Regulated endocytic routing modulates wingless signaling in Drosophila embryos

Steroid hormone-induced wingless ligands tune female intestinal size in Drosophila

Female reproduction comes at great expense to energy metabolism compensated by extensive organ adaptations including intestinal size. Upon mating, endocrine signals orchestrate a 30% net increase of absorptive epithelium. Mating increases production of the steroid hormone Ecdysone released by the Drosophila ovaries that stimulates intestinal stem cell (ISC) divisions. Here, we uncover the transcription factor crooked legs (crol) as an intraepithelial coordinator of Ecdysone-induced ISC mitosis. For the precise investigation of non-autonomous factors on ISC behaviour, we establish Rapport, a spatiotemporally-controlled dual expression and tracing system for the analysis of paracrine genetic manipulation while tracing ISC behaviour. Rapport tracing reveals that Ecdysone-induced Crol controls mitogenic Wnt/Wg-ligand expression from epithelial enterocytes activating ISC mitosis. Paracrine Wg stimulation is counterbalanced by Crol-repression of string/CDC25 and CyclinB autonomously in ISC. Rapport-based ISC tumours confirm paracrine stimulation through the Ecdysone-Crol-Wg axis on mitotic behaviour, whereas the autonomous anti-proliferative role of Crol in ISC is conserved in models of colorectal cancer. Finally, mathematical modelling corroborates increasing enterocyte numbers and Wnt/Wg-degradation to set a stable post-mating intestinal size. Together, our findings provide insights into the complex endocrine growth control mechanisms during mating-induced adaptations and might help untangling pleiotropic hormonal effects observed in gastrointestinal tumorigenesis.

The E3 Ubiquitin Ligase Trip12 attenuates Wnt9a/Fzd9b signaling during hematopoietic stem cell development

Wnt signaling is essential for both the development and homeostasis of diverse cellular lineages, including hematopoietic stem cells. Organism-wide, Wnt signals are tightly regulated, as overactivation of the pathway can lead to tumorigenesis. Although numerous Wnt ligands and Frizzled (Fzd) receptors exist, how particular Wnt/Fzd pairings are established and how their signals are regulated is poorly understood. We have previously identified the requirements of the cognate pairing of Wnt9a and Fzd9b for early hematopoietic stem cell proliferation. However, the specific signals governing activation, but equally important, the molecular mechanisms required to turn the signal 'off,' are unknown. Here, we show that the E3 ubiquitin ligase Trip12 (thyroid hormone receptor interactor 12) is specifically required to ubiquitinate the third intracellular loop of Fzd9b at K437, targeting it for lysosomal degradation. In contrast to other ubiquitin ligases described to regulate the cell surface availability of multiple Fzds broadly, our data indicate that Trip12 is selective for Fzd9b. We further demonstrate that this occurs through ubiquitination at K437 of Fzd9b in the third intracellular loop, ultimately leading to a decrease in Fzd9b receptor availability and in Wnt9a/Fzd9b signaling that impacts hematopoietic stem cell proliferation in zebrafish. Our results point to specific mechanisms driving the availability of different Fzd receptors. Determining how particular Fzd abundance is regulated at the membrane will be critical to developing specific therapies for human intervention.

EGFR-dependent endocytosis of Wnt9a and Fzd9b promotes β-catenin signaling during hematopoietic stem cell development in zebrafish

Cell-to-cell communication through secreted Wnt ligands that bind to members of the Frizzled (Fzd) family of transmembrane receptors is critical for development and homeostasis. Wnt9a signals through Fzd9b, the co-receptor LRP5 or LRP6 (LRP5/6), and the epidermal growth factor receptor (EGFR) to promote early proliferation of zebrafish and human hematopoietic stem cells during development. Here, we developed fluorescently labeled, biologically active Wnt9a and Fzd9b fusion proteins to demonstrate that EGFR-dependent endocytosis of the ligand-receptor complex was required for signaling. In human cells, the Wnt9a-Fzd9b complex was rapidly endocytosed and trafficked through early and late endosomes, lysosomes, and the endoplasmic reticulum. Using small-molecule inhibitors and genetic and knockdown approaches, we found that Wnt9a-Fzd9b endocytosis required EGFR-mediated phosphorylation of the Fzd9b tail, caveolin, and the scaffolding protein EGFR protein substrate 15 (EPS15). LRP5/6 and the downstream signaling component AXIN were required for Wnt9a-Fzd9b signaling but not for endocytosis. Knockdown or loss of EPS15 impaired hematopoietic stem cell development in zebrafish. Other Wnt ligands do not require endocytosis for signaling activity, implying that specific modes of endocytosis and trafficking may represent a method by which Wnt-Fzd specificity is established.

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

Extracellular WNTs: Trafficking, Exosomes, and Ligand-Receptor Interaction

WNT signaling is a key developmental pathway in tissue organization. A recent focus of research is the secretion of WNT proteins from source cells. Research over the past decade on how WNTs are produced and released into the extracellular space has unravelled very specific control mechanisms in the early secretory pathway, specialized trafficking routes, and redundant forms of packaging for delivery to target cells. In this review I discuss the findings that WNT proteins have been found on extracellular vesicles (EVs) such as exosomes and possible functional implications. There is an ongoing debate in the WNT signaling field whether EV are relevant in vivo and can fulfill specific functions, also fueled by the general preconception of EV secretion as cellular garbage disposal. As part of the EV research community, I want to give an overview of what we know and don't know about WNT secretion on EVs and offer a more unifying model that can explain current discrepancies in observations regarding WNT secretion.

Revisiting staining of biological samples for electron microscopy: perspectives for recent research

This review revisits essential staining protocols for electron microscopy focussing on the visualization of active sites, i.e. enzymes, metabolites or proteins, in cells and tissues, which have been developed 50 to 60 years ago, however, never were established as standard protocols being used in electron microscopy in a routine fashion. These approaches offer numerous possibilities to expand the knowledge of cellular function and specifically address the localization of active compounds of these systems. It is our conviction, that many of these techniques are still useful, in particular when applied in conjunction with correlative light and electron microscopy. Revisiting specialized classical electron microscopy staining protocols for use in correlative microscopy is particularly promising, as some of these protocols were originally developed as staining methods for light microscopy. To account for this history, rather than summarizing the most recent achievements in literature, we instead first provide an overview of techniques that have been used in the past. While some of these techniques have been successfully implemented into modern microscopy techniques during recent years already, more possibilities are yet to be re-discovered and provide exciting new perspectives for their future use.

