Novel determinants of NOTCH1 trafficking and signaling in breast epithelial cells

The evolutionarily conserved Notch signaling pathway controls cell-cell communication, enacting cell fate decisions during development and tissue homeostasis. Its dysregulation is associated with a wide range of diseases, including congenital disorders and cancers. Signaling outputs depend on maturation of Notch receptors and trafficking to the plasma membrane, endocytic uptake and sorting, lysosomal and proteasomal degradation, and ligand-dependent and independent proteolytic cleavages. We devised assays to follow quantitatively the trafficking and signaling of endogenous human NOTCH1 receptor in breast epithelial cells in culture. Based on such analyses, we executed a high-content screen of 2,749 human genes to identify new regulators of Notch that might be amenable to pharmacologic intervention. We uncovered 39 new NOTCH1 modulators for NOTCH1 trafficking and signaling. Among them, we find that PTPN23 and HCN2 act as positive NOTCH1 regulators by promoting endocytic trafficking and NOTCH1 maturation in the Golgi apparatus, respectively, whereas SGK3 serves as a negative regulator that can be modulated by pharmacologic inhibition. Our findings might be relevant in the search of new strategies to counteract pathologic Notch signaling.

Computational Analysis of CC2D1A Missense Mutations: Insight into Protein Structure and Interaction Dynamics

CC2D1A is implicated in a range of conditions, including autism spectrum disorder, intellectual disability, seizures, autosomal recessive nonsyndromic intellectual disability, heterotaxy, and ciliary dysfunction. In order to understand the molecular mechanisms underlying these conditions, we focused on the structural and dynamic activity consequences of mutations within this gene. In this study, whole exome sequencing identified the c.1552G > A (GLU518LYS) missense mutation in the CC2D1A in an 18-year-old male, linking it to intellectual disability and autism. In addition to the GLU518LYS mutation, we conducted a comprehensive analysis of other predefined missense mutations (i.e., PRO192LEU, GLN506ARG, PRO532LEU, GLY781VAL, and GLY781GLU) found within the CC2D1A. Utilizing all-atom molecular dynamics (MD) simulations and neighborhood interaction analyses, we delve into the impact of these mutations on protein structure and function at an atomic level, aiming to shed light on their contribution to the pathogenesis of related diseases. The results suggest that GLU518LYS, GLY781VAL, and GLY781GLU mutations did not significantly alter overall global protein structure compared to the wild type, while PRO192LEU, GLN506ARG, and PRO532LEU exhibited slightly higher protein root-mean-square deviation (RMSD) values, which may indicate potential impacts on whole protein stability. Moreover, neighborhood interaction analysis indicated that ASP85 emerges as a unique interaction partner specifically associated with the GLU518LYS mutation, whereas LYS75, which interacts with the ASP85 in the mutated form, is absent in the wild type. This alteration signifies a crucial reconfiguration in the local interaction network at the site of the mutation.

CC2D1A causes ciliopathy, intellectual disability, heterotaxy, renal dysplasia, and abnormal CSF flow

Intellectual and developmental disabilities result from abnormal nervous system development. Over a 1,000 genes have been associated with intellectual and developmental disabilities, driving continued efforts toward dissecting variant functionality to enhance our understanding of the disease mechanism. This report identified two novel variants in CC2D1A in a cohort of four patients from two unrelated families. We used multiple model systems for functional analysis, including Xenopus, Drosophila, and patient-derived fibroblasts. Our experiments revealed that cc2d1a is expressed explicitly in a spectrum of ciliated tissues, including the left-right organizer, epidermis, pronephric duct, nephrostomes, and ventricular zone of the brain. In line with this expression pattern, loss of cc2d1a led to cardiac heterotaxy, cystic kidneys, and abnormal CSF circulation via defective ciliogenesis. Interestingly, when we analyzed brain development, mutant tadpoles showed abnormal CSF circulation only in the midbrain region, suggesting abnormal local CSF flow. Furthermore, our analysis of the patient-derived fibroblasts confirmed defective ciliogenesis, further supporting our observations. In summary, we revealed novel insight into the role of CC2D1A by establishing its new critical role in ciliogenesis and CSF circulation.

Structural Analysis of the ESCRT-III Regulator Lethal(2) Giant Discs/Coiled-Coil and C2 Domain-Containing Protein 1 (Lgd/CC2D1)

Members of the LGD/CC2D1 protein family contain repeats of the family-defining DM14 domains. Via this domain, they interact with members of the CHMP family, which are essential for the ESCRT machinery-mediated formation of intraluminal vesicles during endosome maturation. Here, we investigate the requirement of the DM14 domains for the function of Lgd in detail. We found that although both odd-numbered DM14s can act in a functionally redundant manner, the redundancy is not complete and both contribute to the full function of Lgd. Our analysis indicates that some of the AAs that form the KARRxxR motif of the onDM14s are not exchangeable by similarly charged AAs without loss of function, indicating that they not only provide charge, but also fulfil structural roles. Furthermore, we show that the region of Lgd between DM14-4 and the C2 domain as well as its C-terminal region to the C2 domain are important for protein stability/function. Moreover, we analysed the importance of AAs that are conserved in all DM14 domains. Finally, our analysis of the C. elegans ortholog of Lgd revealed that it has only one DM14 domain that is functionally equivalent to the onDM14s. Altogether, the results further the understanding of how Lgd family members regulate the ESCRT machinery.

Alternative mechanisms of Notch activation by partitioning into distinct endosomal domains

Different membrane microdomain compositions provide unique environments that can regulate signaling receptor function. We identify microdomains on the endosome membrane of Drosophila endosomes, enriched in lipid-raft or clathrin/ESCRT-0, which are associated with Notch activation by distinct, ligand-independent mechanisms. Transfer of Notch between microdomains is regulated by Deltex and Suppressor of deltex ubiquitin ligases and is limited by a gate-keeper role for ESCRT complexes. Ubiquitination of Notch by Deltex recruits it to the clathrin/ESCRT-0 microdomain and enhances Notch activation by an ADAM10-independent/TRPML-dependent mechanism. This requirement for Deltex is bypassed by the downregulation of ESCRT-III. In contrast, while ESCRT-I depletion also activates Notch, it does so by an ADAM10-dependent/TRPML-independent mechanism and Notch is retained in the lipid raft-like microdomain. In the absence of such endosomal perturbation, different activating Notch mutations also localize to different microdomains and are activated by different mechanisms. Our findings demonstrate the interplay between Notch regulators, endosomal trafficking components, and Notch genetics, which defines membrane locations and activation mechanisms.

