No abstract

Little has been understood about the underlying mechanisms that generate the morphological diversity of dendritic trees. Dendritic arborization neurons in Drosophila provide an excellent model system to tackle this question, and they are classified into classes I-IV in order of increasing arbor complexity. Here we have developed transgenic green fluorescent protein markers for class I or class IV cells, which allowed time-lapse recordings of dendritic birth in the embryo, its maturation processes in the larva, and lesion-induced reactions. The two classes used distinct strategies of dendritic emergence from the cell body and branching, which contributed to differences in their basic arbor patterns. In contrast to the class I cells examined, one cell of class IV, which was a focus in this study, continued to elaborate branches throughout larval stages, and it was much more capable of responding to the severing of branches. We also investigated the cellular basis of field formation between adjacent class IV cells. Our results support the fact that class-specific inhibitory interaction is necessary and sufficient for tiling and confirmed that this intercellular communication was at work at individual dendrodendritic interfaces. Finally, this inhibitory signaling appeared to play a central role when arbors of adjacent cells started meeting midway between the cells and until the body wall became partitioned into abutting, minimal-overlapping territories.

Summary In Drosophila embryos and larvae, a small number of identified motor neurons innervate body wall muscles in a highly stereotyped pattern. Although genetic screens have identified many proteins that are required for axon guidance and synaptogenesis in this system, little is known about the mechanisms by which muscle fibers are defined as targets for specific motor axons. To identify potential target labels, we screened 410 genes encoding cell-surface and secreted proteins, searching for those whose overexpression on all muscle fibers causes motor axons to make targeting errors. Thirty such genes were identified, and a number of these were members of a large gene family encoding proteins whose extracellular domains contain leucine-rich repeat (LRR) sequences, which are protein interaction modules. By manipulating gene expression in muscle 12, we showed that four LRR proteins participate in the selection of this muscle as the appropriate synaptic target for the RP5 motor neuron.

Glycosylation modifies protein activities in various biological processes. Here, we report the functions of a novel UDP-sugar transporter (UST74C, an alternative name for Fringe connection (Frc)) localized to the Golgi apparatus in cellular signalling of Drosophila. Mutants in the frc gene exhibit phenotypes resembling wingless and Notch mutants. Both Fringe-dependent and Fringe-independent Notch pathways are affected, and both glycosylation and proteolytic maturation of Notch are defective in mutant larvae. The results suggest that changes in nucleotide-sugar levels can differently affect Wingless and two distinct aspects of Notch signalling.