A dual function of V0-ATPase a1 provides an endolysosomal degradation mechanism in Drosophila melanogaster photoreceptors.

The vesicular adenosine triphosphatase (v-ATPase) is a proton pump that acidifies intracellular compartments. In addition, mutations in components of the membrane-bound v-ATPase V0 sector cause acidification-independent defects in yeast, worm, fly, zebrafish, and mouse. In this study, we present a dual function for the neuron-specific V0 subunit a1 orthologue v100 in Drosophila melanogaster. A v100 mutant that selectively disrupts proton translocation rescues a previously characterized synaptic vesicle fusion defect and vesicle fusion with early endosomes. Correspondingly, V100 selectively interacts with syntaxins on the respective target membranes, and neither synaptic vesicles nor early endosomes require v100 for their acidification. In contrast, V100 is required for acidification once endosomes mature into degradative compartments. As a consequence of the complete loss of this neuronal degradation mechanism, photoreceptors undergo slow neurodegeneration, whereas selective rescue of the acidification-independent function accelerates cell death by increasing accumulations in degradation-incompetent compartments. We propose that V100 exerts a temporally integrated dual function that increases neuronal degradative capacity.

Drosophila acinus encodes a novel regulator of endocytic and autophagic trafficking.

Endosomal trafficking affects many cellular pathways from cell signaling to metabolism, but little is known about how these effects are coordinated. In a genetic screen for mutants affecting endosomal trafficking, we identified Drosophila acinus (dacn; hook-like). Its mammalian homolog Acinus has been implicated in RNA processing and chromatin fragmentation during apoptosis. Loss-of-function analysis of dacn revealed two distinct functions. First, dacn is required for stabilization of early endosomes, thus modulating levels of Notch and Egfr signaling. Second, loss of dacn interferes with cellular starvation responses by inhibiting autophagosome maturation. By contrast, overexpression of dacn causes lethality due to enhanced autophagy. We show that this enhanced autophagy is independent of the Tor pathway. Taken together, our data show that dacn encodes a regulator of endosomal and autophagosomal dynamics, modulating developmental signaling and the cellular response to starvation.

Drosophila Vps16A is required for trafficking to lysosomes and biogenesis of pigment granules.

Mutations that disrupt trafficking to lysosomes and lysosome-related organelles cause multiple diseases, including Hermansky-Pudlak syndrome. The Drosophila eye is a model system for analyzing such mutations. The eye-color genes carnation and deep orange encode two subunits of the Vps-C protein complex required for endosomal trafficking and pigment-granule biogenesis. Here we demonstrate that dVps16A (CG8454) encodes another Vps-C subunit. Biochemical experiments revealed a specific interaction between the dVps16A C-terminus and the Sec1/Munc18 homolog Carnation but not its closest homolog, dVps33B. Instead, dVps33B interacted with a related protein, dVps16B (CG18112). Deep orange bound both Vps16 homologs. Like a deep orange null mutation, eye-specific RNAi-induced knockdown of dVps16A inhibited lysosomal delivery of internalized ligands and interfered with biogenesis of pigment granules. Ubiquitous knockdown of dVps16A was lethal. Together, these findings demonstrate that Drosophila Vps16A is essential for lysosomal trafficking. Furthermore, metazoans have two types of Vps-C complexes with non-redundant functions.

Specification of cell fates within the salivary gland primordium.

The Drosophila salivary gland is a simple tubular organ derived from a contiguous epithelial primordium, which is established by the activities of the homeodomain-containing proteins Sex combs reduced (SCR), Extradenticle (EXD), and Homothorax (HTH). EGF signaling along the ventral midline specifies the salivary duct fate for cells in the center of the primordium, while cells farther away from the source of EGF signal adopt a secretory cell fate. EGF signaling works, at least in part, by repressing expression of secretory cell genes in the duct primordium, including fork head (fkh), which encodes a winged-helix transcription factor. FKH, in turn, represses trachealess (trh), a duct-specific gene initially expressed throughout the salivary gland primordium. trh encodes a basic helix-loop-helix PAS-domain containing transcription factor that has been proposed to specify the salivary duct fate. In conflict with this model, we find that three genes, dead ringer (dri), Serrate (Ser), and trh itself, are expressed in the duct independently of trh. Expression of all three duct genes is repressed in the secretory cells by FKH. We also show that SER in the duct cells signals to the adjacent secretory cells to specify a third cell type, the imaginal ring cells. Thus, localized EGF- and Notch-signaling transform a uniform epithelial sheet into three distinct cell types. In addition, Ser directs formation of actin rings in the salivary duct.