Secreted forms of DELTA and SERRATE define antagonists of Notch signaling in Drosophila

A genetic screen in Drosophila reveals the role of fucosylation in host susceptibility to Candida infection

Candida infections constitute a blind spot in global public health as very few new anti-fungal drugs are being developed. Genetic surveys of host susceptibilities to such infections using mammalian models have certain disadvantages in that obtaining results is time-consuming, owing to relatively long lifespans, and these results have low statistical resolution because sample sizes are usually small. Here, we report a targeted genetic screening of 5698 RNAi lines encompassing 4135 Drosophila genes with human homologues, several of which we identify as important for host survival after Candida albicans infection. These include genes in a variety of functional classes encompassing gene expression, intracellular signalling, metabolism and enzymatic regulation. Analysis of one of the screen hits, the infection-induced α-(1,3)-fucosylase FucTA, showed that N-glycan fucosylation has several targets among proteins involved in host defence, which provides multiple avenues of investigation for the mechanistic analysis of host survival to systemic C. albicans infection.

Comparative and evolutionary analyses reveal conservation and divergence of the notch pathway in lophotrochozoa

Lophotrochozoan species exhibit wide morphological diversity; however, the molecular basis underlying this diversity remains unclear. Here, we explored the evolution of Notch pathway genes across 37 metazoan species via phylogenetic and molecular evolutionary studies with emphasis on the lophotrochozoans. We displayed the components of Notch pathway in metazoans and found that Delta and Hes/Hey-related genes, as well as their functional domains, are duplicated in lophotrochozoans. Comparative transcriptomics analyses allow us to pinpoint sequence divergence of multigene families in the Notch signalling pathway. We identified the duplication mechanism of a mollusc-specific gene, Delta2, and found it displayed complementary expression throughout development. Furthermore, we found the functional diversification not only in expanded genes in the Notch pathway (Delta and Hes/Hey-related genes), but also in evolutionary conservative genes (Notch, Presenilin, and Su(H)). Together, this comprehensive study demonstrates conservation and divergence within the Notch pathway, reveals evolutionary relationships among metazoans, and provides evidence for the occurrence of developmental diversity in lophotrochozoans, as well as a basis for future gene function studies.

Biophysics of Notch Signaling

Notch signaling is a conserved system of communication between adjacent cells, influencing numerous cell fate decisions in the development of multicellular organisms. Aberrant signaling is also implicated in many human pathologies. At its core, Notch has a mechanotransduction module that decodes receptor-ligand engagement at the cell surface under force to permit proteolytic cleavage of the receptor, leading to the release of the Notch intracellular domain (NICD). NICD enters the nucleus and acts as a transcriptional effector to regulate expression of Notch-responsive genes. In this article, we review and integrate current understanding of the detailed molecular basis for Notch signal transduction, highlighting quantitative, structural, and dynamic features of this developmentally central signaling mechanism. We discuss the implications of this mechanistic understanding for the functionality of the signaling pathway in different molecular and cellular contexts.

A modifier screen identifies regulators of cytoskeletal architecture as mediators of Shroom-dependent changes in tissue morphology

Regulation of cell architecture is critical in the formation of tissues during animal development. The mechanisms that control cell shape must be both dynamic and stable in order to establish and maintain the correct cellular organization. Previous work has identified Shroom family proteins as essential regulators of cell morphology during vertebrate development. Shroom proteins regulate cell architecture by directing the subcellular distribution and activation of Rho-kinase, which results in the localized activation of non-muscle myosin II. Because the Shroom-Rock-myosin II module is conserved in most animal model systems, we have utilized Drosophila melanogaster to further investigate the pathways and components that are required for Shroom to define cell shape and tissue architecture. Using a phenotype-based heterozygous F1 genetic screen for modifiers of Shroom activity, we identified several cytoskeletal and signaling protein that may cooperate with Shroom. We show that two of these proteins, Enabled and Short stop, are required for ShroomA-induced changes in tissue morphology and are apically enriched in response to Shroom expression. While the recruitment of Ena is necessary, it is not sufficient to redefine cell morphology. Additionally, this requirement for Ena appears to be context dependent, as a variant of Shroom that is apically localized, binds to Rock, but lacks the Ena binding site, is still capable of inducing changes in tissue architecture. These data point to important cellular pathways that may regulate contractility or facilitate Shroom-mediated changes in cell and tissue morphology.

Summary: Using Drosophila as a model system, we identify F-actin and microtubules as important determinants of how cells and tissues respond to Shroom induced contractility.

Ligand-Induced Cis-Inhibition of Notch Signaling: The Role of an Extracellular Region of Serrate

Cellular development can be controlled by communication between adjacent cells mediated by the highly conserved Notch signaling system. A cell expressing the Notch receptor on one cell can be activated in trans by ligands on an adjacent cell leading to alteration of transcription and cellular fate. Ligands also have the ability to inhibit Notch signaling, and this can be accomplished when both receptor and ligands are coexpressed in cis on the same cell. The manner in which cis-inhibition is accomplished is not entirely clear but it is known to involve several different protein domains of the ligands and the receptor. Some of the protein domains involved in trans-activation are also used for cis-inhibition, but some are used uniquely for each process. In this work, the involvement of various ligand regions and the receptor are discussed in relation to their contributions to Notch signaling.

Contraction of basal filopodia controls periodic feather branching via Notch and FGF signaling

Branching morphogenesis is a general mechanism that increases the surface area of an organ. In chicken feathers, the flat epithelial sheath at the base of the follicle is transformed into periodic branches. How exactly the keratinocytes are organized into this pattern remains unclear. Here we show that in the feather follicle, the pre-branch basal keratinocytes have extensive filopodia, which contract and smooth out after branching. Manipulating the filopodia via small GTPases RhoA/Cdc42 also regulates branch formation. These basal filopodia help interpret the proximal-distal FGF gradient in the follicle. Furthermore, the topological arrangement of cell adhesion via E-Cadherin re-distribution controls the branching process. Periodic activation of Notch signaling drives the differential cell adhesion and contraction of basal filopodia, which occurs only below an FGF signaling threshold. Our results suggest a coordinated adjustment of cell shape and adhesion orchestrates feather branching, which is regulated by Notch and FGF signaling.

Keratinocytes are organised into a periodic pattern in feather branching, but how this is regulated is unclear. Here, the authors show that there is a coordinated change in cell shape and adherence, mediated by Notch, FGF signalling and Rho GTPases, which in turn regulates feather branching.