Activity-dependent endocytosis of Wingless regulates synaptic plasticity in the Drosophila visual system

Neural activity contributes to synaptic regulation in sensory systems, which allows organisms to adjust to changing environments. However, little is known about how synaptic molecular components are regulated to achieve activity-dependent plasticity at central synapses. Previous studies have shown that following prolonged exposure to natural ambient light, the presynaptic active zone (AZ), an area associated with presynaptic neurotransmitter release in Drosophila photoreceptors, undergoes reversible remodeling. Other studies suggest that the secretory protein Wingless (Wg; an ortholog of Wnt-1) can mediate communication between synaptic cells to achieve synaptic remodeling. However, the source of Wg and the mechanism of Wg signal modulation by neuronal activity remained unclear. Here, we found that Wg secreted from glial cells regulates synaptic remodeling in photoreceptors. In addition, antibody staining revealed that Wg changes its localization depending on light conditions. Although Wg is secreted from glial cells, Wg appeared inside photoreceptor axons when flies were kept under light conditions, suggesting that an increase in neuronal activity causes Wg internalization into photoreceptors by endocytosis. Indeed, by blocking endocytosis in photoreceptors, the localization of Wg in photoreceptors disappeared. Interestingly, Wg accumulation was higher in axons with disassembled AZ structure than in axons whose AZ structure was stabilized at the single-cell level, indicating that Wg endocytosis may trigger AZ disassembly. Furthermore, when we genetically activated Wg signaling, Wg accumulation in photoreceptors decreased. Conversely, when we suppressed Wg signaling there was an increase in Wg accumulation. Through RNAi screening of Ca²⁺-binding proteins in photoreceptors, we found that Calcineurin is a key molecule that triggers Wg endocytosis. Overall, we propose that Wg signaling is regulated by a negative feedback loop driven by Wg endocytosis. The increase in neuronal activity is transmitted via calcium signaling, which leads to a decrease in Wg signaling and thereby promotes presynaptic remodeling.

A Picture Worth a Thousand Molecules-Integrative Technologies for Mapping Subcellular Molecular Organization and Plasticity in Developing Circuits

A key challenge in developmental neuroscience is identifying the local regulatory mechanisms that control neurite and synaptic refinement over large brain volumes. Innovative molecular techniques and high-resolution imaging tools are beginning to reshape our view of how local protein translation in subcellular compartments drives axonal, dendritic, and synaptic development and plasticity. Here we review recent progress in three areas of neurite and synaptic study in situ-compartment-specific transcriptomics/translatomics, targeted proteomics, and super-resolution imaging analysis of synaptic organization and development. We discuss synergies between sequencing and imaging techniques for the discovery and validation of local molecular signaling mechanisms regulating synaptic development, plasticity, and maintenance in circuits.

Bile salt-dependent lipase promotes the barrier integrity of Caco-2 cells by activating Wnt/β-catenin signaling via LRP6 receptor

Bile salt-dependent lipase (BSDL) within intestinal lumen can be endocytosed by enterocytes and support the intestinal barrier function. However, the epithelial-supporting effect of this protein has not been verified in a human cell line and neither the direct signaling pathway nor the function of endocytosis in this process has been clearly identified. We sought to investigate the signaling pathway and the membrane receptor through which BSDL might exert these effects using intestinal epithelial cells. Caco-2 cells were treated with recombinant BSDL, and the barrier function, cell proliferation, and activation of the Wnt signaling pathway were assessed. The effect of Wnt signaling activation induced by BSDL and BSDL endocytosis was investigated in LRP6-silenced and non-silenced cells. Moreover, caveolae- and clathrin-dependent endocytosis inhibitors were also applied respectively to analyze their effects on Wnt signaling activation induced by BSDL. BSDL treatment increased the barrier function but not proliferation of Caco-2 cells. It also induced β-catenin nuclear translocation and activated Wnt target gene transcription. Moreover, in the Wnt pathway, BSDL increased the levels of non-phosphorylated-β-catenin (Ser33/37/Thr41) and phosphorylated-β-catenin (Ser552). Notably, the silencing of LRP6 expression impaired BSDL endocytosis and decreased BSDL-induced β-catenin nuclear translocation. The inhibition of BSDL endocytosis induced by caveolae-mediated endocytosis inhibitor was stronger than that by clathrin-mediated endocytosis inhibitor, and the Wnt signaling activation associated with its endocytosis was also most likely caveolae-dependent. Our findings suggested that LRP6, a canonical Wnt pathway co-receptor, can mediate BSDL endocytosis and then activate Wnt signaling in Caco-2 cells.

Ykt6-dependent endosomal recycling is required for Wnt secretion in the Drosophila wing epithelium

Morphogens are important signalling molecules for tissue development and their secretion requires tight regulation. In the wing imaginal disc of flies, the morphogen Wnt/Wingless is apically presented by the secreting cell and re-internalized before final long-range secretion. Why Wnt molecules undergo these trafficking steps and the nature of the regulatory control within the endosomal compartment remain unclear. Here, we have investigated how Wnts are sorted at the level of endosomes by the versatile v-SNARE Ykt6. Using in vivo genetics, proximity-dependent proteomics and in vitro biochemical analyses, we show that most Ykt6 is present in the cytosol, but can be recruited to de-acidified compartments and recycle Wnts to the plasma membrane via Rab4-positive recycling endosomes. Thus, we propose a molecular mechanism by which producing cells integrate and leverage endocytosis and recycling via Ykt6 to coordinate extracellular Wnt levels.

CDK11 negatively regulates Wnt/β-catenin signaling in the endosomal compartment by affecting microtubule stability

Objectives: Improper activation of Wnt/β-catenin signaling has been implicated in human diseases. Beyond the well-studied glycogen synthase kinase 3β (GSK3β) and casein kinase 1 (CK1), other kinases affecting Wnt/β-catenin signaling remain to be defined. Methods:To identify the kinases that modulate Wnt/β-catenin signaling, we applied a kinase small interfering RNA (siRNA) library screen approach. Luciferase assays, immunoblotting, and real-time polymerase chain reaction (PCR) were performed to confirm the regulation of the Wnt/β-catenin signaling pathway by cyclin-dependent kinase 11 (CDK11) and to investigate the underlying mechanism. Confocal immunofluorescence, coimmunoprecipitation (co-IP), and scratch wound assays were used to demonstrate colocalization, detect protein interactions, and explore the function of CDK11. Results: CDK11 was found to be a significant candidate kinase participating in the negative control of Wnt/β-catenin signaling. Down-regulation of CDK11 led to the accumulation of Wnt/β-catenin signaling receptor complexes, in a manner dependent on intact adenomatosis polyposis coli (APC) protein. Further analysis showed that CDK11 modulation of Wnt/β-catenin signaling engaged the endolysosomal machinery, and CDK11 knockdown enhanced the colocalization of Wnt/β-catenin signaling receptor complexes with early endosomes and decreased colocalization with lysosomes. Mechanistically, CDK11 was found to function in Wnt/β-catenin signaling by regulating microtubule stability. Depletion of CDK11 down-regulated acetyl-α-tubulin. Moreover, co-IP assays demonstrated that CDK11 interacts with the α-tubulin deacetylase SIRT2, whereas SIRT2 down-regulation in CDK11-depleted cells reversed the accumulation of Wnt/β-catenin signaling receptor complexes. CDK11 was found to suppress cell migration through altered Wnt/β-catenin signaling. Conclusions: CDK11 is a negative modulator of Wnt/β-catenin signaling that stabilizes microtubules, thus resulting in the dysregulation of receptor complex trafficking from early endosomes to lysosomes.

