At the crossroads of polarity, proliferation and apoptosis: the use of Drosophila to unravel the multifaceted role of endocytosis in tumor suppression

A neuroligin-2-YAP axis regulates progression of pancreatic intraepithelial neoplasia

Pancreatic ductal adenocarcinoma (PDAC) is a tumor with a dismal prognosis that arises from precursor lesions called pancreatic intraepithelial neoplasias (PanINs). Progression from low- to high-grade PanINs is considered as tumor initiation, and a deeper understanding of this switch is needed. Here, we show that synaptic molecule neuroligin-2 (NLGN2) is expressed by pancreatic exocrine cells and plays a crucial role in the regulation of contact inhibition and epithelial polarity, which characterize the switch from low- to high-grade PanIN. NLGN2 localizes to tight junctions in acinar cells, is diffusely distributed in the cytosol in low-grade PanINs and is lost in high-grade PanINs and in a high percentage of advanced PDACs. Mechanistically, NLGN2 is necessary for the formation of the PALS1/PATJ complex, which in turn induces contact inhibition by reducing YAP function. Our results provide novel insights into NLGN2 functions outside the nervous system and can be used to model PanIN progression.

Endocytosis at the Crossroad of Polarity and Signaling Regulation: Learning from Drosophila melanogaster and Beyond

Cellular trafficking through the endosomal–lysosomal system is essential for the transport of cargo proteins, receptors and lipids from the plasma membrane inside the cells and across membranous organelles. By acting as sorting stations, vesicle compartments direct the fate of their content for degradation, recycling to the membrane or transport to the trans-Golgi network. To effectively communicate with their neighbors, cells need to regulate their compartmentation and guide their signaling machineries to cortical membranes underlying these contact sites. Endosomal trafficking is indispensable for the polarized distribution of fate determinants, adaptors and junctional proteins. Conversely, endocytic machineries cooperate with polarity and scaffolding components to internalize receptors and target them to discrete membrane domains. Depending on the cell and tissue context, receptor endocytosis can terminate signaling responses but can also activate them within endosomes that act as signaling platforms. Therefore, cell homeostasis and responses to environmental cues rely on the dynamic cooperation of endosomal–lysosomal machineries with polarity and signaling cues. This review aims to address advances and emerging concepts on the cooperative regulation of endocytosis, polarity and signaling, primarily in Drosophila melanogaster and discuss some of the open questions across the different cell and tissue types that have not yet been fully explored.

Imaging Endocytosis Dynamics in Health and Disease

Endocytosis is a critical process for cell growth and viability. It mediates nutrient uptake, guarantees plasma membrane homeostasis, and generates intracellular signaling cascades. Moreover, it plays an important role in dead cell clearance and defense against external microbes. Finally, endocytosis is an important cellular route for the delivery of nanomedicines for therapeutic treatments. Thus, it is not surprising that both environmental and genetic perturbation of endocytosis have been associated with several human conditions such as cancer, neurological disorders, and virus infections, among others. Over the last decades, a lot of research has been focused on developing advanced imaging methods to monitor endocytosis events with high resolution in living cells and tissues. These include fluorescence imaging, electron microscopy, and correlative and super-resolution microscopy. In this review, we outline the major endocytic pathways and briefly discuss how defects in the molecular machinery of these pathways lead to disease. We then discuss the current imaging methodologies used to study endocytosis in different contexts, highlighting strengths and weaknesses.

Tumorigenesis and cell competition in Drosophila in the absence of polyhomeotic function

Cell competition is a homeostatic process that eliminates by apoptosis unfit or undesirable cells from animal tissues, including tumor cells that appear during the life of the organism. In Drosophila there is evidence that many types of oncogenic cells are eliminated by cell competition. One exception is cells mutant for polyhomeotic (ph), a member of the Polycomb family of genes; most of the isolated mutant ph clones survive and develop tumorous overgrowths in imaginal discs. To characterize the tumorigenic effect of the lack of ph, we first studied the growth of different regions of the wing disc deficient in ph activity and found that the effect is restricted to the proximal appendage. Moreover, we found that ph-deficient tissue is partially refractory to apoptosis. Second, we analyzed the behavior of clones lacking ph function and found that many suffer cell competition but are not completely eliminated. Unexpectedly, we found that nonmutant cells also undergo cell competition when surrounded by ph-deficient cells, indicating that within the same tissue cell competition may operate in opposite directions. We suggest two reasons for the incompleteness of cell competition in ph mutant cells: 1) These cells are partially refractory to apoptosis, and 2) the loss of ph function alters the identity of imaginal cells and subsequently their cell affinities. It compromises the winner/loser interaction, a prerequisite for cell competition.

Cell proliferation control by Notch signalling during imaginal discs development in Drosophila

The Notch signalling pathway is evolutionary conserved and participates in numerous developmental processes, including the control of cell proliferation. However, Notch signalling can promote or restrain cell division depending on the developmental context, as has been observed in human cancer where Notch can function as a tumor suppressor or an oncogene. Thus, the outcome of Notch signalling can be influenced by the cross-talk between Notch and other signalling pathways. The use of model organisms such as Drosophila has been proven to be very valuable to understand the developmental role of the Notch pathway in different tissues and its relationship with other signalling pathways during cell proliferation control. Here we review recent studies in Drosophila that shed light in the developmental control of cell proliferation by the Notch pathway in different contexts such as the eye, wing and leg imaginal discs. We also discuss the autonomous and non-autonomous effects of the Notch pathway on cell proliferation and its interactions with different signalling pathways.

