A Drosophila gain-of-function screen for candidate genes involved in steroid-dependent neuroendocrine cell remodeling

Taking flight, the use of Drosophila melanogaster for neuroscience research in Uruguay

The Sociedad de Neurociencias del Uruguay is celebrating its 30th anniversary, sustained by more than a century of neuroscience research in the country. During this time, different approaches and experimental organisms have been incorporated to study diverse aspects of neurobiology. One of these experimental animals, successfully used in a variety of biological fields, is the fruit fly Drosophila melanogaster. Although Drosophila has been a model organism for neuroscience research worldwide for many decades, its use in Uruguay for that purpose is relatively new and just taking flight. In this special issue article, we will describe some of the research lines that are currently using Drosophila for neuroscience studies, questioning a wide range of issues including thermoreception, neurodegenerative diseases such as Parkinson's, screening of bioactive compounds with a neuroprotective effect, and gene/protein function during development of the nervous system. The consolidation of these research lines has been achieved due to unique features of D. melanogaster as an experimental model. We will review the advantages of using Drosophila to study neurobiology and describe some of its useful genetic tools. Advantages such as having powerful genetics, highly conserved disease pathways, a complete connectome, very low comparative costs, easy maintenance, and the support of a collaborative community allowing access to a vast toolkit, all make D. melanogaster an ideal model organism for neuroscientists in countries with low levels of investment in research and development. This review focuses on the strengths and description of useful techniques to study neurobiology using Drosophila, from the perspective of a Latin-American experience.

Survival, Movement, and Lifespan: Decoding the Roles of Patched-Related in Drosophila melanogaster

Patched-related (Ptr) is a transmembrane protein implicated in developmental processes in Drosophila melanogaster, yet its precise role remains incompletely understood. Here, we use Ptr23c null mutants to investigate the functional significance of Ptr through the entire life cycle monitoring survival during embryonic, larval, pupal and adult development, and studying larval locomotion and muscle structure. We report that Ptr23c larvae displayed impaired hatching, indicative of defective embryonic development. Moreover, mutant larvae exhibited reduced mobility and lethargy, suggesting a potential involvement of Ptr in neuromuscular function. Morphological analysis of somatic muscles in mutant larvae revealed enlarged cell nuclei. Despite high preadult mortality, a subset of Ptr23c mutant adults display an unexpected extension in lifespan compared to controls, implicating Ptr in the regulation of longevity. Our findings provide critical insights into the multifaceted role of Ptr in Drosophila development, highlighting its contributions to post-embryonic survival, neuromuscular function, and lifespan regulation. This study underscores the significance of exploring broader genetic networks to unravel the complexities of developmental processes.

Teratogenic impacts of Antiepileptic drugs on development, behavior and reproduction in Drosophila melanogaster

Clobazam (CLB) and Vigabatrin (VGB) are the two widely used Antiepileptic drugs, which may have teratogenic potentiality and it has been evaluated in the fruit fly Drosophila melanogaster. These different concentrations of CLB (0.156, 0.25, and 0.312 μg/ml) and VGB (17.6, 22, and 44 μg/ml) were used to evaluate the life-history parameters, developmental, and behavioral abnormalities. The results revealed that life-history parameters (fecundity, fertility, larval and pupal mortality) were significantly affected along with varied developmental duration, and pupal and adult deformities in flies on exposure of CLB and VGB in concentration dependent manner. The present study demonstrated that the prenatal treatment of CLB and VGB has displayed clear teratogenic potentiality with various deformities in the fruit fly. The findings could be correlated with the various abnormalities in human caused by the use of AEDs.

SPOC domain proteins in health and disease

Since it was first described >20 yr ago, the SPOC domain (Spen paralog and ortholog C-terminal domain) has been identified in many proteins all across eukaryotic species. SPOC-containing proteins regulate gene expression on various levels ranging from transcription to RNA processing, modification, export, and stability, as well as X-chromosome inactivation. Their manifold roles in controlling transcriptional output implicate them in a plethora of developmental processes, and their misregulation is often associated with cancer. Here, we provide an overview of the biophysical properties of the SPOC domain and its interaction with phosphorylated binding partners, the phylogenetic origin of SPOC domain proteins, the diverse functions of mammalian SPOC proteins and their homologs, the mechanisms by which they regulate differentiation and development, and their roles in cancer.

Patched-Related Is Required for Proper Development of Embryonic Drosophila Nervous System

Patched-related (Ptr), classified primarily as a neuroectodermal gene, encodes a protein with predicted topology and domain organization closely related to those of Patched (Ptc), the canonical receptor of the Hedgehog (Hh) pathway. To investigate the physiological function of Ptr in the developing nervous system, Ptr null mutant embryos were immunolabeled and imaged under confocal microscopy. These embryos displayed severe alterations in the morphology of the primary axonal tracts, reduced number, and altered distribution of the Repo-positive glia as well as peripheral nervous system defects. Most of these alterations were recapitulated by downregulating Ptr expression, specifically in embryonic nerve cells. Because similar nervous system phenotypes have been observed in hh and ptc mutant embryos, we evaluated the Ptr participation in the Hh pathway by performing cell-based reporter assays. Clone-8 cells were transfected with Ptr-specific dsRNA or a Ptr DNA construct and assayed for changes in Hh-mediated induction of a luciferase reporter. The results obtained suggest that Ptr could act as a negative regulator of Hh signaling. Furthermore, co-immunoprecipitation assays from cell culture extracts premixed with a conditioned medium revealed a direct interaction between Ptr and Hh. Moreover, in vivo Ptr overexpression in the domain of the imaginal wing disc where Engrailed and Ptc coexist produced wing phenotypes at the A/P border. Thus, these results strongly suggest that Ptr plays a crucial role in nervous system development and appears to be a negative regulator of the Hh pathway.

