Delta proteins and MAGI proteins: an interaction of Notch ligands with intracellular scaffolding molecules and its significance for zebrafish development

Reversible and bidirectional signaling of notch ligands

The Notch signaling pathway is a direct cell-cell communication system involved in a wide variety of biological processes, and its disruption is observed in several pathologies. The pathway is comprised of a ligand-expressing (sender) cell and a receptor-expressing (receiver) cell. The canonical ligands are members of the Delta/Serrate/Lag-1 (DSL) family of proteins. Their binding to a Notch receptor in a neighboring cell induces a conformational change in the receptor, which will undergo regulated intramembrane proteolysis (RIP), liberating the Notch intracellular domain (NICD). The NICD is translocated to the nucleus and promotes gene transcription. It has been demonstrated that the ligands can also undergo RIP and nuclear translocation, suggesting a function for the ligands in the sender cell and possible bidirectionality of the Notch pathway. Although the complete mechanism of ligand processing is not entirely understood, and its dependence on Notch receptors has not been ruled out. Also, ligands have autonomous functions beyond Notch activation. Here we review the concepts of reverse and bidirectional signalization of DSL proteins and discuss the characteristics that make them more than just ligands of the Notch pathway.

Cleaved Delta like 1 intracellular domain regulates neural development via Notch signal-dependent and -independent pathways

Notch-Delta signaling regulates many developmental processes, including tissue homeostasis and maintenance of stem cells. Upon interaction of juxtaposed cells via Notch and Delta proteins, intracellular domains of both transmembrane proteins are cleaved and translocate to the nucleus. Notch intracellular domain activates target gene expression; however, the role of the Delta intracellular domain remains elusive. Here, we show the biological function of Delta like 1 intracellular domain (D1ICD) by modulating its production. We find that the sustained production of D1ICD abrogates cell proliferation but enhances neurogenesis in the developing dorsal root ganglia (DRG), whereas inhibition of D1ICD production promotes cell proliferation and gliogenesis. D1ICD acts as an integral component of lateral inhibition mechanism by inhibiting Notch activity. In addition, D1ICD promotes neurogenesis in a Notch signaling-independent manner. We show that D1ICD binds to Erk1/2 in neural crest stem cells and inhibits the phosphorylation of Erk1/2. In summary, our results indicate that D1ICD regulates DRG development by modulating not only Notch signaling but also the MAP kinase pathway.

A New Story of the Three Magi: Scaffolding Proteins and lncRNA Suppressors of Cancer

Scaffolding molecules exert a critical role in orchestrating cellular response through the spatiotemporal assembly of effector proteins as signalosomes. By increasing the efficiency and selectivity of intracellular signaling, these molecules can exert (anti/pro)oncogenic activities. As an archetype of scaffolding proteins with tumor suppressor property, the present review focuses on MAGI1, 2, and 3 (membrane-associated guanylate kinase inverted), a subgroup of the MAGUK protein family, that mediate networks involving receptors, junctional complexes, signaling molecules, and the cytoskeleton. MAGI1, 2, and 3 are comprised of 6 PDZ domains, 2 WW domains, and 1 GUK domain. These 9 protein binding modules allow selective interactions with a wide range of effectors, including the PTEN tumor suppressor, the β-catenin and YAP1 proto-oncogenes, and the regulation of the PI3K/AKT, the Wnt, and the Hippo signaling pathways. The frequent downmodulation of MAGIs in various human malignancies makes these scaffolding molecules and their ligands putative therapeutic targets. Interestingly, MAGI1 and MAGI2 genetic loci generate a series of long non-coding RNAs that act as a tumor promoter or suppressor in a tissue-dependent manner, by selectively sponging some miRNAs or by regulating epigenetic processes. Here, we discuss the different paths followed by the three MAGIs to control carcinogenesis.

de novo transcriptome profile of two earthworms Lampito mauritii and Drawida calebi during regeneration

Earthworms have remarkable ability to regenerate its tail and head region. However the list of genes expressed in this regeneration process has been less explored baring a few species. The current study involves the de novo transcriptome sequencing of intact tail and regenerating tail (15 day post amputation) of earthworms belonging to two different genera Lampito mauritii (Kinberg, 1867) and Drawida calebi (Gates, 1945). This study contains one de-novo and one reference based transcriptome analysis each from one genus of two earthworm genera. From a total of 119.92 million (150 × 2) reads, 112.95 million high quality adapter free reads were utilized in analysis. Assembly of high-quality reads was performed separately for Lampito mauritii (LM sample) and Drawida calebi (DC sample) that resulted in 66368 and 1,61,289 transcripts respectively. About 25.21% of transcripts were functionally annotated for DC sample and 38.27% for LM samples against Annelida sequences. A total of 239 genes were expressed exclusively in regenerated tissue compared to intact sample in DC whereas about 241 genes were exclusively expressed in regenerated tissue of LM compared to its intact sample. Majority of genes in Drawida and Lampito were dedicated to immune response, maintenance of cytoskeleton, resisting oxidative stress and promoting neuronal regeneration for cell-cell communication during tail regeneration.

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Upregulation of genes such as beta catenin, Sox, notch, FGF, frizzled.

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Similarity with annelid worm Capitella telesta.

DLL1- and DLL4-Mediated Notch Signaling Is Essential for Adult Pancreatic Islet Homeostasis

Genes of the Notch signaling pathway are expressed in different cell types and organs at different time points during embryonic development and adulthood. The Notch ligand Delta-like 1 (DLL1) controls the decision between endocrine and exocrine fates of multipotent progenitors in the developing pancreas, and loss of Dll1 leads to premature endocrine differentiation. However, the role of Delta-Notch signaling in adult tissue homeostasis is not well understood. Here, we describe the spatial expression pattern of Notch pathway components in adult murine pancreatic islets and show that DLL1 and DLL4 are specifically expressed in β-cells, whereas JAGGED1 is expressed in α-cells. We show that mice lacking both DLL1 and DLL4 in adult β-cells display improved glucose tolerance, increased glucose-stimulated insulin secretion, and hyperglucagonemia. In contrast, overexpression of the intracellular domain of DLL1 in adult murine pancreatic β-cells results in impaired glucose tolerance and reduced insulin secretion, both in vitro and in vivo. These results suggest that Notch ligands play specific roles in the adult pancreas and highlight a novel function of the Delta/Notch pathway in β-cell insulin secretion.