The structure of a tetrameric septin complex reveals a hydrophobic element essential for NC-interface integrity

The septins of the yeast Saccharomyces cerevisiae assemble into hetero-octameric rods by alternating interactions between neighboring G-domains or N- and C-termini, respectively. These rods polymerize end to end into apolar filaments, forming a ring beneath the prospective new bud that expands during the cell cycle into an hourglass structure. The hourglass finally splits during cytokinesis into a double ring. Understanding these transitions as well as the plasticity of the higher order assemblies requires a detailed knowledge of the underlying structures. Here we present the first X-ray crystal structure of a tetrameric Shs1-Cdc12-Cdc3-Cdc10 complex at a resolution of 3.2 Å. Close inspection of the NC-interfaces of this and other septin structures reveals a conserved contact motif that is essential for NC-interface integrity of yeast and human septins in vivo and in vitro. Using the tetrameric structure in combination with AlphaFold-Multimer allowed us to propose a model of the octameric septin rod.

Iron-responsive element of Divalent metal transporter 1 (Dmt1) controls Notch-mediated cell fates

Notch receptor activation is regulated by the intramembrane protease γ-secretase, which cleaves and liberates the Notch intracellular domain (Nicd) that regulates gene transcription. While γ-secretase cleavage is necessary, we demonstrate it is insufficient for Notch activation and requires vesicular trafficking. Here, we report Divalent metal transporter 1 (Dmt1, Slc11A2) as a novel and essential regulator of Notch signalling. Dmt1-deficient cells are defective in Notch signalling and have perturbed endolysosomal trafficking and function. Dmt1 encodes for two isoforms, with and without an iron response element (ire). We show that isoform-specific silencing of Dmt1-ire and Dmt1+ire has opposite consequences on Notch-dependent cell fates in cell lines and intestinal organoids. Loss of Dmt1-ire suppresses Notch activation and promotes differentiation, whereas loss of Dmt1+ire causes Notch activation and maintains stem-progenitor cell fates. Dmt1 isoform expression correlates with Notch and Wnt signalling in Apc-deficient intestinal organoids and human colorectal cancers. Consistently, Dmt1-ire silencing induces Notch-dependent differentiation in colorectal cancer cells. These data identify Dmt1 isoforms as binary switches controlling Notch cell fate decisions in normal and tumour cells.

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.

Ubiquitylation is required for the incorporation of the Notch receptor into intraluminal vesicles to prevent prolonged and ligand-independent activation of the pathway

BACKGROUND

Ubiquitylation of the ligands and the receptor plays an important part in the regulation of the activity of the evolutionary conserved Notch signalling pathway. However, its function for activation of Notch is not completely understood, despite the identification of several E3 ligases devoted to the receptor.

RESULTS

Here we analysed a variant of the Notch receptor where all lysines in its intracellular domain are replaced by arginines. Our analysis of this variant revealed that ubiquitylation of Notch is not essential for its endocytosis. We identified two functions for ubiquitylation of lysines in the Notch receptor. First, it is required for the degradation of free Notch intracellular domain (NICD) in the nucleus, which prevents a prolonged activation of the pathway. More importantly, it is also required for the incorporation of Notch into intraluminal vesicles of maturing endosomes to prevent ligand-independent activation of the pathway from late endosomal compartments.

CONCLUSIONS

The findings clarify the role of lysine-dependent ubiquitylation of the Notch receptor and indicate that Notch is endocytosed by several independent operating mechanisms.

The Role of Intracellular Trafficking of Notch Receptors in Ligand-Independent Notch Activation

Aberrant Notch signaling has been found in a broad range of human malignancies. Consequently, small molecule inhibitors and antibodies targeting Notch signaling in human cancers have been developed and tested; however, these have failed due to limited anti-tumor efficacy because of dose-limiting toxicities in normal tissues. Therefore, there is an unmet need to discover novel regulators of malignant Notch signaling, which do not affect Notch signaling in healthy tissues. This review provides a comprehensive overview of the current knowledge on the role of intracellular trafficking in ligand-independent Notch receptor activation, the possible mechanisms involved, and possible therapeutic opportunities for inhibitors of intracellular trafficking in Notch targeting.

Intracellular trafficking of Notch orchestrates temporal dynamics of Notch activity in the fly brain

While Delta non-autonomously activates Notch in neighboring cells, it autonomously inactivates Notch through cis-inhibition, the molecular mechanism and biological roles of which remain elusive. The wave of differentiation in the Drosophila brain, the ‘proneural wave’, is an excellent model for studying Notch signaling in vivo. Here, we show that strong nonlinearity in cis-inhibition reproduces the second peak of Notch activity behind the proneural wave in silico. Based on this, we demonstrate that Delta expression induces a quick degradation of Notch in late endosomes and the formation of the twin peaks of Notch activity in vivo. Indeed, the amount of Notch is upregulated and the twin peaks are fused forming a single peak when the function of Delta or late endosomes is compromised. Additionally, we show that the second Notch peak behind the wavefront controls neurogenesis. Thus, intracellular trafficking of Notch orchestrates the temporal dynamics of Notch activity and the temporal patterning of neurogenesis.

During Drosophila development, two peaks of Notch activity propagate across the neuroepithelium to generate neuroblasts. Here, the authors show Notch cis-inhibition under the control of intracellular Notch trafficking establishes these two peaks, which temporally control neurogenesis in the brain.

Lethal (2) giant discs (Lgd)/CC2D1 is required for the full activity of the ESCRT machinery

Background

A major task of the endosomal sorting complex required for transport (ESCRT) machinery is the pinching off of cargo-loaded intraluminal vesicles (ILVs) into the lumen of maturing endosomes (MEs), which is essential for the complete degradation of transmembrane proteins in the lysosome. The ESCRT machinery is also required for the termination of signalling through activated signalling receptors, as it separates their intracellular domains from the cytosol. At the heart of the machinery lies the ESCRT-III complex, which is required for an increasing number of processes where membrane regions are abscised away from the cytosol. The core of ESCRT-III, comprising four members of the CHMP protein family, organises the assembly of a homopolymer of CHMP4, Shrub in Drosophila, that is essential for abscission. We and others identified the tumour-suppressor lethal (2) giant discs (Lgd)/CC2D1 as a physical interactor of Shrub/CHMP4 in Drosophila and mammals, respectively.

