Regulation of notch signaling activity

Cell-cell signalling mediated by the receptor Notch is used widely across the metazoans to determine cell fate and regulate pattern formation. Notch signals via a conserved regulated intramembrane proteolysis. Recent analyses of the cell biology of the Notch receptor have identified several fundamental mechanisms that contribute to regulate Notch signaling activity in space and time.

[Cell polarity and asymmetric division in the peripheral nervous system of Drosophila]

During metazoan development, cell fate diversity is generated in part by asymmetric cell divisions, in which mother cells divide to produce two daughter cells with distinct developmental potentials. Adoption of different cell fates often relies on the polarised distribution and unequal segregation of cell-fate determinants. Unequal segregation of cell-fate determinants requires that the mother cell becomes polarised prior to mitosis. In response to this polarisation, cell-fate determinants localise asymmetrically and the mitotic spindle lines up with the pole to which cell-fate determinants accumulate, thereby leading to their unequal partitioning upon cytokinesis. I review here the regulatory mechanisms that establish cell asymmetry and orient this asymmetry relative to the body axis in the sensory organ lineages of Drosophila.

Cell polarity: the ups and downs of the Par6/aPKC complex

A signaling complex in which atypical protein kinase C associates with a regulatory protein, Par6, plays an essential role in establishing cell polarity. Recent studies in organisms ranging from worms to mammals have highlighted some of the conserved mechanisms by which the assembly, localization and activity of this complex are regulated. Recent work is also beginning to unravel how this complex acts in concert with additional molecular complexes to establish and maintain polarity.

Unequal segregation of Neuralized biases Notch activation during asymmetric cell division

In Drosophila, Notch signaling regulates binary fate decisions at each asymmetric division in sensory organ lineages. Following division of the sensory organ precursor cell (pI), Notch is activated in one daughter cell (pIIa) and inhibited in the other (pIIb). We report that the E3 ubiquitin ligase Neuralized localizes asymmetrically in the dividing pI cell and unequally segregates into the pIIb cell, like the Notch inhibitor Numb. Furthermore, Neuralized upregulates endocytosis of the Notch ligand Delta in the pIIb cell and acts in the pIIb cell to promote activation of Notch in the pIIa cell. Thus, Neuralized is a conserved regulator of Notch signaling that acts as a cell fate determinant. Polarization of the pI cell directs the unequal segregation of both Neuralized and Numb. We propose that coordinated upregulation of ligand activity by Neuralized and inhibition of receptor activity by Numb results in a robust bias in Notch signaling.

Notch signaling: endocytosis makes delta signal better

Endocytosis of cell surface receptors is involved in down-regulation of receptor activity. Recent findings indicate that, paradoxically, endocytosis of a membrane-spanning ligand may up-regulate receptor activity: the zebrafish E3 ligase Mind bomb promotes the endocytosis of Delta and is required for efficient activation of Notch.

Snail is required for Delta endocytosis and Notch-dependent activation of single-minded expression

In the Drosophila embryo, the mesectoderm corresponds to a single row of cells abutting the mesoderm. It is specified by the expression of the single-minded (sim) gene. The information that precisely positions the sim-expressing cells along the dorso-ventral axis is incompletely understood. Previous studies have shown that Dorsal and Twist activate sim expression in a large ventral domain, while two negative regulators, Snail (Sna) and Suppressor of Hairless [Su(H)], repress sim expression in the mesoderm and neuroectoderm, respectively. Repression by Su(H) is relieved in the presumptive mesectoderm by Notch signaling. In this paper, we show that Sna also has a positive regulatory function on sim expression in the presumptive mesectoderm. This positive effect of Sna depends on the Su(H)-binding sites within the sim promoter, suggesting that Sna regulates Notch signaling. In addition, we find that Delta is endocytosed together with the extracellular domain of Notch. The endocytosis of Delta is only seen in the mesoderm and requires the activity of the sna and neuralized (neur) genes. Interestingly, the Neur-mediated endocytosis of Delta has recently been shown to be sufficient for the non-autonomous activation of Notch target genes in wing imaginal discs. We therefore propose a novel model in which Sna positions the mesectoderm via its dual regulatory activity. In this model, Sna cell-autonomously represses sim expression in the mesoderm and relieves Su(H)-dependent repression in a cell non-autonomous fashion by promoting the Neur-dependent endocytosis of Delta in the mesoderm.