Loss of Drosophila nucleostemin 2 (NS2) blocks nucleolar release of the 60S subunit leading to ribosome stress

Four nucleostemin-like proteins (nucleostemin (NS) 1-4) were identified previously in Drosophila melanogaster. NS1 and NS2 are nucleolar proteins, while NS3 and NS4 are cytoplasmic proteins. We showed earlier that NS1 (homologous to human GNL3) enriches within the granular components (GCs) of Drosophila nucleoli and is required for efficient maturation or nucleolar release of the 60S subunit. Here, we show that NS2 is homologous to the human nucleostemin-like protein, Ngp1 (GNL2), and that endogenous NS2 is expressed in both progenitor and terminally differentiated cell types. Exogenous GFP-NS2 enriched within nucleolar GCs versus endogenous fibrillarin that marked the dense fibrillar components (DFCs). Like NS1, depletion of NS2 in midgut cells blocked the release of the 60S subunit as detected by the accumulation of GFP-RpL11 within nucleoli, and this likely led to the general loss of 60S subunits as shown by immunoblot analyses of RpL23a and RpL34. At the ultrastructural level, nucleoli in midgut cells depleted of NS2 displayed enlarged GCs not only on the nucleolar periphery but interspersed within the DFCs. Depletion of NS2 caused ribosome stress: larval midgut cells displayed prominent autophagy marked by the appearance of autolysosomes containing mCherry-ATG8a and the appearance of rough endoplasmic reticulum (rER)-derived isolation membranes. Larval imaginal wing disc cells depleted of NS2 induced apoptosis as marked by anti-caspase 3 labeling; loss of these progenitor cells resulted in defective adult wings. We conclude that nucleolar proteins NS1 and NS2 have similar but non-overlapping roles in the final maturation or nucleolar release of 60S ribosomal subunits.

Mutational analysis of the Notch2 negative regulatory region identifies key structural elements for mechanical stability

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Mutations within heterodimerization (HD) domain of Notch2 reduce mechanical stability.

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Structural changes are observed within the LNRA:B linker region/LNRB and αC helix.

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The LNRC:HD domain interaction is also reduced in stability.

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Changes in mechanical stability versus chemical stability are highlighted.

The Notch signalling pathway is fundamental to cell differentiation in developing and self-renewing tissues. Notch is activated upon ligand-induced conformational change of the Notch negative regulatory region (NRR), unmasking a key proteolytic site (S2) and facilitating downstream events. The favoured model requires endocytosis of a tightly bound ligand to transmit force to the NRR region, sufficient to cause a structural change that exposes the S2 site. We have previously shown, using atomic force microscopy and molecular dynamics simulations, that application of force to the N-terminus of the Notch2 NRR facilitates metalloprotease cleavage at an early stage in the unfolding process. Here, mutations are made within the heterodimerization (HD) domain of the NRR that are known to cause constitutive activation of Notch1 whilst having no effect on the chemical stability of Notch2. Comparison of the mechanical stability and simulated forced unfolding of recombinant Notch2 NRR proteins demonstrates a reduced stability following mutation and identifies two critical structural elements of the NRR in its response to force – the linker region between Lin12-Notch repeats LNRA and LNRB and the α3 helix within the HD domain – both of which mask the S2 cleavage site prior to Notch activation. In two mutated proteins, the LNRC:HD domain interaction is also reduced in stability. The observed changes to mechanical stability following these HD domain mutations highlight key regions of the Notch2 NRR that are important for mechanical, but not chemical, stability. This research could also help determine the fundamental differences in the NRRs of Notch1 and Notch2.

E-cadherin-defective gastric cancer cells depend on Laminin to survive and invade

Epithelial-cadherin (Ecad) deregulation affects cell-cell adhesion and results in increased invasiveness of distinct human carcinomas. In gastric cancer, loss of Ecad expression is a common event and is associated with disease aggressiveness and poor prognosis. However, the molecular mechanisms underlying the invasive process associated to Ecad dysfunction are far from understood. We hypothesized that deregulation of cell-matrix interactions could play an important role during this process. Thus, we focussed on LM-332, which is a major matrix component, and in Ecad/LM-332 crosstalk in the process of Ecad-dependent invasion. To verify whether matrix deregulation was triggered by Ecad loss, we used the Drosophila model. To dissect the key molecules involved and unveil their functional significance, we used gastric cancer cell lines. The relevance of this relationship was then confirmed in human primary tumours. In vivo, Ecad knockdown induced apoptosis; nonetheless, at the invasive front, cells ectopically expressed Laminin A and βPS integrin. In vitro, we demonstrated that, in two different gastric cancer cell models, Ecad-defective cells overexpressed Laminin γ2 (LM-γ2), β1 and β4 integrin, when compared with Ecad-competent ones. We showed that LM-γ2 silencing impaired invasion and enhanced cell death, most likely via pSrc and pAkt reduction, and JNK activation. In human gastric carcinomas, we found a concomitant decrease in Ecad and increase in LM-γ2. This is the first evidence that ectopic Laminin expression depends on Ecad loss and allows Ecad-dysfunctional cells to survive and invade. This opens new avenues for using LM-γ2 signalling regulators as molecular targets to impair gastric cancer progression.

Lighting Up the Force: Investigating Mechanisms of Mechanotransduction Using Fluorescent Tension Probes

The ability of cells to sense the physical nature of their surroundings is critical to the survival of multicellular organisms. Cellular response to physical cues from adjacent cells and the extracellular matrix leads to a dynamic cycle in which cells respond by remodeling their local microenvironment, fine-tuning cell stiffness, polarity, and shape. Mechanical regulation is important in cellular development, normal morphogenesis, and wound healing. The mechanisms by which these finely balanced mechanotransduction events occur, however, are not well understood. In large part, this is due to the limited availability of tools to study molecular mechanotransduction events in live cells. Several classes of molecular tension probes have been recently developed which are rapidly transforming the study of mechanotransduction. Molecular tension probes are primarily based on fluorescence resonance energy transfer (FRET) and report on piconewton scale tension events in live cells. In this minireview, we describe the two main classes of tension probes, genetically encoded tension sensors and immobilized tension sensors, and discuss the advantages and limitations of each type. We discuss future opportunities to address major biological questions and outline the challenges facing the next generation of molecular tension probes.

Phenotype-based clustering of glycosylation-related genes by RNAi-mediated gene silencing

Glycan structures are synthesized by a series of reactions conducted by glycosylation-related (GR) proteins such as glycosyltransferases, glycan-modifying enzymes, and nucleotide-sugar transporters. For example, the common core region of glycosaminoglycans (GAGs) is sequentially synthesized by peptide-O-xylosyltransferase, β1,4-galactosyltransferase I, β1,3-galactosyltransferase II, and β1,3-glucuronyltransferase. This raises the possibility that functional impairment of GR proteins involved in synthesis of the same glycan might result in the same phenotypic abnormality. To examine this possibility, comprehensive silencing of genes encoding GR and proteoglycan core proteins was conducted in Drosophila. Drosophila GR candidate genes (125) were classified into five functional groups for synthesis of GAGs, N-linked, O-linked, Notch-related, and unknown glycans. Spatiotemporally regulated silencing caused a range of malformed phenotypes that fell into three types: extra veins, thick veins, and depigmentation. The clustered phenotypes reflected the biosynthetic pathways of GAGs, Fringe-dependent glycan on Notch, and glycans placed at or near nonreducing ends (herein termed terminal domains of glycans). Based on the phenotypic clustering, CG33145 was predicted to be involved in formation of terminal domains. Our further analysis showed that CG33145 exhibited galactosyltransferase activity in synthesis of terminal N-linked glycans. Phenotypic clustering, therefore, has potential for the functional prediction of novel GR genes.

