Prospero distinguishes sibling cell fate without asymmetric localization in the Drosophila adult external sense organ lineage

The adult external sense organ precursor (SOP) lineage is a model system for studying asymmetric cell division. Adult SOPs divide asymmetrically to produce IIa and IIb daughter cells; IIa generates the external socket (tormogen) and hair (trichogen) cells, while IIb generates the internal neuron and sheath (thecogen) cells. Here we investigate the expression and function of prospero in the adult SOP lineage. Although Prospero is asymmetrically localized in embryonic SOP lineage, this is not observed in the adult SOP lineage: Prospero is first detected in the IIb nucleus and, during IIb division, it is cytoplasmic and inherited by both neuron and sheath cells. Subsequently, Prospero is downregulated in the neuron but maintained in the sheath cell. Loss of prospero function leads to 'double bristle' sense organs (reflecting a IIb-to-IIa transformation) or 'single bristle' sense organs with abnormal neuronal differentiation (reflecting defective IIb development). Conversely, ectopic prospero expression results in duplicate neurons and sheath cells and a complete absence of hair/socket cells (reflecting a IIa-to-IIb transformation). We conclude that (1) despite the absence of asymmetric protein localization, prospero expression is restricted to the IIb cell but not its IIa sibling, (2) prospero promotes IIb cell fate and inhibits IIa cell fate, and (3) prospero is required for proper axon and dendrite morphology of the neuron derived from the IIb cell. Thus, prospero plays a fundamental role in establishing binary IIa/IIb sibling cell fates without being asymmetrically localized during SOP division. Finally, in contrast to previous studies, we find that the IIb cell divides prior to the IIa cell in the SOP lineage.     

The regulation of apoptosis by Numb/Notch signaling in the serotonin lineage of Drosophila

Apoptosis is prevalent during development of the central nervous system (CNS), yet very little is known about the signals that specify an apoptotic cell fate. In this paper, we examine the role of Numb/Notch signaling in the development of the serotonin lineage of Drosophila and show that it is necessary for regulating apoptosis. Our results indicate that when Numb inhibits Notch signaling, cells undergo neuronal differentiation, whereas cells that maintain Notch signaling initiate apoptosis. The apoptosis inhibitor p35 can counteract Notch-mediated apoptosis and rescue cells within the serotonin lineage that normally undergo apoptosis. Furthermore, we observe tumor-like overproliferation of cells in the CNS when Notch signaling is reduced. These data suggest that the distribution of Numb during terminal mitotic divisions of the CNS can distinguish between a neuronal cell fate and programmed cell death.     

Numb and alpha-Adaptin regulate Sanpodo endocytosis to specify cell fate in Drosophila external sensory organs

During asymmetric cell division in Drosophila sensory organ precursors (SOPs), the Numb protein segregates into one of the two daughter cells, in which it inhibits Notch signalling to specify pIIb cell fate. We show here that Numb acts in SOP cells by inducing the endocytosis of Sanpodo, a four-pass transmembrane protein that has previously been shown to regulate Notch signalling in the central nervous system. In sanpodo mutants, SOP cells divide symmetrically into two pIIb cells. We show that Sanpodo is cortical in pIIa, but colocalizes with Notch and Delta in Rab5- and Rab7-positive endocytic vesicles in pIIb. Sanpodo endocytosis requires alpha-Adaptin, a Numb-binding partner involved in clathrin-mediated endocytosis. In numb or alpha-adaptin mutants, Sanpodo is not endocytosed. Surprisingly, this defect is observed already before and during mitosis, which suggests that Numb not only acts in pIIb, but also regulates endocytosis throughout the cell cycle. Numb binds to Sanpodo by means of its phosphotyrosine-binding domain, a region that is essential for Numb function. Our results establish numb- and alpha-adaptin-dependent endocytosis of Sanpodo as the mechanism by which Notch is regulated during external sensory organ development.     

Lethal giant larvae acts together with numb in notch inhibition and cell fate specification in the Drosophila adult sensory organ precursor lineage

The tumor suppressor genes lethal giant larvae (lgl) and discs large (dlg) act together to maintain the apical basal polarity of epithelial cells in the Drosophila embryo. Neuroblasts that delaminate from the embryonic epithelium require lgl to promote formation of a basal Numb and Prospero crescent, which will be asymmetrically segregated to the basal daughter cell upon division to specify cell fate. Sensory organ precursors (SOPs) also segregate Numb asymmetrically at cell division. Numb functions to inhibit Notch signaling and to specify the fates of progenies of the SOP that constitute the cellular components of the adult sensory organ. We report here that, in contrast to the embryonic neuroblast, lgl is not required for asymmetric localization of Numb in the dividing SOP. Nevertheless, mosaic analysis reveals that lgl is required for cell fate specification within the SOP lineage; SOPs lacking Lgl fail to specify internal neurons and glia. Epistasis studies suggest that Lgl acts to inhibit Notch signaling by functioning downstream or in parallel with Numb. These findings uncover a previously unknown function of Lgl in the inhibition of Notch and reveal different modes of action by which Lgl can influence cell fate in the neuroblast and SOP lineages.     