Ultraspiracle-independent anti-apoptotic function of ecdysone receptors is required for the survival of larval peptidergic neurons via suppression of grim expression in Drosophila melanogaster

In Drosophila melanogaster a significant number of heterogenous larval neurons in the central nervous system undergo metamorphosis-associated programmed cell death, termed metamorphoptosis. Interestingly distinct groups of doomed larval neurons are eliminated at different metamorphic phases. Although ecdysone hormonal signaling via nuclear ecdysone receptors (EcRs) is known to orchestrate the neuronal metamorphoptosis, little is known about how this signaling controls such diverse neuronal responses. Crustacean cardioactive peptide (CCAP)-producing neurons in the ventral nerve cord are developmentally programmed to die shortly after adult emergence. In this study, we show that disruption of endogenous EcR function by ectopic expression of dominant negative forms of EcRs (EcR^DN) causes premature death of larval CCAP neurons in a caspase-dependent manner. This event is rescued by co-expression of individual EcR isoforms. Furthermore, larval CCAP neurons are largely normal in ecr mutants lacking either EcR-A or EcR-B isoforms, suggesting that EcR isoforms redundantly function to protect larval CCAP neurons. Of surprise, a role of Ultraspiracle (Usp), a canonical partner of EcR, is dispensable in the protection of CCAP neurons, whereas both EcR and Usp are required for inducing metamorphoptosis of vCrz neurons shortly after prepupal formation. As a downstream, grim is an essential cell death gene for the EcR^DN-mediated CCAP neuronal death, while either hid or rpr function is dispensable. Together, our results suggest that Usp-independent EcR actions protect CCAP neurons from their premature death by repressing grim expression until their normally scheduled apoptosis at post-emergence. Our studies highlight two opposite roles played by EcR function for metamorphoptosis of two different peptidergic neuronal groups, proapoptotic (vCrz) versus antiapoptotic (CCAP), and propose that distinct death timings of doomed larval neurons are determined by differential signaling mechanisms involving EcR.

Cbt modulates Foxo activation by positively regulating insulin signaling in Drosophila embryos

In late Drosophila embryos, the epidermis exhibits a dorsal hole as a consequence of germ band retraction. It is sealed during dorsal closure (DC), a morphogenetic process in which the two lateral epidermal layers converge towards the dorsal midline and fuse. We previously demonstrated the involvement of the Cbt transcription factor in Drosophila DC. However its molecular role in the process remained obscure. In this study, we used genomic approaches to identify genes regulated by Cbt as well as its direct targets during late embryogenesis. Our results reveal a complex transcriptional circuit downstream of Cbt and evidence that it is functionally related with the Insulin/insulin-like growth factor signaling pathway. In this context, Cbt may act as a positive regulator of the pathway, leading to the repression of Foxo activity. Our results also suggest that the DC defects observed in cbt embryos could be partially due to Foxo overactivation and that a regulatory feedback loop between Foxo and Cbt may be operating in the DC context.

Direct measurement of pervasive weak repression by microRNAs and their role at the network level

BACKGROUND

A gene regulatory network (GRN) comprises many weak links that are often regulated by microRNAs. Since miRNAs rarely repress their target genes by more than 30%, doubts have been expressed about the biological relevance of such weak effects. These doubts raise the possibility of under-estimation as miRNA repression is usually estimated indirectly from equilibrium expression levels.

RESULTS

To measure miRNA repression directly, we inhibited transcript synthesis in Drosophila larvae and collected time-course data on mRNA abundance, the decline of which reflects transcript degradation. The rate of target degradation in the absence of miR310s, a moderately expressed miRNA family, was found to decrease by 5 to 15%. A conventional analysis that does not remove transcript synthesis yields an estimate of 6.5%, within the range of the new estimates. These data permit further examinations of the repression mechanisms by miRNAs including seed matching types, APA (alternative polyadenylation) sites, effects of other highly-expressed miRNAs and the length of 3'UTR. Our direct measurements suggest the latter two factors have a measurable effect on decay rate.

CONCLUSION

The direct measurement confirms pervasive weak repression by miRNAs, supporting the conclusions based on indirect assays. The confirmation suggests that this weak repression may indeed be miRNAs' main function. In this context, we discuss the recent proposal that weak repression is "cumulatively powerful" in stabilizing GRNs.

Shep regulates Drosophila neuronal remodeling by controlling transcription of its chromatin targets

Neuronal remodeling is crucial for formation of the mature nervous system and disruption of this process can lead to neuropsychiatric diseases. Global gene expression changes in neurons during remodeling as well as the factors that regulate these changes remain poorly defined. To elucidate this process, we performed RNA-seq on isolated Drosophila larval and pupal neurons and found upregulated synaptic signaling and downregulated gene expression regulators as a result of normal neuronal metamorphosis. We further tested the role of alan shepard (shep), which encodes an evolutionarily conserved RNA-binding protein required for proper neuronal remodeling. Depletion of shep in neurons prevents the execution of metamorphic gene expression patterns, and shep-regulated genes correspond to Shep chromatin and/or RNA-binding targets. Reduced expression of a Shep-inhibited target gene that we identified, brat, is sufficient to rescue neuronal remodeling defects of shep knockdown flies. Our results reveal direct regulation of transcriptional programs by Shep to regulate neuronal remodeling during metamorphosis.

The large and small SPEN family proteins stimulate axon outgrowth during neurosecretory cell remodeling in Drosophila

Split ends (SPEN) is the founding member of a well conserved family of nuclear proteins with critical functions in transcriptional regulation and the post-transcriptional processing and nuclear export of transcripts. In animals, the SPEN proteins fall into two size classes that perform either complementary or antagonistic functions in different cellular contexts. Here, we show that the two Drosophila representatives of this family, SPEN and Spenito (NITO), regulate metamorphic remodeling of the CCAP/bursicon neurosecretory cells. CCAP/bursicon cell-targeted overexpression of SPEN had no effect on the larval morphology or the pruning back of the CCAP/bursicon cell axons at the onset of metamorphosis. During the subsequent outgrowth phase of metamorphic remodeling, overexpression of either SPEN or NITO strongly inhibited axon extension, axon branching, peripheral neuropeptide accumulation, and soma growth. Cell-targeted loss-of-function alleles for both spen and nito caused similar reductions in axon outgrowth, indicating that the absolute levels of SPEN and NITO activity are critical to support the developmental plasticity of these neurons. Although nito RNAi did not affect SPEN protein levels, the phenotypes produced by SPEN overexpression were suppressed by nito RNAi. We propose that SPEN and NITO function additively or synergistically in the CCAP/bursicon neurons to regulate multiple aspects of neurite outgrowth during metamorphic remodeling.

