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

Asymmetric cell divisions in the epidermis

Generation of three-dimensional tissues with distinct cell types is required for the development of all organs. On its own, mitotic spindle orientation allows tissues to change in length or shape. In combination with intrinsic or extrinsic cues, this can also be coupled to the generation of diverse cell fates-a process known as asymmetric cell division (ACD). Understanding ACDs has been greatly aided by studies in invertebrate model systems, where genetics and live imaging have provided the basis for much of what we know. ACDs also drive the development and differentiation of the epidermis in mammals. While similar to the invertebrate models, the epidermis is distinct in balancing symmetric and asymmetric divisions to yield a tissue of the correct surface area and thickness. Here, we review the roles of spindle orientation in driving both morphogenesis and cell fate decisions. We highlight the epidermis as a unique model system to study not only basic mechanisms of ACD but also their regulation during development.

Genetic identification of intracellular trafficking regulators involved in notch dependent binary cell fate acquisition following asymmetric cell division

Notch signaling is involved in numerous cellular processes during development and throughout adult life. Although ligands and receptors are largely expressed in the whole organism, activation of Notch receptors only takes place in a subset of cells and/or tissues and is accurately regulated in time and space. Previous studies have demonstrated that endocytosis and recycling of both ligands and/or receptors are essential for this regulation. However, the precise endocytic routes, compartments and regulators involved in the spatio temporal regulation are largely unknown.

In order to identify Notch signaling intracellular trafficking regulators, we have undertaken a tissue-specific dsRNA genetic screen against candidates potentially involved in endocytosis and recycling within the endolysosomal pathway. dsRNA against 418 genes was induced in Drosophila melanogaster sensory organ lineage in which Notch signaling regulates binary cell fate acquisition. Gain- or loss-of Notch signaling phenotypes were observed in adult sensory organs for 113 of them. Furthermore, 26 genes presented a change in the steady state localization of Notch, Sanpodo, a Notch co-factor, and/or Delta in the pupal lineage. In particular, we identified 20 genes with previously unknown function in Drosophila melanogaster intracellular trafficking. Among them, we identified CG2747 and show that it regulates the localization of clathrin adaptor AP-1 complex, a negative regulator of Notch signaling. All together, our results further demonstrate the essential function of intracellular trafficking in regulating Notch signaling-dependent binary cell fate acquisition and constitute an additional step toward the elucidation of the routes followed by Notch receptor and ligands to signal.

Musashi expression in β-cells coordinates insulin expression, apoptosis and proliferation in response to endoplasmic reticulum stress in diabetes

Diabetes is associated with the death and dysfunction of insulin-producing pancreatic β-cells. In other systems, Musashi genes regulate cell fate via Notch signaling, which we recently showed regulates β-cell survival. Here we show for the first time that human and mouse adult islet cells express mRNA and protein of both Musashi isoforms, as well Numb/Notch/Hes/neurogenin-3 pathway components. Musashi expression was observed in insulin/glucagon double-positive cells during human fetal development and increased during directed differentiation of human embryonic stem cells (hESCs) to the pancreatic lineage. De-differentiation of β-cells with activin A increased Msi1 expression. Endoplasmic reticulum (ER) stress increased Msi2 and Hes1, while it decreased Ins1 and Ins2 expression, revealing a molecular link between ER stress and β-cell dedifferentiation in type 2 diabetes. These effects were independent of changes in Numb protein levels and Notch activation. Overexpression of MSI1 was sufficient to increase Hes1, stimulate proliferation, inhibit apoptosis and reduce insulin expression, whereas Msi1 knockdown had the converse effects on proliferation and insulin expression. Overexpression of MSI2 resulted in a decrease in MSI1 expression. Taken together, these results demonstrate overlapping, but distinct roles for Musashi-1 and Musashi-2 in the control of insulin expression and β-cell proliferation. Our data also suggest that Musashi is a novel link between ER stress and the compensatory β-cell proliferation and the loss of β-cell gene expression seen in specific phases of the progression to type 2 diabetes.

Genome-wide analysis of self-renewal in Drosophila neural stem cells by transgenic RNAi

The balance between stem cell self-renewal and differentiation is precisely controlled to ensure tissue homeostasis and prevent tumorigenesis. Here we use genome-wide transgenic RNAi to identify 620 genes potentially involved in controlling this balance in Drosophila neuroblasts. We quantify all phenotypes and derive measurements for proliferation, lineage, cell size, and cell shape. We identify a set of transcriptional regulators essential for self-renewal and use hierarchical clustering and integration with interaction data to create functional networks for the control of neuroblast self-renewal and differentiation. Our data identify key roles for the chromatin remodeling Brm complex, the spliceosome, and the TRiC/CCT-complex and show that the alternatively spliced transcription factor Lola and the transcriptional elongation factors Ssrp and Barc control self-renewal in neuroblast lineages. As our data are strongly enriched for genes highly expressed in murine neural stem cells, they are likely to provide valuable insights into mammalian stem cell biology as well.

Apical-basal polarity in Drosophila neuroblasts is independent of vesicular trafficking

Cell polarity in epithelia depends on the PAR proteins, which interact with the machinery for exocytic and endocytic vesicular trafficking. Polarity in Drosophila neural stem cells is independent of vesicular trafficking, although it depends on the PAR proteins, revealing different mechanisms of how polarity is controlled.

The possession of apical–basal polarity is a common feature of epithelia and neural stem cells, so-called neuroblasts (NBs). In Drosophila, an evolutionarily conserved protein complex consisting of atypical protein kinase C and the scaffolding proteins Bazooka/PAR-3 and PAR-6 controls the polarity of both cell types. The components of this complex localize to the apical junctional region of epithelial cells and form an apical crescent in NBs. In epithelia, the PAR proteins interact with the cellular machinery for polarized exocytosis and endocytosis, both of which are essential for the establishment of plasma membrane polarity. In NBs, many cortical proteins show a strongly polarized subcellular localization, but there is little evidence for the existence of distinct apical and basolateral plasma membrane domains, raising the question of whether vesicular trafficking is required for polarization of NBs. We analyzed the polarity of NBs mutant for essential regulators of the main exocytic and endocytic pathways. Surprisingly, we found that none of these mutations affected NB polarity, demonstrating that NB cortical polarity is independent of plasma membrane polarity and that the PAR proteins function in a cell type–specific manner.