Transmembrane protein 215 promotes angiogenesis by maintaining endothelial cell survival

Sprouting angiogenesis is a major form of neovascularization of tissues suffering from hypoxia and other related stress. Endothelial cells (ECs) undergo proliferation, differentiation, programmed death, and migration during angiogenic sprouting, but the underlying molecular mechanisms regulating ECs in angiogenesis have been incompletely elucidated. Here we report that the transmembrane protein 215 (TMEM215) is involved in angiogenesis by regulating EC survival. The murine TMEM215 gene, which possesses two transcriptional starting sites as determined by 5′‐rapid amplification of complementary DNA (cDNA) ends (RACE), encodes a two‐pass TMEM. The TMEM215 transcripts were detected in ECs in addition to other tissues by quantitative reverse transcription‐polymerase chain reaction. Immunofluorescence showed that TMEM215 was expressed in the vasculature in retina, liver, and tumor, and colocalized with EC markers. We show that knockdown of TMEM215 in ECs induced strong cell death of ECs in vitro without affecting cell proliferation and migration, suggesting that TMEM215 was required for EC survival. Downregulation of TMEM215 expression compromised lumen formation and sprouting capacities of ECs in vitro. Moreover, intravitreous injection of TMEM215 small interfering RNA resulted in delayed and abnormal development of retinal vasculature with poor perfusion. These results identified TMEM215 as a novel molecule involved in angiogenesis by regulating the survival of ECs.

MPDZ promotes DLL4-induced Notch signaling during angiogenesis

Angiogenesis is coordinated by VEGF and Notch signaling. DLL4-induced Notch signaling inhibits tip cell formation and vessel branching. To ensure proper Notch signaling, receptors and ligands are clustered at adherens junctions. However, little is known about factors that control Notch activity by influencing the cellular localization of Notch ligands. Here, we show that the multiple PDZ domain protein (MPDZ) enhances Notch signaling activity. MPDZ physically interacts with the intracellular carboxyterminus of DLL1 and DLL4 and enables their interaction with the adherens junction protein Nectin-2. Inactivation of the MPDZ gene leads to impaired Notch signaling activity and increased blood vessel sprouting in cellular models and the embryonic mouse hindbrain. Tumor angiogenesis was enhanced upon endothelial-specific inactivation of MPDZ leading to an excessively branched and poorly functional vessel network resulting in tumor hypoxia. As such, we identified MPDZ as a novel modulator of Notch signaling by controlling ligand recruitment to adherens junctions.

The AP-1 transcription factor component Fosl2 potentiates the rate of myocardial differentiation from the zebrafish second heart field

The vertebrate heart forms through successive phases of cardiomyocyte differentiation. Initially, cardiomyocytes derived from first heart field (FHF) progenitors assemble the linear heart tube. Thereafter, second heart field (SHF) progenitors differentiate into cardiomyocytes that are accreted to the poles of the heart tube over a well-defined developmental window. Although heart tube elongation deficiencies lead to life-threatening congenital heart defects, the variables controlling the initiation, rate and duration of myocardial accretion remain obscure. Here, we demonstrate that the AP-1 transcription factor, Fos-like antigen 2 (Fosl2), potentiates the rate of myocardial accretion from the zebrafish SHF. fosl2 mutants initiate accretion appropriately, but cardiomyocyte production is sluggish, resulting in a ventricular deficit coupled with an accumulation of SHF progenitors. Surprisingly, mutant embryos eventually correct the myocardial deficit by extending the accretion window. Overexpression of Fosl2 also compromises production of SHF-derived ventricular cardiomyocytes, a phenotype that is consistent with precocious depletion of the progenitor pool. Our data implicate Fosl2 in promoting the progenitor to cardiomyocyte transition and uncover the existence of regulatory mechanisms to ensure appropriate SHF-mediated cardiomyocyte contribution irrespective of embryonic stage.

Jagged1 intracellular domain-mediated inhibition of Notch1 signalling regulates cardiac homeostasis in the postnatal heart

Aims

Notch1 signalling in the heart is mainly activated via expression of Jagged1 on the surface of cardiomyocytes. Notch controls cardiomyocyte proliferation and differentiation in the developing heart and regulates cardiac remodelling in the stressed adult heart. Besides canonical Notch receptor activation in signal-receiving cells, Notch ligands can also activate Notch receptor-independent responses in signal-sending cells via release of their intracellular domain. We evaluated therefore the importance of Jagged1 (J1) intracellular domain (ICD)-mediated pathways in the postnatal heart.

Methods and results

In cardiomyocytes, Jagged1 releases J1ICD, which then translocates into the nucleus and down-regulates Notch transcriptional activity. To study the importance of J1ICD in cardiac homeostasis, we generated transgenic mice expressing a tamoxifen-inducible form of J1ICD, specifically in cardiomyocytes. Using this model, we demonstrate that J1ICD-mediated Notch inhibition diminishes proliferation in the neonatal cardiomyocyte population and promotes maturation. In the neonatal heart, a response via Wnt and Akt pathway activation is elicited as an attempt to compensate for the deficit in cardiomyocyte number resulting from J1ICD activation. In the stressed adult heart, J1ICD activation results in a dramatic reduction of the number of Notch signalling cardiomyocytes, blunts the hypertrophic response, and reduces the number of apoptotic cardiomyocytes. Consistently, this occurs concomitantly with a significant down-regulation of the phosphorylation of the Akt effectors ribosomal S6 protein (S6) and eukaryotic initiation factor 4E binding protein1 (4EBP1) controlling protein synthesis.

Conclusions

Altogether, these data demonstrate the importance of J1ICD in the modulation of physiological and pathological hypertrophy, and reveal the existence of a novel pathway regulating cardiac homeostasis.