Independent Negative Prognostic Role of TCF1 Expression within the Wnt/β-Catenin Signaling Pathway in Primary Breast Cancer Patients

The Wnt pathway is involved in the progression of breast cancer (BC). We aimed to evaluate the expression of some components of the Wnt pathway (β-catenin, FZD4 (frizzled receptor 4), LRP5 (low-density lipoprotein receptor-related protein 5), LRP6, and TCF1 (T-cell factor 1)) to detect potential associations with NHERF1 (Na+/H+ exchanger regulatory factor 1) protein. Besides, we assessed their impact on patients’ clinical outcome. We evaluated 220 primary BC samples by immunohistochemistry (IHC) and protein localization by immunofluorescence. We found a significant correlation between NHERF1 and FZD4, LRP5, LRP6, and TCF1. Univariate analysis showed that the overexpression of β-catenin (p < 0.0001), FZD4 (p = 0.0001), LRP5, LRP6, and TCF1 (p < 0.0001 respectively) was related to poor disease-free survival (DFS). A Kaplan-Meier analysis confirmed univariate data and showed a poor DFS for cNHERF1+/FZD4+ (p = 0.0007), cNHERF1+/LRP5+ (p = 0.0002), cNHERF1+/LRP6+ (p < 0.0001), and cNHERF1+/TCF1+ phenotypes (p = 0.0034). In multivariate analysis, the expression of TCF1 and β-catenin was an independent prognostic variable of worse DFS (p = 0.009 and p = 0.027, respectively). In conclusion, we found that the overexpression of β-catenin, FZD4, LRP5, LRP6, and TCF1 was associated with poor prognosis. Furthermore, we first identified TCF1 as an independent prognostic factor of poor outcome, indicating it as a new potential biomarker for the management of BC patients. Also, the expression of Wnt pathway proteins, both alone and in association with NHERF1, suggests original associations of biological significance for new studies.

Vps11 and Vps18 of Vps-C membrane traffic complexes are E3 ubiquitin ligases and fine-tune signalling

In response to extracellular signals, many signalling proteins associated with the plasma membrane are sorted into endosomes. This involves endosomal fusion, which depends on the complexes HOPS and CORVET. Whether and how their subunits themselves modulate signal transduction is unknown. We show that Vps11 and Vps18 (Vps11/18), two common subunits of the HOPS/CORVET complexes, are E3 ubiquitin ligases. Upon overexpression of Vps11/Vps18, we find perturbations of ubiquitination in signal transduction pathways. We specifically demonstrate that Vps11/18 regulate several signalling factors and pathways, including Wnt, estrogen receptor α (ERα), and NFκB. For ERα, we demonstrate that the Vps11/18-mediated ubiquitination of the scaffold protein PELP1 impairs the activation of ERα by c-Src. Thus, proteins involved in membrane traffic, in addition to performing their well-described role in endosomal fusion, fine-tune signalling in several different ways, including through ubiquitination.

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.

Quintuple labeling in the electron microscope with genetically encoded enhanced horseradish peroxidase

Genetic encoded multilabeling is essential for modern cell biology. In fluorescence microscopy this need has been satisfied by the development of numerous color-variants of the green fluorescent protein. In electron microscopy, however, true genetic encoded multilabeling is currently not possible. Here, we introduce combinatorial cell organelle type-specific labeling as a strategy for multilabeling. First, we created a reliable and high sensitive label by evolving the catalytic activity of horseradish peroxidase (HRP). We then built fusion proteins that targeted our new enhanced HRP (eHRP) to three cell organelles whose labeling pattern did not overlap with each other. The labeling of the endoplasmic reticulum, synaptic vesicles and the plasma membrane consequently allowed for triple labeling in the EM. The combinatorial expression of the three organelle-specific constructs increased the number of clearly distinguishable labels to seven. This strategy of multilabeling for EM closes a significant gap in our tool set and has a broad application range in cell biology.

Bile salt dependent lipase promotes intestinal adaptation in rats with massive small bowel resection

Intestinal adaptation is important for the short bowel syndrome (SBS) patients. Growing evidence has suggested that bile salt dependent lipase (BSDL) not only has the lipolytic activity, but also the immune-modulating and pro-proliferative activities. The purpose of the present study was to investigate the effects of BSDL on intestinal adaptive growth and gut barrier function in a rat model of SBS. Twenty-four male Sprague-Dawley rats were randomly divided into three experimental groups: sham group (rats underwent bowel transection and re-anastomosis), SBS group (rats underwent 80% bowel resection), SBS-BSDL group (SBS rats orally administered BSDL). The animals were weighed daily. The intestinal morpho-histochemical changes and intestinal barrier function were determined 14 days after the operations. Meanwhile, the expressions of Wnt signaling molecules in enterocytes were also analyzed by immunohistochemistry and Western blot. The postoperative weight gain was faster in the SBS rats treated with BSDL than in the SBS/untreated group. The SBS rats treated with BSDL had significantly greater villus height, crypt depth, and enterocyte proliferation in their residual intestines, as compared with the SBS/untreated group. The recovery of intestinal barrier function was promoted and the expressions of tight-junction proteins were increased in the SBS rats treated with BSDL. Additionally, the data indicated that the proadaptive activities of BSDL might be mediated by Wnt signaling activation in the enterocytes. These observations suggested that enteral BSDL administration promoted intestinal adaptive growth and barrier repairing by activating Wnt signaling pathway in SBS rats.