Reduction of endocytic activity accelerates cell elimination during tissue remodeling of the Drosophila epidermal epithelium

Epithelial tissues undergo cell turnover both during development and for homeostatic maintenance. Cells that are no longer needed are quickly removed without compromising the barrier function of the tissue. During metamorphosis, insects undergo developmentally programmed tissue remodeling. However, the mechanisms that regulate this rapid tissue remodeling are not precisely understood. Here, we show that the temporal dynamics of endocytosis modulate physiological cell properties to prime larval epidermal cells for cell elimination. Endocytic activity gradually reduces as tissue remodeling progresses. This reduced endocytic activity accelerates cell elimination through the regulation of Myosin II subcellular reorganization, junctional E-cadherin levels, and caspase activation. Whereas the increased Myosin II dynamics accelerates cell elimination, E-cadherin plays a protective role against cell elimination. Reduced E-cadherin is involved in the amplification of caspase activation by forming a positive-feedback loop with caspase. These findings reveal the role of endocytosis in preventing cell elimination and in the cell-property switching initiated by the temporal dynamics of endocytic activity to achieve rapid cell elimination during tissue remodeling.

The Initial Stage of Tumorigenesis in Drosophila Epithelial Tissues

Cancer development originates in a single mutant cell transformed from a normal cell, including further evolution of pro-tumor cells through additional mutations into malignant cancer tissues. Data from recent studies, however, suggest that most pro-tumor cells do not develop into tumors but remain dormant within or are prophylactically eliminated from tissues unless bestowed with additional driver mutations. Drosophila melanogaster has provided very efficient model systems, such as imaginal discs and ovarian follicular epithelia, to study the initial stage of tumorigenesis. This review will focus on the behaviors of emerging pro-tumor cells surrounded by normal cells and situations where they initiate tumor development.

Polarity protein Canoe mediates overproliferation via modulation of JNK, Ras-MAPK and Hippo signalling

Objectives

Over the past decade an intriguing connection between cell polarity and tumorigenesis has emerged. Multiple core components of the junction complexes that help to form and maintain cell polarity display both pro‐ and anti‐tumorigenic functions in a context‐dependent manner, with the underlying mechanisms poorly understood.

Materials and Methods

With transgenic fly lines that overexpress or knock down specific signalling components, we perform genetic analysis to investigate the precise role of the polarity protein Canoe (Cno) in tumorigenesis and the downstream pathways.

Results

We show that overexpression of cno simultaneously activates JNK and Ras‐MEK‐ERK signalling, resulting in mixed phenotypes of both overproliferation and cell death in the Drosophila wing disc. Moderate alleviation of JNK activation eliminates the effect of Cno on cell death, leading to organ overgrowth and cell migration that mimic the formation and invasion of tumours. In addition, we find that the Hippo pathway acts downstream of JNK and Ras signalling to mediate the effect of Cno on cell proliferation.

Conclusions

Our work reveals an oncogenic role of Cno and creates a new type of Drosophila tumour model for cancer research.

Common Deregulation of Seven Biological Processes by MicroRNAs in Gastrointestinal Cancers

MicroRNAs are frequently dysregulated in human neoplasms, including gastrointestinal cancers. Nevertheless, the global influence of microRNA dysregulation on cellular signaling is still unknown. Here we sought to elucidate cellular signaling dysregulation by microRNAs in gastrointestinal cancers at the systems biology level followed by experimental validation. Signature dysregulated microRNAs in gastric, colorectal and liver cancers were defined based on our previous studies. Targets of signature dysregulated miRNAs were predicted using multiple computer algorithms followed by gene enrichment analysis to identify biological processes perturbed by dysregulated microRNAs. Effects of microRNAs on endocytosis were measured by epidermal growth factor (EGF) internalization assay. Our analysis revealed that, aside from well-established cancer-related signaling pathways, several novel pathways, including axon guidance, neurotrophin/nerve growth factor signaling, and endocytosis, were found to be involved in the pathogenesis of gastrointestinal cancers. The regulation of EGF receptor (EGFR) endocytosis by two predicted miRNAs, namely miR-17 and miR-145, was confirmed experimentally. Functionally, miR-145, which blocked EGFR endocytosis, prolonged EGFR membrane signaling and altered responsiveness of colon cancer cells to EGFR-targeting drugs. In conclusion, our analysis depicts a comprehensive picture of cellular signaling dysregulation, including endocytosis, by microRNAs in gastrointestinal cancers.

Non-mammalian models of multiple endocrine neoplasia type 2

Twenty-five years ago, RET was identified as the primary driver of multiple endocrine neoplasia type 2 (MEN2) syndrome. MEN2 is characterized by several transformation events including pheochromocytoma, parathyroid adenoma and, especially penetrant, medullary thyroid carcinoma (MTC). Overall, MTC is a rare but aggressive type of thyroid cancer for which no effective treatment currently exists. Surgery, radiation, radioisotope treatment and chemotherapeutics have all shown limited success, and none of these approaches have proven durable in advanced disease. Non-mammalian models that incorporate the oncogenic RET isoforms associated with MEN2 and other RET-associated diseases have been useful in delineating mechanisms underlying disease progression. These models have also identified novel targeted therapies as single agents and as combinations. These studies highlight the importance of modeling disease in the context of the whole animal, accounting for the complex interplay between tumor and normal cells in controlling disease progression as well as response to therapy. With convenient access to whole genome sequencing data from expanded thyroid cancer patient cohorts, non-mammalian models will become more complex, sophisticated and continue to complement future mammalian studies. In this review, we explore the contributions of non-mammalian models to our understanding of thyroid cancer including MTC, with a focus on Danio rerio and Drosophila melanogaster (fish and fly) models.