Regulatory Mechanisms of Metamorphic Neuronal Remodeling Revealed Through a Genome-Wide Modifier Screen in Drosophila melanogaster

During development, neuronal remodeling shapes neuronal connections to establish fully mature and functional nervous systems. Our previous studies have shown that the RNA-binding factor alan shepard (shep) is an important regulator of neuronal remodeling during metamorphosis in Drosophila melanogaster, and loss of shep leads to smaller soma size and fewer neurites in a stage-dependent manner. To shed light on the mechanisms by which shep regulates neuronal remodeling, we conducted a genetic modifier screen for suppressors of shep-dependent wing expansion defects and cellular morphological defects in a set of peptidergic neurons, the bursicon neurons, that promote posteclosion wing expansion. Out of 702 screened deficiencies that covered 86% of euchromatic genes, we isolated 24 deficiencies as candidate suppressors, and 12 of them at least partially suppressed morphological defects in shep mutant bursicon neurons. With RNA interference and mutant alleles of individual genes, we identified Daughters against dpp (Dad) and Olig family (Oli) as shep suppressor genes, and both of them restored the adult cellular morphology of shep-depleted bursicon neurons. Dad encodes an inhibitory Smad protein that inhibits bone morphogenetic protein (BMP) signaling, raising the possibility that shep interacted with BMP signaling through antagonism of Dad. By manipulating expression of the BMP receptor tkv, we found that activated BMP signaling was sufficient to rescue loss-of-shep phenotypes. These findings reveal mechanisms of shep regulation during neuronal development, and they highlight a novel genetic shep interaction with the BMP signaling pathway that controls morphogenesis in mature, terminally differentiated neurons during metamorphosis.

A fly's view of neuronal remodeling

Developmental neuronal remodeling is a crucial step in sculpting the final and mature brain connectivity in both vertebrates and invertebrates. Remodeling includes degenerative events, such as neurite pruning, that may be followed by regeneration to form novel connections during normal development. Drosophila provides an excellent model to study both steps of remodeling since its nervous system undergoes massive and stereotypic remodeling during metamorphosis. Although pruning has been widely studied, our knowledge of the molecular and cellular mechanisms is far from complete. Our understanding of the processes underlying regrowth is even more fragmentary. In this review, we discuss recent progress by focusing on three groups of neurons that undergo stereotypic pruning and regrowth during metamorphosis, the mushroom body γ neurons, the dendritic arborization neurons and the crustacean cardioactive peptide peptidergic neurons. By comparing and contrasting the mechanisms involved in remodeling of these three neuronal types, we highlight the common themes and differences as well as raise key questions for future investigation in the field. WIREs Dev Biol 2016, 5:618–635. doi: 10.1002/wdev.241

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As the fat flies: The dynamic lipid droplets of Drosophila embryos

Research into lipid droplets is rapidly expanding, and new cellular and organismal roles for these lipid-storage organelles are continually being discovered. The early Drosophila embryo is particularly well suited for addressing certain questions in lipid-droplet biology and combines technical advantages with unique biological phenomena. This review summarizes key features of this experimental system and the techniques available to study it, in order to make it accessible to researchers outside this field. It then describes the two topics most heavily studied in this system, lipid-droplet motility and protein sequestration on droplets, discusses what is known about the molecular players involved, points to open questions, and compares the results from Drosophila embryo studies to what it is known about lipid droplets in other systems.

Neuronal remodeling during metamorphosis is regulated by the alan shepard (shep) gene in Drosophila melanogaster

Peptidergic neurons are a group of neuronal cells that synthesize and secrete peptides to regulate a variety of biological processes. To identify genes controlling the development and function of peptidergic neurons, we conducted a screen of 545 splice-trap lines and identified 28 loci that drove expression in peptidergic neurons when crossed to a GFP reporter transgene. Among these lines, an insertion in the alan shepard (shep) gene drove expression specifically in most peptidergic neurons. shep transcripts and SHEP proteins were detected primarily and broadly in the central nervous system (CNS) in embryos, and this expression continued into the adult stage. Loss of shep resulted in late pupal lethality, reduced adult life span, wing expansion defects, uncoordinated adult locomotor activities, rejection of males by virgin females, and reduced neuropil area and reduced levels of multiple presynaptic markers throughout the adult CNS. Examination of the bursicon neurons in shep mutant pharate adults revealed smaller somata and fewer axonal branches and boutons, and all of these cellular phenotypes were fully rescued by expression of the most abundant wild-type shep isoform. In contrast to shep mutant animals at the pharate adult stage, shep mutant larvae displayed normal bursicon neuron morphologies. Similarly, shep mutant adults were uncoordinated and weak, while shep mutant larvae displayed largely, although not entirely, normal locomotor behavior. Thus, shep played an important role in the metamorphic development of many neurons.

Insulin signaling regulates neurite growth during metamorphic neuronal remodeling

Although the growth capacity of mature neurons is often limited, some neurons can shift through largely unknown mechanisms from stable maintenance growth to dynamic, organizational growth (e.g. to repair injury, or during development transitions). During insect metamorphosis, many terminally differentiated larval neurons undergo extensive remodeling, involving elimination of larval neurites and outgrowth and elaboration of adult-specific projections. Here, we show in the fruit fly, Drosophila melanogaster (Meigen), that a metamorphosis-specific increase in insulin signaling promotes neuronal growth and axon branching after prolonged stability during the larval stages. FOXO, a negative effector in the insulin signaling pathway, blocked metamorphic growth of peptidergic neurons that secrete the neuropeptides CCAP and bursicon. RNA interference and CCAP/bursicon cell-targeted expression of dominant-negative constructs for other components of the insulin signaling pathway (InR, Pi3K92E, Akt1, S6K) also partially suppressed the growth of the CCAP/bursicon neuron somata and neurite arbor. In contrast, expression of wild-type or constitutively active forms of InR, Pi3K92E, Akt1, Rheb, and TOR, as well as RNA interference for negative regulators of insulin signaling (PTEN, FOXO), stimulated overgrowth. Interestingly, InR displayed little effect on larval CCAP/bursicon neuron growth, in contrast to its strong effects during metamorphosis. Manipulations of insulin signaling in many other peptidergic neurons revealed generalized growth stimulation during metamorphosis, but not during larval development. These findings reveal a fundamental shift in growth control mechanisms when mature, differentiated neurons enter a new phase of organizational growth. Moreover, they highlight strong evolutionarily conservation of insulin signaling in neuronal growth regulation.

