The endocytic protein alpha-Adaptin is required for numb-mediated asymmetric cell division in Drosophila

Ubiquitin E3 Ligase LNX2 is Critical for Osteoclastogenesis In Vitro by Regulating M-CSF/RANKL Signaling and Notch2

The Notch signaling pathway plays a crucial role in skeletal development and homeostasis by regulating the proliferation and differentiation of osteoblasts and osteoclasts. However, the molecular mechanisms modulating the level and activity of Notch receptors in bone cells remain unknown. In this study, we uncovered that LNX2, an E3 ubiquitin ligase and Notch inhibitor Numb binding protein, was up-regulated during osteoclast differentiation. Knocking-down LNX2 expression in bone marrow macrophages by lentivirus-mediated short hairpin RNAs markedly inhibited osteoclast formation. Decreased LNX2 expression attenuated macrophage colony-stimulating factor (M-CSF)-induced ERK and AKT activation and RANKL-stimulated activation of NF-κB and JNK pathways; therefore, accelerated osteoclast apoptosis. Additionally, loss of LNX2 led to an increased accumulation of Numb, which promoted the degradation of Notch and caused a reduction of the expression of the Notch downstream target gene, Hes1. We conclude that LNX2 regulates M-CSF/RANKL and the Notch signaling pathways during osteoclastogenesis.

Adapting for endocytosis: roles for endocytic sorting adaptors in directing neural development

Proper cortical development depends on the orchestrated actions of a multitude of guidance receptors and adhesion molecules and their downstream signaling. The levels of these receptors on the surface and their precise locations can greatly affect guidance outcomes. Trafficking of receptors to a particular surface locale and removal by endocytosis thus feed crucially into the final guidance outcomes. In addition, endocytosis of receptors can affect downstream signaling (both quantitatively and qualitatively) and regulated endocytosis of guidance receptors is thus an important component of ensuring proper neural development. We will discuss the cell biology of regulated endocytosis and the impact on neural development. We focus our discussion on endocytic accessory proteins (EAPs) (such as numb and disabled) and how they regulate endocytosis and subsequent post-endocytic trafficking of their cognate receptors (such as Notch, TrkB, β-APP, VLDLR, and ApoER2).

Numb is not a critical regulator of Notch-mediated cell fate decisions in the developing chick inner ear

The Notch signaling pathway controls differentiation of hair cells and supporting cells in the vertebrate inner ear. Here, we have investigated whether Numb, a known regulator of Notch activity in Drosophila, is involved in this process in the embryonic chick. The chicken homolog of Numb is expressed throughout the otocyst at early stages of development and is concentrated at the basal pole of the cells. It is asymmetrically allocated at some cell divisions, as in Drosophila, suggesting that it could act as a determinant inherited by one of the two daughter cells and favoring adoption of a hair-cell fate. To test the implication of Numb in hair cell fate decisions and the regulation of Notch signaling, we used different methods to overexpress Numb at different stages of inner ear development. We found that sustained or late Numb overexpression does not promote hair cell differentiation, and Numb does not prevent the reception of Notch signaling. Surprisingly, none of the Numb-overexpressing cells differentiated into hair cells, suggesting that high levels of Numb protein could interfere with intracellular processes essential for hair cell survival. However, when Numb was overexpressed early and more transiently during ear development, no effect on hair cell formation was seen. These results suggest that in the inner ear at least, Numb does not significantly repress Notch activity and that its asymmetric distribution in dividing precursor cells does not govern the choice between hair cell and supporting cell fates.

Epithelial-to-Mesenchymal Plasticity Harnesses Endocytic Circuitries

The ability of cells to alter their phenotypic and morphological characteristics, known as cellular plasticity, is critical in normal embryonic development and adult tissue repair and contributes to the pathogenesis of diseases, such as organ fibrosis and cancer. The epithelial-to-mesenchymal transition (EMT) is a type of cellular plasticity. This transition involves genetic and epigenetic changes as well as alterations in protein expression and post-translational modifications. These changes result in reduced cell-cell adhesion, enhanced cell adhesion to the extracellular matrix, and altered organization of the cytoskeleton and of cell polarity. Among these modifications, loss of cell polarity represents the nearly invariable, distinguishing feature of EMT that frequently precedes the other traits or might even occur in their absence. EMT transforms cell morphology and physiology, and hence cell identity, from one typical of cells that form a tight barrier, like epithelial and endothelial cells, to one characterized by a highly motile mesenchymal phenotype. Time-resolved proteomic and phosphoproteomic analyses of cells undergoing EMT recently identified thousands of changes in proteins involved in many cellular processes, including cell proliferation and motility, DNA repair, and - unexpectedly - membrane trafficking (1). These results have highlighted a picture of great complexity. First, the EMT transition is not an all-or-none response but rather a gradual process that develops over time. Second, EMT events are highly dynamic and frequently reversible, involving both cell-autonomous and non-autonomous mechanisms. The net results is that EMT generates populations of mixed cells, with partial or full phenotypes, possibly accounting (at least in part) for the physiological as well as pathological cellular heterogeneity of some tissues. Endocytic circuitries have emerged as complex connectivity infrastructures for numerous cellular networks required for the execution of different biological processes, with a primary role in the control of polarized functions. Thus, they may be relevant for controlling EMT or certain aspects of it. Here, by discussing a few paradigmatic cases, we will outline how endocytosis may be harnessed by the EMT process to promote dynamic changes in cellular identity, and to increase cellular flexibility and adaptation to micro-environmental cues, ultimately impacting on physiological and pathological processes, first and foremost cancer progression.

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.