High MYC Levels Favour Multifocal Carcinogenesis

The term "field cancerisation" describes the formation of tissue sub-areas highly susceptible to multifocal tumourigenesis. In the earlier stages of cancer, cells may indeed display a series of molecular alterations that allow them to proliferate faster, eventually occupying discrete tissue regions with irrelevant morphological anomalies. This behaviour recalls cell competition, a process based on a reciprocal fitness comparison: when cells with a growth advantage arise in a tissue, they are able to commit wild-type neighbours to death and to proliferate at their expense. It is known that cells expressing high MYC levels behave as super-competitors, able to kill and replace less performant adjacent cells; given MYC upregulation in most human cancers, MYC-mediated cell competition is likely to pioneer field cancerisation. Here we show that MYC overexpression in a sub-territory of the larval wing epithelium of Drosophila is sufficient to trigger a number of cellular responses specific to mammalian pre-malignant tissues. Moreover, following induction of different second mutations, high MYC-expressing epithelia were found to be susceptible to multifocal growth, a hallmark of mammalian pre-cancerous fields. In summary, our study identified an early molecular alteration implicated in field cancerisation and established a genetically amenable model which may help study the molecular basis of early carcinogenesis.

Germline Proliferation Is Regulated by Somatic Endocytic Genes via JNK and BMP Signaling in Drosophila

Signals derived from the microenvironment contribute greatly to tumorigenesis . The underlying mechanism requires thorough investigation. Here, we use Drosophila testis as a model system to address this question, taking the advantage of the ease to distinguish germline and somatic cells and to track the cell numbers. In an EMS mutagenesis screen, we identified Rab5, a key factor in endocytosis, for its nonautonomous role in germline proliferation. The disruption of Rab5 in somatic cyst cells, which escort the development of germline lineage, induced the overproliferation of underdifferentiated but genetically wild-type germ cells. We demonstrated that this nonautonomous effect was mediated by the transcriptional activation of Dpp [the fly homolog of bone morphogenetic protein (BMP)] by examining the Dpp-reporter expression and knocking down Dpp to block germline overgrowth. Consistently, the protein levels of Bam, the germline prodifferentiation factor normally accumulated in the absence of BMP/Dpp signaling, decreased in the overproliferating germ cells. Further, we discovered that the JNK signaling pathway operated between Rab5 and Dpp, because simultaneously inhibiting the JNK pathway and Rab5 in cyst cells prevented both dpp transcription and germline tumor growth. Additionally, we found that multiple endocytic genes, such as avl, TSG101, Vps25, or Cdc42, were required in the somatic cyst cells to restrict germline amplification. These findings indicate that when the endocytic state of the surrounding cells is impaired, genetically wild-type germ cells overgrow. This nonautonomous model of tumorigenesis provides a simple system to dissect the relation between tumor and its niche.

Endocrinology: advances through omics and related technologies

The rapid development of new omics technologies to measure changes at genetic, transcriptomic, proteomic, and metabolomics levels together with the evolution of methods to analyze and integrate the data at a systems level are revolutionizing the study of biological processes. Here we discuss how new approaches using omics technologies have expanded our knowledge especially in nontraditional models. Our increasing knowledge of these interactions and evolutionary pathway conservation facilitates the use of nontraditional species, both invertebrate and vertebrate, as new model species for biological and endocrinology research. The increasing availability of technology to create organisms overexpressing key genes in endocrine function allows manipulation of complex regulatory networks such as growth hormone (GH) in transgenic fish where disregulation of GH production to produce larger fish has also permitted exploration of the role that GH plays in testis development, suggesting that it does so through interactions with insulin-like growth factors. The availability of omics tools to monitor changes at nearly any level in any organism, manipulate gene expression and behavior, and integrate data across biological levels, provides novel opportunities to explore endocrine function across many species and understand the complex roles that key genes play in different aspects of the endocrine function.

JAK/STAT pathway dysregulation in tumors: a Drosophila perspective

Sustained activation of the JAK/STAT pathway is causal to human cancers. This pathway is less complex in Drosophila, and its dysregulation has been linked to several tumor models in this organism. Here, we discuss models of metastatic epithelial and hematopoietic tumors that are causally linked to dysregulation of JAK/STAT signaling in Drosophila. First, we focus on cancer models in imaginal discs where ectopic expression of the JAK/STAT pathway ligand Unpaired downstream of distinct tumor suppressors has emerged as an unexpected mediator of neoplastic transformation. We also discuss the collaboration between STAT and oncogenic Ras in epithelial transformation. Second, we examine hematopoietic tumors, where mutations that cause hyperactive JAK/STAT signaling are necessary and sufficient for "fly leukemia". We highlight the important contributions that genetic screens in Drosophila have made to understanding the JAK/STAT pathway, its developmental roles, and how its function is co-opted during tumorigenesis.

