During asymmetric stem cell division, both the daughter stem cell and the presumptive intermediate progenitor cell inherit cytoplasm from their parental stem cell. Thus, proper specification of intermediate progenitor cell identity requires an efficient mechanism to rapidly extinguish the activity of self-renewal factors, but the mechanisms remain unknown in most stem cell lineages. During asymmetric division of a type II neural stem cell (neuroblast) in the Drosophila larval brain, the Brain tumor (Brat) protein segregates unequally into the immature intermediate neural progenitor (INP), where it specifies INP identity by attenuating the function of the self-renewal factor Klumpfuss (Klu), but the mechanisms are not understood. Here, we report that Brat specifies INP identity through its N-terminal B-boxes via a novel mechanism that is independent of asymmetric protein segregation. Brat-mediated specification of INP identity is critically dependent on the function of the Wnt destruction complex, which attenuates the activity of β-catenin/Armadillo (Arm) in immature INPs. Aberrantly increasing Arm activity in immature INPs further exacerbates the defects in the specification of INP identity and enhances the supernumerary neuroblast mutant phenotype in brat mutant brains. By contrast, reducing Arm activity in immature INPs suppresses supernumerary neuroblast formation in brat mutant brains. Finally, reducing Arm activity also strongly suppresses supernumerary neuroblasts induced by overexpression of klu. Thus, the Brat-dependent mechanism extinguishes the function of the self-renewal factor Klu in the presumptive intermediate progenitor cell by attenuating Arm activity, balancing stem cell maintenance and progenitor cell specification.

Disruption of Drosophila melanogaster lipid metabolism genes causes tissue overgrowth associated with altered developmental signaling

Developmental patterning requires the precise interplay of numerous intercellular signaling pathways to ensure that cells are properly specified during tissue formation and organogenesis. The spatiotemporal function of many developmental pathways is strongly influenced by the biosynthesis and intracellular trafficking of signaling components. Receptors and ligands must be trafficked to the cell surface where they interact, and their subsequent endocytic internalization and endosomal trafficking is critical for both signal propagation and its down-modulation. In a forward genetic screen for mutations that alter intracellular Notch receptor trafficking in Drosophila melanogaster, we recovered mutants that disrupt genes encoding serine palmitoyltransferase and acetyl-CoA carboxylase. Both mutants cause Notch, Wingless, the Epidermal Growth Factor Receptor (EFGR), and Patched to accumulate abnormally in endosomal compartments. In mosaic animals, mutant tissues exhibit an unusual non-cell-autonomous effect whereby mutant cells are functionally rescued by secreted activities emanating from adjacent wildtype tissue. Strikingly, both mutants display prominent tissue overgrowth phenotypes that are partially attributable to altered Notch and Wnt signaling. Our analysis of the mutants demonstrates genetic links between abnormal lipid metabolism, perturbations in developmental signaling, and aberrant cell proliferation.

The development of complex, multicellular animal tissues requires the coordinated function of many different cell-cell communication pathways, in which secreted or cell-surface-anchored ligands from one cell typically activate a receptor on the surface of other cells, which in turn regulates downstream gene transcription and other cellular processes. We used a genetic approach in the fruit fly Drosophila melanogaster to search directly for mutations that perturb intracellular trafficking of a major signaling receptor, namely the Notch receptor, which controls cell differentiation in various tissue contexts. The Notch signaling pathway, like other key developmental signaling pathways, is evolutionarily conserved and functions in a similar manner in D. melanogaster and mammals, including humans. We recovered and characterized mutations in two genes that encode different enzymes involved in cellular lipid metabolism. Both mutants alter not only Notch signaling but also downstream activity of another highly conserved signaling pathway mediated by the Wingless protein, illustrating that alterations in cellular enzymes of lipid metabolism can exert complex effects on multiple critical signaling pathways. We also found that the new mutants exhibit dramatic cell overproliferation effects, reinforcing findings from mammalian studies suggesting that lipid metabolism might play an important role in oncogenesis and tumor progression.

