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

The meaning of ubiquitylation of the DSL ligand Delta for the development of Drosophila

BACKGROUND

Ubiquitylation (ubi) of the intracellular domain of the Notch ligand Delta (Dl) by the E3 ligases Neuralized (Neur) and Mindbomb1 (Mib1) on lysines (Ks) is thought to be essential for the its signalling activity. Nevertheless, we have previously shown that DlK2R-HA, a Dl variant where all Ks in its intracellular domain (ICD) are replaced by the structurally similar arginine (R), still possess weak activity if over-expressed. This suggests that ubi is not absolutely required for Dl signalling. However, it is not known whether the residual activity of DlK2R-HA is an effect of over-expression and, if not, whether DlK2R can provide sufficient activity for the whole development of Drosophila.

RESULTS

To clarify these issues, we generated and analysed DlattP-DlK2R-HA, a knock-in allele into the Dl locus. Our analysis of this allele reveals that the sole presence of one copy of DlattP-DlK2R-HA can provide sufficient activity for completion of development. It further indicates that while ubi is required for the full activity of Dl in Mib1-dependent processes, it is not essential for Neur-controlled neural development. We identify three modes of Dl signalling that are either dependent or independent of ubi. Importantly, all modes depend on the presence of the endocytic adapter Epsin. During activation of Dl, direct binding of Epsin appears not to be an essential requirement. In addition, our analysis further reveals that the Ks are required to tune down the cis-inhibitory interaction of Dl with Notch.

CONCLUSIONS

Our results indicate that Dl can activate the Notch pathway without ubi of its ICD. It signals via three modes. Ubi is specifically required for the Mib1-dependent processes and the adjustment of cis-inhibition. In contrast to Mib1, Neur can efficiently activate Dl without ubi. Neur probably acts as an endocytic co-adapter in addition to its role as E3 ligase. Endocytosis, regulated in a ubi-dependent or ubi-independent manner is required for signalling and also suppression of cis-inhibition. The findings clarify the role of ubi of the ligands during Notch signalling.

The role of ligand endocytosis in notch signalling

The Notch signalling receptor is a mechanoreceptor that is activated by force. This force elicits a conformational change in Notch that results in the release of its intracellular domain into the cytosol by two consecutive proteolytic cleavages. In most cases, the force is generated by pulling of the ligands on the receptor upon their endocytosis. In this review, we summarise recent work that shed a more detailed light on the role of endocytosis during ligand-dependent Notch activation and discuss the role of ubiquitylation of the ligands during this process.

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

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

Asymmetric cell division in the Drosophila bristle lineage: from the polarization of sensory organ precursor cells to Notch-mediated binary fate decision

Asymmetric cell division (ACD) is a simple and evolutionary conserved process whereby a mother divides to generate two daughter cells with distinct developmental potentials. This process can generate cell fate diversity during development. Fate asymmetry may result from the unequal segregation of molecules and/or organelles between the two daughter cells. Here, I will review how fate asymmetry is regulated in the sensory bristle lineage in Drosophila and focus on the molecular mechanisms underlying ACD of the sensory organ precursor cells (SOPs). WIREs Dev Biol 2015, 4:299–309. doi: 10.1002/wdev.175

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Conflict of interest: The author has declared no conflicts of interest for this article.

Endocytosis and signaling during development

The development of multicellular organisms relies on an intricate choreography of intercellular communication events that pattern the embryo and coordinate the formation of tissues and organs. It is therefore not surprising that developmental biology, especially using genetic model organisms, has contributed significantly to the discovery and functional dissection of the associated signal-transduction cascades. At the same time, biophysical, biochemical, and cell biological approaches have provided us with insights into the underlying cell biological machinery. Here we focus on how endocytic trafficking of signaling components (e.g., ligands or receptors) controls the generation, propagation, modulation, reception, and interpretation of developmental signals. A comprehensive enumeration of the links between endocytosis and signal transduction would exceed the limits of this review. We will instead use examples from different developmental pathways to conceptually illustrate the various functions provided by endocytic processes during key steps of intercellular signaling.