Multiple functions of the SNARE protein Snap29 in autophagy, endocytic, and exocytic trafficking during epithelial formation in Drosophila

How autophagic degradation is linked to endosomal trafficking routes is little known. Here we screened a collection of uncharacterized Drosophila mutants affecting membrane transport to identify new genes that also have a role in autophagy. We isolated a loss of function mutant in Snap29 (Synaptosomal-associated protein 29 kDa), the gene encoding the Drosophila homolog of the human protein SNAP29 and have characterized its function in vivo. Snap29 contains 2 soluble NSF attachment protein receptor (SNARE) domains and a asparagine-proline-phenylalanine (NPF motif) at its N terminus and rescue experiments indicate that both SNARE domains are required for function, whereas the NPF motif is in part dispensable. We find that Snap29 interacts with SNARE proteins, localizes to multiple trafficking organelles, and is required for protein trafficking and for proper Golgi apparatus morphology. Developing tissue lacking Snap29 displays distinctive epithelial architecture defects and accumulates large amounts of autophagosomes, highlighting a major role of Snap29 in autophagy and secretion. Mutants for autophagy genes do not display epithelial architecture or secretion defects, suggesting that the these alterations of the Snap29 mutant are unlikely to be caused by the impairment of autophagy. In contrast, we find evidence of elevated levels of hop-Stat92E (hopscotch-signal transducer and activator of transcription protein at 92E) ligand, receptor, and associated signaling, which might underlie the epithelial defects. In summary, our findings support a role of Snap29 at key steps of membrane trafficking, and predict that signaling defects may contribute to the pathogenesis of cerebral dysgenesis, neuropathy, ichthyosis, and palmoplantar keratoderma (CEDNIK), a human congenital syndrome due to loss of Snap29.

Rabaptin-5 and Rabex-5 are neoplastic tumour suppressor genes that interact to modulate Rab5 dynamics in Drosophila melanogaster

Endocytosis plays an important role in the regulation of tumour growth and metastasis. In Drosophila, a number of endocytic neoplastic tumour suppressor genes have been identified that when mutated cause epithelial disruption and over-proliferation. Here we characterise the Drosophila homologue of the Rab5 effector Rabaptin-5, and show that it is a novel neoplastic tumour suppressor. Its ability to bind Rab5 and modulate early endosomal dynamics is conserved in Drosophila, as is its interaction with the Rab5 GEF Rabex5, for which we also demonstrate neoplastic tumour suppressor characteristics. Surprisingly, we do not observe disruption of apico-basal polarity in Rabaptin-5 and Rabex-5 mutant tissues; instead the tumour phenotype is associated with upregulation of Jun N-terminal Kinase (JNK) and Janus Kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) signalling.

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Drosophila Rabaptin-5 and Rabex-5 are endocytic neoplastic tumour suppressor genes.

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The Rab5 effector function of Rabaptin-5 is highly conserved in Drosophila.

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Rabaptin-5 interacts with Rabex-5 to modulate early endosomal dynamics in vivo.

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Tumour phenotypes are associated with upregulation of JNK and JAK/STAT signalling.

Overexpression of Numb suppresses tumor cell growth and enhances sensitivity to cisplatin in epithelioid malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is a highly aggressive and conventional treatment-resistant tumor with a dismal prognosis. Among the three histological subtypes of MPM, the epithelioid is the most common type. Numb is considered as a tumor suppressor playing a critical role in controlling asymmetric cell division, maintenance of stem cell compartments, ubiquitination of specific substrates and regulating Notch-, Hedgehog- and TP53-activated pathways. The present study was designed to analyze the role of Numb in epithelioid MPM. We investigated the expression of Numb in 39 epithelioid MPM and 22 normal pleural tissues by immunohistochemistry. Furthermore, we overexpressed Numb in NCI-H2452, an epithelioid human MPM cell line, and investigated the effect of Numb overexpression on the proliferation, apoptosis and sensitivity to cisplatin in cells. The expression of Numb was significantly lower in MPM compared to the control group and Numb had an inverse correlation with the ki-67 labeling index. Loss of Numb expression was associated with poor prognosis in epithelioid MPM. Overexpression of Numb in NCI-H2452 cells significantly inhibited proliferation, promoted apoptosis and enhanced sensitivity to cisplatin. Moreover, Numb overexpression activated caspase-9 and caspase-3 through release of cytochrome c as well as downregulation of XIAP and survivin. We speculate that cytochrome c/caspase signaling is a possible mechanism through which Numb enhances the apoptosis of NCI-H2452 cells. These results suggest that Numb may be involved in epithelioid MPM development, and its upregulation may confer sensitivity to cisplatin, suggesting potential therapeutic options for MPM.