Suppression of epithelial folding at actomyosin-enriched compartment boundaries downstream of Wingless signalling in Drosophila

Epithelial folding shapes embryos and tissues during development. Here, we investigate the coupling between epithelial folding and actomyosin-enriched compartmental boundaries. The mechanistic relationship between the two is unclear, because actomyosin-enriched boundaries are not necessarily associated with folds. Also, some cases of epithelial folding occur independently of actomyosin contractility. We investigated the shallow folds called parasegment grooves that form at boundaries between anterior and posterior compartments in the early Drosophila embryo. We demonstrate that formation of these folds requires the presence of an actomyosin enrichment along the boundary cell-cell contacts. These enrichments, which require Wingless signalling, increase interfacial tension not only at the level of the adherens junctions but also along the lateral surfaces. We find that epithelial folding is normally under inhibitory control because different genetic manipulations, including depletion of the Myosin II phosphatase Flapwing, increase the depth of folds at boundaries. Fold depth correlates with the levels of Bazooka (Baz), the Par-3 homologue, along the boundary cell-cell contacts. Moreover, Wingless and Hedgehog signalling have opposite effects on fold depth at the boundary that correlate with changes in Baz planar polarity.

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

ESCRT genes and regulation of developmental signaling

ESCRT (Endosomal Sorting Complex Required for Transport) proteins have been shown to control an increasing number of membrane-associated processes. Some of these, and prominently regulation of receptor trafficking, profoundly shape signal transduction. Evidence in fungi, plants and multiple animal models support the emerging concept that ESCRTs are main actors in coordination of signaling with the changes in cells and tissues occurring during development and homeostasis. Consistent with their pleiotropic function, ESCRTs are regulated in multiple ways to tailor signaling to developmental and homeostatic needs. ESCRT activity is crucial to correct execution of developmental programs, especially at key transitions, allowing eukaryotes to thrive and preventing appearance of congenital defects.

A Wnt-planar polarity pathway instructs neurite branching by restricting F-actin assembly through endosomal signaling

Spatial arrangement of neurite branching is instructed by both attractive and repulsive cues. Here we show that in C. elegans, the Wnt family of secreted glycoproteins specify neurite branching sites in the PLM mechanosensory neurons. Wnts function through MIG-1/Frizzled and the planar cell polarity protein (PCP) VANG-1/Strabismus/Vangl2 to restrict the formation of F-actin patches, which mark branching sites in nascent neurites. We find that VANG-1 promotes Wnt signaling by facilitating Frizzled endocytosis and genetically acts in a common pathway with arr-1/β-arrestin, whose mutation results in defective PLM branching and F-actin patterns similar to those in the Wnt, mig-1 or vang-1 mutants. On the other hand, the UNC-6/Netrin pathway intersects orthogonally with Wnt-PCP signaling to guide PLM branch growth along the dorsal-ventral axis. Our study provides insights for how attractive and repulsive signals coordinate to sculpt neurite branching patterns, which are critical for circuit connectivity.

Phylogenetic and Evolutionary Analyses of the Frizzled Gene Family in Common Carp (Cyprinus carpio) Provide Insights into Gene Expansion from Whole-Genome Duplications

In humans, the frizzled (FZD) gene family encodes 10 homologous proteins that commonly localize to the plasma membrane. Besides being associated with three main signaling pathways for cell development, most FZDs have different physiological effects and are major determinants in the development process of vertebrates and. Here, we identified and annotated the FZD genes in the whole-genome of common carp (Cyprinus carpio), a teleost fish, and determined their phylogenetic relationships to FZDs in other vertebrates. Our analyses revealed extensive gene duplications in the common carp that have led to the 26 FZD genes that we detected in the common carp genome. All 26 FZD genes were assigned orthology to the 10 FZD genes of on-land vertebrates, with none of genes being specific to the fish lineage. We postulated that the expansion of the FZD gene family in common carp was the result of an additional whole genome duplication event and that the FZD gene family in other teleosts has been lost in their evolution history with the reason that the functions of genes are redundant and conservation. Through the expression profiling of FZD genes in common carp, we speculate that the ancestral gene was likely capable of performing all functions and was expressed broadly, while some descendant duplicate genes only performed partial functions and were specifically expressed at certain stages of development.

The Myopic-Ubpy-Hrs nexus enables endosomal recycling of Frizzled

Mop regulates endosomal localization and recycling of Frizzled. Hrs is ubiquitinated and degraded in the absence of Mop. Mop aids in the maintenance of Ubpy to control the ubiquitin homeostasis of Hrs. Mop and Ubpy can rescue each other. Mop’s function is not required in the cell in the absence of the ubiquitin ligase Cbl.

Endosomal trafficking of signaling proteins plays an essential role in cellular homeostasis. The seven-pass transmembrane protein Frizzled (Fz) is a critical component of Wnt signaling. Although Wnt signaling is proposed to be regulated by endosomal trafficking of Fz, the molecular events that enable this regulation are not completely understood. Here we show that the endosomal protein Myopic (Mop) regulates Fz trafficking in the Drosophila wing disk by inhibiting the ubiquitination and degradation of Hrs. Deletion of Mop or Hrs results in endosomal accumulation of Fz and therefore reduced Wnt signaling. The in situ proximity ligation assay revealed a strong association between Mop and Hrs in the Drosophila wing disk. Overexpression of Hrs rescues the trafficking defect caused by mop knockdown. Mop aids in the maintenance of Ubpy, which deubiquitinates (and thus stabilizes) Hrs. In the absence of the ubiquitin ligase Cbl, Mop is dispensable. These findings support a previously unknown role for Mop in endosomal trafficking of Fz in Wnt-receiving cells.

Specialized Cortex Glial Cells Accumulate Lipid Droplets in Drosophila melanogaster

Lipid droplets (LDs) are common organelles of the majority of eukaryotic cell types. Their biological significance has been extensively studied in mammalian liver cells and white adipose tissue. Although the central nervous system contains the highest relative amount and the largest number of different lipid species, neither the spatial nor the temporal distribution of LDs has been described. In this study, we used the brain of the fruitfly, Drosophila melanogaster, to investigate the neuroanatomy of LDs. We demonstrated that LDs are exclusively localised in glial cells but not in neurons in the larval nervous system. We showed that the brain’s LD pool, rather than being constant, changes dynamically during development and reaches its highest value at the beginning of metamorphosis. LDs are particularly enriched in cortex glial cells located close to the brain surface. These specialized superficial cortex glial cells contain the highest amount of LDs among glial cell types and encapsulate neuroblasts and their daughter cells. Superficial cortex glial cells, combined with subperineurial glial cells, express the Drosophila fatty acid binding protein (Dfabp), as we have demonstrated through light- and electron microscopic immunocytochemistry. To the best of our best knowledge this is the first study that describes LD neuroanatomy in the Drosophila larval brain.