A glycosphingolipid binding domain controls trafficking and activity of the mammalian notch ligand delta-like 1

The activity of Notch ligands is tightly regulated by trafficking events occurring both before and after ligand-receptor interaction. In particular endocytosis and recycling have been shown to be required for full signaling activity of the ligands before they encounter the Notch receptor. However little is known about the precise endocytic processes that contribute to ligand internalization. Here we demonstrate that endocytosis contributes to Dll1 signaling activity by preserving the ligand from shedding and degradation. We further show that the glycosphingolipid-binding motif originally identified in Drosophila Notch ligands is conserved in mammals and is necessary for Dll1 internalization. Mutation of its conserved tryptophan residue results in a Dll1 molecule which is rapidly inactivated by shedding and degradation, does not recycle to the cell surface and does not activate Notch signaling. Finally, silencing in the signal-sending cells of glucosylceramide synthase, the enzyme implicated in the initial phase of glycosphingolipid synthesis, down-regulates Notch activation. Our data indicate that glycosphingolipids, by interacting with Dll1, may act as functional co-factors to promote its biological activity.

Drosophila EHBP1 regulates Scabrous secretion during Notch-mediated lateral inhibition

Notch signaling is an evolutionarily conserved pathway that plays a central role in numerous developmental and disease processes. The versatility of the Notch pathway relies on the activity of context-dependent regulators. These include rab11, sec15, arp3 and Drosophila EHBP1 (dEHBP1), which control Notch signaling and cell fate acquisition in asymmetrically dividing mechanosensory lineages by regulating the trafficking of the ligand Delta. Here, we show that dEHBP1 also controls the specification of R8 photoreceptors, as its loss results in the emergence of supernumerary R8 photoreceptors. Given the requirements for Notch signaling during lateral inhibition, we propose that dEHBP1 regulates distinct aspects of Notch signaling in different developmental contexts. We show that dEHBP1 regulates the exocytosis of Scabrous, a positive regulator of Notch signaling. In conclusion, dEHBP1 provides developmental versatility of intercellular signaling by regulating the trafficking of distinct Notch signaling components.

Sanpodo controls sensory organ precursor fate by directing Notch trafficking and binding γ-secretase

In sensory organ precursor cells, Sanpodo can enhance or suppress Notch signaling by promoting interaction with Presenilin or driving receptor internalization, respectively.

In Drosophila peripheral neurogenesis, Notch controls cell fates in sensory organ precursor (SOP) cells. SOPs undergo asymmetric cell division by segregating Numb, which inhibits Notch signaling, into the pIIb daughter cell after cytokinesis. In contrast, in the pIIa daughter cell, Notch is activated and requires Sanpodo, but its mechanism of action has not been elucidated. As Sanpodo is present in both pIIa and pIIb cells, a second role for Sanpodo in regulating Notch signaling in the low-Notch pIIb cell has been proposed. Here we demonstrate that Sanpodo regulates Notch signaling levels in both pIIa and pIIb cells via distinct mechanisms. The interaction of Sanpodo with Presenilin, a component of the γ-secretase complex, was required for Notch activation and pIIa cell fate. In contrast, Sanpodo suppresses Notch signaling in the pIIb cell by driving Notch receptor internalization. Together, these results demonstrate that a single protein can regulate Notch signaling through distinct mechanisms to either promote or suppress signaling depending on the local cellular context.

Functional analysis of the NHR2 domain indicates that oligomerization of Neuralized regulates ubiquitination and endocytosis of Delta during Notch signaling

The Notch pathway plays an integral role in development by regulating cell fate in a wide variety of multicellular organisms. A critical step in the activation of Notch signaling is the endocytosis of the Notch ligands Delta and Serrate. Ligand endocytosis is regulated by one of two E3 ubiquitin ligases, Neuralized (Neur) or Mind bomb. Neur is comprised of a C-terminal RING domain, which is required for Delta ubiquitination, and two Neur homology repeat (NHR) domains. We have previously shown that the NHR1 domain is required for Delta trafficking. Here we show that the NHR1 domain also affects the binding and internalization of Serrate. Furthermore, we show that the NHR2 domain is required for Neur function and that a point mutation in the NHR2 domain (Gly430) abolishes Neur ubiquitination activity and affects ligand internalization. Finally, we provide evidence that Neur can form oligomers in both cultured cells and fly tissues, which regulate Neur activity and, by extension, ligand internalization.