Impaired endolysosomal function disrupts Notch signalling in optic nerve astrocytes

Astrocytes migrate from the optic nerve into the inner retina, forming a template upon which retinal vessels develop. In the Nuc1 rat, mutation in the gene encoding βA3/A1-crystallin disrupts both Notch signalling in astrocytes and formation of the astrocyte template. Here we show that loss of βA3/A1-crystallin in astrocytes does not impede Notch ligand binding or extracellular cleavages. However, it affects vacuolar-type proton ATPase (V-ATPase) activity, thereby compromising acidification of the endolysosomal compartments, leading to reduced γ-secretase-mediated processing and release of the Notch intracellular domain (NICD). Lysosomal-mediated degradation of Notch is also impaired. These defects decrease the level of NICD in the nucleus, inhibiting the expression of Notch target genes. Overexpression of βA3/A1-crystallin in those same astrocytes restored V-ATPase activity and normal endolysosomal acidification, thereby increasing the levels of γ-secretase to facilitate optimal Notch signalling. We postulate that βA3/A1-crystallin is essential for normal endolysosomal acidification, and thereby, normal activation of Notch signalling in astrocytes.

An extracellular region of Serrate is essential for ligand-induced cis-inhibition of Notch signaling

Cell-to-cell communication via the Notch pathway is mediated between the membrane-bound Notch receptor and either of its canonical membrane-bound ligands Delta or Serrate. Notch ligands mediate receptor transactivation between cells and also mediate receptor cis-inhibition when Notch and ligand are co-expressed on the same cell. We demonstrate in Drosophila that removal of any of the EGF-like repeats (ELRs) 4, 5 or 6 results in a Serrate molecule capable of transactivating Notch but exhibiting little or no Notch cis-inhibition capacity. These forms of Serrate require Epsin (Liquid facets) to transduce a signal, suggesting that ELR 4-6-deficient ligands still require endocytosis for Notch activation. We also demonstrate that ELRs 4-6 are responsible for the dominant-negative effects of Serrate ligand forms that lack the intracellular domain and are therefore incapable of endocytosis in the ligand-expressing cell. We find that ELRs 4-6 of Serrate are conserved across species but do not appear to be conserved in Delta homologs.

Endothelial cells promote the colorectal cancer stem cell phenotype through a soluble form of Jagged-1

We report a paracrine effect whereby endothelial cells (ECs) promote the cancer stem cell (CSC) phenotype of human colorectal cancer (CRC) cells. We showed that, without direct cell-cell contact, ECs secrete factors that promoted the CSC phenotype in CRC cells via Notch activation. In human CRC specimens, CD133 and Notch intracellular domain-positive CRC cells colocalized in perivascular regions. An EC-derived, soluble form of Jagged-1, via ADAM17 proteolytic activity, led to Notch activation in CRC cells in a paracrine manner; these effects were blocked by immunodepletion of Jagged-1 in EC-conditioned medium or blockade of ADAM17 activity. Collectively, ECs play an active role in promoting Notch signaling and the CSC phenotype by secreting soluble Jagged-1.

Direct observation of proteolytic cleavage at the S2 site upon forced unfolding of the Notch negative regulatory region

The conserved Notch signaling pathway plays crucial roles in developing and self-renewing tissues. Notch is activated upon ligand-induced conformation change of the Notch negative regulatory region (NRR) unmasking a key proteolytic site (S2) and facilitating downstream events. Thus far, the molecular mechanism of this signal activation is not defined. However, strong indirect evidence favors a model whereby transendocytosis of the Notch extracellular domain, in tight association with ligand into the ligand-bearing cell, exerts a force on the NRR to drive the required structure change. Here, we demonstrate that force applied to the human Notch2 NRR can indeed expose the S2 site and, crucially, allow cleavage by the metalloprotease TACE (TNF-alpha-converting enzyme). Molecular insight into this process is achieved using atomic force microscopy and molecular dynamics simulations on the human Notch2 NRR. The data show near-sequential unfolding of its constituent LNR (Lin12-Notch repeat) and HD (heterodimerization) domains, at forces similar to those observed for other protein domains with a load-bearing role. Exposure of the S2 site is the first force "barrier" on the unfolding pathway, occurring prior to unfolding of any domain, and achieved via removal of the LNRAB linker region from the HD domain. Metal ions increase the resistance of the Notch2 NRR to forced unfolding, their removal clearly facilitating unfolding at lower forces. The results provide direct demonstration of force-mediated exposure and cleavage of the Notch S2 site and thus firmly establish the feasibility of a mechanotransduction mechanism for ligand-induced Notch activation.

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

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

Distinct intracellular motifs of Delta mediate its ubiquitylation and activation by Mindbomb1 and Neuralized

Ubiquitylation of the intracellular domain of Drosophila Delta is necessary for Notch activation.

DSL proteins are transmembrane ligands of the Notch receptor. They associate with a RING (really interesting new gene) family E3 ubiquitin ligase, either Neuralized (Neur) or Mindbomb 1 (Mib1), as a prerequisite to signaling. Although Neur and Mib1 stimulate internalization of DSL ligands, it is not known how ubiquitylation contributes to signaling. We present a molecular dissection of the intracellular domain (ICD) of Drosophila melanogaster Delta (Dl), a prototype DSL protein. Using a cell-based assay, we detected ubiquitylation of Dl by both Neur and Mib1. The two enzymes use distinct docking sites and displayed different acceptor lysine preferences on the Dl ICD. We generated Dl variants that selectively perturb its interactions with Neur or Mib1 and analyzed their signaling activity in two in vivo contexts. We found an excellent correlation between the ability to undergo ubiquitylation and signaling. Therefore, ubiquitylation of the DSL ICD seems to be a necessary step in the activation of Notch.

The role of Adams in Notch signaling

Regulated intramembrane proteolysis (RIP) is a highly conserved signaling paradigm whereby membrane-bound signaling proteins are cleaved in their transmembrane region and then released into the cytoplasm to act as signaling molecules. In most if not all cases intramembrane cleavage is preceded and regulated by a membrane proximal cleavage step called 'ectodomain shedding'. Here we will review the role of ectodomain shedding in RIP of the NOTCH signaling pathway, a highly conserved cell-cell communication pathway that mediates cell fate decisions during development and in adult tissues.

Mutational analysis of JAG1 gene in non-syndromic tetralogy of Fallot children

BACKGROUND

JAG1 is an evolutionarily conserved ligand for Notch receptor and functions in the cell fate decisions, cell-cell interactions throughout the development of heart especially right heart development. Tetralogy of Fallot (TOF) is essentially a right sided heart disease with characteristic features of ventricular septal defect, right ventricular outflow tract obstruction, aortic dextroposition and right ventricular hypertrophy. Hence, the present study was investigated to identify mutations of JAG1 gene in an Indian cohort of patients with TOF.