Drosophila glial development is regulated by genes involved in the control of neuronal cell fate

The Drosophila proneural genes specify neuronal determination among cells within the ectoderm. Here we address the question of whether proneural genes also affect the specification of glia, the most abundant cell type in the nervous system. We provide evidence that the proneural gene daughterless is essential for the formation of two major classes of PNS glia. In contrast, the proneural genes in the achaete-scute complex have no detectable effect on the specification and differentiation of these PNS glia and certain CNS glia. We also show that, as with neuronal development, glial determination is restricted by the neurogenic genes neuralized, Delta, and the genes of the Enhancer of split complex. Finally, we demonstrate that prospero, a gene involved in neuronal differentiation, also affects glial development. These results demonstrate extensive overlap in the genetic control of glial and neuronal development.Abbreviations ß galactosidase - (ß-gal) Alkaline phosphatase - (AP) Central nervous system - (CNS) Peripheral nervous system - (PNS) Home domain binding sites - (HDS) Helix-loop-helix - (HLH) Peripheral glia - (PG) Exit glia - (EG) Dorsal roof glia - (DRG) Intersegmental glia - (ISG) Midline glia - (MG) chordotonal - (CH) Sensory mother cell     

Inscuteable maintains type I neuroblast lineage identity via Numb/Notch signaling in the Drosophila larval brain

In the Drosophila larval brain, type I and type Ⅱ neuroblasts(NBs) undergo a series of asymmetric divisions which give rise to distinct progeny lineages. The intermediate neural progenitors(INPs) exist only in type Ⅱ NB lineages. In this study, we reveal a novel function of Inscuteable(Insc) that acts to maintain type I NB lineage identity. In insc type I NB clones of mosaic analyses with a repressible cell marker(MARCM), the formation of extra Deadpan(Dpn)tNB-like and GMC-like cells is observed. The lack of Insc leads to the defective localization and segregation of Numb during asymmetric cell division. By the end of cytokinesis, this results in insufficient Numb in ganglion mother cells(GMCs). The formation of extra Deadpan(Dpn)tcells in insc clones is prevented by the attenuation of Notch activity. This suggests that Insc functions through the Numb/Notch signaling pathway. We also show that in the absence of Insc in type I NB lineages, the cellular identity of GMCs is altered where they adopt an INP-like cell fate as indicated by the initiation of Dpn expression accompanied by a transient presence of Earmuff(Erm).These INP-like cells have the capacity to divide multiple times. We conclude that Insc is necessary for the maintenance of type I NB lineage identity. Genetic manipulations to eliminate most type I NBs with overproliferating type Ⅱ NBs in the larval brain lead to altered circadian rhythms and defective phototaxis in adult flies. This indicates that the homeogenesis of NB lineages is important for the adult's brain function.     

A dual function of phyllopod in Drosophila external sensory organ development: cell fate specification of sensory organ precursor and its progeny

During Drosophila external sensory organ development, one sensory organ precursor (SOP) arises from a proneural cluster, and undergoes asymmetrical cell divisions to produce an external sensory (es) organ made up of different types of daughter cells. We show that phyllopod (phyl), previously identified to be essential for R7 photoreceptor differentiation, is required in two stages of es organ development: the formation of SOP cells and cell fate specification of SOP progeny. Loss-of-function mutations in phyl result in failure of SOP formation, which leads to missing bristles in adult flies. At a later stage of es organ development, phyl mutations cause the first cell division of the SOP lineage to generate two identical daughters, leading to the fate transformation of neurons and sheath cells to hair cells and socket cells. Conversely, misexpression of phyl promotes ectopic SOP formation, and causes opposite fate transformation in SOP daughter cells. Thus, phyl functions as a genetic switch in specifying the fate of the SOP cells and their progeny. We further show that seven in absentia (sina), another gene required for R7 cell fate differentiation, is also involved in es organ development. Genetic interactions among phyl, sina and tramtrack (ttk) suggest that phyl and sina function in bristle development by antagonizing ttk activity, and ttk acts downstream of phyl. It has been shown previously that Notch (N) mutations induce formation of supernumerary SOP cells, and transformation from hair and socket cells to neurons. We further demonstrate that phyl acts epistatically to N. phyl is expressed specifically in SOP cells and other neural precursors, and its mRNA level is negatively regulated by N signaling. Thus, these analyses demonstrate that phyl acts downstream of N signaling in controlling cell fates in es organ development.     