Results

Here, we show that the loss of function of lgd constitutes a state of reduced activity of Shrub/CHMP4/ESCRT-III. This hypomorphic shrub mutant situation causes a slight decrease in the rate of ILV formation that appears to result in incomplete incorporation of Notch into ILVs. We found that the forced incorporation in ILVs of lgd mutant MEs suppresses the uncontrolled and ligand-independent activation of Notch. Moreover, the analysis of Su(dx) lgd double mutants clarifies their relationship and suggests that they are not operating in a linear pathway. We could show that, despite prolonged lifetime, the MEs of lgd mutants have a similar ILV density as wild-type but less than rab7 mutant MEs, suggesting the rate in lgd mutants is slightly reduced. The analysis of the MEs of wild-type and mutant cells in the electron microscope revealed that the ESCRT-containing electron-dense microdomains of ILV formation at the limiting membrane are elongated, indicating a change in ESCRT activity. Since lgd mutants can be rescued to normal adult flies if extra copies of shrub (or its mammalian ortholog CHMP4B) are added into the genome, we conclude that the net activity of Shrub is reduced upon loss of lgd function. Finally, we show that, in solution, CHMP4B/Shrub exists in two conformations. LGD1/Lgd binding does not affect the conformational state of Shrub, suggesting that Lgd is not a chaperone for Shrub/CHMP4B.

Conclusion

Our results suggest that Lgd is required for the full activity of Shrub/ESCRT-III. In its absence, the activity of the ESCRT machinery is reduced. This reduction causes the escape of a fraction of cargo, among it Notch, from incorporation into ILVs, which in turn leads to an activation of this fraction of Notch after fusion of the ME with the lysosome. Our results highlight the importance of the incorporation of Notch into ILV not only to assure complete degradation, but also to avoid uncontrolled activation of the pathway.

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

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

To be more precise: the role of intracellular trafficking in development and pattern formation

Living cells interpret a variety of signals in different contexts to elucidate functional responses. While the understanding of signalling molecules, their respective receptors and response at the gene transcription level have been relatively well-explored, how exactly does a single cell interpret a plethora of time-varying signals? Furthermore, how their subsequent responses at the single cell level manifest in the larger context of a developing tissue is unknown. At the same time, the biophysics and chemistry of how receptors are trafficked through the complex dynamic transport network between the plasma membrane-endosome-lysosome-Golgi-endoplasmic reticulum are much more well-studied. How the intracellular organisation of the cell and inter-organellar contacts aid in orchestrating trafficking, as well as signal interpretation and modulation by the cells are beginning to be uncovered. In this review, we highlight the significant developments that have strived to integrate endosomal trafficking, signal interpretation in the context of developmental biology and relevant open questions with a few chosen examples. Furthermore, we will discuss the imaging technologies that have been developed in the recent past that have the potential to tremendously accelerate knowledge gain in this direction while shedding light on some of the many challenges.

Unravelling of Hidden Secrets: The Tumour Suppressor Lethal (2) Giant Discs (Lgd)/CC2D1, Notch Signalling and Cancer

The endosomal pathway plays a pivotal role upon signal transduction in the Notch pathway. Recent work on lethal (2) giant discs (lgd) points to an additional critical role in avoiding uncontrolled ligand-independent signalling during trafficking of the Notch receptor through the endosomal pathway to the lysosome for degradation. In this chapter, we will outline the journey of Notch through the endosomal system and present an overview of the current knowledge about Lgd and its mammalian orthologs Lgd1/CC2D1b and Lgd2/CC2D1a. We will then discuss how Notch is activated in the absence of lgd function in Drosophila and ask whether there is evidence that a similar ligand-independent activation of the Notch pathway can also happen in mammals if the orthologs are inactivated.

Notch Pathway: A Journey from Notching Phenotypes to Cancer Immunotherapy

Notch is a key evolutionary conserved pathway, which has fascinated and engaged the work of investigators in an uncountable number of biological fields, from development of metazoans to immunotherapy for cancer. The study of Notch has greatly contributed to the understanding of cancer biology and a substantial effort has been spent in designing Notch-targeting therapies. Due to its broad involvement in cancer, targeting Notch would allow to virtually modulate any aspect of the disease. However, this means that Notch-based therapies must be highly specific to avoid off-target effects. This review will present the newest mechanistic and therapeutic advances in the Notch field and discuss the promises and challenges of this constantly evolving field.

Targeting Notch Trafficking and Processing in Cancers

The Notch family comprises a group of four ligand-dependent receptors that control evolutionarily conserved developmental and homeostatic processes and transmit signals to the microenvironment. NOTCH undergoes remodeling, maturation, and trafficking in a series of post-translational events, including glycosylation, ubiquitination, and endocytosis. The regulatory modifications occurring in the endoplasmic reticulum/Golgi precede the intramembrane γ-secretase proteolysis and the transfer of active NOTCH to the nucleus. Hence, NOTCH proteins coexist in different subcellular compartments and undergo continuous relocation. Various factors, including ion concentration, enzymatic activity, and co-regulatory elements control Notch trafficking. Interfering with these regulatory mechanisms represents an innovative therapeutic way to bar oncogenic Notch signaling. In this review, we briefly summarize the role of Notch signaling in cancer and describe the protein modifications required for NOTCH to relocate across different subcellular compartments. We focus on the functional relationship between these modifications and the corresponding therapeutic options, and our findings could support the development of trafficking modulators as a potential alternative to the well-known γ-secretase inhibitors.

Notch3 in Development, Health and Disease

Notch3 is one of four mammalian Notch proteins, which act as signalling receptors to control cell fate in many developmental and adult tissue contexts. Notch signalling continues to be important in the adult organism for tissue maintenance and renewal and mis-regulation of Notch is involved in many diseases. Genetic studies have shown that Notch3 gene knockouts are viable and have limited developmental defects, focussed mostly on defects in the arterial smooth muscle cell lineage. Additional studies have revealed overlapping roles for Notch3 with other Notch proteins, which widen the range of developmental functions. In the adult, Notch3, in collaboration with other Notch proteins, is involved in stem cell regulation in different tissues in stem cell regulation in different tissues, and it also controls the plasticity of the vascular smooth muscle phenotype involved in arterial vessel remodelling. Overexpression, gene amplification and mis-activation of Notch3 are associated with different cancers, in particular triple negative breast cancer and ovarian cancer. Mutations of Notch3 are associated with a dominantly inherited disease CADASIL (cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy), and there is further evidence linking Notch3 misregulation to hypertensive disease. Here we discuss the distinctive roles of Notch3 in development, health and disease, different views as to the underlying mechanisms of its activation and misregulation in different contexts and potential for therapeutic intervention.

