Dynamic expression of broad-complex isoforms mediates temporal control of an ecdysteroid target gene at the onset of Drosophila metamorphosis

Neuronal identity control at the resolution of a single transcription factor isoform

The brain exhibits remarkable neuronal diversity which is critical for its functional integrity. From the sheer number of cell types emerging from extensive transcriptional, morphological, and connectome datasets, the question arises of how the brain is capable of generating so many unique identities. 'Terminal selectors' are transcription factors hypothesized to determine the final identity characteristics in post-mitotic cells. Which transcription factors function as terminal selectors and the level of control they exert over different terminal characteristics are not well defined. Here, we establish a novel role for the transcription factor broad as a terminal selector in Drosophila melanogaster. We capitalize on existing large sequencing and connectomics datasets and employ a comprehensive characterization of terminal characteristics including Perturb-seq and whole-cell electrophysiology. We find a single isoform broad-z4 serves as the switch between the identity of two visual projection neurons LPLC1 and LPLC2. Broad-z4 is natively expressed in LPLC1, and is capable of transforming the transcriptome, morphology, and functional connectivity of LPLC2 cells into LPLC1 cells when perturbed. Our comprehensive work establishes a single isoform as the smallest unit underlying an identity switch, which may serve as a conserved strategy replicated across developmental programs.

Apocrine secretion in the salivary glands of Drosophilidae and other dipterans is evolutionarily conserved

Apocrine secretion is a transport and secretory mechanism that remains only partially characterized, even though it is evolutionarily conserved among all metazoans, including humans. The excellent genetic model organism Drosophila melanogaster holds promise for elucidating the molecular mechanisms regulating this fundamental metazoan process. Two prerequisites for such investigations are to clearly define an experimental system to investigate apocrine secretion and to understand the evolutionarily and functional contexts in which apocrine secretion arose in that system. To this end, we recently demonstrated that, in D. melanogaster, the prepupal salivary glands utilize apocrine secretion prior to pupation to deliver innate immune and defense components to the exuvial fluid that lies between the metamorphosing pupae and its chitinous case. This finding provided a unique opportunity to appraise how this novel non-canonical and non-vesicular transport and secretory mechanism is employed in different developmental and evolutionary contexts. Here we demonstrate that this apocrine secretion, which is mechanistically and temporarily separated from the exocytotic mechanism used to produce the massive salivary glue secretion (Sgs), is shared across Drosophilidae and two unrelated dipteran species. Screening more than 30 species of Drosophila from divergent habitats across the globe revealed that apocrine secretion is a widespread and evolutionarily conserved cellular mechanism used to produce exuvial fluid. Species with longer larval and prepupal development than D. melanogaster activate apocrine secretion later, while smaller and more rapidly developing species activate it earlier. In some species, apocrine secretion occurs after the secretory material is first concentrated in cytoplasmic structures of unknown origin that we name "collectors." Strikingly, in contrast to the widespread use of apocrine secretion to provide exuvial fluid, not all species use exocytosis to produce the viscid salivary glue secretion that is seen in D. melanogaster. Thus, apocrine secretion is the conserved mechanism used to realize the major function of the salivary gland in fruitflies and related species: it produces the pupal exuvial fluid that provides an active defense against microbial invasion during pupal metamorphosis.

Developmental regulation of epithelial cell cuboidal-to-squamous transition in Drosophila follicle cells

Epithelial cells form continuous membranous structures for organ formation, and these cells are classified into three major morphological categories: cuboidal, columnar, and squamous. It is crucial that cells transition between these shapes during the morphogenetic events of organogenesis, yet this process remains poorly understood. All three epithelial cell shapes can be found in the follicular epithelium of Drosophila egg chamber during oogenesis. Squamous cells (SCs) are initially restricted to the anterior terminus in cuboidal shape. They then rapidly become flattened to assume squamous shape by stretching and expansion in 12 ​h during midoogenesis. Previously, we reported that Notch signaling activated a zinc-finger transcription factor Broad (Br) at the end of early oogenesis. Here we report that ecdysone and JAK/STAT pathways subsequently converge on Br to serve as an important spatiotemporal regulator of this dramatic morphological change of SCs. The early uniform pattern of Br in the follicular epithelium is directly established by Notch signaling at stage 5 of oogenesis. Later, ecdysone and JAK/STAT signaling activities synergize to suppress Br in SCs from stage 8 to 10a, contributing to proper SC squamous shape. During this process, ecdysone signaling is essential for SC stretching, while JAK/STAT regulates SC clustering and cell fate determination. This study reveals an inhibitory role of ecdysone signaling in suppressing Br in epithelial cell remodeling. In this study we also used single-cell RNA sequencing data to highlight the shift in gene expression which occurs as Br is suppressed and cells become flattened.

20-Hydroxyecdysone regulates the prophenoloxidase cascade to immunize Metarhizium anisopliae in Locusta migratoria

BACKGROUND

PPO (prophenoloxidase) cascade plays an important role in resisting invasion of entomogenous fungus. The 20-hydroxyecdysone (20E) exerts potent effect on the innate immunity in many insects. However, whether 20E controls the PPO cascade system against fungi and the regulatory mechanism in insects remains unclear.

RESULTS

In this study, both the proteome and transcriptome of Locusta migratoria were determined followed by the induction of 20E. Pattern recognition receptor GNBP-2 (Gram-negative binding proteins) has been identified that responded to 20E at both messenger RNA (mRNA) and protein levels. The PPO gene expression in fat body and PO (phenoloxidase) activity in plasma was found significantly induced after 20E injection and during the high-20E developmental stage. However, when 20E signal was blocked by RNA interference (RNAi) of ecdysone receptor, the expression level of PPO and PO activity failed to be increased by 20E. Thus, 20E could not significantly induce the expression of PPO gene and PO activity after RNAi of GNBP-2. Furthermore, 20E treatment notably enhanced the resistance of L. migratoria against Metarhizium anisopliae. Followed by of GNBP-2 silencing, the mortality of nymphs was significantly increased under the stress of Metarhizium anisopliae, and 20E injection could not increase the resistance.

CONCLUSION

The 20E regulates the PPO system to resist fungal invasion via regulating GNBP-2 in worldwide pest L. migratoria. Our results provide insight into the mechanism of how 20E enhances the antimicrobial immunity, and will be beneficial for modification of entomogenous fungi targeting on hormones and the immune system. © 2020 Society of Chemical Industry.

let-7-Complex MicroRNAs Regulate Broad-Z3, Which Together with Chinmo Maintains Adult Lineage Neurons in an Immature State

During Drosophila melanogaster metamorphosis, arrested immature neurons born during larval development differentiate into their functional adult form. This differentiation coincides with the downregulation of two zinc-finger transcription factors, Chronologically Inappropriate Morphogenesis (Chinmo) and the Z3 isoform of Broad (Br-Z3). Here, we show that br-Z3 is regulated by two microRNAs, let-7 and miR-125, that are activated at the larval-to-pupal transition and are known to also regulate chinmo. The br-Z3 3′UTR contains functional binding sites for both let-7 and miR-125 that confers sensitivity to both of these microRNAs, as determined by deletion analysis in reporter assays. Forced expression of let-7 and miR-125 miRNAs leads to early silencing of Br-Z3 and Chinmo and is associated with inappropriate neuronal sprouting and outgrowth. Similar phenotypes were observed by the combined but not separate depletion of br-Z3 and chinmo. Because persistent Br-Z3 was not detected in let-7-C mutants, this work suggests a model in which let-7 and miR-125 activation at the onset of metamorphosis may act as a failsafe mechanism that ensures the coordinated silencing of both br-Z3 and chinmo needed for the timely outgrowth of neurons arrested during larval development. The let-7 and miR-125 binding site sequences are conserved across Drosophila species and possibly other insects as well, suggesting that this functional relationship is evolutionarily conserved.