Comprehensive identification of phosphorylation sites on the Numb endocytic adaptor protein

Numb is an adaptor protein that functions in the endocytosis and intracellular trafficking of membrane receptors and adhesion molecules. Previous studies have indicated that Numb localization and function are regulated through phosphorylation by atypical protein kinase C at several key sites. Here, using LC-MS/MS, we report the identification of 25 serine/threonine Numb phosphorylation sites, and a single tyrosine phosphorylation site. Amino acid sequences flanking several of the sites identified conform to consensus motifs for cyclin-dependent kinase 5 (CDK5). In vitro kinase assays and immunoblotting confirmed that CDK5 phosphorylates Numb. LC-MS/MS analysis identified specific CDK5-directed phosphorylation of Numb at position S288 and at two additional regions. Therefore, Numb is likely to exist in multiple phospho-isoforms, and may be subject to phosphorylation-mediated regulation downstream of CDK5. These findings provide a basis for further investigations into the complex role of Numb phosphorylation in regulating its subcellular localization, protein interactions, and function. All MS data have been deposited in the ProteomeXchange with identifier PXD000997 (http://proteomecentral.proteomexchange.org/dataset/PXD000997).

A fluorescent tagging approach in Drosophila reveals late endosomal trafficking of Notch and Sanpodo

Signaling and endocytosis are highly integrated processes that regulate cell fate. In the Drosophila melanogaster sensory bristle lineages, Numb inhibits the recycling of Notch and its trafficking partner Sanpodo (Spdo) to regulate cell fate after asymmetric cell division. In this paper, we have used a dual GFP/Cherry tagging approach to study the distribution and endosomal sorting of Notch and Spdo in living pupae. The specific properties of GFP, i.e., quenching at low pH, and Cherry, i.e., slow maturation time, revealed distinct pools of Notch and Spdo: cargoes exhibiting high GFP/low Cherry fluorescence intensities localized mostly at the plasma membrane and early/sorting endosomes, whereas low GFP/high Cherry cargoes accumulated in late acidic endosomes. These properties were used to show that Spdo is sorted toward late endosomes in a Numb-dependent manner. This dual-tagging approach should be generally applicable to study the trafficking dynamics of membrane proteins in living cells and tissues.

Notch signaling acts before cell division to promote asymmetric cleavage and cell fate of neural precursor cells

Asymmetric cell divisions in the central nervous system generate neurons of diverse fates. In Drosophila melanogaster, the protein Numb localizes asymmetrically to dividing neural precursor cells such that only one daughter cell inherits Numb. Numb inhibits Notch signaling in this daughter cell, resulting in a different cell fate from the Notch-induced fate in the other-Numb-negative-daughter cell. Precursor cells undergo asymmetric cytokinesis generating daughter cells of different sizes. I found that inactivation of Notch in fly embryonic neural precursor cells disrupted the asymmetric positioning of the cleavage furrow and produced daughter cells of the same size and fate. Moreover, inactivation of Notch at different times altered the degree of asymmetric Numb localization, such that earlier inactivation of Notch caused symmetric distribution of Numb and later inactivation produced incomplete asymmetric localization of Numb. The extent of asymmetrically localized Numb positively correlated with the degree of asymmetric cytokinesis and the size disparity in daughter cells. Loss of Numb or expression of constitutively active Notch led to premature specification of the precursor cells into the fate of one of the daughter cells. Thus, in addition to its role in the specification of daughter cell fate after division, Notch controls Numb localization in the precursor cells to determine the size and fate of daughter cells. Numb also inhibits Notch signaling in precursor cells to prevent Notch-induced differentiation of the precursor cell, forming an autoregulatory loop.

Unconventional functions for clathrin, ESCRTs, and other endocytic regulators in the cytoskeleton, cell cycle, nucleus, and beyond: links to human disease

The roles of clathrin, its regulators, and the ESCRT (endosomal sorting complex required for transport) proteins are well defined in endocytosis. These proteins can also participate in intracellular pathways that are independent of endocytosis and even independent of the membrane trafficking function of these proteins. These nonendocytic functions involve unconventional biochemical interactions for some endocytic regulators, but can also exploit known interactions for nonendocytic functions. The molecular basis for the involvement of endocytic regulators in unconventional functions that influence the cytoskeleton, cell cycle, signaling, and gene regulation are described here. Through these additional functions, endocytic regulators participate in pathways that affect infection, glucose metabolism, development, and cellular transformation, expanding their significance in human health and disease.

NUMB inhibition of NOTCH signalling as a therapeutic target in prostate cancer

Prostate cancer is among the most prevalent life-threatening cancers diagnosed in the male population today. Various methods have been exploited in an attempt to treat this disease but these treatments, alongside preventative tactics, have been insufficient to control mortality rates and have usually resulted in detrimental adverse events. An opportunity to devise more-specific and potentially more-effective approaches for the eradication of prostate tumours can be found by targeting specific biological pathways. NUMB (protein numb homologue), a key regulator of cell fate, represents an attractive, actionable target in prostate cancer. NUMB participates in the observed deregulation of NOTCH (neurogenic locus notch homologue protein) signalling in prostate tumours, and the NUMB-NOTCH interaction regulates cell fate. NUMB has potential both as a target for control of prostate tumorigenesis and as a biomarker for identification of patients with prostate cancer who are likely to benefit from NOTCH inhibition.

Uninflatable and Notch control the targeting of Sara endosomes during asymmetric division

Cell fate decision during asymmetric division is mediated by the biased partition of cell fate determinants during mitosis [1-6]. In the case of the asymmetric division of the fly sensory organ precursor cells, directed Notch signaling from pIIb to the pIIa daughter endows pIIa with its distinct fate [1-6]. We have previously shown that Notch/Delta molecules internalized in the mother cell traffic through Sara endosomes and are directed to the pIIa daughter [6]. Here we show that the receptor Notch itself is required during the asymmetric targeting of the Sara endosomes to pIIa. Notch binds Uninflatable, and both traffic together through Sara endosomes, which is essential to direct asymmetric endosomes motility and Notch-dependent cell fate assignation. Our data uncover a part of the core machinery required for the asymmetric motility of a vesicular structure that is essential for the directed dispatch of Notch signaling molecules during asymmetric mitosis.