Binary cell death decision regulated by unequal partitioning of Numb at mitosis

An important issue in Metazoan development is to understand the mechanisms that lead to stereotyped patterns of programmed cell death. In particular, cells programmed to die may arise from asymmetric cell divisions. The mechanisms underlying such binary cell death decisions are unknown. We describe here a Drosophila sensory organ lineage that generates a single multidentritic neuron in the embryo. This lineage involves two asymmetric divisions. Following each division, one of the two daughter cells expresses the pro-apoptotic genes reaper and grim and subsequently dies. The protein Numb appears to be specifically inherited by the daughter cell that does not die. Numb is necessary and sufficient to prevent apoptosis in this lineage. Conversely, activated Notch is sufficient to trigger death in this lineage. These results show that binary cell death decision can be regulated by the unequal segregation of Numb at mitosis. Our study also indicates that regulation of programmed cell death modulates the final pattern of sensory organs in a segment-specific manner.

Drosophila E-cadherin regulates the orientation of asymmetric cell division in the sensory organ lineage

BACKGROUND

Generation of cell-fate diversity in Metazoan depends in part on asymmetric cell divisions in which cell-fate determinants are asymmetrically distributed in the mother cell and unequally partitioned between daughter cells. The polarization of the mother cell is a prerequisite to the unequal segregation of cell-fate determinants. In the Drosophila bristle lineage, two distinct mechanisms are known to define the axis of polarity of the pI and pIIb cells. Frizzled (Fz) signaling regulates the planar orientation of the pI division, while Inscuteable (Insc) directs the apical-basal polarity of the pIIb cell. The orientation of the asymmetric division of the pIIa cell is identical to the one of its mother cell, the pI cell, but, in contrast, is regulated by an unknown Insc- and Fz-independent mechanism.

RESULTS

DE-Cadherin-Catenin complexes are shown to localize at the cell contact between the two cells born from the asymmetric division of the pI cell. The mitotic spindle of the dividing pIIa cell rotates to line up with asymmetrically localized DE-Cadherin-Catenin complexes. While a complete loss of DE-Cadherin function disrupts the apical-basal polarity of the epithelium, both a partial loss of DE-Cadherin function and expression of a dominant-negative form of DE-Cadherin affect the orientation of the pIIa division. Furthermore, expression of dominant-negative DE-Cadherin also affects the position of Partner of Inscuteable (Pins) and Bazooka, two asymmetrically localized proteins known to regulate cell polarity. These results show that asymmetrically distributed Cad regulates the orientation of asymmetric cell division.

CONCLUSIONS

We describe a novel mechanism involving a specialized Cad-containing cortical region by which a daughter cell divides with the same orientation as its mother cell.

The Partner of Inscuteable/Discs-large complex is required to establish planar polarity during asymmetric cell division in Drosophila

Frizzled (Fz) signaling regulates cell polarity in both vertebrates and invertebrates. In Drosophila, Fz orients the asymmetric division of the sensory organ precursor cell (pI) along the antero-posterior axis of the notum. Planar polarization involves a remodeling of the apical-basal polarity of the pI cell. The Discs-large (Dlg) and Partner of Inscuteable (Pins) proteins accumulate at the anterior cortex, while Bazooka (Baz) relocalizes to the posterior cortex. Dlg interacts directly with Pins and regulates the localization of Pins and Baz. Pins acts with Fz to localize Baz posteriorly, but Baz is not required to localize Pins anteriorly. Finally, Baz and the Dlg/Pins complex are required for the asymmetric localization of Numb. Thus, the Dlg/Pins complex responds to Fz signaling to establish planar asymmetry in the pI cell.

Lineage diversity in the Drosophila nervous system

The detailed descriptions of cellular lineages in the Drosophila nervous system have provided the foundations for an in-depth genetic analysis of the mechanisms that regulate fate decisions at every cell cycle.