Silencing Br-C impairs larval development and chitin synthesis in Lymantria dispar larvae

In insects, 20-hydroxyecdysone (20E) mediates developmental transitions and regulates molting processes through activation of a series of transcription factors. Broad-Complex (Br-C), a vital gene in the 20E signalling pathway, plays crucial roles during insect growth processes. However, whether Br-C affects chitin synthesis in insects remains unclear. In the present study, the Br-C gene from Lymantria dispar, a notorious defoliator of forestry, was identified based on transcriptome data, and subjected to bioinformatic analysis. The regulatory functions of LdBr-C in chitin synthesis and metabolism in L. dispar larvae were analysed by RNA interference (RNAi). The full-length LdBr-C gene (1431 bp) encodes a 477 amino acid (aa) polypeptide containing a common BRcore region (391 aa) at the N-terminus and a C-terminal Zinc finger domain (56 aa) harbouring two characteristic C2H2 motifs (CXXC and HXXXXH). Phylogenetic analyses showed that LdBr-C shares highest homology and identity with Br-C isoform 7 (83.12%) of Helicoverpa armigera. Expression profiles indicate that LdBr-C was expressed throughout larval and pupal stages, and highly expressed in prepupal and pupal stages. Furthermore, LdBr-C expression was strongly induced by exogenous 20E, and suppressed dramatically after application of dsLdBr-C. Bioassay results showed that knockdown of LdBr-C caused larval developmental deformity, significant weight loss, and a mortality rate of 67.18%. Knockdown of LdBr-C significantly down-regulated transcription levels of eight critical genes (LdTre1, LdTre2, LdG6PI, LdUAP, LdCHS1, LdCHS2, LdTPS and LdCHT) related to chitin synthesis and metabolism, thereby lowering the chitin content in the midgut and epidermis. Our findings demonstrate that Br-C knockdown impairs larval development and chitin synthesis in L. dispar.

cis-regulatory variation modulates susceptibility to enteric infection in the Drosophila genetic reference panel

BACKGROUND

Resistance to enteric pathogens is a complex trait at the crossroads of multiple biological processes. We have previously shown in the Drosophila Genetic Reference Panel (DGRP) that resistance to infection is highly heritable, but our understanding of how the effects of genetic variants affect different molecular mechanisms to determine gut immunocompetence is still limited.

RESULTS

To address this, we perform a systems genetics analysis of the gut transcriptomes from 38 DGRP lines that were orally infected with Pseudomonas entomophila. We identify a large number of condition-specific, expression quantitative trait loci (local-eQTLs) with infection-specific ones located in regions enriched for FOX transcription factor motifs. By assessing the allelic imbalance in the transcriptomes of 19 F1 hybrid lines from a large round robin design, we independently attribute a robust cis-regulatory effect to only 10% of these detected local-eQTLs. However, additional analyses indicate that many local-eQTLs may act in trans instead. Comparison of the transcriptomes of DGRP lines that were either susceptible or resistant to Pseudomonas entomophila infection reveals nutcracker as the only differentially expressed gene. Interestingly, we find that nutcracker is linked to infection-specific eQTLs that correlate with its expression level and to enteric infection susceptibility. Further regulatory analysis reveals one particular eQTL that significantly decreases the binding affinity for the repressor Broad, driving differential allele-specific nutcracker expression.

CONCLUSIONS

Our collective findings point to a large number of infection-specific cis- and trans-acting eQTLs in the DGRP, including one common non-coding variant that lowers enteric infection susceptibility.

Developmental regulation of regenerative potential in Drosophila by ecdysone through a bistable loop of ZBTB transcription factors

In many organisms, the regenerative capacity of tissues progressively decreases as development progresses. However, the developmental mechanisms that restrict regenerative potential remain unclear. In Drosophila, wing imaginal discs become unable to regenerate upon damage during the third larval stage (L3). Here, we show that production of ecdysone after larvae reach their critical weight (CW) terminates the window of regenerative potential by acting on a bistable loop composed of two antagonistic Broad-complex/Tramtrack/Bric-à-brac Zinc-finger (ZBTB) genes: chinmo and broad (br). Around mid L3, ecdysone signaling silences chinmo and activates br to switch wing epithelial progenitors from a default self-renewing to a differentiation-prone state. Before mid L3, Chinmo promotes a strong regenerative response upon tissue damage. After mid L3, Br installs a nonpermissive state that represses regeneration. Transient down-regulation of ecdysone signaling or Br in late L3 larvae enhances chinmo expression in damaged cells that regain the capacity to regenerate. This work unveils a mechanism that ties the self-renewing and regenerative potential of epithelial progenitors to developmental progression.

This study finds that the loss of regeneration potential in Drosophila wing imaginal discs is induced by the production of the steroid hormone ecdysone after the larva reaches its critical weight. Manipulating ecdysone signaling or the downstream transcription factors can uncouple regenerative properties from developmental progression.

While some organisms exhibit remarkable regenerative abilities throughout their life, many animals, including mammals, present limited regenerative potential that progressively decreases during development. Understanding the mechanisms underlying this progressive loss is important to devise therapeutic approaches aiming at facilitating the regeneration of a damaged tissue throughout life. The fruitfly Drosophila is a powerful model organism to address such questions. Indeed, while tissues, such as imaginal discs, can fully regenerate if damaged during early development, they fail to do so upon damages during late development. We show here that restriction of regenerative potential occurring during midlarval stages is due to the production of a steroid hormone, named ecdysone. By genetically manipulating ecdysone signaling, we can uncouple regenerative abilities from developmental progression. In particular, we show that ecdysone signaling triggers a switch in the sequential expression of two transcription factors, Chinmo and Broad, that positively and negatively regulate the competence for imaginal disc regeneration, respectively. Our work therefore identifies a key developmental signal that restricts regenerative potential in insects and opens new perspectives on elucidating how regeneration-permissive transcriptional programs are locked as development progresses.

Temporospatial induction of homeodomain gene cut dictates natural lineage reprogramming

Understanding how cellular identity naturally interconverts with high efficiency and temporospatial precision is crucial for regenerative medicine. Here, we revealed a natural midgut-to-renal lineage conversion event during Drosophila metamorphosis and identified the evolutionarily-conserved homeodomain protein Cut as a master switch in this process. A steep Wnt/Wingless morphogen gradient intersects with a pulse of steroid hormone ecdysone to induce cut expression in a subset of midgut progenitors and reprogram them into renal progenitors. Molecularly, ecdysone-induced temporal factor Broad physically interacts with cut enhancer-bound Wnt pathway effector TCF/β-catenin and likely bridges the distant enhancer and promoter region of cut through its self-association. Such long-range enhancer-promoter looping could subsequently trigger timely cut transcription. Our results therefore led us to propose an unexpected poising-and-bridging mechanism whereby spatial and temporal cues intersect, likely via chromatin looping, to turn on a master transcription factor and dictate efficient and precise lineage reprogramming.