Drosophila neuroblasts as a new model for the study of stem cell self-renewal and tumour formation

Drosophila larval brain stem cells (neuroblasts) have emerged as an important model for the study of stem cell asymmetric division and the mechanisms underlying the transformation of neural stem cells into tumour-forming cancer stem cells. Each Drosophila neuroblast divides asymmetrically to produce a larger daughter cell that retains neuroblast identity, and a smaller daughter cell that is committed to undergo differentiation. Neuroblast self-renewal and differentiation are tightly controlled by a set of intrinsic factors that regulate ACD (asymmetric cell division). Any disruption of these two processes may deleteriously affect the delicate balance between neuroblast self-renewal and progenitor cell fate specification and differentiation, causing neuroblast overgrowth and ultimately lead to tumour formation in the fly. In this review, we discuss the mechanisms underlying Drosophila neural stem cell self-renewal and differentiation. Furthermore, we highlight emerging evidence in support of the notion that defects in ACD in mammalian systems, which may play significant roles in the series of pathogenic events leading to the development of brain cancers.

Earmuff restricts progenitor cell potential by attenuating the competence to respond to self-renewal factors

Despite expressing stem cell self-renewal factors, intermediate progenitor cells possess restricted developmental potential, which allows them to give rise exclusively to differentiated progeny rather than stem cell progeny. Failure to restrict the developmental potential can allow intermediate progenitor cells to revert into aberrant stem cells that might contribute to tumorigenesis. Insight into stable restriction of the developmental potential in intermediate progenitor cells could improve our understanding of the development and growth of tumors, but the mechanisms involved remain largely unknown. Intermediate neural progenitors (INPs), generated by type II neural stem cells (neuroblasts) in fly larval brains, provide an in vivo model for investigating the mechanisms that stably restrict the developmental potential of intermediate progenitor cells. Here, we report that the transcriptional repressor protein Earmuff (Erm) functions temporally after Brain tumor (Brat) and Numb to restrict the developmental potential of uncommitted (immature) INPs. Consistently, endogenous Erm is detected in immature INPs but undetectable in INPs. Erm-dependent restriction of the developmental potential in immature INPs leads to attenuated competence to respond to all known neuroblast self-renewal factors in INPs. We also identified that the BAP chromatin-remodeling complex probably functions cooperatively with Erm to restrict the developmental potential of immature INPs. Together, these data led us to conclude that the Erm-BAP-dependent mechanism stably restricts the developmental potential of immature INPs by attenuating their genomic responses to stem cell self-renewal factors. We propose that restriction of developmental potential by the Erm-BAP-dependent mechanism functionally distinguishes intermediate progenitor cells from stem cells, ensuring the generation of differentiated cells and preventing the formation of progenitor cell-derived tumor-initiating stem cells.

Development of the embryonic and larval peripheral nervous system of Drosophila

The peripheral nervous system (PNS) of embryonic and larval stage Drosophila consists of diverse types of sensory neurons positioned along the body wall. Sensory neurons, and associated end organs, show highly stereotyped locations and morphologies. Many powerful genetic tools for gene manipulation available in Drosophila make the PNS an advantageous system for elucidating basic principles of neural development. Studies of the Drosophila PNS have provided key insights into molecular mechanisms of cell fate specification, asymmetric cell division, and dendritic morphogenesis. A canonical lineage gives rise to sensory neurons and associated organs, and cells within this lineage are diversified through asymmetric cell divisions. Newly specified sensory neurons develop specific dendritic patterns, which are controlled by numerous factors including transcriptional regulators, interactions with neighboring neurons, and intracellular trafficking systems. In addition, sensory axons show modality specific terminations in the central nervous system, which are patterned by secreted ligands and their receptors expressed by sensory axons. Modality-specific axon projections are critical for coordinated larval behaviors. We review the molecular basis for PNS development and address some of the instances in which the mechanisms and molecules identified are conserved in vertebrate development.

Tools and methods for studying Notch signaling in Drosophila melanogaster

Notch signaling involves a highly conserved pathway that mediates communication between neighboring cells. Activation of Notch by its ligands, results in the release of the Notch intracellular domain (NICD), which enters the nucleus and regulates transcription. This pathway has been implicated in many developmental decisions and diseases (including cancers) over the past decades. The simplicity of the Notch pathway in Drosophila melanogaster, in combination with the availability of powerful genetics, make this an attractive model for studying fundamental principles of Notch regulation and function. In this article we present some of the established and emerging tools that are available to monitor and manipulate the Notch pathway in Drosophila and discuss their strengths and weaknesses.

PKCζ regulates Notch receptor routing and activity in a Notch signaling-dependent manner

Activation of Notch signaling requires intracellular routing of the receptor, but the mechanisms controlling the distinct steps in the routing process is poorly understood. We identify PKCζ as a key regulator of Notch receptor intracellular routing. When PKCζ was inhibited in the developing chick central nervous system and in cultured myoblasts, Notch-stimulated cells were allowed to undergo differentiation. PKCζ phosphorylates membrane-tethered forms of Notch and regulates two distinct routing steps, depending on the Notch activation state. When Notch is activated, PKCζ promotes re-localization of Notch from late endosomes to the nucleus and enhances production of the Notch intracellular domain, which leads to increased Notch activity. In the non-activated state, PKCζ instead facilitates Notch receptor internalization, accompanied with increased ubiquitylation and interaction with the endosomal sorting protein Hrs. Collectively, these data identify PKCζ as a key regulator of Notch trafficking and demonstrate that distinct steps in intracellular routing are differentially modulated depending on Notch signaling status.