Caenorhabditis elegans numb inhibits endocytic recycling by binding TAT-1 aminophospholipid translocase

Numb regulates endocytosis in many metazoans, but the mechanism by which it functions is not completely understood. Here we report that the Caenorhabditis elegans Numb ortholog, NUM-1A, a regulator of endocytic recycling, binds the C isoform of transbilayer amphipath transporter-1 (TAT-1), a P4 family adenosine triphosphatase and putative aminophospholipid translocase that is required for proper endocytic trafficking. We demonstrate that TAT-1 is differentially spliced during development and that TAT-1C-specific splicing occurs in the intestine where NUM-1A is known to function. NUM-1A and TAT-1C colocalize in vivo. We have mapped the binding site to an NXXF motif in TAT-1C. This motif is not required for TAT-1C function but is required for NUM-1A's ability to inhibit recycling. We demonstrate that num-1A and tat-1 defects are both suppressed by the loss of the activity of PSSY-1, a phosphatidylserine (PS) synthase. PS is mislocalized in intestinal cells with defects in tat-1 or num-1A function. We propose that NUM-1A inhibits recycling by inhibiting TAT-1C's ability to translocate PS across the membranes of recycling endosomes.

Numb controls E-cadherin endocytosis through p120 catenin with aPKC

Cadherin trafficking controls tissue morphogenesis and cell polarity. The endocytic adaptor Numb participates in apicobasal polarity by acting on intercellular adhesions in epithelial cells. However, it remains largely unknown how Numb controls cadherin-based adhesion. Here, we found that Numb directly interacted with p120 catenin (p120), which is known to interact with E-cadherin and prevent its internalization. Numb accumulated at intercellular adhesion sites and the apical membrane in epithelial cells. Depletion of Numb impaired E-cadherin internalization, whereas depletion of p120 accelerated internalization. Expression of the Numb-binding fragment of p120 inhibited E-cadherin internalization in a dominant-negative fashion, indicating that Numb interacts with the E-cadherin/p120 complex and promotes E-cadherin endocytosis. Impairment of Numb induced mislocalization of E-cadherin from the lateral membrane to the apical membrane. Atypical protein kinase C (aPKC), a member of the PAR complex, phosphorylated Numb and inhibited its association with p120 and α-adaptin. Depletion or inhibition of aPKC accelerated E-cadherin internalization. Wild-type Numb restored E-cadherin internalization in the Numb-depleted cells, whereas a phosphomimetic mutant or a mutant with defective α-adaptin-binding ability did not restore the internalization. Thus, we propose that aPKC phosphorylates Numb to prevent its binding to p120 and α-adaptin, thereby attenuating E-cadherin endocytosis to maintain apicobasal polarity.

Neurogenesis at the brain-cerebrospinal fluid interface

Cerebral cortical progenitor cells can be classified into several different types, and each progenitor type integrates cell-intrinsic and cell-extrinsic cues to regulate neurogenesis. On one hand, cell-intrinsic mechanisms that depend upon appropriate apical-basal polarity are established by adherens junctions and apical complex proteins and are particularly important in progenitors with apical processes contacting the lateral ventricle. The apical protein complexes themselves are concentrated at the ventricular surface, and apical complex proteins regulate mitotic spindle orientation and cell fate. On the other hand, remarkably little is known about how cell-extrinsic cues signal to progenitors and couple with cell-intrinsic mechanisms to instruct neurogenesis. Recent research shows that the cerebrospinal fluid, which contacts apical progenitors at the ventricular surface and bathes the apical complex of these cells, provides growth- and survival-promoting cues for neural progenitor cells in developing and adult brain. This review addresses how the apical-basal polarity of progenitor cells regulates cell fate and allows progenitors to sample diffusible signals distributed by the cerebrospinal fluid. We also review several classes of signaling factors that the cerebrospinal fluid distributes to the developing brain to instruct neurogenesis.

Numb links extracellular cues to intracellular polarity machinery to promote chemotaxis

Cell polarization is essential throughout development for proliferation, migration, and differentiation. However, it is not known how extracellular cues correctly orient cell polarity at distinct stages of development. Here, we show that the endocytic adaptor protein Numb, previously characterized for its role in cell proliferation, subsequently plays an important role in cell migration. In neural precursors stimulated with the chemotactic factor BDNF, Numb binds to activated TrkB, the BDNF receptor, and functions both as an endocytic regulator for TrkB and as a scaffold for aPKC (aPKC). Thus, Numb promotes BDNF-dependent aPKC activation. Interestingly, Numb is also a substrate of aPKC. When phosphorylated, Numb exhibits increased efficacy in binding TrkB and in promoting a chemotactic response to BDNF. Therefore, Numb functions in a feed-forward loop to promote chemotaxis of neural precursors, linking BDNF, an extracellular cue, to aPKC, a critical component of the intrinsic polarity machinery.

Role of stem cells in cardiovascular biology

This review article addresses the controversy as to whether the adult heart possesses an intrinsic growth reserve. If myocyte renewal takes place in healthy and diseased organs, the reconstitution of the damaged tissue lost upon pathological insults might be achieved by enhancing a natural occurring process. Evidence in support of the old and new view of cardiac biology is critically discussed in an attempt to understand whether the heart is a static or dynamic organ. According to the traditional concept, the heart exerts its function until death of the organism with the same or lesser number of cells that are present at birth. This paradigm was challenged by documentation of the cell cycle activation and nuclear and cellular division in a subset of myocytes. These observations raised the important question of the origin of replicating myocytes. Several theories have been proposed and are presented in this review article. Newly formed myocytes may derive from a pre-existing pool of cells that has maintained the ability to divide. Alternatively, myocytes may be generated by activation and commitment of resident cardiac stem cells or by migration of progenitor cells from distant organs. In all cases, parenchymal cell turnover throughout lifespan results in a heterogeneous population consisting of young, adult, and senescent myocytes. With time, accumulation of old myocytes has detrimental effects on cardiac performance and may cause the development of an aging myopathy.

