The Drosophila mushroom body is a quadruple structure of clonal units each of which contains a virtually identical set of neurones and glial cells

Modeling spinal muscular atrophy in Drosophila links Smn to FGF signaling

FGF signaling in neurons is regulated by Survival Motor Neuron, a component of a complex that regulates snRNP biogenesis and FGF receptor expression.

Spinal muscular atrophy (SMA), a devastating neurodegenerative disorder characterized by motor neuron loss and muscle atrophy, has been linked to mutations in the Survival Motor Neuron (SMN) gene. Based on an SMA model we developed in Drosophila, which displays features that are analogous to the human pathology and vertebrate SMA models, we functionally linked the fibroblast growth factor (FGF) signaling pathway to the Drosophila homologue of SMN, Smn. Here, we characterize this relationship and demonstrate that Smn activity regulates the expression of FGF signaling components and thus FGF signaling. Furthermore, we show that alterations in FGF signaling activity are able to modify the neuromuscular junction defects caused by loss of Smn function and that muscle-specific activation of FGF is sufficient to rescue Smn-associated abnormalities.

The female-specific doublesex isoform regulates pleiotropic transcription factors to pattern genital development in Drosophila

Regulatory networks driving morphogenesis of animal genitalia must integrate sexual identity and positional information. Although the genetic hierarchy that controls somatic sexual identity in the fly Drosophila melanogaster is well understood, there are very few cases in which the mechanism by which it controls tissue-specific gene activity is known. In flies, the sex-determination hierarchy terminates in the doublesex (dsx) gene, which produces sex-specific transcription factors via alternative splicing of its transcripts. To identify sex-specifically expressed genes downstream of dsx that drive the sexually dimorphic development of the genitalia, we performed genome-wide transcriptional profiling of dissected genital imaginal discs of each sex at three time points during early morphogenesis. Using a stringent statistical threshold, we identified 23 genes that have sex-differential transcript levels at all three time points, of which 13 encode transcription factors, a significant enrichment. We focus here on three sex-specifically expressed transcription factors encoded by lozenge (lz), Drop (Dr) and AP-2. We show that, in female genital discs, Dsx activates lz and represses Dr and AP-2. We further show that the regulation of Dr by Dsx mediates the previously identified expression of the fibroblast growth factor Branchless in male genital discs. The phenotypes we observe upon loss of lz or Dr function in genital discs explain the presence or absence of particular structures in dsx mutant flies and thereby clarify previously puzzling observations. Our time course of expression data also lays the foundation for elucidating the regulatory networks downstream of the sex-specifically deployed transcription factors.

Regulation of somatic myosin activity by protein phosphatase 1β controls Drosophila oocyte polarization

The Drosophila body axes are established in the oocyte during oogenesis. Oocyte polarization is initiated by Gurken, which signals from the germline through the epidermal growth factor receptor (Egfr) to the posterior follicle cells (PFCs). In response the PFCs generate an unidentified polarizing signal that regulates oocyte polarity. We have identified a loss-of-function mutation of flapwing, which encodes the catalytic subunit of protein phosphatase 1β (PP1β) that disrupts oocyte polarization. We show that PP1β, by regulating myosin activity, controls the generation of the polarizing signal. Excessive myosin activity in the PFCs causes oocyte mispolarization and defective Notch signaling and endocytosis in the PFCs. The integrated activation of JAK/STAT and Egfr signaling results in the sensitivity of PFCs to defective Notch. Interestingly, our results also demonstrate a role of PP1β in generating the polarizing signal independently of Notch, indicating a direct involvement of somatic myosin activity in axis formation.

A transient expression of Prospero promotes cell cycle exit of Drosophila postembryonic neurons through the regulation of Dacapo

Cell proliferation, specification and terminal differentiation must be precisely coordinated during brain development to ensure the correct production of different neuronal populations. Most Drosophila neuroblasts (NBs) divide asymmetrically to generate a new NB and an intermediate progenitor called ganglion mother cell (GMC) which divides only once to generate two postmitotic cells called ganglion cells (GCs) that subsequently differentiate into neurons. During the asymmetric division of NBs, the homeodomain transcription factor PROSPERO is segregated into the GMC where it plays a key role as cell fate determinant. Previous work on embryonic neurogenesis has shown that PROSPERO is not expressed in postmitotic neuronal progeny. Thus, PROSPERO is thought to function in the GMC by repressing genes required for cell-cycle progression and activating genes involved in terminal differentiation. Here we focus on postembryonic neurogenesis and show that the expression of PROSPERO is transiently upregulated in the newly born neuronal progeny generated by most of the larval NBs of the OL and CB. Moreover, we provide evidence that this expression of PROSPERO in GCs inhibits their cell cycle progression by activating the expression of the cyclin-dependent kinase inhibitor (CKI) DACAPO. These findings imply that PROSPERO, in addition to its known role as cell fate determinant in GMCs, provides a transient signal to ensure a precise timing for cell cycle exit of prospective neurons, and hence may link the mechanisms that regulate neurogenesis and those that control cell cycle progression in postembryonic brain development.

JAK/STAT and the GATA factor Pannier control hemocyte maturation and differentiation in Drosophila

The lymph gland is the major site of hematopoiesis in Drosophila. During late larval stages three types of hemocytes are produced, plasmatocytes, crystal cells, and lamellocytes, and their differentiation is tightly controlled by conserved factors and signaling pathways. JAK/STAT is one of these pathways which have essential roles in vertebrate and fly hematopoiesis. We show that Stat has opposing cell-autonomous and non-autonomous functions in hemocyte differentiation. Using a clonal approach we established that loss of Stat in a set of prohemocytes in the cortical zone induces plasmatocyte maturation in adjacent hemocytes. Hemocytes lacking Stat fail to differentiate into plasmatocytes, indicating that Stat positively and cell-autonomously controls plasmatocyte differentiation. We also identified the GATA factor pannier (pnr) as a downstream target of Stat. By analyzing the phenotypes resulting from clonal loss and over-expression of pnr in lymph glands, we find that Pnr is positively regulated by Stat and specifically required for the differentiation of plasmatocytes. Stat and Pnr represent two essential factors controlling blood cell maturation in the developing lymph gland and exert their functions both in a cell-autonomous and non-cell-autonomous manner.