Endocytosis and control of Notch signaling

The Notch signaling pathway controls patterning and cell fate decisions during development in metazoans, and is associated with human diseases such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) and certain cancers. Studies over the last several years have revealed sophisticated regulation of both the membrane-bound Notch receptor and its ligands by vesicle trafficking. This is perhaps most evident in neural progenitor cells in Drosophila, which divide asymmetrically to segregate Numb, an endocytic adaptor protein that acts as a Notch pathway inhibitor, to one daughter cell. Here, we discuss recent findings addressing how receptor and ligand trafficking to specific membrane compartments control activation of the Notch pathway in asymmetrically dividing cells and other tissues.

SNX17 regulates Notch pathway and pancreas development through the retromer-dependent recycling of Jag1

BACKGROUND

Notch is one of the most important signaling pathways involved in cell fate determination. Activation of the Notch pathway requires the binding of a membrane-bound ligand to the Notch receptor in the adjacent cell which induces proteolytic cleavages and the activation of the receptor. A unique feature of the Notch signaling is that processes such as modification, endocytosis or recycling of the ligand have been reported to play critical roles during Notch signaling, however, the underlying molecular mechanism appears context-dependent and often controversial.

RESULTS

Here we identified SNX17 as a novel regulator of the Notch pathway. SNX17 is a sorting nexin family protein implicated in vesicular trafficking and we find it is specifically required in the ligand-expressing cells for Notch signaling. Mechanistically, SNX17 regulates the protein level of Jag1a on plasma membrane by binding to Jag1a and facilitating the retromer-dependent recycling of the ligand. In zebrafish, inhibition of this SNX17-mediated Notch signaling pathway results in defects in neurogenesis as well as pancreas development.

CONCLUSIONS

Our results reveal that SNX17, by acting as a cargo-specific adaptor, promotes the retromer dependent recycling of Jag1a and Notch signaling and this pathway is involved in cell fate determination during zebrafish neurogenesis and pancreas development.

Notch ligand endocytosis generates mechanical pulling force dependent on dynamin, epsins, and actin

Notch signaling induced by cell surface ligands is critical to development and maintenance of many eukaryotic organisms. Notch and its ligands are integral membrane proteins that facilitate direct cell-cell interactions to activate Notch proteolysis and release the intracellular domain that directs Notch-specific cellular responses. Genetic studies suggest that Notch ligands require endocytosis, ubiquitylation, and epsin endocytic adaptors to activate signaling, but the exact role of ligand endocytosis remains unresolved. Here we characterize a molecularly distinct mode of clathrin-mediated endocytosis requiring ligand ubiquitylation, epsins, and actin for ligand cells to activate signaling in Notch cells. Using a cell-bead optical tweezers system, we obtained evidence for cell-mediated mechanical force dependent on this distinct mode of ligand endocytosis. We propose that the mechanical pulling force produced by endocytosis of Notch-bound ligand drives conformational changes in Notch that permit activating proteolysis.

Optical tweezers studies on Notch: single-molecule interaction strength is independent of ligand endocytosis

Notch signaling controls diverse cellular processes critical to development and disease. Cell surface ligands bind Notch on neighboring cells but require endocytosis to activate signaling. The role ligand endocytosis plays in Notch activation has not been established. Here we integrate optical tweezers with cell biological and biochemical methods to test the prevailing model that ligand endocytosis facilitates recycling to enhance ligand interactions with Notch necessary to trigger signaling. Specifically, single-molecule measurements indicate that interference of ligand endocytosis and/or recycling does not alter the force required to rupture bonds formed between cells expressing the Notch ligand Delta-like1 (Dll1) and laser-trapped Notch1 beads. Together, our analyses eliminate roles for ligand endocytosis and recycling in Dll1-Notch1 interactions and indicate that recycling indirectly affects signaling by regulating the accumulation of cell surface ligand. Importantly, our study demonstrates the utility of optical tweezers to test a role for ligand endocytosis in generating cell-mediated mechanical force.