Pebble/ECT2 RhoGEF negatively regulates the Wingless/Wnt signaling pathway

Wingless (Wg)/Wnt signaling is essential for patterning invertebrate and vertebrate embryos, and inappropriate Wnt activity is associated with a variety of human cancers. Despite intensive study, Wnt pathway mechanisms are not fully understood. We have discovered a new mechanism for regulating the Wnt pathway: activity of a Rho guanine nucleotide exchange factor (GEF) encoded by pebble (pbl) in Drosophila and ECT2 in humans. This RhoGEF has an essential role in cytokinesis, but also plays an unexpected, conserved role in inhibiting Wg/Wnt activity. Loss and gain of pbl function in Drosophila embryos cause pattern defects that indicate altered Wg activity. Both Pbl and ECT2 repress Wg/Wnt target gene expression in cultured Drosophila and human cells. The GEF activity is required for Wnt regulation, whereas other protein domains important for cytokinesis are not. Unlike most negative regulators of Wnt activity, Pbl/ECT2 functions downstream of Armadillo (Arm)/beta-catenin stabilization. Our results indicate GTPase regulation at a novel point in Wg/Wnt signal transduction, and provide new insight into the categorization of ECT2 as a human proto-oncogene.

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.

The miR-310/13 cluster antagonizes β-catenin function in the regulation of germ and somatic cell differentiation in the Drosophila testis

MicroRNAs (miRNAs) are regulators of global gene expression and function in a broad range of biological processes. Recent studies have suggested that miRNAs can function as tumor suppressors or oncogenes by modulating the activities of evolutionarily conserved signaling pathways that are commonly dysregulated in cancer. We report the identification of the miR-310 to miR-313 (miR-310/13) cluster as a novel antagonist of Wingless (Drosophila Wnt) pathway activity in a functional screen for Drosophila miRNAs. We demonstrate that miR-310/13 can modulate Armadillo (Arm; Drosophila β-catenin) expression and activity by directly targeting the 3'-UTRs of arm and pangolin (Drosophila TCF) in vivo. Notably, the miR-310/13-deficient flies exhibit abnormal germ and somatic cell differentiation in the male gonad, which can be rescued by reducing Arm protein levels or activity. Our results implicate a previously unrecognized function for miR-310/13 in dampening the activity of Arm in early somatic and germline progenitor cells, whereby inappropriate/sustained activation of Arm-mediated signaling or cell adhesion may impact normal differentiation in the Drosophila male gonad.

The ETS domain transcriptional repressor Anterior open inhibits MAP kinase and Wingless signaling to couple tracheal cell fate with branch identity

Cells at the tips of budding branches in the Drosophila tracheal system generate two morphologically different types of seamless tubes. Terminal cells (TCs) form branched lumenized extensions that mediate gas exchange at target tissues, whereas fusion cells (FCs) form ring-like connections between adjacent tracheal metameres. Each tracheal branch contains a specific set of TCs, FCs, or both, but the mechanisms that select between the two tip cell types in a branch-specific fashion are not clear. Here, we show that the ETS domain transcriptional repressor anterior open (aop) is dispensable for directed tracheal cell migration, but plays a key role in tracheal tip cell fate specification. Whereas aop globally inhibits TC and FC specification, MAPK signaling overcomes this inhibition by triggering degradation of Aop in tip cells. Loss of aop function causes excessive FC and TC specification, indicating that without Aop-mediated inhibition, all tracheal cells are competent to adopt a specialized fate. We demonstrate that Aop plays a dual role by inhibiting both MAPK and Wingless signaling, which induce TC and FC fate, respectively. In addition, the branch-specific choice between the two seamless tube types depends on the tracheal branch identity gene spalt major, which is sufficient to inhibit TC specification. Thus, a single repressor, Aop, integrates two different signals to couple tip cell fate selection with branch identity. The switch from a branching towards an anastomosing tip cell type may have evolved with the acquisition of a main tube that connects separate tracheal primordia to generate a tubular network.

Drosophila apc regulates delamination of invasive epithelial clusters

Border Cells in the Drosophila ovaries are a useful genetic model for understanding the molecular events underlying epithelial cell motility. During stage 9 of egg chamber development they detach from neighboring stretched cells and migrate between the nurse cells to reach the oocyte. RNAi screening allowed us to identify the dapc1 gene as being critical in this process. Clonal and live analysis showed a requirement of dapc1 in both outer border cells and contacting stretched cells for delamination. This mutant phenotype was rescued by dapc1 or dapc2 expression. Loss of dapc1 function was associated with an abnormal lasting accumulation of β-catenin/Armadillo and E-cadherin at the boundary between migrating border and stretched cells. Moreover, β-catenin/armadillo or E-cadherin downregulation rescued the dapc1 loss of function phenotype. Altogether these results indicate that Drosophila Apc1 is required for dynamic remodeling of β-catenin/Armadillo and E-cadherin adhesive complexes between outer border cells and stretched cells regulating proper delamination and invasion of migrating epithelial clusters.