Distinct roles for Mind bomb, Neuralized and Epsin in mediating DSL endocytosis and signaling in Drosophila

Auxilin is essential for Delta signaling

Endocytosis regulates Notch signaling in both signaling and receiving cells. A puzzling observation is that endocytosis of transmembrane ligand by the signaling cells is required for Notch activation in adjacent receiving cells. A key to understanding why signaling depends on ligand endocytosis lies in identifying and understanding the functions of crucial endocytic proteins. One such protein is Epsin, an endocytic factor first identified in vertebrate cells. Here, we show in Drosophila that Auxilin, an endocytic factor that regulates Clathrin dynamics, is also essential for Notch signaling. Auxilin, a co-factor for the ATPase Hsc70, brings Hsc70 to Clathrin cages. Hsc70/Auxilin functions in vesicle scission and also in uncoating Clathrin-coated vesicles. We find that like Epsin, Auxilin is required in Notch signaling cells for ligand internalization and signaling. Results of several experiments suggest that the crucial role of Auxilin in signaling is, at least in part, the generation of free Clathrin. We discuss these observations in the light of current models for the role of Epsin in ligand endocytosis and the role of ligand endocytosis in Notch signaling.

The neuralized homology repeat 1 domain of Drosophila neuralized mediates nuclear envelope association and delta-dependent inhibition of nuclear import

Signaling by the Notch (N) pathway is critical for many developmental processes and requires complex trafficking of both the N receptor and its transmembrane ligands, Delta (Dl) and Serrate. neuralized encodes an E3 ubiquitin ligase required for N ligand internalization. Neuralized (Neur) is conserved from worms to humans and comprises two Neur homology repeat (NHR) domains, NHR1 and NHR2, and a carboxyl-terminal RING domain. We have previously shown that the RING domain is required for ubiquitin ligase activity and that NHR1 mediates binding to Dl, a ubiquitination target. In Drosophila, Neur associates with the plasma membrane and hepatocyte responsive serum phosphoprotein-positive endosomes. Here we demonstrate that Neur also exhibits nuclear envelope localization. We have determined that Neur subcellular localization is regulated by nuclear trafficking and that inhibition of chromosome region maintenance 1, a nuclear export receptor, interferes with Neur nuclear export, trapping Neur in the nucleus. Moreover, we demonstrate that nuclear envelope localization is mediated by the Neur NHR1 domain. Interestingly, Dl expression in Schneider cells is sufficient to inhibit Neur nuclear import and inhibition occurs in an NHR1-dependent manner, suggesting that Neur nuclear localization occurs in contexts where Dl expression is either low or absent. Consistent with this, we found that Neur exhibits nuclear trafficking and associates with the nuclear envelope in the secretory cells of the larval salivary gland and that overexpression of Dl can reduce Neur localization to the nucleus. Altogether, our data demonstrate that Neur localizes to the nuclear envelope and that this localization can be negatively regulated by Dl, suggesting a possible nuclear function for Neur in Drosophila.

Neuralized contains a phosphoinositide-binding motif required downstream of ubiquitination for delta endocytosis and notch signaling

The Notch signaling pathway, which plays a critical role in cell-fate decisions throughout development, is regulated by endocytosis of both the ligand and receptor. Endocytosis of the Drosophila ligands, Delta and Serrate, is required in the signaling cell for signal initiation and requires one of two ubiquitin ligases, Neuralized or Mind bomb. Through in vitro binding assays we have identified an interaction between Neuralized and phosphoinositides, modified membrane lipids that mediate membrane trafficking and signaling. We show that interactions between phosphoinositides and Neuralized contribute to the membrane localization of Neuralized in the absence of Delta, and that the phosphoinositide-binding motif is required for Neuralized to endocytose Delta downstream of Delta ubiquitination. Lastly, we provide evidence that this interaction may also be important for vertebrate Neuralized function. These results demonstrate that, through interactions with Neuralized, phosphoinositides may regulate Delta endocytosis and, by extension, Notch signal transduction.

Wing vein patterning in Drosophila and the analysis of intercellular signaling

The positioning and elaboration of ectodermal veins in the wing of Drosophila melanogaster rely on widely utilized developmental signals, including those mediated by EGF, BMP, Hedgehog, Notch, and Wnt. Analysis of vein patterning mutants, using the molecular and genetic mosaic techniques available in Drosophila, has provided important insights into how a combination of short-range and long-range signaling can pattern a simple epidermal tissue. Moreover, venation has become a powerful system for isolating and analyzing novel components in these signaling pathways. I here review the basic events of vein patterning and give examples of how changes in venation have been used to identify important features of cell signaling pathways.

Neuralized-like 1 (Neurl1) targeted to the plasma membrane by N-myristoylation regulates the Notch ligand Jagged1

Notch signaling constitutes an evolutionarily conserved mechanism that mediates cell-cell interactions in various developmental processes. Numerous regulatory proteins interact with the Notch receptor and its ligands and control signaling at multiple levels. Ubiquitination and endocytosis followed by endosomal sorting of both the receptor and its ligands is essential for Notch-mediated signaling. The E3 ubiquitin ligases, Neuralized (Neur) and Mind Bomb (Mib1), are crucial for regulating the activity and stability of Notch ligands in Drosophila; however, biochemical evidence that the Notch ligands are directly targeted for ubiquitination by Neur and/or Mib1 has been lacking. In this report, we explore the function of Neurl1, a mouse ortholog of Drosophila Neur. We show that Neurl1 can function as an E3 ubiquitin ligase to activate monoubiquitination in vitro of Jagged1, but not other mammalian Notch ligands. Neurl1 expression decreases Jagged1 levels in cells and blocks signaling from Jagged1-expressing cells to neighboring Notch-expressing cells. We demonstrate that Neurl1 is myristoylated at its N terminus, and that myristoylation of Neurl1 targets it to the plasma membrane. Point mutations abolishing either Neurl1 myristoylation and plasma membrane localization or Neurl1 ubiquitin ligase activity impair its ability to down-regulate Jagged1 expression and to block signaling. Taken together, our results argue that Neurl1 at the plasma membrane can affect the signaling activity of Jagged1 by directly enhancing its ubiquitination and subsequent turnover.