METHODS

The clinical data and blood samples from 84 unrelated subjects with TOF were collected and evaluated in comparison with 87 healthy individuals. PCR based single strand conformation polymorphism analysis and subsequent bidirectional DNA sequencing of conformers was carried in the exon 6 of JAG1 gene.

RESULTS

The DNA sequences aligned with NCBI-BLAST led to the identification of four novel variations including one nonsense 765 C>A, two missense 814 G>T, 834 G>T; and one silent alteration 816 G>T in TOF patients. The protein structure of JAG1 predicts that these variations effect first and second epidermal growth factor like repeat and might disturb ligand-receptor binding ability. The presence of similar variations was not observed in healthy controls. The software CLUSTAL-W showed the inter species conservation of altered amino acids in missense mutations.

CONCLUSION

Disease-associating novel JAG1 gene variations were found in TOF patients, and seem to play an important role in the causation of the disease.

Force-induced unfolding simulations of the human Notch1 negative regulatory region: possible roles of the heterodimerization domain in mechanosensing

Notch receptors are core components of the Notch signaling pathway and play a central role in cell fate decisions during development as well as tissue homeostasis. Upon ligand binding, Notch is sequentially cleaved at the S2 site by an ADAM protease and at the S3 site by the γ-secretase complex. Recent X-ray structures of the negative regulatory region (NRR) of the Notch receptor reveal an auto-inhibited fold where three protective Lin12/Notch repeats (LNR) of the NRR shield the S2 cleavage site housed in the heterodimerization (HD) domain. One of the models explaining how ligand binding drives the NRR conformation from a protease-resistant state to a protease-sensitive one invokes a mechanical force exerted on the NRR upon ligand endocytosis. Here, we combined physics-based atomistic simulations and topology-based coarse-grained modeling to investigate the intrinsic and force-induced folding and unfolding mechanisms of the human Notch1 NRR. The simulations support that external force applied to the termini of the NRR disengages the LNR modules from the heterodimerization (HD) domain in a well-defined, largely sequential manner. Importantly, the mechanical force can further drive local unfolding of the HD domain in a functionally relevant fashion that would provide full proteolytic access to the S2 site prior to heterodimer disassociation. We further analyzed local structural features, intrinsic folding free energy surfaces, and correlated motions of the HD domain. The results are consistent with a model in which the HD domain possesses inherent mechanosensing characteristics that could be utilized during Notch activation. This potential role of the HD domain in ligand-dependent Notch activation may have implications for understanding normal and aberrant Notch signaling.

The cell giveth and the cell taketh away: an overview of Notch pathway activation by endocytic trafficking of ligands and receptors

Notch signaling is firmly established as a form of cell-to-cell communication that is critical throughout development. Dysregulation of Notch has been linked to cancer and developmental disorders, making it an important target for therapeutic intervention. One aspect of this pathway that sets it apart from others is its apparent reliance on endocytosis by signal-sending and signal-receiving cells. The subtle details of endocytosis-mediated molecular processing within both ligand- and receptor-presenting cells that are required for the Notch signal to maintain fidelity remain unclear. The endosomal system has long been known to play an important role in terminating signal transduction by directing lysosomal trafficking and degradation of cell surface receptors. More recently, endocytic trafficking has also been shown to be critical for activation of signaling. This review highlights four models of endocytic processing in the context of the Notch pathway. In ligand-presenting cells, endocytosis may be required for pre-processing of ligands to make them competent for interaction with Notch receptors and/or for exerting a pulling force on the ligand/Notch complex. In receptor-presenting cells, endocytosis may be a prerequisite for Notch cleavage and thus activation and/or it could be a means of limiting ligand-independent Notch activation. Recent advances in our understanding of how and why endocytosis of Notch receptors and ligands is required for activation and termination of signaling during normal development and in disease states are discussed.

The functions of auxilin and Rab11 in Drosophila suggest that the fundamental role of ligand endocytosis in notch signaling cells is not recycling

Notch signaling requires ligand internalization by the signal sending cells. Two endocytic proteins, epsin and auxilin, are essential for ligand internalization and signaling. Epsin promotes clathrin-coated vesicle formation, and auxilin uncoats clathrin from newly internalized vesicles. Two hypotheses have been advanced to explain the requirement for ligand endocytosis. One idea is that after ligand/receptor binding, ligand endocytosis leads to receptor activation by pulling on the receptor, which either exposes a cleavage site on the extracellular domain, or dissociates two receptor subunits. Alternatively, ligand internalization prior to receptor binding, followed by trafficking through an endosomal pathway and recycling to the plasma membrane may enable ligand activation. Activation could mean ligand modification or ligand transcytosis to a membrane environment conducive to signaling. A key piece of evidence supporting the recycling model is the requirement in signaling cells for Rab11, which encodes a GTPase critical for endosomal recycling. Here, we use Drosophila Rab11 and auxilin mutants to test the ligand recycling hypothesis. First, we find that Rab11 is dispensable for several Notch signaling events in the eye disc. Second, we find that Drosophila female germline cells, the one cell type known to signal without clathrin, also do not require auxilin to signal. Third, we find that much of the requirement for auxilin in Notch signaling was bypassed by overexpression of both clathrin heavy chain and epsin. Thus, the main role of auxilin in Notch signaling is not to produce uncoated ligand-containing vesicles, but to maintain the pool of free clathrin. Taken together, these results argue strongly that at least in some cell types, the primary function of Notch ligand endocytosis is not for ligand recycling.

RNAi knockdown of dRNaseZ, the Drosophila homolog of ELAC2, impairs growth of mitotic and endoreplicating tissues

The Drosophila RNase Z(L) (dRNaseZ) gene encodes a member of the ELAC1/ELAC2 protein family with homologs in every living organism. All RNase Z proteins tested so far were found to possess endoribonuclease activity, which is responsible for the removal of a 3' trailer from a primary tRNA transcript. Given that tRNA 3'-end processing has been delineated using in vitro, bacterial and cell culture models, its relevance to RNase Z functions in vivo has yet to be established. In this study, we employed heritable RNA interference (RNAi) in combination with the GAL4/UAS system to spatiotemporally knockdown the dRNaseZ gene and study its biological role in cells of a developing fruit fly. We found that dRNaseZ is an essential gene, as ubiquitous knockdown caused growth arrest and early larval lethality. Molecular analysis confirmed that dRNaseZ is necessary for 3'-end processing of nuclear and mitochondrial tRNAs: knockdown larvae displayed significant accumulation of both types of processing intermediates with extensions at the 3' end. This is the first in vivo demonstration of RNase Z(L) dependent tRNA processing in the context of a metazoan model organism. Using tissue-specific GAL4 drivers, we also showed that in mitotically growing imaginal discs dRNaseZ is required for cell proliferation and/or viability, but not for the maintenance of their differentiated progeny. In endoreplicating salivary glands, dRNaseZ controls organ size by supporting cell growth but not DNA replication. Although the mechanisms remain unclear, our results support the notion that RNase Z(L) is involved in biological pathways regulating cell growth and proliferation.