Diverse in- and output polarities and high complexity of local synaptic and non-synaptic signaling within a chemically defined class of peptidergic Drosophila neurons

Peptidergic neurons are not easily integrated into current connectomics concepts, since their peptide messages can be distributed via non-synaptic paracrine signaling or volume transmission. Moreover, the polarity of peptidergic interneurons in terms of in- and out-put sites can be hard to predict and is very little explored. We describe in detail the morphology and the subcellular distribution of fluorescent vesicle/dendrite markers in CCAP neurons (N_CCAP), a well defined set of peptidergic neurons in the Drosophila larva. N_CCAP can be divided into five morphologically distinct subsets. In contrast to other subsets, serial homologous interneurons in the ventral ganglion show a mixed localization of in- and output markers along ventral neurites that defy a classification as dendritic or axonal compartments. Ultrastructurally, these neurites contain both pre- and postsynaptic sites preferably at varicosities. A significant portion of the synaptic events are due to reciprocal synapses. Peptides are mostly non-synaptically or parasynaptically released, and dense-core vesicles and synaptic vesicle pools are typically well separated. The responsiveness of the N_CCAP to ecdysis-triggering hormone may be at least partly dependent on a tonic synaptic inhibition, and is independent of ecdysteroids. Our results reveal a remarkable variety and complexity of local synaptic circuitry within a chemically defined set of peptidergic neurons. Synaptic transmitter signaling as well as peptidergic paracrine signaling and volume transmission from varicosities can be main signaling modes of peptidergic interneurons depending on the subcellular region. The possibility of region-specific variable signaling modes should be taken into account in connectomic studies that aim to dissect the circuitry underlying insect behavior and physiology, in which peptidergic neurons act as important regulators.

Novel isoforms of the transport regulator klar

Klar is a regulator of microtubule-motor dependent transport processes in Drosophila, including nuclear migration, vesicle motility, and lipid-droplet transport. The single klar locus gives rise to multiple isoforms that presumably have unique functions. Up to now, three Klar isoforms (α, β, γ) were known. Here we describe two novel isoforms, δ and ε, whose expression depends on a previously uncharacterized promoter. Klar δ and/or ε are widely expressed during development, including in the embryonic and larval nervous system as well as in ovaries. When we specifically ablate Klar δ and ε expression genetically, no gross organismal phenotypes are apparent. However, ectopic expression of these isoforms causes nuclear mispositioning in developing photoreceptors and in oocytes, demonstrating their biological activity. Our analysis identifies novel forms of the Klar protein and provides new tools for functionally dissecting the complex klar locus.

TDP-43 loss-of-function causes neuronal loss due to defective steroid receptor-mediated gene program switching in Drosophila

TDP-43 proteinopathy is strongly implicated in the pathogenesis of amyotrophic lateral sclerosis and related neurodegenerative disorders. Whether TDP-43 neurotoxicity is caused by a novel toxic gain-of-function mechanism of the aggregates or by a loss of its normal function is unknown. We increased and decreased expression of TDP-43 (dTDP-43) in Drosophila. Although upregulation of dTDP-43 induced neuronal ubiquitin and dTDP-43-positive inclusions, both up- and downregulated dTDP-43 resulted in selective apoptosis of bursicon neurons and highly similar transcriptome alterations at the pupal-adult transition. Gene network analysis and genetic validation showed that both up- and downregulated dTDP-43 directly and dramatically increased the expression of the neuronal microtubule-associated protein Map205, resulting in cytoplasmic accumulations of the ecdysteroid receptor (EcR) and a failure to switch EcR-dependent gene programs from a pupal to adult pattern. We propose that dTDP-43 neurotoxicity is caused by a loss of its normal function.

How clocks and hormones act in concert to control the timing of insect development

During the last century, insect model systems have provided fascinating insights into the endocrinology and developmental biology of all animals. During the insect life cycle, molts and metamorphosis delineate transitions from one developmental stage to the next. In most insects, pulses of the steroid hormone ecdysone drive these developmental transitions by activating signaling cascades in target tissues. In holometabolous insects, ecdysone triggers metamorphosis, the remarkable remodeling of an immature larva into a sexually mature adult. The input from another developmental hormone, juvenile hormone (JH), is required to repress metamorphosis by promoting juvenile fates until the larva has acquired sufficient nutrients to survive metamorphosis. Ecdysone and JH act together as key endocrine timers to precisely control the onset of developmental transitions such as the molts, pupation, or eclosion. In this review, we will focus on the role of the endocrine system and the circadian clock, both individually and together, in temporally regulating insect development. Since this is not a coherent field, we will review recent developments that serve as examples to illuminate this complex topic. First, we will consider studies conducted in Rhodnius that revealed how circadian pathways exert temporal control over the production and release of ecdysone. We will then take a look at molecular and genetic data that revealed the presence of two circadian clocks, located in the brain and the prothoracic gland, that regulate eclosion rhythms in Drosophila. In this context, we will also review recent developments that examined how the ecdysone hierarchy delays the differentiation of the crustacean cardioactive peptide (CCAP) neurons, an event that is critical for the timing of ecdysis and eclosion. Finally, we will discuss some recent findings that transformed our understanding of JH function.

