Ap-let neurons--a peptidergic circuit potentially controlling ecdysial behavior in Drosophila

Branching gene regulatory network dictating different aspects of a neuronal cell identity

The nervous system displays a daunting cellular diversity. Neuronal sub-types differ from each other in several aspects, including their neurotransmitter expression and axon projection. These aspects can converge, but can also diverge, such that neurons expressing the same neurotransmitter may project axons to different targets. It is not well understood how regulatory programs converge/diverge to associate/dissociate different cell fate features. Studies of the Drosophila Tv1 neurons have identified a regulatory cascade; ladybird early -> collier -> apterous/eyes absent -> dimmed, which specifies Tv1 neurotransmitter expression. Here, we conduct genetic and transcriptome analysis to address how other aspects of Tv1 cell fate is governed. We find that an initiator terminal selector gene triggers a feedforward loop which branches into different subroutines, each of which establishes different features of this one unique neuronal cell fate.

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.

Neuronal cell fate specification by the molecular convergence of different spatio-temporal cues on a common initiator terminal selector gene

The extensive genetic regulatory flows underlying specification of different neuronal subtypes are not well understood at the molecular level. The Nplp1 neuropeptide neurons in the developing Drosophila nerve cord belong to two sub-classes; Tv1 and dAp neurons, generated by two distinct progenitors. Nplp1 neurons are specified by spatial cues; the Hox homeotic network and GATA factor grn, and temporal cues; the hb -> Kr -> Pdm -> cas -> grh temporal cascade. These spatio-temporal cues combine into two distinct codes; one for Tv1 and one for dAp neurons that activate a common terminal selector feedforward cascade of col -> ap/eya -> dimm -> Nplp1. Here, we molecularly decode the specification of Nplp1 neurons, and find that the cis-regulatory organization of col functions as an integratory node for the different spatio-temporal combinatorial codes. These findings may provide a logical framework for addressing spatio-temporal control of neuronal sub-type specification in other systems.

Neuronal cell fate diversification controlled by sub-temporal action of Kruppel

During Drosophila embryonic nervous system development, neuroblasts express a programmed cascade of five temporal transcription factors that govern the identity of cells generated at different time-points. However, these five temporal genes fall short of accounting for the many distinct cell types generated in large lineages. Here, we find that the late temporal gene castor sub-divides its large window in neuroblast 5–6 by simultaneously activating two cell fate determination cascades and a sub-temporal regulatory program. The sub-temporal program acts both upon itself and upon the determination cascades to diversify the castor window. Surprisingly, the early temporal gene Kruppel acts as one of the sub-temporal genes within the late castor window. Intriguingly, while the temporal gene castor activates the two determination cascades and the sub-temporal program, spatial cues controlling cell fate in the latter part of the 5–6 lineage exclusively act upon the determination cascades.

DOI:http://dx.doi.org/10.7554/eLife.19311.001

As a nervous system develops, stem cells generate different types of nerve cells at different times. This series of events follows a fixed schedule in developing embryos, and even a single stem cell that is removed and then grown outside the body will follow the same schedule. This strongly suggests that stem cells have a built-in clock that controls their development.

Studies of the developing nervous system of fruit flies reveal that this clock works by switching genes on in specific sequences, which defines which nerve cells are produced at different stages of development. However, a clock built from the genes that are currently known to be involved in the process is simply not fine-tuned enough to explain how so many different types of nerve cell develop at such precise times. This implies that scientists do not yet know all of the genes that are involved.

Using genetic experiments in stem cells from fruit flies, Stratmann, Gabilondo et al. now identify additional clock genes that act to divide broad windows of time during development into smaller, more precise ones. Genes that define broad windows of time switch on the “small window” genes at specific times – a bit like large cogs turning small cogs in a clock. One small window gene, called Kruppel, works at different stages of development and it is possible that other small window genes multi-task in similar ways in other developmental clocks, such as those found in more complex organisms like humans.

It is clear that many genes work in sequence in the developing nervous system to ensure that developmental stages happen at precise times. Stratmann, Gabilondo et al. will next investigate the molecular details of this timing, specifically how genes in sequential time windows connect together like cogs in the developmental clock.

DOI:http://dx.doi.org/10.7554/eLife.19311.002

A critical role for the Drosophila dopamine D1-like receptor Dop1R2 at the onset of metamorphosis

BACKGROUND

Insect metamorphosis relies on temporal and spatial cues that are precisely controlled. Previous studies in Drosophila have shown that untimely activation of genes that are essential to metamorphosis results in growth defects, developmental delay and death. Multiple factors exist that safeguard these genes against dysregulated expression. The list of identified negative regulators that play such a role in Drosophila development continues to expand.

RESULTS

By using RNAi transgene-induced gene silencing coupled to spatio/temporal assessment, we have unraveled an important role for the Drosophila dopamine 1-like receptor, Dop1R2, in development. We show that Dop1R2 knockdown leads to pre-adult lethality. In adults that escape death, abnormal wing expansion and/or melanization defects occur. Furthermore we show that salivary gland expression of this GPCR during the late larval/prepupal stage is essential for the flies to survive through adulthood. In addition to RNAi-induced effects, treatment of larvae with the high affinity D1-like receptor antagonist flupenthixol, also results in developmental arrest, and in morphological defects comparable to those seen in Dop1R2 RNAi flies. To examine the basis for pupal lethality in Dop1R2 RNAi flies, we carried out transcriptome analysis. These studies revealed up-regulation of genes that respond to ecdysone, regulate morphogenesis and/or modulate defense/immunity.

CONCLUSION

Taken together our findings suggest a role for Dop1R2 in the repression of genes that coordinate metamorphosis. Premature release of this inhibition is not tolerated by the developing fly.

Neuronal Cell Fate Specification by the Convergence of Different Spatiotemporal Cues on a Common Terminal Selector Cascade

Specification of the myriad of unique neuronal subtypes found in the nervous system depends upon spatiotemporal cues and terminal selector gene cascades, often acting in sequential combinatorial codes to determine final cell fate. However, a specific neuronal cell subtype can often be generated in different parts of the nervous system and at different stages, indicating that different spatiotemporal cues can converge on the same terminal selectors to thereby generate a similar cell fate. However, the regulatory mechanisms underlying such convergence are poorly understood. The Nplp1 neuropeptide neurons in the Drosophila ventral nerve cord can be subdivided into the thoracic-ventral Tv1 neurons and the dorsal-medial dAp neurons. The activation of Nplp1 in Tv1 and dAp neurons depends upon the same terminal selector cascade: col>ap/eya>dimm>Nplp1. However, Tv1 and dAp neurons are generated by different neural progenitors (neuroblasts) with different spatiotemporal appearance. Here, we find that the same terminal selector cascade is triggered by Kr/pdm>grn in dAp neurons, but by Antp/hth/exd/lbe/cas in Tv1 neurons. Hence, two different spatiotemporal combinations can funnel into a common downstream terminal selector cascade to determine a highly related cell fate.

A study of neuropeptide neurons in the Drosophila nervous system reveals that two different combinations of spatiotemporal cues—active in different progenitors—converge on a common terminal selector gene to trigger a similar neuronal subtype identity.

A fundamental challenge in developmental neurobiology is to understand how the great diversity of neuronal subtypes is generated during nervous system development. Neuronal subtype cell fate is established in a stepwise manner, starting with spatial and temporal cues that confer distinct identities to neural progenitors and trigger expression of terminal selector genes in the early-born neurons. Terminal selectors are those that determine the final neuronal subtype cell fate. Intriguingly, similar neuronal subtypes can be generated by different progenitors and under the control of different spatiotemporal cues; thus, we wondered how such convergence is achieved. To address this issue, we have decoded the specification of two highly related neuropeptide neurons, which are generated at different locations and time-points in the Drosophila nervous system. We find that two different combinations of spatiotemporal cues, in two different neural progenitors, funnel onto the same terminal selector gene, which in turn activates a shared regulatory cascade, ultimately resulting in the specification of a similar neuronal cell subtype identity.

Plug-and-play genetic access to drosophila cell types using exchangeable exon cassettes

Genetically encoded effectors are important tools for probing cellular function in living animals, but improved methods for directing their expression to specific cell types are required. Here, we introduce a simple, versatile method for achieving cell-type-specific expression of transgenes that leverages the untapped potential of "coding introns" (i.e., introns between coding exons). Our method couples the expression of a transgene to that of a native gene expressed in the cells of interest using intronically inserted "plug-and-play" cassettes (called "Trojan exons") that carry a splice acceptor site followed by the coding sequences of T2A peptide and an effector transgene. We demonstrate the efficacy of this approach in Drosophila using lines containing suitable MiMIC (Minos-mediated integration cassette) transposons and a palette of Trojan exons capable of expressing a range of commonly used transcription factors. We also introduce an exchangeable, MiMIC-like Trojan exon construct that can be targeted to coding introns using the Crispr/Cas system.

Peptidergic cell-specific synaptotagmins in Drosophila: localization to dense-core granules and regulation by the bHLH protein DIMMED

Bioactive peptides are packaged in large dense-core secretory vesicles, which mediate regulated secretion by exocytosis. In a variety of tissues, the regulated release of neurotransmitters and hormones is dependent on calcium levels and controlled by vesicle-associated synaptotagmin (SYT) proteins. Drosophila express seven SYT isoforms, of which two (SYT-α and SYT-β) were previously found to be enriched in neuroendocrine cells. Here we show that SYT-α and SYT-β tissue expression patterns are similar, though not identical. Furthermore, both display significant overlap with the bHLH transcription factor DIMM, a known neuroendocrine (NE) regulator. RNAi-mediated knockdown indicates that both SYT-α and SYT-β functions are essential in identified NE cells as these manipulations phenocopy loss-of-function states for the indicated peptide hormones. In Drosophila cell culture, both SYT-α and neuropeptide cargo form DIMM-dependent fluorescent puncta that are coassociated by super-resolution microscopy. DIMM is required to maintain SYT-α and SYT-β protein levels in DIMM-expressing cells in vivo. In neurons normally lacking all three proteins (DIMM(-)/SYT-α(-)/SYT-β(-)), DIMM misexpression conferred accumulation of endogenous SYT-α and SYT-β proteins. Furthermore transgenic SYT-α does not appreciably accumulate in nonpeptidergic neurons in vivo but does so if DIMM is comisexpressed. Among Drosophila syt genes, only syt-α and syt-β RNA levels are upregulated by DIMM overexpression. Together, these data suggest that SYT-α and SYT-β are important for NE cell physiology, that one or both are integral membrane components of the large dense-core vesicles, and that they are closely regulated by DIMM at a post-transcriptional level.

Atlas-builder software and the eNeuro atlas: resources for developmental biology and neuroscience

A major limitation in understanding embryonic development is the lack of cell type-specific markers. Existing gene expression and marker atlases provide valuable tools, but they typically have one or more limitations: a lack of single-cell resolution; an inability to register multiple expression patterns to determine their precise relationship; an inability to be upgraded by users; an inability to compare novel patterns with the database patterns; and a lack of three-dimensional images. Here, we develop new 'atlas-builder' software that overcomes each of these limitations. A newly generated atlas is three-dimensional, allows the precise registration of an infinite number of cell type-specific markers, is searchable and is open-ended. Our software can be used to create an atlas of any tissue in any organism that contains stereotyped cell positions. We used the software to generate an 'eNeuro' atlas of the Drosophila embryonic CNS containing eight transcription factors that mark the major CNS cell types (motor neurons, glia, neurosecretory cells and interneurons). We found neuronal, but not glial, nuclei occupied stereotyped locations. We added 75 new Gal4 markers to the atlas to identify over 50% of all interneurons in the ventral CNS, and these lines allowed functional access to those interneurons for the first time. We expect the atlas-builder software to benefit a large proportion of the developmental biology community, and the eNeuro atlas to serve as a publicly accessible hub for integrating neuronal attributes - cell lineage, gene expression patterns, axon/dendrite projections, neurotransmitters--and linking them to individual neurons.

Klumpfuss controls FMRFamide expression by enabling BMP signaling within the NB5-6 lineage

A number of transcription factors that are expressed within most, if not all, embryonic neuroblast (NB) lineages participate in neural subtype specification. Some have been extensively studied in several NB lineages (e.g. components of the temporal gene cascade) whereas others only within specific NB lineages. To what extent they function in other lineages remains unknown. Klumpfuss (Klu), the Drosophila ortholog of the mammalian Wilms tumor 1 (WT1) protein, is one such transcription factor. Studies in the NB4-2 lineage have suggested that Klu functions to ensure that the two ganglion mother cells (GMCs) in this embryonic NB lineage acquire different fates. Owing to limited lineage marker availability, these observations were made only for the NB4-2 lineage. Recent findings reveal that Klu is necessary for larval neuroblast growth and self-renewal. We have extended the study of Klu to the well-known embryonic NB5-6T lineage and describe a novel role for Klu in the Drosophila embryonic CNS. Our results demonstrate that Klu is expressed specifically in the postmitotic Ap4/FMRFa neuron, promoting its differentiation through the initiation of BMP signaling. Our findings indicate a pleiotropic function of Klu in Ap cluster specification in general and particularly in Ap4 neuron differentiation, indicating that Klu is a multitasking transcription factor. Finally, our studies indicate that a transitory downregulation of klu is crucial for the specification of the Ap4/FMRFa neuron. Similar to WT1, klu seems to have either self-renewal or differentiation-promoting functions, depending on the developmental context.

Temporal patterning of neural progenitors in Drosophila

Drosophila has recently become a powerful model system to understand the mechanisms of temporal patterning of neural progenitors called neuroblasts (NBs). Two different temporal sequences of transcription factors (TFs) have been found to be sequentially expressed in NBs of two different systems: the Hunchback, Krüppel, Pdm1/Pdm2, Castor, and Grainyhead sequence in the Drosophila ventral nerve cord; and the Homothorax, Klumpfuss, Eyeless, Sloppy-paired, Dichaete, and Tailless sequence that patterns medulla NBs. In addition, the intermediate neural progenitors of type II NB lineages are patterned by a different sequence: Dichaete, Grainyhead, and Eyeless. These three examples suggest that temporal patterning of neural precursors by sequences of TFs is a common theme to generate neural diversity. Cross-regulations, including negative feedback regulation and positive feedforward regulation among the temporal factors, can facilitate the progression of the sequence. However, there are many remaining questions to understand the mechanism of temporal transitions. The temporal sequence progression is intimately linked to the progressive restriction of NB competence, and eventually determines the end of neurogenesis. Temporal identity has to be integrated with spatial identity information, as well as with the Notch-dependent binary fate choices, in order to generate specific neuron fates.

Seven up acts as a temporal factor during two different stages of neuroblast 5-6 development

Drosophila embryonic neuroblasts generate different cell types at different time points. This is controlled by a temporal cascade of Hb→Kr→Pdm→Cas→Grh, which acts to dictate distinct competence windows sequentially. In addition, Seven up (Svp), a member of the nuclear hormone receptor family, acts early in the temporal cascade, to ensure the transition from Hb to Kr, and has been referred to as a ‘switching factor’. However, Svp is also expressed in a second wave within the developing CNS, but here, the possible role of Svp has not been previously addressed. In a genetic screen for mutants affecting the last-born cell in the embryonic NB5-6T lineage, the Ap4/FMRFamide neuron, we have isolated a novel allele of svp. Expression analysis shows that Svp is expressed in two distinct pulses in NB5-6T, and mutant analysis reveals that svp plays two distinct roles. In the first pulse, svp acts to ensure proper downregulation of Hb. In the second pulse, which occurs in a Cas/Grh double-positive window, svp acts to ensure proper sub-division of this window. These studies show that a temporal factor may play dual roles, acting at two different stages during the development of one neural lineage.

Generating neuronal diversity in the Drosophila central nervous system

Generating diverse neurons in the central nervous system involves three major steps. First, heterogeneous neural progenitors are specified by positional cues at early embryonic stages. Second, neural progenitors sequentially produce neurons or intermediate precursors that acquire different temporal identities based on their birth-order. Third, sister neurons produced during asymmetrical terminal mitoses are given distinct fates. Determining the molecular mechanisms underlying each of these three steps of cellular diversification will unravel brain development and evolution. Drosophila has a relatively simple and tractable CNS, and previous studies on Drosophila CNS development have greatly advanced our understanding of neuron fate specification. Here we review those studies and discuss how the lessons we have learned from fly teach us the process of neuronal diversification in general.

Molecular organization of Drosophila neuroendocrine cells by Dimmed

BACKGROUND

In Drosophila, the basic-helix-loop-helix protein DIMM coordinates the molecular and cellular properties of all major neuroendocrine cells, irrespective of the secretory peptides they produce. When expressed by nonneuroendocrine neurons, DIMM confers the major properties of the regulated secretory pathway and converts such cells away from fast neurotransmission and toward a neuroendocrine state.

RESULTS

We first identified 134 transcripts upregulated by DIMM in embryos and then evaluated them systematically using diverse assays (including embryo in situ hybridization, in vivo chromatin immunoprecipitation, and cell-based transactivation assays). We conclude that of eleven strong candidates, six are strongly and directly controlled by DIMM in vivo. The six targets include several large dense-core vesicle (LDCV) proteins, but also proteins in non-LDCV compartments such as the RNA-associated protein Maelstrom. In addition, a functional in vivo assay, combining transgenic RNA interference with MS-based peptidomics, revealed that three DIMM targets are especially critical for its action. These include two well-established LDCV proteins, the amidation enzyme PHM and the ascorbate-regenerating electron transporter cytochrome b(561-1). The third key DIMM target, CAT-4 (CG13248), has not previously been associated with peptide neurosecretion-it encodes a putative cationic amino acid transporter, closely related to the Slimfast arginine transporter. Finally, we compared transcripts upregulated by DIMM with those normally enriched in DIMM neurons of the adult brain and found an intersection of 18 DIMM-regulated genes, which included all six direct DIMM targets.

CONCLUSIONS

The results provide a rigorous molecular framework with which to describe the fundamental regulatory organization of diverse neuroendocrine cells.

Segment-specific generation of Drosophila Capability neuropeptide neurons by multi-faceted Hox cues

In the Drosophila ventral nerve cord, the three pairs of Capability neuropeptide-expressing Va neurons are exclusively found in the second, third and fourth abdominal segments (A2–A4). To address the underlying mechanisms behind such segment-specific cell specification, we followed the developmental specification of these neurons. We find that Va neurons are initially generated in all ventral nerve cord segments and progress along a common differentiation path. However, their terminal differentiation only manifests itself in A2–A4, due to two distinct mechanisms: segment-specific programmed cell death (PCD) in posterior segments, and differentiation to an alternative identity in segments anterior to A2. Genetic analyses reveal that the Hox homeotic genes are involved in the segment-specific appearance of Va neurons. In posterior segments, the Hox gene Abdominal-B exerts a pro-apoptotic role on Va neurons, which involves the function of several RHG genes. Strikingly, this role of Abd-B is completely opposite to its role in the segment-specific apoptosis of other classes of neuropeptide neurons, the dMP2 and MP1 neurons, where Abd-B acts in an anti-apoptotic manner. In segments A2–A4 we find that abdominal A is important for the terminal differentiation of Va cell fate. In the A1 segment, Ultrabithorax acts to specify an alternate Va neuron fate. In contrast, in thoracic segments, Antennapedia suppresses the Va cell fate. Thus, Hox genes act in a multi-faceted manner to control the segment-specific appearance of the Va neuropeptide neurons in the ventral nerve cord.

Segment-specific neuronal subtype specification by the integration of anteroposterior and temporal cues

To address the question of how neuronal diversity is achieved throughout the CNS, this study provides evidence of modulation of neural progenitor cell “output” along the body axis by integration of local anteroposterior and temporal cues.

The generation of distinct neuronal subtypes at different axial levels relies upon both anteroposterior and temporal cues. However, the integration between these cues is poorly understood. In the Drosophila central nervous system, the segmentally repeated neuroblast 5–6 generates a unique group of neurons, the Apterous (Ap) cluster, only in thoracic segments. Recent studies have identified elaborate genetic pathways acting to control the generation of these neurons. These insights, combined with novel markers, provide a unique opportunity for addressing how anteroposterior and temporal cues are integrated to generate segment-specific neuronal subtypes. We find that Pbx/Meis, Hox, and temporal genes act in three different ways. Posteriorly, Pbx/Meis and posterior Hox genes block lineage progression within an early temporal window, by triggering cell cycle exit. Because Ap neurons are generated late in the thoracic 5–6 lineage, this prevents generation of Ap cluster cells in the abdomen. Thoracically, Pbx/Meis and anterior Hox genes integrate with late temporal genes to specify Ap clusters, via activation of a specific feed-forward loop. In brain segments, “Ap cluster cells” are present but lack both proper Hox and temporal coding. Only by simultaneously altering Hox and temporal gene activity in all segments can Ap clusters be generated throughout the neuroaxis. This study provides the first detailed analysis, to our knowledge, of an identified neuroblast lineage along the entire neuroaxis, and confirms the concept that lineal homologs of truncal neuroblasts exist throughout the developing brain. We furthermore provide the first insight into how Hox/Pbx/Meis anteroposterior and temporal cues are integrated within a defined lineage, to specify unique neuronal identities only in thoracic segments. This study reveals a surprisingly restricted, yet multifaceted, function of both anteroposterior and temporal cues with respect to lineage control and cell fate specification.

An animal's nervous system contains a wide variety of neuronal subtypes generated from neural progenitor (“stem”) cells, which generate different types of neurons at different axial positions and time points. Hence, the generation and specification of unique neuronal subtypes is dependent upon the integration of both spatial and temporal cues within distinct stem cells. The nature of this integration is poorly understood. We have addressed this issue in the Drosophila neuroblast 5–6 lineage. This stem cell is generated in all 18 segments of the central nervous system, stretching from the brain down to the abdomen of the fly, but a larger lineage containing a well-defined set of cells—the Apterous (Ap) cluster—is generated only in thoracic segments. We show that segment-specific generation of the Ap cluster neurons is achieved by the integration of the anteroposterior and temporal cues in several different ways. Generation of the Ap neurons in abdominal segments is prevented by anteroposterior cues stopping the cell cycle in the stem cell at an early stage. In brain segments, late-born neurons are generated, but are differently specified due to the presence of different anteroposterior and temporal cues. Finally, in thoracic segments, the temporal and spatial cues integrate on a highly limited set of target genes to specify the Ap cluster neurons.

Transcriptional orchestration of the regulated secretory pathway in neurons by the bHLH protein DIMM

BACKGROUND

The Drosophila basic helix-loop-helix (bHLH) gene dimmed (dimm) promotes a neurosecretory/neuroendocrine phenotype in cells but is not associated with specific neuropeptides or neurohormones. Rather, it is expressed by those peptidergic neurons that project long axons and appear to produce large amounts of secretory peptides. Here, we genetically transform nonpeptidergic neurons in Drosophila to study DIMM's action mechanisms.

RESULTS

Nonpeptidergic neurons normally fail to accumulate ectopic neuropeptides. We now show that they will do so when they are also forced to express ectopic DIMM. Furthermore, mass spectrometry shows that photoreceptors, which are normally nonpeptidergic, fail to process an ectopic neuropeptide precursor to make bioactive peptides but will do so efficiently when DIMM is co-misexpressed. Likewise, photoreceptors, which normally package the fast neurotransmitter histamine within small clear synaptic vesicles, produce numerous large dense-core vesicles (LDCVs) when they misexpress DIMM. These novel LDCVs accumulate ectopic neuropeptide when photoreceptors co-misexpress a neuropeptide transgene. DIMM-expressing photoreceptors no longer accumulate histamine and lose synaptic organelles critical to their normal physiology.

CONCLUSIONS

These findings indicate that DIMM suppresses conventional fast neurotransmission and promotes peptidergic neurosecretory properties. We conclude that DIMM normally provides a comprehensive transcriptional control to direct the differentiation of dedicated neuroendocrine neurons.

Neuronal subtype specification within a lineage by opposing temporal feed-forward loops

Neural progenitors generate distinct cell types at different stages, but the mechanisms controlling these temporal transitions are poorly understood. In the Drosophila CNS, a cascade of transcription factors, the "temporal gene cascade," has been identified that acts to alter progenitor competence over time. However, many CNS lineages display broad temporal windows, and it is unclear how broad windows progress into subwindows that generate unique cell types. We have addressed this issue in an identifiable Drosophila CNS lineage and find that a broad castor temporal window is subdivided by two different feed-forward loops, both of which are triggered by castor itself. The first loop acts to specify a unique cell fate, whereas the second loop suppresses the first loop, thereby allowing for the generation of alternate cell fates. This mechanism of temporal and "subtemporal" genes acting in opposing feed-forward loops may be used by many stem cell lineages to generate diversity.

The proprotein convertase amontillado (amon) is required during Drosophila pupal development

Peptide hormones governing many developmental processes are generated via endoproteolysis of inactive precursor molecules by a family of subtilisin-like proprotein convertases (SPCs). We previously identified mutations in the Drosophila amontillado (amon) gene, a homolog of the vertebrate neuroendocrine-specific Prohormone Convertase 2 (PC2) gene, and showed that amon is required during embryogenesis, early larval development, and larval molting. Here, we define amon requirements during later developmental stages using a conditional rescue system and find that amon is required during pupal development for head eversion, leg and wing disc extension, and abdominal differentiation. Immuno-localization experiments show that amon protein is expressed in a subset of central nervous system cells but does not co-localize with peptide hormones known to elicit molting behavior, suggesting the involvement of novel regulatory peptides in this process. The amon protein is expressed in neuronal cells that innervate the corpus allatum and corpora cardiaca of the ring gland, an endocrine organ which is the release site for many key hormonal signals. Expression of amon in a subset of these cell types using the GAL4/UAS system in an amon mutant background partially rescues larval molting and growth. Our results show that amon is required for pupal development and identify a subset of neuronal cell types in which amon function is sufficient to rescue developmental progression and growth defects shown by amon mutants. The results are consistent with a model that the amon protein acts to proteolytically process a diverse suite of peptide hormones that coordinate larval and pupal growth and development.

Peptidergic neurosecretory cells in insects: organization and control by the bHLH protein DIMMED

This review considers evidence that defines a role for the transcription factor DIMMED in the regulation of insect neurosecretory cells. Genetic anatomical and molecular data all suggest DIMMED is a dedicated controller of the regulated secretory pathway. DIMM is normally expressed within diverse neuropeptide-expressing cells and appears highly correlated with a neurosecretory cell fate. Loss of DIMM is associated with deficits in display of neuropeptides and neuropeptide-associated enzymes. Gain of DIMM promotes such display in peptidergic cells and can confer such neurosecretory properties onto conventional neurons. We review models proposed to explain how DIMMED regulates these essential cellular properties.

Cloning of neuropeptide-like precursor 1 in the gray flesh fly and peptide identification and expression

The neuropeptide-like precursor 1 (NPLP1) was first identified in a peptidomics experiment on Drosophila melanogaster. Limited data on this novel neuropeptide precursor suggest a role in the regulation of ecdysis in holometabolous larvae. In this study, we characterized the NPLP1 precursor in the gray flesh fly, Neobellieria bullata, which is an excellent model for physiological assays and hence to discover the role of the NPLP1 peptides. Antisera against three of the D. melanogaster NPLP1 neuropeptides stained an identical set of neurons in the central nervous system of N. bullata compared to D. melanogaster. A novel approach was applied to identify the N. bullata NPLP1 orthologs. Using a combination of affinity chromatography, mass spectrometry, cDNA cloning and RACE experiments, we obtained almost the complete coding sequence of the NPLP1 mRNA. Three encoded NPLP1 peptides were identified in central nervous system extracts by mass spectrometry. Neither doses of 25-250pmol of synthetic Neb-MGYamide and Neb-PQNamide peptides, nor the NPLP1 antisera did affect the speed of retraction, contraction and tanning in the pupariation bioassay.

Synaptic transmission in neurons that express the Drosophila atypical soluble guanylyl cyclases, Gyc-89Da and Gyc-89Db, is necessary for the successful completion of larval and adult ecdysis

Insect ecdysis is a precisely coordinated series of behavioral and hormonal events that occur at the end of each molt. A great deal is known about the hormonal events that underlie this process, although less is known about the neuronal circuitry involved. In this study we identified two populations of neurons that are required for larval and adult ecdyses in the fruit fly, Drosophila melanogaster (Meigen). These neurons were identified by using the upstream region of two genes that code for atypical soluble guanylyl cyclases to drive tetanus toxin in the neurons that express these cyclases to block their synaptic activity. Expression of tetanus toxin in neurons that express Gyc-89Da blocked adult eclosion whereas expression of tetanus toxin in neurons that express Gyc-89Db prevented the initiation of the first larval ecdysis. Expression of tetanus toxin in the Gyc-89Da neurons also resulted in about 50% lethality just prior to pupariation; however, this was probably due to suffocation in the food as lethality was prevented by stopping the larvae from burrowing deep within the food. This result is consistent with our model that the atypical soluble guanylyl cyclases can act as molecular oxygen detectors. The expression pattern of these cyclases did not overlap with any of the neurons containing peptides known to regulate ecdysis and eclosion behaviors. By using the conditional expression of tetanus toxin we were also able to demonstrate that synaptic activity in the Gyc-89Da and Gyc-89Db neurons is required during early adult development for adult eclosion.

Drosophila, a genetic model system to study cocaine-related behaviors: a review with focus on LIM-only proteins

In the last decade, the fruit fly Drosophila melanogaster, highly accessible to genetic, behavioral and molecular analyses, has been introduced as a novel model organism to help decipher the complex genetic, neurochemical, and neuroanatomical underpinnings of behaviors induced by drugs of abuse. Here we review these data, focusing specifically on cocaine-related behaviors. Several of cocaine's most characteristic properties have been recapitulated in Drosophila. First, cocaine induces motor behaviors in flies that are remarkably similar to those observed in mammals. Second, repeated cocaine administration induces behavioral sensitization a form of behavioral plasticity believed to underlie certain aspects of addiction. Third, a key role for dopaminergic systems in mediating cocaine's effects has been demonstrated through both pharmacological and genetic methods. Finally, and most importantly, unbiased genetic screens, feasible because of the simplicity and scale with which flies can be manipulated in the laboratory, have identified several novel genes and pathways whose role in cocaine behaviors had not been anticipated. Many of these genes and pathways have been validated in mammalian models of drug addiction. We focus in this review on the role of LIM-only proteins in cocaine-induced behaviors.

Mapping peptidergic cells in Drosophila: where DIMM fits in

The bHLH transcription factor DIMMED has been associated with the differentiation of peptidergic cells in Drosophila. However, whether all Drosophila peptidergic cells express DIMM, and the extent to which all DIMM cells are peptidergic, have not been determined. To address these issues, we have mapped DIMM expression in the central nervous system (CNS) and periphery in the late larval stage Drosophila. At 100 hr after egg-laying, DIMM immunosignals are largely congruent with a dimm-promoter reporter (c929-GAL4) and they present a stereotyped pattern of 306 CNS cells and 52 peripheral cells. We assigned positional values for all DIMM CNS cells with respect to reference gene expression patterns, or to patterns of secondary neuroblast lineages. We could assign provisional peptide identities to 68% of DIMM-expressing CNS cells (207/306) and to 73% of DIMM-expressing peripheral cells (38/52) using a panel of 24 markers for Drosophila neuropeptide genes. Furthermore, we found that DIMM co-expression was a prevalent feature within single neuropeptide marker expression patterns. Of the 24 CNS neuropeptide gene patterns we studied, six patterns are >90% DIMM-positive, while 16 of 22 patterns are >40% DIMM-positive. Thus most or all DIMM cells in Drosophila appear to be peptidergic, and many but not all peptidergic cells express DIMM. The co-incidence of DIMM-expression among peptidergic cells is best explained by a hypothesis that DIMM promotes a specific neurosecretory phenotype we term LEAP. LEAP denotes Large cells that display Episodic release of Amidated Peptides.

The Drosophila basic helix-loop-helix protein DIMMED directly activates PHM, a gene encoding a neuropeptide-amidating enzyme

The basic helix-loop-helix (bHLH) protein DIMMED (DIMM) supports the differentiation of secretory properties in numerous peptidergic cells of Drosophila melanogaster. DIMM is coexpressed with diverse amidated neuropeptides and with the amidating enzyme peptidylglycine alpha-hydroxylating monooxygenase (PHM) in approximately 300 cells of the late embryo. Here we confirm that DIMM has transcription factor activity in transfected HEK 293 cells and that the PHM gene is a direct target. The mammalian DIMM orthologue MIST1 also transactivated the PHM gene. DIMM activity was dependent on the basic region of the protein and on the sequences of three E-box sites within PHM's first intron; the sites make different contributions to the total activity. These data suggest a model whereby the three E boxes interact cooperatively and independently to produce high PHM transcriptional activation. This DIMM-controlled PHM regulatory region displayed similar properties in vivo. Spatially, its expression mirrored that of the DIMM protein, and its activity was largely dependent on dimm. Further, in vivo expression was highly dependent on the sequences of the same three E boxes. This study supports the hypothesis that DIMM is a master regulator of a peptidergic cell fate in Drosophila and provides a detailed transcriptional mechanism of DIMM action on a defined target gene.

D1 dopamine receptor dDA1 is required in the mushroom body neurons for aversive and appetitive learning in Drosophila

Drosophila has robust behavioral plasticity to avoid or prefer the odor that predicts punishment or food reward, respectively. Both types of plasticity are mediated by the mushroom body (MB) neurons in the brain, in which various signaling molecules play crucial roles. However, important yet unresolved molecules are the receptors that initiate aversive or appetitive learning cascades in the MB. We have shown previously that D1 dopamine receptor dDA1 is highly enriched in the MB neuropil. Here, we demonstrate that dDA1 is a key receptor that mediates both aversive and appetitive learning in pavlovian olfactory conditioning. We identified two mutants, dumb1 and dumb2, with abnormal dDA1 expression. When trained with the same conditioned stimuli, both dumb alleles showed negligible learning in electric shock-mediated conditioning while they exhibited moderately impaired learning in sugar-mediated conditioning. These phenotypes were not attributable to anomalous sensory modalities of dumb mutants because their olfactory acuity, shock reactivity, and sugar preference were comparable to those of control lines. Remarkably, the dumb mutant's impaired performance in both paradigms was fully rescued by reinstating dDA1 expression in the same subset of MB neurons, indicating the critical roles of the MB dDA1 in aversive as well as appetitive learning. Previous studies using dopamine receptor antagonists implicate the involvement of D1/D5 receptors in various pavlovian conditioning tasks in mammals; however, these have not been supported by the studies of D1- or D5-deficient animals. The findings described here unambiguously clarify the critical roles of D1 dopamine receptor in aversive and appetitive pavlovian conditioning.

Locomotor activity is regulated by D2-like receptors in Drosophila: an anatomic and functional analysis

In mammals, dopamine 2-like receptors are expressed in distinct pathways within the central nervous system, as well as in peripheral tissues. Selected neuronal D2-like receptors play a critical role in modulating locomotor activity and, as such, represent an important therapeutic target (e.g. in Parkinson's disease). Previous studies have established that proteins required for dopamine (DA) neurotransmission are highly conserved between mammals and the fruit fly Drosophila melanogaster. These include a fly dopamine 2-like receptor (DD2R; Hearn et al. PNAS 2002 99(22):14554) that has structural and pharmacologic similarity to the human D2-like (D2R). In the current study, we define the spatial expression pattern of DD2R, and functionally characterize flies with reduced DD2 receptor levels. We show that DD2R is expressed in the larval and adult nervous systems, in cell groups that include the Ap-let cohort of peptidergic neurons, as well as in peripheral tissues including the gut and Malpighian tubules. To examine DD2R function in vivo, we generated RNA-interference (RNAi) flies with reduced DD2R expression. Behavioral analysis revealed that these flies show significantly decreased locomotor activity, similar to the phenotype observed in mammals with reduced D2R expression. The fly RNAi phenotype can be rescued by administration of the DD2R synthetic agonist bromocriptine, indicating specificity for the RNAi effect. These results suggest Drosophila as a useful system for future studies aimed at identifying modifiers of dopaminergic signaling/locomotor function.

Specification of neuronal identities by feedforward combinatorial coding

Neuronal specification is often seen as a multistep process: earlier regulators confer broad neuronal identity and are followed by combinatorial codes specifying neuronal properties unique to specific subtypes. However, it is still unclear whether early regulators are re-deployed in subtype-specific combinatorial codes, and whether early patterning events act to restrict the developmental potential of postmitotic cells. Here, we use the differential peptidergic fate of two lineage-related peptidergic neurons in the Drosophila ventral nerve cord to show how, in a feedforward mechanism, earlier determinants become critical players in later combinatorial codes. Amongst the progeny of neuroblast 5–6 are two peptidergic neurons: one expresses FMRFamide and the other one expresses Nplp1 and the dopamine receptor DopR. We show the HLH gene collier functions at three different levels to progressively restrict neuronal identity in the 5–6 lineage. At the final step, collier is the critical combinatorial factor that differentiates two partially overlapping combinatorial codes that define FMRFamide versus Nplp1/DopR identity. Misexpression experiments reveal that both codes can activate neuropeptide gene expression in vast numbers of neurons. Despite their partially overlapping composition, we find that the codes are remarkably specific, with each code activating only the proper neuropeptide gene. These results indicate that a limited number of regulators may constitute a potent combinatorial code that dictates unique neuronal cell fate, and that such codes show a surprising disregard for many global instructive cues.

By studying the differential peptidergic fate of two lineage-related neurons in theDrosophila ventral nerve cord, the authors provide deeper insights into how, in a feedforward mechanism, earlier developmental determinants become critical players in later combinatorial codes defining cell identity.

The nervous system contains a daunting number of different cell types, perhaps as many as 10,000 in mammals, far outnumbering regulatory genes in many animal species. Studies of the determinants of cell fate in many systems during the last decade have supported the conclusion that cell fate is not determined by any one regulatory gene, but results from the combinatorial action of several regulators. Many questions about the nature of such codes, however, remain. It is not known, for example, how complex such codes are or how they are established. It is also unclear whether they are confined in their action or if they act outside of their normal context. To address these outstanding issues, we have used two unique subsets of Drosophila neurons, identifiable by their specific expression of two different neuropeptide genes. We have identified two partially overlapping and relatively simple codes, consisting of four to seven regulators that act to specify these two cell types. Intriguingly, specification is achieved in a feedforward manner such that A activates B, followed by A/B activating C, and A/B/C activating D. Each code is surprisingly potent, and can ectopically activate neuropeptide gene expression in a variety of neurons, with a surprising disregard for many early patterning events.

Sex-peptide-regulated female sexual behavior requires a subset of ascending ventral nerve cord neurons

BACKGROUND

Male-derived Sex-peptide (SP) elicits egg laying and rejection of courting males in mated Drosophila females. Little is known about the genes that specify the underlying neuronal circuits and mediate this switch in female sexual behavior.

RESULTS

Here we show that the egghead gene involved in glycosphingolipid biosynthesis provides an essential component to the SP response. We have isolated viable alleles of the vital egghead gene that abolish egghead expression from a distal promoter resulting in the absence of the largest transcript of this complex transcription unit. Temporally and spatially restricted expression of egghead revealed a requirement for egghead early in the development of apterous-expressing ventral nerve cord neurons to rescue the SP response. In viable egghead alleles, these ascending interneurons, three per abdominal and seven per thoracic hemisegment, fail to innervate the central brain. egghead expression in apterous neurons rescues neuronal targeting and the response to SP. Furthermore, neurotransmission in apterous neurons is required to elicit the SP response.

CONCLUSION

Together with the former finding of SP binding to afferent nerves , these results suggest that SP-mediated modification of sensory input switches female sexual behavior from the virgin to the mated state.

Transcriptional regulation of neuropeptide and peptide hormone expression by the Drosophila dimmed and cryptocephal genes

The regulation of neuropeptide and peptide hormone gene expression is essential for the development and function of neuroendocrine cells in integrated physiological networks. In insects, a decline in circulating ecdysteroids triggers the activation of a neuroendocrine system to stimulate ecdysis, the behaviors used to shed the old cuticle at the culmination of each molt. Here we show that two evolutionarily conserved transcription factor genes, the basic helix-loop-helix (bHLH) gene dimmed (dimm) and the basic-leucine zipper (bZIP) gene cryptocephal (crc), control expression of diverse neuropeptides and peptide hormones in Drosophila. Central nervous system expression of three neuropeptide genes, Dromyosuppressin, FMRFamide-related and Leucokinin, is activated by dimm. Expression of Ecdysis triggering hormone (ETH) in the endocrine Inka cells requires crc; homozygous crc mutant larvae display markedly reduced ETH levels and corresponding defects in ecdysis. crc activates ETH expression though a 382 bp enhancer, which completely recapitulates the ETH expression pattern. The enhancer contains two evolutionarily conserved regions, and both are imperfect matches to recognition elements for activating transcription factor-4 (ATF-4), the vertebrate ortholog of the CRC protein and an important intermediate in cellular responses to endoplasmic reticulum stress. These regions also contain a putative ecdysteroid response element and a predicted binding site for the products of the E74 ecdysone response gene. These results suggest that convergence between ATF-related signaling and an important intracellular steroid response pathway may contribute to the neuroendocrine regulation of insect molting.

Regulation of secretory protein expression in mature cells by DIMM, a basic helix-loop-helix neuroendocrine differentiation factor

During differentiation, neuroendocrine cells acquire highly amplified capacities to synthesize neuropeptides to overcome dilution of these signals in the general circulation. Once mature, the normal functioning of integrated physiological systems requires that neuroendocrine cells remain plastic to dramatically alter neuropeptide expression for long periods in response to hormonal and electrical cues. The mechanisms underlying the long-term regulation of neuroendocrine systems are poorly understood. Here we show that the Drosophila basic helix-loop-helix protein DIMM, a critical regulator of neuroendocrine cell differentiation, controls secretory capacity in mature neurons. DIMM expression began embryonically but persisted in adults. Through spatial and temporal manipulation of transgene expression in vivo, we defined two phases of prosecretory DIMM activity. During an embryonic critical window, DIMM controlled the differentiation of amplified expression of the neuropeptide leucokinin. At the onset of metamorphosis, levels of DIMM decreased in the insulin-producing cells (IPCs) in parallel with a marked reduction in levels of Drosophila insulin-like peptide 2 and a key neuropeptide biosynthetic enzyme peptidylglycine alpha-monooxygenase (PHM). Overexpression of DIMM in the IPCs prevented the decrease in PHM levels at this stage. In addition, transient overexpression of DIMM in adults produced a dramatic increase in PHM levels in numerous neurons located throughout the brain. These findings provide insights into the mechanisms controlling the maintenance of differentiated cell states, and they suggest an effective means for dynamically adjusting the strength of hormonal signals in diverse homeostatic systems.

Regulators acting in combinatorial codes also act independently in single differentiating neurons

In the Drosophila ventral nerve cord, a small number of neurons express the LIM-homeodomain gene apterous (ap). These ap neurons can be subdivided based upon axon pathfinding and their expression of neuropeptidergic markers. ap, the zinc finger gene squeeze, the bHLH gene dimmed, and the BMP pathway are all required for proper specification of these cells. Here, using several ap neuron terminal differentiation markers, we have resolved how each of these factors contributes to ap neuron diversity. We find that these factors interact genetically and biochemically in subtype-specific combinatorial codes to determine certain defining aspects of ap neuron subtype identity. However, we also find that ap, dimmed, and squeeze additionally act independently of one another to specify certain other defining aspects of ap neuron subtype identity. Therefore, within single neurons, we show that single regulators acting in numerous molecular contexts differentially specify multiple subtype-specific traits.

Genetic and biochemical strategies for identifying Drosophila genes that function in circadian control

Explicit biochemical models have been elaborated for the circadian oscillators of cyanobacterial, fungal, insect, and mammalian species. In contrast, much remains to be learned about how such circadian oscillators regulate rhythmic physiological processes. This article summarizes contemporary genetic and biochemical strategies that are useful for identifying gene products that have a role in circadian control.

Lmo mutants reveal a novel role for circadian pacemaker neurons in cocaine-induced behaviors

Drosophila has been developed recently as a model system to investigate the molecular and neural mechanisms underlying responses to drugs of abuse. Genetic screens for mutants with altered drug-induced behaviors thus provide an unbiased approach to define novel molecules involved in the process. We identified mutations in the Drosophila LIM-only (LMO) gene, encoding a regulator of LIM-homeodomain proteins, in a genetic screen for mutants with altered cocaine sensitivity. Reduced Lmo function increases behavioral responses to cocaine, while Lmo overexpression causes the opposite effect, reduced cocaine responsiveness. Expression of Lmo in the principal Drosophila circadian pacemaker cells, the PDF-expressing ventral lateral neurons (LN(v)s), is sufficient to confer normal cocaine sensitivity. Consistent with a role for Lmo in LN(v)function,Lmomutants also show defects in circadian rhythms of behavior. However, the role for LN(v)s in modulating cocaine responses is separable from their role as pacemaker neurons: ablation or functional silencing of the LN(v)s reduces cocaine sensitivity, while loss of the principal circadian neurotransmitter PDF has no effect. Together, these results reveal a novel role for Lmo in modulating acute cocaine sensitivity and circadian locomotor rhythmicity, and add to growing evidence that these behaviors are regulated by shared molecular mechanisms. The finding that the degree of cocaine responsiveness is controlled by the Drosophila pacemaker neurons provides a neuroanatomical basis for this overlap. We propose that Lmo controls the responsiveness of LN(v)s to cocaine, which in turn regulate the flies' behavioral sensitivity to the drug.

Drosophila uses two distinct neuropeptide amidating enzymes, dPAL1 and dPAL2

Neuropeptide alpha-amidation is a common C-terminal modification of secretory peptides, frequently required for biological activity. In mammals, amidation is catalyzed by the sequential actions of two enzymes [peptidylglycine-alpha-hydroxylating monooxygenase (PHM) and peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL)] that are co-synthesized within a single bifunctional precursor. The Drosophila genome predicts expression of one monofunctional PHM gene and two monofunctional PAL genes. Drosophila PHM encodes an active enzyme that is required for peptide amidation in vivo. Here we initiate studies of the two Drosophila PAL genes. dPAL1 has two predicted transmembrane domains, whereas dPAL2 is predicted to be soluble and secreted. dPAL2 expressed in heterologous cells is secreted readily and co-localized with hormone. In contrast, dPAL1 is secreted poorly, even when expressed with a cleaved signal replacing the predicted transmembrane domains; the majority of dPAL1 stays in the endoplasmic reticulum. Both proteins display PAL enzymatic activity. Compared to the catalytic core of rat PAL, the two Drosophila lyases have higher K(m) values, higher pH optima and similarly broad divalent metal ion requirements. Antibodies to dPAL1 and dPAL2 reveal co-expression in many identified neuroendocrine neurons. Although dPAL1 is broadly expressed, dPAL2 is found in only a limited subset of neurons. dPAL1 expression is highly correlated with the non-amidated peptide proctolin. Tissue immunostaining demonstrates that dPAL1 is largely localized to the cell soma, whereas dPAL2 is distributed throughout neuronal processes.