Adaptor proteins involved in polarized sorting

Polarized cells such as epithelial cells and neurons exhibit different plasma membrane domains with distinct protein compositions. Recent studies have shown that sorting of transmembrane proteins to the basolateral domain of epithelial cells and the somatodendritic domain of neurons is mediated by recognition of signals in the cytosolic domains of the proteins by adaptors. These adaptors are components of protein coats associated with the trans-Golgi network and/or recycling endosomes. The clathrin-associated adaptor protein 1 (AP-1) complex plays a preeminent role in this process, although other adaptors and coat proteins, such as AP-4, ARH, Numb, exomer, and retromer, have also been implicated.

Polarized cells, polarized views: asymmetric cell division in hematopoietic cells

It has long been recognized that alterations in cell shape and polarity play important roles in coordinating lymphocyte functions. In the last decade, a new aspect of lymphocyte polarity has attracted much attention, termed asymmetric cell division (ACD). ACD has previously been shown to dictate or influence many aspects of development in model organisms such as the worm and the fly, and to be disrupted in disease. Recent observations that ACD also occurs in lymphocytes led to exciting speculations that ACD might influence lymphocyte differentiation and function, and leukemia. Dissecting the role that ACD might play in these activities has not been straightforward, and the evidence to date for a functional role in lymphocyte fate determination has been controversial. In this review, we discuss the evidence to date for ACD in lymphocytes, and how it might influence lymphocyte fate. We also discuss current gaps in our knowledge, and suggest approaches to definitively test the physiological role of ACD in lymphocytes.

Genetic screens to identify new Notch pathway mutants in Drosophila

Notch signaling controls a wide range of developmental processes, including proliferation, apoptosis, and cell fate specification during both development and adult tissue homeostasis. The functional versatility of the Notch signaling pathway is tightly linked with the complexity of its regulation in different cellular contexts. To unravel the complexity of Notch signaling, it is important to identify the different components of the Notch signaling pathway. A powerful strategy to accomplish this task is based on genetic screens. Given that the developmental context of signaling is important, these screens should be customized to specific cell populations or tissues. Here, I describe how to perform F1 clonal forward genetic screens in Drosophila to identify novel components of the Notch signaling pathway. These screens combine a classical EMS (ethyl methanesulfonate) chemical mutagenesis protocol along with clonal analysis via FRT-mediated mitotic recombination. These F1 clonal screens allow rapid phenotypic screening within clones of mutant cells induced at specific developmental stages and in tissues of interest, bypassing the pleiotropic effects of isolated mutations. More importantly, since EMS mutations have been notoriously difficult to map to specific genes in the past, I briefly discuss mapping methods that allow rapid identification of the causative mutations.

Cargo recognition in clathrin-mediated endocytosis

The endosomal system is expansive and complex, characterized by swift morphological transitions, dynamic remodeling of membrane constituents, and intracellular positioning changes. To properly navigate this ever-altering membrane labyrinth, transmembrane protein cargoes typically require specific sorting signals that are decoded by components of protein coats. The best-characterized sorting process within the endosomal system is the rapid internalization of select transmembrane proteins within clathrin-coated vesicles. Endocytic signals consist of linear motifs, conformational determinants, or covalent modifications in the cytosolic domains of transmembrane cargo. These signals are interpreted by a diverse set of clathrin-associated sorting proteins (CLASPs) that translocate from the cytosol to the inner face of the plasma membrane. Signal recognition by CLASPs is highly cooperative, involving additional interactions with phospholipids, Arf GTPases, other CLASPs, and clathrin, and is regulated by large conformational changes and covalent modifications. Related sorting events occur at other endosomal sorting stations.

Cell biology: Notch recycling is numbed

During asymmetric cell division, the Notch regulator Numb segregates unequally to establish different cell fates in the two daughter cells. Numb is thought to act as an endocytic protein. Two new studies show that Numb antagonizes Notch signaling by also regulating recycling of Sanpodo-Notch complexes via AP-1.

Structural disorder provides increased adaptability for vesicle trafficking pathways

Vesicle trafficking systems play essential roles in the communication between the organelles of eukaryotic cells and also between cells and their environment. Endocytosis and the late secretory route are mediated by clathrin-coated vesicles, while the COat Protein I and II (COPI and COPII) routes stand for the bidirectional traffic between the ER and the Golgi apparatus. Despite similar fundamental organizations, the molecular machinery, functions, and evolutionary characteristics of the three systems are very different. In this work, we compiled the basic functional protein groups of the three main routes for human and yeast and analyzed them from the structural disorder perspective. We found similar overall disorder content in yeast and human proteins, confirming the well-conserved nature of these systems. Most functional groups contain highly disordered proteins, supporting the general importance of structural disorder in these routes, although some of them seem to heavily rely on disorder, while others do not. Interestingly, the clathrin system is significantly more disordered (∼23%) than the other two, COPI (∼9%) and COPII (∼8%). We show that this structural phenomenon enhances the inherent plasticity and increased evolutionary adaptability of the clathrin system, which distinguishes it from the other two routes. Since multi-functionality (moonlighting) is indicative of both plasticity and adaptability, we studied its prevalence in vesicle trafficking proteins and correlated it with structural disorder. Clathrin adaptors have the highest capability for moonlighting while also comprising the most highly disordered members. The ability to acquire tissue specific functions was also used to approach adaptability: clathrin route genes have the most tissue specific exons encoding for protein segments enriched in structural disorder and interaction sites. Overall, our results confirm the general importance of structural disorder in vesicle trafficking and suggest major roles for this structural property in shaping the differences of evolutionary adaptability in the three routes.