DREF is critical for Drosophila bristle development by regulating endoreplication in shaft cells

DREF (DNA replication-related element-binding factor) plays important roles in replication and proliferation in vivo by regulating transcription of various genes. However, due to a lack of appropriate cell biological studies in vivo, roles of DREF during a single cell development are poorly understood. To address this question, we focused our attention on macrochaetes bristle development system. Utilizing cell lineage analysis focusing on a single posterior scutellar (PSC) macrochaete sensory organ precursor (SOP) lineages in combination with GAL4/UAS targeted expression system for DREF double strand RNA, we revealed that DREF plays no apparent role in differentiation process during SOP formation. Rather, DREF regulates the timing of asymmetric cell division but perhaps plays no direct role in differentiation during asymmetric cell division. Most importantly, DREF affected replication and growth in shaft cells and/or socket cells. Further analysis revealed that DREF is necessary but not sufficient for nuclear growth and protein synthesis in shaft cells. Finally, it could be demonstrated that DREF plays a critical role in regulating pcna transcription in endocycling shaft cells. All these results provide evidence that DREF plays critical roles, especially in endoreplication process of bristle development, at least in part by regulating the pcna gene expression.

Concentric zones, cell migration and neuronal circuits in the Drosophila visual center

The Drosophila optic lobe comprises a wide variety of neurons, which form laminar neuropiles with columnar units and topographic projections from the retina. The Drosophila optic lobe shares many structural characteristics with mammalian visual systems. However, little is known about the developmental mechanisms that produce neuronal diversity and organize the circuits in the primary region of the optic lobe, the medulla. Here, we describe the key features of the developing medulla and report novel phenomena that could accelerate our understanding of the Drosophila visual system. The identities of medulla neurons are pre-determined in the larval medulla primordium, which is subdivided into concentric zones characterized by the expression of four transcription factors: Drifter, Runt, Homothorax and Brain-specific homeobox (Bsh). The expression pattern of these factors correlates with the order of neuron production. Once the concentric zones are specified, the distribution of medulla neurons changes rapidly. Each type of medulla neuron exhibits an extensive but defined pattern of migration during pupal development. The results of clonal analysis suggest homothorax is required to specify the neuronal type by regulating various targets including Bsh and cell-adhesion molecules such as N-cadherin, while drifter regulates a subset of morphological features of Drifter-positive neurons. Thus, genes that show the concentric zones may form a genetic hierarchy to establish neuronal circuits in the medulla.

Sulfated is a negative feedback regulator of wingless in Drosophila

Drosophila Wingless (Wg) acts as a morphogen to control pattern formation in a concentration dependent manner. Previous studies demonstrated important roles of heparan sulfate proteoglycans (HSPGs) in controlling Wg signaling and distribution. Here, we examined the role of Sulfated (Sulf1), a Drosophila homolog of vertebrate heparan sulfate 6-O endosulfatase, in Wg signaling and distribution. We show that sulf1 is specifically up-regulated by Wg signaling in the wing disc. We found that expression of Wg target gene senseless (sens) was elevated in the sulf1 mutant wing discs. Sulf1 also negatively regulate extracellular levels of Wg. Genetic interaction experiments indicate that Wg antagonist Notum may work synergistically with Sulf1 to restrict Wg signaling, and Dally, a member of Drosophila HSPGs, is a potential target of Sulf1. Our results demonstrate that sulf1 is a novel Wg target gene and by a feedback mechanism, it negatively regulated Wg signaling and distribution in vivo.

Flybow: genetic multicolor cell labeling for neural circuit analysis in Drosophila melanogaster

To facilitate studies of neural network architecture and formation, we generated three Drosophila melanogaster variants of the mouse Brainbow-2 system, called Flybow. Sequences encoding different membrane-tethered fluorescent proteins were arranged in pairs within cassettes flanked by recombination sites. Flybow combines the Gal4-upstream activating sequence binary system to regulate transgene expression and an inducible modified Flp-FRT system to drive inversions and excisions of cassettes. This provides spatial and temporal control over the stochastic expression of one of two or four reporters within one sample. Using the visual system, the embryonic nervous system and the wing imaginal disc, we show that Flybow in conjunction with specific Gal4 drivers can be used to visualize cell morphology with high resolution. Finally, we demonstrate that this labeling approach is compatible with available Flp-FRT-based techniques, such as mosaic analysis with a repressible cell marker; this could further support the genetic analysis of neural circuit assembly and function.

The bHLH factor deadpan is a direct target of Notch signaling and regulates neuroblast self-renewal in Drosophila

A defining feature of stem cells is their capacity to renew themselves at each division while producing differentiated progeny. How these cells balance self-renewal versus differentiation is a fundamental issue in developmental and cancer biology. The Notch signaling pathway has long been known to influence cell fate decisions during development. Indeed, there is a great deal of evidence correlating its function with the regulation of neuroblast (NB) self-renewal during larval brain development in Drosophila. However, little is known about the transcription factors regulated by this pathway during this process. Here we show that deadpan (dpn), a gene encoding a bHLH transcription factor, is a direct target of the Notch signaling pathway during type II NB development. Type II NBs undergo repeated asymmetric divisions to self-renew and to produce immature intermediate neural progenitors. These cells mature into intermediate neural progenitors (INPs) that have the capacity to undergo multiple rounds of asymmetric division to self-renew and to generate GMCs and neurons. Our results indicate that the expression of dpn at least in INPs cells depends on Notch signaling. The ectopic expression of dpn in immature INP cells can transform these cells into NBs-like cells that divide uncontrollably causing tumor over-growth. We show that in addition to dpn, Notch signaling must be regulating other genes during this process that act redundantly with dpn.

MAP4K3 is a component of the TORC1 signalling complex that modulates cell growth and viability in Drosophila melanogaster

Background

MAP4K3 is a conserved Ser/Thr kinase that has being found in connection with several signalling pathways, including the Imd, EGFR, TORC1 and JNK modules, in different organisms and experimental assays. We have analyzed the consequences of changing the levels of MAP4K3 expression in the development of the Drosophila wing, a convenient model system to characterize gene function during epithelial development.

