Mapping Drosophila mutations with molecularly defined P element insertions

Pellino enhances innate immunity in Drosophila

The innate immune response is a defense mechanism against infectious agents in both vertebrates and invertebrates, and is in part mediated by the Toll pathway. Toll receptor activation upon exposure to bacteria causes stimulation of Pelle/IRAK kinase, eventually resulting in translocation of the transcription factor NF-kappaB to the nucleus. Here we show that Pellino, a highly conserved protein interacting with activated Pelle/IRAK, acts as a positive regulator of innate immunity in Drosophila.

The transcriptional co-factor Chip acts with LIM-homeodomain proteins to set the boundary of the eye field in Drosophila

Development involves the establishment of boundaries between fields specified to differentiate into distinct tissues. The Drosophila larval eye-antennal imaginal disc must be subdivided into regions that differentiate into the adult eye, antenna and head cuticle. We have found that the transcriptional co-factor Chip is required for cells at the ventral eye-antennal disc border to take on a head cuticle fate; clones of Chip mutant cells in this region instead form outgrowths that differentiate into ectopic eye tissue. Chip acts independently of the transcription factor Homothorax, which was previously shown to promote head cuticle development in the same region. Chip and its vertebrate CLIM homologues have been shown to form complexes with LIM-homeodomain transcription factors, and the domain of Chip that mediates these interactions is required for its ability to suppress the eye fate. We show that two LIM-homeodomain proteins, Arrowhead and Lim1, are expressed in the region of the eye-antennal disc affected in Chip mutants, and that both require Chip for their ability to suppress photoreceptor differentiation when misexpressed in the eye field. Loss-of-function studies support the model that Arrowhead and Lim1 act redundantly, using Chip as a co-factor, to prevent retinal differentiation in regions of the eye disc destined to become ventral head tissue.

Regulation of notch signaling via O-glucosylation insights from Drosophila studies

Recent work using Drosophila melanogaster has shown that a protein O-glucosyltransferase called Rumi regulates Notch signaling. Studies on several alleles of rumi identified in a forward genetic screen indicated that Rumi is a temperature-sensitive regulator of Notch signaling in flies. Further genetic and rescue experiments demonstrated that Rumi is a general regulator of Drosophila Notch signaling. Biochemical analyses showed that Rumi adds glucose to specific EGF repeats in the extracellular domain of Notch receptor in the Drosophila S2 cell line. Furthermore, RNAi-mediated knockdown of Rumi in this cell line resulted in a severe decrease in the level of O-linked glucose on Notch. In this chapter, we discuss the genetic and biochemical methods used to determine the role of Rumi in the regulation of Notch signaling in flies.

Tweek, an evolutionarily conserved protein, is required for synaptic vesicle recycling

Synaptic vesicle endocytosis is critical for maintaining synaptic communication during intense stimulation. Here we describe Tweek, a conserved protein that is required for synaptic vesicle recycling. tweek mutants show reduced FM1-43 uptake, cannot maintain release during intense stimulation, and harbor larger than normal synaptic vesicles, implicating it in vesicle recycling at the synapse. Interestingly, the levels of a fluorescent PI(4,5)P(2) reporter are reduced at tweek mutant synapses, and the probe is aberrantly localized during stimulation. In addition, various endocytic adaptors known to bind PI(4,5)P(2) are mislocalized and the defects in FM1-43 dye uptake and adaptor localization are partially suppressed by removing one copy of the phosphoinositide phosphatase synaptojanin, suggesting a role for Tweek in maintaining proper phosphoinositide levels at synapses. Our data implicate Tweek in regulating synaptic vesicle recycling via an action mediated at least in part by the regulation of PI(4,5)P(2) levels or availability at the synapse.

A synaptic vesicle-associated Ca2+ channel promotes endocytosis and couples exocytosis to endocytosis

Synaptic vesicle (SV) exo- and endocytosis are tightly coupled to sustain neurotransmission in presynaptic terminals, and both are regulated by Ca(2+). Ca(2+) influx triggered by voltage-gated Ca(2+) channels is necessary for SV fusion. However, extracellular Ca(2+) has also been shown to be required for endocytosis. The intracellular Ca(2+) levels (<1 microM) that trigger endocytosis are typically much lower than those (>10 microM) needed to induce exocytosis, and endocytosis is inhibited when the Ca(2+) level exceeds 1 microM. Here, we identify and characterize a transmembrane protein associated with SVs that, upon SV fusion, localizes at periactive zones. Loss of Flower results in impaired intracellular resting Ca(2+) levels and impaired endocytosis. Flower multimerizes and is able to form a channel to control Ca(2+) influx. We propose that Flower functions as a Ca(2+) channel to regulate synaptic endocytosis and hence couples exo- with endocytosis.

Massively parallel resequencing of the isogenic Drosophila melanogaster strain w(1118); iso-2; iso-3 identifies hotspots for mutations in sensory perception genes

We used the Illumina reversible-short sequencing technology to obtain 17-fold average depth (s.d. approximately 8) of approximately 94% of the euchromatic genome and approximately 1-5% of the heterochromatin sequence of the Drosophila melanogaster isogenic strain w(1118); iso-2; iso-3. We show that this strain has a approximately 9 kb deletion that uncovers the first exon of the white (w) gene, approximately 4 kb of downstream promoter sequences, and most of the first intron, thus demonstrating that whole-genome sequencing can be used for mutation characterization. We chose this strain because there are thousands of transposon insertion lines and hundreds of isogenic deficiency lines available with this genetic background, such as the Exelixis, Inc., and the DrosDEL collections. We compared our sequence to Release 5 of the finished reference genome sequence which was made from the isogenic strain y(1); cn(1) bw(1) sp(1) and identified 356,614 candidate SNPs in the approximately 117 Mb unique sequence genome, which represents a substitution rate of approximately 1/305 nucleotides ( approximately 0.30%). The distribution of SNPs is not uniform, but rather there is a approximately 2-fold increase in SNPs on the autosome arms compared with the X chromosome and a approximately 7-fold increase when compared to the small 4(th) chromosome. This is consistent with previous analyses that demonstrated a correlation between recombination frequency and SNP frequency. An unexpected finding was a SNP hotpot in a approximately 20 Mb central region of the 4(th) chromosome, which might indicate higher than expected recombination frequency in this region of this chromosome. Interestingly, genes involved in sensory perception are enriched in SNP hotspots and genes encoding developmental genes are enriched in SNP coldspots, which suggests that recombination frequencies might be proportional to the evolutionary selection coefficient. There are currently 12 Drosophila species sequenced, and this represents one of many isogenic Drosophila melanogaster genome sequences that are in progress. Because of the dramatic increase in power in using isogenic lines rather than outbred individuals, the SNP information should be valuable as a test bed for understanding genotype-by-environment interactions in human population studies.

The Arp2/3 complex and WASp are required for apical trafficking of Delta into microvilli during cell fate specification of sensory organ precursors

Cell fate decisions mediated by the Notch signalling pathway require direct cell-cell contact between adjacent cells. In Drosophila melanogaster, an external sensory organ (ESO) develops from a single sensory organ precursor (SOP) and its fate specification is governed by differential Notch activation. Here we show that mutations in actin-related protein-3 (Arp3) compromise Notch signalling, leading to a fate transformation of the ESO. Our data reveal that during ESO fate specification, most endocytosed vesicles containing the ligand Delta traffic to a prominent apical actin-rich structure (ARS) formed in the SOP daughter cells. Using immunohistochemistry and transmission electron microscopy (TEM) analyses, we show that the ARS contains numerous microvilli on the apical surface of SOP progeny. In Arp2/3 and WASp mutants, the surface area of the ARS is substantially reduced and there are significantly fewer microvilli. More importantly, trafficking of Delta-positive vesicles from the basal area to the apical portion of the ARS is severely compromised. Our data indicate that WASp-dependent Arp2/3 actin polymerization is crucial for apical presentation of Delta, providing a mechanistic link between actin polymerization and Notch signalling.

Importin 13 regulates neurotransmitter release at the Drosophila neuromuscular junction

In an unbiased genetic screen designed to isolate mutations that affect synaptic transmission, we have isolated homozygous lethal mutations in Drosophila importin 13 (imp13). Imp13 is expressed in and around nuclei of both neurons and muscles. At the larval neuromuscular junction (NMJ), imp13 affects muscle growth and formation of the subsynaptic reticulum without influencing any presynaptic structural features. In the absence of imp13, the probability of release of neurotransmitter and quantal content is increased, yet the abundance of the postsynaptic receptors and the amplitude of miniature excitatory junctional potentials are not affected. Interestingly, imp13 is required in the muscles to control presynaptic release. Thus, imp13 is a novel factor that affects neurotransmitter release at the fly NMJ. Its role in the context of synaptic homeostasis is discussed.

Sequoia regulates cell fate decisions in the external sensory organs of adult Drosophila

The adult Drosophila external sensory organ (ESO), comprising the hair, socket, neuron, sheath and glia cells, arises through the asymmetric division of sensory organ precursor cells (SOPs). In a mosaic screen designed to identify new components in ESO development, we isolated mutations in sequoia, which encodes a putative zinc-finger transcription factor that has previously been shown to have a role in dendritogenesis. Here, we show that adult clones mutant for seq exhibit a loss of hair cells and a gain of socket cells. We propose that the seq mutant phenotype arises, in part, owing to the loss of several crucial transcription factors known to be important in peripheral nervous system development such as D-Pax2, Prospero and Hamlet. Thus, Sequoia is a new upstream regulator of genes that orchestrates cell fate specification during development of the adult ESO lineage.

Variants of the elongator protein 3 (ELP3) gene are associated with motor neuron degeneration

Amyotrophic lateral sclerosis (ALS) is a spontaneous, relentlessly progressive motor neuron disease, usually resulting in death from respiratory failure within 3 years. Variation in the genes SOD1 and TARDBP accounts for a small percentage of cases, and other genes have shown association in both candidate gene and genome-wide studies, but the genetic causes remain largely unknown. We have performed two independent parallel studies, both implicating the RNA polymerase II component, ELP3, in axonal biology and neuronal degeneration. In the first, an association study of 1884 microsatellite markers, allelic variants of ELP3 were associated with ALS in three human populations comprising 1483 people (P = 1.96 × 10⁻⁹). In the second, an independent mutagenesis screen in Drosophila for genes important in neuronal communication and survival identified two different loss of function mutations, both in ELP3 (R475K and R456K). Furthermore, knock down of ELP3 protein levels using antisense morpholinos in zebrafish embryos resulted in dose-dependent motor axonal abnormalities [Pearson correlation: −0.49, P = 1.83 × 10⁻¹² (start codon morpholino) and −0.46, P = 4.05 × 10⁻⁹ (splice-site morpholino), and in humans, risk-associated ELP3 genotypes correlated with reduced brain ELP3 expression (P = 0.01). These findings add to the growing body of evidence implicating the RNA processing pathway in neurodegeneration and suggest a critical role for ELP3 in neuron biology and of ELP3 variants in ALS.

Ero1L, a thiol oxidase, is required for Notch signaling through cysteine bridge formation of the Lin12-Notch repeats in Drosophila melanogaster

Notch-mediated cell–cell communication regulates numerous developmental processes and cell fate decisions. Through a mosaic genetic screen in Drosophila melanogaster, we identified a role in Notch signaling for a conserved thiol oxidase, endoplasmic reticulum (ER) oxidoreductin 1–like (Ero1L). Although Ero1L is reported to play a widespread role in protein folding in yeast, in flies Ero1L mutant clones show specific defects in lateral inhibition and inductive signaling, two characteristic processes regulated by Notch signaling. Ero1L mutant cells accumulate high levels of Notch protein in the ER and induce the unfolded protein response, suggesting that Notch is misfolded and fails to be exported from the ER. Biochemical assays demonstrate that Ero1L is required for formation of disulfide bonds of three Lin12-Notch repeats (LNRs) present in the extracellular domain of Notch. These LNRs are unique to the Notch family of proteins. Therefore, we have uncovered an unexpected requirement for Ero1L in the maturation of the Notch receptor.

The dominant cold-sensitive Out-cold mutants of Drosophila melanogaster have novel missense mutations in the voltage-gated sodium channel gene paralytic

Here we report the molecular characterization of Out-cold (Ocd) mutants of Drosophila melanogaster, which produce a dominant, X-linked, cold-sensitive paralytic phenotype. From its initial 1.5-Mb cytological location within 13F1-16A2, P-element and SNP mapping reduced the Ocd critical region to <100 kb and to six candidate genes: hangover, CG9947, CG4420, eIF2a, Rbp2, and paralytic (para). Complementation testing with para null mutations strongly suggests Ocd and para are allelic, as does gene rescue of Ocd semilethality with a wild-type para transgene. Pesticide resistance and electrophysiological phenotypes of Ocd mutants support this conclusion. The para gene encodes a voltage-gated sodium channel. Sequencing the Ocd lines revealed mutations within highly conserved regions of the para coding sequence, in the transmembrane segment S6 of domain III (I1545M and T1551I), and in the linker between domains III and IV (G1571R), the location of the channel inactivation gate. The G1571R mutation is of particular interest as mutations of the orthologous residue (G1306) in the human skeletal muscle sodium channel gene SCN4A are associated with cases of periodic paralysis and myotonia, including the human cold-sensitive disorder paramyotonia congenita. The mechanisms by which sodium channel mutations cause cold sensitivity are not well understood. Therefore, in the absence of suitable vertebrate models, Ocd provides a system in which genetic, molecular, physiological, and behavioral tools can be exploited to determine mechanisms underlying sodium channel periodic paralyses.

Myopic acts in the endocytic pathway to enhance signaling by the Drosophila EGF receptor

Endocytosis of activated receptors can control signaling levels by exposing the receptors to novel downstream molecules or by instigating their degradation. Epidermal growth factor receptor (EGFR) signaling has crucial roles in development and is misregulated in many cancers. We report here that Myopic, the Drosophila homolog of the Bro1-domain tyrosine phosphatase HD-PTP, promotes EGFR signaling in vivo and in cultured cells. myopic is not required in the presence of activated Ras or in the absence of the ubiquitin ligase Cbl, indicating that it acts on internalized EGFR, and its overexpression enhances the activity of an activated form of EGFR. Myopic is localized to intracellular vesicles adjacent to Rab5-containing early endosomes, and its absence results in the enlargement of endosomal compartments. Loss of Myopic prevents cleavage of the EGFR cytoplasmic domain, a process controlled by the endocytic regulators Cbl and Sprouty. We suggest that Myopic promotes EGFR signaling by mediating its progression through the endocytic pathway.

straightjacket is required for the synaptic stabilization of cacophony, a voltage-gated calcium channel alpha1 subunit

In a screen to identify genes involved in synaptic function, we isolated mutations in Drosophila melanogaster straightjacket (stj), an α₂δ subunit of the voltage-gated calcium channel. stj mutant photoreceptors develop normal synaptic connections but display reduced “on–off” transients in electroretinogram recordings, indicating a failure to evoke postsynaptic responses and, thus, a defect in neurotransmission. stj is expressed in neurons but excluded from glia. Mutants exhibit endogenous seizure-like activity, indicating altered neuronal excitability. However, at the synaptic level, stj larval neuromuscular junctions exhibit approximately fourfold reduction in synaptic release compared with controls stemming from a reduced release probability at these synapses. These defects likely stem from destabilization of Cacophony (Cac), the primary presynaptic α₁ subunit in D. melanogaster. Interestingly, neuronal overexpression of cac partially rescues the viability and physiological defects in stj mutants, indicating a role for the α₂δ Ca²⁺ channel subunit in mediating the proper localization of an α₁ subunit at synapses.

Rumi is a CAP10 domain glycosyltransferase that modifies Notch and is required for Notch signaling

Notch signaling is broadly used to regulate cell-fate decisions. We have identified a gene, rumi, with a temperature-sensitive Notch phenotype. At 28 degrees C-30 degrees C, rumi clones exhibit a full-blown loss of Notch signaling in all tissues tested. However, at 18 degrees C only a mild Notch phenotype is evident. In vivo analyses reveal that the target of Rumi is the extracellular domain of Notch. Notch accumulates intracellularly and at the cell membrane of rumi cells but fails to be properly cleaved, despite normal binding to Delta. Rumi is an endoplasmic reticulum-retained protein with a highly conserved CAP10 domain. Our studies show that Rumi is a protein O-glucosyltransferase, capable of adding glucose to serine residues in Notch EGF repeats with the consensus C1-X-S-X-P-C2 sequence. These data indicate that by O-glucosylating Notch in the ER, Rumi regulates its folding and/or trafficking and allows signaling at the cell membrane.

Huntingtin-interacting protein 14, a palmitoyl transferase required for exocytosis and targeting of CSP to synaptic vesicles

Posttranslational modification through palmitoylation regulates protein localization and function. In this study, we identify a role for the Drosophila melanogaster palmitoyl transferase Huntingtin-interacting protein 14 (HIP14) in neurotransmitter release. hip14 mutants show exocytic defects at low frequency stimulation and a nearly complete loss of synaptic transmission at higher temperature. Interestingly, two exocytic components known to be palmitoylated, cysteine string protein (CSP) and SNAP25, are severely mislocalized at hip14 mutant synapses. Complementary DNA rescue and localization experiments indicate that HIP14 is required solely in the nervous system and is essential for presynaptic function. Biochemical studies indicate that HIP14 palmitoylates CSP and that CSP is not palmitoylated in hip14 mutants. Furthermore, the hip14 exocytic defects can be suppressed by targeting CSP to synaptic vesicles using a chimeric protein approach. Our data indicate that HIP14 controls neurotransmitter release by regulating the trafficking of CSP to synapses.

The E1 ubiquitin-activating enzyme Uba1 in Drosophila controls apoptosis autonomously and tissue growth non-autonomously

Ubiquitination is an essential process regulating turnover of proteins for basic cellular processes such as the cell cycle and cell death (apoptosis). Ubiquitination is initiated by ubiquitin-activating enzymes (E1), which activate and transfer ubiquitin to ubiquitin-conjugating enzymes (E2). Conjugation of target proteins with ubiquitin is then mediated by ubiquitin ligases (E3). Ubiquitination has been well characterized using mammalian cell lines and yeast genetics. However, the consequences of partial or complete loss of ubiquitin conjugation in a multi-cellular organism are not well understood. Here, we report the characterization of Uba1, the only E1 in Drosophila. We found that weak and strong Uba1 alleles behave genetically differently with sometimes opposing phenotypes. Whereas weak Uba1 alleles protect cells from cell death, clones of strong Uba1 alleles are highly apoptotic. Strong Uba1 alleles cause cell cycle arrest which correlates with failure to reduce cyclin levels. Surprisingly, clones of strong Uba1 mutants stimulate neighboring wild-type tissue to undergo cell division in a non-autonomous manner giving rise to overgrowth phenotypes of the mosaic fly. We demonstrate that the non-autonomous overgrowth is caused by failure to downregulate Notch signaling in Uba1 mutant clones. In summary, the phenotypic analysis of Uba1 demonstrates that impaired ubiquitin conjugation has significant consequences for the organism, and may implicate Uba1 as a tumor suppressor gene.

Cytoplasmic and mitochondrial protein translation in axonal and dendritic terminal arborization

We identified a mutation in Aats-gly (also known as gars or glycyl-tRNA synthetase), the Drosophila melanogaster ortholog of the human GARS gene that is associated with Charcot-Marie-Tooth neuropathy type 2D (CMT2D), from a mosaic genetic screen. Loss of gars in Drosophila neurons preferentially affects the elaboration and stability of terminal arborization of axons and dendrites. The human and Drosophila genes each encode both a cytoplasmic and a mitochondrial isoform. Using additional mutants that selectively disrupt cytoplasmic or mitochondrial protein translation, we found that cytoplasmic protein translation is required for terminal arborization of both dendrites and axons during development. In contrast, disruption of mitochondrial protein translation preferentially affects the maintenance of dendritic arborization in adults. We also provide evidence that human GARS shows equivalent functions in Drosophila, and that CMT2D causal mutations show loss-of-function properties. Our study highlights different demands of protein translation for the development and maintenance of axons and dendrites.

Drosophila NMNAT maintains neural integrity independent of its NAD synthesis activity

Wallerian degeneration refers to a loss of the distal part of an axon after nerve injury. Wallerian degeneration slow (Wld^s) mice overexpress a chimeric protein containing the NAD synthase NMNAT (nicotinamide mononucleotide adenylyltransferase 1) and exhibit a delay in axonal degeneration. Currently, conflicting evidence raises questions as to whether NMNAT is the protecting factor and whether its enzymatic activity is required for such a possible function. Importantly, the link between nmnat and axon degeneration is at present solely based on overexpression studies of enzymatically active protein. Here we use the visual system of Drosophila as a model system to address these issues. We have isolated the first nmnat mutations in a multicellular organism in a forward genetic screen for synapse malfunction in Drosophila. Loss of nmnat causes a rapid and severe neurodegeneration that can be attenuated by blocking neuronal activity. Furthermore, in vivo neuronal expression of mutated nmnat shows that enzymatically inactive NMNAT protein retains strong neuroprotective effects and rescues the degeneration phenotype caused by loss of nmnat. Our data indicate an NAD-independent requirement of NMNAT for maintaining neuronal integrity that can be exploited to protect neurons from neuronal activity-induced degeneration by overexpression of the protein.

The first mutant analysis of NMNAT (nicotinamide mononucleotide adenylyltransferase 1) reveals an essential neuronal protective role that functions independently of NMNAT's enzymatic activity. NMNAT can also be exploited to protect neurons against activity-induced neurodegeneration.

A genetic screen in Drosophila for genes interacting with senseless during neuronal development identifies the importin moleskin

Senseless (Sens) is a conserved transcription factor required for normal development of the Drosophila peripheral nervous system. In the Drosophila retina, sens is necessary and sufficient for differentiation of R8 photoreceptors and interommatidial bristles (IOBs). When Sens is expressed in undifferentiated cells posterior to the morphogenetic furrow, ectopic IOBs are formed. This phenotype was used to identify new members of the sens pathway in a dominant modifier screen. Seven suppressor and three enhancer complementation groups were isolated. Three groups from the screen are the known genes Delta, lilliputian, and moleskin/DIM-7 (msk), while the remaining seven groups represent novel genes with previously undefined functions in neural development. The nuclear import gene msk was identified as a potent suppressor of the ectopic interommatidial bristle phenotype. In addition, msk mutant adult eyes are extremely disrupted with defects in multiple cell types. Reminiscent of the sens mutant phenotype, msk eyes demonstrate reductions in the number of R8 photoreceptors due to an R8 to R2,5 fate switch, providing genetic evidence that Msk is a component of the sens pathway. Interestingly, in msk tissue, the loss of R8 fate occurs earlier than with sens and suggests a previously unidentified stage of R8 development between atonal and sens.

Lethal giant discs, a novel C2-domain protein, restricts notch activation during endocytosis

The Notch signaling pathway plays a central role in animal growth and patterning, and its deregulation leads to many human diseases, including cancer. Mutations in the tumor suppressor lethal giant discs (lgd) induce strong Notch activation and hyperplastic overgrowth of Drosophila imaginal discs. However, the gene that encodes Lgd and its function in the Notch pathway have not yet been identified. Here, we report that Lgd is a novel, conserved C2-domain protein that regulates Notch receptor trafficking. Notch accumulates on early endosomes in lgd mutant cells and signals in a ligand-independent manner. This phenotype is similar to that seen when cells lose endosomal-pathway components such as Erupted and Vps25. Interestingly, Notch activation in lgd mutant cells requires the early endosomal component Hrs, indicating that Hrs is epistatic to Lgd. These data suggest that Lgd affects Notch trafficking between the actions of Hrs and the late endosomal component Vps25. Taken together, our data identify Lgd as a novel tumor-suppressor protein that regulates Notch signaling by targeting Notch for degradation or recycling.

Genome-wide P-element screen for Drosophila synaptogenesis mutants

A molecular understanding of synaptogenesis is a critical step toward the goal of understanding how brains "wire themselves up," and then "rewire" during development and experience. Recent genomic and molecular advances have made it possible to study synaptogenesis on a genomic scale. Here, we describe the results of a screen for genes involved in formation and development of the glutamatergic Drosophila neuromuscular junction (NMJ). We screened 2185 P-element transposon mutants representing insertions in approximately 16% of the entire Drosophila genome. We first identified recessive lethal mutants, based on the hypothesis that mutations causing severe disruptions in synaptogenesis are likely to be lethal. Two hundred twenty (10%) of all insertions were homozygous lethal. Two hundred five (93%) of these lethal mutants developed at least through late embryogenesis and formed neuromusculature. We examined embryonic/larval NMJs in 202 of these homozygous mutants using immunocytochemistry and confocal microscopy. We identified and classified 88 mutants with altered NMJ morphology. Insertion loci in these mutants encode several different types of proteins, including ATP- and GTPases, cytoskeletal regulators, cell adhesion molecules, kinases, phosphatases, RNA regulators, regulators of protein formation, transcription factors, and transporters. Thirteen percent of insertions are in genes that encode proteins of novel or unknown function. Complementation tests and RT-PCR assays suggest that approximately 51% of the insertion lines carry background mutations. Our results reveal that synaptogenesis requires the coordinated action of many different types of proteins--perhaps as much as 44% of the entire genome--and that transposon mutageneses carry important caveats that must be respected when interpreting results generated using this method.

vps25 mosaics display non-autonomous cell survival and overgrowth, and autonomous apoptosis

Appropriate cell-cell signaling is crucial for proper tissue homeostasis. Protein sorting of cell surface receptors at the early endosome is important for both the delivery of the signal and the inactivation of the receptor, and its alteration can cause malignancies including cancer. In a genetic screen for suppressors of the pro-apoptotic gene hid in Drosophila, we identified two alleles of vps25, a component of the ESCRT machinery required for protein sorting at the early endosome. Paradoxically, although vps25 mosaics were identified as suppressors of hid-induced apoptosis, vps25 mutant cells die. However, we provide evidence that a non-autonomous increase of Diap1 protein levels, an inhibitor of apoptosis, accounts for the suppression of hid. Furthermore, before they die, vps25 mutant clones trigger non-autonomous proliferation through a failure to downregulate Notch signaling, which activates the mitogenic JAK/STAT pathway. Hid and JNK contribute to apoptosis of vps25 mutant cells. Inhibition of cell death in vps25 clones causes dramatic overgrowth phenotypes. In addition, Hippo signaling is increased in vps25 clones, and hippo mutants block apoptosis in vps25 clones. In summary, the phenotypic analysis of vps25 mutants highlights the importance of receptor downregulation by endosomal protein sorting for appropriate tissue homeostasis, and may serve as a model for human cancer.

Site-specific transformation of Drosophila via phiC31 integrase-mediated cassette exchange

Position effects can complicate transgene analyses. This is especially true when comparing transgenes that have inserted randomly into different genomic positions and are therefore subject to varying position effects. Here, we introduce a method for the precise targeting of transgenic constructs to predetermined genomic sites in Drosophila using the C31 integrase system in conjunction with recombinase-mediated cassette exchange (RMCE). We demonstrate the feasibility of this system using two donor cassettes, one carrying the yellow gene and the other carrying GFP. At all four genomic sites tested, we observed exchange of donor cassettes with an integrated target cassette carrying the mini-white gene. Furthermore, because RMCE-mediated integration of the donor cassette is necessarily accompanied by loss of the target cassette, we were able to identify integrants simply by the loss of mini-white eye color. Importantly, this feature of the technology will permit integration of unmarked constructs into Drosophila, even those lacking functional genes. Thus, C31 integrase-mediated RMCE should greatly facilitate transgene analysis as well as permit new experimental designs.

An essential Drosophila glutamate receptor subunit that functions in both central neuropil and neuromuscular junction

A Drosophila forward genetic screen for mutants with defective synaptic development identified bad reception (brec). Homozygous brec mutants are embryonic lethal, paralyzed, and show no detectable synaptic transmission at the glutamatergic neuromuscular junction (NMJ). Genetic mapping, complementation tests, and genomic sequencing show that brec mutations disrupt a previously uncharacterized ionotropic glutamate receptor subunit, named here "GluRIID." GluRIID is expressed in the postsynaptic domain of the NMJ, as well as widely throughout the synaptic neuropil of the CNS. In the NMJ of null brec mutants, all known glutamate receptor subunits are undetectable by immunocytochemistry, and all functional glutamate receptors are eliminated. Thus, we conclude that GluRIID is essential for the assembly and/or stabilization of glutamate receptors in the NMJ. In null brec mutant embryos, the frequency of periodic excitatory currents in motor neurons is significantly reduced, demonstrating that CNS motor pattern activity is regulated by GluRIID. Although synaptic development and molecular differentiation appear otherwise unperturbed in null mutants, viable hypomorphic brec mutants display dramatically undergrown NMJs by the end of larval development, suggesting that GluRIID-dependent central pattern activity regulates peripheral synaptic growth. These studies reveal GluRIID as a newly identified glutamate receptor subunit that is essential for glutamate receptor assembly/stabilization in the peripheral NMJ and required for properly patterned motor output in the CNS.

Sec15, a component of the exocyst, promotes notch signaling during the asymmetric division of Drosophila sensory organ precursors

Asymmetric division of sensory organ precursors (SOPs) in Drosophila generates different cell types of the mature sensory organ. In a genetic screen designed to identify novel players in this process, we have isolated a mutation in Drosophila sec15, which encodes a component of the exocyst, an evolutionarily conserved complex implicated in intracellular vesicle transport. sec15(-) sensory organs contain extra neurons at the expense of support cells, a phenotype consistent with loss of Notch signaling. A vesicular compartment containing Notch, Sanpodo, and endocytosed Delta accumulates in basal areas of mutant SOPs. Based on the dynamic traffic of Sec15, its colocalization with the recycling endosomal marker Rab11, and the aberrant distribution of Rab11 in sec15 clones, we propose that a defect in Delta recycling causes cell fate transformation in sec15(-) sensory lineages. Our data indicate that Sec15 mediates a specific vesicle trafficking event to ensure proper neuronal fate specification in Drosophila.

A major bristle QTL from a selected population of Drosophila uncovers the zinc-finger transcription factor poils-au-dos, a repressor of achaete-scute

Traditional screens aiming at identifying genes regulating development have relied on mutagenesis. Here, we describe a new gene involved in bristle development, identified through the use of natural variation and selection. Drosophila melanogaster bears a pattern of 11 macrochaetes per heminotum. From a population initially sampled in Marrakech, a strain was selected for an increased number of thoracic macrochaetes. Using recombination and single nucleotide polymorphisms, the factor responsible was mapped to a single locus on the third chromosome, poils au dos, that encodes a zinc-finger-ZAD protein. The original, as well as new, presumed null, alleles of poils au dos, is associated with ectopic achaete-scute expression that results in the additional bristles. This suggests a possible role for Poils au dos as a repressor of achaete and scute. Ectopic expression appears to be independent of the activity of known cis-regulatory enhancer sequences at the achaete-scute complex that mediate activation at specific sites on the notum. The target sequences for Poils au dos activity were mapped to a 14 kb region around scute. In addition, we show that pad interacts synergistically with the repressor hairy and with Dpp signaling in posterior and anterior regions of the notum, respectively.

Aberrant lysosomal carbohydrate storage accompanies endocytic defects and neurodegeneration in Drosophila benchwarmer

Lysosomal storage is the most common cause of neurodegenerative brain disease in preadulthood. However, the underlying cellular mechanisms that lead to neuronal dysfunction are unknown. Here, we report that loss of Drosophila benchwarmer (bnch), a predicted lysosomal sugar carrier, leads to carbohydrate storage in yolk spheres during oogenesis and results in widespread accumulation of enlarged lysosomal and late endosomal inclusions. At the bnch larval neuromuscular junction, we observe similar inclusions and find defects in synaptic vesicle recycling at the level of endocytosis. In addition, loss of bnch slows endosome-to-lysosome trafficking in larval garland cells. In adult bnch flies, we observe age-dependent synaptic dysfunction and neuronal degeneration. Finally, we find that loss of bnch strongly enhances tau neurotoxicity in a dose-dependent manner. We hypothesize that, in bnch, defective lysosomal carbohydrate efflux leads to endocytic defects with functional consequences in synaptic strength, neuronal viability, and tau neurotoxicity.

Localization, mobility and fidelity of retrotransposed Group II introns in rRNA genes

We previously showed that the group II Lactococcus lactis Ll.LtrB intron could retrotranspose into ectopic locations on the genome of its native host. Two integration events, which had been mapped to unique sequences, were localized in the present study to separate copies of the six L.lactis 23S rRNA genes, within operon B or D. Although further movement within the bacterial chromosome was undetectable, the retrotransposed introns were able to re-integrate into their original homing site provided on a plasmid. This finding indicates not only that retrotransposed group II introns retain mobility properties, but also that movement occurs back into sequence that is heterologous to the sequence of the chromosomal location. Sequence analysis of the retrotransposed introns and the secondary mobility events back to the homing site showed that the introns retain sequence integrity. These results are illuminating, since the reverse transcriptase (RT) of the intron-encoded protein, LtrA, has no known proofreading function, yet the mobility events have a low error rate. Enzymatic digests were used to monitor sequence changes from the wild-type intron. The results indicate that retromobility events have approximately 10(-5) misincorporations per nucleotide inserted. In contrast to the high RT error rates for retroviruses that must escape host defenses, the infrequent mutations of group II introns would ensure intron spread through retention of sequences essential for mobility.

The v-ATPase V0 subunit a1 is required for a late step in synaptic vesicle exocytosis in Drosophila

The V(0) complex forms the proteolipid pore of an ATPase that acidifies vesicles. In addition, an independent function in membrane fusion has been proposed largely based on yeast vacuolar fusion experiments. We have isolated mutations in the largest V(0) component vha100-1 in flies in an unbiased genetic screen for synaptic malfunction. The protein is only required in neurons, colocalizes with markers for synaptic vesicles as well as active zones, and interacts with t-SNAREs. Loss of vha100-1 leads to vesicle accumulation in synaptic terminals, suggesting a deficit in release. The amplitude of spontaneous release events and release with hypertonic stimulation indicate normal levels of neurotransmitter loading, yet mutant embryos display severe defects in evoked synaptic transmission and FM1-43 uptake. Our data suggest that Vha100-1 functions downstream of SNAREs in synaptic vesicle fusion.