Transcriptional repression by Suppressor of Hairless involves the binding of a Hairless-dCtBP complex in Drosophila

Notch is the receptor for a conserved signaling pathway that regulates numerous cell fate decisions during development [1]. Signal transduction involves the presenilin-dependent intracellular processing of Notch and the nuclear translocation of the intracellular domain of Notch, NICD [2-6]. NICD associates with Suppressor of Hairless [Su(H)], a DNA binding protein, and Mastermind (Mam), a transcriptional coactivator [7-9]. In the absence of Notch signaling, Su(H) acts as a transcriptional repressor [10, 11]. Repression by Su(H) is relieved by the activation of Notch [12-16]. In the Drosophila embryo, this transcriptional switch from repression to activation is important for patterning the expression of the single-minded (sim) gene along the dorsoventral axis [12]. Here, we investigate the mechanisms by which Su(H) inhibits the expression of Notch target genes in Drosophila. We show that Hairless, an antagonist of Notch signaling [17-19], is required to repress the transcription of the sim gene. Hairless forms a DNA-bound complex with Su(H). Furthermore, it directly binds the Drosophila C-terminal Binding Protein (dCtBP), which acts as a transcriptional corepressor. The dCtBP binding motif of Hairless is essential for the function of Hairless in vivo. We propose that Hairless mediates transcriptional repression by Su(H) via the recruitment of dCtBP.

Lineage, cell polarity and inscuteable function in the peripheral nervous system of the Drosophila embryo

The stereotyped pattern of the Drosophila embryonic peripheral nervous system (PNS) makes it an ideal system to use to identify mutations affecting cell polarity during asymmetric cell division. However, the characterisation of such mutations requires a detailed description of the polarity of the asymmetric divisions in the sensory organ lineages. We describe the pattern of cell divisions generating the vp1-vp4a mono-innervated external sense (es) organs. Each sensory organ precursor (SOP) cell follows a series of four asymmetric cell divisions that generate the four es organs cells (the socket, shaft, sheath cells and the es neurone) together with one multidendritic (md) neurone. This lineage is distinct from any of the previously proposed es lineages. Strikingly, the stereotyped pattern of cell divisions in this lineage is identical to those described for the embryonic chordotonal organ lineage and for the adult thoracic bristle lineage. Our analysis reveals that the vp2-vp4a SOP cells divide with a planar polarity to generate a dorsal pIIa cell and a ventral pIIb cell. The pIIb cell next divides with an apical-basal polarity to generate a basal daughter cell that differentiates as an md neurone. We found that Inscuteable specifically accumulated at the apical pole of the dividing pIIb cell and regulated the polarity of the pIIb division. This study establishes for the first time the function of Inscuteable in the PNS, and provides the basis for studying the mechanisms controlling planar and apical-basal cell polarities in the embryonic sensory organ lineages.

Frizzled regulates localization of cell-fate determinants and mitotic spindle rotation during asymmetric cell division

Cell-fate diversity is generated in part by the unequal segregation of cell-fate determinants during asymmetric cell divisions. In the Drosophila pupa, the pI sense organ precursor cell is polarized along the anterior-posterior axis of the fly and divides asymmetrically to generate a posterior pIIa cell and an anterior pIIb cell. The anterior pIIb cell specifically inherits the determinant Numb and the adaptor protein Partner of Numb (Pon). By labelling both the Pon crescent and the microtubules in living pupae, we show that determinants localize at the anterior cortex before mitotic-spindle formation, and that the spindle forms with random orientation and rotates to line up with the Pon crescent. By imaging living frizzled (fz) mutant pupae we show that Fz regulates the orientation of the polarity axis of pI, the initiation of spindle rotation and the unequal partitioning of determinants. We conclude that Fz participates in establishing the polarity of pI.

Cell polarity: fixing cell polarity with Pins

A protein complex is assembled in a step-wise manner at the apical pole of Drosophila neuroblasts. This complex organizes the apical-basal polarity of asymmetrically dividing neuroblasts, and may act via G-protein signalling.