Wasp, the Drosophila Wiskott-Aldrich syndrome gene homologue, is required for cell fate decisions mediated by Notch signaling

Wiskott-Aldrich syndrome proteins, encoded by the Wiskott-Aldrich syndrome gene family, bridge signal transduction pathways and the microfilament-based cytoskeleton. Mutations in the Drosophila homologue, Wasp (Wsp), reveal an essential requirement for this gene in implementation of cell fate decisions during adult and embryonic sensory organ development. Phenotypic analysis of Wsp mutant animals demonstrates a bias towards neuronal differentiation, at the expense of other cell types, resulting from improper execution of the program of asymmetric cell divisions which underlie sensory organ development. Generation of two similar daughter cells after division of the sensory organ precursor cell constitutes a prominent defect in the Wsp sensory organ lineage. The asymmetric segregation of key elements such as Numb is unaffected during this division, despite the misassignment of cell fates. The requirement for Wsp extends to additional cell fate decisions in lineages of the embryonic central nervous system and mesoderm. The nature of the Wsp mutant phenotypes, coupled with genetic interaction studies, identifies an essential role for Wsp in lineage decisions mediated by the Notch signaling pathway.     

Cell lineage and cell fate specification in the embryonic CNS of Drosophila

The Drosophila CNS derives from a population of neural stem cells, called neuroblasts (NBs), which delaminate individually from the neurogenic region of the ectoderm. In the embryonic ventral nerve cord each NB can be uniquely identified and gives rise to a specific lineage consisting of neurons and/or glial cells. This 'NB identity' is dependent on the position of the progenitor cells in the neuroectoderm before delamination. The positional information is provided by the products of segment polarity and dorsoventral (D/V) patterning genes. Subsequently, 'cell fate genes' like huckebein (hkb) and eagle (eg) contribute to the generation of specific NB lineages. These genes act downstream of segment polarity and D/V patterning genes and regulate different processes such as the generation of glial cells and the determination of serotonergic neurons.     

The Notch gene, adhesion, and developmental fate in the Drosophila embryo

The Notch locus in Drosophila encodes a transmembrane protein required for the determination of cell fate in ectodermal cells. When these cells are faced with a choice of two possible fates, Notch enables some of them to advance from the fate that represents a "default" state. The decision appears to involve both an interaction between cells and the presence of the Notch product on the surface of those advancing from the default state. The timing of gene action suggests that Notch plays a role in the stabilization of the choice of cell fate and that the Notch-mediated interaction occurs between presumptive epidermal cells. Several properties of the Notch product are characteristic of an adhesion molecule, and thus cell adhesion may play a role in the determination of cell fate.     

Mother-daughter precursor cell fate transformation after Cdc2 down-regulation in the Drosophila bristle lineage

The Drosophila bristle lineage is an excellent system in which to study how cell cycle and fate determination are synchronized in invariant cell lineages. In this model, five different cells arise from a single precursor cell, pI, after four asymmetric cell divisions. Cell diversity is achieved by the asymmetric segregation of cell determinants, such as Numb and Neuralized (Neur), resulting in differential activation of the Notch (N) pathway. We show that down-regulation of Cdc2, by over-expressing Tribbles, Dwee1, and Dmyt1 (three negative regulators of Cdc2) or by using thermo-sensitive Cdc2 mutant flies, delayed pI mitosis, and altered the polarity and the number of subsequent cell divisions. These modifications were associated with a mother-daughter cell fate transformation as the pI cell acquired the identity of the secondary precursor cell, pIIb. This type of change in cell identity only occurred when the N signaling pathway was inactive since ectopic N signaling transformed pI to pIIa-progeny fate. These transformations in cell identity suggest that, although synchronized, cell cycle and fate determination are independent phenomena in the bristle lineage.     

Two types of asymmetric divisions in the Drosophila sensory organ precursor cell lineage

Asymmetric partitioning of cell-fate determinants during development requires coordinating the positioning of these determinants with orientation of the mitotic spindle. In the Drosophila peripheral nervous system, sensory organ progenitor cells (SOPs) undergo several rounds of division to produce five cells that give rise to a complete sensory organ. Here we have observed the asymmetric divisions that give rise to these cells in the developing pupae using green fluorescent protein fusion proteins. We find that spindle orientation and determinant localization are tightly coordinated at each division. Furthermore, we find that two types of asymmetric divisions exist within the sensory organ precursor cell lineage: the anterior-posterior pI cell-type division, where the spindle remains symmetric throughout mitosis, and the strikingly neuroblast-like apical-basal division of the pIIb cell, where the spindle exhibits a strong asymmetry at anaphase. In both these divisions, the spindle reorientates to position itself perpendicular to the region of the cortex containing the determinant. On the basis of these observations, we propose that two distinct mechanisms for controlling asymmetric cell divisions occur within the same lineage in the developing peripheral nervous system in Drosophila.     

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.