Drosophila mind bomb2 is required for maintaining muscle integrity and survival

We report that the Drosophila mind bomb2 (mib2) gene is a novel regulator of muscle development. Unlike its paralogue, mib1, zygotic expression of mib2 is restricted to somatic and visceral muscle progenitors, and their respective differentiated musculatures. We demonstrate that in embryos that lack functional Mib2, muscle detachment is observed beginning in mid stage 15 and progresses rapidly, culminating in catastrophic degeneration and loss of most somatic muscles by stage 17. Notably, the degenerating muscles are positive for apoptosis markers, and inhibition of apoptosis in muscles prevents to a significant degree the muscle defects. Rescue experiments with Mib1 and Neuralized show further that these E3 ubiquitin ligases are not capable of ameliorating the muscle mutant phenotype of mib2. Our data suggest strongly that mib2 is involved in a novel Notch- and integrin-independent pathway that maintains the integrity of fully differentiated muscles and prevents their apoptotic degeneration.

Setting the tempo in development: an investigation of the zebrafish somite clock mechanism

The somites of the vertebrate embryo are clocked out sequentially from the presomitic mesoderm (PSM) at the tail end of the embryo. Formation of each somite corresponds to one cycle of oscillation of the somite segmentation clock--a system of genes whose expression switches on and off periodically in the cells of the PSM. We have previously proposed a simple mathematical model explaining how the oscillations, in zebrafish at least, may be generated by a delayed negative feedback loop in which the products of two Notch target genes, her1 and her7, directly inhibit their own transcription, as well as that of the gene for the Notch ligand DeltaC; Notch signalling via DeltaC keeps the oscillations of neighbouring cells in synchrony. Here we subject the model to quantitative tests. We show how to read temporal information from the spatial pattern of stripes of gene expression in the anterior PSM and in this way obtain values for the biosynthetic delays and molecular lifetimes on which the model critically depends. Using transgenic lines of zebrafish expressing her1 or her7 under heat-shock control, we confirm the regulatory relationships postulated by the model. From the timing of somite segmentation disturbances following a pulse of her7 misexpression, we deduce that although her7 continues to oscillate in the anterior half of the PSM, it governs the future somite segmentation behaviour of the cells only while they are in the posterior half. In general, the findings strongly support the mathematical model of how the somite clock works, but they do not exclude the possibility that other oscillator mechanisms may operate upstream from the her7/her1 oscillator or in parallel with it.

Regulation of vascular morphogenesis by Notch signaling

The Notch pathway is a versatile regulator of cell fate specification, growth, differentiation, and patterning processes in metazoan organisms. In the vertebrate cardiovascular system, multiple Notch family receptors and several of their Jagged and Delta-like ligands are expressed during critical stages of embryonic and postnatal development. Functional studies in mice, fish, tumor models, and cell culture systems have shown that the angiogenic growth of the blood vessel network, the proliferation of endothelial cells, and the differentiation of arteries and veins are controlled by Notch signaling. Moreover, Notch pathway components play important roles in human pathological conditions involving the vasculature, namely CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy) and Alagille syndrome. Recent findings highlight the Notch ligand Delta-like 4 as a key regulator of tumor angiogenesis and suggest that this protein might be a promising target for cancer therapy.

Neuronal morphogenesis is regulated by the interplay between cyclin-dependent kinase 5 and the ubiquitin ligase mind bomb 1

Neuronal communication requires the coordinated assembly of polarized structures including axons, dendrites, and synapses. Here, we report the identification of a ubiquitin ligase mind bomb 1 (Mib1) in the postsynaptic density and the characterization of its role in neuronal morphogenesis. Expression of Mib1 inhibits neurite outgrowth in cell culture and its gene deletion enhances synaptic growth at the neuromuscular junction in Drosophila. The analysis of Mib1 interactome by mass spectrometry revealed that Mib1 primarily interacts with membrane trafficking proteins [e.g., EEA1 (early endosomal antigen 1), Rab11-interacting proteins, and SNAP25 (synaptosomal-associated protein of 25 kDa)-like protein] and cell adhesion components (e.g., catenin, coronin, dystrobrevin, and syndecan), consistent with its previously reported function in protein sorting. More interestingly, Mib1 is associated with deubiquitinating enzymes, BRCC36 and the mammalian ortholog of fat facets, and a number of kinases, such as casein kinase II, MARK (microtubule affinity regulating kinase)/PAR1, and cyclin-dependent kinase 5 (CDK5). Further characterization of the Mib1-CDK5 interaction indicated that the N-terminal domain of Mib1 directly binds to the regulatory subunit p35 of the CDK5 complex. In cell culture, Mib1 induces the relocalization of p35/CDK5 without affecting its degradation. Surprisingly, p35/CDK5 downregulates the protein level of Mib1 by its kinase activity, and completely rescues the Mib1-induced inhibitory effect on neurite morphology. p35/CDK5 also genetically interacts with Mib1 in the fly according to the rough-eye phenotype. The data strongly support that the negative interplay between Mib1 and p35/CDK5 may integrate the activities of multiple pathways during neuronal development.