Studying apoptosis in the Zebrafish

Zebrafish (Danio rerio) have been extensively used to study apoptotic cell death during normal development and under a wide range of experimental manipulations. A number of features make zebrafish a particularly powerful model organism: (1) embryos are small in size, develop rapidly outside the mother, and are optically transparent; (2) tools are readily available for rapid knockdown and overexpression of genes; and (3) embryos can be arrayed into multiwell plates and are permeable to a wide range of drugs and small molecules. The molecular machinery underlying the intrinsic and extrinsic apoptosis pathways appears to be highly conserved between zebrafish and mammals. In this chapter, techniques are described for detecting apoptotic cells in situ in both fixed and live zebrafish embryos. Methods for inducing and inhibiting apoptosis and for functionally manipulating genes involved in apoptotic signaling are also discussed.

Tissue factor pathway inhibitor-2: a novel gene involved in zebrafish central nervous system development

Tissue factor pathway inhibitor-2 (Tfpi-2) is an important serine protease inhibitor in the extracellular matrix (ECM), but its precise physiological significance remains unknown. This work is part of a series of studies intended to investigate functional roles of Tfpi-2 and explore the underlying molecular mechanisms. First, we cloned and identified zebrafish Tfpi-2 (zTfpi-2) as an evolutionarily conserved protein essential for zebrafish development. We also demonstrated that ztfpi-2 is mainly expressed in the central nervous system (CNS) of zebrafish, and embryonic depletion of ztfpi-2 caused severe CNS defects. In addition, changes of neural markers, including pax2a, egr2b, huC, ngn1, gfap and olig2, confirmed the presence of developmental abnormalities in the relevant regions of ztfpi-2 morphants. Using microarray analysis, we found that members of the Notch pathway, especially her4 and mib, which mediate lateral inhibition in CNS development, were also downregulated. Intriguingly, both her4 and mib were able to partially rescue the ztfpi-2 morphant phenotype. Furthermore, Morpholino knockdown of ztfpi-2 resulted in upregulation of neuronal markers while downregulation of glial markers, providing evidence that the Notch pathway is probably involved in ztfpi-2-mediated CNS development.

No evidence for a functional role of bi-directional Notch signaling during angiogenesis

The Delta-Notch pathway is a signal exchanger between adjacent cells to regulate numerous differentiation steps during embryonic development. Blood vessel formation by sprouting angiogenesis requires high expression of the Notch ligand DLL4 in the leading tip cell, while Notch receptors in the trailing stalk cells are activated by DLL4 to achieve strong Notch signaling activity. Upon ligand binding, Notch receptors are cleaved by ADAM proteases and gamma-secretase. This releases the intracellular Notch domain that acts as a transcription factor. There is evidence that also Notch ligands (DLL1, DLL4, JAG1, JAG2) are processed upon receptor binding to influence transcription in the ligand-expressing cell. Thus, the existence of bi-directional Delta-Notch signaling has been proposed. We report here that the Notch ligands DLL1 and JAG1 are processed in endothelial cells in a gamma-secretase-dependent manner and that the intracellular ligand domains accumulate in the cell nucleus. Overexpression of JAG1 intracellular domain (ICD) as well as DLL1-ICD, DLL4-ICD and NOTCH1-ICD inhibited endothelial proliferation. Whereas NOTCH1-ICD strongly repressed endothelial migration and sprouting angiogenesis, JAG1-ICD, DLL1-ICD and DLL4-ICD had no significant effects. Consistently, global gene expression patterns were only marginally affected by the processed Notch ligands. In addition to its effects as a transcription factor, NOTCH1-ICD promotes cell adhesion to the extracellular matrix in a transcription-independent manner. However, JAG1-ICD, DLL1-ICD and DLL4-ICD did not influence endothelial cell adhesion. In summary, reverse signaling of Notch ligands appears to be dispensable for angiogenesis in cellular systems.

The expression of Delta ligands in the sponge Amphimedon queenslandica suggests an ancient role for Notch signaling in metazoan development

Background

Intercellular signaling via the Notch pathway regulates cell fate, patterning, differentiation and proliferation, and is essential for the proper development of bilaterians and cnidarians. To investigate the origins of the Notch pathway, we are studying its deployment in a representative of an early branching lineage, the poriferan Amphimedon queenslandica. The A. queenslandica genome encodes a single Notch receptor and five membrane-bound Delta ligands, as well as orthologs of many genes that enact and regulate canonical Notch signaling events in other animals.

Methods

In the present report we analyze the structure of the five A. queenslandica Deltas using bioinformatic methods, and characterize their developmental expression via whole mount in situ hybridization and histological staining.

Results

Sequence analysis of the A. queenslandica Delta ligands highlights the conservation of their extracellular domains. This contrasts with the divergence of their intracellular regions, each of which is predicted to bear a unique repertoire of protein interaction motifs. In keeping with this diversity, these ligands are expressed differentially and dynamically throughout A. queenslandica embryogenesis, both in cell type specific patterns and broader regional domains. Notably, this expression coincides with the development of the photosensitive larval pigment ring, the non-ciliated cuboidal cells located at the anterior pole of the larva, and the intraepithelial flask cells and globular cells that are presumed to have sensory and/or secretory roles.

Conclusions

Based on the dynamic and complex patterns of expression of these Delta ligands and the Notch receptor, we propose that the Notch signaling pathway is involved in regulating the development of diverse cell types in A. queenslandica. From these observations we infer that Notch signaling is a conserved feature of metazoan development, ancestrally contributing to cell determination, patterning and differentiation processes.

Pharmacogenomic study of side-effects for antidepressant treatment options in STAR*D

BACKGROUND

Understanding individual differences in susceptibility to antidepressant therapy side-effects is essential to optimize the treatment of depression.

METHOD

We performed genome-wide association studies (GWAS) to search for genetic variation affecting the susceptibility to side-effects. The analysis sample consisted of 1439 depression patients, successfully genotyped for 421K single nucleotide polymorphisms (SNPs), from the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study. Outcomes included four indicators of side-effects: general side-effect burden, sexual side-effects, dizziness and vision/hearing-related side-effects. Our criterion for genome-wide significance was a prespecified threshold ensuring that, on average, only 10% of the significant findings are false discoveries.