Expression of a human variant of CHMP2B linked to neurodegeneration in Drosophila external sensory organs leads to cell fate transformations associated with increased Notch activity

Proper function of cell signaling pathways is dependent upon regulated membrane trafficking events that lead to the endocytosis, recycling, and degradation of cell surface receptors. The endosomal complexes required for transport (ESCRT) genes play a critical role in the sorting of ubiquitinated cell surface proteins. CHMP2B^Intron5 , a truncated form of a human ESCRT-III protein, was discovered in a Danish family afflicted by a hereditary form of frontotemporal dementia (FTD). Although the mechanism by which the CHMP2B mutation in this family causes FTD is unknown, the resulting protein has been shown to disrupt normal endosomal-lysosomal pathway function and leads to aberrant regulation of signaling pathways. Here we have misexpressed CHMP2B^Intron5 in the developing Drosophila external sensory (ES) organ lineage and demonstrate that it is capable of altering cell fates. Each of the cell fate transformations seen is compatible with an increase in Notch signaling. Furthermore, this interpretation is supported by evidence that expression of CHMP2B^Intron5 in the notum environment is capable of raising the levels of Notch signaling. As such, these results add to a growing body of evidence that CHMP2B^Intron5 can act rapidly to disrupt normal cellular function via the misregulation of critical cell surface receptor function.

The auxiliary ESCRT complexes provide robustness to cold in poikilothermic organisms

The ESCRT pathway, comprising the in sequence acting ESCRT-0, -I, -II, -III and Vps4 complexes, conducts the abscission of membranes away from the cytosol. Whereas the components of the central ESCRT-III core complex have been thoroughly investigated, the function of the components of the associated two auxiliary ESCRT sub-complexes are not well-understood in metazoans, especially at the organismal level. We here present the developmental analysis of the Drosophila orthologs of the auxiliary ESCRTs Chmp5 and Ist1, DChmp5 and DIst1, which belong to the two auxiliary sub-complexes. While each single null mutant displayed mild defects in development, the Dist1 Dchmp5 double mutant displayed a severe defect, indicating that the two genes act synergistically, but in separate pathways. Moreover, the presented results indicate that the auxiliary ESCRTs provide robustness against cold during development of diverse poikilothermic organisms, probably by preventing the accumulation of the ESCRT-III core component Shrub on the endosomal membrane.

Summary: The analysis of Chmp5 and Ist1, which belong to the two ESCRT auxiliary sub-complexes in Drosophila, suggests that these ESCRT proteins provide robustness against cold in diverse poikilothermic organisms.

The anti-malarial drug chloroquine sensitizes oncogenic NOTCH1 driven human T-ALL to γ-secretase inhibition

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer arising from T-cell progenitors. Although current treatments, including chemotherapy and glucocorticoids, have significantly improved survival, T-ALL remains a fatal disease and new treatment options are needed. Since more than 60% of T-ALL cases bear oncogenic NOTCH1 mutations, small molecule inhibitors of NOTCH1 signalling; γ-secretase inhibitors (GSI), are being actively investigated for the treatment of T-ALL. Unfortunately, GSI have shown limited clinical efficacy and dose-limiting toxicities. We hypothesized that by combining known drugs, blocking NOTCH activity through another mechanism, may synergize with GSI enabling equal efficacy at a lower concentration. Here, we show that the clinically used anti-malarial drug chloroquine (CQ), an inhibitor of lysosomal function and autophagy, decreases T-ALL cell viability and proliferation. This effect of CQ was not observed in GSI-resistant T-ALL cell lines. Mechanistically, CQ impairs the redox balance, induces ds DNA breaks and activates the DNA damage response. CQ also interferes with intracellular trafficking and processing of oncogenic NOTCH1. Interestingly, we show for the first time that the addition of CQ to γ-secretase inhibition has a synergistic therapeutic effect on T-ALL and reduces the concentration of GSI required to obtain a reduction in cell viability and a block of proliferation. Overall, our results suggest that CQ may be a promising repurposed drug in the treatment of T-ALL, as a single treatment or in combination with GSI, increasing the therapeutic ratio.

Mechanisms of Notch signaling: a simple logic deployed in time and space

Most cells in our body communicate during development and throughout life via Notch receptors and their ligands. Notch receptors relay information from the cell surface to the genome via a very simple mechanism, yet Notch plays multiple roles in development and disease. Recent studies suggest that this versatility in Notch function may not necessarily arise from complex and context-dependent integration of Notch signaling with other developmental signals, but instead arises, in part, from signaling dynamics. Here, we review recent findings on the core Notch signaling mechanism and discuss how spatial-temporal dynamics contribute to Notch signaling output.

Combining genetic and biophysical approaches to probe the structure and function relationships of the notch receptor

Notch is a conserved cell signalling receptor regulating many aspects of development and tissue homeostasis. Notch is activated by ligand-induced proteolytic cleavages that release the Notch intracellular domain, which relocates to the nucleus to regulate gene transcription. Proteolytic activation first requires mechanical force to be applied to the Notch extracellular domain through an endocytic pulling mechanism transmitted through the ligand/receptor interface. This exposes the proteolytic cleavage site allowing the signal to be initiated following removal of the Notch extracellular domain. Ligands can also act, when expressed in the same cell, through non-productive cis-interactions to inhibit Notch activity. Furthermore, ligand selectivity and Notch activation are regulated by numerous post-translational modifications of the extracellular domain. Additional non-canonical trans and cis interactions with other regulatory proteins may modulate alternative mechanisms of Notch activation that depend on endocytic trafficking of the full-length receptor and proteolytic release of the intracellular domain from endo-lysosomal surface. Mutations of Notch, located in different regions of the protein, are associated with a spectrum of different loss and gain of function phenotypes and offer the possibility to dissect distinct regulatory interactions and mechanisms, particularly when combined with detailed structural analysis of Notch in complex with various regulatory partners.

The retromer complex safeguards against neural progenitor-derived tumorigenesis by regulating Notch receptor trafficking

The correct establishment and maintenance of unidirectional Notch signaling are critical for the homeostasis of various stem cell lineages. However, the molecular mechanisms that prevent cell-autonomous ectopic Notch signaling activation and deleterious cell fate decisions remain unclear. Here we show that the retromer complex directly and specifically regulates Notch receptor retrograde trafficking in Drosophila neuroblast lineages to ensure the unidirectional Notch signaling from neural progenitors to neuroblasts. Notch polyubiquitination mediated by E3 ubiquitin ligase Itch/Su(dx) is inherently inefficient within neural progenitors, relying on retromer-mediated trafficking to avoid aberrant endosomal accumulation of Notch and cell-autonomous signaling activation. Upon retromer dysfunction, hypo-ubiquitinated Notch accumulates in Rab7⁺ enlarged endosomes, where it is ectopically processed and activated in a ligand-dependent manner, causing progenitor-originated tumorigenesis. Our results therefore unveil a safeguard mechanism whereby retromer retrieves potentially harmful Notch receptors in a timely manner to prevent aberrant Notch activation-induced neural progenitor dedifferentiation and brain tumor formation.