Keeping Wnt signalosome in check by vesicular traffic

Wg/Wnts are paracrine and autocrine ligands that activate distinct signaling pathways while being internalized through surface receptors. Converging and contrasting views are shaping our understanding of whether, where, and how endocytosis may modulate Wnt signaling. We gather considerable amount of evidences to elaborate the point that signal-receiving cells utilize distinct, flexible, and sophisticated vesicular trafficking mechanisms to keep Wnt signaling activity in check. Same molecules in a highly context-dependent fashion serve as regulatory hub for various signaling purposes: amplification, maintenance, inhibition, and termination. Updates are provided for the regulatory mechanisms related to the three critical cell surface complexes, Wnt-Fzd-LRP6, Dkk1-Kremen-LRP6, and R-spondin-LGR5-RNF43, which potently influence Wnt signaling. We pay particular attentions to how cells achieve sustained and delicate control of Wnt signaling strength by employing comprehensive aspects of vesicular trafficking.

Axonal wrapping in the Drosophila PNS is controlled by glia-derived neuregulin homolog Vein

Efficient neuronal conductance requires that axons are insulated by glial cells. For this, glial membranes need to wrap around axons. Invertebrates show a relatively simple extension of glial membranes around the axons, resembling Remak fibers formed by Schwann cells in the mammalian peripheral nervous system. To unravel the molecular pathways underlying differentiation of glial cells that provide axonal wrapping, we are using the genetically amenable Drosophila model. At the end of larval life, the wrapping glia differentiates into very large cells, spanning more than 1 mm of axonal length. The extension around axonal membranes is not influenced by the caliber of the axon or its modality. Using cell type-specific gene knockdown we show that the extension of glial membranes around the axons is regulated by an autocrine activation of the EGF receptor through the neuregulin homolog Vein. This resembles the molecular mechanism employed during cell-autonomous reactivation of glial differentiation after injury in mammals. We further demonstrate that Vein, produced by the wrapping glia, also regulates the formation of septate junctions in the abutting subperineurial glia. Moreover, the wrapping glia indirectly controls the proliferation of the perineurial glia. Thus, the wrapping glia appears center stage to orchestrate the development of the different glial cell layers in a peripheral nerve.

Ultrastructural visualization of trans-ciliary rhodopsin cargoes in mammalian rods

Background

Cilia are vital to various cellular and sensory functions. The pathway by which ciliary membrane proteins translocate through the transition zone is not well understood. Direct morphological characterization of ciliary cargoes in transit remains lacking. In the vertebrate photoreceptor, rhodopsin is synthesized and transported from the inner segment to the disc membranes of the outer segment (OS), which is a modified cilium. To date, the membrane topology of the basal OS and the mechanisms by which rhodopsin is transported through the transition zone (i.e., connecting cilium) and by which nascent disc membranes are formed remain controversial.

Results

Using an antibody recognizing its cytoplasmic C-terminus, we localize rhodopsin on both the plasma membrane and lumen of the connecting cilium by immuno-electron microscopy (EM). We also use transmission EM to visualize the electron-dense enzymatic products derived from the rhodopsin-horseradish peroxidase (HRP) fusion in transfected rodent rods. In the connecting cilium, rhodopsin is not only expressed in the plasma membrane but also in the lumen on two types of membranous carriers, long smooth tubules and small, coated, filament-bound vesicles. Additionally, membrane-bound rhodopsin carriers are also found in close proximity to the nascent discs at the basal OS axoneme and in the distal inner segment. This topology-indicative HRP-rhodopsin reporter shows that the nascent basalmost discs and the mature discs have the same membrane topology, with no indication of evagination or invagination from the basal OS plasma membranes. Serial block face and focus ion beam scanning EM analyses both indicate that the transport carriers enter the connecting cilium lumen from either the basal body lumen or cytoplasmic space between the axonemal microtubules and the ciliary plasma membrane.

Conclusions

Our results suggest the existence of multiple ciliary gate entry pathways in rod photoreceptors. Rhodopsin is likely transported across the connecting cilium on the plasma membrane and through the lumens on two types of tubulovesicular carriers produced in the inner segment. Our findings agree with a previous model that rhodopsin carriers derived from the cell body may fuse directly onto nascent discs as they grow and mature.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-015-0013-1) contains supplementary material, which is available to authorized users.

Wnt5a promotes cancer cell invasion and proliferation by receptor-mediated endocytosis-dependent and -independent mechanisms, respectively

Wnt5a activates the Wnt/β-catenin-independent pathway and its overexpression is associated with tumor aggressiveness enhancing invasive activity. For this action, Wnt5a-induced receptor endocytosis with clathrin is required. Wnt5a expression was previously believed to be associated with cancer cell motility but not proliferation. Recently, it was reported that Wnt5a is also implicated in cancer cell proliferation, but the mechanism was not clear. In this study, we generated a neutralizing anti-Wnt5a monoclonal antibody (mAb5A16) to investigate the mechanism by which Wnt5a regulates cancer cell proliferation. Wnt5a stimulated both invasion and proliferation of certain types of cancer cells, including HeLaS3 cervical cancer cells and A549 lung cancer cells although Wnt5a promoted invasion but not proliferation in other cancer cells such as KKLS gastric cancer cells. mAb5A16 did not affect the binding of Wnt5a to its receptor, but it suppressed Wnt5a-induced receptor-mediated endocytosis. mAb5A16 inhibited invasion but not proliferation of HeLaS3 and A549 cells. Wnt5a activated Src family kinases (SFKs) and Wnt5a-dependent cancer cell proliferation was dependent on SFKs, yet blockade of receptor-mediated endocytosis did not affect cancer cell proliferation and SFK activity. These results suggest that Wnt5a promotes invasion and proliferation of certain types of cancer cells through receptor-mediated endocytosis-dependent and -independent mechanisms, respectively.

A genetically specified connectomics approach applied to long-range feeding regulatory circuits

Synaptic connectivity and molecular composition provide a blueprint for information processing in neural circuits. Detailed structural analysis of neural circuits requires nanometer resolution, which can be obtained with serial-section electron microscopy. However, this technique remains challenging for reconstructing molecularly defined synapses. We used a genetically encoded synaptic marker for electron microscopy (GESEM) based on intra-vesicular generation of electron-dense labeling in axonal boutons. This approach allowed the identification of synapses from Cre recombinase-expressing or GAL4-expressing neurons in the mouse and fly with excellent preservation of ultrastructure. We applied this tool to visualize long-range connectivity of AGRP and POMC neurons in the mouse, two molecularly defined hypothalamic populations that are important for feeding behavior. Combining selective ultrastructural reconstruction of neuropil with functional and viral circuit mapping, we characterized some basic features of circuit organization for axon projections of these cell types. Our findings demonstrate that GESEM labeling enables long-range connectomics with molecularly defined cell types.