RESULTS

Thirty-four SNPs satisfied this criterion. The top finding indicated that 10 SNPs in SACM1L mediated the effects of bupropion on sexual side-effects (p = 4.98 × 10(-7), q = 0.023). Suggestive findings were also found for SNPs in MAGI2, DTWD1, WDFY4 and CHL1.

CONCLUSIONS

Although our findings require replication and functional validation, this study demonstrates the potential of GWAS to discover genes and pathways that could mediate adverse effects of antidepressant medication.

The conserved clusterin gene is expressed in the developing choroid plexus under the regulation of notch but not IGF signaling in zebrafish

Recent genome-wide association studies have implicated the clusterin gene in the etiology of Alzheimer's disease. The expression and function of clusterin in the developing brain, however, is poorly understood. In this study, we have characterized the zebrafish clusterin gene and determined its structural conservation, developmental expression, and physiological regulation. The structure of the zebrafish clusterin gene and protein is similar to its human orthologue. Biochemical assays show that zebrafish Clusterin is a secreted protein that cannot bind IGFs. In adult zebrafish, clusterin mRNA is detected in many tissues. In early development, clusterin mRNA becomes detectable at 12 h postfertilization, and its levels gradually increase thereafter. In situ hybridization analysis indicates that clusterin mRNA is specifically expressed in the developing diencephalic and myelencephalic choroid plexus. Among various stresses tested, heat shock, but not hypoxic or ionic stresses, increases the levels of clusterin mRNA. Inhibition of the IGF-I receptor-mediated signaling or overexpression of IGF ligands did not change clusterin mRNA levels. In comparison, inhibition or targeted knockdown of Notch signaling significantly increased clusterin mRNA expression in choroid plexus. These results suggest that clusterin is a marker of choroid plexus in zebrafish, and its expression in the developing choroid plexus is under the regulation of Notch but not IGF signaling.

Sox11 is expressed in early progenitor human multipotent stromal cells and decreases with extensive expansion of the cells

There has been considerable interest in developing new therapies with adult multipotent progenitor stromal cells or mesenchymal stem cells (MSCs) in organ replacement and repair. To be effectively seeded into scaffolds for therapy, large numbers of cells are needed, but concerns remain regarding their chromatin stability in long-term culture. We therefore expanded four donors of human MSCs (hMSCs) from bone marrow aspirates with a protocol that maintains the cells at low density. MSCs initially proliferated at average doubling times of 24  h and then gradually reached senescence after 8-15 passages (33-55 population doublings) without evidence of immortalization. Comparative genomic hybridization assays of two preparations revealed no abnormalities through 33 population doublings. One preparation had a small amplification of unknown significance in chromosome 7 (7q21:11) after 55 population doublings. Microarray assays demonstrated progressive changes in the transcriptome of the cells. However, the transcriptomes clustered more closely over time within a single passage, rather than with passage number, indicating a partial reversibility of the patterns of gene expression. One of the largest changes was a decrease in mRNA for Sox11, a transcription factor previously identified in neural progenitor cells. Knockdown of Sox11 with siRNA decreased the proliferation and osteogenic differentiation potential of hMSCs. The results suggested that assays for Sox11 may provide a biomarker for early progenitor hMSCs.

Up-regulation of the homophilic adhesion molecule sidekick-1 in podocytes contributes to glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a leading cause of nephrotic syndrome and end-stage renal disease worldwide. Although the mechanisms underlying this important disease are poorly understood, the glomerular podocyte clearly plays a central role in disease pathogenesis. In the current work, we demonstrate that the homophilic adhesion molecule sidekick-1 (sdk-1) is up-regulated in podocytes in FSGS both in rodent models and in human kidney biopsy samples. Transgenic mice that have podocyte-specific overexpression of sdk-1 develop gradually progressive heavy proteinuria and severe FSGS. We also show that sdk-1 associates with the slit diaphragm linker protein MAGI-1, which is already known to interact with several critical podocyte proteins including synaptopodin, alpha-actinin-4, nephrin, JAM4, and beta-catenin. This interaction is mediated through a direct interaction between the carboxyl terminus of sdk-1 and specific PDZ domains of MAGI-1. In vitro expression of sdk-1 enables a dramatic recruitment of MAGI-1 to the cell membrane. Furthermore, a truncated version of sdk-1 that is unable to bind to MAGI-1 does not induce podocyte dysfunction when overexpressed. We conclude that the up-regulation of sdk-1 in podocytes is an important pathogenic factor in FSGS and that the mechanism involves disruption of the actin cytoskeleton possibly via alterations in MAGI-1 function.

Delta1 family members are involved in filopodial actin formation and neuronal cell migration independent of Notch signaling

Delta family proteins are transmembrane molecules that bind Notch receptors and activate downstream signaling events in neighboring cells. In addition to serving as Notch ligands, Notch-independent roles for Delta have been suggested but are not fully understood. Here, we demonstrate a previously unrecognized role for Delta in filopodial actin formation. Delta1 and Delta4, but not Delta3, exhibit filopodial protrusive activity, and this activity is independent of Notch signaling. The filopodial activity of Delta1 does not depend on the PDZ-binding domain at the C-terminus; however, the intracellular membrane-proximal region that is anchored to the plasma membrane plays an important role in filopodial activity. We further identified a Notch-independent role of DeltaD in neuronal cell migration in zebrafish. These findings suggest a possible functional link between Notch-independent filopodial activity of Delta and the control of cell motility.

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.

Distinct biological roles for the notch ligands Jagged-1 and Jagged-2

Notch signaling is activated in a subset of non-small cell lung cancer cells because of overexpression of Notch3, but the role of Notch ligands has not been fully defined. On the basis of gene expression profiling of a panel of non-small cell lung cancer cell lines, we found that the predominant Notch ligands were JAG1, JAG2, DLL1, and DLL3. Given that Notch ligands reportedly have overlapping receptor binding specificities, we postulated that they have redundant biological roles. Arguing against this hypothesis, we found that JAG1 and JAG2 were differentially regulated; JAG1 expression was dependent upon epidermal growth factor receptor (EGFR) activation in HCC827 cells, which require EGFR for survival, whereas JAG2 expression was EGFR-independent in these cells. Furthermore, HCC827 cells underwent apoptosis following depletion of JAG1 but not JAG2, whereas co-culture experiments revealed that depletion of JAG2, but not JAG1, enhanced the ability of HCC827 cells to chemoattract THP-1 human monocytes. JAG2-depleted HCC827 cells expressed high levels of inflammation-related genes, including interleukin 1 (IL1) and a broad range of IL1-regulated cytokines, which was attenuated by inhibition of IL1 receptor (IL1R). Our findings suggest that JAG1 and JAG2 have distinct biological roles including a previously undiscovered role for JAG2 in regulating the expression of cytokines that can promote antitumor immunity.