The adaptor-associated kinase 1, AAK1, is a positive regulator of the Notch pathway

The Notch pathway is involved in cell-cell signaling during development and adulthood from invertebrates to higher eukaryotes. Activation of the Notch receptor by its ligands relies upon a multi-step processing. The extracellular part of the receptor is removed by a metalloprotease of the ADAM family and the remaining fragment is cleaved within its transmembrane domain by a presenilin-dependent γ-secretase activity. γ-Secretase processing of Notch has been shown to depend upon monoubiquitination as well as clathrin-mediated endocytosis (CME). We show here that AAK1, the adaptor-associated kinase 1, directly interacts with the membrane-tethered active form of Notch released by metalloprotease cleavage. Active AAK1 acts upstream of the γ-secretase cleavage by stabilizing both the membrane-tethered activated form of Notch and its monoubiquitinated counterpart. We propose that AAK1 acts as an adaptor for Notch interaction with components of the clathrin-mediated pathway such as Eps15b. Moreover, transfected AAK1 increases the localization of activated Notch to Rab5-positive endocytic vesicles, while AAK1 depletion or overexpression of Numb, an inhibitor of the pathway, interferes with this localization. These results suggest that after ligand-induced activation of Notch, the membrane-tethered form can be directed to different endocytic pathways leading to distinct fates.

NUMB-ing down cancer by more than just a NOTCH

The protein Numb does not live up to its name. This passive-sounding protein is anything but spent. Originally identified as a cell-fate determinant in Drosophila development, Numb received a good deal of attention as an inhibitor of the Notch receptor signaling pathway. It turns out, however, that Numb does a lot more than simply regulate Notch. It has been implicated in a variety of biochemical pathways connected with signaling (it regulates Notch-, Hedgehog- and TP53-activated pathways), endocytosis (it is involved in cargo internalization and recycling), determination of polarity (it interacts with the PAR complex, and regulates adherens and tight junctions), and ubiquitination (it exploits this mechanism to regulate protein function and stability). This complex biochemical network lies at the heart of Numb's involvement in diverse cellular phenotypes, including cell fate developmental decisions, maintenance of stem cell compartments, regulation of cell polarity and adhesion, and migration. Considering its multifaceted role in cellular homeostasis, it is not surprising that Numb has been implicated in cancer as a tumor suppressor. Our major goal here is to explain the cancer-related role of Numb based on our understanding of its role in cell physiology. We will attempt to do this by reviewing the present knowledge of Numb at the biochemical and functional level, and by integrating its apparently heterogeneous functions into a unifying scenario, based on our recently proposed concept of the "endocytic matrix". Finally, we will discuss the role of Numb in the maintenance of the normal stem cell compartment, as a starting point to interpret the tumor suppressor function of Numb in the context of the cancer stem cell hypothesis.

Asymmetric cell division: recent developments and their implications for tumour biology

The ability of cells to divide asymmetrically is essential for generating diverse cell types during development. The past 10 years have seen tremendous progress in our understanding of this important biological process. We have learned that localized phosphorylation events are responsible for the asymmetric segregation of cell fate determinants in mitosis and that centrosomes and microtubules play important parts in this process. The relevance of asymmetric cell division for stem cell biology has added a new dimension to the field, and exciting connections between asymmetric cell division and tumorigenesis have begun to emerge.

Notch signaling regulates neuroepithelial stem cell maintenance and neuroblast formation in Drosophila optic lobe development

Notch signaling mediates multiple developmental decisions in Drosophila. In this study, we have examined the role of Notch signaling in Drosophila larval optic lobe development. Loss of function in Notch or its ligand Delta leads to loss of the lamina and a smaller medulla. The neuroepithelial cells in the optic lobe in Notch or Delta mutant brains do not expand but instead differentiate prematurely into medulla neuroblasts, which lead to premature neurogenesis in the medulla. Clonal analyses of loss-of-function alleles for the pathway components, including N, Dl, Su(H), and E(spl)-C, indicate that the Delta/Notch/Su(H) pathway is required for both maintaining the neuroepithelial stem cells and inhibiting medulla neuroblast formation while E(spl)-C is only required for some aspects of the inhibition of medulla neuroblast formation. Conversely, Notch pathway overactivation promotes neuroepithelial cell expansion while suppressing medulla neuroblast formation and neurogenesis; numb loss of function mimics Notch overactivation, suggesting that Numb may inhibit Notch signaling activity in the optic lobe neuroepithelial cells. Thus, our results show that Notch signaling plays a dual role in optic lobe development, by maintaining the neuroepithelial stem cells and promoting their expansion while inhibiting their differentiation into medulla neuroblasts. These roles of Notch signaling are strikingly similar to those of the JAK/STAT pathway in optic lobe development, raising the possibility that these pathways may collaborate to control neuroepithelial stem cell maintenance and expansion, and their differentiation into the progenitor cells.