The apoptotic engulfment protein Ced-6 participates in clathrin-mediated yolk uptake in Drosophila egg chambers

During oogenesis in Drosophila, the phagocytic engulfment protein Ced-6 recognizes the atypical endocytic sorting signal within the vitellogenin receptor Yolkless. Because Ced-6 displays all of the features of an authentic clathrin adaptor, an unrecognized clathrin dependence for Ced-6/Gulp operation during phagocytosis is possible.

Clathrin-mediated endocytosis and phagocytosis are both selective surface internalization processes but have little known mechanistic similarity or interdependence. Here we show that the phosphotyrosine-binding (PTB) domain protein Ced-6, a well-established phagocytosis component that operates as a transducer of so-called “eat-me” signals during engulfment of apoptotic cells and microorganisms, is expressed in the female Drosophila germline and that Ced-6 expression correlates with ovarian follicle development. Ced-6 exhibits all the known biochemical properties of a clathrin-associated sorting protein, yet ced-6–null flies are semifertile despite massive accumulation of soluble yolk precursors in the hemolymph. This is because redundant sorting signals within the cytosolic domain of the Drosophila vitellogenin receptor Yolkless, a low density lipoprotein receptor superfamily member, occur; a functional atypical dileucine signal binds to the endocytic AP-2 clathrin adaptor directly. Nonetheless, the Ced-6 PTB domain specifically recognizes the noncanonical Yolkless FXNPXA sorting sequence and in HeLa cells promotes the rapid, clathrin-dependent uptake of a Yolkless chimera lacking the distal dileucine signal. Ced-6 thus operates in vivo as a clathrin adaptor. Because the human Ced-6 orthologue GULP similarly binds to clathrin machinery, localizes to cell surface clathrin-coated structures, and is enriched in placental clathrin-coated vesicles, new possibilities for Ced-6/Gulp operation during phagocytosis must be considered.

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

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

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

Notch ligand endocytosis: mechanistic basis of signaling activity

Regulation of Notch signaling is critical to development and maintenance of most eukaryotic organisms. The Notch receptors and ligands are integral membrane proteins and direct cell-cell interactions are needed to activate signaling. Ligand-expressing cells activate Notch signaling through an unusual mechanism involving Notch proteolysis to release the intracellular domain from the membrane, allowing the Notch receptor to function directly as the downstream signal transducer. In the absence of ligand, the Notch receptor is maintained in an autoinhibited, protease resistant state. Genetic studies suggest that Notch ligands require ubiquitylation, epsin endocytic adaptors and dynamin-dependent endocytosis for signaling activity. Here we discuss potential models and supporting evidence to account for the absolute requirement for ligand endocytosis to activate signaling in Notch cells. Specifically, we focus on a role for ligand-mediated endocytic force to unfold Notch, override the autoinhibited state, and activate proteolysis to direct Notch-specific cellular responses.

Drosophila Epsin's role in Notch ligand cells requires three Epsin protein functions: the lipid binding function of the ENTH domain, a single Ubiquitin interaction motif, and a subset of the C-terminal protein binding modules

Epsin is an endocytic protein that binds Clathrin, the plasma membrane, Ubiquitin, and also a variety of other endocytic proteins through well-characterized motifs. Although Epsin is a general endocytic factor, genetic analysis in Drosophila and mice revealed that Epsin is essential specifically for internalization of ubiquitinated transmembrane ligands of the Notch receptor, a process required for Notch activation. Epsin's mechanism of function is complex and context-dependent. Consequently, how Epsin promotes ligand endocytosis and thus Notch signaling is unclear, as is why Notch signaling is uniquely dependent on Epsin. Here, by generating Drosophila lines containing transgenes that express a variety of different Epsin deletion and substitution variants, we tested each of the five protein or lipid interaction modules for a role in Notch activation by each of the two ligands, Serrate and Delta. There are five main results of this work that impact present thinking about the role of Epsin in ligand cells. First, we discovered that deletion or mutation of both UIMs destroyed Epsin's function in Notch signaling and had a greater negative impact on Epsin activity than removal of any other module type. Second, only one of Epsin's two UIMs was essential. Third, the lipid-binding function of the ENTH domain was required only for maximal Epsin activity. Fourth, although the C-terminal Epsin modules that interact with Clathrin, the adapter protein complex AP-2, or endocytic accessory proteins were necessary collectively for Epsin activity, their functions were highly redundant; most unexpected was the finding that Epsin's Clathrin binding motifs were dispensable. Finally, we found that signaling from either ligand, Serrate or Delta, required the same Epsin modules. All of these observations are consistent with a model where Epsin's essential function in ligand cells is to link ubiquitinated Notch ligands to Clathrin-coated vesicles through other Clathrin adapter proteins. We propose that Epsin's specificity for Notch signaling simply reflects its unique ability to interact with the plasma membrane, Ubiquitin, and proteins that bind Clathrin.