Vesicle trafficking systems are fundamental among cellular transport mechanisms; various cargo molecules are transported via different coated vesicles to their specific destinations in every eukaryotic cell. Clathrin-coated vesicles mediate endocytosis and the late secretory route, while the COat Protein I and II (COPI and COPII) vesicle trafficking routes are responsible for the bidirectional traffic between the ER and the Golgi apparatus. Despite similar basic principles, regulatory mechanisms and structural features of the three systems, their molecular machinery, functions, and evolutionary characteristics vastly differ. We investigated and compared these three routes and their basic functional protein groups from the structural disorder point of view, since disordered protein regions could provide a broad variety of functional and evolutionary advantages for them. We found that structurally disordered protein segments are most abundant in the clathrin system, which might explain the observed inherent plasticity, increased adaptability and exceptional robustness of this route. We support our hypothesis by two analyses on protein multi-functionality and tissue specificity, both being indicative of evolutionary adaptability. Clathrin pathway proteins stand out in both measures, with their disordered regions being largely responsible for their outstanding capabilities.

Regulation of centrosome movements by Numb and the Collapsin Response Mediator Protein during Drosophila sensory progenitor asymmetric division

Asymmetric cell division generates cell fate diversity during development and adult life. Recent findings have demonstrated that during stem cell divisions, the movement of centrosomes is asymmetric in prophase and that such asymmetry participates in mitotic spindle orientation and cell polarization. Here, we have investigated the dynamics of centrosomes during Drosophila sensory organ precursor asymmetric divisions and find that centrosome movements are asymmetric during cytokinesis. We demonstrate that centrosome movements are controlled by the cell fate determinant Numb, which does not act via its classical effectors, Sanpodo and α-Adaptin, but via the Collapsin Response Mediator Protein (CRMP). Furthermore, we find that CRMP is necessary for efficient Notch signalling and that it regulates the duration of the pericentriolar accumulation of Rab11-positive endosomes, through which the Notch ligand, Delta is recycled. Our work characterizes an additional mode of asymmetric centrosome movement during asymmetric divisions and suggests a model whereby the asymmetry in centrosome movements participates in differential Notch activation to regulate cell fate specification.

Expression and function of NUMB in odontogenesis

NUMB is a multifunctional protein implicated to function in self-renewal and differentiation of progenitors in several tissues. To characterize the transcripts and to analyze the expression pattern of NUMB in odontogenesis, we isolated 2 full-length clones for NUMB from mouse dental pulp mRNA. One novel sequence contained 200 bp insertion in the phosphotyrosine binding domain (PTB). Confocal microscopy analysis showed strong NUMB expression in human dental pulp stem cells (hDPSC) and preameloblasts. Western blot analysis indicated that NUMB isoforms were differentially expressed in various dental tissues. Immunohistochemical analysis showed that in postnatal mouse tooth germs, NUMB was differentially expressed in the preameloblasts, odontoblasts, cervical loop region, and in the dental pulp stem cells during development. Interestingly, overexpression of NUMB in HAT-7, a preameloblast cell line, had dramatic antagonizing effects on the protein expression level of activated Notch 1. Further analysis of the Notch signaling pathway showed that NUMB significantly downregulates sonic hedgehog (Shh) expression in preameloblasts. Therefore, we propose that NUMB maintains ameloblast progenitor phenotype at the cervical loop by downregulating the activated Notch1 protein and thereby inhibiting the mRNA expression of Shh.

Notch signaling at a glance

Cell-cell interactions define a quintessential aspect of multicellular development. Metazoan morphogenesis depends on a handful of fundamental, conserved cellular interaction mechanisms, one of which is defined by the Notch signaling pathway. Signals transmitted through the Notch surface receptor have a unique developmental role: Notch signaling links the fate of one cell with that of a cellular neighbor through physical interactions between the Notch receptor and the membrane-bound ligands that are expressed in an apposing cell. The developmental outcome of Notch signals is strictly dependent on the cellular context and can influence differentiation, proliferation and apoptotic cell fates. The Notch pathway is conserved across species (Artavanis-Tsakonas et al., 1999; Bray, 2006; Kopan and Ilagan, 2009). In humans, Notch malfunction has been associated with a diverse range of diseases linked to changes in cell fate and cell proliferation including cancer (Louvi and Artavanis-Tsakonas, 2012). In this Cell Science at a Glance article and the accompanying poster we summarize the molecular biology of Notch signaling, its role in development and its relevance to disease.

The mortal strand hypothesis: non-random chromosome inheritance and the biased segregation of damaged DNA

If a eukaryotic cell is to reproduce, it must duplicate its genetic information in the form of DNA, and faithfully segregate that information during a complex process of cell division. During this division process, the resulting cells inherit one, and only one, copy of each chromosome. Over thirty years ago, it was predicted that the segregation of sister chromosomes could occur non-randomly, such that a daughter cell would preferentially inherit one of the two sister chromosomes according to some characteristic of that chromosome's template DNA strand. Although this prediction has been confirmed in studies of various cell-types, we know little of both the mechanism by which the asymmetric inheritance occurs and the significance it has to cells. In this essay, we propose a new model of non-random chromosome segregation-the mortal strand hypothesis-and discuss tests of the model that will provide insight into the molecular choreography of this intriguing phenomenon.

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.

Numb localizes at endosomes and controls the endosomal sorting of notch after asymmetric division in Drosophila

Numb acts as a cell-fate determinant during asymmetric and stem cell divisions in both vertebrates and invertebrates [1, 2]. In Drosophila, Numb is unequally segregated in asymmetrically dividing sensory organ precursor cells (SOPs). Numb is inherited by the pIIb cell (Notch OFF) and is absent from the pIIa cell (Notch ON) [3, 4]. Numb is required to establish directional Notch signaling during cytokinesis [3, 5-7]. Using real-time imaging of a functional GFP-tagged Numb, we show that Numb relocalizes during cytokinesis from the basal cortex of pIIb to subapical endosomes. This relocalization appeared to depend on its interaction with the α-adaptin [8, 9]. Live imaging of Sanpodo (Spdo), a membrane protein interacting with Numb and regulating the trafficking of Notch [6, 7, 10-15], revealed that Spdo is internalized during cytokinesis and coaccumulates with Numb in pIIb endosomes. Using a GFP-tagged Notch [6], we found that Notch coaccumulates with Spdo in a Numb-dependent manner in these pIIb endosomes. Numb was, however, dispensable for the internalization of Notch and Spdo. We propose that Numb interacts with internalized Spdo-Notch oligomers at sorting endosomes and inhibits the recycling of Notch, thereby creating an asymmetry in Notch distribution along the pIIa-pIIb interface and regulating binary fate choice.