Most cells in the animal body are tailored to perform particular tasks, but stem cells have not yet made their choice. Instead, they have unlimited capacity to divide and, with the right signals, they can start to specialize to become a given type of cells. In the brain, this process starts with a stem cell dividing. One of the daughters will remain a stem cell, while the other, the neural progenitor, will differentiate to form a mature cell such as a neuron. Keeping this tight balance is crucial for the health of the organ: if the progenitor reverts back to being a stem cell, there will be a surplus of undifferentiated cells that can lead to a tumor.

A one-way signal driven by the protein Notch partly controls the distinct fates of the two daughter cells. While the neural progenitor carries Notch at its surface, its neural stem cell sister has a Notch receptor on its membrane instead. This ensures that the Notch signaling goes in one direction, from the cell with Notch to the one sporting the receptor.

When a stem cell divides, one daughter gets more of a protein called Numb than the other. Numb pulls Notch receptors away from the external membrane and into internal capsules called endosomes. This guarantees that only one of the siblings will be carrying the receptors at its surface. Yet, sometimes the Notch receptors can get activated in the endosomes, which may make neural progenitors revert to being stem cells. It is still unclear what tools the cells have to stop this abnormal activation.

Here, Li et al. screened brain cells from fruit fly larvae to find out the genes that might play a role in suppressing the inappropriate Notch signaling. This highlighted a protein complex known as the retromer, which normally helps to transport proteins in the cell. Experiments showed that, in progenitors, the retromer physically interacts with Notch receptors and retrieves them from the endosomes back to the cell surface. If the retromer is inactive, the Notch receptors accumulate in the endosomes, where they can be switched on. It seems that, in fruit flies, the retromer acts as a bomb squad that recognizes and retrieves potentially harmful Notch receptors, thereby preventing brain tumor formation.

Several retromer components are less present in patients with various cancers, including glioblastoma, an aggressive form of brain cancer. The results by Li et al. may therefore shed light on the link between the protein complex and the emergence of the disease in humans.

Structural Basis for Regulation of ESCRT-III Complexes by Lgd

The ESCRT-III complex induces outward membrane budding and fission through homotypic polymerization of its core component Shrub/CHMP4B. Shrub activity is regulated by its direct interaction with a protein called Lgd in flies, or CC2D1A or B in humans. Here, we report the structural basis for this interaction and propose a mechanism for regulation of polymer assembly. The isolated third DM14 repeat of Lgd binds Shrub, and an Lgd fragment containing only this DM14 repeat and its C-terminal C2 domain is sufficient for in vivo function. The DM14 domain forms a helical hairpin with a conserved, positively charged tip, that, in the structure of a DM14 domain-Shrub complex, occupies a negatively charged surface of Shrub that is otherwise used for homopolymerization. Lgd mutations at this interface disrupt its function in flies, confirming functional importance. Together, these data argue that Lgd regulates ESCRT activity by controlling access to the Shrub self-assembly surface.

Integration of Drosophila and Human Genetics to Understand Notch Signaling Related Diseases

Notch signaling research dates back to more than one hundred years, beginning with the identification of the Notch mutant in the fruit fly Drosophila melanogaster. Since then, research on Notch and related genes in flies has laid the foundation of what we now know as the Notch signaling pathway. In the 1990s, basic biological and biochemical studies of Notch signaling components in mammalian systems, as well as identification of rare mutations in Notch signaling pathway genes in human patients with rare Mendelian diseases or cancer, increased the significance of this pathway in human biology and medicine. In the 21st century, Drosophila and other genetic model organisms continue to play a leading role in understanding basic Notch biology. Furthermore, these model organisms can be used in a translational manner to study underlying mechanisms of Notch-related human diseases and to investigate the function of novel disease associated genes and variants. In this chapter, we first briefly review the major contributions of Drosophila to Notch signaling research, discussing the similarities and differences between the fly and human pathways. Next, we introduce several biological contexts in Drosophila in which Notch signaling has been extensively characterized. Finally, we discuss a number of genetic diseases caused by mutations in genes in the Notch signaling pathway in humans and we expand on how Drosophila can be used to study rare genetic variants associated with these and novel disorders. By combining modern genomics and state-of-the art technologies, Drosophila research is continuing to reveal exciting biology that sheds light onto mechanisms of disease.

Mechanisms of Non-canonical Signaling in Health and Disease: Diversity to Take Therapy up a Notch?

Non-canonical Notch signaling encompasses a wide range of cellular processes, diverging considerably from the established paradigm. It can dispense of ligand, proteolytic or nuclear activity. Non-canonical Notch signaling events have been studied mostly in the fruit fly Drosophila melanogaster, the organism in which Notch was identified first and a powerful model for understanding signaling outcomes. However, non-canonical events are ill-defined and their involvement in human physiology is not clear, hampering our understanding of diseases arising from Notch signaling alterations. At a time in which therapies based on specific targeting of Notch signaling are still an unfulfilled promise, detailed understanding of non-canonical Notch events might be key to devising more specific and less toxic pharmacologic options. Based on the blueprint of non-canonical signaling in Drosophila, here, we review and rationalize current evidence about non-canonical Notch signaling. Our effort might inform Notch biologists developing new research avenues and clinicians seeking future treatment of Notch-dependent diseases.

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.

ESCRT-III Membrane Trafficking Misregulation Contributes To Fragile X Syndrome Synaptic Defects

The leading cause of heritable intellectual disability (ID) and autism spectrum disorders (ASD), Fragile X syndrome (FXS), is caused by loss of the mRNA-binding translational suppressor Fragile X Mental Retardation Protein (FMRP). In the Drosophila FXS disease model, we found FMRP binds shrub mRNA (human Chmp4) to repress Shrub expression, causing overexpression during the disease state early-use critical period. The FXS hallmark is synaptic overelaboration causing circuit hyperconnectivity. Testing innervation of a central brain learning/memory center, we found FMRP loss and Shrub overexpression similarly increase connectivity. The ESCRT-III core protein Shrub has a central role in endosome-to-multivesicular body membrane trafficking, with synaptic requirements resembling FMRP. Consistently, we found FMRP loss and Shrub overexpression similarly elevate endosomes and result in the arrested accumulation of enlarged intraluminal vesicles within synaptic boutons. Importantly, genetic correction of Shrub levels in the FXS model prevents synaptic membrane trafficking defects and strongly restores innervation. These results reveal a new molecular mechanism underpinning the FXS disease state.