MAGI-1, a candidate stereociliary scaffolding protein, associates with the tip-link component cadherin 23

Inner ear hair-cell mechanoelectrical transduction is mediated by a largely unidentified multiprotein complex associated with the stereociliary tips of hair bundles. One identified component of tip links, which are the extracellular filamentous connectors implicated in gating the mechanoelectrical transduction channels, is the transmembrane protein cadherin 23 (Cdh23), more specifically, the hair- cell-specific Cdh23(+68) splice variant. Using the intracellular domain of Cdh23(+68) as bait, we identified in a cochlear cDNA library MAGI-1, a MAGUK (membrane-associated guanylate kinase) protein. MAGI-1 binds via its PDZ4 domain to a C-terminal PDZ-binding site on Cdh23. MAGI-1 immunoreactivity was detectable throughout neonatal stereocilia in a distribution similar to that of Cdh23. As development proceeded, MAGI-1 occurred in a punctate staining pattern on stereocilia, which was maintained into adulthood. Previous reports suggest that Cdh23 interacts via an internal PDZ-binding site with the PDZ1 domain of the stereociliary protein harmonin, and potentially via a weaker binding of its C terminus with harmonin's PDZ2 domain. We propose that MAGI-1 has the ability to replace harmonin's PDZ2 binding at Cdh23's C terminus. Moreover, the strong interaction between PDZ1 of harmonin and Cdh23 is interrupted by a 35 aa insertion in the hair-cell-specific Cdh23(+68) splice variant, which puts forward MAGI-1 as an attractive candidate for an intracellular scaffolding partner of this tip-link protein. Our results consequently support a role of MAGI-1 in the tip-link complex, where it could provide a sturdy connection with the cytoskeleton and with other components of the mechanoelectrical transduction complex.

Development and Notch signaling requirements of the zebrafish choroid plexus

BACKGROUND

The choroid plexus (CP) is an epithelial and vascular structure in the ventricular system of the brain that is a critical part of the blood-brain barrier. The CP has two primary functions, 1) to produce and regulate components of the cerebral spinal fluid, and 2) to inhibit entry into the brain of exogenous substances. Despite its importance in neurobiology, little is known about how this structure forms.

METHODOLOGY AND PRINCIPAL FINDINGS

Here we show that the transposon-mediated enhancer trap zebrafish line Et(Mn16) expresses green fluorescent protein within a population of cells that migrate toward the midline and coalesce to form the definitive CP. We further demonstrate the development of the integral vascular network of the definitive CP. Utilizing pharmacologic pan-notch inhibition and specific morpholino-mediated knockdown, we demonstrate a requirement for Notch signaling in choroid plexus development. We identify three Notch signaling pathway members as mediating this effect, notch1b, deltaA, and deltaD.

CONCLUSIONS AND SIGNIFICANCE

This work is the first to identify the zebrafish choroid plexus and to characterize its epithelial and vasculature integration. This study, in the context of other comparative anatomical studies, strongly indicates a conserved mechanism for development of the CP. Finally, we characterize a requirement for Notch signaling in the developing CP. This establishes the zebrafish CP as an important new system for the determination of key signaling pathways in the formation of this essential component of the vertebrate brain.

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.

ADAM proteases: ligand processing and modulation of the Notch pathway

ADAM metalloproteases play important roles in development and disease. One of the key functions of ADAMs is the proteolytic processing of Notch receptors and their ligands. ADAM-mediated cleavage of Notch represents the first step in regulated intramembrane proteolysis of the receptor, leading to activation of the Notch pathway. Recent reports indicate that the transmembrane Notch ligands also undergo ADAM-mediated processing in cultured cells and in vivo. The proteolytic processing of Notch ligands modulates the strength and duration of Notch signals, leads to generation of soluble intracellular domains of the ligands, and may support a bi-directional signaling between cells.

Two deltaC splice-variants have distinct signaling abilities during somitogenesis and midline patterning

Notch signaling is required for many developmental processes, yet differences in the signaling abilities of various Notch ligands are poorly understood. Here, we have isolated a splice variant of the zebrafish Notch ligand deltaC in which the inclusion of the last intron leads to a truncation of the C-terminal 39 amino acids (deltaC(tv2)). We show that, unlike deltaC(tv1), deltaC(tv2) cannot function effectively in somitogenesis but has an enhanced ability to signal during midline development. Additionally, over-expression of deltaC(tv2) preferentially affects anterior midline development, while another Notch ligand, deltaD, shows a posterior bias. Using chimeric Deltas we show that the intracellular domain is responsible for the strength of signal in midline development, while the extracellular domain influences the anterior-posterior bias of the effect. Together our data show that different deltas can signal in biologically distinct ways in both midline formation and somitogenesis. Moreover, it illustrates the importance of cell-type-dependent modifiers of Notch signaling in providing ligand specificity.

Controlling morpholino experiments: don't stop making antisense

One of the most significant problems facing developmental biologists who do not work on an organism with well-developed genetics - and even for some who do - is how to inhibit the action of a gene of interest during development so as to learn about its normal biological function. A widely adopted approach is to use antisense technologies, and especially morpholino antisense oligonucleotides. In this article, we review the use of such reagents and present examples of how they have provided insights into developmental mechanisms. We also discuss how the use of morpholinos can lead to misleading results, including off-target effects, and we suggest controls that will allow researchers to interpret morpholino experiments correctly.