The importance of structured noise in the generation of self-organizing tissue patterns through contact-mediated cell-cell signalling

Lateral inhibition provides the basis for a self-organizing patterning system in which distinct cell states emerge from an otherwise uniform field of cells. The development of the microchaete bristle pattern on the notum of the fruitfly, Drosophila melanogaster, has long served as a popular model of this process. We recently showed that this bristle pattern depends upon a population of dynamic, basal actin-based filopodia, which span multiple cell diameters. These protrusions establish transient signalling contacts between non-neighbouring cells, generating a type of structured noise that helps to yield a well-ordered and spaced pattern of bristles. Here, we develop a general model of protrusion-based patterning to analyse the role of noise in this process. Using a simple asynchronous cellular automata rule-based model we show that this type of structured noise drives the gradual refinement of lateral inhibition-mediated patterning, as the system moves towards a stable configuration in which cells expressing the inhibitory signal are near-optimally packed. By analysing the effects of introducing thresholds required for signal detection in this model of lateral inhibition, our study shows how filopodia-mediated cell–cell communication can generate complex patterns of spots and stripes, which, in the presence of signalling noise, align themselves across a patterning field. Thus, intermittent protrusion-based signalling has the potential to yield robust self-organizing tissue-wide patterns without the need to invoke diffusion-mediated signalling.

Impaired angiogenesis and altered Notch signaling in mice overexpressing endothelial Egfl7

Epidermal growth factor-like domain 7 (Egfl7) is important for regulating tubulogenesis in zebrafish, but its role in mammals remains unresolved. We show here that endothelial overexpression of Egfl7 in transgenic mice leads to partial lethality, hemorrhaging, and altered cardiac morphogenesis. These defects are accompanied by abnormal vascular patterning and remodeling in both the embryonic and postnatal vasculature. Egfl7 overexpression in the neonatal retina results in a hyperangiogenic response, and EGFL7 knockdown in human primary endothelial cells suppresses endothelial cell proliferation, sprouting, and migration. These phenotypes are reminiscent of Notch inhibition. In addition, our results show that EGFL7 and endothelial-specific NOTCH physically interact in vivo and strongly suggest that Egfl7 antagonizes Notch in both the postnatal retina and in primary endothelial cells. Specifically, Egfl7 inhibits Notch reporter activity and down-regulates the level of Notch target genes when overexpressed. In conclusion, we have uncovered a critical role for Egfl7 in vascular development and have shown that some of these functions are mediated through modulation of Notch signaling.

Dynamic filopodia transmit intermittent Delta-Notch signaling to drive pattern refinement during lateral inhibition

The organization of bristles on the Drosophila notum has long served as a popular model of robust tissue patterning. During this process, membrane-tethered Delta activates intracellular Notch signaling in neighboring epithelial cells, which inhibits Delta expression. This induces lateral inhibition, yielding a pattern in which each Delta-expressing mechanosensory organ precursor cell in the epithelium is surrounded on all sides by cells with active Notch signaling. Here, we show that conventional models of Delta-Notch signaling cannot account for bristle spacing or the gradual refinement of this pattern. Instead, the pattern refinement we observe using live imaging is dependent upon dynamic, basal actin-based filopodia and can be quantitatively reproduced by simulations of lateral inhibition incorporating Delta-Notch signaling by transient filopodial contacts between nonneighboring cells. Significantly, the intermittent signaling induced by these filopodial dynamics generates a type of structured noise that is uniquely suited to the generation of well-ordered, tissue-wide epithelial patterns.

Inhibition of proteolysis of Delta-like-1 does not promote or reduce T-cell developmental potential

Notch signaling is critical for T-cell generation in the thymus. Notch signaling is linear in nature and is highly regulated through differential gene expression and post-translational modification. Upon ligand binding, the Notch receptor is sequentially cleaved, first via extracellular ADAM protease-mediated cleavage, followed by an intracellular presenilin-dependent cleavage to release the Notch intracellular domain and activate transcription. Delta-like-1 (Dll1) is a Notch ligand that positively regulates T-cell development. Dll1 is proteolytically processed in a similar manner to the Notch receptor, and it has been speculated to participate in bidirectional signaling. We hypothesized that inhibition of Dll1 processing in Notch signal sending cells would lead to changes in their ability to support thymopoiesis. We used the OP9 in vitro co-culture system, and transduced OP9s with full length, cleavable Dll1 or a non-cleavable mutant (NC-Dll1) lacking the ADAM protease cleavage site. OP9-NC-Dll1 cells were able to support T-cell development with similar efficacy to OP9-Dll1 cells. Interestingly, expression of the Notch target gene Hes5 was more highly induced in T-cell progenitors by NC-Dll1, whereas expression of Hes1, Deltex1, and pre-Tα were similar to controls. Furthermore, a reduced ability of hematopoietic progenitors to assume the granulocyte cell fate in OP9-NC-Dll1 cultures was noted. Taken together, these findings show that proteolytic cleavage of Dll1 in Notch signal sending cells is dispensable for murine T-cell development, differentially affects expression of Notch target genes, and might be a mechanism that regulates myelopoiesis.

The H3K27me3 demethylase dUTX is a suppressor of Notch- and Rb-dependent tumors in Drosophila

Trimethylated lysine 27 of histone H3 (H3K27me3) is an epigenetic mark for gene silencing and can be demethylated by the JmjC domain of UTX. Excessive H3K27me3 levels can cause tumorigenesis, but little is known about the mechanisms leading to those cancers. Mutants of the Drosophila H3K27me3 demethylase dUTX display some characteristics of Trithorax group mutants and have increased H3K27me3 levels in vivo. Surprisingly, dUTX mutations also affect H3K4me1 levels in a JmjC-independent manner. We show that a disruption of the JmjC domain of dUTX results in a growth advantage for mutant cells over adjacent wild-type tissue due to increased proliferation. The growth advantage of dUTX mutant tissue is caused, at least in part, by increased Notch activity, demonstrating that dUTX is a Notch antagonist. Furthermore, the inactivation of Retinoblastoma (Rbf in Drosophila) contributes to the growth advantage of dUTX mutant tissue. The excessive activation of Notch in dUTX mutant cells leads to tumor-like growth in an Rbf-dependent manner. In summary, these data suggest that dUTX is a suppressor of Notch- and Rbf-dependent tumors in Drosophila melanogaster and may provide a model for UTX-dependent tumorigenesis in humans.