Plasticity of polyubiquitin recognition as lysosomal targeting signals by the endosomal sorting machinery

Lysosomal targeting is fundamental for the regulated disposal of ubiquitinated membrane proteins from the cell surface. To elucidate ubiquitin (Ub) configurations that are necessary and sufficient as multivesicular body (MVB)/lysosomal-sorting motifs, the intraendosomal destination and transport kinetics of model transmembrane cargo molecules bearing monoubiquitinated, multi-monoubiquitinated, or polyubiquitinated cytoplasmic tails were determined. Monomeric CD4 chimeras with K63-linked poly-Ub chains and tetrameric CD4-mono-Ub chimeras were rapidly targeted to the lysosome. In contrast, lysosomal delivery of CD4 chimeras exposing K48-linked Ub chains was delayed, whereas delivery of monoubiquitinated CD4 chimeras was undetectable. Similar difference was observed in the lysosomal targeting of mono- versus polyubiquitinated invariant chain and CD4 ubiquitinated by the MARCH (membrane-associated RING-CH) IV Ub ligase. Consistent with this, Hrs (hepatocyte growth factor regulated tyrosine kinase phosphorylated substrate), an endosomal sorting adaptor, binds preferentially to K63-Ub chain and negligibly to mono-Ub. These results highlight the plasticity of Ub as a sorting signal and its recognition by the endosomal sorting machinery, and together with previous data, suggest a regulatory role for assembly and disassembly of Ub chains of specific topology in lysosomal cargo sorting.

Delta and Egfr expression are regulated by Importin-7/Moleskin in Drosophila wing development

Drosophila DIM-7 (encoded by the moleskin gene, msk) is the orthologue of vertebrate Importin-7. Both Importin-7 and Msk/DIM-7 function as nuclear import cofactors, and have been implicated in the control of multiple signal transduction pathways, including the direct nuclear import of the activated (phosphorylated) form of MAP kinase. We performed two genetic deficiency screens to identify deficiencies that similarly modified Msk overexpression phenotypes in both eyes and wings. We identified 11 total deficiencies, one of which removes the Delta locus. In this report, we show that Delta loss-of-function alleles dominantly suppress Msk gain-of-function phenotypes in the developing wing. We find that Msk overexpression increases both Delta protein expression and Delta transcription, though Msk expression alone is not sufficient to activate Delta protein function. We also find that Msk overexpression increases Egfr protein levels, and that msk gene function is required for proper Egfr expression in both developing wings and eyes. These results indicate a novel function for Msk in Egfr expression. We discuss the implications of these data with respect to the integration of Egfr and Delta/Notch signaling, specifically through the control of MAP kinase subcellular localization.

Notch signaling--constantly on the move

The Notch pathway is a highly conserved and ubiquitous signaling system that functions in determining a diverse array of cell fates and regulates many cellular processes during embryonic development and throughout adulthood. Links to cancer, stroke and Alzheimer's disease underscore the need to define the molecular basis of Notch activation. Notch signaling is induced through direct cell-cell interactions that promote receptor activation following engagement with a membrane-bound Delta, Serrate, Lag-2 (DSL) ligand on adjacent cells. Cells take on distinct fates because Notch signaling is consistently activated in only one of the two interacting cells, highlighting the importance of establishing and maintaining signaling polarity. Studies in flies and worms have identified positive and negative transcriptional feedback mechanisms that amplify small differences in Notch and DSL ligand expression to bias which cells send or receive signals. However, endocytosis by signal-sending and signal-receiving cells also appears critical for directing and regulating Notch activation. In particular, endocytosis and membrane trafficking of DSL ligands, Notch and modulators can determine the competence of cells to send or receive signals that ensure reproducibility in generating cell types regulated by Notch signaling.

Decoding ubiquitin sorting signals for clathrin-dependent endocytosis by CLASPs

Cargo selectivity is a hallmark of clathrin-mediated endocytosis. A wide range of structurally unrelated internalization signals specify the preferential clustering of transmembrane cargo into clathrin coats forming on the plasma membrane. Intriguingly, the classical endocytic adaptor AP-2 appears to recognize only a subset of these endocytic sorting signals. New data now reveal the molecular basis for recognition of other internalization signals, including post-translationally appended ubiquitin, by clathrin-coat-associated sorting proteins (CLASPs). Curiously, structurally related ubiquitin-recognition modules are shared by select CLASPs and the 26S proteasome, and recent work indicates that both display similar requirements for ubiquitin binding. During endocytosis, these modules engage oligoubiquitylated cargo in the form of polyubiquitin chains and/or multiple single ubiquitin molecules appended to different acceptor lysines. Functional separation between clathrin-mediated endocytosis and proteasome-dependent proteolysis is probably ensured by temporally regulated, local assembly of ubiquitin-tagged membrane cargo at sorting stations on the cell surface, shielding ubiquitin sorting signals from the proteasome. Thus, an expanded repertoire of CLASPs couples the process of clathrin-coat assembly with high-fidelity incorporation of assorted, cargo-specific sorting signals.

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.

Getting the edge: neural precursor selection

A key issue in development is how to specify single isolated precursor cells to adopt a distinct fate from a group of naive cells. Studies on the development of Drosophila external sensory (ES) organs have revealed multiple mechanisms to specify single sensory organ precursors (SOPs) from clusters of cells with equivalent neural potential. Initially single SOPs are selected in part through cell-cell competition from clusters of ectodermal cells that express proneural proteins. To reinforce the singularity, lateral inhibition through the Delta/Notch system and feedback regulations lead to exclusive expression of proneural proteins in SOPs. As transcriptional activators, proneural proteins execute a genetic program in SOP cells for the development of an eventually ES organ. In this article, we will summarize recent advances on how transcriptional regulation, protein degradation, endocytosis and gene silencing by microRNA participate in SOP specification.