The role of ubiquitylation in receptor endocytosis and endosomal sorting

Ligand-induced activation of transmembrane receptors activates intracellular signaling cascades that control vital cellular processes, such as cell proliferation, differentiation, migration and survival. Receptor signaling is modulated by several mechanisms to ensure that the correct biological outcome is achieved. One such mechanism, which negatively regulates receptor signaling, involves the modification of receptors with ubiquitin. This post-translational modification can promote receptor endocytosis and targets receptors for lysosomal degradation, thereby ensuring termination of receptor signaling. In this Commentary, we review the roles of ubiquitylation in receptor endocytosis and degradative endosomal sorting by drawing on the epidermal growth factor receptor (EGFR) as a well-studied example. Furthermore, we elaborate on the molecular basis of ubiquitin recognition along the endocytic pathway through compartment-specific ubiquitin-binding proteins and highlight how endocytic sorting machineries control these processes. In addition, we discuss the importance of ubiquitin-dependent receptor endocytosis for the maintenance of cellular homeostasis and in the prevention of diseases such as cancer.

The HSP90 inhibitor geldanamycin perturbs endosomal structure and drives recycling ErbB2 and transferrin to modified MVBs/lysosomal compartments

The ErbB2 receptor is a validated cancer target whose internalization and trafficking remain poorly understood. The authors propose that ErbB2 internalization upon geldanamycin (GA) occurs predominantly via clathrin-mediated endocytosis and that GA affects endosomal structure and sorting, forcing recycling cargoes toward mixed endo/lysosomal compartments, irrespective of their HSP90 interaction.

The ErbB2 receptor is a clinically validated cancer target whose internalization and trafficking mechanisms remain poorly understood. HSP90 inhibitors, such as geldanamycin (GA), have been developed to target the receptor to degradation or to modulate downstream signaling. Despite intense investigations, the entry route and postendocytic sorting of ErbB2 upon GA stimulation have remained controversial. We report that ErbB2 levels inversely impact cell clathrin-mediated endocytosis (CME) capacity. Indeed, the high levels of the receptor are responsible for its own low internalization rate. GA treatment does not directly modulate ErbB2 CME rate but it affects ErbB2 recycling fate, routing the receptor to modified multivesicular endosomes (MVBs) and lysosomal compartments, by perturbing early/recycling endosome structure and sorting capacity. This activity occurs irrespective of the cargo interaction with HSP90, as both ErbB2 and the constitutively recycled, HSP90-independent, transferrin receptor are found within modified endosomes, and within aberrant, elongated recycling tubules, leading to modified MVBs/lysosomes. We propose that GA, as part of its anticancer activity, perturbs early/recycling endosome sorting, routing recycling cargoes toward mixed endosomal compartments.

Scribble acts in the Drosophila fat-hippo pathway to regulate warts activity

Epithelial cells are the major cell-type for all organs in multicellular organisms. In order to achieve correct organ size, epithelial tissues need mechanisms that limit their proliferation, and protect tissues from damage caused by defective epithelial cells. Recently, the Hippo signaling pathway has emerged as a major mechanism that orchestrates epithelial development. Hippo signaling is required for cells to stop proliferation as in the absence of Hippo signaling tissues continue to proliferate and produce overgrown organs or tumors. Studies in Drosophila have led the way in providing a framework for how Hippo alters the pattern of gene transcription in target cells, leading to changes in cell proliferation, survival, and other behaviors. Scribble (Scrib) belongs to a class of neoplastic tumor suppressor genes that are required to establish apical-basal cell polarity. The disruption of apical-basal polarity leads to uncontrolled cell proliferation of epithelial cells. The interaction of apical basal polarity genes with the Hippo pathway has been an area of intense investigation. Loss of scrib has been known to affect Hippo pathway targets, however, its functions in the Hippo pathway still remain largely unknown. We investigated the interactions of Scrib with the Hippo pathway. We present data suggesting that Drosophila scrib acts downstream of the Fat (Ft) receptor, and requires Hippo signaling for its growth regulatory functions. We show that Ft requires Scrib to interact with Expanded (Ex) and Dachs (D), and for regulating Warts (Wts) levels and stability, thus placing Scrib in the Hippo pathway network.

Transgenic expression of the Helicobacter pylori virulence factor CagA promotes apoptosis or tumorigenesis through JNK activation in Drosophila

Gastric cancer development is strongly correlated with infection by Helicobacter pylori possessing the effector protein CagA. Using a transgenic Drosophila melanogaster model, we show that CagA expression in the simple model epithelium of the larval wing imaginal disc causes dramatic tissue perturbations and apoptosis when CagA-expressing and non-expressing cells are juxtaposed. This cell death phenotype occurs through activation of JNK signaling and is enhanced by loss of the neoplastic tumor suppressors in CagA-expressing cells or loss of the TNF homolog Eiger in wild type neighboring cells. We further explored the effects of CagA-mediated JNK pathway activation on an epithelium in the context of oncogenic Ras activation, using a Drosophila model of metastasis. In this model, CagA expression in epithelial cells enhances the growth and invasion of tumors in a JNK-dependent manner. These data suggest a potential role for CagA-mediated JNK pathway activation in promoting gastric cancer progression.

The gastric pathogen Helicobacter pylori infects an estimated 50% of the world's population and is a major risk factor for the development of gastric cancer. Strains of H. pylori that can inject the CagA effector protein into host cells are known to be more virulent, but the potential contributions of host genetics to pathogenesis are not well-understood. Using transgenic Drosophila melanogaster, we show that the genetic context of both the host cells in which CagA is expressed and their neighboring cells changes CagA's effects on epithelial tissue. When CagA is expressed in a subset of cells within an epithelium, it disrupts tissue integrity and induces apoptosis through activation of JNK signaling, a pathway that functions to remove aberrant cells from an epithelium. CagA's proapoptotic effects are inhibited by neoplastic tumor suppressor genes in CagA-expressing cells, and by the tumor necrosis factor homolog Eiger in neighboring cells. In contrast, when CagA is coexpressed with oncogenic Ras in a Drosophila model of metastasis, it enhances the growth and invasion of tumors in a JNK-dependent manner. Our study demonstrates how changes in host genetics can cooperate with activation of JNK signaling by the bacterial virulence factor CagA to promote tumorigenesis.