dEHBP1 controls exocytosis and recycling of Delta during asymmetric divisions

Drosophila EHBP1 is a novel regulator of Notch signaling that may function as an adaptor protein during the exocytosis and recycling of the Notch ligand Delta.

Notch signaling governs binary cell fate determination in asymmetrically dividing cells. Through a forward genetic screen we identified the fly homologue of Eps15 homology domain containing protein-binding protein 1 (dEHBP1) as a novel regulator of Notch signaling in asymmetrically dividing cells. dEHBP1 is enriched basally and at the actin-rich interface of pII cells of the external mechanosensory organs, where Notch signaling occurs. Loss of function of dEHBP1 leads to up-regulation of Sanpodo, a regulator of Notch signaling, and aberrant trafficking of the Notch ligand, Delta. Furthermore, Sec15 and Rab11, which have been previously shown to regulate the localization of Delta, physically interact with dEHBP1. We propose that dEHBP1 functions as an adaptor molecule for the exocytosis and recycling of Delta, thereby affecting cell fate decisions in asymmetrically dividing cells.

Notch receptor-ligand interactions during T cell development, a ligand endocytosis-driven mechanism

Notch signaling plays an important role during the development of different cell types and tissues. The role of Notch signaling in lymphocyte development, in particular in the development and commitment to the T cell lineage, has been the focus of research for many years. Notch signaling is absolutely required during the commitment and early stages of T cell development. Activation of the Notch signaling pathway is initiated by ligand-receptor interactions and appears to require active endocytosis of Notch ligands. Studies addressing the mechanism underlying endocytosis of Notch ligands have helped to identify the main players important and necessary for this process. Here, we review the Notch ligands, and the proposed models of Notch activation by Notch ligand endocytosis, highlighting key molecules involved. In particular, we discuss recent studies on Notch ligands involved in T cell development, current studies aimed at elucidating the relevance of Notch ligand endocytosis during T cell development and the identification of key players necessary for ligand endocytosis in the thymus and during T cell development.

Notch ligand ubiquitylation: what is it good for?

In the first volume of Developmental Cell, it was reported that the classic Drosophila neurogenic gene neuralized encodes a ubiquitin ligase that monoubiquitylates the Notch ligand Delta, thus promoting Delta endocytosis. A requirement for ligand internalization by the signal-sending cell, although counterintuitive, remains to date a feature unique to Notch signaling. Ten years and many ubiquitin ligases later, we discuss sequels to these three papers with an eye toward reviewing the development of ideas for how ligand ubiquitylation and endocytosis propel Notch signaling.

Regulation of developmental intercellular signalling by intracellular trafficking

Universal trafficking components within the cell can be recruited to coordinate and regulate the developmental signalling cascades. We will present ways in which the intracellular trafficking machinery is used to affect and modulate the outcome of signal transduction in developmental contexts, thus regulating multicellular development. Each of the signalling components must reach its proper intracellular destination, in a form that is properly folded and modified. In many instances, the ability to bring components together or segregate them into distinct compartments within the cell actually provides the switch mechanism to turn developmental signalling pathways on or off. The review will begin with a focus on the signal-sending cells, and the ways in which ligand trafficking can impinge on the signalling outcome, via processing, endocytosis and recycling. We will then turn to the signal-receiving cell, and discuss mechanisms by which endocytosis can affect the spatial features of the signal, and the compartmentalization of components downstream to the receptor.