The glycosyltransferase Fringe promotes Delta-Notch signaling between neurons and glia, and is required for subtype-specific glial gene expression

The development, organization and function of central nervous systems depend on interactions between neurons and glial cells. However, the molecular signals that regulate neuron-glial communication remain elusive. In the ventral nerve cord of Drosophila, the close association of the longitudinal glia (LG) with the neuropil provides an excellent opportunity to identify and characterize neuron-glial signals in vivo. We have found that the activity and restricted expression of the glycosyltransferase Fringe (Fng)renders a subset of LG sensitive to activation of signaling through the Notch(N) receptor. This is the first report showing that modulation of N signaling by Fng is important for central nervous system development in any organism. In each hemisegment of the nerve cord the transcription factor Prospero (Pros) is selectively expressed in the six most anterior LG. Pros expression is specifically reduced in fng mutants, and is blocked by antagonism of the N pathway. The N ligand Delta (Dl), which is expressed by a subset of neurons, cooperates with Fng for N signaling in the anterior LG, leading to subtype-specific expression of Pros. Furthermore, ectopic Pros expression in posterior LG can be triggered by Fng, and by Dl derived from neurons but not glia. This effect can be mimicked by direct activation of the N pathway within glia. Our genetic studies suggest that Fng sensitizes N on glia to axon-derived Dl and that enhanced neuron-glial communication through this ligand-receptor pair is required for the proper molecular diversity of glial cell subtypes in the developing nervous system.

The characterization of zebrafish antimorphic mib alleles reveals that Mib and Mind bomb-2 (Mib2) function redundantly

Both mind bomb (mib) and mind bomb-2 (mib2) encode RING E3 ubiquitin ligases that promote Delta ubiquitylation and endocytosis in Notch activation. Detailed morphological and molecular examinations revealed that zebrafish mib(ta52b) (missense mutation in the C-terminal RING Finger (RF), M1013R) and mib(m132) (nonsense mutation resulting in a truncated protein that loses all three RFs, C785stop) are strong and weak antimorphic alleles, respectively, compared to the null allele, mib(tfi91) (nonsense mutation resulting in a truncated protein of only 60 amino acids, Y60stop). Zebrafish mib2 ortholog was identified in this study. Zebrafish Mib and Mib2 are colocalized in transfected cells and function redundantly in regulating Notch signaling in embryos. Mib(ta52b) and Mib(m132) have a dosage-dependent dominant-negative effect, at least, on Mib2, which is a molecular basis for the antimorphic phenotypes. It was also shown that Notch signaling negatively regulates mib expression in a Su(H)-dependent manner, forming a negative feedback loop in modulating Notch activation.

Frizzled/PCP-dependent asymmetric neuralized expression determines R3/R4 fates in the Drosophila eye

Planar cell polarity (PCP) is a common feature in many epithelia, reflected in cellular organization within the plane of an epithelium. In the Drosophila eye, Frizzled (Fz)/PCP signaling induces cell-fate specification of the R3/R4 photoreceptors through regulation of Notch activation in R4. Except for Dl upregulation in R3, the mechanism of how Fz/PCP signaling regulates Notch in this context is not understood. We demonstrate that the E3-ubiquitin ligase Neuralized (Neur), required for Dl-N signaling, is asymmetrically expressed within the R3/R4 pair. It is required in R3, where it is also upregulated in a Fz/PCP-dependent manner. As is the case for Dl, N activity in R4 further represses neur expression, thus, reinforcing the asymmetry. We demonstrate that Neur asymmetry is instructive in correct R3/R4 specification. Our data indicate that Fz/PCP-dependent Neur expression in R3 ensures the proper directionality of Dl-N signaling during R3/R4 specification.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Identification of genes that interact with Drosophila liquid facets

We have performed mutagenesis screens of the Drosophila X chromosome and the autosomes for dominant enhancers of the rough eye resulting from overexpression of liquid facets. The liquid facets gene encodes the homolog of vertebrate endocytic Epsin, an endocytic adapter protein. In Drosophila, Liquid facets is a core component of the Notch signaling pathway required in the signaling cells for ligand endocytosis and signaling. Why ligand internalization by the signaling cells is essential for signaling is a mystery. The requirement for Liquid facets is a hint at the answer, and the genes identified in this screen provide further clues. Mutant alleles of clathrin heavy chain, Rala, split ends, and auxilin were identified as enhancers. We describe the mutant alleles and mutant phenotypes of Rala and aux. We discuss the relevance of all of these genetic interactions to the function of Liquid facets in Notch signaling.

Zebrafish Mib and Mib2 are mutual E3 ubiquitin ligases with common and specific delta substrates

It was already known that both mind bomb (mib) and mind bomb-2 (mib2) encode E3 ubiquitin ligases that target Delta in Notch activation. Here we further demonstrated that zebrafish Mib and Mib2, similar to their mouse orthologs, have a C-terminal-most RING finger-dependent E3 ubiquitin ligase activity. Mib and Mib2 are reciprocal E3 ubiquitin ligases and substrates. They function similarly in Notch signaling by using DeltaC as a common substrate. However, Mib2 behaves differently from Mib in DeltaD internalization. In addition, Mib and Mib2 bind differently to extracellular and intracellular parts of DeltaA and DeltaC. Finally, mutant Mibs, Mib(ta52b) with a missense mutation in the C-terminal-most RING finger (M1013R) and Mib(m132) with a premature stop codon that leads to a deletion of three RING fingers (C785stop), act dominant-negatively and compete with Mib2 in DeltaC ubiquitylation and internalization, suggesting a molecular basis for the antimorphic phenotypes (stronger than the null phenotypes) observed in zebrafish mib(ta52b) and mib(m132) alleles.