Keeping an eye on the fly genome

With its unique structure and dynamic development, the Drosophila eye has been a powerful genetic model system for studying molecular mechanisms of cell fate specification and differentiation. Hundreds of genes that function in a complex genetic network controlling this process have been identified during the past two decades. To further advance our understanding of the molecular mechanisms of eye development, it is increasingly important to place the current genetic pathway into a whole-genome perspective. Here, we review emerging technologies and strategies that will help to achieve this goal, including generation of a complete mutant set in Drosophila, genome-wide transcription factor target identification, and systematic studies of gene function aided by computational biology.

Mutations in Drosophila sec15 reveal a function in neuronal targeting for a subset of exocyst components

The exocyst is a complex of proteins originally identified in yeast that has been implicated in polarized secretion. Components of the exocyst have been implicated in neurite outgrowth, cell polarity, and cell viability. We have isolated an exocyst component, sec15, in a screen for genes required for synaptic specificity. Loss of sec15 causes a targeting defect of photoreceptors that coincides with mislocalization of specific cell adhesion and signaling molecules. Additionally, sec15 mutant neurons fail to localize other exocyst members like Sec5 and Sec8, but not Sec6, to neuronal terminals. However, loss of sec15 does not cause cell lethality in contrast to loss of sec5 or sec6. Our data suggest a role of Sec15 in an exocyst-like subcomplex for the targeting and subcellular distribution of specific proteins. The data also show that functions of other exocyst components persist in the absence of sec15, suggesting that different exocyst components have separable functions.

A genetic screen in Drosophila for identifying novel components of the hedgehog signaling pathway

The Hedgehog signaling pathway plays an essential role in the pattern formation and development of metazoan animals. Misregulation of Hedgehog signaling has also been associated with the formation of multiple types of cancer. For these reasons, the Hedgehog pathway has attracted considerable interest. Many proteins required in the Hedgehog pathway have been identified, and while much has been learned about their function in signal transduction, it is clear that this complement of proteins does not comprise the full set necessary for Hedgehog signal transduction. Because significant gaps remain in our knowledge of the molecules required for Hedgehog signaling, we performed an enhancer/suppressor screen in Drosophila melanogaster to identify novel components of the pathway. In addition to the isolation of new alleles of the known pathway components patched and smoothened, this screen identified 14 novel complementation groups and a larger number of loci represented by single alleles. These groups include mutations in the genes encoding the translation factors eRF1 and eIF1A and the kinesin-like protein Pavarotti. It also identified mutations in a gene whose product is necessary for the movement of Hedgehog protein through tissues.

Emerging technologies for gene manipulation in Drosophila melanogaster

The popularity of Drosophila melanogaster as a model for understanding eukaryotic biology over the past 100 years has been accompanied by the development of numerous tools for manipulating the fruitfly genome. Here we review some recent technologies that will allow Drosophila melanogaster to be manipulated more easily than any other multicellular organism. These developments include the ability to create molecularly designed deletions, improved genetic mapping technologies, strategies for creating targeted mutations, new transgenic approaches and the means to clone and modify large fragments of DNA.

Research resources for Drosophila: the expanding universe

Drosophila melanogaster has been the subject of research into central questions about biological mechanisms for almost a century. The experimental tools and resources that are available or under development for D. melanogaster and its related species, particularly those for genomic analysis, are truly outstanding. Here we review three types of resource that have been developed for D. melanogaster research: databases and other sources of information, biological materials and experimental services. These resources are there to be exploited and we hope that this guide will encourage new uses for D. melanogaster information, materials and services, both by those new to flies and by experienced D. melanogaster researchers.

Synaptojanin is recruited by endophilin to promote synaptic vesicle uncoating

We describe the isolation and characterization of Drosophila synaptojanin (synj) mutants. synj encodes a phosphatidylinositol phosphatase involved in clathrin-mediated endocytosis. We show that Synj is specifically localized to presynaptic terminals and is associated with synaptic vesicles. The electrophysiological and ultrastructural defects observed in synj mutants are strikingly similar to those found in endophilin mutants, and Synj and Endo colocalize and interact biochemically. Moreover, synj; endo double mutant synaptic terminals exhibit properties that are very similar to terminals of each single mutant, and overexpression of Endophilin can partially rescue the functional defects in partial loss-of-function synj mutants. Interestingly, Synj is mislocalized and destabilized at synapses devoid of Endophilin, suggesting that Endophilin recruits and stabilizes Synj on newly formed vesicles to promote vesicle uncoating. Our data also provide further evidence that kiss-and-run is able to maintain neurotransmitter release when synapses are not extensively challenged.