Methodology and Principal Findings

Using loss-of-function mutants and over-expression conditions we find that MAP4K3 activity affects cell growth and viability in the Drosophila wing. These requirements are related to the modulation of the TORC1 and JNK signalling pathways, and are best detected when the larvae grow in a medium with low protein concentration (TORC1) or are exposed to irradiation (JNK). We also show that MAP4K3 display strong genetic interactions with different components of the InR/Tor signalling pathway, and can interact directly with the GTPases RagA and RagC and with the multi-domain kinase Tor.

Conclusions and Significance

We suggest that MAP4K3 has two independent functions during wing development, one related to the activation of the JNK pathway in response to stress and other in the assembling or activation of the TORC1 complex, being critical to modulate cellular responses to changes in nutrient availability.

Neural architecture of the primary gustatory center of Drosophila melanogaster visualized with GAL4 and LexA enhancer-trap systems

Gustatory information is essential for animals to select edible foods and avoid poisons. Whereas mammals detect tastants with their taste receptor cells, which convey gustatory signals to the brain indirectly via the taste sensory neurons, insect gustatory receptor neurons (GRNs) send their axons directly to the primary gustatory center in the suboesophageal ganglion (SOG). In spite of this relatively simple architecture, the precise structure of the insect primary gustatory center has not been revealed in enough detail. To obtain comprehensive anatomical knowledge about this brain area, we screened the Drosophila melanogaster GAL4 enhancer-trap strains that visualize specific subsets of the gustatory neurons as well as putative mechanosensory neurons associated with the taste pegs. Terminals of these neurons form three branches in the SOG. To map the positions of their arborization areas precisely, we screened newly established LexA::VP16 enhancer-trap strains and obtained a driver line that labels a large subset of peripheral sensory neurons. By double-labeling specific and landmark neurons with GAL4 and LexA strains, we were able to distinguish 11 zones in the primary gustatory center, among which 5 zones were identified newly in this study. Arborization areas of various known GRNs on the labellum, oesophagus, and legs were also mapped in this framework. The putative mechanosensory neurons terminate exclusively in three zones of these areas, supporting the notion of segregated primary centers that are specialized for chemosensory and mechanosensory signals associated with gustatory sensation.

Drosophila neural stem cells: cell cycle control of self-renewal, differentiation, and termination in brain development

The wealth of neurons that make up the brain are generated through the proliferative activity of neural stem cells during development. This neurogenesis activity involves complex cell cycle control of proliferative self-renewal, differentiation, and termination processes in these cells. Considerable progress has been made in understanding these processes in the neural stem cell-like neuroblasts which generate the brain in the genetic model system Drosophila. Neuroblasts in the developing fly brain generate neurons through repeated series of asymmetrical cell divisions, which balance self-renewal of the neuroblast with generation of differentiated progeny through the segregation of cell fate determinants such as Numb, Prospero, and Brat to the neural progeny. A number of classical cell cycle regulators such as cdc2/CDK1, Polo, Aurora A, and cyclin E are implicated in the control of asymmetric divisions in neuroblasts linking the cell cycle to the asymmetrical division machinery. The cellular and molecular identity of the postmitotic neurons produced by proliferating neuroblasts is influenced by the timing of their exit from the cell cycle through the action of a temporal expression series of transcription factors, which include Hunchback, Kruppel, Pdm, and Castor. This temporal series is also implicated in the control of termination of neuroblast proliferation which is effected by two different cell cycle exit strategies, terminal differentiative division or programmed cell death of the neuroblast. Defects in the asymmetric division machinery which interfere with the termination of proliferation can result in uncontrolled tumorigenic overgrowth. These findings in Drosophila brain development are likely to have general relevance in neural stem cell biology and may apply to cell cycle control in mammalian brain development as well.

Atg1-mediated myosin II activation regulates autophagosome formation during starvation-induced autophagy

Autophagy is a membrane-mediated degradation process of macromolecule recycling. Although the formation of double-membrane degradation vesicles (autophagosomes) is known to have a central role in autophagy, the mechanism underlying this process remains elusive. The serine/threonine kinase Atg1 has a key role in the induction of autophagy. In this study, we show that overexpression of Drosophila Atg1 promotes the phosphorylation-dependent activation of the actin-associated motor protein myosin II. A novel myosin light chain kinase (MLCK)-like protein, Spaghetti-squash activator (Sqa), was identified as a link between Atg1 and actomyosin activation. Sqa interacts with Atg1 through its kinase domain and is a substrate of Atg1. Significantly, myosin II inhibition or depletion of Sqa compromised the formation of autophagosomes under starvation conditions. In mammalian cells, we found that the Sqa mammalian homologue zipper-interacting protein kinase (ZIPK) and myosin II had a critical role in the regulation of starvation-induced autophagy and mammalian Atg9 (mAtg9) trafficking when cells were deprived of nutrients. Our findings provide evidence of a link between Atg1 and the control of Atg9-mediated autophagosome formation through the myosin II motor protein.

A conserved function of the chromatin ATPase Kismet in the regulation of hedgehog expression

The development of the Drosophila melanogaster wing depends on its subdivision into anterior and posterior compartments, which constitute two independent cell lineages since their origin in the embryonic ectoderm. The anterior-posterior compartment boundary is the place where signaling by the Hedgehog pathway takes place, and this requires pathway activation in anterior cells by ligand expressed exclusively in posterior cells. Several mechanisms ensure the confinement of hedgehog expression to posterior cells, including repression by Cubitus interruptus, the co-repressor Groucho and Master of thick veins. In this work we identified Kismet, a chromodomain-containing protein of the SNF2-like family of ATPases, as a novel component of the hedgehog transcriptional repression mechanism in anterior compartment cells. In kismet mutants, hedgehog is ectopically expressed in a domain of anterior cells close to the anterior-posterior compartment boundary, causing inappropriate activation of the pathway and changes in the development of the central region of the wing. The contribution of Kismet to the silencing of hedgehog expression is limited to anterior cells with low levels of the repressor form of Cubitus interruptus. We also show that knockdown of CHD8, the kismet homolog in Xenopus tropicalis, is also associated with ectopic sonic hedgehog expression and up-regulation of one of its target genes in the eye, Pax2, indicating the evolutionary conservation of Kismet/CHD8 function in negatively controlling hedgehog expression.