FIH-1 disrupts an LRRK1/EGFR complex to positively regulate keratinocyte migration

Factor inhibiting hypoxia-inducible factor 1 (FIH-1; official symbol HIF1AN) is a hydroxylase that negatively regulates hypoxia-inducible factor 1α but also targets other ankyrin repeat domain-containing proteins such as Notch receptor to limit epithelial differentiation. We show that FIH-1 null mutant mice exhibit delayed wound healing. Importantly, in vitro scratch wound assays demonstrate that the positive role of FIH-1 in migration is independent of Notch signaling, suggesting that this hydroxylase targets another ankyrin repeat domain-containing protein to positively regulate motogenic signaling pathways. Accordingly, FIH-1 increases epidermal growth factor receptor (EGFR) signaling, which in turn enhances keratinocyte migration via mitogen-activated protein kinase pathway, leading to extracellular signal-regulated kinase 1/2 activation. Our studies identify leucine-rich repeat kinase 1 (LRRK1), a key regulator of the EGFR endosomal trafficking and signaling, as an FIH-1 binding partner. Such an interaction prevents the formation of an EGFR/LRRK1 complex, necessary for proper EGFR turnover. The identification of LRRK1 as a novel target for FIH-1 provides new insight into how FIH-1 functions as a positive regulator of epithelial migration.

Sara endosomes and the asymmetric division of intestinal stem cells

Tissue homeostasis is maintained by adult stem cells, which self-renew and give rise to differentiating cells. The generation of daughter cells with different fates is mediated by signalling molecules coming from an external niche or being asymmetrically dispatched between the two daughters upon stem cell mitosis. In the adult Drosophila midgut, the intestinal stem cell (ISC) divides to generate a new ISC and an enteroblast (EB) differentiating daughter. Notch signalling activity restricted to the EB regulates intestinal cell fate decision. Here, we show that ISCs divide asymmetrically, and Sara endosomes in ISCs are specifically dispatched to the presumptive EB. During ISC mitosis, Notch and Delta traffic through Sara endosomes, thereby contributing to Notch signalling bias, as revealed in Sara mutants: Sara itself contributes to the control of the ISC asymmetric division. Our data uncover an intrinsic endosomal mechanism during ISC mitosis, which participates in the maintenance of the adult intestinal lineage.

Migrating cells mediate long-range WNT signaling

In amniotes, it is widely accepted that WNTs secreted by the dorsal neural tube form a concentration gradient that regulates early somite patterning and myotome organization. Here we demonstrate in the chicken embryo that WNT protein is not secreted to act at a distance, but rather loaded onto migrating neural crest cells that deliver it to somites. Inhibiting neural crest migration or ablating their population has a profound impact on the WNT response in somites. Furthermore, we show that a central player in the efficient delivery of WNT to somites is the heparan sulfate proteoglycan GPC4, expressed by neural crest. Together, our data describe a novel mode of signaling whereby WNT proteins hitch a ride on migratory neural crest cells to pattern the somites at a distance from its source.

Endocytosis and signaling during development

The development of multicellular organisms relies on an intricate choreography of intercellular communication events that pattern the embryo and coordinate the formation of tissues and organs. It is therefore not surprising that developmental biology, especially using genetic model organisms, has contributed significantly to the discovery and functional dissection of the associated signal-transduction cascades. At the same time, biophysical, biochemical, and cell biological approaches have provided us with insights into the underlying cell biological machinery. Here we focus on how endocytic trafficking of signaling components (e.g., ligands or receptors) controls the generation, propagation, modulation, reception, and interpretation of developmental signals. A comprehensive enumeration of the links between endocytosis and signal transduction would exceed the limits of this review. We will instead use examples from different developmental pathways to conceptually illustrate the various functions provided by endocytic processes during key steps of intercellular signaling.

Epidermal cells are the primary phagocytes in the fragmentation and clearance of degenerating dendrites in Drosophila

During developmental remodeling, neurites destined for pruning often degenerate on-site. Physical injury also induces degeneration of neurites distal to the injury site. Prompt clearance of degenerating neurites is important for maintaining tissue homeostasis and preventing inflammatory responses. Here we show that in both dendrite pruning and dendrite injury of Drosophila sensory neurons, epidermal cells rather than hemocytes are the primary phagocytes in clearing degenerating dendrites. Epidermal cells act via Draper-mediated recognition to facilitate dendrite degeneration and to engulf and degrade degenerating dendrites. Using multiple dendritic membrane markers to trace phagocytosis, we show that two members of the CD36 family, croquemort (crq) and debris buster (dsb), act at distinct stages of phagosome maturation for dendrite clearance. Our finding reveals the physiological importance of coordination between neurons and their surrounding epidermis, for both dendrite fragmentation and clearance.

Patterning and growth control by membrane-tethered Wingless

Wnts are evolutionarily conserved secreted signalling proteins that, in various developmental contexts, spread from their site of synthesis to form a gradient and activate target-gene expression at a distance. However, the requirement for Wnts to spread has never been directly tested. Here we used genome engineering to replace the endogenous wingless gene, which encodes the main Drosophila Wnt, with one that expresses a membrane-tethered form of the protein. Surprisingly, the resulting flies were viable and produced normally patterned appendages of nearly the right size, albeit with a delay. We show that, in the prospective wing, prolonged wingless transcription followed by memory of earlier signalling allows persistent expression of relevant target genes. We suggest therefore that the spread of Wingless is dispensable for patterning and growth even though it probably contributes to increasing cell proliferation.

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.

Sphingolipid depletion impairs endocytic traffic and inhibits Wingless signaling

Sphingolipids are an important part of the plasma membrane and implicated in a multitude of cellular processes. However, little is known about the role of sphingolipids in an epithelial context and their potential influence on the activity of signaling pathways. To shed light on these aspects we analyzed the consequences of changing ceramide levels in vivo in the Drosophila wing disc: an epithelial tissue in which the most fundamental signaling pathways, including the Wnt/Wg signaling pathway, are well characterized. We found that downregulation of Drosophila's only ceramide synthase gene schlank led to defects in the endosomal trafficking of proteins. One of the affected proteins is the Wnt ligand Wingless (Wg) that accumulated. Unexpectedly, although Wg protein levels were raised, signaling activity of the Wg pathway was impaired. Recent work has spotlighted the central role of the endocytic trafficking in the transduction of the Wnt signal. Our results underscore this and support the view that sphingolipid levels are crucial in orchestrating epithelial endocytic trafficking in vivo. They further demonstrate that ceramide/sphingolipid levels can affect Wnt signaling.