Genetic analysis of ecdysis behavior in Drosophila reveals partially overlapping functions of two unrelated neuropeptides

Ecdysis behavior allows insects to shed their old exoskeleton at the end of every molt. It is controlled by a suite of interacting hormones and neuropeptides, and has served as a useful behavior for understanding how bioactive peptides regulate CNS function. Previous findings suggest that crustacean cardioactive peptide (CCAP) activates the ecdysis motor program; the hormone bursicon is believed to then act downstream of CCAP to inflate, pigment, and harden the exoskeleton of the next stage. However, the exact roles of these signaling molecules in regulating ecdysis remain unclear. Here we use a genetic approach to investigate the functions of CCAP and bursicon in Drosophila ecdysis. We show that null mutants in CCAP express no apparent defects in ecdysis and postecdysis, producing normal adults. By contrast, a substantial fraction of flies genetically null for one of the two subunits of bursicon [encoded by the partner of bursicon gene (pburs)] show severe defects in ecdysis, with escaper adults exhibiting the expected failures in wing expansion and exoskeleton pigmentation and hardening. Furthermore, flies lacking both CCAP and bursicon show much more severe defects at ecdysis than do animals null for either neuropeptide alone. Our results show that the functions thought to be subserved by CCAP are partially effected by bursicon, and that bursicon plays an important and heretofore undescribed role in ecdysis behavior itself. These findings have important implications for understanding the regulation of this vital insect behavior and the mechanisms by which hormones and neuropeptides control the physiology and behavior of animals.

Temporally tuned neuronal differentiation supports the functional remodeling of a neuronal network in Drosophila

During insect metamorphosis, neuronal networks undergo extensive remodeling by restructuring their connectivity and recruiting newborn neurons from postembryonic lineages. The neuronal network that directs the essential behavior, ecdysis, generates a distinct behavioral sequence at each developmental transition. Larval ecdysis replaces the cuticle between larval stages, and pupal ecdysis externalizes and expands the head and appendages to their adult position. However, the network changes that support these differences are unknown. Crustacean cardioactive peptide (CCAP) neurons and the peptide hormones they secrete are critical for ecdysis; their targeted ablation alters larval ecdysis progression and results in a failure of pupal ecdysis. In this study, we demonstrate that the CCAP neuron network is remodeled immediately before pupal ecdysis by the emergence of 12 late CCAP neurons. All 12 are CCAP efferents that exit the central nervous system. Importantly, these late CCAP neurons were found to be entirely sufficient for wild-type pupal ecdysis, even after targeted ablation of all other 42 CCAP neurons. Our evidence indicates that late CCAP neurons are derived from early, likely embryonic, lineages. However, they do not differentiate to express their peptide hormone battery, nor do they project an axon via lateral nerve trunks until pupariation, both of which are believed to be critical for the function of CCAP efferent neurons in ecdysis. Further analysis implicated ecdysone signaling via ecdysone receptors A/B1 and the nuclear receptor ftz-f1 as the differentiation trigger. These results demonstrate the utility of temporally tuned neuronal differentiation as a hard-wired developmental mechanism to remodel a neuronal network to generate a scheduled change in behavior.

Transcriptional activity and nuclear localization of Cabut, the Drosophila ortholog of vertebrate TGF-β-inducible early-response gene (TIEG) proteins

Background

Cabut (Cbt) is a C₂H₂-class zinc finger transcription factor involved in embryonic dorsal closure, epithelial regeneration and other developmental processes in Drosophila melanogaster. Cbt orthologs have been identified in other Drosophila species and insects as well as in vertebrates. Indeed, Cbt is the Drosophila ortholog of the group of vertebrate proteins encoded by the TGF-ß-inducible early-response genes (TIEGs), which belong to Sp1-like/Krüppel-like family of transcription factors. Several functional domains involved in transcriptional control and subcellular localization have been identified in the vertebrate TIEGs. However, little is known of whether these domains and functions are also conserved in the Cbt protein.

Methodology/Principal Findings

To determine the transcriptional regulatory activity of the Drosophila Cbt protein, we performed Gal4-based luciferase assays in S2 cells and showed that Cbt is a transcriptional repressor and able to regulate its own expression. Truncated forms of Cbt were then generated to identify its functional domains. This analysis revealed a sequence similar to the mSin3A-interacting repressor domain found in vertebrate TIEGs, although located in a different part of the Cbt protein. Using β-Galactosidase and eGFP fusion proteins, we also showed that Cbt contains the bipartite nuclear localization signal (NLS) previously identified in TIEG proteins, although it is non-functional in insect cells. Instead, a monopartite NLS, located at the amino terminus of the protein and conserved across insects, is functional in Drosophila S2 and Spodoptera exigua Sec301 cells. Last but not least, genetic interaction and immunohistochemical assays suggested that Cbt nuclear import is mediated by Importin-α2.

Conclusions/Significance

Our results constitute the first characterization of the molecular mechanisms of Cbt-mediated transcriptional control as well as of Cbt nuclear import, and demonstrate the existence of similarities and differences in both aspects of Cbt function between the insect and the vertebrate TIEG proteins.

Membrane phospholipid asymmetry counters the adverse effects of sterol overloading in the Golgi membrane of Drosophila

Cholesterol and phospholipids serve as structural and functional components of cellular membranes in all eukaryotes. Heterogeneity in cholesterol and phospholipid content both within and between different organelles is an important characteristic of eukaryotic membranes. How this heterogeneity is achieved and orchestrated to maintain proper cellular physiology remains poorly understood. We previously found that overexpression of the Drosophila oxysterol-binding protein (OSBP) leads to sterol accumulation in the Golgi apparatus. Here, we show that Osbp overexpression in a set of neuroendocrine neurons compromises the function of the Golgi apparatus. It impairs trafficking of the neuropeptide bursicon and results in post-eclosion behavior defects characterized by unexpanded wings. We performed a genetic screen to identify modifiers that suppress the unexpanded wing phenotype. A putative phospholipid flippase-encoding gene, CG33298, was validated, suggesting that a membrane-asymmetry-directed mechanism balances cholesterol chaos within the Golgi membranes. Since the functional connection between cholesterol metabolism and the activity of phospholipid flippase has been implicated in studies in yeast and worms, our findings here support an evolutionarily conserved causal link between cholesterol homeostasis and phospholipid asymmetry that maintains normal cellular physiology.