Repression by suppressor of hairless and activation by Notch are required to define a single row of single-minded expressing cells in the Drosophila embryo

Notch signal transduction appears to involve the ligand-induced intracellular processing of Notch, and the formation of a processed Notch-Suppressor of Hairless complex that binds DNA and activates the transcription of Notch target genes. This suggests that loss of either Notch or Su(H) activities should lead to similar cell fate changes. However, previous data indicate that, in the Drosophila blastoderm embryo, mesectoderm specification requires Notch but not Su(H) activity. The determination of the mesectodermal fate is specified by Single-minded (Sim), a transcription factor expressed in a single row of cells abutting the mesoderm. The molecular mechanisms by which the dorsoventral gradient of nuclear Dorsal establishes the single-cell wide territory of sim expression are not fully understood. We have found that Notch activity is required for sim expression in cellularizing embryos. In contrast, at this stage, Su(H) has a dual function. Su(H) activity was required to up-regulate sim expression in the mesectoderm, and to prevent the ectopic expression of sim dorsally in the neuroectoderm. We have shown that repression of sim transcription by Su(H) is direct and independent of Notch activity. Conversely, activation of sim transcription by Notch requires the Su(H)-binding sites. Thus, Notch signalling appears to relieve the repression exerted by Su(H) and to up-regulate sim transcription in the mesectoderm. We propose a model in which repression by Su(H) and derepression by Notch are essential to allow for the definition of a single row of mesectodermal cells in the blastoderm embryo.

Repression by Suppressor of Hairless and activation by Notch are required to define a single row of single-minded expressing cells in the Drosophila embryo

Notch signal transduction appears to involve the ligand-induced intracellular processing of Notch, and the formation of a processed Notch-Suppressor of Hairless complex that binds DNA and activates the transcription of Notch target genes. This suggests that loss of eitherNotch or Su(H) activities should lead to similar cell fate changes. However, previous data indicate that, in theDrosophila blastoderm embryo, mesectoderm specification requires Notch but not Su(H) activity. The determination of the mesectodermal fate is specified by Single-minded (Sim), a transcription factor expressed in a single row of cells abutting the mesoderm. The molecular mechanisms by which the dorsoventral gradient of nuclear Dorsal establishes the single-cell wide territory of sim expression are not fully understood. We have found that Notch activity is required for simexpression in cellularizing embryos. In contrast, at this stage,Su(H) has a dual function. Su(H) activity was required to up-regulate sim expression in the mesectoderm, and to prevent the ectopic expression of sim dorsally in the neuroectoderm. We have shown that repression of simtranscription by Su(H) is direct and independent of Notchactivity. Conversely, activation of sim transcription by Notch requires the Su(H)-binding sites. Thus, Notch signalling appears to relieve the repression exerted by Su(H) and to up-regulate simtranscription in the mesectoderm. We propose a model in which repression by Su(H) and derepression by Notch are essential to allow for the definition of a single row of mesectodermal cells in the blastoderm embryo.

Covalent modification of the transcriptional repressor tramtrack by the ubiquitin-related protein Smt3 in Drosophila flies

The ubiquitin-related SUMO-1 modifier can be covalently attached to a variety of proteins. To date, four substrates have been characterized in mammalian cells: RanGAP1, IkappaBalpha, and the two nuclear body-associated PML and Sp100 proteins. SUMO-1 modification has been shown to be involved in protein localization and/or stabilization and to require the activity of specialized E1-activating and E2 Ubc9-conjugating enzymes. SUMO-1 homologues have been identified in various species and belong to the so-called Smt3 family of proteins. Here we have characterized the Drosophila homologues of mammalian SUMO-1 and Ubc9 (termed dSmt3 and dUbc9, respectively). We show that dUbc9 is the conjugating enzyme for dSmt3 and that dSmt3 can covalently modify a number of proteins in Drosophila cells in addition to the human PML substrate. The dSmt3 transcript and protein are maternally deposited in embryos, where the protein accumulates predominantly in nuclei. Similar to its human counterpart, dSmt3 protein is observed in a punctate nuclear pattern. We demonstrate that Tramtrack 69 (Ttk69), a repressor of neuronal differentiation, is a bona fide in vivo substrate for dSmt3 conjugation. Finally, we show that both the modified and unmodified forms of Ttk69 can bind to a Ttk69 binding site in vitro. Moreover, dSmt3 and Ttk69 proteins colocalize on polytene chromosomes, indicating that the dSmt3-conjugated Ttk69 species can bind at sites of Ttk69 action in vivo. Altogether, these data indicate a high conservation of the Smt3 conjugation pathway and further suggest that this mechanism may play a role in the transcriptional regulation of cell differentiation in Drosophila flies.