Asymmetric cell division: recent developments and their implications for tumour biology

The ability of cells to divide asymmetrically is essential for generating diverse cell types during development. The past 10 years have seen tremendous progress in our understanding of this important biological process. We have learned that localized phosphorylation events are responsible for the asymmetric segregation of cell fate determinants in mitosis and that centrosomes and microtubules play important parts in this process. The relevance of asymmetric cell division for stem cell biology has added a new dimension to the field, and exciting connections between asymmetric cell division and tumorigenesis have begun to emerge.

Embryonic multipotent progenitors remodel the Drosophila airways during metamorphosis

Adult structures in holometabolous insects such as Drosophila are generated by groups of imaginal cells dedicated to the formation of different organs. Imaginal cells are specified in the embryo and remain quiescent until the larval stages, when they proliferate and differentiate to form organs. The Drosophila tracheal system is extensively remodeled during metamorphosis by a small number of airway progenitors. Among these, the spiracular branch tracheoblasts are responsible for the generation of the pupal and adult abdominal airways. To understand the coordination of proliferation and differentiation during organogenesis of tubular organs, we analyzed the remodeling of Drosophila airways during metamorphosis. We show that the embryonic spiracular branch tracheoblasts are multipotent cells that express the homeobox transcription factor Cut, which is necessary for their survival and normal development. They give rise to three distinct cell populations at the end of larval development, which generate the adult tracheal tubes, the spiracle and the epidermis surrounding the spiracle. Our study establishes the series of events that lead to the formation of an adult tubular structure in Drosophila.

Dorsal eye selector pannier (pnr) suppresses the eye fate to define dorsal margin of the Drosophila eye

Axial patterning is crucial for organogenesis. During Drosophila eye development, dorso-ventral (DV) axis determination is the first lineage restriction event. The eye primordium begins with a default ventral fate, on which the dorsal eye fate is established by expression of the GATA-1 transcription factor pannier (pnr). Earlier, it was suggested that loss of pnr function induces enlargement in the dorsal eye due to ectopic equator formation. Interestingly, we found that in addition to regulating DV patterning, pnr suppresses the eye fate by downregulating the core retinal determination genes eyes absent (eya), sine oculis (so) and dacshund (dac) to define the dorsal eye margin. We found that pnr acts downstream of Ey and affects the retinal determination pathway by suppressing eya. Further analysis of the "eye suppression" function of pnr revealed that this function is likely mediated through suppression of the homeotic gene teashirt (tsh) and is independent of homothorax (hth), a negative regulator of eye. Pnr expression is restricted to the peripodial membrane on the dorsal eye margin, which gives rise to head structures around the eye, and pnr is not expressed in the eye disc proper that forms the retina. Thus, pnr has dual function, during early developmental stages pnr is involved in axial patterning whereas later it promotes the head specific fate. These studies will help in understanding the developmental regulation of boundary formation of the eye field on the dorsal eye margin.

Negative regulation of Drosophila JAK-STAT signalling by endocytic trafficking

Appropriate regulation of signal transduction pathways is essential for normal development and is often disrupted in disease. Therefore, many regulatory mechanisms and feedback loops have evolved to ensure appropriate signalling. One mechanism previously suggested to modulate a range of signal transduction pathways involves the internalisation and destruction of transmembrane receptors by the endocytic trafficking machinery. Strikingly, a recent report has suggested that the endocytic trafficking of the Drosophila JAK-STAT pathway receptor Domeless (Dome) does not act to downregulate pathway activity, but rather is necessary for in vivo signalling. Here, we examine this relationship to address the interaction of Drosophila JAK-STAT pathway signalling and endocytic trafficking. We show that Dome is trafficked through clathrin-mediated endocytosis and a directed RNAi screen identified several components of the endocytic machinery as negative regulators of pathway signalling. We demonstrate that Dome signals both from the plasma membrane and internalised vesicles and show, using knockdown experiments, that endocytic components negatively regulate JAK-STAT signalling in vivo. As such, disruption in endocytic trafficking represents a potent negative regulator of the disease relevant JAK-STAT signalling cascade.

The cytoskeletal regulator zyxin is required for viability in Drosophila melanogaster

The zyxin family of proteins function as cytoskeletal regulators in adhesion, actin assembly, and cell motility. Though fibroblasts derived from zyxin-null mice show striking defects in motility and response to mechanical stimuli, the mice are viable and fertile. In Drosophila melanogaster, the family is represented by a single homologue, Zyx102. To study the role of zyxin during development, we generated a zyx102 RNA-interference transgenic line that allows for the conditional knockdown of Zyx102. When UAST-zyx102-dsRNAi expression is driven broadly by Actin5C-GAL4, loss of Zyx102 results in lethality during the pharate adult stage, a narrow developmental window during which the fly must molt, resorb molting fluid, fill adult trachea with air, and execute a behavioral program to eclose. Zyx102 knockdown animals attempt to emerge, but their adult trachea do not fill with air. If dissected from the pupal case, knockdown individuals appear morphologically normal, but remain inviable.

Coordinated sequential action of EGFR and Notch signaling pathways regulates proneural wave progression in the Drosophila optic lobe

During neurogenesis in the medulla of the Drosophila optic lobe, neuroepithelial cells are programmed to differentiate into neuroblasts at the medial edge of the developing optic lobe. The wave of differentiation progresses synchronously in a row of cells from medial to the lateral regions of the optic lobe, sweeping across the entire neuroepithelial sheet; it is preceded by the transient expression of the proneural gene lethal of scute [l(1)sc] and is thus called the proneural wave. We found that the epidermal growth factor receptor (EGFR) signaling pathway promotes proneural wave progression. EGFR signaling is activated in neuroepithelial cells and induces l(1)sc expression. EGFR activation is regulated by transient expression of Rhomboid (Rho), which is required for the maturation of the EGF ligand Spitz. Rho expression is also regulated by the EGFR signal. The transient and spatially restricted expression of Rho generates sequential activation of EGFR signaling and assures the directional progression of the differentiation wave. This study also provides new insights into the role of Notch signaling. Expression of the Notch ligand Delta is induced by EGFR, and Notch signaling prolongs the proneural state. Notch signaling activity is downregulated by its own feedback mechanism that permits cells at proneural states to subsequently develop into neuroblasts. Thus, coordinated sequential action of the EGFR and Notch signaling pathways causes the proneural wave to progress and induce neuroblast formation in a precisely ordered manner.

Recognition of pre- and postsynaptic neurons via nephrin/NEPH1 homologs is a basis for the formation of the Drosophila retinotopic map

Topographic maps, which maintain the spatial order of neurons in the order of their axonal connections, are found in many parts of the nervous system. Here, we focus on the communication between retinal axons and their postsynaptic partners, lamina neurons, in the first ganglion of the Drosophila visual system, as a model for the formation of topographic maps. Post-mitotic lamina precursor cells differentiate upon receiving Hedgehog signals delivered through newly arriving retinal axons and, before maturing to extend neurites, extend short processes toward retinal axons to create the lamina column. The lamina column provides the cellular basis for establishing stereotypic synapses between retinal axons and lamina neurons. In this study, we identified two cell-adhesion molecules: Hibris, which is expressed in post-mitotic lamina precursor cells; and Roughest, which is expressed on retinal axons. Both proteins belong to the nephrin/NEPH1 family. We provide evidence that recognition between post-mitotic lamina precursor cells and retinal axons is mediated by interactions between Hibris and Roughest. These findings revealed mechanisms by which axons of presynaptic neurons deliver signals to induce the development of postsynaptic partners at the target area. Postsynaptic partners then recognize the presynaptic axons to make ensembles, thus establishing a topographic map along the anterior/posterior axis.

Characterization of a dominant-active STAT that promotes tumorigenesis in Drosophila

Little is known about the molecular mechanisms by which STAT proteins promote tumorigenesis. Drosophila is an ideal system for investigating this issue, as there is a single STAT (Stat92E), and its hyperactivation causes overgrowths resembling human tumors. Here we report the first identification of a dominant-active Stat92E protein, Stat92E(DeltaNDeltaC), which lacks both N- and C-termini. Mis-expression of Stat92E(DeltaNDeltaC)in vivo causes melanotic tumors, while in vitro it transactivates a Stat92E-luciferase reporter in the absence of stimulation. These gain-of-function phenotypes require phosphorylation of Y(711) and dimer formation with full-length Stat92E. Furthermore, a single point mutation, an R(442P) substitution in the DNA-binding domain, abolishes Stat92E function. Recombinant Stat92E(R442P) translocates to the nucleus following activation but fails to function in all assays tested. Interestingly, R(442) is conserved in most STATs in higher organisms, suggesting conservation of function. Modeling of Stat92E indicates that R(442) may contact the minor groove of DNA via invariant TC bases in the consensus binding element bound by all STAT proteins. We conclude that the N- and C- termini function unexpectedly in negatively regulating Stat92E activity, possibly by decreasing dimer dephosphorylation or increasing stability of DNA interaction, and that Stat92E(R442) has a nuclear function by altering dimer:DNA binding.

The role of Drosophila Lamin C in muscle function and gene expression

The inner side of the nuclear envelope (NE) is lined with lamins, a meshwork of intermediate filaments that provides structural support for the nucleus and plays roles in many nuclear processes. Lamins, classified as A- or B-types on the basis of biochemical properties, have a conserved globular head, central rod and C-terminal domain that includes an Ig-fold structural motif. In humans, mutations in A-type lamins give rise to diseases that exhibit tissue-specific defects, such as Emery-Dreifuss muscular dystrophy. Drosophila is being used as a model to determine tissue-specific functions of A-type lamins in development, with implications for understanding human disease mechanisms. The GAL4-UAS system was used to express wild-type and mutant forms of Lamin C (the presumed Drosophila A-type lamin), in an otherwise wild-type background. Larval muscle-specific expression of wild type Drosophila Lamin C caused no overt phenotype. By contrast, larval muscle-specific expression of a truncated form of Lamin C lacking the N-terminal head (Lamin C DeltaN) caused muscle defects and semi-lethality, with adult 'escapers' possessing malformed legs. The leg defects were due to a lack of larval muscle function and alterations in hormone-regulated gene expression. The consequences of Lamin C association at a gene were tested directly by targeting a Lamin C DNA-binding domain fusion protein upstream of a reporter gene. Association of Lamin C correlated with localization of the reporter gene at the nuclear periphery and gene repression. These data demonstrate connections among the Drosophila A-type lamin, hormone-induced gene expression and muscle function.

Dopamine reveals neural circuit mechanisms of fly memory

A goal of memory research is to understand how changing the weight of specific synapses in neural circuits in the brain leads to an appropriate learned behavioral response. Finding the relevant synapses should allow investigators to probe the underlying physiological and molecular operations that encode memories and permit their retrieval. In this review I discuss recent work in Drosophila that implicates specific subsets of dopaminergic (DA) neurons in aversive reinforcement and appetitive motivation. The zonal architecture of these DA neurons is likely to reveal the functional organization of aversive and appetitive memory in the mushroom bodies. Combinations of fly DA neurons might code negative and positive value, consistent with a motivational systems role as proposed in mammals.

Development-based compartmentalization of the Drosophila central brain

The neuropile of the Drosophila brain is subdivided into anatomically discrete compartments. Compartments are rich in terminal neurite branching and synapses; they are the neuropile domains in which signal processing takes place. Compartment boundaries are defined by more or less dense layers of glial cells as well as long neurite fascicles. These fascicles are formed during the larval period, when the approximately 100 neuronal lineages that constitute the Drosophila central brain differentiate. Each lineage forms an axon tract with a characteristic trajectory in the neuropile; groups of spatially related tracts congregate into the brain fascicles that can be followed from the larva throughout metamorphosis into the adult stage. Here we provide a map of the adult brain compartments and the relevant fascicles defining compartmental boundaries. We have identified the neuronal lineages contributing to each fascicle, which allowed us to compare compartments of the larval and adult brain directly. Most adult compartments can be recognized already in the early larval brain, where they form a "protomap" of the later adult compartments. Our analysis highlights the morphogenetic changes shaping the Drosophila brain; the data will be important for studies that link early-acting genetic mechanisms to the adult neuronal structures and circuits controlled by these mechanisms.

Role of lipid metabolism in smoothened derepression in hedgehog signaling

The binding of Hedgehog (Hh) to its receptor Patched causes derepression of Smoothened (Smo), resulting in the activation of the Hh pathway. Here, we show that Smo activation is dependent on the levels of the phospholipid phosphatidylinositol-4 phosphate (PI4P). Loss of STT4 kinase, which is required for the generation of PI4P, exhibits hh loss-of-function phenotypes, whereas loss of Sac1 phosphatase, which is required for the degradation of PI4P, results in hh gain-of-function phenotypes in multiple settings during Drosophila development. Furthermore, loss of Ptc function, which results in the activation of Hh pathway, also causes an increase in PI4P levels. Sac1 functions downstream of STT4 and Ptc in the regulation of Smo membrane localization and Hh pathway activation. Taken together, our results suggest a model in which Ptc directly or indirectly functions to suppress the accumulation of PI4P. Binding of Hh to Ptc derepresses the levels of PI4P, which, in turn, promotes Smo activation.

Loss of Trx-2 enhances oxidative stress-dependent phenotypes in Drosophila

Overexpression of thioredoxin (TRX) confers oxidative stress resistance and extends lifespan in mammals and insects. However, less is known about phenotypes associated with loss of TRX. We investigated loss-of-function phenotypes of Trx-2 in Drosophila, and found that the mutant flies are hyper-susceptible to paraquat, a free radical generator, but not to hydrogen peroxide. They contain a high amount of protein carbonyl, which dramatically increases with age. Trx-2 mutants express high levels of anti-oxidant genes, such as superoxide dismutase, catalase, and glutathione synthetase. This is the first demonstration of biochemical and physiological consequences caused by loss of Trx-2 in Drosophila.

Bap170, a subunit of the Drosophila PBAP chromatin remodeling complex, negatively regulates the EGFR signaling

BAP and PBAP constitute the two different forms of the Drosophila melanogaster Brahma chromatin remodelers. A common multisubunit core, containing the Brahma ATPase, can associate either with Osa to form the BAP complex or with Bap170, Bap180, and Sayp to constitute the PBAP complex. Although required for many biological processes, recent genetic analyses revealed that one role of the BAP complex during Drosophila wing development is the proper regulation of EGFR target genes. Here, we show that Bap170, a distinctive subunit of the PBAP complex, participates instead in the negative regulation of EGFR signaling. In adults, loss of Bap170 generates phenotypes similar to the defects induced by hyperactivation of the EGFR pathway, such as overrecruitment of cone and photoreceptor cells and formation extra veins. In genetic interactions, bap170 mutations suppress the loss of veins and photoreceptors caused by mutations affecting the activity of the EGFR pathway. Our results suggest a dual requirement of the PBAP complex: for transcriptional repression of rhomboid and for efficient expression of argos. Interestingly, genetic evidence also indicates that Bap170-mediated repression of rho is inhibited by EGFR signaling, suggesting a scenario of mutual antagonism between EGFR signaling and PBAP function.

The Drosophila neural lineages: a model system to study brain development and circuitry

In Drosophila, neurons of the central nervous system are grouped into units called lineages. Each lineage contains cells derived from a single neuroblast. Due to its clonal nature, the Drosophila brain is a valuable model system to study neuron development and circuit formation. To better understand the mechanisms underlying brain development, genetic manipulation tools can be utilized within lineages to visualize, knock down, or over-express proteins. Here, we will introduce the formation and development of lineages, discuss how one can utilize this model system, offer a comprehensive list of known lineages and their respective markers, and then briefly review studies that have utilized Drosophila neural lineages with a look at how this model system can benefit future endeavors.

Cell lineage tracing reveals the plasticity of the hemocyte lineages and of the hematopoietic compartments in Drosophila melanogaster

Much of our knowledge on hematopoiesis, hematopoietic compartments, hematopoietic cell lineages and immunity has been derived from studies on the vertebrate immune system. The sophisticated innate immunity of insects, the phylogenetic conservation and the power of Drosophila genetics allowed the investigation of immune cell (hemocyte) lineage relationships in Drosophila melanogaster. The development of the hemocyte lineages in Drosophila is a result of a precisely regulated succession of intracellular and intercellular events, though the nature and extent of these interactions are not known. We describe here a cell lineage tracing system set up to analyze the development of hemocyte lineages and functionally distinct hemocyte subsets. This system allowed us to distinguish two major embryonic hemocyte lineages, the crq and Dot lineages, in two, physically separated compartments, the embryonic macrophages and the embryonic lymph gland. We followed the fate and development of these lineages in the construction of the larval hematopoietic compartments and during the cell-mediated immune response, the encapsulation reaction. Our results revealed the considerable plasticity and concerted action of the hematopoietic compartments and the hemocyte lineages in the development of the innate immune system and in the course of the cell-mediated immune response in Drosophila.

The olfactory circuit of the fruit fly Drosophila melanogaster

The olfactory circuit of the fruit fly Drosophila melanogaster has emerged in recent years as an excellent paradigm for studying the principles and mechanisms of information processing in neuronal circuits. We discuss here the organizational principles of the olfactory circuit that make it an attractive model for experimental manipulations, the lessons that have been learned, and future challenges.

Opposing interactions between Drosophila cut and the C/EBP encoded by slow border cells direct apical constriction and epithelial invagination

Stage 10 of Drosophila oogenesis can be subdivided into stages 10A and 10B based on a change in the morphology of the centripetal follicle cells (FC) from a columnar to an apically constricted shape. This coordinated cell shape change drives epithelial cell sheet involution between the oocyte and nurse cell complex which patterns the operculum structure of the mature eggshell. We have shown previously that proper centripetal FC migration requires transient expression of the C/EBP encoded by slow border cells (slbo) at 10A, due in part to Notch activation followed by slbo autorepression (Levine et al., 2007). Here we show that decreased slbo expression in the centripetal FC coincides with increased expression of the transcription factor Cut, a Cut/Cux/CDP family member, at 10B. The 10A/10B temporal switch from Slbo to Cut expression is refined by both cross repression between Slbo and Cut, Slbo auto repression and Cut auto activation. High Cut levels are necessary and sufficient to direct polarized, supracellular accumulation of Actin, DE-cadherin and Armadillo associated with apical constriction of the centripetal FC. Separately, Slbo in the border cell rosette and Cut in the pole cells have antagonistic interactions to restrict Fas2 accumulation to the pole cells, which is important for proper border cell migration. The opposing effects of Cut and Slbo in these two tissues reflect the opposing interactions between their respective mammalian homologs CAAT Displacement Protein (CDP; now CUX1) and CAAT Enhancer Binding Protein (C/EBP) in tissue culture.