Notch signaling maintains proliferation and survival of the HL60 human promyelocytic leukemia cell line and promotes the phosphorylation of the Rb protein

The Notch signaling pathway has been implicated in the development of several leukemia and lymphoma. In order to investigate the relationship between Notch signaling and acute myeloid leukemia (AML), in this study, we expressed a recombinant Notch ligand protein, the DSL domain of the human Jagged1 fused with GST (GST-Jag1). GST-Jag1 could activate Notch signaling in the human promyelocytic leukemia cell line HL60, as shown by a reporter assay and the induced expression of Notch effector gene Hes1 and Hes5. However, GST-Jag1 had no effect on the proliferation and survival of HL60 cells. HL60 cells expressed both Notch ligands and receptors, and had a potential of reciprocal stimulation of Notch signaling between cells. We, therefore, blocked Notch signaling in cultured HL60 cells using a gamma-secretase inhibitor (GSI). We found that GSI inhibited the proliferation of HL60 cells significantly by blocking the cell-cycle progression in the G1 phase. Furthermore, GSI induced remarkably apoptosis of HL60 cells. These changes in GSI-treated HL60 cells correlated with the down-regulation of c-Myc and Bcl2, and the low phosphorylation of the Rb protein. These results suggested that reciprocal Notch signaling might be necessary for the proliferation and survival of AML cells, possibly through the maintenance of the expression of c-Myc and Bcl2, as well as the phosphorylation of the Rb protein.

Canonical and non-canonical Notch ligands

Notch signaling induced by canonical Notch ligands is critical for normal embryonic development and tissue homeostasis through the regulation of a variety of cell fate decisions and cellular processes. Activation of Notch signaling is normally tightly controlled by direct interactions with ligand-expressing cells, and dysregulated Notch signaling is associated with developmental abnormalities and cancer. While canonical Notch ligands are responsible for the majority of Notch signaling, a diverse group of structurally unrelated noncanonical ligands has also been identified that activate Notch and likely contribute to the pleiotropic effects of Notch signaling. Soluble forms of both canonical and noncanonical ligands have been isolated, some of which block Notch signaling and could serve as natural inhibitors of this pathway. Ligand activity can also be indirectly regulated by other signaling pathways at the level of ligand expression, serving to spatiotemporally compartmentalize Notch signaling activity and integrate Notch signaling into a molecular network that orchestrates developmental events. Here, we review the molecular mechanisms underlying the dual role of Notch ligands as activators and inhibitors of Notch signaling. Additionally, evidence that Notch ligands function independent of Notch is presented. We also discuss how ligand posttranslational modification, endocytosis, proteolysis, and spatiotemporal expression regulate their signaling activity.

Mechanistic insights into Notch receptor signaling from structural and biochemical studies

Notch proteins are the receptors in a highly conserved signal transduction system used to communicate signals between cells that contact each other. Studies investigating structure-function relationships in Notch signaling have gained substantial momentum in recent years. Here, we summarize the current understanding of the molecular logic of Notch signal transduction, emphasizing structural and biochemical studies of Notch receptors, their ligands, and complexes of intracellular Notch proteins with their target transcription factors. Recent advances in the structure-based modulation of Notch-signaling activity are also discussed.

Endocytosis and intracellular trafficking of Notch and its ligands

Notch signaling occurs through direct interaction between Notch, the receptor, and its ligands, presented on the surface of neighboring cells. Endocytosis has been shown to be essential for Notch signal activation in both signal-sending and signal-receiving cells, and numerous genes involved in vesicle trafficking have recently been shown to act as key regulators of the pathway. Defects in vesicle trafficking can lead to gain- or loss-of-function defects in a context-dependent manner. Here, we discuss how endocytosis and vesicle trafficking regulate Notch signaling in both signal-sending and signal-receiving cells. We will introduce the key players in different trafficking steps, and further illustrate how they impact the signal outcome. Some of these players act as general factors and modulate Notch signaling in all contexts, whereas others modulate signaling in a context-specific fashion. We also discuss Notch signaling during mechanosensory organ development in the fly to exemplify how endocytosis and vesicle trafficking are effectively used to determine correct cell fates. In summary, endocytosis plays an essential role in Notch signaling, whereas intracellular vesicle trafficking often plays a context-dependent or regulatory role, leading to divergent outcomes in different developmental contexts.

Origin and evolution of the Notch signalling pathway: an overview from eukaryotic genomes

Background

Of the 20 or so signal transduction pathways that orchestrate cell-cell interactions in metazoans, seven are involved during development. One of these is the Notch signalling pathway which regulates cellular identity, proliferation, differentiation and apoptosis via the developmental processes of lateral inhibition and boundary induction. In light of this essential role played in metazoan development, we surveyed a wide range of eukaryotic genomes to determine the origin and evolution of the components and auxiliary factors that compose and modulate this pathway.

Results

We searched for 22 components of the Notch pathway in 35 different species that represent 8 major clades of eukaryotes, performed phylogenetic analyses and compared the domain compositions of the two fundamental molecules: the receptor Notch and its ligands Delta/Jagged. We confirm that a Notch pathway, with true receptors and ligands is specific to the Metazoa. This study also sheds light on the deep ancestry of a number of genes involved in this pathway, while other members are revealed to have a more recent origin. The origin of several components can be accounted for by the shuffling of pre-existing protein domains, or via lateral gene transfer. In addition, certain domains have appeared de novo more recently, and can be considered metazoan synapomorphies.

Conclusion

The Notch signalling pathway emerged in Metazoa via a diversity of molecular mechanisms, incorporating both novel and ancient protein domains during eukaryote evolution. Thus, a functional Notch signalling pathway was probably present in Urmetazoa.

Study and simulation of reaction-diffusion systems affected by interacting signaling pathways

Possible effects of interaction (cross-talk) between signaling pathways is studied in a system of Reaction-Diffusion (RD) equations. Furthermore, the relevance of spontaneous neurite symmetry breaking and Turing instability has been examined through numerical simulations. The interaction between Retinoic Acid (RA) and Notch signaling pathways is considered as a perturbation to RD system of axon-forming potential for N2a neuroblastoma cells. The present work suggests that large increases to the level of RA-Notch interaction can possibly have substantial impacts on neurite outgrowth and on the process of axon formation. This can be observed by the numerical study of the homogeneous system showing that in the absence of RA-Notch interaction the unperturbed homogeneous system may exhibit different saddle-node bifurcations that are robust under small perturbations by low levels of RA-Notch interactions, while large increases in the level of RA-Notch interaction result in a number of transitions of saddle-node bifurcations into Hopf bifurcations. It is speculated that near a Hopf bifurcation, the regulations between the positive and negative feedbacks change in such a way that spontaneous symmetry breaking takes place only when transport of activated Notch protein takes place at a faster rate.