Dominant-negative mutation in the beta2 and beta6 proteasome subunit genes affect alternative cell fate decisions in the Drosophila sense organ lineage

In Drosophila, dominant-negative mutations in the beta2 and beta6 proteasome catalytic subunit genes have been identified as dominant temperature-sensitive (DTS) mutations. At restrictive temperature, beta2 and beta6 DTS mutations confer lethality at the pupal stage. I investigate here the role of proteasome activity in regulating cell fate decisions in the sense organ lineage at the early pupal stage. Temperature-shift experiments in beta2 and beta6 DTS mutant pupae occasionally resulted in external sense organs with two sockets and no shaft. This double-socket phenotype was strongly enhanced in conditions in which Notch signaling was up-regulated. Furthermore, conditional overexpression of the beta6 dominant-negative mutant subunit led to shaft-to-socket and to neuron-to-sheath cell fate transformations, which are both usually associated with increased Notch signaling activity. Finally, expression of the beta6 dominant-negative mutant subunit led to the stabilization of an ectopically expressed nuclear form of Notch in imaginal wing discs. This study demonstrates that mutations affecting two distinct proteasome catalytic subunits affect two alternative cell fate decisions and enhance Notch signaling activity in the sense organ lineage. These findings raise the possibility that the proteasome targets an active form of the Notch receptor for degradation in Drosophila.

Revisiting the Drosophila microchaete lineage: a novel intrinsically asymmetric cell division generates a glial cell

The bristle mechanosensory organs of the adult fly are composed of four different cells that originate from a single precursor cell, pI, via two rounds of asymmetric cell division. Here, we have examined the pattern of cell divisions in this lineage by time-lapse confocal microscopy using GFP imaging and by immunostaining analysis. pI divided within the plane of the epithelium and along the anteroposterior axis to give rise to an anterior cell, pIIb, and a posterior cell, pIIa. pIIb divided prior to pIIa to generate a small subepithelial cell and a larger daughter cell, named pIIIb. This unequal division, oriented perpendicularly to the epithelium plane, has not been described previously. pIIa divided after pIIb, within the plane of the epithelium and along the AP axis, to produce a posterior socket cell and an anterior shaft cell. Then pIIIb divided perpendicularly to the epithelium plane to generate a basal neurone and an apical sheath cell. The small subepithelial pIIb daughter cell was identified as a sense organ glial cell: it expressed glial cell missing, a selector gene for the glial fate and migrated away from the sensory cluster along extending axons. We propose that mechanosensory organ glial cells, the origin of which was until now unknown, are generated by the asymmetric division of pIIb cells. Both Numb and Prospero segregated specifically into the basal glial and neuronal cells during the pIIb and pIIIb divisions, respectively. This revised description of the sense organ lineage provides the basis for future studies on how polarity and fate are regulated in asymmetrically dividing cells.

Presenilins in their infancy

Familial forms of Alzheimer's disease are caused by mutations in the genes encoding the presenilins, which are integral membrane proteins. Presenilins have been shown to interact with beta-amyloid precursor proteins and Notch receptors. Several recent studies have examined the role of presenilins in Notch processing.