As an embryo develops, an organism transforms from a single cell into an organized collection of different cells, tissues and organs. Regulated by genes and messenger molecules, non-specialized cells known as precursor cells, move, divide and adapt to produce the different cells in the adult body.

However, sometimes already-specialized adult cells can acquire a new role in a process known as lineage reprogramming. Finding ways to artificially induce and control lineage reprogramming could be useful in regenerative medicine. This would allow cells to be reprogrammed to replace those that are lost or damaged. So far, scientists have been unable to develop a clear view of how lineage reprogramming happens naturally.

Here, Xu et al. identified a cell-conversion event in the developing fruit fly. As the fly larva develops into an adult, a group of cells in the midgut reprogramme to become renal cells – the equivalent to human kidney cells. The experiments revealed that a combination of signals from a cell messenger system important for cell specialization (called Wnt) and the hormone that controls molting in insects, activate a gene called cut, which controls the midgut-to-renal lineage reprogramming.

Together, Wnt and the hormone ensure that cut is activated only in a small, specific group of midgut precursor cells at a precise time. The reprogrammed cells then move into the excretory organs, the renal tubes, where they give rise to renal cells. Midgut precursor cells in which cut had been experimentally removed, still traveled into the renal tubes. However, they failed to switch their identity and gave rise to midgut cells instead.

Further examination revealed that both Wnt and the ecdysone hormone are needed to activate the cut gene. This is probably achieved by creating loops in the DNA to bring together the two distantly located key regulatory elements of cut gene expression. If this mechanism can be seen in other contexts it may be possible to adapt it for medical purposes. The ability to reprogramme groups of cells with high specificity could transform medicine. It would make it easier for our bodies to regenerate and repair.

20-Hydroxyecdysone activates PGRP-SA mediated immune response in Locusta migratoria

20-hydroxyecdysone (20E) has been implicated in regulating the immune response in insects. Conflicting conclusions on 20E regulating immunity have been reported in model holometabolous species. However, in hemimetabolous insects, the role of 20E as an immune-suppressor or activator and the mechanism remains unclear. The migratory locust Locusta migratoria is a representative member of hemimetabolous insects. Here, digital gene expression (DGE) profiles of Locusta migratoria treated with 20E were analyzed. Pattern recognition receptors [peptidoglycan recognition protein (PGRP-SA), PGRP-LE, and gram-negative binding protein (GNBP3)] and antimicrobial peptides (defensin, diptericin, and i-type lysozyme) were significantly induced by 20E in fat body. These immune-related genes significantly increased their mRNA levels during the high-20E stage. Antibacterial activities in plasma were enhanced after 20E injection and during the high-20E developmental stage. Conversely, when 20E signal was suppressed by RNAi of EcR (ecdysone receptor), the expression levels of these genes and antibacterial activities failed to be increased by 20E injection and during the high-20E developmental stage, and the mortality increased after being infected by entomogenous fungus. The knockdown of PGRP-SA inhibited the expression level of defensin, diptericin and i-type lysozyme in fat body and reduced antibacterial activities in plasma. 20E injection could not significantly induce the expression of antimicrobial peptides after RNAi of PGRP-SA. These results demonstrated that 20E enhanced the immune response by activating PGRP-SA in L. migratoria.

Combgap Promotes Ovarian Niche Development and Chromatin Association of EcR-Binding Regions in BR-C

The development of niches for tissue-specific stem cells is an important aspect of stem cell biology. Determination of niche size and niche numbers during organogenesis involves precise control of gene expression. How this is achieved in the context of a complex chromatin landscape is largely unknown. Here we show that the nuclear protein Combgap (Cg) supports correct ovarian niche formation in Drosophila by controlling ecdysone-Receptor (EcR)- mediated transcription and long-range chromatin contacts in the broad locus (BR-C). Both cg and BR-C promote ovarian growth and the development of niches for germ line stem cells. BR-C levels were lower when Combgap was either reduced or over-expressed, indicating an intricate regulation of the BR-C locus by Combgap. Polytene chromosome stains showed that Cg co-localizes with EcR, the major regulator of BR-C, at the BR-C locus and that EcR binding to chromatin was sensitive to changes in Cg levels. Proximity ligation assay indicated that the two proteins could reside in the same complex. Finally, chromatin conformation analysis revealed that EcR-bound regions within BR-C, which span ~30 KBs, contacted each other. Significantly, these contacts were stabilized in an ecdysone- and Combgap-dependent manner. Together, these results highlight Combgap as a novel regulator of chromatin structure that promotes transcription of ecdysone target genes and ovarian niche formation.

Germ line stem cells (GSCs) supply either eggs or sperm throughout the life- time of many organisms, including mammals. For their function, GSCs require input from somatic niche cells. Understanding how niches form during development is an important initial step in understanding how stem cell units form, and by extension, how they may regenerate. In this work we describe a new function for the chromatin binding protein Combgap in ovarian niche formation of the model organism Drosophila melanogaster. Combgap is required for the correct expression of another factor, BR-C, in somatic ovarian cells. BR-C is one of the central target genes of the steroid hormone ecdysone, and its expression is controlled by the ecdysone receptor (EcR). Interestingly, EcR-enriched regions within the BR-C locus are engaged in long-range contacts that are stabilized by ecdysone in a Combgap-depended manner. We also found that EcR binding to chromatin depends on WT levels of Combgap. BR-C regulates GSC unit establishment, intestinal stem cells, immune responses, and many other processes. Understanding Combgaps’ function in shaping the BR-C chromatin landscape is a first step towards better appreciation of how this important locus is controlled, and the general machinery coupling gene expression to 3D chromatin structure.

Transcriptional regulation of cuticular protein glycine-rich13 gene expression in wing disc of Bombyx mori, Lepidoptera

Cuticular protein genes are good models to study the molecular mechanisms of signaling by ecdysteroids, which regulate molting and metamorphosis in insects. The present research demonstrates on hormonal regulation and analysis of the regulatory sequences and transcription factors important for Bombyx mori cuticular protein glycine-rich13 (CPG13) gene expression. Expression of CPG13 was strong at prepupal stage in wing tissues of B. mori. CPG13 expression was induced by the addition of 20E, which was inhibited by cycloheximide in the wing disc. The upstream region of the CPG13 gene was analyzed using a transient reporter assay with a gene gun system and identified two BR-Z2 binding sites to be important cis-acting elements for the transcription activation of the luciferase reporter gene by an ecdysone pulse. Site-directed mutagenesis of these sites, followed by introduction into wing discs, significantly decreased the reporter activity. It was found that the regions carrying the binding sites for the ecdysone-responsive transcription factor BR-Z2 were responsible for the hormonal enhancement of the reporter gene activity in wing discs. Mutation of the BR-Z2 binding sites decreased the reporter activity suggesting that the BR-Z2 isoform can bind to the upstream region of the cuticle protein gene, CPG13 and activates its expression.

The COP9 signalosome converts temporal hormone signaling to spatial restriction on neural competence

During development, neural competence is conferred and maintained by integrating spatial and temporal regulations. The Drosophila sensory bristles that detect mechanical and chemical stimulations are arranged in stereotypical positions. The anterior wing margin (AWM) is arrayed with neuron-innervated sensory bristles, while posterior wing margin (PWM) bristles are non-innervated. We found that the COP9 signalosome (CSN) suppresses the neural competence of non-innervated bristles at the PWM. In CSN mutants, PWM bristles are transformed into neuron-innervated, which is attributed to sustained expression of the neural-determining factor Senseless (Sens). The CSN suppresses Sens through repression of the ecdysone signaling target gene broad (br) that encodes the BR-Z1 transcription factor to activate sens expression. Strikingly, CSN suppression of BR-Z1 is initiated at the prepupa-to-pupa transition, leading to Sens downregulation, and termination of the neural competence of PWM bristles. The role of ecdysone signaling to repress br after the prepupa-to-pupa transition is distinct from its conventional role in activation, and requires CSN deneddylating activity and multiple cullins, the major substrates of deneddylation. Several CSN subunits physically associate with ecdysone receptors to represses br at the transcriptional level. We propose a model in which nuclear hormone receptors cooperate with the deneddylation machinery to temporally shutdown downstream target gene expression, conferring a spatial restriction on neural competence at the PWM.

A critical step in building a functional nervous system is to generate neurons at the appropriate locations. Neural competence is acquired at the precursor stage with the expression of specific transcription factors. One such critical factor is Senseless (Sens), as precursors lacking Sens fail to develop to neurons. Here we describe the critical role of protein complex COP9 signalosome (CSN) that regulates Sens expression by integrating temporal and spatial information. This was studied in developing Drosophila wing tissues, in which the anterior wing margin develops neuron-innervated bristles, while the posterior wing margin develops non-innervated bristles. The CSN complex is required for the anterior-posterior difference in spatial patterning of neuron formation, and posterior cells lacking CSN develop innervated bristles like anterior cells. CSN accomplishes this by transforming the temporal hormonal ecdysone signaling from activation to repression of downstream target BR-Z1. As BR-Z1 itself is a transcription activator, repression of BR-Z1 in turn leads to repression of Sens in posterior wing margin, eventually terminating the neural competence. Repression of BR-Z1 expression requires the interaction between the CSN complex and the ecdysone receptors. Our results suggest a novel CSN-mediated regulation that converts temporal hormone signaling to the patterning of neurons at the right place.

Expression of the pupal determinant broad during metamorphic and neotenic development of the strepsipteran Xenos vesparum Rossi

Derived members of the endoparasitic order Strepsiptera have acquired an extreme form of sexual dimorphism whereby males undergo metamorphosis and exist as free-living adults while females remain larviform, reaching sexual maturity within their hosts. Expression of the transcription factor, broad (br) has been shown to be required for pupal development in insects in which both sexes progress through metamorphosis. A surge of br expression appears in the last larval instar, as the epidermis begins pupal development. Here we ask if br is also up-regulated in the last larval instar of male Xenos vesparum Rossi (Stylopidae), and whether such expression is lost in neotenic larviform females. We clone three isoforms of br from X. vesparum (Xv’br), and show that they share greatest similarity to the Z1, Z3 and Z4 isoforms of other insect species. By monitoring Xv’br expression throughout development, we detect elevated levels of total br expression and the Xv’Z1, Xv’Z3, and Xv’Z4 isoforms in the last larval instar of males, but not females. By focusing on Xv’br expression in individual samples, we show that the levels of Xv’BTB and Xv’Z3 in the last larval instar of males are bimodal, with some males expressing 3X greater levels of Xv’br than fourth instar femlaes. Taken together, these data suggest that neoteny (and endoparasitism) in females of Strepsiptera Stylopidia could be linked to the suppression of pupal determination. Our work identifies a difference in metamorphic gene expression that is associated with neoteny, and thus provides insights into the relationship between metamorphic and neotenic development.

Upregulation of the expression of prodeath serine/threonine protein kinase for programmed cell death by steroid hormone 20-hydroxyecdysone

Serine/threonine protein kinases phosphorylate protein substrates to initiate further cellular events. Different serine/threonine protein kinases have varied functions despite their highly conserved homology. We propose prodeath-S/TK, a prodeath serine/threonine protein kinase from the lepidopteran insect Helicoverpa armigera, promotes programmed cell death (PCD) during metamorphosis. Prodeath-S/TK is expressed in various tissues with a high expression level during molting and metamorphosis by 20-hydroxyecdysone (20E) induction. Prodeath-S/TK is localized in the larval midgut during metamorphosis. Prodeath-S/TK knockdown by injecting dsRNA into larval hemocoel suppresses the 20E-induced metamorphosis and PCD, as well as downregulates a set of genes involved in the PCD and 20E signaling pathway. 20E upregulates prodeath-S/TK expression through its nuclear receptor EcR-B1 and USP1. Prodeath-S/TK overexpression in the epidermal cell line leads to PCD with DNA fragmentation and the activation of caspases 3 and 7. Prodeath-S/TK plays role in the cytoplasm. The N-terminal and C-terminal sequences of prodeath-S/TK determine its subcellular location. These data indicate that prodeath-S/TK participates in PCD by regulating gene expression in the 20E signaling pathway.

Ecdysone triggered PGRP-LC expression controls Drosophila innate immunity

Throughout the animal kingdom, steroid hormones have been implicated in the defense against microbial infection, but how these systemic signals control immunity is unclear. Here, we show that the steroid hormone ecdysone controls the expression of the pattern recognition receptor PGRP-LC in Drosophila, thereby tightly regulating innate immune recognition and defense against bacterial infection. We identify a group of steroid-regulated transcription factors as well as two GATA transcription factors that act as repressors and activators of the immune response and are required for the proper hormonal control of PGRP-LC expression. Together, our results demonstrate that Drosophila use complex mechanisms to modulate innate immune responses, and identify a transcriptional hierarchy that integrates steroid signalling and immunity in animals.

Characterization of a Trichoplusia ni hexamerin-derived promoter in the AcMNPV baculovirus vector

The promoter sequences of the encoding genes for the three most abundant hexamerins of the Lepidoptera Trichoplusia ni were isolated and cloned into the Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-derived baculovirus expression vector. From the sequences analyzed, the DNA region driving the expression of the Basic juvenile hormone-suppressible protein 2 (BJHSP-2), denominated pB2, presented the highest promoter strength in the context of the baculovirus vector in Sf21 insect cells. This promoter activity occurred earlier in baculovirus-infected cells than that achieved by a conventional polyhedrin promoter (polh), but surprisingly stopped at 48h post-infection. A mapping of pB2 essential promoter elements determined that a region of about 400bp, denominated pB29, retained and even increased the transcriptional activity with respect to the parental full-length sequence. Finally, several chimeric combinations of the insect-derived pB2 with the virus-derived conventional polh or p10 promoters were constructed and incorporated into an AcMNPV baculovirus vector. The pB2-p10 combination showed increased recombinant protein expression at early times post-infection and similar expression levels at very late times post-infection in Sf21 cells with respect to conventional late promoters. To the best of our knowledge, pB2 is the first promoter isolated from the Lepidoptera T. ni, the natural host of AcMNPV, to be assayed in a baculovirus expression vector.

The BTB/POZ zinc finger protein Broad-Z3 promotes dendritic outgrowth during metamorphic remodeling of the peripheral stretch receptor dbd

Background

Various members of the family of BTB/POZ zinc-finger transcription factors influence patterns of dendritic branching. One such member, Broad, is notable because its BrZ3 isoform is widely expressed in Drosophila in immature neurons around the time of arbor outgrowth. We used the metamorphic remodeling of an identified sensory neuron, the dorsal bipolar dendrite sensory neuron (dbd), to examine the effects of BrZ3 expression on the extent and pattern of dendrite growth during metamorphosis.

Results

Using live imaging of dbd in Drosophila pupae, we followed its normal development during metamorphosis and the effect of ectopic expression of BrZ3 on this development. After migration of its cell body, dbd extends a growth-cone that grows between two muscle bands followed by branching and turning back on itself to form a compact dendritic bundle. The ectopic expression of the BrZ3 isoform, using the GAL4/UAS system, caused dbd's dendritic tree to transform from its normal, compact, fasciculated form into a comb-like arbor that spread over on the body wall. Time-lapse analysis revealed that the expression of BrZ3 caused the premature extension of the primary dendrite onto immature myoblasts, ectopic growth past the muscle target region, and subsequent elaboration onto the epidermis. To control the timing of expression of BrZ3, we used a temperature-sensitive GAL80 mutant. When BrZ3 expression was delayed until after the extension of the primary dendrite, then a normal arbor was formed. By contrast, when BrZ3 expression was confined to only the early outgrowth phase, then ectopic arbors were subsequently formed and maintained on the epidermis despite the subsequent absence of BrZ3.

Conclusions

The adult arbor of dbd is a highly branched arbor whose branches self-fasciculate to form a compact dendritic bundle. The ectopic expression of BrZ3 in this cell causes a premature extension of its growth-cone, resulting in dendrites that extend beyond their normal muscle substrate and onto the epidermis, where they form a comb-shaped, ectopic arbor. Our quantitative data suggest that new ectopic arbor represents an 'unpacking' of the normally fasciculated arbor onto the epidermis. These data suggest that the nature of their local environment can change dendrite behavior from self-adhesion to self-avoidance.

Broad Complex isoforms have unique distributions during central nervous system metamorphosis in Drosophila melanogaster

Broad Complex (BRC) is a highly conserved, ecdysone-pathway gene essential for metamorphosis in Drosophila melanogaster, and possibly all holometabolous insects. Alternative splicing among duplicated exons produces several BRC isoforms, each with one zinc-finger DNA-binding domain (Z1, Z2, Z3, or Z4), highly expressed at the onset of metamorphosis. BRC-Z1, BRC-Z2, and BRC-Z3 represent distinct genetic functions (BRC complementation groups rbp, br, and 2Bc, respectively) and are required at discrete stages spanning final-instar larva through very young pupa. We showed previously that morphogenetic movements necessary for adult CNS maturation require BRC-Z1, -Z2, and -Z3, but not at the same time: BRC-Z1 is required in the mid-prepupa, BRC-Z2 and -Z3 are required earlier, at the larval-prepupal transition. To explore how BRC isoforms controlling the same morphogenesis events do so at different times, we examined their central nervous system (CNS) expression patterns during the approximately 16 hours bracketing the hormone-regulated start of metamorphosis. Each isoform had a unique pattern, with BRC-Z3 being the most distinctive. There was some colocalization of isoform pairs, but no three-way overlap of BRC-Z1, -Z2, and -Z3. Instead, their most prominent expression was in glia (BRC-Z1), neuroblasts (BRC-Z2), or neurons (BRC-Z3). Despite sequence similarity to BRC-Z1, BRC-Z4 was expressed in a unique subset of neurons. These data suggest a switch in BRC isoform choice, from BRC-Z2 in proliferating cells to BRC-Z1, BRC-Z3, or BRC-Z4 in differentiating cells. Together with isoform-selective temporal requirements and phenotype considerations, this cell-type-selective expression suggests a model of BRC-dependent CNS morphogenesis resulting from intercellular interactions, culminating in BRC-Z1-controlled, glia-mediated CNS movements in late prepupa.

Temporal patterns of broad isoform expression during the development of neuronal lineages in Drosophila

Background

During the development of the central nervous system (CNS) of Drosophila, neuronal stem cells, the neuroblasts (NBs), first generate a set of highly diverse neurons, the primary neurons that mature to control larval behavior, and then more homogeneous sets of neurons that show delayed maturation and are primarily used in the adult. These latter, 'secondary' neurons show a complex pattern of expression of broad, which encodes a transcription factor usually associated with metamorphosis, where it acts as a key regulator in the transitions from larva and pupa.

Results

The Broad-Z3 (Br-Z3) isoform appears transiently in most central neurons during embryogenesis, but persists in a subset of these cells through most of larval growth. Some of the latter are embryonic-born secondary neurons, whose development is arrested until the start of metamorphosis. However, the vast bulk of the secondary neurons are generated during larval growth and bromodeoxyuridine incorporation shows that they begin expressing Br-Z3 about 7 hours after their birth, approximately the time that they have finished outgrowth to their initial targets. By the start of metamorphosis, the oldest secondary neurons have turned off Br-Z3 expression, while the remainder, with the exception of the very youngest, maintain Br-Z3 while they are interacting with potential partners in preparation for neurite elaboration. That Br-Z3 may be involved in early sprouting is suggested by ectopically expressing this isoform in remodeling primary neurons, which do not normally express Br-Z3. These cells now sprout into ectopic locations. The expression of Br-Z3 is transient and seen in all interneurons, but two other isoforms, Br-Z4 and Br-Z1, show a more selective expression. Analysis of MARCM clones shows that the Br-Z4 isoform is expressed by neurons in virtually all lineages, but only in those cells born during a window during the transition from the second to the third larval instar. Br-Z4 expression is then maintained in this temporal cohort of cells into the adult.

Conclusion

These data show the potential for diverse functions of Broad within the developing CNS. The Br-Z3 isoform appears in all interneurons, but not motoneurons, when they first begin to interact with potential targets. Its function during this early sorting phase needs to be defined. Two other Broad isoforms, by contrast, are stably expressed in cohorts of neurons in all lineages and are the first examples of persisting molecular 'time-stamps' for Drosophila postembryonic neurons.

betaFTZ-F1 and Broad-Complex positively regulate the transcription of the wing cuticle protein gene, BMWCP5, in wing discs of Bombyx mori

The present study was undertaken to clarify the mechanism regulating cuticle protein gene expression. Expression of BMWCP5 was strong at around pupation and weak at the mid-pupal stage in wing tissues of Bombyx mori. We analyzed the upstream region of the BMWCP5 gene using a transient reporter assay with a gene gun system to identify the regulatory elements responsible for its unique expression pattern. We identified two betaFTZ-F1 binding sites to be important cis-acting elements for the transcription activation of the luciferase reporter gene by an ecdysone pulse. Site-directed mutagenesis of these sites, followed by introduction into wing discs, significantly decreased the reporter activity. We also found that the regions carrying the binding sites for the ecdysone-responsive factor BR-C Z4 (BR-Z4) were responsible for the hormonal enhancement of the reporter gene activity in wing discs. Mutation of the BR-Z4 binding sites decreased the reporter activity. The nuclear proteins that bound to these betaFTZ-F1 and BR-Z4 sites were identified by an electrophoretic mobility shift assay (EMSA). The results demonstrate for the first time that the BR-Z4 isoform can bind to the upstream region of the cuticle protein gene, BMWCP5, and activate its expression. The results also suggest that the BMWCP5 transcription is primarily regulated by the ecdysone pulse through betaFTZ-F1, and the stage-specific enhancement is brought about through BR-Z4.

Elucidation of the regulation of an adult cuticle gene Acp65A by the transcription factor Broad

Broad (BR), an ecdysone-inducible transcription factor, is a major determinant of the pupal stage. The misexpression of BR-Z1 isoform (BR-Z1) during adult development of Drosophila melanogaster prevents the expression of the adult cuticle protein 65A gene (Acp65A). We found that the proximal 237 bp of the 5' flanking region of Acp65A were sufficient to mediate this suppression. A targeted point mutation of a putative BR-Z1 response element (BRE) within this region showed that it was not involved. Drosophila hormone receptor-like 38 (DHR38) is required for Acp65A expression. We found that BR-Z1 repressed DHR38 expression and that BR's inhibition of Acp65A expression was rescued by exogenous expression of DHR38. Thus, BR-Z1 suppresses Acp65A expression by preventing the normal up-regulation of DHR38 at the time of adult cuticle formation.

The ecdysone receptor controls the post-critical weight switch to nutrition-independent differentiation in Drosophila wing imaginal discs

In holometabolous insects, a species-specific size, known as critical weight, needs to be reached for metamorphosis to be initiated in the absence of further nutritional input. Previously, we found that reaching critical weight depends on the insulin-dependent growth of the prothoracic glands (PGs) in Drosophila larvae. Because the PGs produce the molting hormone ecdysone, we hypothesized that ecdysone signaling switches the larva to a nutrition-independent mode of development post-critical weight. Wing discs from pre-critical weight larvae [5 hours after third instar ecdysis (AL3E)] fed on sucrose alone showed suppressed Wingless (WG), Cut (CT) and Senseless (SENS) expression. Post-critical weight, a sucrose-only diet no longer suppressed the expression of these proteins. Feeding larvae that exhibit enhanced insulin signaling in their PGs at 5 hours AL3E on sucrose alone produced wing discs with precocious WG, CT and SENS expression. In addition, knocking down the Ecdysone receptor (EcR) selectively in the discs also promoted premature WG, CUT and SENS expression in the wing discs of sucrose-fed pre-critical weight larvae. EcR is involved in gene activation when ecdysone is present, and gene repression in its absence. Thus, knocking down EcR derepresses genes that are normally repressed by unliganded EcR, thereby allowing wing patterning to progress. In addition, knocking down EcR in the wing discs caused precocious expression of the ecdysone-responsive gene broad. These results suggest that post-critical weight, EcR signaling switches wing discs to a nutrition-independent mode of development via derepression.

Overexpression of HmgD causes the failure of pupariation in Drosophila by affecting ecdysone receptor pathway

HmgD encodes Drosophila homologue of high mobility group proteins (HMGD), which are thought to have an architectural function in chromatin organization. However, current opinions about the function of HMGD in Drosophila development are controversial. Our previous studies have shown that ubiquitous overexpression of HmgD caused the formation of melanotic tumors in the Drosophila larvae by prematurely activating the Ras-MAPK pathway. Here we report that under maternal control, the viability of flies links with overexpression of HmgD, while under ubiquitous control, ActGal4, overexpressing HmgD animals, which display prolonged larval stages around day 13, developmentally stagnate in the larva-white pupa transition. Ecdysone feeding did not rescue overexpressing HmgD animals. RT-PCR analyses show that overexpression of HmgD does not affect the temporal expression pattern of ecdysone receptor gene EcR, whereas transcriptional patterns of some key regulatory genes, such as E74A, E74B, E75A, E75B, betaFTZ-F1, are changed greatly. These results suggest that ubiquitous overexpression of HmgD results in the failure of pupariation neither by affecting the process of ecdysone synthesis and release nor by abnormal EcR transcription, but by causing expression of EcR regulatory nuclear receptors out of schedule. The results led us to postulate that overexpression of HMGD likely changes the signaling cascade of Drosophila metamorphosis by an interaction between HMGD and DNA strands, and subsequently by an error of DNA binding abilities and transcriptional activities of some nuclear receptor genes. Arch. Insect Biochem. Physiol. 2008.

Broad-Complex acts downstream of Met in juvenile hormone signaling to coordinate primitive holometabolan metamorphosis

Metamorphosis of holometabolous insects, an elaborate change of form between larval, pupal and adult stages, offers an ideal system to study the regulation of morphogenetic processes by hormonal signals. Metamorphosis involves growth and differentiation, tissue remodeling and death, all of which are orchestrated by the morphogenesis-promoting ecdysteroids and the antagonistically acting juvenile hormone (JH), whose presence precludes the metamorphic changes. How target tissues interpret this combinatorial effect of the two hormonal cues is poorly understood, mainly because JH does not prevent larval-pupal transformation in the derived Drosophila model, and because the JH receptor is unknown. We have recently used the red flour beetle Tribolium castaneum to show that JH controls entry to metamorphosis via its putative receptor Methoprene-tolerant (Met). Here, we demonstrate that Met mediates JH effects on the expression of the ecdysteroid-response gene Broad-Complex (BR-C). Using RNAi and a classical mutant, we show that Tribolium BR-C is necessary for differentiation of pupal characters. Furthermore, heterochronic combinations of retarded and accelerated phenotypes caused by impaired BR-C function suggest that besides specifying the pupal fate, BR-C operates as a temporal coordinator of hormonally regulated morphogenetic events across epidermal tissues. Similar results were also obtained when using the lacewing Chrysopa perla (Neuroptera), a member of another holometabolous group with a primitive type of metamorphosis. The tissue coordination role of BR-C may therefore be a part of the Holometabola groundplan.

Anciently duplicated Broad Complex exons have distinct temporal functions during tissue morphogenesis

Broad Complex (BRC) is an essential ecdysone-pathway gene required for entry into and progression through metamorphosis in Drosophila melanogaster. Mutations of three BRC complementation groups cause numerous phenotypes, including a common suite of morphogenesis defects involving central nervous system (CNS), adult salivary glands (aSG), and male genitalia. These defects are phenocopied by the juvenile hormone mimic methoprene. Four BRC isoforms are produced by alternative splicing of a protein-binding BTB/POZ-encoding exon (BTBBRC) to one of four tandemly duplicated, DNA-binding zinc-finger-encoding exons (Z1BRC, Z2BRC, Z3BRC, Z4BRC). Highly conserved orthologs of BTBBRC and all four ZBRC were found among published cDNA sequences or genome databases from Diptera, Lepidoptera, Hymenoptera, and Coleoptera, indicating that BRC arose and underwent internal exon duplication before the split of holometabolous orders. Tramtrack subfamily members, abrupt, tramtrack, fruitless, longitudinals lacking (lola), and CG31666 were characterized throughout Holometabola and used to root phylogenetic analyses of ZBRC exons, which revealed that the ZBRC clade includes Zabrupt. All four ZBRC domains, including Z4BRC, which has no known essential function, are evolving in a manner consistent with selective constraint. We used transgenic rescue to explore how different BRC isoforms contribute to shared tissue-morphogenesis functions. As predicted from earlier studies, the common CNS and aSG phenotypes were rescued by BRC-Z1 in rbp mutants, BRC-Z2 in br mutants, and BRC-Z3 in 2Bc mutants. However, the isoforms are required at two different developmental stages, with BRC-Z2 and -Z3 required earlier than BRC-Z1. The sequential action of BRC isoforms indicates subfunctionalization of duplicated ZBRC exons even when they contribute to common developmental processes.

Distinct roles of Broad isoforms in regulation of the 20-hydroxyecdysone effector gene, Vitellogenin, in the mosquito Aedes aegypti

We investigated the role of the mosquito broad (br) gene in regulating the 20-hydroxyecdysone (20E) effector Vitellogenin (Vg) gene. Injection of double-stranded RNA corresponding to the BR isoform Z2 led to a significant decrease in expression of the Vg gene at 8 and 24h post-blood meal. Knockdown of Z1 or Z4 resulted in enhanced Vg expression beyond its normal expression time. In vitro studies suggested that the effects of BR require its direct binding to the Vg promoter, as well as protein-protein interaction between BR and the ecdysone receptor complex. The BR isoforms are therefore essential for a proper stage-specific biological response to 20E in the adult female mosquito. In particular, the isoform Z2 is required for 20E-mediated activation of Vg, while isoforms Z1 and Z4 serve as repressors to ensure appropriate termination of Vg expression.

Broad-Complex,E74, andE75early genes control DNA puffBhC4-1expression in prepupal salivary glands

The DNA puff BhC4-1 gene of the sciarid Bradysia hygida is induced in salivary glands prior to the pupal molt as a secondary response to the increase in ecdysone titers. Previous studies demonstrated that the BhC4-1 promoter is activated in transgenic Drosophila melanogaster salivary glands as a late response to the ecdysone peak that triggers metamorphosis, revealing that this aspect of BhC4-1 transcriptional regulation is conserved in the Drosophila background. To identify regulators of BhC4-1 expression, we utilized a candidate gene approach and tested the roles of the ecdysone-induced genes BR-C, E74, and E75. Our results reveal that the BR-C Z3 isoform is essential for BhC4-1-lacZ induction in prepupal salivary glands and constitute the first demonstration of the participation of early genes products on DNA puff genes regulation.

Transcription of Myocyte enhancer factor-2 in adult Drosophila myoblasts is induced by the steroid hormone ecdysone

The steroid hormone 20-hydroxyecdysone (ecdysone) activates a relatively small number of immediate-early genes during Drosophila pupal development, yet is able to orchestrate distinct differentiation events in a wide variety of tissues. Here, we demonstrate that expression of the muscle differentiation gene Myocyte enhancer factor-2 (Mef2) is normally delayed in twist-expressing adult myoblasts until the end of the third larval instar. The late up-regulation of Mef2 transcription in larval myoblasts is an ecdysone-dependent event which acts upon an identified Mef2 enhancer, and we identify enhancer sequences required for up-regulation. We also present evidence that the ecdysone-induced Broad Complex of zinc finger transcription factor genes is required for full activation of the myogenic program in these cells. Since forced early expression of Mef2 in adult myoblasts leads to premature muscle differentiation, our results explain how and why the adult muscle differentiation program is attenuated prior to pupal development. We propose a mechanism for the initiation of adult myogenesis, whereby twist expression in myoblasts provides a cellular context upon which an extrinsic signal builds to control muscle-specific differentiation events, and we discuss the general relevance of this model for gene regulation in animals.

Interaction between hormonal signaling pathways in Drosophila melanogaster as revealed by genetic interaction between methoprene-tolerant and broad-complex

Juvenile hormone (JH) regulates insect development by a poorly understood mechanism. Application of JH agonist insecticides to Drosophila melanogaster during the ecdysone-driven onset of metamorphosis results in lethality and specific morphogenetic defects, some of which resemble those in mutants of the ecdysone-regulated Broad-Complex (BR-C). The Methoprene-tolerant (Met) bHLH-PAS gene mediates JH action, and Met mutations protect against the lethality and defects. To explore relationships among these two genes and JH, double mutants were constructed between Met alleles and alleles of each of the BR-C complementation groups: broad (br), reduced bristles on palpus (rbp), and 2Bc. Defects in viability and oogenesis were consistently more severe in rbp Met or br Met double mutants than would be expected if these genes act independently. Additionally, complementation between BR-C mutant alleles often failed when MET was absent. Patterns of BRC protein accumulation during metamorphosis revealed essentially no difference between wild-type and Met-null individuals. JH agonist treatment did not block accumulation of BRC proteins. We propose that MET and BRC interact to control transcription of one or more downstream effector genes, which can be disrupted either by mutations in Met or BR-C or by application of JH/JH agonist, which alters MET interaction with BRC.

Hormonal control of a metamorphosis-specific transcriptional factor Broad-Complex in silkworm

Insect metamorphosis is induced by the steroid 20-hydroxyecdysone (20E) in the absence of sesquiterpenoid juvenile hormone (JH). In Drosophila melanogaster, the Broad-Complex (BR-C) transcriptional factor plays critical roles during metamorphosis. We isolated cDNAs encoding BR-C in the silkworm Bombyx mori and examined their mRNA expression. cDNAs for three BR-C isoforms with zinc finger pairs (Z1, Z2 and Z4) and four isoforms lacking them were cloned. Their mRNAs were expressed in multiple tissues at the larval-pupal metamorphosis. In the anterior silk gland, BR-C mRNAs were expressed at the end of the last larval instar but not expressed during the penultimate instar. 20E administration induced BR-C mRNA expression and JH suppressed 20E-induced BR-C expression in this tissue both in vivo and in vitro. Thus, BR-C mRNAs are inducible by 20E only in the absence of JH, a finding that explains their metamorphosis-specific expression.

Translating available food into the number of eggs laid by Drosophila melanogaster

In Drosophila and other insects egg production is related to the nutrients available. Somehow the nutritional status of the environment is translated into hormonal signs that can be "read" by each individual egg chamber, influencing the decision to either develop into an egg or die. We have shown that BR-C is a control gene during oogenesis and that the differential expression of BR-C isoforms plays a key role in controlling whether the fate of the egg chamber is to develop or undergo apoptosis.

The steroid hormone-regulated geneBroad Complex is required for dendritic growth of motoneurons during metamorphosis ofDrosophila

Dendrites are subject to subtle modifications as well as extensive remodeling during the assembly and maturation of neural circuits in a wide variety of organisms. During metamorphosis, Drosophila flight motoneurons MN1-MN4 undergo dendritic regression, followed by regrowth, whereas MN5 differentiates de novo (Consoulas et al. [2002] J. Neurosci. 22:4906-4917). Many cellular changes during metamorphosis are triggered and orchestrated by the steroid hormone 20-hydroxyecdysone, which initiates a cascade of coordinated gene expression. Broad Complex (BRC), a primary response gene in the ecdysone cascade, encodes a family of transcription factors (BRC-Z1-Z4) that are essential for metamorphic reorganization of the central nervous system (CNS). Using neuron-filling techniques that reveal cellular morphology with very high resolution, we tested the hypothesis that BRC is required for metamorphic development of MN1-MN5. Through a combination of loss-of-function mutant analyses, genetic mapping, and transgenic rescue experiments, we found that 2Bc function, mediated by BRC-Z3, is required selectively for motoneuron dendritic regrowth (MN1-MN4) and de novo outgrowth (MN5), as well as for soma expansion of MN5. In contrast, larval development and dendritic regression of MN1-MN4 are BRC-independent. Surprisingly, BRC proteins are not expressed in the motoneurons, suggesting that BRC-Z3 exerts its effect in a non-cell-autonomous manner. The 2Bc mutants display no gross defects in overall thoracic CNS structure, or in peripheral structures such as target muscles or sensory neurons. Candidates for mediating the effect of BRC-Z3 on dendritic growth of MN1-MN5 include their synaptic inputs and non-neuronal CNS cells that interact with them through direct contact or diffusible factors.

Overexpression of broad: a new insight into its role in the Drosophila prothoracic gland cells

Insect molting is triggered by ecdysteroids, which are produced in the prothoracic glands (PG). The broad (br) gene is one of the`early genes' directly regulated by ecdysteroids. Ectopic expression of the BR-Z3 isoform in early second instar Drosophila larvae (L2) before the rise of the ecdysteroid titer prevented molting to the third instar, but the larvae subsequently formed L2 prepupae after prolonged feeding. When these larvae were fed on diet containing 20-hydroxyecdysone (20E), they formed pharate third instar larvae. The critical weight for normal L3 pupariation of w1118 larvae was found to be 0.8 mg and that for L2 pupariation was 0.45 mg. We also defined a threshold weight for metamorphosis of 0.3 mg, above which L2 larvae will metamorphose when provided with 20E. BR-Z3 apparently works through the PG cells of the ring gland but not the putative neurosecretory cells that drive ecdysone secretion, because ectopic expression of BR-Z3 specifically in the ring gland caused 53% of the larvae to become permanent first instar larvae. Driving other BR isoforms in the ring gland prevented larval molting or pupariation to varying degrees. These molting defects were rescued by feeding 20E. Overexpression of each of the BR isoforms caused degeneration of the PG cells but on different time courses,indicating that BR is a signal for the degeneration of the PG cells that normally occurs during the pupal–adult transition.

Insights into the molecular basis of the hormonal control of molting and metamorphosis from Manduca sexta and Drosophila melanogaster

This short review summarizes our current knowledge about the role of transcription factors regulated by ecdysteroids and juvenile hormone (JH) in larval molting and metamorphosis in the tobacco hornworm, Manduca sexta, and Drosophila melanogaster. We show new evidence that EcR-A/USP-2 and E75A contribute to the down-regulation of MHR3 after the peak of ecdysteroid. Also, there is suggestive evidence that both MHR4 and betaFTZ-F1 may regulate the expression of dopa decarboxylase as the ecdysteroid titer declines. We summarize the regulation by JH of the Broad transcription factor that normally appears in the epidermis in the final larval instar and specifies pupal cuticle formation at the metamorphic molt. Premature expression of different Broad isoforms also is shown to cause precocious degeneration of the prothoracic glands as well as to prevent ecdysteroid release during its presence.

Isoform specific control of gene activity in vivo by the Drosophila ecdysone receptor

The steroid hormone 20-hydroxyecdysone induces metamorphosis in insects. The receptor for the hormone is the ecdysone receptor, a heterodimer of two nuclear receptors, EcR and USP. In Drosophila the EcR gene encodes 3 isoforms (EcR-A, EcR-B1 and EcR-B2) that vary in their N-terminal region but not in their DNA binding and ligand binding domains. The stage and tissue specific distribution of the isoforms during metamorphosis suggests distinct functions for the different isoforms. By over-expressing the three isoforms in animals we present results supporting this hypothesis. We tested for the ability of the different isoforms to rescue the lack of dendritic pruning that is characteristic of mutants lacking both EcR-B1 and EcR-B2. By expressing the different isoforms specifically in the affected neurons, we found that both EcR-B isoforms were able to rescue the neuronal defect cell autonomously, but that EcR-A was less effective. We also analyzed the effect of over-expressing the isoforms in a wild-type background. We determined a sensitive period when high levels of either EcR-B isoform were lethal, indicating that the low levels of EcR-B at this time are crucial to ensure normal development. Over-expressing EcR-A in contrast had no detrimental effect. However, high levels of EcR-A expressed in the posterior compartment suppressed puparial tanning, and resulted in down-regulation of some of the tested target genes in the posterior compartment of the wing disc. EcR-B1 or EcR-B2 over-expression had little or no effect.

Control of Dopa decarboxylase gene expression by the Broad-Complex during metamorphosis in Drosophila

The induction of the Dopa decarboxylase gene (Ddc) in the epidermis of Drosophila at pupariation is a receptor-mediated response to the steroid molting hormone, ecdysone. Activity is also dependent on the Broad-Complex (BR-C), an early ecdysone response gene that functions during metamorphosis. BR-C encodes a family of zinc-finger protein isoforms, BR-C(Z1-Z4). Genetic experiments have shown that the Z2 isoform is required for epidermal Ddc to reach maximum expression at pupariation. In this paper, we report that BR-C regulates Ddc expression at two different developmental stages through two different cis-acting regions. At pupariation, BR-C acts synergistically with the ecdysone receptor to up-regulate Ddc. DNase I foot printing has identified four binding sites of the predominant Z2 isoform within a distal regulatory element that is required for maximal Ddc activity. The sites share a conserved core sequence with a set of BR-C sites that had been mapped previously to within the first Ddc intron. Using variously deleted Ddc genomic regions to drive reporter gene expression in transgenic organisms, we show that the intronic binding sites are required for Ddc expression at eclosion. At both pupariation and eclosion, BR-C releases Ddc from an active silencing mechanism, operating through two distinct cis-acting regions of the Ddc genomic domain at these stages. Transgenes, bearing a Ddc fragment from which one of the cis-acting silencers has been deleted, exhibit beta-galactosidase reporter activity in the epidermal cells prior to the appearance of endogenous DDC. Our finding that BR-C is required for Ddc activation at eclosion is the first evidence to suggest that this important regulator of the early metamorphic events, also regulates target gene expression at the end of metamorphosis.

Temporal regulation of Drosophila salivary gland degeneration by the Broad-Complex transcription factors

The destruction of obsolete larval tissues at the onset of insect metamorphosis is a complex process triggered by the steroid hormone ecdysone. Among the genes required for the implementation of salivary gland (SG) degeneration the reduced bristles on palpus (rbp) gene of the Broad-Complex (BR-C) locus plays a critical role. This gene encodes the BR-C Z1 transcription factor and its expression is directly regulated by ecdysone through the ecdysone receptor (EcR/Usp). The BR-C locus encodes four major protein isoforms, including BR-C Z1, Z2, Z3, and Z4. With the exceptions of mutations in BR-C Z1 all mutations affecting the other BR-C isoforms produce pupal lethality. To gain insight into the function of the different BR-C isoforms on the process of SG degeneration, we used transgenes expressing each of the four major BR-C isoform proteins. This study revealed that, depending upon the period of expression relative to the major peak of ecdysone production, BR-C Z1, Z2, and Z4 first inhibited and then stimulated the process of SG degeneration. In contrast, BR-C Z3 exerted all time points an inhibition on SG degeneration.

Broad specifies pupal development and mediates the ‘status quo’ action of juvenile hormone on the pupal-adult transformation inDrosophilaandManduca

The understanding of the molecular basis of the endocrine control of insect metamorphosis has been hampered by the profound differences in responses of the Lepidoptera and the Diptera to juvenile hormone (JH). In both Manduca and Drosophila, the broad (br) gene is expressed in the epidermis during the formation of the pupa, but not during adult differentiation. