Bendless alters thoracic musculature in Drosophila

Single-cell type analysis of wing premotor circuits in the ventral nerve cord of Drosophila melanogaster

To perform most behaviors, animals must send commands from higher-order processing centers in the brain to premotor circuits that reside in ganglia distinct from the brain, such as the mammalian spinal cord or insect ventral nerve cord. How these circuits are functionally organized to generate the great diversity of animal behavior remains unclear. An important first step in unraveling the organization of premotor circuits is to identify their constituent cell types and create tools to monitor and manipulate these with high specificity to assess their functions. This is possible in the tractable ventral nerve cord of the fly. To generate such a toolkit, we used a combinatorial genetic technique (split-GAL4) to create 195 sparse transgenic driver lines targeting 196 individual cell types in the ventral nerve cord. These included wing and haltere motoneurons, modulatory neurons, and interneurons. Using a combination of behavioral, developmental, and anatomical analyses, we systematically characterized the cell types targeted in our collection. In addition, we identified correspondences between the cells in this collection and a recent connectomic data set of the ventral nerve cord. Taken together, the resources and results presented here form a powerful toolkit for future investigations of neuronal circuits and connectivity of premotor circuits while linking them to behavioral outputs.

Drosophila Uev1a is dually required for Ben-dependent DNA-damage response and fly mobility

K63-linked polyubiquitination requires the ubiquitin-conjugating enzyme Ubc13 and a Ubc/E2 variant Uev. Lower eukaryotic organisms contain one UEV gene required for DNA-damage tolerance, while vertebrates and higher plants contain multiple UEV genes with distinct functions. In contrast, Drosophila contains only one UEV gene designated dUev1a. Here we report that dUev1a forms a stable heterodimer with Ben, the Drosophila Ubc13 ortholog, that dUev1a-F15E completely abolishes the interaction, and that a conserved dUev1a-F15Y substitution severely reduces its interaction with Ben. dUev1a functionally rescues the corresponding yeast mms2 null mutant from killing by various DNA-damaging agents in a Ben-dependent manner, and the heterozygous dUev1a mutant flies are more sensitive to DNA-damaging agent, indicating that the function of UEV in DNA-damage response is conserved throughout eukaryotes. Meanwhile, dUev1a^+/- mutant flies displayed reduced mobility characteristic of defects in the central nervous system and reminiscent of the bendless phenotypes, suggesting that dUev1a acts together with Ben in this process. Our observations collectively imply that dUev1a is dually required for DNA-damage response and neurological signaling in Drosophila, and that these processes are mediated by the Ben-dUev1a complex that promotes K63-linked polyubiquitination.

Ubc13: the Lys63 ubiquitin chain building machine

Ubc13 is an ubiquitin E2 conjugating enzyme that participates with many different E3 ligases to form lysine 63-linked (Lys63) ubiquitin chains that are critical to signaling in inflammatory and DNA damage response pathways. Recent studies have suggested Ubc13 as a potential therapeutic target for intervention in various human diseases including several different cancers, alleviation of anti-cancer drug resistance, chronic inflammation, and viral infections. Understanding a potential therapeutic target from different angles is important to assess its usefulness and potential pitfalls. Here we present a global review of Ubc13 from its structure, function, and cellular activities, to its natural and chemical inhibition. The aim of this article is to review the literature that directly implicates Ubc13 in a biological function, and to integrate structural and mechanistic insights into the larger role of this critical E2 enzyme. We discuss observations of multiple Ubc13 structures that suggest a novel mechanism for activation of Ubc13 that involves conformational change of the active site loop.

Morphogenesis of the somatic musculature in Drosophila melanogaster

In Drosophila melanogaster, the somatic muscle system is first formed during embryogenesis, giving rise to the larval musculature. Later during metamorphosis, this system is destroyed and replaced by an entirely new set of muscles in the adult fly. Proper formation of the larval and adult muscles is critical for basic survival functions such as hatching and crawling (in the larva), walking and flying (in the adult), and feeding (at both larval and adult stages). Myogenesis, from mononucleated muscle precursor cells to multinucleated functional muscles, is driven by a number of cellular processes that have begun to be mechanistically defined. Once the mesodermal cells destined for the myogenic lineage have been specified, individual myoblasts fuse together iteratively to form syncytial myofibers. Combining cytoplasmic contents demands a level of intracellular reorganization that, most notably, leads to redistribution of the myonuclei to maximize internuclear distance. Signaling from extending myofibers induces terminal tendon cell differentiation in the ectoderm, which results in secure muscle-tendon attachments that are critical for muscle contraction. Simultaneously, muscles become innervated and undergo sarcomerogenesis to establish the contractile apparatus that will facilitate movement. The cellular mechanisms governing these morphogenetic events share numerous parallels to mammalian development, and the basic unit of all muscle, the myofiber, is conserved from flies to mammals. Thus, studies of Drosophila myogenesis and comparisons to muscle development in other systems highlight conserved regulatory programs of biomedical relevance to general muscle biology and studies of muscle disease.

no poles encodes a predicted E3 ubiquitin ligase required for early embryonic development of Drosophila

In a screen for cell-cycle regulators, we identified a Drosophila maternal effect-lethal mutant that we named ;no poles' (nopo). Embryos from nopo females undergo mitotic arrest with barrel-shaped, acentrosomal spindles during the rapid S-M cycles of syncytial embryogenesis. We identified CG5140, which encodes a candidate RING domain-containing E3 ubiquitin ligase, as the nopo gene. A conserved residue in the RING domain is altered in our EMS-mutagenized allele of nopo, suggesting that E3 ligase activity is crucial for NOPO function. We show that mutation of a DNA checkpoint kinase, CHK2, suppresses the spindle and developmental defects of nopo-derived embryos, revealing that activation of a DNA checkpoint operational in early embryos contributes significantly to the nopo phenotype. CHK2-mediated mitotic arrest has been previously shown to occur in response to mitotic entry with DNA damage or incompletely replicated DNA. Syncytial embryos lacking NOPO exhibit a shorter interphase during cycle 11, suggesting that they may enter mitosis prior to the completion of DNA replication. We show that Bendless (BEN), an E2 ubiquitin-conjugating enzyme, interacts with NOPO in a yeast two-hybrid assay; furthermore, ben-derived embryos arrest with a nopo-like phenotype during syncytial divisions. These data support our model that an E2-E3 ubiquitination complex consisting of BEN-UEV1A (E2 heterodimer) and NOPO (E3 ligase) is required for the preservation of genomic integrity during early embryogenesis.

A mechanism distinct from highwire for the Drosophila ubiquitin conjugase bendless in synaptic growth and maturation

The signaling mechanisms that allow the conversion of a growth cone into a mature and stable synapse are yet to be completely understood. Ubiquitination plays key regulatory roles in synaptic development and may be involved in this process. Previous studies identified the Drosophila ubiquitin conjugase bendless (ben) to be important for central synapse formation, but the precise role it plays has not been elucidated. Our studies indicate that Ben plays a pivotal role in synaptic growth and maturation. We have determined that an incipient synapse is present with a high penetrance in ben mutants, suggesting that Ben is required for a developmental step after target recognition. We used cell-autonomous rescue experiments to show that Ben has a presynaptic role in synapse growth. We then harnessed the TARGET system to transiently express UAS (upstream activating sequence)-ben in a ben mutant background and identified a well defined critical period for Ben function in establishing a full-grown, mature synaptic terminal. We demonstrate that the protein must be present at a time point before but not during the actual growth process. We also provide phenotypic evidence demonstrating that Ben is not a part of the signal transduction pathway involving the well characterized ubiquitin ligase highwire. We conclude that Bendless functions as a novel developmental switch that permits the transition from axonal growth and incipient synapse formation to synaptic growth and maturation in the CNS.

Ariadne-1: a vital Drosophila gene is required in development and defines a new conserved family of ring-finger proteins

We report the identification and functional characterization of ariadne-1 (ari-1), a novel and vital Drosophila gene required for the correct differentiation of most cell types in the adult organism. Also, we identify a sequence-related gene, ari-2, and the corresponding mouse and human homologues of both genes. All these sequences define a new protein family by the Acid-rich, RING finger, B-box, RING finger, coiled-coil (ARBRCC) motif string. In Drosophila, ari-1 is expressed throughout development in all tissues. The mutant phenotypes are most noticeable in cells that undergo a large and rapid membrane deposition, such as rewiring neurons during metamorphosis, large tubular muscles during adult myogenesis, and photoreceptors. Occasional survivors of null alleles exhibit reduced life span, motor impairments, and short and thin bristles. Single substitutions at key cysteines in each RING finger cause lethality with no survivors and a drastic reduction of rough endoplasmic reticulum that can be observed in the photoreceptors of mosaic eyes. In yeast two-hybrid assays, the protein ARI-1 interacts with a novel ubiquitin-conjugating enzyme, UbcD10, whose sequence is also reported here. The N-terminal RING-finger motif is necessary and sufficient to mediate this interaction. Mouse and fly homologues of both ARI proteins and the Ubc can substitute for each other in the yeast two-hybrid assay, indicating that ARI represents a conserved novel mechanism in development. In addition to ARI homologues, the RBR signature is also found in the Parkinson-disease-related protein Parkin adjacent to an ubiquitin-like domain, suggesting that the study of this mechanism could be relevant for human pathology.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

The Drosophila bendless gene encodes a neural protein related to ubiquitin-conjugating enzymes

The bendless (ben) mutation of Drosophila alters synaptic connectivity between a subset of CNS neurons. Here, we show that ben also causes morphological abnormalities within the visual system, suggesting that ben functions in a number of different developmental processes. We show that the ben gene encodes a protein which is closely related to ubiquitin-conjugating enzymes and that a missense mutation in the highly conserved active site region is associated with the ben mutation. High levels of ben expression are restricted to the nervous system during development. These results suggest a role for ubiquitin-mediated protein modification in nervous system development, including, but not exclusive to, the regulation of synaptic connectivity.