Chronic AMD3100 antagonism of SDF-1alpha-CXCR4 exacerbates cardiac dysfunction and remodeling after myocardial infarction

The role of the SDF-1alpha-CXCR4 axis in response to myocardial infarction is unknown. We addressed it using the CXCR4 antagonist, AMD3100, to block SDF-1alpha interaction with CXCR4 after chronic coronary artery ligation. Chronic AMD3100 treatment decreased ejection fraction and fractional shortening in mice 20days after myocardial infarction compared with vehicle-treated mice (echocardiography). Morphometric analysis showed hearts of AMD3100-treated infarcted mice to have expanded scar, to be hypertrophic (confirmed by myocyte cross-section area) and dilated, with increased LV end systolic and end diastolic dimensions, and to have decreased scar collagen content; p-AKT levels were attenuated and this was accompanied by increased apoptosis. Despite increased injury, c-kit(pos) cardiac progenitor cells (CPCs) were increased in the risk region of AMD3100-treated infarcted mice; CPCs were CD34(neg)/CD45(neg) with the majority undergoing symmetric cell division. c-kit(pos)/MHC(pos) CPCs also increased in the risk region of the AMD3100-treated infarcted group. In this group, GSK-3beta signaling was attenuated compared to vehicle-treated, possibly accounting for increased proliferation and increased cardiac committed MHC(pos) CPCs. Increased proliferation following AMD3100 treatment was supported by increased levels of cyclin D1, a consequence of increased prolyl isomerase, Pin1, and decreased cyclin D1 phosphorylation. In summary, pharmacologic antagonism of CXCR4 demonstrates that SDF-1alpha-CXCR4 signaling plays an important role during and after myocardial infarction and that it exerts pleiotropic salubrious effects, protecting the myocardium from apoptotic cell death, facilitating scar formation, restricting CPC proliferation, and directing CPCs toward a cardiac fate.

The endocytic adaptor Numb regulates thymus size by modulating pre-TCR signaling during asymmetric division

Stem cells must proliferate and differentiate to generate the lineages that shape mature organs; understanding these 2 processes and their interaction is one of the central themes in current biomedicine. An intriguing aspect is asymmetric division, by which 2 daughter cells with different fates are generated. Several cell fate determinants participate in asymmetric division, with the endocytic adaptor Numb as the best-known example. Here, we have explored the role of asymmetric division in thymocyte development, visualizing the differential segregation of Numb and pre-TCR in thymic precursors. Analysis of mice where Numb had been inhibited by expressing a dominant negative revealed enhanced pre-T-cell receptor (TCR) signaling and a smaller thymus. Conversely, Numb overexpression resulted in loss of asymmetric division and a larger thymus. The conclusion is that Numb determines the levels of pre-TCR signaling in dividing thymocytes and, ultimately, the size of the pool from which mature T lymphocytes are selected.

Regulation of skeletal myogenesis by Notch

Notch signaling has emerged as a key player in skeletal muscle development and regeneration. Simply stated, Notch signaling inhibits differentiation. Accordingly, fine-tuning the pathway is essential for proper muscle homeostasis. This review will address various aspects of Notch signaling, including our current views of the core pathway, its effects in muscle, its interactions with other signaling pathways, and its relationship with ageing.

The endocytic matrix

Endocytosis has long been thought of as simply a way for cells to internalize nutrients and membrane-associated molecules. But an explosive growth in knowledge has given a new dimension to our understanding of this process. It now seems that endocytosis is a master organizer of signalling circuits, with one of its main roles being the resolution of signals in space and time. Many of the functions of endocytosis that are emerging from recent research cannot yet be reconciled with the canonical view of intracellular trafficking but, instead, point to endocytosis being integrated at a deeper level in the cellular 'master plan' (the cellular network of signalling circuits that lie at the base of the cell's make-up). Deconvolution of this level, which we call the 'endocytic matrix', might uncover a fundamental aspect of how a cell is built.

Numb independently antagonizes Sanpodo membrane targeting and Notch signaling in Drosophila sensory organ precursor cells

How Numb regulates Notch signaling following asymmetric cell division is unclear. Numb directly binds and blocks membrane localization of Sanpodo, a protein essential for Notch signaling in Drosophila. Uncoupling Sanpodo from Numb results in accumulation of Sanpodo at the membrane, but this surprisingly does not appear to promote Notch signaling.

In Drosophila, mitotic neural progenitor cells asymmetrically segregate the cell fate determinant Numb in order to block Notch signaling in only one of the two daughter cells. Sanpodo, a membrane protein required for Notch signaling in asymmetrically dividing cells, is sequestered from the plasma membrane to intracellular vesicles in a Numb-dependent way after neural progenitor cell mitosis. However, the significance of Numb-dependent Sanpodo regulation is unclear. In this study, we conducted a structure–function analysis to identify the determinants of Sanpodo targeting in vivo. We identified an NPAF motif in the amino-terminal cytoplasmic tail of Sanpodo, which is conserved among insect Sanpodo homologues. The Sanpodo NPAF motif is predicted to bind directly to the Numb phosphotyrosine-binding domain and is critical for Numb binding in vitro. Deletion or mutation of the NPAF motif results in accumulation of Sanpodo at the plasma membrane in Numb-positive cells in vivo. Genetic analysis of Sanpodo NPAF mutants shows that Numb-dependent Sanpodo endocytic targeting can be uncoupled from Notch signaling regulation. Our findings demonstrate that Sanpodo contains an evolutionarily conserved motif that has been linked to Numb-dependent regulation in vertebrates and further support the model that Numb regulates Notch signaling independently of Sanpodo membrane trafficking in neural progenitor cells.

Asymmetric cell divisions and asymmetric cell fates

The regulation of self-renewal, cell diversity, and differentiation can occur by modulating symmetric and asymmetric cell divisions. Remarkably, asymmetric cell divisions can arise through multiple processes in which molecules in the cytoplasm and nucleus, as well as template "immortal" DNA strands, can segregate to one daughter cell during cell division. Explaining how these events direct distinct daughter cell fates is a major challenge to understanding how the organism is assembled and maintained for a lifetime. Numerous technical issues that are associated with assessing how distinct cell fates are executed in vivo have resulted in divergent interpretations of experimental findings. This review addresses some of these points and considers different developmental model systems that attempt to investigate how cell fate decisions are determined, as well as the molecules that guide these choices.

Endocytosis and intracellular trafficking of Notch and its ligands

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

Dividing cellular asymmetry: asymmetric cell division and its implications for stem cells and cancer

Cell division is commonly thought to involve the equal distribution of cellular components into the two daughter cells. During many cell divisions, however, proteins, membrane compartments, organelles, or even DNA are asymmetrically distributed between the two daughter cells. Here, we review the various types of asymmetries that have been described in yeast and in animal cells. Asymmetric segregation of protein determinants is particularly relevant for stem cell biology. We summarize the relevance of asymmetric cell divisions in various stem cell systems and discuss why defects in asymmetric cell division can lead to the formation of tumors.