Wnt regulation of planar cell polarity (PCP)

Planar cell polarity (PCP), a process controlling coordinated, uniformly polarized cellular behaviors in a field of cells, has been identified to be critically required for many fundamental developmental processes. However, a global directional cue that establishes PCP in a three-dimensional tissue or organ with respect to the body axes remains elusive. In vertebrate, while Wnt-secreted signaling molecules have been implicated in regulating PCP in a β-catenin-independent manner, whether they function permissively or act as a global cue to convey directional information is not clearly defined. In addition, the underlying molecular mechanism by which Wnt signal is transduced to core PCP proteins is largely unknown. In this chapter, I review the roles of Wnt signaling in regulating PCP during vertebrate development and update our knowledge of its regulatory mechanism.

Wnt secretion and gradient formation

Concentration gradients formed by the lipid-modified morphogens of the Wnt family are known for their pivotal roles during embryogenesis and adult tissue homeostasis. Wnt morphogens are also implicated in a variety of human diseases, especially cancer. Therefore, the signaling cascades triggered by Wnts have received considerable attention during recent decades. However, how Wnts are secreted and how concentration gradients are formed remains poorly understood. The use of model organisms such as Drosophila melanogaster has provided important advances in this area. For instance, we have previously shown that the lipid raft-associated reggie/flotillin proteins influence Wnt secretion and spreading in Drosophila. Our work supports the notion that producing cells secrete Wnt molecules in at least two pools: a poorly diffusible one and a reggie/flotillin-dependent highly diffusible pool which allows morphogen spreading over long distances away from its source of production. Here we revise the current views of Wnt secretion and spreading, and propose two models for the role of the reggie/flotillin proteins in these processes: (i) reggies/flotillins regulate the basolateral endocytosis of the poorly diffusible, membrane-bound Wnt pool, which is then sorted and secreted to apical compartments for long-range diffusion, and (ii) lipid rafts organized by reggies/flotillins serve as “dating points” where extracellular Wnt transiently interacts with lipoprotein receptors to allow its capture and further spreading via lipoprotein particles. We further discuss these processes in the context of human breast cancer. A better understanding of these phenomena may be relevant for identification of novel drug targets and therapeutic strategies.

The way Wnt works: components and mechanism

The canonical Wnt/β-catenin pathway is an ancient and evolutionarily conserved signaling pathway that is required for the proper development of all metazoans, from the basal demosponge Amphimedon queenslandica to humans. Misregulation of Wnt signaling is implicated in many human diseases, making this pathway an intense area of research in industry as well as academia. In this review, we explore our current understanding of the molecular steps involved in the transduction of a Wnt signal. We will focus on how the critical Wnt pathway component, β-catenin, is in a "futile cycle" of constant synthesis and degradation and how this cycle is disrupted upon pathway activation. We describe the role of the Wnt pathway in major human cancers and in the control of stem cell self-renewal in the developing organism and in adults. Finally, we describe well-accepted criteria that have been proposed as evidence for the involvement of a molecule in regulating the canonical Wnt pathway.

Drosophila heparan sulfate 6-O-endosulfatase Sulf1 facilitates wingless (Wg) protein degradation

Heparan sulfate proteoglycans regulate various physiological and developmental processes through interactions with a number of protein ligands. Heparan sulfate (HS)-ligand binding depends on the amount and patterns of sulfate groups on HS, which are controlled by various HS sulfotransferases in the Golgi apparatus as well as extracellular 6-O-endosulfatases called "Sulfs." Sulfs are a family of secreted molecules that specifically remove 6-O-sulfate groups within the highly sulfated regions on HS. Vertebrate Sulfs promote Wnt signaling, whereas the only Drosophila homologue of Sulfs, Sulf1, negatively regulates Wingless (Wg) signaling. To understand the molecular mechanism for the negative regulation of Wg signaling by Sulf1, we studied the effects of Sulf1 on HS-Wg interaction and Wg stability. Sulf1 overexpression strongly inhibited the binding of Wg to Dally, a potential target heparan sulfate proteoglycan of Sulf1. This effect of Drosophila Sulf1 on the HS-Wg interaction is similar to that of vertebrate Sulfs. Using in vitro, in vivo, and ex vivo systems, we show that Sulf1 reduces extracellular Wg protein levels, at least partly by facilitating Wg degradation. In addition, expression of human Sulf1 in the Drosophila wing disc lowers the levels of extracellular Wg protein, as observed for Drosophila Sulf1. Our study demonstrates that vertebrate and Drosophila Sulfs have an intrinsically similar activity and that the function of Sulfs in the fate of Wnt/Wg ligands is context-dependent.

Chemistry with spatial control using particles and streams()

Spatial control of chemical reactions, with micro- and nanometer scale resolution, has important consequences for one pot synthesis, engineering complex reactions, developmental biology, cellular biochemistry and emergent behavior. We review synthetic methods to engineer this spatial control using chemical diffusion from spherical particles, shells and polyhedra. We discuss systems that enable both isotropic and anisotropic chemical release from isolated and arrayed particles to create inhomogeneous and spatially patterned chemical fields. In addition to such finite chemical sources, we also discuss spatial control enabled with laminar flow in 2D and 3D microfluidic networks. Throughout the paper, we highlight applications of spatially controlled chemistry in chemical kinetics, reaction-diffusion systems, chemotaxis and morphogenesis.

Drosophila TRPML is required for TORC1 activation

Loss-of-function mutations in TRPML1 (transient receptor potential mucolipin 1) cause the lysosomal storage disorder, mucolipidosis type IV (MLIV). Here, we report that flies lacking the TRPML1 homolog displayed incomplete autophagy and reduced viability during the pupal period--a phase when animals rely on autophagy for nutrients. We show that TRPML was required for fusion of amphisomes with lysosomes, and its absence led to accumulation of vesicles of significantly larger volume and higher luminal Ca(2+). We also found that trpml(1) mutant cells showed decreased TORC1 (target of rapamycin complex 1) signaling and a concomitant upregulation of autophagy induction. Both of these defects in the mutants were reversed by genetically activating TORC1 or by feeding the larvae a high-protein diet. The high-protein diet also reduced the pupal lethality and the increased volume of acidic vesicles. Conversely, further inhibition of TORC1 activity by rapamycin exacerbated the mutant phenotypes. Finally, TORC1 exerted reciprocal control on TRPML function. A high-protein diet caused cortical localization of TRPML, and this effect was blocked by rapamycin. Our findings delineate the interrelationship between the TRPML and TORC1 pathways and raise the intriguing possibility that a high-protein diet might reduce the severity of MLIV.