The Drosophila STIM1 orthologue, dSTIM, has roles in cell fate specification and tissue patterning

Background

Mammalian STIM1 and STIM2 and the single Drosophila homologue dSTIM have been identified as key regulators of store-operated Ca²⁺ entry in cells. STIM proteins function both as molecular sensors of Ca²⁺concentration in the endoplasmic reticulum (ER) and the molecular triggers that activate SOC channels in the plasma membrane. Ca²⁺ is a crucial intracellular messenger utilised in many cellular processes, and regulators of Ca²⁺ homeostasis in the ER and cytosol are likely to play important roles in developmental processes. STIM protein expression is altered in several tumour types but the role of these proteins in developmental signalling pathways has not been thoroughly examined.

Results

We have investigated the expression and developmental function of dSTIM in Drosophila and shown that dSTIM is widely expressed in embryonic and larval tissues. Using the UAS-Gal4 induction system, we have expressed full-length dSTIM protein and a dsRNAi construct in different tissues. We demonstrate an essential role for dSTIM in larval development and survival, and a tissue-specific role in specification of mechanosensory bristles in the notum and specification of wing vein thickness.

Conclusion

Our studies show that dSTIM regulates growth and patterning of imaginal discs and indicate potential interactions with the Notch and Wingless signaling pathways. These interactions may be relevant to studies implicating STIM family proteins in tumorigenesis.

Notch and BCR signaling synergistically promote the proliferation of Raji B-lymphoma cells

The evolutionarily conserved Notch signaling pathway plays a pivotal role in cell proliferation, apoptosis, and cell fate decision from invertebrates to vertebrates, and is oncogenic in some human hematopoietic malignancies. To study the role of Notch signaling in B-lymphoma, we expressed a soluble fragment of human Delta-like1 (hDll1) in E. coli, which was shown to activate the Notch signaling. Incubation of Burkitt's lymphoma Raji cells with the soluble hDll1 led to gamma-secretase-dependent up-regulation of a Notch downstream gene, Hes1. This treatment synergized with B-cell receptor (BCR)-mediated signaling to promote proliferation of Raji cells in vitro, which was cancelled by GSI. We further showed that Notch signaling significantly repressed, while gamma-secretase inhibitor (GSI) enhanced, "natural" apoptosis of Raji cells. Because c-myc is a downstream gene of both Notch signaling and BCR signaling, and GSI blocked c-myc expression in the presence of hDll1 and anti-IgM, Notch signaling might interact with BCR signaling at the level of c-myc expression to regulate proliferation and apoptosis of B-lymphoma cells.

The many facets of Notch ligands

The Notch signaling pathway regulates a diverse array of cell types and cellular processes and is tightly regulated by ligand binding. Both canonical and noncanonical Notch ligands have been identified that may account for some of the pleiotropic nature associated with Notch signaling. This review focuses on the molecular mechanisms by which Notch ligands function as signaling agonists and antagonists, and discusses different modes of activating ligands as well as findings that support intrinsic ligand signaling activity independent of Notch. Post-translational modification, proteolytic processing, endocytosis and membrane trafficking, as well as interactions with the actin cytoskeleton may contribute to the recently appreciated multifunctionality of Notch ligands. The regulation of Notch ligand expression by other signaling pathways provides a mechanism to coordinate Notch signaling with multiple cellular and developmental cues. The association of Notch ligands with inherited human disorders and cancer highlights the importance of understanding the molecular nature and activities intrinsic to Notch ligands. Oncogene (2008) 27, 5148-5167; doi:10.1038/onc.2008.229.

The atypical mammalian ligand Delta-like homologue 1 (Dlk1) can regulate Notch signalling in Drosophila

BACKGROUND

Mammalian Delta-like 1 (Dlk-1) protein shares homology with Notch ligands but lacks a critical receptor-binding domain. Thus it is unclear whether it is able to interact with Notch in vivo. Unlike mammals, Drosophila have a single Notch receptor allowing a simple in vivo assay for mammalian Dlk1 function.

RESULTS

Here we show that membrane-bound DLK1 can regulate Notch leading to altered cellular distribution of Notch itself and inhibiting expression of Notch target genes. The resulting adult phenotypes are indicative of reduced Notch function and are enhanced by Notch mutations, confirming that DLK1 action is antagonistic. In addition, cells expressing an alternative Dlk1 isoform exhibit alterations in cell size, functions previously not attributed to Notch suggesting that DLK1 might also act via an alternative target.

CONCLUSION

Our results demonstrate that DLK1 can regulate the Notch receptor despite its atypical structure.

Delta-like 4 Notch ligand regulates tumor angiogenesis, improves tumor vascular function, and promotes tumor growth in vivo

The vascular endothelial growth factor (VEGF) plays a key role in tumor angiogenesis. However, clinical trials targeting the VEGF pathway are often ineffective, suggesting that other factors/pathways are also important in tumor angiogenesis. We have previously shown that the Notch ligand Delta-like 4 (DLL4) is up-regulated in tumor vasculature. Here, we show that DLL4, when expressed in tumor cells, functions as a negative regulator of tumor angiogenesis by reducing the number of blood vessels in all five types of xenografts, but acts as a positive driver for tumor growth in two of them (human glioblastoma and prostate cancer). The growth of in vivo models was not related to the effects on growth in vitro. DLL4 expressed in the tumor cells activated Notch signaling in host stromal/endothelial cells, increased blood vessel size, and improved vascular function within tumors. The promotion of tumor growth was, to some extent, due to a reduction of tumor hypoxia and apoptosis. DLL4-expressing tumor cells responded to anti-VEGF therapy with bevacizumab. A soluble form of DLL4 (D4ECD-Fc) blocked tumor growth in both bevacizumab-sensitive and bevacizumab-resistant tumors by disrupting vascular function despite increased tumor vessel density. In addition, we show that DLL4 is up-regulated in tumor cells and tumor endothelial cells of human glioblastoma. Our findings provide a rational basis for the development of novel antiangiogenic strategies via blockade of DLL4/Notch signaling and suggest that combined approaches for interrupting both DLL4 and VEGF pathways may improve antiangiogenic therapy.

Modulation of Notch signaling by antibodies specific for the extracellular negative regulatory region of NOTCH3

The Notch pathway regulates the development of many tissues and cell types and is involved in a variety of human diseases, making it an attractive potential therapeutic target. This promise has been limited by the absence of potent inhibitors or agonists that are specific for individual human Notch receptors (NOTCH1-4). Using an unbiased functional screening, we identified monoclonal antibodies that specifically inhibit or induce activating proteolytic cleavages in NOTCH3. Remarkably, the most potent inhibitory and activating antibodies bind to overlapping epitopes within a juxtamembrane negative regulatory region that protects NOTCH3 from proteolysis and activation in its resting autoinhibited state. The inhibitory antibodies revert phenotypes conveyed on 293T cells by NOTCH3 signaling, such as increased cellular proliferation, survival, and motility, whereas the activating antibody mimics some of the effects of ligand-induced Notch activation. These findings provide insights into the mechanisms of Notch autoinhibition and activation and pave the way for the further development of specific antibody-based modulators of the Notch receptors, which are likely to be of utility in a wide range of experimental and therapeutic settings.