Anterior PAR proteins function during cytokinesis and maintain DYN-1 at the cleavage furrow in Caenorhabditis elegans

PAR proteins are key regulators of cellular polarity and have links to the endocytic machinery and the actin cytoskeleton. Our data suggest a unique role for PAR proteins in cytokinesis. We have found that at the onset of cytokinesis, anterior PAR-6 and posterior PAR-2 proteins are redistributed to the furrow membrane in a temporal and spatial manner. PAR-6 and PAR-2 localize to the furrow membrane during ingression but PAR-2-GFP is distinct in that it is excluded from the extreme tip of the furrow. Once the midbody has formed, PAR-2-GFP becomes restricted to the midbody region (the midbody plus the membrane flanking it). Depletion of both anterior PAR proteins, PAR-3 and PAR-6, led to an increase in multinucleate embryos, suggesting that the anterior PAR proteins are necessary during cytokinesis and that PAR-3 and PAR-6 function in cytokinesis may be partially redundant. Lastly, anterior PAR proteins play a role in the maintenance of DYN-1 in the cleavage furrow. Our data indicate that the PAR proteins are involved in the events that occur during cytokinesis and may play a role in promoting the membrane trafficking and remodeling events that occur during this time.

AP-1 is required for the maintenance of apico-basal polarity in the C. elegans intestine

Epithelial tubes perform functions that are essential for the survival of multicellular organisms. Understanding how their polarised features are maintained is therefore crucial. By analysing the function of the clathrin adaptor AP-1 in the C. elegans intestine, we found that AP-1 is required for epithelial polarity maintenance. Depletion of AP-1 subunits does not affect epithelial polarity establishment or the formation of the intestinal lumen. However, the loss of AP-1 affects the polarised distribution of both apical and basolateral transmembrane proteins. Moreover, it triggers de novo formation of ectopic apical lumens between intestinal cells along the lateral membranes later during embryogenesis. We also found that AP-1 is specifically required for the apical localisation of the small GTPase CDC-42 and the polarity determinant PAR-6. Our results demonstrate that AP-1 controls an apical trafficking pathway required for the maintenance of epithelial polarity in vivo in a tubular epithelium.

Spatial regulation of receptor tyrosine kinases in development and cancer

During development and tissue homeostasis, patterns of cellular organization, proliferation and movement are highly choreographed. Receptor tyrosine kinases (RTKs) have a crucial role in establishing these patterns. Individual cells and tissues exhibit tight spatial control of the RTKs that they express, enabling tissue morphogenesis and function, while preventing unwarranted cell division and migration that can contribute to tumorigenesis. Indeed, RTKs are deregulated in most human cancers and are a major focus of targeted therapeutics. A growing appreciation of the essential role of spatial RTK regulation during development prompts the realization that spatial deregulation of RTKs is likely to contribute broadly to cancer development and may affect the sensitivity and resistance of cancer to pharmacological RTK inhibitors.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Annexin B9 binds to β(H)-spectrin and is required for multivesicular body function in Drosophila

The role of the cytoskeleton in protein trafficking is still being defined. Here, we describe a relationship between the small Ca(2+)-dependent membrane-binding protein Annexin B9 (AnxB9), apical β(Heavy)-spectrin (β(H)) and the multivesicular body (MVB) in Drosophila. AnxB9 binds to a subset of β(H) spliceoforms, and loss of AnxB9 results in an increase in basolateral β(H) and its appearance on cytoplasmic vesicles that overlap with the MVB markers Hrs, Vps16 and EPS15. Similar colocalizations are seen when β(H)-positive endosomes are generated either by upregulation of β(H) in pak mutants or through the expression of the dominant-negative version of β(H). In common with other mutations disrupting the MVB, we also show that there is an accumulation of ubiquitylated proteins and elevated EGFR signaling in the absence of AnxB9 or β(H). Loss of AnxB9 or β(H) function also causes the redistribution of the DE-Cadherin (encoded by shotgun) to endosomal vesicles, suggesting a rationale for the previously documented destabilization of the zonula adherens in karst (which encodes β(H)) mutants. Reduction of AnxB9 results in degradation of the apical-lateral boundary and the appearance of the basolateral proteins Coracle and Dlg on internal vesicles adjacent to β(H). These results indicate that AnxB9 and β(H) are intimately involved in endosomal trafficking to the MVB and play a role in maintaining high-fidelity segregation of the apical and lateral domains.