Crosstalk Between Vascular Endothelial Growth Factor, Notch, and Transforming Growth Factor-β in Vascular Morphogenesis

Vascular morphogenesis encompasses a temporally regulated set of morphological changes that endothelial cells undergo to generate a network of interconnected tubules. Such a complex process inevitably involves multiple cell signaling pathways that must be tightly coordinated in time and space. The formation of a new capillary involves endothelial cell activation, migration, alignment, proliferation, tube formation, branching, anastomosis, and maturation of intercellular junctions and the surrounding basement membrane. Each of these stages is either known or suspected to fall under the influence of the vascular endothelial growth factor, notch, and transforming growth factor-β/bone morphogenetic protein signaling pathways. Vascular endothelial growth factor is essential for initiation of angiogenic sprouting, and also regulates migration of capillary tip cells, proliferation of trunk cells, and gene expression in both. Notch has been implicated in the regulation of cell fate decisions in the vasculature, especially the choice between arterial and venular endothelial cells, and between tip and trunk cell phenotype. Transforming growth factor-β regulates cell migration and proliferation, as well as matrix synthesis. In this review, we emphasize how crosstalk between these pathways is essential for proper patterning of the vasculature and offer a transcriptional oscillator model to explain how these pathways might interact to generate new tip cells during retinal angiogenesis.

How to create the vascular tree? (Latest) help from the zebrafish

The cardiovascular system provides oxygen, nutrients and hormones to organs, it directs traffic of metabolites and it maintains tissue homeostasis. It is one of the first organs assembled during vertebrate development and it is essential to life from early stages to adult. For these reasons, the process of vessel formation has being studied for more than a century, but it is only in the late eighties that there has been an explosion of research in the field with the employment of various in vitro and in vivo model systems. The zebrafish (Danio rerio) offers several advantages for in vivo studies; it played a fundamental role in new discoveries and helped to refine our knowledge of the vascular system. This review recapitulates the zebrafish data on vasculogenesis and angiogenesis, including the specification of the haemangioblasts from the mesoderm, their migration to form the vascular cord followed by axial vessels specification, the primary and secondary sprouting of intersomitic vessels, the formation of the lumen, the arterial versus venous specification and patterning. To emphasize the strengths of the zebrafish system in the vascular field, we summarize main tools, such as gene expression and mutagenesis screens, knock down technologies, transgenic lines and imaging, which played a major role in the development of the field and allowed significant discoveries, for instance the recent visualization of the lymphatic system in zebrafish. This information contributes to the prospective of drug discovery to cure human diseases linked to angiogenesis, not last tumours.

Divergent functions and distinct localization of the Notch ligands DLL1 and DLL3 in vivo

The Notch ligands Dll1 and Dll3 are coexpressed in the presomitic mesoderm of mouse embryos. Despite their coexpression, mutations in Dll1 and Dll3 cause strikingly different defects. To determine if there is any functional equivalence, we replaced Dll1 with Dll3 in mice. Dll3 does not compensate for Dll1; DLL1 activates Notch in Drosophila wing discs, but DLL3 does not. We do not observe evidence for antagonism between DLL1 and DLL3, or repression of Notch activity in mice or Drosophila. In vitro analyses show that differences in various domains of DLL1 and DLL3 individually contribute to their biochemical nonequivalence. In contrast to endogenous DLL1 located on the surface of presomitic mesoderm cells, we find endogenous DLL3 predominantly in the Golgi apparatus. Our data demonstrate distinct in vivo functions for DLL1 and DLL3. They suggest that DLL3 does not antagonize DLL1 in the presomitic mesoderm and warrant further analyses of potential physiological functions of DLL3 in the Golgi network.

Syntenin mediates Delta1-induced cohesiveness of epidermal stem cells in culture

In human interfollicular epidermis, stem cell clusters express high levels of the Notch ligand Delta1. Delta1 stimulates neighbouring cells to differentiate and also promotes stem cell clustering. Although Notch signalling is known to stimulate epidermal differentiation, little is known about the mechanism by which Delta1 promotes epidermal cell cohesiveness. This is an important issue, because the location of stem cells determines the local microenvironmental signals they receive. We now show that mutation of the Delta1 PDZ-binding domain abolishes Delta1-mediated keratinocyte cohesiveness, stimulates Notch transcriptional activity and promotes epidermal differentiation. A yeast two-hybrid screen revealed that Delta1 binds to the adaptor protein syntenin - an interaction dependent on the Delta1 PDZ-binding domain. Syntenin, like Delta1, is upregulated in the stem cell clusters of human interfollicular epidermis. Knockdown of syntenin in cells overexpressing full-length Delta1 had the same effects on Notch signalling, epidermal differentiation and adhesion as overexpressing Delta1 with a mutated PDZ-binding domain. Syntenin has previously been reported to regulate membrane traffic, and mutation of the Delta1 PDZ-binding domain or knockdown of syntenin led to rapid internalisation of Delta1. We propose that syntenin binding to Delta1 plays a dual role in promoting intercellular adhesion and regulating Notch signalling.

The intracellular region of Notch ligands: does the tail make the difference?

The cytoplasmic tail of Notch ligands drives endocytosis, mediates association with proteins implicated in the organization of cell-cell junctions and, through regulated intra-membrane proteolysis, is released from the membrane as a signaling fragment. We survey these findings and discuss the role of Notch ligands intracellular region in bidirectional signaling and possibly in signal modulation in mammals.

This article was reviewed by Frank Eisenhaber, L Aravind, and Eugene V. Koonin.

Amphioxus AmphiDelta: evolution of Delta protein structure, segmentation, and neurogenesis

The amphioxus genome has a single Delta gene (AmphiDelta) encoding a protein 766 amino acids long. Comparison of Delta proteins of amphioxus and other animals indicates that AmphiDelta retains features of a basal bilaterian Delta protein--in having nine epidermal growth factor (EGF) repeats and also in having characteristic numbers of amino acids separating successive cysteines between and within EGF repeats. During development, AmphiDelta is expressed in the forming somites, in some regions of pharyngeal endoderm, and in cells (presumably differentiating neurons) scattered in both the neural plate and ectoderm. Expression is strongly associated with cells initiating movements to separate themselves from parent epithelia, either en masse by evagination (endoderm and mesoderm) or by delamination as isolated cells (ectoderm). The AmphiDelta-expressing cells delaminating from the ectoderm apparently migrate beneath it as they begin differentiating into probable sensory neurons, suggesting a scenario for the evolutionary origin of the placode-derived neurons of vertebrate cranial ganglia.