Regulation of Notch Signaling Through Intracellular Transport

The highly conserved Notch-signaling pathway performs a central role in cell differentiation, survival, and proliferation. A major mechanism by which cells modulate signaling is by controlling the intracellular transport itinerary of Notch. Indeed, Notch removal from the cell surface and its targeting to the lysosome for degradation is one way in which Notch activity is downregulated since it limits receptor exposure to ligand. In contrast, Notch-signaling capacity is maintained through repeated rounds of receptor recycling and redelivery of Notch to the cell surface from endosomal stores. This review discusses the molecular mechanisms by which Notch transit through the endosome is controlled and how various intracellular sorting decisions are thought to impact signaling activity.

The Mammalian Orthologs of Drosophila Lgd, CC2D1A and CC2D1B, Function in the Endocytic Pathway, but Their Individual Loss of Function Does Not Affect Notch Signalling

CC2D1A and CC2D1B belong to the evolutionary conserved Lgd protein family with members in all multi-cellular animals. Several functions such as centrosomal cleavage, involvement in signalling pathways, immune response and synapse maturation have been described for CC2D1A. Moreover, the Drosophila melanogaster ortholog Lgd was shown to be involved in the endosomal trafficking of the Notch receptor and other transmembrane receptors and physically interacts with the ESCRT-III component Shrub/CHMP4. To determine if this function is conserved in mammals we generated and characterized Cc2d1a and Cc2d1b conditional knockout mice. While Cc2d1b deficient mice displayed no obvious phenotype, we found that Cc2d1a deficient mice as well as conditional mutants that lack CC2D1A only in the nervous system die shortly after birth due to respiratory distress. This finding confirms the suspicion that the breathing defect is caused by the central nervous system. However, an involvement in centrosomal function could not be confirmed in Cc2d1a deficient MEF cells. To analyse an influence on Notch signalling, we generated intestine specific Cc2d1a mutant mice. These mice did not display any alterations in goblet cell number, proliferating cell number or expression of the Notch reporter Hes1-emGFP, suggesting that CC2D1A is not required for Notch signalling. However, our EM analysis revealed that the average size of endosomes of Cc2d1a mutant cells, but not Cc2d1b mutant cells, is increased, indicating a defect in endosomal morphogenesis. We could show that CC2D1A and its interaction partner CHMP4B are localised on endosomes in MEF cells, when the activity of the endosomal protein VPS4 is reduced. This indicates that CC2D1A cycles between the cytosol and the endosomal membrane. Additionally, in rescue experiments in D. melanogaster, CC2D1A and CC2D1B were able to functionally replace Lgd. Altogether our data suggest a functional conservation of the Lgd protein family in the ESCRT-III mediated process in metazoans.

The proteins of the Lgd/CC2D1 family are conserved in all multicellular animals. The Drosophila melanogaster ortholog Lgd is involved in the regulation of signalling receptor degradation via the endosomal pathway. Loss of lgd function causes ectopic ligand-independent activation of the Notch signalling pathway due to a defect in the endosomal pathway. For the mammalian proteins no endosomal function has been defined so far. Here, we asked whether the function of Lgd is conserved in mammals with the focus on the question whether its orthologs are also involved in the endosomal pathway and regulation of Notch pathway activity. Therefore, we generated and characterised Cc2d1a and Cc2d1b conditional knockout mice. We found that the loss of Cc2d1b does not lead to an obvious phenotype, while the known lethality of Cc2d1a deficient newborns is nervous system dependent. In experiments with MEFs isolated from knockout animals we provide evidence that both CC2D1 proteins are involved in the function of the ESCRT-III complex in a similar manner as Lgd in D. melanogaster. Moreover, we found that the loss of one CC2D1 protein is not sufficient to cause ectopic activation of Notch signalling.

Ligand-Independent Mechanisms of Notch Activity

Interaction between the Notch receptor and Delta-Serrate-Lag2 (DSL) ligands is generally deemed to be the starting point of the Notch signaling cascade, after which, Notch is cleaved and the intracellular domain acts as a transcriptional coactivator. By contrast, Notch protein can become activated independent of ligand stimulus through recently identified endosomal trafficking routes as well as through aberrant regulation of Notch components during Notch trafficking, ubiquitination, and degradation. In this review, we summarize genes implicated in ligand-independent Notch activity and remark on the mechanisms by which this process could occur.

Rme-8 depletion perturbs Notch recycling and predisposes to pathogenic signaling

The retromer-associated DNAJ protein Rme-8 is necessary for normal Notch recycling, and reductions in Rme-8 sensitize cells so that additional loss-of-sorting retromer or ESCRT-0 components have catastrophic effects.

Notch signaling is a major regulator of cell fate, proliferation, and differentiation. Like other signaling pathways, its activity is strongly influenced by intracellular trafficking. Besides contributing to signal activation and down-regulation, differential fluxes between trafficking routes can cause aberrant Notch pathway activation. Investigating the function of the retromer-associated DNAJ protein Rme-8 in vivo, we demonstrate a critical role in regulating Notch receptor recycling. In the absence of Rme-8, Notch accumulated in enlarged tubulated Rab4-positive endosomes, and as a consequence, signaling was compromised. Strikingly, when the retromer component Vps26 was depleted at the same time, Notch no longer accumulated and instead was ectopically activated. Likewise, depletion of ESCRT-0 components Hrs or Stam in combination with Rme-8 also led to high levels of ectopic Notch activity. Together, these results highlight the importance of Rme-8 in coordinating normal endocytic recycling route and reveal that its absence predisposes toward conditions in which pathological Notch signaling can occur.

[The ESCRT complex: from endosomal transport to the development of multicellular organisms]

Since its discovery more than 50 years ago, the endo-lysosomal system has emerged as a central integrator of different cellular activities. This vesicular trafficking apparatus governs processes as diverse as the transduction of stimuli by growth factor receptors, the recycling and secretion of signaling molecules and the regulation of cellular homeostasis through autophagy. Accordingly, dysfunctions of the vesicular transport machinery have been linked to a growing number of pathologies. In this review we take the "Endosomal Sorting Complex Required for Transport" (ESCRT) as an example to illustrate the multiple functions of an evolutionarily conserved endosomal transport machinery. We describe the major concepts that have emerged from the study of this machinery at the level of the development and the physiology of multi-cellular organisms. In particular, we highlight the essential contributions of ESCRT proteins on the regulation of three biological processes: the endocytic regulation of cell signaling, autophagy and its role in neuronal morphogenesis and finally the biogenesis and function of extracellular vesicles.

Lgd regulates the activity of the BMP/Dpp signalling pathway during Drosophila oogenesis

The tumour suppressor gene lethal (2) giant discs (lgd) is involved in endosomal trafficking of transmembrane proteins in Drosophila. Loss of function results in the ligand-independent activation of the Notch pathway in all imaginal disc cells and follicle cells. Analysis of lgd loss of function has largely been restricted to imaginal discs and suggests that no other signalling pathway is affected. The devotion of Lgd to the Notch pathway was puzzling given that lgd loss of function also affects trafficking of components of other signalling pathways, such as the Dpp (a Drosophila BMP) pathway. Moreover, Lgd physically interacts with Shrub, a fundamental component of the ESCRT trafficking machinery, whose loss of function results in the activation of several signalling pathways. Here, we show that during oogenesis lgd loss of function causes ectopic activation of the Drosophila BMP signalling pathway. This activation occurs in somatic follicle cells as well as in germline cells. The activation in germline cells causes an extra round of division, producing egg chambers with 32 instead of 16 cells. Moreover, more germline stem cells were formed. The lgd mutant cells are defective in endosomal trafficking, causing an accumulation of the type I Dpp receptor Thickveins in maturing endosomes, which probably causes activation of the pathway. Taken together, these results show that lgd loss of function causes various effects among tissues and can lead to the activation of signalling pathways other than Notch. They further show that there is a role for the endosomal pathway during oogenesis.

Drosophila Vps4 promotes Epidermal growth factor receptor signaling independently of its role in receptor degradation

Endocytic trafficking of signaling receptors is an important mechanism for limiting signal duration. Components of the Endosomal Sorting Complexes Required for Transport (ESCRT), which target ubiquitylated receptors to intra-lumenal vesicles (ILVs) of multivesicular bodies, are thought to terminate signaling by the epidermal growth factor receptor (EGFR) and direct it for lysosomal degradation. In a genetic screen for mutations that affect Drosophila eye development, we identified an allele of Vacuolar protein sorting 4 (Vps4), which encodes an AAA ATPase that interacts with the ESCRT-III complex to drive the final step of ILV formation. Photoreceptors are largely absent from Vps4 mutant clones in the eye disc, and even when cell death is genetically prevented, the mutant R8 photoreceptors that develop fail to recruit surrounding cells to differentiate as R1-R7 photoreceptors. This recruitment requires EGFR signaling, suggesting that loss of Vps4 disrupts the EGFR pathway. In imaginal disc cells mutant for Vps4, EGFR and other receptors accumulate in endosomes and EGFR target genes are not expressed; epistasis experiments place the function of Vps4 at the level of the receptor. Surprisingly, Vps4 is required for EGFR signaling even in the absence of Shibire, the Dynamin that internalizes EGFR from the plasma membrane. In ovarian follicle cells, in contrast, Vps4 does not affect EGFR signaling, although it is still essential for receptor degradation. Taken together, these findings indicate that Vps4 can promote EGFR activity through an endocytosis-independent mechanism.

The vesicle-associated function of NOD2 as a link between Crohn's disease and mycobacterial infection

Although Crohn’s disease (CD) etiology remains unclear, a growing body of evidence suggests that CD may include an infectious component, with Mycobacterium avium subsp. paratuberculosis (MAP) being the most likely candidate for this role. However, the molecular mechanism of the MAP involvement in CD pathogenesis remains unclear. The polymorphism of the NOD2 gene, coding for an intracellular pattern recognition receptor, is a factor of predisposition to mycobacterial infections and CD. Recent findings on NOD2 interactions and functions provide the missing pieces in the puzzle of a NOD2-mediated mechanism common for mycobacterial infections and CD. Implications of these new findings for the development of a better understanding and treatments of CD and mycobacterial infections are discussed.

Cis-interactions between Notch and its ligands block ligand-independent Notch activity

The Notch pathway is integrated into numerous developmental processes and therefore is fine-tuned on many levels, including receptor production, endocytosis, and degradation. Notch is further characterized by a twofold relationship with its Delta-Serrate (DSL) ligands, as ligands from opposing cells (trans-ligands) activate Notch, whereas ligands expressed in the same cell (cis-ligands) inhibit signaling. We show that cells without both cis- and trans-ligands can mediate Notch-dependent developmental events during Drosophila oogenesis, indicating ligand-independent Notch activity occurs when the receptor is free of cis- and trans-ligands. Furthermore, cis-ligands can reduce Notch activity in endogenous and genetically induced situations of elevated trans-ligand-independent Notch signaling. We conclude that cis-expressed ligands exert their repressive effect on Notch signaling in cases of trans-ligand-independent activation, and propose a new function of cis-inhibition which buffers cells against accidental Notch activity.

DOI:http://dx.doi.org/10.7554/eLife.04415.001

Many biological processes require cells to send messages to one another. Typically, this is achieved when molecules are released from one cell and make contact with companion molecules on another cell. This triggers a chemical or biological reaction in the receiving cell.

One of the most common examples of this is the Notch pathway, which is used throughout the animal kingdom and plays an important role in helping cells and embryos to develop. The Notch protein itself is a ‘receptor’ protein that is embedded in the surface of a cell, and relays signals from outside the cell to activate certain genes inside the cell. In fruit flies, two proteins called Serrate and Delta act as ‘ligands’ for Notch—by binding to Notch, they can change how this receptor works.

If Serrate or Delta are present on the outside of one cell, they can activate Notch (and hence the Notch signaling pathway) in an adjacent cell. However, if the Serrate or Delta ligands are present on the surface of the same cell as Notch they turn the receptor off, rather than activate it. Notch can also work without being activated by Serrate or Delta, but whether the ligands can inhibit this ‘ligand-independent’ Notch activation if they are on the surface of the same cell as the Notch receptor was unknown.

Palmer et al. study Notch signaling in the fruit fly equivalent of the ovary, in cells that are naturally deficient in Serrate and from which Delta was artificially removed. The Notch protein was activated when these ligands were not present. Furthermore, the developmental processes that are activated by Notch were able to proceed as normal when triggered by ligand-independent Notch signaling. In total, Palmer et al. investigated three different types of fruit fly cell, and found that ligand-independent Notch signaling can occur in all of them.

Reintroducing Delta to the same cell as Notch turns the receptor off, suggesting that ligands on the surface of the same cell as the receptor can inhibit ligand-independent Notch activity. Many genetic diseases and cancers have been linked to Notch being activated when it should not be; therefore, understanding how Notch is controlled could help guide the development of new treatments for these conditions.