The TBC1D15 oncoprotein controls stem cell self-renewal through destabilization of the Numb-p53 complex

Stem cell populations are maintained through self-renewing divisions in which one daughter cell commits to a specific fate while the other retains the multipotent characteristics of its parent. The p53 tumor suppressor, in conjunction with its interacting partner protein Numb, preserves this asymmetry and functions as a vital barrier against the unchecked expansion of tumor stem cell pools; however, little is known about the biological control of the Numb-p53 interaction. We show here that Numb and p53 are the constituents of a high molecular mass complex, which is disintegrated upon activation of aPKCζ, a Numb kinase. Using large-scale affinity purification and tandem mass spectrometry, we identify TBC1D15 as a Numb-associated protein and demonstrate that its amino-terminal domain disengages p53 from Numb, triggering p53 proteolysis and promoting self-renewal and pluripotency. Cellular levels of TBC1D15 are diminished upon acute nutrient deprivation through autophagy-mediated degradation, indicating that TBC1D15 serves as a conduit through which cellular metabolic status is linked to self-renewal. The profound deregulation of TBC1D15 expression exhibited in a diverse array of patient tumors underscores its proposed function as an oncoprotein.

Identification and selected reaction monitoring (SRM) quantification of endocytosis factors associated with Numb

Numb is an endocytic adaptor protein that regulates the endocytosis and trafficking of transmembrane receptors including Notch, E-cadherin, and integrins. Vertebrate Numb is alternatively spliced at exons 3 and 9 to give rise to four protein isoforms. Expression of these isoforms varies at different developmental stages, and although the function of Numb isoforms containing exon 3 has been studied, the role of exon 9 inclusion has not been shown. Here we use affinity purification and tandem mass spectrometry to identify Numb associated proteins, including novel interactions with REPS1, BMP2K, and BCR. In vitro binding measurements indicated exon 9-independent Numb interaction with REPS1 and Eps15 EH domains. Selected reaction monitoring mass spectrometry was used to quantitatively compare the proteins associated with the p72 and p66 Numb isoforms, which differ by the exon 9 region. This showed that significantly more EPS15 and three AP-2 subunit proteins bound Numb isoforms containing exon 9. The EPS15 preference for exon 9-containing Numb was confirmed in intact cells by using a proximity ligation assay. Finally, we used multiplexed selected reaction monitoring mass spectrometry to assess the dynamic regulation of Numb association with endocytic proteins. Numb hyper-phosphorylation resulted in disassociation of Numb endocytic complexes, while inhibition of endocytosis did not alter Numb association with the AP-2 complex but altered recruitment of EPS15, REPS1, and BMP2K. Hence, quantitative mass spectrometric analysis of Numb protein-protein interactions has provided new insights into the assembly and regulation of protein complexes important in development and cancer.

The ascidian numb gene involves in the formation of neural tissues

Notch signaling plays fundamental roles in various animal development. It has been suggested that Hr-Notch, a Notch homologue in the ascidian Halocynthia roretzi, is involved in the formation of peripheral neurons by suppressing the neural fates and promoting the epidermal differentiation. However, roles of Notch signaling remain controversial in the formation of nervous system in ascidian embryos. To precisely investigate functions of Notch signaling, we have isolated and characterized Hr-Numb, a Numb homologue which is a negative regulator of Notch signaling, in H. roretzi. Maternal expression of Hr-Numb mRNAs was detected in egg cytoplasm and the transcripts were inherited by the animal blastomeres. Its zygotic expression became evident by the early neurula stage and the transcripts were detected in dorsal neural precursor cells. Suppression of Hr-Numb function by an antisense morpholino oligonucleotide resulted in larvae with defect in brain vesicle and palps formation. Similar results have been obtained by overexpression of the constitutively activated Hr-Notch forms. Therefore, these results suggest that Hr-Numb is involved in Notch signaling during ascidian embryogenesis.

Uncovering the link between malfunctions in Drosophila neuroblast asymmetric cell division and tumorigenesis

Asymmetric cell division is a developmental process utilized by several organisms. On the most basic level, an asymmetric division produces two daughter cells, each possessing a different identity or fate. Drosophila melanogaster progenitor cells, referred to as neuroblasts, undergo asymmetric division to produce a daughter neuroblast and another cell known as a ganglion mother cell (GMC). There are several features of asymmetric division in Drosophila that make it a very complex process, and these aspects will be discussed at length. The cell fate determinants that play a role in specifying daughter cell fate, as well as the mechanisms behind setting up cortical polarity within neuroblasts, have proved to be essential to ensuring that neurogenesis occurs properly. The role that mitotic spindle orientation plays in coordinating asymmetric division, as well as how cell cycle regulators influence asymmetric division machinery, will also be addressed. Most significantly, malfunctions during asymmetric cell division have shown to be causally linked with neoplastic growth and tumor formation. Therefore, it is imperative that the developmental repercussions as a result of asymmetric cell division gone awry be understood.

Sending the right signal: Notch and stem cells

BACKGROUND

Notch signaling plays a critical role in multiple developmental programs and not surprisingly, the Notch pathway has also been implicated in the regulation of many adult stem cells, such as those in the intestine, skin, lungs, hematopoietic system, and muscle.