Building the central complex in Drosophila: the generation and development of distinct neural subsets

The distinctive midline neuropil, the central complex (CX), is one of the most prominent features of the insect brain. We investigated the development of the four CX structures and several sets of CX neurons in the Drosophila brain using immunostaining for two cell adhesion molecules, DN-cadherin and Echinoid, and a set of seven enhancer trap lines. Our results showed that the CX is first identifiable in the third instar larva and that it elaborates over the first 48 hours of metamorphosis. The first identifiable structures to appear in their immature form are the protocerebral bridge and fan-shaped body, which are present in the brain of the third instar larva, followed by the noduli (from P12h), and finally the ellipsoid body (from P24h). We observed that neurons are added incrementally to the developing CX structure, with sets of small-field neurons projecting to the CX prior to the large-field neurons. The small-field neurons first project to the developing fan-shaped body, before arborizing or extending to the other structures. We found evidence to suggest that small-field neurons exist in sets of 16 and that they originate from eight common clusters of perikarya in the cortex, suggesting a common origin. We also identified a novel set of pontine neurons that connect contralateral segments in the fan-shaped body.

aPKC phosphorylation of Bazooka defines the apical/lateral border in Drosophila epithelial cells

Bazooka (PAR-3), PAR-6, and aPKC form a complex that plays a key role in the polarization of many cell types. In epithelial cells, however, Bazooka localizes below PAR-6 and aPKC at the apical/lateral junction. Here, we show that Baz is excluded from the apical aPKC domain in epithelia by aPKC phosphorylation, which disrupts the Baz/aPKC interaction. Removal of Baz from the complex is epithelial-specific because it also requires the Crumbs complex, which prevents the Baz/PAR-6 interaction. In the absence of Crumbs or aPKC phosphorylation of Baz, mislocalized Baz recruits adherens junction components apically, leading to a loss of the apical domain and an expansion of lateral. Thus, apical exclusion of Baz by Crumbs and aPKC defines the apical/lateral border. Although Baz acts as an aPKC targeting and specificity factor in nonepithelial cells, our results reveal that it performs a complementary function in positioning the adherens junction in epithelia.

chinmo is a functional effector of the JAK/STAT pathway that regulates eye development, tumor formation, and stem cell self-renewal in Drosophila

The Drosophila STAT transcription factor Stat92E regulates diverse functions, including organ development and stem cell self-renewal. However, the Stat92E functional effectors that mediate these processes are largely unknown. Here we show that chinmo is a cell-autonomous, downstream mediator of Stat92E that shares numerous functions with this protein. Loss of either gene results in malformed eyes and head capsules due to defects in eye progenitor cells. Hyperactivation of Stat92E or misexpression of Chinmo results in blood cell tumors. Both proteins are expressed in germline (GSCs) and cyst stem cells (CySCs) in the testis. While Stat92E is required for the self-renewal of both populations, chinmo is only required in CySCs, indicating that Stat92E regulates self-renewal in different stem cells through independent effectors. Like hyperactivated Stat92E, Chinmo misexpression in CySCs is sufficient to maintain GSCs nonautonomously. Chinmo is therefore a key effector of JAK/STAT signaling in a variety of developmental and pathological contexts.

Identification of genes affecting wing patterning through a loss-of-function mutagenesis screen and characterization of med15 function during wing development

The development of the Drosophila melanogaster wing depends on the correct regulation of cell survival, growth, proliferation, differentiation, and pattern formation. These processes, and the genes controlling then, are common to the development of epithelia in many different organisms. To identify additional genes contributing to wing development we have carried out a genetic screen in mosaic wings carrying clones of homozygous mutant cells. We obtained 12 complementation groups corresponding to genes with a proven role in wing formation such as smoothened, thick veins, mothers against dpp, expanded, and fat and 71 new complementation groups affecting the pattern of veins and the size of wing. We mapped one of these groups to the mediator15 gene (med15), a component of the Mediator complex. We show that Med15 and other members of the Mediator complex are required, among other processes, for the transcription of decapentaplegic target genes.

The novel endosomal membrane protein Ema interacts with the class C Vps-HOPS complex to promote endosomal maturation

Endosomal maturation is critical for accurate and efficient cargo transport through endosomal compartments. Here we identify a mutation of the novel Drosophila gene, ema (endosomal maturation defective) in a screen for abnormal synaptic overgrowth and defective protein trafficking. Ema is an endosomal membrane protein required for trafficking of fluid-phase and receptor-mediated endocytic cargos. In the ema mutant, enlarged endosomal compartments accumulate as endosomal maturation fails, with early and late endosomes unable to progress into mature degradative late endosomes and lysosomes. Defective endosomal down-regulation of BMP signaling is responsible for the abnormal synaptic overgrowth. Ema binds to and genetically interacts with Vps16A, a component of the class C Vps-HOPS complex that promotes endosomal maturation. The human orthologue of ema, Clec16A, is a candidate susceptibility locus for autoimmune disorders, and its expression rescues the Drosophila mutant demonstrating conserved function. Characterizing this novel gene family identifies a new component of the endosomal pathway and provides insights into class C Vps-HOPS complex function.