DOI:http://dx.doi.org/10.7554/eLife.04415.002

Analysis of the expression patterns, subcellular localisations and interaction partners of Drosophila proteins using a pigP protein trap library

Although we now have a wealth of information on the transcription patterns of all the genes in the Drosophila genome, much less is known about the properties of the encoded proteins. To provide information on the expression patterns and subcellular localisations of many proteins in parallel, we have performed a large-scale protein trap screen using a hybrid piggyBac vector carrying an artificial exon encoding yellow fluorescent protein (YFP) and protein affinity tags. From screening 41 million embryos, we recovered 616 verified independent YFP-positive lines representing protein traps in 374 genes, two-thirds of which had not been tagged in previous P element protein trap screens. Over 20 different research groups then characterized the expression patterns of the tagged proteins in a variety of tissues and at several developmental stages. In parallel, we purified many of the tagged proteins from embryos using the affinity tags and identified co-purifying proteins by mass spectrometry. The fly stocks are publicly available through the Kyoto Drosophila Genetics Resource Center. All our data are available via an open access database (Flannotator), which provides comprehensive information on the expression patterns, subcellular localisations and in vivo interaction partners of the trapped proteins. Our resource substantially increases the number of available protein traps in Drosophila and identifies new markers for cellular organelles and structures.

Lgl regulates Notch signaling via endocytosis, independently of the apical aPKC-Par6-Baz polarity complex

BACKGROUND

The Drosophila melanogaster junctional neoplastic tumor suppressor, Lethal-2-giant larvae (Lgl), is a regulator of apicobasal cell polarity and tissue growth. We have previously shown in the developing Drosophila eye epithelium that, without affecting cell polarity, depletion of Lgl results in ectopic cell proliferation and blockage of developmental cell death due to deregulation of the Hippo signaling pathway.

RESULTS

Here, we show that Notch signaling is increased in lgl-depleted eye tissue, independently of Lgl's function in apicobasal cell polarity. The upregulation of Notch signaling is ligand dependent and correlates with accumulation of cleaved Notch. Concomitant with higher cleaved Notch levels in lgl- tissue, early endosomes (Avalanche [Avl+]), recycling endosomes (Rab11+), early multivesicular bodies (Hrs+), and acidified vesicles, but not late endosomal markers (Car+ and Rab7+), accumulate. Colocalization studies revealed that Lgl associates with early to late endosomes and lysosomes. Upregulation of Notch signaling in lgl- tissue requires dynamin- and Rab5-mediated endocytosis and vesicle acidification but is independent of Hrs/Stam or Rab11 activity. Furthermore, Lgl regulates Notch signaling independently of the aPKC-Par6-Baz apical polarity complex.

CONCLUSIONS

Altogether, our data show that Lgl regulates endocytosis to restrict vesicle acidification and prevent ectopic ligand-dependent Notch signaling. This Lgl function is independent of the aPKC-Par6-Baz polarity complex and uncovers a novel attenuation mechanism of ligand-activated Notch signaling during Drosophila eye development.

Compensatory flux changes within an endocytic trafficking network maintain thermal robustness of Notch signaling

Developmental signaling is remarkably robust to environmental variation, including temperature. For example, in ectothermic animals such as Drosophila, Notch signaling is maintained within functional limits across a wide temperature range. We combine experimental and computational approaches to show that temperature compensation of Notch signaling is achieved by an unexpected variety of endocytic-dependent routes to Notch activation which, when superimposed on ligand-induced activation, act as a robustness module. Thermal compensation arises through an altered balance of fluxes within competing trafficking routes, coupled with temperature-dependent ubiquitination of Notch. This flexible ensemble of trafficking routes supports Notch signaling at low temperature but can be switched to restrain Notch signaling at high temperature and thus compensates for the inherent temperature sensitivity of ligand-induced activation. The outcome is to extend the physiological range over which normal development can occur. Similar mechanisms may provide thermal robustness for other developmental signals.

Oncogenic programmes and Notch activity: an 'organized crime'?

The inappropriate Notch signalling can influence virtually all aspect of cancer, including tumour-cell growth, survival, apoptosis, angiogenesis, invasion and metastasis, although it does not do this alone. Hence, elucidating the partners of Notch that are active in cancer is now the focus of much intense research activity. The genetic toolkits available, coupled to the small size and short life of the fruit fly Drosophila melanogaster, makes this an inexpensive and effective animal model, suited to large-scale cancer gene discovery studies. The fly eye is not only a non-vital organ but its stereotyped size and disposition also means it is easy to screen for mutations that cause tumours and metastases and provides ample opportunities to test cancer theories and to unravel unanticipated nexus between Notch and other cancer genes, or to discover unforeseen Notch's partners in cancer. These studies suggest that Notch's oncogenic capacity is brought about not simply by increasing signal strength but through partnerships, whereby oncogenes gain more by cooperating than acting individually, as in a ring 'organized crime'.

ESCRT-0 is not required for ectopic Notch activation and tumor suppression in Drosophila

Multivesicular endosome (MVE) sorting depends on proteins of the Endosomal Sorting Complex Required for Transport (ESCRT) family. These are organized in four complexes (ESCRT-0, -I, -II, -III) that act in a sequential fashion to deliver ubiquitylated cargoes into the internal luminal vesicles (ILVs) of the MVE. Drosophila genes encoding ESCRT-I, -II, -III components function in sorting signaling receptors, including Notch and the JAK/STAT signaling receptor Domeless. Loss of ESCRT-I, -II, -III in Drosophila epithelia causes altered signaling and cell polarity, suggesting that ESCRTs genes are tumor suppressors. However, the nature of the tumor suppressive function of ESCRTs, and whether tumor suppression is linked to receptor sorting is unclear. Unexpectedly, a null mutant in Hrs, encoding one of the components of the ESCRT-0 complex, which acts upstream of ESCRT-I, -II, -III in MVE sorting is dispensable for tumor suppression. Here, we report that two Drosophila epithelia lacking activity of Stam, the other known components of the ESCRT-0 complex, or of both Hrs and Stam, accumulate the signaling receptors Notch and Dome in endosomes. However, mutant tissue surprisingly maintains normal apico-basal polarity and proliferation control and does not display ectopic Notch signaling activation, unlike cells that lack ESCRT-I, -II, -III activity. Overall, our in vivo data confirm previous evidence indicating that the ESCRT-0 complex plays no crucial role in regulation of tumor suppression, and suggest re-evaluation of the relationship of signaling modulation in endosomes and tumorigenesis.

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.