SCOPE OF REVIEW

In this review, we will first describe molecular mechanisms of Notch component modulation including recent advances in this field and introduce the fundamental principles of Notch signaling controlling cell fate decisions. We will then illustrate its important and varied functions in major stem cell model systems including: Drosophila and mammalian intestinal stem cells and mammalian skin, lung, hematopoietic and muscle stem cells.

MAJOR CONCLUSIONS

The Notch receptor and its ligands are controlled by endocytic processes that regulate activation, turnover, and recycling. Glycosylation of the Notch extracellular domain has important modulatory functions on interactions with ligands and on proper receptor activity. Notch can mediate cell fate decisions including proliferation, lineage commitment, and terminal differentiation in many adult stem cell types. Certain cell fate decisions can have precise requirements for levels of Notch signaling controlled through modulatory regulation.

GENERAL SIGNIFICANCE

We describe the current state of knowledge of how the Notch receptor is controlled through its interaction with ligands and how this is regulated by associated factors. The functional consequences of Notch receptor activation on cell fate decisions are discussed. We illustrate the importance of Notch's role in cell fate decisions in adult stem cells using examples from the intestine, skin, lung, blood, and muscle. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Notch signaling: numb makes the difference

During Drosophila sensory organ precursor cell development, Numb segregates asymmetrically and functions as a cell fate determinant. Recent work now demonstrates in vivo that Numb inactivates Notch by promoting its endocytosis.

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.

Numb expression and asymmetric versus symmetric cell division in distal embryonic lung epithelium

Proper balance between self-renewal and differentiation of lung-specific progenitors is absolutely required for normal lung morphogenesis/regeneration. Therefore, understanding the behavior of lung epithelial stem/progenitor cells could identify innovative solutions for restoring normal lung morphogenesis and/or regeneration. The Notch inhibitor Numb is a key determinant of asymmetric or symmetric cell division and hence cell fate. Yet Numb proximal-distal expression pattern and symmetric versus asymmetric division are uncharacterized during lung epithelial development. Herein, the authors find that the cell fate determinant Numb is highly expressed and asymmetrically distributed at the apical side of distal epithelial progenitors and segregated to one daughter cell in most mitotic cells. Knocking down Numb in MLE15 epithelial cells significantly increased the number of cells expressing the progenitor cell markers Sox9/Id2. Furthermore, cadherin hole analysis revealed that most distal epithelial stem/progenitor cells in embryonic lungs divide asymmetrically; with their cleavage, planes are predicted to bypass the cadherin hole, resulting in asymmetric distribution of the cadherin hole to the daughter cells. These novel findings provide evidence for asymmetric cell division in distal epithelial stem/progenitor cells of embryonic lungs and a framework for future translationally oriented studies in this area.

Neural stem cells in Drosophila: molecular genetic mechanisms underlying normal neural proliferation and abnormal brain tumor formation

Neural stem cells in Drosophila are currently one of the best model systems for understanding stem cell biology during normal development and during abnormal development of stem cell-derived brain tumors. In Drosophila brain development, the proliferative activity of neural stem cells called neuroblasts gives rise to both the optic lobe and the central brain ganglia, and asymmetric cell divisions are key features of this proliferation. The molecular mechanisms that underlie the asymmetric cell divisions by which these neuroblasts self-renew and generate lineages of differentiating progeny have been studied extensively and involve two major protein complexes, the apical complex which maintains polarity and controls spindle orientation and the basal complex which is comprised of cell fate determinants and their adaptors that are segregated into the differentiating daughter cells during mitosis. Recent molecular genetic work has established Drosophila neuroblasts as a model for neural stem cell-derived tumors in which perturbation of key molecular mechanisms that control neuroblast proliferation and the asymmetric segregation of cell fate determinants lead to brain tumor formation. Identification of novel candidate genes that control neuroblast self-renewal and differentiation as well as functional analysis of these genes in normal and tumorigenic conditions in a tissue-specific manner is now possible through genome-wide transgenic RNAi screens. These cellular and molecular findings in Drosophila are likely to provide valuable genetic links for analyzing mammalian neural stem cells and tumor biology.

Lewis X-carrying N-glycans regulate the proliferation of mouse embryonic neural stem cells via the Notch signaling pathway

Neural stem cells (NSCs) possess high proliferative potential and the capacity for self-renewal with retention of multipotency to differentiate into brain-forming cells. Several signaling pathways have been shown to be involved in the fate determination process of NSCs, but the molecular mechanisms underlying the maintenance of neural cell stemness remain largely unknown. Our previous study showed that human natural killer carbohydrate epitopes expressed specifically by mouse NSCs modulate the Ras-MAPK pathway, raising the possibility of regulatory roles of glycoprotein glycans in the specific signaling pathways involved in NSC fate determination. To address this issue, we performed comparative N-glycosylation profiling of NSCs before and after differentiation in a comprehensive and quantitative manner. We found that Lewis X-carrying N-glycans were specifically displayed on undifferentiated cells, whereas pauci-mannose-type N-glycans were predominantly expressed on differentiated cells. Furthermore, by knocking down a fucosyltransferase 9 with short interfering RNA, we demonstrated that the Lewis X-carrying N-glycans were actively involved in the proliferation of NSCs via modulation of the expression level of Musashi-1, which is an activator of the Notch signaling pathway. Our findings suggest that Lewis X carbohydrates, which have so far been characterized as undifferentiation markers, actually operate as activators of the Notch signaling pathway for the maintenance of NSC stemness during brain development.