Indirect evidence for Delta-dependent intracellular processing of notch in Drosophila embryos

Cell-cell signaling mediated by the receptor Notch regulates the differentiation of a wide variety of cell types in invertebrate and vertebrate species, but the mechanism of signal transduction following receptor activation is unknown. A recent model proposes that ligand binding induces intracellular processing of Notch; the processed intracellular form of Notch then translocates to the nucleus and interacts with DNA-bound Suppressor of Hairless (Su(H)), a transcription factor required for target gene expression. As intracellular processing of endogenous Notch has so far escaped immunodetection, we devised a sensitive nuclear-activity assay to monitor indirectly the processing of an engineered Notch in vivo. First, we show that the intracellular domain of Notch, fused to the DNA-binding domain of Gal4, regulated transcription, in a delta-independent manner. Second, we show that full-length Notch, containing the Gal4 DNA-binding domain inserted 27 amino acids carboxy-terminal to the transmembrane domain, activated transcription in a delta-dependent manner. These results provide indirect evidence for a ligand-dependent intracellular processing event in vivo, supporting the view that Su(H)-dependent Notch signaling involves intracellular cleavage, and transcriptional regulation by processed Notch.

Frizzled signalling controls orientation of asymmetric sense organ precursor cell divisions in Drosophila

During metazoan development, cell-fate diversity is brought about, in part, by asymmetric cell divisions. In Drosophila, bristle mechanosensory organs are composed of four different cells that originate from a single precursor cell, pI, after two rounds of asymmetric division. At each division, distinct fates are conferred on sister cells by the asymmetric segregation of Numb, a negative regulator of Notch signalling. Here we show that the orientation of the mitotic spindles and the localization of the Numb crescent follow a stereotyped pattern. Mitosis of pI is orientated parallel to the anteroposterior axis of the fly. We show that signalling mediated by the Frizzled receptor polarizes pI along this axis, thereby specifying the orientation of the mitotic spindle and positioning the Numb crescent. The mitoses of the two cells produced by mitosis of pI are orientated parallel and orthogonal, respectively, to the division axis of pI. This difference in cell-division orientation is largely independent of the identity of the secondary precursor cells, and is regulated by Frizzled-independent mechanisms.

The activity of Drosophila Hairless is required in pupae but not in embryos to inhibit Notch signal transduction

Drosophila Hairless (H) encodes a negative regulator of Notch signalling. H activity antagonizes Notch (N) signalling during bristle development at the pupal stage. We show here by clonal analysis that H acts by inhibiting signal transduction rather than by promoting signal production, during both selection of microchaete precursors in the notum and vein cell differentiation in the wing. Allele-specific interactions further suggest that H inhibits Notch signal transduction by interacting directly with Suppressor of Hairless. Unexpectedly, this regulatory function of H appears to be essential only during imaginal development. Using a null allele of H that corresponds to a deletion of the H coding sequence, we show that embryos devoid of both maternal and zygotic gene products develop similarly to wild-type embryos. Thus, H activity is not strictly required to regulate N-mediated cell fate choices in the embryo.

Role of suppressor of hairless in the delta-activated Notch signaling pathway

The Notch protein (N) acts as a transmembrane receptor for intercellular signals controlling cell fate choices in vertebrates and invertebrates. Genetical and molecular evidence indicates that, during Drosophila neurogenesis, an evolutionarily conserved transcription factor, Suppressor of Hairless [Su(H)], transduces the signal of N activation by its ligand Delta (D1). Su(H) plays a direct role in the immediate response of the genome to N signaling by up-regulating the transcription of the Enhancer of split Complex [E(spl)-C] genes. These findings suggest that the N transduction pathway can be described as a simple, linear cascade of molecular activation. At the molecular level, the mechanism of Su(H) "activation" is yet unknown. Two non-exclusive models have been proposed. In the first one, Su(H) binds to inactive N at the membrane. The binding of D1 to N in the extracellular space somehow interferes with the N-mediated cytoplasmic retention of Su(H), resulting in the nuclear translocation and "activation" of Su(H). In the second model, DNA-bound Su(H) is proposed to be "activated" in the nucleus by the direct binding of a processed form of N, acting as a transcriptional coactivator. This nuclear N protein would be generated by the ligand-induced proteolytic cleavage of the N transmembrane receptor.

[Signalling by Notch family receptors]

From nematode to man, the transmembrane receptors of the Notch family act throughout embryonic and post-embryonic development to regulate the acquisition and/or maintenance of specific differentiative states. We will review here our current state of knowledge on Notch receptors structure and signalling activity.