Genomic DNA pooling for whole-genome association scans in complex disease: empirical demonstration of efficacy in rheumatoid arthritis

A pragmatic approach that balances the benefit of a whole-genome association (WGA) experiment against the cost of individual genotyping is to use pooled genomic DNA samples. We aimed to determine the feasibility of this approach in a WGA scan in rheumatoid arthritis (RA) using the validated human leucocyte antigen (HLA) and PTPN22 associations as test loci. A total of 203 269 single-nucleotide polymorphisms (SNPs) on the Affymetrix 100K GeneChip and Illumina Infinium microarrays were examined. A new approach to the estimation of allele frequencies from Affymetrix hybridization intensities was developed involving weighting for quality signals from the probe quartets. SNPs were ranked by z-scores, combined from United Kingdom and New Zealand case-control cohorts. Within a 1.7 Mb HLA region, 33 of the 257 SNPs and at PTPN22, 21 of the 45 SNPs, were ranked within the top 100 associated SNPs genome wide. Within PTPN22, individual genotyping of SNP rs1343125 within MAGI3 confirmed association and provided some evidence for association independent of the PTPN22 620W variant (P=0.03). Our results emphasize the feasibility of using genomic DNA pooling for the detection of association with complex disease susceptibility alleles. The results also underscore the importance of the HLA and PTPN22 loci in RA aetiology.

Role of conserved intracellular motifs in Serrate signalling, cis-inhibition and endocytosis

Notch is the receptor in a signalling pathway that operates in a diverse spectrum of developmental processes. Its ligands (e.g. Serrate) are transmembrane proteins whose signalling competence is regulated by the endocytosis-promoting E3 ubiquitin ligases, Mindbomb1 and Neuralized. The ligands also inhibit Notch present in the same cell (cis-inhibition). Here, we identify two conserved motifs in the intracellular domain of Serrate that are required for efficient endocytosis. The first, a dileucine motif, is dispensable for trans-activation and cis-inhibition despite the endocytic defect, demonstrating that signalling can be separated from bulk endocytosis. The second, a novel motif, is necessary for interactions with Mindbomb1/Neuralized and is strictly required for Serrate to trans-activate and internalise efficiently but not for it to inhibit Notch signalling. Cis-inhibition is compromised when an ER retention signal is added to Serrate, or when the levels of Neuralized are increased, and together these data indicate that cis-inhibitory interactions occur at the cell surface. The balance of ubiquitinated/unubiquitinated ligand will thus affect the signalling capacity of the cell at several levels.

Notch signalling: a simple pathway becomes complex

A small number of signalling pathways are used iteratively to regulate cell fates, cell proliferation and cell death in development. Notch is the receptor in one such pathway, and is unusual in that most of its ligands are also transmembrane proteins; therefore signalling is restricted to neighbouring cells. Although the intracellular transduction of the Notch signal is remarkably simple, with no secondary messengers, this pathway functions in an enormous diversity of developmental processes and its dysfunction is implicated in many cancers.

Methylmercury induces activation of Notch signaling

Methylmercury (MeHg) toxicity in humans manifests deficits in neurological function. Cases of prenatal exposure to mercury have established that the developing nervous system is most highly susceptible to perturbation by MeHg. At a cellular level, MeHg-induced defects result from altered neuronal proliferation, migration and pathfinding. However, the molecular targets of MeHg that give rise to these outcomes are not fully understood. In an overall effort to identify the fundamental molecular targets of MeHg in neural development, we have explored the effects of MeHg on cell surface receptor function using the simplified Drosophila model. In this study, we investigated the potential role of MeHg to alter activity of the Notch receptor pathway, a highly conserved cell-cell signaling mechanism that controls cell fate decisions, proliferation, migration and neurite outgrowth in neural development. Notch receptor activation requires proteolysis by a cell surface ADAM metalloprotease. ADAM proteases are required for normal neural development and are activated by organomercurials, thus presenting a possible mechanism for MeHg neurotoxicity. Here, we demonstrate a concentration- and time-dependent increase in Notch receptor activity with MeHg exposure in three distinct Drosophila cell lines. Ten micromolar MeHg results in a 4-5.5-fold increase in Notch signaling as measured by the upregulation of two enhancer of split (E(spl)) target genes. MeHg-induced Notch activity also correlates with receptor proteolysis. Targeted knockdown of Notch protein expression demonstrates that MeHg induced E(spl) activation specifically requires the Notch receptor. Furthermore, MeHg-induced Notch activity is partially attenuated by the metalloprotease inhibitor, GM6001, consistent with a model in which MeHg promotes activation of ADAM metalloproteases. Finally, we demonstrate that inorganic HgCl(2) is significantly less active in inducing Notch activity, suggesting a mechanism specific to organic species of mercury. Overall, these data identify Notch as a potential target for MeHg toxicity in the developing nervous system.

Interaction between X-Delta-2 and Hox genes regulates segmentation and patterning of the anteroposterior axis

In vertebrates, the paraxial mesoderm already exhibits a complex Hox gene pattern by the time that segmentation occurs and somites are formed. The anterior boundaries of the Hox genes are always maintained at the same somite number, suggesting coordination between somite formation and Hox expression. To study this interaction, we used morpholinos to knockdown either the somitogenesis gene X-Delta-2 or the complete Hox paralogous group 1 (PG1) in Xenopus laevis. When X-Delta-2 is knocked down, Hox genes from different paralogous groups are downregulated from the beginning of their expression at gastrula stages. This effect is not via the canonical Notch pathway, as it is independent of the Notch effector Su(H). We also reveal for the first time a clear role for Hox genes in somitogenesis, as loss of PG1 gene function results in the perturbation of somite formation and downregulation of the X-Delta-2 expression in the PSM. This effect on X-Delta-2 expression is also observed during neurula stages, before the somites are formed. These results show that somitogenesis and patterning of the anteroposterior axis are closely linked via a feedback loop involving Hox genes and X-Delta-2, suggesting the existence of a coordination mechanism between somite formation and anteroposterior patterning. Such a mechanism is likely to be functional during gastrulation, before the formation of the first pair of somites, as suggested by the early X-Delta-2 regulation of the Hox genes.