Sanpodo: a context-dependent activator and inhibitor of Notch signaling during asymmetric divisions

Asymmetric cell divisions generate sibling cells of distinct fates ('A', 'B') and constitute a fundamental mechanism that creates cell-type diversity in multicellular organisms. Antagonistic interactions between the Notch pathway and the intrinsic cell-fate determinant Numb appear to regulate asymmetric divisions in flies and vertebrates. During these divisions, productive Notch signaling requires sanpodo, which encodes a novel transmembrane protein. Here, we demonstrate that Drosophila sanpodo plays a dual role to regulate Notch signaling during asymmetric divisions - amplifying Notch signaling in the absence of Numb in the 'A' daughter cell and inhibiting Notch signaling in the presence of Numb in the 'B' daughter cell. In so doing, sanpodo ensures the asymmetry in Notch signaling levels necessary for the acquisition of distinct fates by the two daughter cells. These findings answer long-standing questions about the restricted ability of Numb and Sanpodo to inhibit and to promote, respectively, Notch signaling during asymmetric divisions.

Identifying genes that interact with Drosophila presenilin and amyloid precursor protein

The gamma-secretase complex is involved in cleaving transmembrane proteins such as Notch and one of the genes targeted in Alzheimer's disease known as amyloid precursor protein (APP). Presenilins function within the catalytic core of gamma-secretase, and mutated forms of presenilins were identified as causative factors in familial Alzheimer's disease. Recent studies show that in addition to Notch and APP, numerous signal transduction pathways are modulated by presenilins, including intracellular calcium signaling. Thus, presenilins appear to have diverse roles. To further understand presenilin function, we searched for Presenilin-interacting genes in Drosophila by performing a genetic modifier screen for enhancers and suppressors of Presenilin-dependent Notch-related phenotypes. We identified 177 modifiers, including known members of the Notch pathway and genes involved in intracellular calcium homeostasis. We further demonstrate that 53 of these modifiers genetically interacted with APP. Characterization of these genes may provide valuable insights into Presenilin function in development and disease.

Breast cancer cells expressing stem cell markers CD44+ CD24 lo are eliminated by Numb-1 peptide-activated T cells

Cancer stem cells (CSC) are resistant to chemo- and radiotherapy. To eliminate cells with phenotypic markers of CSC-like we characterized: (1) expression of CD44, CD24, CD133 and MIC-A/B (NKG2 receptors) in breast (MCF7) and ovarian (SK-OV-3) cells resistant to gemcitabine (GEM), paclitaxel (PTX) and 5-fluorouracil (5-FU) and (2) their elimination by Numb- and Notch-peptide activated CTL. The number of cells in all populations with the luminal CSC phenotype [epithelial specific antigen(+) (ESA) CD44(hi) CD24(lo), CD44(hi) CD133(+), and CD133(+) CD24(lo)] increased in drug-resistant MCF7 and SK-OV-3 cells. Similarly, the number of cells with expressed MIC-A/B increased 4 times in drug-resistant tumor cells compared with drug-sensitive cells. GEM(Res) MCF7 cells had lower levels of the Notch-1-extracellular domain (NECD) and Notch trans-membrane intracellular domain (TMIC) than GEM(Sens) MCF7. The levels of Numb, and Numb-L-[P]-Ser(265) were similar in GEM(Res) and GEM(Sens) MCF7 cells. Only the levels of Numb-L (long)-Ser(295) decreased slightly. This finding suggests that Notch-1 cleavage to TMIC is inhibited in GEM(Res) MCF7 cells. PBMC activated by natural immunogenic peptides Notch-1 (2112-2120) and Numb-1 (87-95) eliminated NICD(positive), CD24(hi) CD24(lo) MCF7 cells. It is likely that the immunogenic Numb-1 peptide in MCF7 cells originated from Numb, [P]-lated by an unknown kinase, because staurosporine but not wortmannin and MAPK-inhibitors decreased peptide presentation. Numb and Notch are antagonistic proteins which degrade each other to stop and activate cell proliferation, respectively. Their peptides are presented alternatively. Targeting both antagonistic proteins should be useful to prevent metastases in patients whose tumors are resistant to conventional treatments.

Endocytic regulation of Notch activation and down-regulation (Review)

Notch receptor signalling plays a central role in development and its misfunction has been linked to a number of diseases. In the cannonical Notch signalling pathway, ligand binding to Notch activates a series of proteolytic cleavages that release the Notch intracellular domain for trafficking to the nucleus, where it activates the transcription factor, Suppressor of Hairless (Su(H)). A number of recent papers have demonstrated the importance of endocytic trafficking of Notch and its ligands for both the activation and the down-regulation of the Notch receptor. These reports highlight uncertainty regarding the whereabouts in the cell where Notch activation occurs, and the form of the ligand that can induce signalling. In this review we speculate that, decision points between alternative trafficking pathways represent important regulatory nodes that may allow Notch signalling levels to be modulated by other developmental signals, providing context-dependency to Notch activation. We also review data that suggest that key proteolytic events, associated with Notch activation, may occur within the endocytic pathway or require prior endocytosis and recycling of Notch and its ligands to the cell surface. Sorting within the endocytic pathway, regulated by several different ubiquitin ligase proteins, may be involved in ensuring whether ligand and receptor are competent to signal. Furthermore, the utilisation of an alternative mechanism of Notch signalling, independent of Su(H), may depend on driving endocytic Notch into a specific compartment, in response to the activity of the ring finger domain protein, Deltex.

Clathrin-mediated endocytosis and Alzheimer's disease: an update

Thanks to new evidence we are now a step closer to understanding how neurons produce amyloid-beta peptide (Abeta)-the chief culprit of Alzheimer's disease. As importance of clathrin-mediated endocytosis to normal neurons has become clearer, so has its role in pathology of neurological disorders. Here we update recent evidence that endocytosis plays a central role in the production of Abeta in neurons.

Numb regulates post-endocytic trafficking and degradation of Notch1

Notch is a transmembrane receptor that controls cell fate decisions during development and tissue homeostasis. Both activation and attenuation of the Notch signal are tightly regulated by endocytosis. The adaptor protein Numb acts as an inhibitor of Notch and is known to function within the intracellular trafficking pathways. However, a role for Numb in regulating Notch trafficking has not been defined. Here we show that mammalian Notch1 is constitutively internalized and trafficked to both recycling and late endosomal compartments, and we demonstrate that changes in Numb expression alter the dynamics of Notch1 trafficking. Overexpression of Numb promotes sorting of Notch1 through late endosomes for degradation, whereas depletion of Numb facilitates Notch1 recycling. Numb mutants that do not interact with the ubiquitin-protein isopeptide ligase, Itch, or that lack motifs important for interaction with endocytic proteins fail to promote Notch1 degradation. Our data suggest that Numb inhibits Notch1 activity by regulating post-endocytic sorting events that lead to Notch1 degradation.

The Arp2/3 complex and WASp are required for apical trafficking of Delta into microvilli during cell fate specification of sensory organ precursors

Cell fate decisions mediated by the Notch signalling pathway require direct cell-cell contact between adjacent cells. In Drosophila melanogaster, an external sensory organ (ESO) develops from a single sensory organ precursor (SOP) and its fate specification is governed by differential Notch activation. Here we show that mutations in actin-related protein-3 (Arp3) compromise Notch signalling, leading to a fate transformation of the ESO. Our data reveal that during ESO fate specification, most endocytosed vesicles containing the ligand Delta traffic to a prominent apical actin-rich structure (ARS) formed in the SOP daughter cells. Using immunohistochemistry and transmission electron microscopy (TEM) analyses, we show that the ARS contains numerous microvilli on the apical surface of SOP progeny. In Arp2/3 and WASp mutants, the surface area of the ARS is substantially reduced and there are significantly fewer microvilli. More importantly, trafficking of Delta-positive vesicles from the basal area to the apical portion of the ARS is severely compromised. Our data indicate that WASp-dependent Arp2/3 actin polymerization is crucial for apical presentation of Delta, providing a mechanistic link between actin polymerization and Notch signalling.

Notch signaling: the core pathway and its posttranslational regulation

Notch signaling controls numerous cell-fate specification events in multicellular organisms, and dysregulated Notch signaling causes several diseases with underlying developmental defects. A key step in Notch receptor activation is its intramembrane proteolysis, which releases an intracellular fragment that participates directly in transcriptional regulation of nuclear target genes. Despite the apparent simplicity of this mechanism, a host of posttranslational processes regulate Notch activity during its synthesis and secretion, ligand-dependent activation at the surface, endocytic trafficking, and degradation. This review describes the core developmental logic of Notch signaling and how regulatory mechanisms tailor Notch pathway outputs to specific developmental scenarios.

Endocytosis, asymmetric cell division, stem cells and cancer: unus pro omnibus, omnes pro uno

Carcinogenesis can be initiated in adult stem cells, suggesting that tumours arise as a consequence of stem-cell dysfunction. In the fruitfly, cancer arises in stem cells that fail to undergo asymmetric cell division. In flies and mammals, a specific regulation of the endocytic trafficking machinery allows stem cells to self-renew and generate the differentiating cells required to form and maintain mature organs. We review recent findings suggesting that an understanding of the relationship between endocytosis, asymmetric cell division, stem cells and cancer will be crucial to unravel the cell biological basis of tumourigenesis.

The TRIM-NHL protein TRIM32 activates microRNAs and prevents self-renewal in mouse neural progenitors

In the mouse neocortex, neural progenitor cells generate both differentiating neurons and daughter cells that maintain progenitor fate. Here, we show that the TRIM-NHL protein TRIM32 regulates protein degradation and microRNA activity to control the balance between those two daughter cell types. In both horizontally and vertically dividing progenitors, TRIM32 becomes polarized in mitosis and is concentrated in one of the two daughter cells. TRIM32 overexpression induces neuronal differentiation while inhibition of TRIM32 causes both daughter cells to retain progenitor cell fate. TRIM32 ubiquitinates and degrades the transcription factor c-Myc but also binds Argonaute-1 and thereby increases the activity of specific microRNAs. We show that Let-7 is one of the TRIM32 targets and is required and sufficient for neuronal differentiation. TRIM32 is the mouse ortholog of Drosophila Brat and Mei-P26 and might be part of a protein family that regulates the balance between differentiation and proliferation in stem cell lineages.

The molecular programme of tumour reversion: the steps beyond malignant transformation

How cells become malignant has preoccupied scientists for over a century. However, the converse question is also valid: are tumour cells capable of reverting from their malignant state? Askanazy's studies in 1907 indicated that teratoma cells could differentiate into normal somatic tissues and current evidence indicates that some tumour cells have acquired the molecular circuitry that results in the negation of chromosomal instability, translocations, oncogene activation and loss of tumour suppressor genes. Studying these extremely rare events of tumour reversion and deciphering these pathways, which involve SIAH1, presenilin 1, TSAP6 and translationally controlled tumour protein (TCTP), could lead to new avenues in cancer treatment.

Drosophila Past1 is involved in endocytosis and is required for germline development and survival of the adult fly

Endocytosis, which is a key process in eukaryotic cells, has a central role in maintaining cellular homeostasis, nutrient uptake, development and downregulation of signal transduction. This complex process depends on several protein-protein interactions mediated by specific modules. One such module is the EH domain. The EH-domain-containing proteins comprise a family that includes four vertebrate members (EHD1-EHD4) and one Drosophila ortholog, Past1. We used Drosophila as a model to understand the physiological role of this family of proteins. We observed that the two predicted Past1 transcripts are differentially expressed both temporally and spatially during the life cycle of the fly. Endogenous Past1 as well as Past1A and Past1B, expressed from plasmids, were localized mainly to the membrane of Drosophila-derived cells. We generated mutants in the Past1 gene by excising a P-element inserted in it. The Past1 mutants reached adulthood but died precociously. They were temperature sensitive and infertile because of lesions in the reproductive system. Garland cells that originated from Past1 mutants exhibited a marked decrease in their ability to endocytose fluorescently labeled avidin. Genetic interaction was found between Past1 and members of the Notch signaling pathway, suggesting a role for Past1 in this developmentally crucial signaling pathway.