Interaction of Notch signaling modulator Numb with α-Adaptin regulates endocytosis of Notch pathway components and cell fate determination of neural stem cells

The ability to balance self-renewal and differentiation is a hallmark of stem cells. In Drosophila neural stem cells (NSCs), Numb/Notch (N) signaling plays a key role in this process. However, the molecular and cellular mechanisms underlying Numb function in a stem cell setting remain poorly defined. Here we show that α-Adaptin (α-Ada), a subunit of the endocytic AP-2 complex, interacts with Numb through a new mode of interaction to regulate NSC homeostasis. In α-ada mutants, N pathway component Sanpodo and the N receptor itself exhibited altered trafficking, and N signaling was up-regulated in the intermediate progenitors of type II NSC lineages, leading to their transformation into ectopic NSCs. Surprisingly, although the Ear domain of α-Ada interacts with the C terminus of Numb and is important for α-Ada function in the sensory organ precursor lineage, it was dispensable in the NSCs. Instead, α-Ada could regulate Sanpodo, N trafficking, and NSC homeostasis by interacting with Numb through new domains in both proteins previously not known to mediate their interaction. This interaction could be bypassed when α-Ada was directly fused to the phospho-tyrosine binding domain of Numb. Our results identify a critical role for the AP-2-mediated endocytosis in regulating NSC behavior and reveal a new mechanism by which Numb regulates NSC behavior through N. These findings are likely to have important implications for cancer biology.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Asymmetric segregation and self-renewal of hematopoietic stem and progenitor cells with endocytic Ap2a2

The stem cell–intrinsic model of self-renewal via asymmetric cell division (ACD) posits that fate determinants be partitioned unequally between daughter cells to either activate or suppress the stemness state. ACD is a purported mechanism by which hematopoietic stem cells (HSCs) self-renew, but definitive evidence for this cellular process remains open to conjecture. To address this issue, we chose 73 candidate genes that function within the cell polarity network to identify potential determinants that may concomitantly alter HSC fate while also exhibiting asymmetric segregation at cell division. Initial gene-expression profiles of polarity candidates showed high and differential expression in both HSCs and leukemia stem cells. Altered HSC fate was assessed by our established in vitro to in vivo screen on a subcohort of candidate polarity genes, which revealed 6 novel positive regulators of HSC function: Ap2a2, Gpsm2, Tmod1, Kif3a, Racgap1, and Ccnb1. Interestingly, live-cell videomicroscopy of the endocytic protein AP2A2 shows instances of asymmetric segregation during HSC/progenitor cell cytokinesis. These results contribute further evidence that ACD is functional in HSC self-renewal, suggest a role for Ap2a2 in HSC activity, and provide a unique opportunity to prospectively analyze progeny from HSC asymmetric divisions.

Non-canonical Notch signaling: emerging role and mechanism

Notch is an ancient transmembrane receptor with crucial roles in cell-fate choices. Although the 'canonical' Notch pathway and its core members are well established - involving ligand-induced cleavage of Notch for transcriptional regulation - it has been unclear whether Notch can also function independently of ligand and transcription ('non-canonically') through a common mechanism. Recent studies suggest that Notch can non-canonically exert its biological functions by post-translationally targeting Wnt/β-catenin signaling, an important cellular and developmental regulator. The non-canonical Notch pathway appears to be highly conserved from flies to mammals. Here, we discuss the emerging conserved mechanism and role of ligand/transcription-independent Notch signaling in cell and developmental biology.

Regulation of cell growth by Notch signaling and its differential requirement in normal vs. tumor-forming stem cells in Drosophila

Cancer stem cells (CSCs) are postulated to be a small subset of tumor cells with tumor-initiating ability that shares features with normal tissue-specific stem cells. The origin of CSCs and the mechanisms underlying their genesis are poorly understood, and it is uncertain whether it is possible to obliterate CSCs without inadvertently damaging normal stem cells. Here we show that a functional reduction of eukaryotic translation initiation factor 4E (eIF4E) in Drosophila specifically eliminates CSC-like cells in the brain and ovary without having discernable effects on normal stem cells. Brain CSC-like cells can arise from dedifferentiation of transit-amplifying progenitors upon Notch hyperactivation. eIF4E is up-regulated in these dedifferentiating progenitors, where it forms a feedback regulatory loop with the growth regulator dMyc to promote cell growth, particularly nucleolar growth, and subsequent ectopic neural stem cell (NSC) formation. Cell growth regulation is also a critical component of the mechanism by which Notch signaling regulates the self-renewal of normal NSCs. Our findings highlight the importance of Notch-regulated cell growth in stem cell maintenance and reveal a stronger dependence on eIF4E function and cell growth by CSCs, which might be exploited therapeutically.

Balancing self-renewal and differentiation by asymmetric division: insights from brain tumor suppressors in Drosophila neural stem cells

Balancing self-renewal and differentiation of stem cells is an important issue in stem cell and cancer biology. Recently, the Drosophila neuroblast (NB), neural stem cell has emerged as an excellent model for stem cell self-renewal and tumorigenesis. It is of great interest to understand how defects in the asymmetric division of neural stem cells lead to tumor formation. Here, we review recent advances in asymmetric division and the self-renewal control of Drosophila NBs. We summarize molecular mechanisms of asymmetric cell division and discuss how the defects in asymmetric division lead to tumor formation. Gain-of-function or loss-of-function of various proteins in the asymmetric machinery can drive NB overgrowth and tumor formation. These proteins control either the asymmetric protein localization or mitotic spindle orientation of NBs. We also discuss other mechanisms of brain tumor suppression that are beyond the control of asymmetric division.

Endocytosis by Numb breaks Notch symmetry at cytokinesis

Cell-fate diversity can be generated by the unequal segregation of the Notch regulator Numb at mitosis in both vertebrates and invertebrates. Whereas the mechanisms underlying unequal inheritance of Numb are understood, how Numb antagonizes Notch has remained unsolved. Live imaging of Notch in sensory organ precursor cells revealed that nuclear Notch is detected at cytokinesis in the daughter cell that does not inherit Numb. Numb and Sanpodo act together to regulate Notch trafficking and establish directional Notch signalling at cytokinesis. We propose that unequal segregation of Numb results in increased endocytosis in one daughter cell, hence asymmetry of Notch at the cytokinetic furrow, directional signalling and binary fate choice.

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.