Balancing self-renewal and differentiation by asymmetric division: insights from brain tumor suppressors in Drosophila neural stem cells

Balancing self-renewal and differentiation of stem cells is an important issue in stem cell and cancer biology. Recently, the Drosophila neuroblast (NB), neural stem cell has emerged as an excellent model for stem cell self-renewal and tumorigenesis. It is of great interest to understand how defects in the asymmetric division of neural stem cells lead to tumor formation. Here, we review recent advances in asymmetric division and the self-renewal control of Drosophila NBs. We summarize molecular mechanisms of asymmetric cell division and discuss how the defects in asymmetric division lead to tumor formation. Gain-of-function or loss-of-function of various proteins in the asymmetric machinery can drive NB overgrowth and tumor formation. These proteins control either the asymmetric protein localization or mitotic spindle orientation of NBs. We also discuss other mechanisms of brain tumor suppression that are beyond the control of asymmetric division.

Drosophila endocytic neoplastic tumor suppressor genes regulate Sav/Wts/Hpo signaling and the c-Jun N-terminal kinase pathway

Genetic screens in the fruit fly Drosophila melanogaster have identified a class of neoplastic tumor suppressor genes (endocytic nTSGs), which encode proteins that localize to endosomes and facilitate the trafficking of membrane-bound receptors and adhesion molecules into the degradative lysosome. Loss of endocytic nTSGs transforms imaginal disc epithelia into highly proliferative, invasive tissues that fail to differentiate and display defects in cellular apicobasal polarity, adhesion and tissue architecture. As vertebrate homologs of some Drosophila nTSGs are linked to tumor formation, identifying molecular changes in signaling associated with nTSG loss could inform understanding of neoplastic transformation in vertebrates. Here we show that mutations in genes that act at multiple steps of the endolysosomal pathway lead to autonomous activation of the Sav/Wts/Hpo (SWH) transcriptional effector Yki (YAP/TAZ in vertebrates) and the Jun N-terminal kinase (JNK), which is known to promote Yki activity in cells with disrupted polarity. Yki and JNK activity are elevated by mutations at multiple steps in the endolysosomal pathway including mutations in the AP-2σ gene, which encodes a component of the AP-2 adaptor complex that recruits cargoes into clathrin-coated pits for subsequent internalization. Moreover, reduction of JNK activity can decrease elevated Yki-signaling caused by altered endocytosis. These studies reveal a broad requirement for components of the endocytic pathway in regulating SWH and JNK outputs, and place Drosophila endocytic nTSGs into a network that involving two major signaling pathways implicated in oncogenesis.

Cell polarity and migration: emerging role for the endosomal sorting machinery

The endosomal sorting complex required for transport (ESCRT) machinery has been implicated in the regulation of endosomal sorting, cell division, viral budding, autophagy, and cell signaling. Here, we review recent evidence that implicates ESCRTs in cell polarity and cell migration, and discuss the potential role of ESCRTs as tumor suppressors.

Loss of the Drosophila cell polarity regulator Scribbled promotes epithelial tissue overgrowth and cooperation with oncogenic Ras-Raf through impaired Hippo pathway signaling

Background

Epithelial neoplasias are associated with alterations in cell polarity and excessive cell proliferation, yet how these neoplastic properties are related to one another is still poorly understood. The study of Drosophila genes that function as neoplastic tumor suppressors by regulating both of these properties has significant potential to clarify this relationship.

Results

Here we show in Drosophila that loss of Scribbled (Scrib), a cell polarity regulator and neoplastic tumor suppressor, results in impaired Hippo pathway signaling in the epithelial tissues of both the eye and wing imaginal disc. scrib mutant tissue overgrowth, but not the loss of cell polarity, is dependent upon defective Hippo signaling and can be rescued by knockdown of either the TEAD/TEF family transcription factor Scalloped or the transcriptional coactivator Yorkie in the eye disc, or reducing levels of Yorkie in the wing disc. Furthermore, loss of Scrib sensitizes tissue to transformation by oncogenic Ras-Raf signaling, and Yorkie-Scalloped activity is required to promote this cooperative tumor overgrowth. The inhibition of Hippo signaling in scrib mutant eye disc clones is not dependent upon JNK activity, but can be significantly rescued by reducing aPKC kinase activity, and ectopic aPKC activity is sufficient to impair Hippo signaling in the eye disc, even when JNK signaling is blocked. In contrast, warts mutant overgrowth does not require aPKC activity. Moreover, reducing endogenous levels of aPKC or increasing Scrib or Lethal giant larvae levels does not promote increased Hippo signaling, suggesting that aPKC activity is not normally rate limiting for Hippo pathway activity. Epistasis experiments suggest that Hippo pathway inhibition in scrib mutants occurs, at least in part, downstream or in parallel to both the Expanded and Fat arms of Hippo pathway regulation.

Conclusions

Loss of Scrib promotes Yorkie/Scalloped-dependent epithelial tissue overgrowth, and this is also important for driving cooperative tumor overgrowth with oncogenic Ras-Raf signaling. Whether this is also the case in human cancers now warrants investigation since the cell polarity function of Scrib and its capacity to restrain oncogene-mediated transformation, as well as the tissue growth control function of the Hippo pathway, are conserved in mammals.

Functional ESCRT machinery is required for constitutive recycling of claudin-1 and maintenance of polarity in vertebrate epithelial cells

Drosophila ESCRT mutants lose epithelial polarity and show increased proliferation, suggesting that ESCRT proteins act as tumor suppressors. In this study, we show for the first time to our knowledge that ESCRT proteins are required to maintain polarity in mammalian epithelial cells, supporting the idea that ESCRT proteins are tumor suppressors.