The Conserved C2 Domain Protein Lethal (2) Giant Discs Regulates Protein Trafficking in Drosophila

Drosophila sensory organ precursor (SOP) cells undergo several rounds of asymmetric cell division to generate the four different cell types that make up external sensory organs. Establishment of different fates among daughter cells of the SOP relies on differential regulation of the Notch pathway. Here, we identify the protein Lethal (2) giant discs (Lgd) as a critical regulator of Notch signaling in the SOP lineage. We show that lgd encodes a conserved C2 domain protein that binds to phospholipids present on early endosomes. When Lgd function is compromised, Notch and other transmembrane proteins accumulate in enlarged early endosomal compartments. These enlarged endosomes are positive for Rab5 and Hrs, a protein involved in trafficking into the degradative pathway. Our experiments suggest that Lgd is a critical regulator of endocytosis that is not present in yeast and acts in the degradative pathway after Hrs.

The Drosophila Notch Inhibitor and Tumor Suppressor Gene lethal (2) giant discs Encodes a Conserved Regulator of Endosomal Trafficking

Notch signaling is involved in many developmental and pathological processes, and its activity must be precisely controlled in order to prevent aberrant development and disease. We have previously shown that the tumor suppressor gene lethal (2) giant discs (lgd) is required to prevent ectopic activation of Notch in developmental processes in Drosophila. Here we show that lgd is required in all imaginal disc cells to suppress the activity of the Notch pathway. lgd encodes a member of a poorly characterized protein family present in all animals, which includes a member that is involved in an inheritable form of mental retardation in humans. Our analysis reveals that Lgd is required for endosomal trafficking of Notch and other proteins. In the absence of Lgd, Notch is activated in a ligand-independent manner in probably all imaginal disc cells in an endosomal compartment downstream of the block in hrs mutants.

The NHR1 domain of Neuralized binds Delta and mediates Delta trafficking and Notch signaling

Notch signaling, which is crucial to metazoan development, requires endocytosis of Notch ligands, such as Delta and Serrate. Neuralized is a plasma membrane-associated ubiquitin ligase that is required for neural development and Delta internalization. Neuralized is comprised of three domains that include a C-terminal RING domain and two neuralized homology repeat (NHR) domains. All three domains are conserved between organisms, suggesting that these regions of Neuralized are functionally important. Although the Neuralized RING domain has been shown to be required for Delta ubiquitination, the function of the NHR domains remains elusive. Here we show that neuralized, a well-characterized neurogenic allele, exhibits a mutation in a conserved residue of the NHR1 domain that results in mislocalization of Neuralized and defects in Delta binding and internalization. Furthermore, we describe a novel isoform of Neuralized and show that it is recruited to the plasma membrane by Delta and that this is mediated by the NHR1 domain. Finally, we show that the NHR1 domain of Neuralized is both necessary and sufficient to bind Delta. Altogether, our data demonstrate that NHR domains can function in facilitating protein-protein interactions and in the case of Neuralized, mediate binding to its ubiquitination target, Delta.

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.

Structure-function analysis of delta trafficking, receptor binding and signaling in Drosophila

The transmembrane proteins Delta and Notch act as ligand and receptor in a conserved signaling pathway required for a variety of cell fate specification events in many organisms. Binding of Delta to Notch results in a proteolytic cascade that releases the Notch intracellular domain, allowing it to participate in transcriptional activation in the nucleus. Recent research has implicated the endocytic and ubiquitylation machinery as essential components of Delta-Notch signaling. Our analysis of chimeric and missense Delta variants has delineated a number of structural requirements for Delta trafficking, receptor binding, and signaling. We find that while the Delta N-terminal domain is necessary and sufficient for binding to Notch, the integrity of the epidermal-growth-factor-like repeat (ELR) 2 is also required for Notch binding. Screening of 117 Delta mutant lines for proteins that exhibit aberrant subcellular trafficking has led to the identification of 18 Delta alleles (DlTD alleles) that encode "trafficking-defective" Delta proteins. We find, unexpectedly, that many DlTD alleles contain missense mutations in ELRs within the Delta extracellular domain. Finally, we find that two DlTD alleles contain lysine missense mutations within the Delta intracellular domain (DeltaICD) that may identify residues important for DeltaICD mono-ubiquitylation and subsequent Delta endocytosis and signaling.

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.

Neuralized-2 regulates a Notch ligand in cooperation with Mind bomb-1

Mutations in Drosophila neuralized (Dneur) result in a variety of developmental defects that closely resemble those of Notch mutants and other Notch pathway mutants. However, mice with disrupted neur1 do not show any aberrant cell fate specifications in neurogenesis and somitogenesis. Thus, we speculated that other vertebrate neur homolog(s) might compensate for loss of the neur gene. Here, we report the paralog of mouse Neur1, named Neuralized-2 (Neur2), which is a ubiquitin-protein isopeptide ligase (E3) that interacts with and ubiquitinates Delta. Both murine Neur1 and Neur2 have similar degrees of homology to DNeur, and neur2 is expressed in patterns similar to those of neur1 in embryos, suggesting potential functional redundancy. Interestingly, two distinct classes of E3 ligases, Mind bomb-1 (Mib1) and Neur2, have cooperative but distinct roles in Delta endocytosis to Hrs-positive vesicles, i.e. Mib1 functions in the initial step of Delta endocytosis, and Neur2 is required for targeting endocytosed Delta to Hrs-positive vesicles. Thus, our study provides a new insight into how distinct E3 ligases work together in the endocytic pathways for Notch signaling.