Requirement of Split ends for epigenetic regulation of Notch signal-dependent genes during infection-induced hemocyte differentiation

Drosophila producing a mutant form of the putative transcription coregulator, Split ends (Spen), originally identified in the analysis of neuronal development, display diverse immune defects. In order to understand the role of Spen in the innate immune response, we analyzed the transcriptional defects associated with spen mutant hemocytes and their relationship to the Notch signaling pathways. Spen is regulated by the Notch pathway in the lymph glands and is required for Notch-dependent activation of a large number of genes involved in energy metabolism and differentiation. Analysis of the epigenetic marks associated with Spen-dependent genes indicates that Spen performs its function as a coactivator by regulating chromatin modification. Intriguingly, expression of the Spen-dependent genes was transiently downregulated in a Notch-dependent manner by the Dif activated upon recognition of pathogen-associated molecules, demonstrating the existence of cross talk between hematopoietic regulation and the innate immune response. Our observations reveal a novel connection between the Notch and Toll/IMD signaling pathways and demonstrate a coactivating role for Spen in activating Notch-dependent genes in differentiating cells.

Ero1L, a thiol oxidase, is required for Notch signaling through cysteine bridge formation of the Lin12-Notch repeats in Drosophila melanogaster

Notch-mediated cell–cell communication regulates numerous developmental processes and cell fate decisions. Through a mosaic genetic screen in Drosophila melanogaster, we identified a role in Notch signaling for a conserved thiol oxidase, endoplasmic reticulum (ER) oxidoreductin 1–like (Ero1L). Although Ero1L is reported to play a widespread role in protein folding in yeast, in flies Ero1L mutant clones show specific defects in lateral inhibition and inductive signaling, two characteristic processes regulated by Notch signaling. Ero1L mutant cells accumulate high levels of Notch protein in the ER and induce the unfolded protein response, suggesting that Notch is misfolded and fails to be exported from the ER. Biochemical assays demonstrate that Ero1L is required for formation of disulfide bonds of three Lin12-Notch repeats (LNRs) present in the extracellular domain of Notch. These LNRs are unique to the Notch family of proteins. Therefore, we have uncovered an unexpected requirement for Ero1L in the maturation of the Notch receptor.

Regulation of neurogenesis by interkinetic nuclear migration through an apical-basal notch gradient

The different cell types in the central nervous system develop from a common pool of progenitor cells. The nuclei of progenitors move between the apical and basal surfaces of the neuroepithelium in phase with their cell cycle, a process termed interkinetic nuclear migration (INM). In the retina of zebrafish mikre oko (mok) mutants, in which the motor protein Dynactin-1 is disrupted, interkinetic nuclei migrate more rapidly and deeply to the basal side and more slowly to the apical side. We found that Notch signaling is predominantly activated on the apical side in both mutants and wild-type. Mutant progenitors are, thus, less exposed to Notch and exit the cell cycle prematurely. This leads to an overproduction of early-born retinal ganglion cells (RGCs) at the expense of later-born interneurons and glia. Our data indicate that the function of INM is to balance the exposure of progenitor nuclei to neurogenic versus proliferative signals.

The ENTH and C-terminal domains of Dictyostelium epsin cooperate to regulate the dynamic interaction with clathrin-coated pits

Epsin contains a phospholipid-binding ENTH domain coupled to C-terminal domain motifs that bind coated pit proteins. We examined how these domains interact to influence epsin function and localization in Dictyostelium. Although not required for global clathrin function, epsin was essential for constructing oval spores during development. Within the epsin protein, we found that features important for essential function were distinct from features targeting epsin to clathrin-coated pits. On its own, the phospholipid-binding ENTH domain could rescue the epsin-null phenotype. Although necessary and sufficient for function, the isolated ENTH domain was not targeted within clathrin-coated pits. The C-terminal domain containing the coated-pit motif was also insufficient, highlighting a requirement for both domains for targeting to coated pits. Replacement of the ENTH domain by an alternative membrane-binding domain resulted in epsin that sequestered clathrin and AP2 and ablated clathrin function, supporting a modulatory role for the ENTH domain. Within the ENTH domain, residues important for PtdIns(4,5)P2 binding were essential for both epsin localization and function, whereas residue T107 was essential for function but not coated pit localization. Our results support a model where the ENTH domain coordinates with the clathrin-binding C-terminal domain to allow a dynamic interaction of epsin with coated pits.

Caenorhabditis elegans num-1 negatively regulates endocytic recycling

Much of the material taken into cells by endocytosis is rapidly returned to the plasma membrane by the endocytic recycling pathway. Although recycling is vital for the correct localization of cell membrane receptors and lipids, the molecular mechanisms that regulate recycling are only partially understood. Here we show that in Caenorhabditis elegans endocytic recycling is inhibited by NUM-1A, the nematode Numb homolog. NUM-1AGFP fusion protein is localized to the baso-lateral surfaces of many polarized epithelial cells, including the hypodermis and the intestine. We show that increased NUM-1A levels cause morphological defects in these cells similar to those caused by loss-of-function mutations in rme-1, a positive regulator of recycling in both C. elegans and mammals. We describe the isolation of worms lacking num-1A activity and show that, consistent with a model in which NUM-1A negatively regulates recycling in the intestine, loss of num-1A function bypasses the requirement for RME-1. Genetic epistasis analysis with rab-10, which is required at an early part of the recycling pathway, suggests that loss of num-1A function does not affect the uptake of material by endocytosis but rather inhibits baso-lateral recycling downstream of rab-10.