Genetic screens in Drosophila have identified regulators of endocytic trafficking as neoplastic tumor suppressor genes. For example, Drosophila endosomal sorting complex required for transport (ESCRT) mutants lose epithelial polarity and show increased cell proliferation, suggesting that ESCRT proteins could function as tumor suppressors. In this study, we show for the for the first time to our knowledge that ESCRT proteins are required to maintain polarity in mammalian epithelial cells. Inhibition of ESCRT function caused the tight junction protein claudin-1 to accumulate in intracellular vesicles. In contrast E-cadherin and occludin localization was unaffected. We investigated the cause of this accumulation and show that claudin-1 is constitutively recycled in kidney, colon, and lung epithelial cells, identifying claudin-1 recycling as a newly described feature of diverse epithelial cell types. This recycling requires ESCRT function, explaining the accumulation of intracellular claudin-1 when ESCRT function is inhibited. We further demonstrate that small interfering RNA knockdown of the ESCRT protein Tsg101 causes epithelial monolayers to lose their polarized organization and interferes with the establishment of a normal epithelial permeability barrier. ESCRT knockdown also reduces the formation of correctly polarized three-dimensional cysts. Thus, in mammalian epithelial cells, ESCRT function is required for claudin-1 trafficking and for epithelial cell polarity, supporting the hypothesis that ESCRT proteins function as tumor suppressors.

UVRAG is required for organ rotation by regulating Notch endocytosis in Drosophila

Heterotaxy characterized by abnormal left-right body asymmetry causes diverse congenital anomalies. Organ rotation is a crucial developmental process to establish the left-right patterning during animal development. However, the molecular basis of how organ rotation is regulated is poorly understood. Here we report that Drosophila UV-resistance associated gene (UVRAG), a tumor suppressor that regulates autophagy and endocytosis, plays unexpected roles in controlling organ rotation. Loss-of-function mutants of UVRAG show seriously impaired organ rotation phenotypes, which are associated with defects in endocytic trafficking rather than autophagy. Blunted endocytic degradation by UVRAG deficiency causes endosomal accumulation of Notch, resulting in abnormally enhanced Notch activity. Knockdown of Notch itself or expression of a dominant negative form of Notch transcriptional co-activator Mastermind is sufficient to rescue the rotation defect in UVRAG mutants. Consistently, UVRAG-mutated heterotaxy patient cells also display highly increased Notch protein levels. These results suggest evolutionarily conserved roles of UVRAG in organ rotation by regulating Notch endocytic degradation.

NUMB-ing down cancer by more than just a NOTCH

The protein Numb does not live up to its name. This passive-sounding protein is anything but spent. Originally identified as a cell-fate determinant in Drosophila development, Numb received a good deal of attention as an inhibitor of the Notch receptor signaling pathway. It turns out, however, that Numb does a lot more than simply regulate Notch. It has been implicated in a variety of biochemical pathways connected with signaling (it regulates Notch-, Hedgehog- and TP53-activated pathways), endocytosis (it is involved in cargo internalization and recycling), determination of polarity (it interacts with the PAR complex, and regulates adherens and tight junctions), and ubiquitination (it exploits this mechanism to regulate protein function and stability). This complex biochemical network lies at the heart of Numb's involvement in diverse cellular phenotypes, including cell fate developmental decisions, maintenance of stem cell compartments, regulation of cell polarity and adhesion, and migration. Considering its multifaceted role in cellular homeostasis, it is not surprising that Numb has been implicated in cancer as a tumor suppressor. Our major goal here is to explain the cancer-related role of Numb based on our understanding of its role in cell physiology. We will attempt to do this by reviewing the present knowledge of Numb at the biochemical and functional level, and by integrating its apparently heterogeneous functions into a unifying scenario, based on our recently proposed concept of the "endocytic matrix". Finally, we will discuss the role of Numb in the maintenance of the normal stem cell compartment, as a starting point to interpret the tumor suppressor function of Numb in the context of the cancer stem cell hypothesis.

At the crossroads of polarity, proliferation and apoptosis: the use of Drosophila to unravel the multifaceted role of endocytosis in tumor suppression

Endocytosis is an important regulator of cell-cell signaling and endocytic trafficking has been increasingly implicated in control of tumor suppression. Recent insights from Drosophila indicate that impairment of multiple trafficking steps which lead to receptor degradation can cause tumor formation in epithelial organs. These tumors are characterized by sustained activation of a number of mitogenic signaling pathways, and by subversion of epithelial polarity and the apoptotic response. Cooperation between such alterations, as well as tumor-host interactions, is also observed. The recapitulation of several hallmarks of human cancers in fly tumors provides a framework to understand the role of defective endocytosis in cancer.

Endocytic proteins in the regulation of nuclear signaling, transcription and tumorigenesis

Accumulating evidence argues that many proteins governing membrane sorting during endocytosis participate also in nuclear signaling and transcriptional regulation, mostly by modulating the activity of various nuclear factors. Some adaptors and accessory proteins acting in clathrin-mediated internalization, as well as endosomal sorting proteins can undergo nuclear translocation and affect gene expression directly, while for others the effects may be more indirect. Although it is often unclear to what extent the endocytic and nuclear functions are interrelated, several of such proteins are implicated in the regulation of cell proliferation and tumorigenesis, arguing that their dual-function nature may be of physiological importance.