Drosophila melanogaster auxilin regulates the internalization of Delta to control activity of the Notch signaling pathway

We have isolated mutations in the Drosophila melanogaster homologue of auxilin, a J-domain-containing protein known to cooperate with Hsc70 in the disassembly of clathrin coats from clathrin-coated vesicles in vitro. Consistent with this biochemical role, animals with reduced auxilin function exhibit genetic interactions with Hsc70 and clathrin. Interestingly, the auxilin mutations interact specifically with Notch and disrupt several Notch-mediated processes. Genetic evidence places auxilin function in the signal-sending cells, upstream of Notch receptor activation, suggesting that the relevant cargo for this auxilin-mediated endocytosis is the Notch ligand Delta. Indeed, the localization of Delta protein is disrupted in auxilin mutant tissues. Thus, our data suggest that auxilin is an integral component of the Notch signaling pathway, participating in the ubiquitin-dependent endocytosis of Delta. Furthermore, the fact that auxilin is required for Notch signaling suggests that ligand endocytosis in the signal-sending cells needs to proceed past coat disassembly to activate Notch.

Notch pathway repression by vestigial is required to promote indirect flight muscle differentiation in Drosophila melanogaster

Drosophila dorsal longitudinal muscles develop during metamorphosis by fusion of myoblasts with larval templates. It has been shown that both vestigial and Notch are crucial for correct formation of these muscles. We investigated the relationship between vestigial and the Notch pathway during this process. Using Enhancer of Split Region Transcript m6 gene expression as a reporter of Notch pathway activity, we were able to demonstrate that this pathway is only active in myoblasts. Moreover, close examination of the cellular location of several of the main actors of the N pathway (Notch, Delta, neuralized, Serrate, Mind bomb1 and fringe) during dorsal longitudinal muscle development enabled us to find that Notch receptor can play multiple roles in adult myogenesis. We report that the locations of the two Notch ligands (Delta and Serrate) are different. Interestingly, we found that fringe, which encodes a glycosyltransferase that modifies the affinity of the Notch receptor for its ligands, is expressed in muscle fibers and in a subset of myoblasts. In addition, we demonstrate that fringe expression is essential for Notch pathway inhibition and muscle differentiation. Lastly, we report that, in vestigial mutants, fringe expression is lost, and when fringe is overexpressed, a significant rescue of indirect flight muscle degeneration is obtained. Altogether, our data show that a vestigial-differentiating function is achieved through the inhibition of the Notch pathway.

Endosome dynamics during development

Endocytosis has traditionally been studied in isolated cells. More recently, however, the analysis of protein trafficking in whole organisms has revealed that it plays exciting roles during development. Endocytic trafficking of cell adhesion molecules regulates epithelial polarity and cell migration. Developmental signaling pathways are regulated by the trafficking of receptors and their ligands through the endocytic pathway. Finally, impairment of the endocytic machinery can affect proliferation control and contribute to tumor development.

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.

In vivo imaging and differential localization of lipid-modified GFP-variant fusions in embryonic stem cells and mice

The visualization of live cell behaviors operating in situ combined with the power of mouse genetics represents a major step toward understanding the mechanisms regulating embryonic development, homeostasis, and disease progression in mammals. The availability of genetically encoded fluorescent protein reporters, combined with improved optical imaging modalities, have led to advances in our ability to examine cells in vivo. We developed a series of lipid-modified fluorescent protein fusions that are targeted to and label the secretory pathway and the plasma membrane, and that are amenable for use in mice. Here we report the generation of two strains of mice, each expressing a spectrally distinct lipid-modified GFP-variant fluorescent protein fusion. The CAG::GFP-GPI strain exhibited widespread expression of a glycosylphosphatidylinositol-tagged green fluorescent protein (GFP) fusion, while the CAG::myr-Venus strain exhibited widespread expression of a myristoyl-Venus yellow fluorescent protein fusion. Imaging of live transgenic embryonic stem (ES) cells, either live or fixed embryos and postnatal tissues demonstrated that glycosylphosphatidyl inositol- and myristoyl-tagged GFP-variant fusion proteins are targeted to and serve as markers of the plasma membrane. Moreover, our data suggest that these two lipid-modified protein fusions are dynamically targeted both to overlapping as well as distinct lipid-enriched compartments within cells. These transgenic strains not only represent high-contrast reporters of cell morphology and plasma membrane dynamics, but also may be used as in vivo sensors of lipid localization. Furthermore, combining these reporters with the study of mouse mutants will be a step forward in understanding the inter- and intracellular behaviors underlying morphogenesis in both normal and mutant contexts.