Numb endocytic adapter proteins regulate the transport and processing of the amyloid precursor protein in an isoform-dependent manner: implications for Alzheimer disease pathogenesis

Central to the pathogenesis of Alzheimer disease is the aberrant processing of the amyloid precursor protein (APP) to generate amyloid beta-peptide (Abeta), the principle component of amyloid plaques. The cell fate determinant Numb is a phosphotyrosine binding domain (PTB)-containing endocytic adapter protein that interacts with the carboxyl-terminal domain of APP. The physiological relevance of this interaction is unknown. Mammals produce four alternatively spliced variants of Numb that differ in the length of their PTB and proline-rich region. In the current study, we determined the influence of the four human Numb isoforms on the intracellular trafficking and processing of APP. Stable expression of Numb isoforms that differ in the PTB but not in the proline-rich region results in marked differences in the sorting of APP to the recycling and degradative pathways. Neural cells expressing Numb isoforms that lack the insert in the PTB (short PTB (SPTB)) exhibited marked accumulation of APP in Rab5A-labeled early endosomal and recycling compartments, whereas those expressing isoforms with the insertion in the PTB (long PTB (LPTB)) exhibited reduced amounts of cellular APP and its proteolytic derivatives relative to parental control cells. Neither the activities of the beta- and gamma-secretases nor the expression of APP mRNA were significantly different in the stably transfected cells, suggesting that the differential effects of the Numb proteins on APP metabolism is likely to be secondary to altered APP trafficking. In addition, the expression of SPTB-Numb increases at the expense of LPTB-Numb in neuronal cultures subjected to stress, suggesting a role for Numb in stress-induced Abeta production. Taken together, these results suggest distinct roles for the human Numb isoforms in APP metabolism and may provide a novel potential link between altered Numb isoform expression and increased Abeta generation.

AAK1 regulates Numb function at an early step in clathrin-mediated endocytosis

Numb is an endocytic protein that is proposed to influence clathrin-coated pit assembly, although its mode of action and the mechanisms that regulate its activity are unknown. In this study, we show that Numb binds to and is phosphorylated by adaptor-associated kinase 1 (AAK1), a key endocytic kinase. We find that AAK1 redistributes Numb to perinuclear endosomes when overexpressed, while kinase depletion causes Numb to accumulate at the plasma membrane. Overexpression of a Numb point mutant (T102A) that lacks the AAK1 phosphorylation site potently disrupts transferrin and low-density lipoprotein internalization but does not impact EGF uptake. Consistent with Numb redistribution results, we find that T102A Numb no longer localizes to perinuclear endosomes. Instead, it is enriched at the plasma membrane where it shows elevated levels of colocalization with coated pit markers. Collectively, these observations demonstrate that Numb endocytic activity is regulated by AAK1 and that phosphorylation may be a critical step in promoting coated pit maturation.

Increased expression of the adult stem cell marker Musashi-1 in endometriosis and endometrial carcinoma

Adult stem cells are thought to be responsible for the high regenerative capacity of the human endometrium, and have been implicated in the pathology of endometriosis and endometrial carcinoma. The RNA-binding protein Musashi-1 is associated with maintenance and asymmetric cell division of neural and epithelial progenitor cells. We investigated expression and localization of Musashi-1 in endometrial, endometriotic and endometrial carcinoma tissue specimens of 46 patients. qPCR revealed significantly increased Musashi-1 mRNA expression in the endometrium compared to the myometrium. Musashi-1 protein expression presented as nuclear or cytoplasmic immunohistochemical staining of single cells in endometrial glands, and of single cells and cell groups in the endometrial stroma. Immunofluorescence microscopy revealed colocalization of Musashi-1 with its molecular target Notch-1 and telomerase. In proliferative endometrium, the proportion of Musashi-1-positive cells in the basalis layer was significantly increased 1.5-fold in the stroma, and three-fold in endometrial glands compared to the functionalis. The number of Musashi-1 expressing cell groups was significantly increased (four-fold) in proliferative compared to secretory endometrium. Musashi-1 expressing stromal cell and cell group numbers were significantly increased (five-fold) in both endometriotic and endometrial carcinoma tissue compared to secretory endometrium. A weak to moderate, diffuse cytoplasmic glandular staining was observed in 50% of the endometriosis cases and in 75% of the endometrioid carcinomas compared to complete absence in normal endometrial samples. Our results emphasize the role of Musashi-1-expressing endometrial progenitor cells in proliferating endometrium, endometriosis and endometrioid uterine carcinoma, and support the concept of a stem cell origin of endometriosis and endometrial carcinoma.

Exosomes from bronchoalveolar fluid of tolerized mice prevent allergic reaction

Exosomes are nanovesicles originating from multivesicular bodies that are secreted by a variety of cell types. The dual capability of exosomes to promote immunity or to induce tolerance has prompted their clinical use as vehicles for vaccination against different human diseases. In the present study, the effect of allergen-specific exosomes from tolerized mice on the development of allergen-induced allergic response was determined using a mouse model. Mice were tolerized by respiratory exposure to the olive pollen allergen Ole e 1. Exosome-like vesicles were isolated from bronchoalveolar lavage fluid of the animals by the well-established filtration and ultracentrifugation procedure, characterized by electron microscopy, Western blot, and FACS analyses, and assessed in a prophylactic protocol. To this end, BALB/c mice were intranasally treated with tolerogenic exosomes or naive exosomes as control, 1 wk before sensitization/challenge to Ole e 1. Blood, lungs, and spleen were collected and analyzed for immune responses. Intranasal administration of tolerogenic exosomes inhibited the development of IgE response, Th2 cytokine production, and airway inflammation--cardinal features of allergy--and maintained specific long-term protection in vivo. This protective effect was associated with a concomitant increase in the expression of the regulatory cytokine TGF-beta. These observations demonstrate that exosomes can induce tolerance and protection against allergic sensitization in mice. Thus, exosome-based vaccines could represent an alternative to conventional therapy for allergic diseases in humans.