Wnt/Wingless signaling in Drosophila

The Wingless (Wg) pathway represents one of the best-characterized intercellular signaling networks. Studies performed in Drosophila over the last 30 years have contributed to our understanding of the role of Wg signaling in the regulation of tissue growth, polarity, and patterning. These studies have revealed mechanisms conserved in the vertebrate Wnt pathways and illustrate the elegance of using the Drosophila model to understand evolutionarily conserved modes of gene regulation. In this article, we describe the function of Wg signaling in patterning the Drosophila embryonic epidermis and wing imaginal disc. As well, we present an overview of the establishment of the Wg morphogen gradient and discuss the differential modes of Wg-regulated gene expression.

The role of endocytosis in activating and regulating signal transduction

Endocytosis is increasingly understood to play crucial roles in most signaling pathways, from determining which signaling components are activated, to how the signal is subsequently transduced and/or terminated. Whether a receptor-ligand complex is internalized via a clathrin-dependent or clathrin-independent endocytic route, and the complexes' subsequent trafficking through specific endocytic compartments, to then be recycled or degraded, has profound effects on signaling output. This review discusses the roles of endocytosis in three markedly different signaling pathways: the Wnt, Notch, and Eph/Ephrin pathways. These offer fundamentally different signaling systems: (1) diffusible ligands inducing signaling in one cell, (2) membrane-tethered ligands inducing signaling in a contacting receptor cell, and (3) bi-directional receptor-ligand signaling in two contacting cells. In each of these systems, endocytosis controls signaling in fascinating ways, and comparison of their similarities and dissimilarities will help to expand our understanding of endocytic control of signal transduction across multiple signaling pathways.

Intracellular trafficking in Drosophila visual system development: a basis for pattern formation through simple mechanisms

Intracellular trafficking underlies cellular functions ranging from membrane remodeling to receptor activation. During multicellular organ development, these basic cell biological functions are required as both passive machinery and active signaling regulators. Exocytosis, endocytosis, and recycling of several key signaling receptors have long been known to actively regulate morphogenesis and pattern formation during Drosophila eye development. Hence, intracellular membrane trafficking not only sets the cell biological stage for receptor-mediated signaling but also actively controls signaling through spatiotemporally regulated receptor localization. In contrast to eye development, the role of intracellular trafficking for the establishment of the eye-to-brain connectivity map has only recently received more attention. It is still poorly understood how guidance receptors are spatiotemporally regulated to serve as meaningful synapse formation signals. Yet, the Drosophila visual system provides some of the most striking examples for the regulatory role of intracellular trafficking during multicellular organ development. In this review we will first highlight the experimental and conceptual advances that motivate the study of intracellular trafficking during Drosophila visual system development. We will then illuminate the development of the eye, the eye-to-brain connectivity map and the optic lobe from the perspective of cell biological dynamics. Finally, we provide a conceptual framework that seeks to explain how the interplay of simple genetically encoded intracellular trafficking events governs the seemingly complex cellular behaviors, which in turn determine the developmental product.

Effect of Thiovit® Jet on the structure of thoracic microtrichia/trichomes in Drosophila melanogaster

Widely used fungicides and pesticides are known to have profound effect on several nontarget organisms, which is a cause of concern. The present study aims to demonstrate the effect of a fungicide, Thiovit® Jet on the structure of epidermal microtrichia (trichome) of the dorsal thorax in Drosophila melanogaster. External morphology and structural variations of thoracic appendages have been extensively studied using scanning electron microscope from flies treated with different concentrations of Thiovit Jet (20, 30, 40 or 200 μg/ml). Similar to the effect of other fungicides like captan and captafol which are reported to produce somatic mutations in the same organism, the present study successfully demonstrates variation in the trichome/microtrichia structure of the dorsal thorax of D. melanogaster. Structural variations were observed to be associated with different concentrations of Thiovit Jet (30, 40 and 200 μg/ml), but the maximum notable change was found with 40 μg/ml treatment. The gross abnormality in the trichome structure may be due to mutation in proteins associated with normal cuticular deposition.

Picking faces out of a crowd: genetic labels for identification of proteins in correlated light and electron microscopy imaging

Correlated light and electron microscopic (CLEM) imaging is a powerful method for dissecting cell and tissue function at high resolution. Each imaging mode provides unique information, and the combination of the two can contribute to a better understanding of the spatiotemporal patterns of protein expression, trafficking, and function. Critical to these methods is the use of genetically appended tags that highlight specific proteins of interest in order to be able to pick them out of their complex cellular environment. Here we review and discuss the current generation of genetic labels for direct protein identification by CLEM, addressing their relative strengths and weaknesses and in what experiments they would be most useful.

Secreted Wnt "inhibitors" are not just inhibitors: regulation of extracellular Wnt by secreted Frizzled-related proteins

Gradient formation and signaling ranges of secreted proteins are crucial problems to understand how morphogens work for positional information and patterning in animal development. Yet, extracellular behaviors of secreted signaling molecules remain unexplored compared to their downstream pathways inside the cell. Recent advances in bioimaging make it possible to directly visualize morphogen molecules, and this simple strategy has, at least partly, succeeded in uncovering molecular behaviors of morphogens, such as Wnt (wingless-type MMTV integration site family member) and BMP (bone morphogenetic protein) as well as secreted Wnt binding proteins, sFRPs (secreted Frizzled-related proteins), in embryonic tissues. Here, we review the regulation of Wnt signaling by sFRPs, focusing on extracellular regulation of Wnt ligands in comparison with other morphogens. We also discuss evolutionary aspects with comprehensive syntenic and phylogenetic information about vertebrate sfrp genes. We newly annotated several sfrp genes including sfrp2-like 1 (sfrp2l1) in frogs and fishes and crescent in mammals.

LGR4 and LGR5 are R-spondin receptors mediating Wnt/β-catenin and Wnt/PCP signalling

R-spondins are secreted Wnt signalling agonists, which regulate embryonic patterning and stem cell proliferation, but whose mechanism of action is poorly understood. Here we show that R-spondins bind to the orphan G-protein-coupled receptors LGR4 and LGR5 by their Furin domains. Gain- and loss-of-function experiments in mammalian cells and Xenopus embryos indicate that LGR4 and LGR5 promote R-spondin-mediated Wnt/β-catenin and Wnt/PCP signalling. R-spondin-triggered β-catenin signalling requires Clathrin, while Wnt3a-mediated β-catenin signalling requires Caveolin-mediated endocytosis, suggesting that internalization has a mechanistic role in R-spondin signalling.