Bazooka/Par3 cooperates with Sanpodo for the assembly of Notch clusters following asymmetric division of Drosophila sensory organ precursor cells

In multiple cell lineages, Delta-Notch signalling regulates cell fate decisions owing to unidirectional signalling between daughter cells. In Drosophila pupal sensory organ lineage, Notch regulates the intra-lineage pIIa/pIIb fate decision at cytokinesis. Notch and Delta that localise apically and basally at the pIIa-pIIb interface are expressed at low levels and their residence time at the plasma membrane is in the order of minutes. How Delta can effectively interact with Notch to trigger signalling from a large plasma membrane area remains poorly understood. Here, we report that the signalling interface possesses a unique apicobasal polarity with Par3/Bazooka localising in the form of nano-clusters at the apical and basal level. Notch is preferentially targeted to the pIIa-pIIb interface, where it co-clusters with Bazooka and its cofactor Sanpodo. Clusters whose assembly relies on Bazooka and Sanpodo activities are also positive for Neuralized, the E3 ligase required for Delta-activity. We propose that the nano-clusters act as snap buttons at the new pIIa-pIIb interface to allow efficient intra-lineage signalling.

The color pattern inducing gene wingless is expressed in specific cell types of campaniform sensilla of a polka-dotted fruit fly, Drosophila guttifera

A polka-dotted fruit fly, Drosophila guttifera, has a unique pigmentation pattern on its wings and is used as a model for evo-devo studies exploring the mechanism of evolutionary gain of novel traits. In this species, a morphogen-encoding gene, wingless, is expressed in species-specific positions and induces a unique pigmentation pattern. To produce some of the pigmentation spots on wing veins, wingless is thought to be expressed in developing campaniform sensilla cells, but it was unknown which of the four cell types there express(es) wingless. Here we show that two of the cell types, dome cells and socket cells, express wingless, as indicated by in situ hybridization together with immunohistochemistry. This is a unique case in which non-neuronal SOP (sensory organ precursor) progeny cells produce Wingless as an inducer of pigmentation pattern formation. Our finding opens a path to clarifying the mechanism of evolutionary gain of a unique wingless expression pattern by analyzing gene regulation in dome cells and socket cells.

Bazooka/Par3 cooperates with Sanpodo for the assembly of Notch signaling clusters following asymmetric division of Drosophila sensory organ precursor cells

SummaryIn multiple cell lineages, Delta-Notch signaling regulates cell fate decisions owing to unidirectional signaling between daughter cells. In Drosophila pupal sensory organ lineage, Notch regulates pIIa/pIIb fate decision at cytokinesis. Notch and Delta that localize apically and basally at the pIIa-pIIb interface, are expressed at low levels and their residence time at the plasma membrane is in the order of the minute. How Delta can effectively interact with Notch to trigger signaling from a large plasma membrane remains poorly understood. Here, we report that the signaling interface possesses a unique apicobasal polarity with Par3/Bazooka localizing in the form of nano-clusters at the apical and basal level. Notch is preferentially targeted to the pIIa-pIIb interface where it co-clusters with Bazooka and the Notch cofactor Sanpodo. Clusters whose assembly relies on Bazooka and Sanpodo activities, are also positive for Neuralized, the E3 ligase required for Delta-activity. We propose that the nano-clusters act as snap buttons at the new pIIa-pIIb interface to allow efficient intra-lineage signaling.

Clathrin adaptor AP-1 and Stratum act in parallel pathways to control Notch activation in Drosophila Sensory Organ Precursor Cells

Drosophila sensory organ precursors divide asymmetrically to generate pIIa/pIIb cells whose identity relies on activation of Notch at cytokinesis. While Notch is present apically and basally relative to the midbody at the pIIa-pIIb interface, the basal pool of Notch is reported to be the main contributor for Notch activation in the pIIa cell. Intra-lineage signaling requires appropriate apico-basal targeting of Notch, its ligand Delta and its trafficking partner Sanpodo. We previously reported that AP-1 and Stratum regulate the trafficking of Notch and Sanpodo from the trans-Golgi network to the basolateral membrane. Loss of AP-1 or Stratum caused mild Notch gain-of-function phenotypes. Here, we report that their concomitant loss results in a penetrant Notch gain-of-function phenotype indicating that they control parallel pathways. While unequal partitioning of cell fate determinants and cell polarity were unaffected, we observed increased amounts of signaling-competent Notch as well as Delta and Sanpodo at the apical pIIa-pIIb interface at the expense of the basal pool of Notch. We propose that AP-1 and Stratum operate in parallel pathways to localize Notch and control where receptor activation takes place.

Clathrin adaptor AP-1 and Stratum act in parallel pathways to control Notch activation in Drosophila Sensory Organ Precursor Cells

Drosophila sensory organ precursors divide asymmetrically to generate pIIa/pIIb cells whose identity relies on the differential activation of Notch at cytokinesis. While Notch is present apically and basally relative to the midbody at the pIIa-pIIb interface, only the basal pool of Notch is reported to contribute to Notch activation in the pIIa cell. Correct intra-lineage signalling requires appropriate apico-basal targeting of Notch, its ligand Delta and its trafficking partner Sanpodo. We previously reported that AP-1 and Stratum regulate the intracellular trafficking of Notch and Sanpodo from the trans-Golgi network to the basolateral membrane. Loss of AP-1 or Stratum caused mild Notch gain-of-function phenotypes. Here, we report that the concomitant loss of AP-1 and Stratum results in a much more penetrant Notch gain-of-function phenotype indicating that AP-1 and Strat control two parallel pathways. While unequal partitioning of cell fate determinants and cell polarity were unaffected, Numb-mediated symmetry breaking is impaired. We further observed increased amounts of signaling competent Notch as well as Delta and Sanpodo at the apical pIIa-pIIb interface and the loss of the basal pool of Notch. We propose that AP-1 and Stratum operate in two parallel pathways to ensure the correct apico-basal localization of Notch controlling where receptor activation takes place.

Numb regulates the balance between Notch recycling and late-endosome targeting inDrosophilaneural progenitor cells

The Notch signaling pathway plays essential roles in both animal development and human disease. Regulation of Notch receptor levels in membrane compartments has been shown to affect signaling in a variety of contexts. Here we used steady-state and pulse-labeling techniques to follow Notch receptors in sensory organ precursor cells in Drosophila. We find that the endosomal adaptor protein Numb regulates levels of Notch receptor trafficking to Rab7-labeled late endosomes but not early endosomes. Using an assay we developed that labels different pools of Notch receptors as they move through the endocytic system, we show that Numb specifically suppresses a recycled Notch receptor subpopulation and that excess Notch signaling in numb mutants requires the recycling endosome GTPase Rab11 activity. Our data therefore suggest that Numb controls the balance between Notch receptor recycling and receptor targeting to late endosomes to regulate signaling output after asymmetric cell division in Drosophila neural progenitors.