Retrograde BMP signaling controls Drosophila behavior through regulation of a peptide hormone battery

Retrograde BMP signaling in neurons plays conserved roles in synaptic efficacy and subtype-specific gene expression. However, a role for retrograde BMP signaling in the behavioral output of neuronal networks has not been established. Insect development proceeds through a series of stages punctuated by ecdysis, a complex patterned behavior coordinated by a dedicated neuronal network. In Drosophila, larval ecdysis sheds the old cuticle between larval stages, and pupal ecdysis everts the head and appendages to their adult external position during metamorphosis. Here, we found that mutants of the type II BMP receptor wit exhibited a defect in the timing of larval ecdysis and in the completion of pupal ecdysis. These phenotypes largely recapitulate those previously observed upon ablation of CCAP neurons, an integral subset of the ecdysis neuronal network. Here, we establish that retrograde BMP signaling in only the efferent subset of CCAP neurons (CCAP-ENs) is required to cell-autonomously upregulate expression of the peptide hormones CCAP, Mip and Bursicon β. In wit mutants, restoration of wit exclusively in CCAP neurons significantly rescued peptide hormone expression and ecdysis phenotypes. Moreover, combinatorial restoration of peptide hormone expression in CCAP neurons in wit mutants also significantly rescued wit ecdysis phenotypes. Collectively, our data demonstrate a novel role for retrograde BMP signaling in maintaining the behavioral output of a neuronal network and uncover the underlying cellular and gene regulatory substrates.

Fragile X mental retardation protein is required for programmed cell death and clearance of developmentally-transient peptidergic neurons

Fragile X syndrome (FXS), caused by loss of fragile X mental retardation 1 (FMR1) gene function, is the most common heritable cause of intellectual disability and autism spectrum disorders. The FMR1 product (FMRP) is an RNA-binding protein best established to function in activity-dependent modulation of synaptic connections. In the Drosophila FXS disease model, loss of functionally-conserved dFMRP causes synaptic overgrowth and overelaboration in pigment dispersing factor (PDF) peptidergic neurons in the adult brain. Here, we identify a very different component of PDF neuron misregulation in dfmr1 mutants: the aberrant retention of normally developmentally-transient PDF tritocerebral (PDF-TRI) neurons. In wild-type animals, PDF-TRI neurons in the central brain undergo programmed cell death and complete, processive clearance within days of eclosion. In the absence of dFMRP, a defective apoptotic program leads to constitutive maintenance of these peptidergic neurons. We tested whether this apoptotic defect is circuit-specific by examining crustacean cardioactive peptide (CCAP) and bursicon circuits, which are similarly developmentally-transient and normally eliminated immediately post-eclosion. In dfmr1 null mutants, CCAP/bursicon neurons also exhibit significantly delayed clearance dynamics, but are subsequently eliminated from the nervous system, in contrast to the fully persistent PDF-TRI neurons. Thus, the requirement of dFMRP for the retention of transitory peptidergic neurons shows evident circuit specificity. The novel defect of impaired apoptosis and aberrant neuron persistence in the Drosophila FXS model suggests an entirely new level of "pruning" dysfunction may contribute to the FXS disease state.

Targeting the motor regulator Klar to lipid droplets

Background

In Drosophila, the transport regulator Klar displays tissue-specific localization: In photoreceptors, it is abundant on the nuclear envelope; in early embryos, it is absent from nuclei, but instead present on lipid droplets. Differential targeting of Klar appears to be due to isoform variation. Droplet targeting, in particular, has been suggested to occur via a variant C-terminal region, the LD domain. Although the LD domain is necessary and sufficient for droplet targeting in cultured cells, lack of specific reagents had made it previously impossible to analyze its role in vivo.

Results

Here we describe a new mutant allele of klar with a lesion specifically in the LD domain; this lesion abolishes both droplet localization of Klar and the ability of Klar to regulate droplet motion. It does not disrupt Klar's function for nuclear migration in photoreceptors. Using a GFP-LD fusion, we show that the LD domain is not only necessary but also sufficient for droplet targeting in vivo; it mediates droplet targeting in embryos, in ovaries, and in a number of somatic tissues.

Conclusions

Our analysis demonstrates that droplet targeting of Klar occurs via a cis-acting sequence and generates a new tool for monitoring lipid droplets in living tissues of Drosophila.

Expression of Drosophila Cabut during early embryogenesis, dorsal closure and nervous system development

cabut (cbt) encodes a transcription factor involved in Drosophila dorsal closure (DC), and it is expressed in embryonic epithelial sheets and yolk cell during this process upon activation of the Jun N-terminal kinase (JNK) signaling pathway. Additional studies suggest that cbt may have a role in multiple developmental processes. To analyze Cbt localization through embryogenesis, we generated a Cbt specific antibody that has allowed detecting new Cbt expression patterns. Immunohistochemical analyses on syncytial embryos and S2 cells reveal that Cbt is localized on the surface of mitotic chromosomes at all mitotic phases. During DC, Cbt is expressed in the yolk cell, in epidermal cells and in the hindgut, but also in amnioserosal cells, which also contribute to the process, albeit cbt transcripts were not detected in that tissue. At later embryonic stages, Cbt is expressed in neurons and glial cells in the central nervous system, and is detected in axons of the central and peripheral nervous systems. Most of these expression patterns are recapitulated by GFP reporter gene constructs driven by different cbt genomic regions. Moreover, they have been further validated by immunostainings of embryos from other Drosophila species, thus suggesting that Cbt function during embryogenesis appears to be conserved in evolution.