Distinct effects of the soluble versus membrane-bound forms of the notch ligand delta-4 on human CD34+CD38low cell expansion and differentiation

Although Notch ligands are considered to activate signaling through direct cell-cell contact, the existence of soluble forms has been demonstrated. However, their roles remain controversial: soluble forms have been reported to mimic the biological activity of membrane-bound form, whereas other studies rather suggested an antagonistic activity toward their full-length counterparts. We previously observed that membrane-bound Delta4-expressing S17 stroma (mbD4/S17) reduced human CD34+CD38(low) cell proliferation and favored self-renewal. Here, we assessed the effects of a soluble form of Delta4 (solD4) by exposing CD34+CD38(low) cells to S17 feeders engineered to express solD4 (solD4/S17). In contrast to mbD4/S17, (a) solD4/S17 increased 10-fold cell production after 2 weeks, through enhanced cell proliferation, and (b) it did not preserve colony-forming cell and long-term culture-initiating cell potential of output CD34+ cells. mbD4 and solD4 appeared to also differ in their signaling. Indeed, mbD4, but not solD4, strongly activated both CSL (the nuclear mediator of Notch signaling) in Hela cells overexpressing Notch1 and transcription of some classic Notch target genes in CD34+CD38(low) cells. Furthermore, both biological effects and CSL activation elicited by mbD4 were strictly dependent upon the gamma-secretase complex, whereas solD4 enhanced cell expansion in a partially gamma-secretase-independent manner. Altogether, these results suggest that part of solD4 activity did not rely upon canonical Notch pathway.

An obligatory role of mind bomb-1 in notch signaling of mammalian development

BACKGROUND

The Notch signaling pathway is an evolutionarily conserved intercellular signaling module essential for cell fate specification that requires endocytosis of Notch ligands. Structurally distinct E3 ubiquitin ligases, Neuralized (Neur) and Mind bomb (Mib), cooperatively regulate the endocytosis of Notch ligands in Drosophila. However, the respective roles of the mammalian E3 ubiquitin ligases, Neur1, Neur2, Mib1, and Mib2, in mammalian development are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

Through extensive use of mammalian genetics, here we show that Neur1 and Neur2 double mutants and Mib2(-/-) mice were viable and grossly normal. In contrast, conditional inactivation of Mib1 in various tissues revealed the representative Notch phenotypes: defects of arterial specification as deltalike4 mutants, abnormal cerebellum and skin development as jagged1 conditional mutants, and syndactylism as jagged2 mutants.

CONCLUSIONS/SIGNIFICANCE

Our data provide the first evidence that Mib1 is essential for Jagged as well as Deltalike ligand-mediated Notch signaling in mammalian development, while Neur1, Neur2, and Mib2 are dispensable.

Lola regulates cell fate by antagonizing Notch induction in the Drosophila eye

Lola is a transcription repressor that regulates axon guidance in the developing embryonic nervous system of Drosophila. Here, we show that Lola regulates two binary cell fate decisions guided by Notch inductive signaling in the developing eye: the R3-R4 and the R7-cone cell fate choices. Lola is required cell-autonomously in R3 for its specification, and Lola transforms R4 into R3 if overexpressed. Lola also promotes R7 fate at the expense of cone cell fate. Lola antagonizes Notch-dependent gene expression and Notch-dependent fate transformation. Expression analysis shows that Lola is constitutively present in all photoreceptors and cone cells. We propose that when a precursor cell receives a weak Notch inductive signal, it is not sufficiently strong to overcome the constitutive repression of target gene transcription by Lola. A precursor that receives a strong Notch inductive signal expresses target genes despite a constitutive repression by Lola. The predicted consequence of this mechanism is to sharpen a cell's responsiveness to Notch signaling by creating a threshold.

Structural basis for autoinhibition of Notch

Notch receptors transmit signals between adjacent cells. Signaling is initiated when ligand binding induces metalloprotease cleavage of Notch within an extracellular negative regulatory region (NRR). We present here the X-ray structure of the human NOTCH2 NRR, which adopts an autoinhibited conformation. Extensive interdomain interactions within the NRR bury the metalloprotease site, showing that a substantial conformational movement is necessary to expose this site during activation by ligand. Leukemia-associated mutations in NOTCH1 probably release autoinhibition by destabilizing the conserved hydrophobic core of the NRR.

Overexpression of Notch1 ectodomain in myeloid cells induces vascular malformations through a paracrine pathway

We previously reported that truncation of Notch1 (N1) by provirus insertion leads to overexpression of both the intracellular (N1(IC)) and the extracellular (N1(EC)) domains. We produced transgenic (Tg) mice expressing N1(EC) in T cells and in cells of the myeloid lineage under the regulation of the CD4 gene. These CD4C/N1(EC) Tg mice developed vascular disease, predominantly in the liver: superficial distorted vessels, cavernae, lower branching of parenchymal vessels, capillarized sinusoids, and aberrant smooth muscle/endothelial cell topography. The disease developed in lethally irradiated normal mice transplanted with Tg bone marrow or fetal liver cells as well as in Rag-/- Tg mice. In nude mice transplanted with fetal liver cells from (ROSA26 x CD4C/N1(EC)) F1 Tg mice, abnormal vessels were of recipient origin. Transplantation of Tg peritoneal macrophages into normal recipients also induced abnormal vessels. These Tg macrophages showed impaired functions, and their conditioned medium inhibited the proliferation of liver sinusoid endothelial cells in vitro. The Egr-1 gene and some of its targets (Jag1, FIII, FXIII-A, MCP-1, and MCP-5), previously implicated in hemangioma or vascular malformations, were overexpressed in Tg macrophages. These results show that myeloid cells can be reprogrammed by N1(EC) to induce vascular malformations through a paracrine pathway.

Jedi—a novel transmembrane protein expressed in early hematopoietic cells

Self-renewal and differentiation of hematopoietic stem and progenitor cells are defined by the ensembles of genes expressed by these cells. Here we report identification of a novel gene named Jedi, which is expressed predominantly in short- and long-term repopulating stem cells when compared to more mature bone marrow progenitors. Jedi mRNA encodes a transmembrane protein that contains multiple EGF-like repeats. Jedi and two earlier reported proteins, MEGF10 and MEGF11, share a substantial homology and are likely to represent a novel protein family. Studies of the potential role of Jedi in hematopoietic regulation demonstrated that the retrovirally mediated expression of Jedi in bone marrow cells decreased the number of myeloid progenitors in in vitro clonogenic assays. In addition, expression of Jedi in NIH 3T3 fibroblasts resulted in a decreased number of late and early myeloid progenitors in the non-adherent co-cultured bone marrow cells. Jedi shares a number of structural features with the Jagged/Serrate/Delta family of Notch ligands, and our experiments indicate that the extracellular domain of Jedi, similar to the corresponding domain of Jagged1, inhibits Notch signaling. On the basis of obtained results, we suggest that Jedi is involved in the fine regulation of the early stages of hematopoietic differentiation, presumably through the Notch signaling pathway.