Echinoid regulates Flamingo endocytosis to control ommatidial rotation in the Drosophila eye

Planar cell polarity (PCP) refers to a second polarity axis orthogonal to the apicobasal axis in the plane of the epithelium. The molecular link between apicobasal polarity and PCP is largely unknown. During Drosophila eye development, differentiated photoreceptors form clusters that rotate independently of the surrounding interommatidial cells (ICs). Here, we demonstrate that both Echinoid (Ed), an adherens junction-associated cell adhesion molecule, and Flamingo (Fmi), a PCP determinant, are endocytosed via a clathrin-mediated pathway in ICs. Interestingly, we found that Ed binds the AP-2 adaptor and is required for the internalization of Fmi into ICs. Loss of ed led to increased amounts of Fmi on the cell membrane of non-rotating ICs and also to the misrotation of photoreceptor clusters. Importantly, overexpression of fmi in ICs alone was sufficient to cause misrotation of the adjacent photoreceptor clusters. Together, we propose that Ed, when internalized by AP-2, undergoes co-endocytosis with, and thereby decreases, Fmi levels on non-rotating ICs to permit correct rotation of ommatidial clusters. Thus, co-endocytosis of Ed and Fmi provides a link between apicobasal polarity and PCP.

The unfulfilled gene is required for the development of mushroom body neuropil in Drosophila

BACKGROUND

The mushroom bodies (MBs) of Drosophila are required for complex behaviors and consist of three types of neurons, gamma, alpha'/beta' and alpha/beta. Previously, roles for transcription factors in MB neuronal differentiation have only been described for a subset of MB neurons. We are investigating the roles of unfulfilled (unf; HR51, CG16801) in MB development. unf encodes a nuclear receptor that is orthologous to the nuclear receptors fasciculation of axons defective 1 (FAX-1) of the nematode and photoreceptor specific nuclear receptor (PNR) of mammals. Based on our previous observations that unf transcripts accumulate in MB neurons at all developmental stages and the presence of axon pathfinding defects in fax-1 mutants, we hypothesized that unf regulates MB axon growth and pathfinding.

RESULTS

We show that unf mutants exhibit a range of highly penetrant axon stalling phenotypes affecting all neurons of the larval and adult MBs. Phenotypic analysis of unfX1 mutants revealed that alpha'/beta' and alpha/beta neurons initially project axons but stall prior to the formation of medial or dorsal MB lobes. unfZ0001 mutants form medial lobes, although these axons fail to branch, which results in a failure to form the alpha or alpha' dorsal lobes. In either mutant background, gamma neurons fail to develop larval-specific dorsal projections. These mutant gamma neurons undergo normal pruning, but fail to re-extend axons medially during pupal development. unfRNAi animals displayed phenotypes similar to those seen in unfZ0001 mutants. Unique asymmetrical phenotypes were observed in unfX1/unfZ0001 compound heterozygotes. Expression of UAS-unf transgenes in MB neurons rescues the larval and adult unf mutant phenotypes.

CONCLUSIONS

These data support the hypothesis that unf plays a common role in the development of all types of MB neurons. Our data indicate that unf is necessary for MB axon extension and branching and that the formation of dorsal collaterals is more sensitive to the loss of unf function than medial projections. The asymmetrical phenotypes observed in compound heterozygotes support the hypothesis that the earliest MB axons may serve as pioneers for the later-born MB neurons, providing evidence for pioneer MB axon guidance in post-embryonic development.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

Synaptic organization in the adult Drosophila mushroom body calyx

Insect mushroom bodies are critical for olfactory associative learning. We have carried out an extensive quantitative description of the synaptic organization of the calyx of adult Drosophila melanogaster, the main olfactory input region of the mushroom body. By using high-resolution confocal microscopy, electron microscopy-based three-dimensional reconstructions, and genetic labeling of the neuronal populations contributing to the calyx, we resolved the precise connections between large cholinergic boutons of antennal lobe projection neurons and the dendrites of Kenyon cells, the mushroom body intrinsic neurons. Throughout the calyx, these elements constitute synaptic complexes called microglomeruli. By single-cell labeling, we show that each Kenyon cell's claw-like dendritic specialization is highly enriched in filamentous actin, suggesting that this might be a site of plastic reorganization. In fact, Lim kinase (LimK) overexpression in the Kenyon cells modifies the shape of the microglomeruli. Confocal and electron microscopy indicate that each Kenyon cell claw enwraps a single bouton of a projection neuron. Each bouton is contacted by a number of such claw-like specializations as well as profiles of gamma-aminobutyric acid-positive neurons. The dendrites of distinct populations of Kenyon cells involved in different types of memory are partially segregated within the calyx and contribute to different subsets of microglomeruli. Our analysis suggests, though, that projection neuron boutons can contact more than one type of Kenyon cell. These findings represent an important basis for the functional analysis of the olfactory pathway, including the formation of associative olfactory memories.

Genetic and biochemical definition of the Hedgehog receptor

Although the transporter-like protein Patched (Ptc) is genetically implicated in reception of the extracellular Hedgehog (Hh) protein signal, a clear definition of the Hh receptor is complicated by the existence of additional Hh-binding proteins and, in Drosophila, by the lack of physical evidence for direct binding of Hh to Ptc. Here we show that activity of Ihog (Interference hedgehog), or of its close relative Boi (Brother of Ihog), is absolutely required for Hh biological response and for sequestration of the Hh protein to limit long-range signaling. We demonstrate that Ihog interacts directly with Ptc, is required for presentation of Ptc on the cell surface, and that Ihog and Ptc are both required for high-affinity Hh binding. On the basis of their joint roles in ligand binding, signal transduction, and receptor trafficking, we conclude that Ihog and Ptc together constitute the Drosophila Hh receptor.

Signal strength and signal duration define two distinct aspects of JNK-regulated axon stability

Signaling proteins often control multiple aspects of cell morphogenesis. Yet the mechanisms that govern their pleiotropic behavior are often unclear. Here we show activity levels and timing mechanisms determine distinct aspects of Jun N-terminal kinase (JNK) pathway dependent axonal morphogenesis in Drosophila mushroom body (MB) neurons. In the complete absence of Drosophila JNK (Basket), MB axons fail to stabilize, leading to their subsequent degeneration. However, with a partial loss of Basket (Bsk), or of one of the upstream JNK kinases, Hemipterous or Mkk4, these axons overextend. This suggests that Bsk activity prevents axons from destabilizing, resulting in degeneration and overextension beyond their terminal targets. These distinct phenotypes require different threshold activities involving the convergent action of two distinct JNK kinases. We show that sustained Bsk signals are essential throughout development and act additively but are dispensable at adulthood. We also suggest that graded Bsk inputs are translated into AP-1 transcriptional outputs consisting of Fos and Jun proteins.