Deletion in Xp22.11: PTCHD1 is a candidate gene for X-linked intellectual disability with or without autism

Submicroscopic chromosomal anomalies play an important role in the aetiology of intellectual disability (ID) and have been shown to account for up to 10% of non-syndromic forms. We present a family with two affected boys compatible with X-linked inheritance of a phenotype of severe neurodevelopmental disorder co-segregating with a deletion in Xp22.11 exclusively containing the PTCHD1 gene. Although the exact function of this gene is unknown to date, the structural overlap of its encoded patched domain-containing protein 1, the transmembrane protein involved in the sonic hedgehog pathway, and its expression in human cortex and cerebellum as well as in mice and drosophila brain suggests a causative role of its nullisomy in the developmental phenotype of our family. Our findings support the recent notions that PTCHD1 may play a role in X-linked intellectual disability (XLID) and autism disorders.

The essential role of bursicon during Drosophila development

Background

The protective external cuticle of insects does not accommodate growth during development. To compensate for this, the insect life cycle is punctuated by a series of molts. During the molt, a new and larger cuticle is produced underneath the old cuticle. Replacement of the smaller, old cuticle culminates with ecdysis, a stereotyped sequence of shedding behaviors. Following each ecdysis, the new cuticle must expand and harden. Studies from a variety of insect species indicate that this cuticle hardening is regulated by the neuropeptide bursicon. However, genetic evidence from Drosophila melanogaster only supports such a role for bursicon after the final ecdysis, when the adult fly emerges. The research presented here investigates the role that bursicon has at stages of Drosophila development which precede adult ecdysis.

Results

We addressed the mechanism and timing of hormonal release from bursicon-positive motor neurons at the larval neuromuscular junction. Our findings indicate that vesicle membrane proteins which are required for classical neurotransmitter release are also expressed at these peptidergic motor neuron terminals; and that these terminals secrete hormones including bursicon at the neuromuscular junction, coinciding with larval ecdysis. This release surprisingly occurs in two waves, indicating bursicon secretion preceding and following the ecdysis sequence. Next, we addressed the functional significance of bursicon signaling during development, by disrupting the expression of its receptor, rickets, in different target tissues. We determined that rickets is developmentally required in the epidermis and imaginal discs for proper formation of the prepupa. It is also required to harden the pharate adult cuticle before eclosion. Significantly, we have also found that the available rickets mutants are not genetic nulls as previously believed, which necessitated the use of targeted RNA interference to disrupt rickets expression.

Conclusions

Our results are consistent with the view that bursicon is the insect tanning hormone. However, this is the first study to rigorously demonstrate both its release and function during development. Importantly, we provide new evidence that bursicon release can precede the initiation of larval ecdysis, and that bursicon tans the puparium. Our results firmly establish bursicon signaling as essential to insect growth and development.

Adverse interactions between micro-RNAs and target genes from different species

It is commonly assumed but not proven that microRNAs (miRNAs) and their targets coevolve. Under this assumption, miRNAs and targets from different species may interact adversely, resulting in reduced fitness. However, the strength of the adverse interactions may not be detectable because even outright deletions of miRNAs often manifest only subtle fitness effects. We tested and measured the strength of heterospecific interactions by carrying out transgenic experiments across Drosophila species by overexpressing the miR310s cluster of Drosophila melanogaster (Dm310s) and Drosophila pseudoobscura (Dp310s) in D. melanogaster. Flies overexpressing the heterospecific Dp310s are only one-third as viable as those overexpressing the conspecific Dm310s. The viability effect is easily detectable in comparison to the effect of the deletion of miR310s. The number of genes significantly misexpressed under the influence of Dp310s is 3-10 times greater than under Dm310s. Importantly, the numbers of predicted targets are similar between them. Expression analysis of the predicted target genes suggests that miRNAs may sometimes function to buffer fluctuations in the transcriptome output. After the buffering function has evolved, heterospecific combinations may cause adverse effects.

Regulation of axonal development by the nuclear protein hindsight (pebbled) in the Drosophila visual system

The molecules and networks involved in the process of acquisition and maintenance of the form of a mature neuron are not completely known. Using a misexpression screen we identified the gene hindsight as a gene involved in the process of acquisition of the neuronal morphogenesis in the Drosophila adult nervous system. hindsight encodes a transcription factor known for its role in early developmental processes such as embryonic germ band retraction and dorsal closure, as well as in the establishment of cell morphology, planar cell polarity, and epithelial integrity during retinal development. We describe here a novel function for HNT by showing that both loss and gain of function of HNT affects the pathfinding of the photoreceptors axons. By manipulating the timing and level of HNT expression, together with the number of cells manipulated we show here that the function of HNT in axonal guidance is independent of the HNT functions previously reported in retinal cells. Based on genetic interaction experiments we show that part of HNT function in axonal development is exerted through the regulation of genes involved in the dynamics of the actin cytoskeleton.

Expression profiling of prospero in the Drosophila larval chemosensory organ: Between growth and outgrowth

Background

The antenno-maxilary complex (AMC) forms the chemosensory system of the Drosophila larva and is involved in gustatory and olfactory perception. We have previously shown that a mutant allele of the homeodomain transcription factor Prospero (prosVoila1, V1), presents several developmental defects including abnormal growth and altered taste responses. In addition, many neural tracts connecting the AMC to the central nervous system (CNS) were affected. Our earlier reports on larval AMC did not argue in favour of a role of pros in cell fate decision, but strongly suggested that pros could be involved in the control of other aspect of neuronal development. In order to identify these functions, we used microarray analysis of larval AMC and CNS tissue isolated from the wild type, and three other previously characterised prospero alleles, including the V1 mutant, considered as a null allele for the AMC.

Results

A total of 17 samples were first analysed with hierarchical clustering. To determine those genes affected by loss of pros function, we calculated a discriminating score reflecting the differential expression between V1 mutant and other pros alleles. We identified a total of 64 genes in the AMC. Additional manual annotation using all the computed information on the attributed role of these genes in the Drosophila larvae nervous system, enabled us to identify one functional category of potential Prospero target genes known to be involved in neurite outgrowth, synaptic transmission and more specifically in neuronal connectivity remodelling. The second category of genes found to be differentially expressed between the null mutant AMC and the other alleles concerned the development of the sensory organs and more particularly the larval olfactory system. Surprisingly, a third category emerged from our analyses and suggests an association of pros with the genes that regulate autophagy, growth and insulin pathways. Interestingly, EGFR and Notch pathways were represented in all of these three functional categories. We now propose that Pros could perform all of these different functions through the modulation of these two antagonistic and synergic pathways.