Why is delta endocytosis required for effective activation of notch?

A number of recent studies have shown that endocytosis of Notch ligands is required for activation of Notch. There are at least two broad models that account for how Delta endocytosis in one cell might contribute to activation of Notch in the neighboring cell. The first class of models is related to the possibility that Delta endocytosis facilitates S2 cleavage and removal of the Notch extracellular domain, a critical step in Notch activation. A second class of models is related to the possibility that Delta ubiquitylation and endocytosis facilitates interactions between Delta and Notch. In the second set of models, Delta undergoes endocytosis and its subsequent trafficking back to the surface, following modifications or some change in the context in which it is presented, makes Delta a more effective ligand. Though it is still not clear how either or both mechanisms contribute, recent evidence points to the importance of both endocytosis and recycling in Delta signaling.

Up-regulation of the Notch ligand Delta-like 4 inhibits VEGF-induced endothelial cell function

Delta-like 4 (Dll4), a membrane-bound ligand for Notch1 and Notch4, is selectively expressed in the developing endothelium and in some tumor endothelium, and it is induced by vascular endothelial growth factor (VEGF)-A and hypoxia. Gene targeting studies have shown that Dll4 is required for normal embryonic vascular remodeling, but the mechanisms underlying Dll4 regulatory functions are currently not defined. In this study, we generated primary human endothelial cells that overexpress Dll4 protein to study Dll4 function and mechanism of action. Human umbilical vein endothelial cells retrovirally transduced with Dll4 displayed reduced proliferative and migratory responses selectively to VEGF-A. Expression of VEGF receptor-2, the principal signaling receptor for VEGF-A in endothelial cells, and coreceptor neuropilin-1 was significantly decreased in Dll4-transduced endothelial cells. Consistent with Dll4 signaling through Notch, expression of HEY2, one of the transcription factors that mediates Notch function, was significantly induced in Dll4-overexpressing endothelial cells. The gamma-secretase inhibitor L-685458 significantly reconstituted endothelial cell proliferation inhibited by immobilized extracellular Dll4 and reconstituted VEGFR2 expression in Dll4-overexpressing endothelial cells. These results identify the Notch ligand Dll4 as a selective inhibitor of VEGF-A biologic activities down-regulating 2 VEGF receptors expressed on endothelial cells and raise the possibility that Dll4 may be exploited therapeutically to modulate angiogenesis.

The divergent DSL ligand Dll3 does not activate Notch signaling but cell autonomously attenuates signaling induced by other DSL ligands

Mutations in the DSL (Delta, Serrate, Lag2) Notch (N) ligand Delta-like (Dll) 3 cause skeletal abnormalities in spondylocostal dysostosis, which is consistent with a critical role for N signaling during somitogenesis. Understanding how Dll3 functions is complicated by reports that DSL ligands both activate and inhibit N signaling. In contrast to other DSL ligands, we show that Dll3 does not activate N signaling in multiple assays. Consistent with these findings, Dll3 does not bind to cells expressing any of the four N receptors, and N1 does not bind Dll3-expressing cells. However, in a cell-autonomous manner, Dll3 suppressed N signaling, as was found for other DSL ligands. Therefore, Dll3 functions not as an activator as previously reported but rather as a dedicated inhibitor of N signaling. As an N antagonist, Dll3 promoted Xenopus laevis neurogenesis and inhibited glial differentiation of mouse neural progenitors. Finally, together with the modulator lunatic fringe, Dll3 altered N signaling levels that were induced by other DSL ligands.

Influence of Notch on dorsoventral compartmentalization and actin organization in the Drosophila wing

Compartment boundaries play key roles in tissue organization by separating cell populations. Activation of the Notch receptor is required for dorsoventral (DV) compartmentalization of the Drosophila wing, but the nature of its requirement has been controversial. Here, we provide additional evidence that a stripe of Notch activation is sufficient to establish a sharp separation between cell populations, irrespective of their dorsal or ventral identities. We further find that cells at the DV compartment boundary are characterized by a distinct shape, a smooth interface, and an accumulation of F-actin at the adherens junction. Genetic manipulation establishes that a stripe of Notch activation is both necessary and sufficient for this DV boundary cell phenotype, and supports the existence of a non-transcriptional branch of the Notch pathway that influences F-actin. Finally, we identify a distinct requirement for a regulator of actin polymerization, capulet, in DV compartmentalization. These observations imply that Notch effects compartmentalization through a novel mechanism, which we refer to as a fence, that does not depend on the establishment of compartment-specific cell affinities, but does depend on the organization of the actin cytoskeleton.

MAGI1 recruits Dll1 to cadherin-based adherens junctions and stabilizes it on the cell surface

Delta-Notch signaling plays an essential role in cell fate determination in many tissue types, including the central nervous system. Although the signaling mechanism of Notch has been extensively studied, the behaviors of its ligands are not well understood. In the present study, we found that, in the developing neural tube, Dll1(Delta-like 1) was mainly localized on the processes extending from nascent neurons toward both the pia and the ventricle and accumulated at apical termini, where adherens junctions (AJs) were formed. To understand the mechanism of Dll1 localization, we searched for binding proteins for Dll1 and identified a scaffolding molecule, MAGI1. In the developing spinal cord, MAGI1 mRNA was highly expressed in the ventricular zone, where Dll1 mRNA was expressed. MAGI1 protein accumulated at the AJs formed around the termini of apically extending processes and was partially colocalized with Dll1. MAGI1 bound not only to Dll1 but also to N-cadherin-beta-catenin complexes. In cultured AJ-forming fibroblasts, MAGI1 was localized at AJs, and Dll1 was recruited to these AJs through binding to MAGI1. In addition, Dll1 was stabilized on the cell surface by MAGI1. Taken together, these results suggest that Dll1 is presented on the surface of AJs formed at the apical termini of processes through interaction with MAGI1 to activate Notch on neighboring cells in the developing central nervous system.