Epsin 1 is a polyubiquitin-selective clathrin-associated sorting protein

Epsin 1 engages several core components of the endocytic clathrin coat, yet the precise mode of operation of the protein remains controversial. The occurrence of tandem ubiquitin-interacting motifs (UIMs) suggests that epsin could recognize a ubiquitin internalization tag, but the association of epsin with clathrin-coat components or monoubiquitin is reported to be mutually exclusive. Here, we show that endogenous epsin 1 is clearly an integral component of clathrin coats forming at the cell surface and is essentially absent from caveolin-1-containing structures under normal conditions. The UIM region of epsin 1 associates directly with polyubiquitin chains but has extremely poor affinity for monoubiquitin. Polyubiquitin binding is retained when epsin synchronously associates with phosphoinositides, the AP-2 adaptor complex and clathrin. The enrichment of epsin within clathrin-coated vesicles purified from different tissue sources varies and correlates with sorting of multiubiquitinated cargo, and in cultured cells, polyubiquitin, rather than non-conjugable monoubiquitin, promotes rapid internalization. As epsin interacts with eps15, which also contains a UIM region that binds to polyubiquitin, epsin and eps15 appear to be central components of the vertebrate poly/multiubiquitin-sorting endocytic clathrin machinery.

A ubiquitin ligase, skeletrophin, is a negative regulator of melanoma invasion

Skeletrophin (mindbomb homolog 2 (MIB2)) is a RING (Really Interesting New Gene) finger-dependent ubiquitin ligase, which targets the intracellular region of Notch ligands. A previous immunohistochemical study demonstrated that skeletrophin was downregulated in many melanomas. In the present study, we have identified a promoter region of skeletrophin on a CpG island and detected aberrant methylation of this region in six of 31 invasive melanomas, but in none of 25 benign nevi or five non-invasive superficial spreading melanomas. Subsequently, we found that a zinc-finger transcriptional factor Snail, which is overexpressed in many melanoma cells, repressed the skeletrophin promoter activity via an E-box-related element and was involved in downregulation of skeletrophin. An activator protein-2, which has a tumor suppressor-like role in melanoma, increased skeletrophin expression. Interestingly, exogenously expressed skeletrophin reduced melanoma cell invasion in vitro and in vivo. Colony formation in soft agar was also reduced in a RING motif-dependent manner, without affecting cell growth. We also found that skeletrophin downregulated transcription of the Met oncogene, which encodes the hepatocyte growth factor receptor and plays a role in the determination of the invasive phenotype of many malignant tumors. Finally, exogenously expressed skeletrophin, but not its RING mutant, increased transcription of Hes1 gene, a downstream effector of Notch pathway in melanoma cells. The present findings indicate that skeletrophin might be a novel suppressor factor for melanoma invasion.

Cell fate-specific regulation of EGF receptor trafficking during Caenorhabditis elegans vulval development

By controlling the subcellular localization of growth factor receptors, cells can modulate the activity of intracellular signal transduction pathways. During Caenorhabditis elegans vulval development, a ternary complex consisting of the LIN-7, LIN-2 and LIN-10 PDZ domain proteins localizes the epidermal growth factor receptor (EGFR) to the basolateral compartment of the vulval precursor cells (VPCs) to allow efficient receptor activation by the inductive EGF signal from the anchor cell. We have identified EGFR substrate protein-8 (EPS-8) as a novel component of the EGFR localization complex that links receptor trafficking to cell fate specification. EPS-8 expression is upregulated in the primary VPCs, where it creates a positive feedback loop in the EGFR/RAS/MAPK pathway. The membrane-associated guanylate kinase LIN-2 recruits EPS-8 into the receptor localization complex to retain the EGFR on the basolateral plasma membrane, and thus allow maximal receptor activation in the primary cell lineage. Low levels of EPS-8 in the neighboring secondary VPCs result in the rapid degradation of the EGFR, allowing these cells to adopt the secondary cell fate. Extracellular signals thus regulate EGFR trafficking in a cell type-specific manner to control pattern formation during organogenesis.

Endocytosis conducts the cell signaling orchestra

Endocytosis is used by eukaryotic cells to regulate nutrient internalization, signal transduction, and the composition of the plasma membrane. However, a more complex picture is emerging, in which endocytic pathways integrate diverse signals, thereby contributing to a higher level of cellular and organismal organization. In this way, endocytosis and cell signaling are intertwined in many biological processes, such as cell motility and cell fate determination.

Bearded family members inhibit Neuralized-mediated endocytosis and signaling activity of Delta in Drosophila

Endocytosis of Notch receptor ligands in signaling cells is essential for Notch receptor activation. In Drosophila, the E3 ubiquitin ligase Neuralized (Neur) promotes the endocytosis and signaling activity of the ligand Delta (Dl). In this study, we identify proteins of the Bearded (Brd) family as interactors of Neur. We show that Tom, a prototypic Brd family member, inhibits Neur-dependent Notch signaling. Overexpression of Tom inhibits the endocytosis of Dl and interferes with the interaction of Dl with Neur. Deletion of the Brd gene complex results in ectopic endocytosis of Dl in dorsal cells of stage 5 embryos. This defect in Dl trafficking is associated with ectopic expression of the single-minded gene, a direct Notch target gene that specifies the mesectoderm. We propose that inhibition of Neur by Brd proteins is important for precise spatial regulation of Dl signaling.

Senseless and Daughterless confer neuronal identity to epithelial cells in the Drosophila wing margin

The basic helix-loop-helix (bHLH) proneural proteins Achaete and Scute cooperate with the class I bHLH protein Daughterless to specify the precursors of most sensory bristles in Drosophila. However, the mechanosensory bristles at the Drosophila wing margin have been reported to be unaffected by mutations that remove Achaete and Scute function. Indeed, the proneural gene(s) for these organs is not known. Here, we show that the zinc-finger transcription factor Senseless, together with Daughterless, plays the proneural role for the wing margin mechanosensory precursors, whereas Achaete and Scute are required for the survival of the mechanosensory neuron and support cells in these lineages. We provide evidence that Senseless and Daughterless physically interact and synergize in vivo and in transcription assays. Gain-of-function studies indicate that Senseless and Daughterless are sufficient to generate thoracic sensory organs (SOs) in the absence of achaete-scute gene complex function. However, analysis of senseless loss-of-function clones in the thorax implicates Senseless not in the primary SO precursor (pI) selection, but in the specification of pI progeny. Therefore, although Senseless and bHLH proneural proteins are employed during the development of all Drosophila bristles, they play fundamentally different roles in different subtypes of these organs. Our data indicate that transcription factors other than bHLH proteins can also perform the proneural function in the Drosophila peripheral nervous system.

Regulation of Notch signalling by endocytosis and endosomal sorting

Cell-cell signalling is an essential process in the formation of multicellular organisms. Notch is the receptor of an evolutionarily conserved signalling pathway regulating numerous developmental decisions. Indeed, its misregulation is linked to multiple developmental and physiological disorders. Notch and its ligands are distributed widely throughout development, yet Notch activity is highly controlled and restricted in time and space. Recent advances have highlighted that endocytosis followed by endosomal sorting of both the Notch receptor and its ligands is an essential mechanism by which Notch-mediated signalling is developmentally controlled.

Presenilin-based genetic screens in Drosophila melanogaster identify novel notch pathway modifiers

Presenilin is the enzymatic component of gamma-secretase, a multisubunit intramembrane protease that processes several transmembrane receptors, such as the amyloid precursor protein (APP). Mutations in human Presenilins lead to altered APP cleavage and early-onset Alzheimer's disease. Presenilins also play an essential role in Notch receptor cleavage and signaling. The Notch pathway is a highly conserved signaling pathway that functions during the development of multicellular organisms, including vertebrates, Drosophila, and C. elegans. Recent studies have shown that Notch signaling is sensitive to perturbations in subcellular trafficking, although the specific mechanisms are largely unknown. To identify genes that regulate Notch pathway function, we have performed two genetic screens in Drosophila for modifiers of Presenilin-dependent Notch phenotypes. We describe here the cloning and identification of 19 modifiers, including nicastrin and several genes with previously undescribed involvement in Notch biology. The predicted functions of these newly identified genes are consistent with extracellular matrix and vesicular trafficking mechanisms in Presenilin and Notch pathway regulation and suggest a novel role for gamma-tubulin in the pathway.

Sec15, a component of the exocyst, promotes notch signaling during the asymmetric division of Drosophila sensory organ precursors

Asymmetric division of sensory organ precursors (SOPs) in Drosophila generates different cell types of the mature sensory organ. In a genetic screen designed to identify novel players in this process, we have isolated a mutation in Drosophila sec15, which encodes a component of the exocyst, an evolutionarily conserved complex implicated in intracellular vesicle transport. sec15(-) sensory organs contain extra neurons at the expense of support cells, a phenotype consistent with loss of Notch signaling. A vesicular compartment containing Notch, Sanpodo, and endocytosed Delta accumulates in basal areas of mutant SOPs. Based on the dynamic traffic of Sec15, its colocalization with the recycling endosomal marker Rab11, and the aberrant distribution of Rab11 in sec15 clones, we propose that a defect in Delta recycling causes cell fate transformation in sec15(-) sensory lineages. Our data indicate that Sec15 mediates a specific vesicle trafficking event to ensure proper neuronal fate specification in Drosophila.

Evidence that nervy, the Drosophila homolog of ETO/MTG8, promotes mechanosensory organ development by enhancing Notch signaling

In the imaginal tissue of developing fruit flies, achaete (ac) and scute (sc) expression defines a group of neurally-competent cells called the proneural cluster (PNC). From the PNC, a single cell, the sensory organ precursor (SOP), is selected as the adult mechanosensory organ precursor. The SOP expresses high levels of ac and sc and sends a strong Delta (Dl) signal, which activates the Notch (N) receptor in neighboring cells, preventing them from also adopting a neural fate. Previous work has determined how ac and sc expression in the PNC and SOP is regulated, but less is known about SOP-specific factors that promote SOP fate. Here, we describe the role of nervy (nvy), the Drosophila homolog of the mammalian proto-oncogene ETO, in mechanosensory organ formation. Nvy is specifically expressed in the SOP, where it interacts with the Ac and Sc DNA binding partner Daughterless (Da) and affects the expression of Ac and Sc targets. nvy loss- and gain-of-function experiments suggest that nvy reinforces, but is not absolutely required for, the SOP fate. We propose a model in which nvy acts downstream of ac and sc to promote the SOP fate by transiently strengthening the Dl signal emanating from the SOP.

The interplay between DSL proteins and ubiquitin ligases in Notch signaling

Lateral inhibition is a pattern refining process that generates single neural precursors from a field of equipotent cells and is mediated via Notch signaling. Of the two Notch ligands Delta and Serrate, only the former was thought to participate in this process. We now show that macrochaete lateral inhibition involves both Delta and Serrate. In this context, Serrate interacts with Neuralized, a ubiquitin ligase that was heretofore thought to act only on Delta. Neuralized physically associates with Serrate and stimulates its endocytosis and signaling activity. We also characterize a mutation in mib1, a Drosophila homolog of mind bomb, another Delta-targeting ubiquitin ligase from zebrafish. Mib1 affects the signaling activity of Delta and Serrate in both lateral inhibition and wing dorsoventral boundary formation. Simultaneous absence of neuralized and mib1 completely abolishes Notch signaling in both aforementioned contexts, making it likely that ubiquitination is a prerequisite for Delta/Serrate signaling.