Conclusions

The current data contribute to the clarification of the prospero function in the larval AMC and show that pros regulates different function in larvae as compared to those controlled by this gene in embryos. In the future, the possible mechanism by which Pros could achieve its function in the AMC will be explored in detail.

Targeted gain-of-function screening in Drosophila using GAL4-UAS and random transposon insertions

Alterations in the activity level or temporal expression of key signalling genes elicit profound patterning effects during development. Consequently, gain-of-function genetic schemes that overexpress or misexpress such loci can identify novel candidates for functions essential for a developmental process. GAL4-Upstream Activating Sequence (UAS)-targeted regulation of gene expression in Drosophila has allowed rapid analyses of coding sequences for potential roles in specific tissues at particular developmental stages. GAL4 has also been combined with randomly mobilized transposons capable of UAS-directed misexpression or overexpression of flanking sequences. This combination has produced a genetic screening system that can uncover novel loci refractory to standard loss of function genetic approaches, such as redundant genes. Available libraries of strains with sequenced insertion sites can allow direct correlation of phenotypes to genetic function. These techniques have also been applied to genetic interaction screening, where a GAL4 driver and UAS-regulated insertion collection are combined with an extant mutant genotype. In this article, we summarize studies that have utilized GAL4-UAS overexpression or misexpression of random loci to screen for candidates involved in specific developmental processes.

The genetic basis of transgressive ovary size in honeybee workers

Ovarioles are the functional unit of the female insect reproductive organs and the number of ovarioles per ovary strongly influences egg-laying rate and fecundity. Social evolution in the honeybee (Apis mellifera) has resulted in queens with 200-360 total ovarioles and workers with usually 20 or less. In addition, variation in ovariole number among workers relates to worker sensory tuning, foraging behavior, and the ability to lay unfertilized male-destined eggs. To study the genetic architecture of worker ovariole number, we performed a series of crosses between Africanized and European bees that differ in worker ovariole number. Unexpectedly, these crosses produced transgressive worker phenotypes with extreme ovariole numbers that were sensitive to the social environment. We used a new selective pooled DNA interval mapping approach with two Africanized backcrosses to identify quantitative trait loci (QTL) underlying the transgressive ovary phenotype. We identified one QTL on chromosome 11 and found some evidence for another QTL on chromosome 2. Both QTL regions contain plausible functional candidate genes. The ovariole number of foragers was correlated with the sugar concentration of collected nectar, supporting previous studies showing a link between worker physiology and foraging behavior. We discuss how the phenotype of extreme worker ovariole numbers and the underlying genetic factors we identified could be linked to the development of queen traits.

Bursicon functions within the Drosophila CNS to modulate wing expansion behavior, hormone secretion, and cell death

Hormones are often responsible for synchronizing somatic physiological changes with changes in behavior. Ecdysis (i.e., the shedding of the exoskeleton) in insects has served as a useful model for elucidating the molecular and cellular mechanisms of this synchronization, and has provided numerous insights into the hormonal coordination of body and behavior. An example in which the mechanisms have remained enigmatic is the neurohormone bursicon, which, after the final molt, coordinates the plasticization and tanning of the initially folded wings with behaviors that drive wing expansion. The somatic effects of the hormone are governed by bursicon that is released into the blood from neurons in the abdominal ganglion (the B(AG)), which die after wing expansion. How bursicon induces the behavioral programs required for wing expansion, however, has remained unknown. Here we show by targeted suppression of excitability that a pair of bursicon-immunoreactive neurons distinct from the B(AG) and located within the subesophageal ganglion in Drosophila (the B(SEG)) is involved in controlling wing expansion behaviors. Unlike the B(AG), the B(SEG) arborize widely in the nervous system, including within the abdominal neuromeres, suggesting that, in addition to governing behavior, they also may modulate the B(AG.) Indeed, we show that animals lacking bursicon receptor function have deficits both in the humoral release of bursicon and in posteclosion apoptosis of the B(AG). Our results reveal novel neuromodulatory functions for bursicon and support the hypothesis that the B(SEG) are essential for orchestrating both the behavioral and somatic processes underlying wing expansion.

Chip is required for posteclosion behavior in Drosophila

Neurons acquire their molecular, neurochemical, and connectional features during development as a result of complex regulatory mechanisms. Here, we show that a ubiquitous, multifunctional protein cofactor, Chip, plays a critical role in a set of neurons in Drosophila that control the well described posteclosion behavior. Newly eclosed flies normally expand their wings and display tanning and hardening of their cuticle. Using multiple approaches to interfere with Chip function, we find that these processes do not occur without normal activity of this protein. Furthermore, we identified the nature of the deficit to be an absence of Bursicon in the hemolymph of newly eclosed flies, whereas the responsivity to Bursicon in these flies remains normal. Chip interacts with transcription factors of the LIM-HD (LIM-homeodomain) family, and we identified one member, dIslet, as a potential partner of Chip in this process. Our findings provide the first evidence of transcriptional mechanisms involved in the development of the neuronal circuit that regulates posteclosion behavior in Drosophila.

The buzz on fly neuronal remodeling

Hormone-dependent rewiring of axons and dendrites is a conserved feature of nervous system development and plasticity. During metamorphosis in insects, steroid hormones (the ecdysteroids) and terpenoid hormones (the juvenile hormones) regulate extensive remodeling of the nervous system. These changes retool the nervous system for new behavioral and physiological functions that are required for the adult stage of the life cycle. In honey bees and other highly social insects, hormones also regulate behavioral changes and neuronal plasticity associated with transitions between social caste roles. This review focuses on recent work in fruit flies and honey bees that reveals hormonal and molecular mechanisms underlying metamorphic and caste-dependent neuronal remodeling, with specific emphasis on the role of Krüppel homolog 1.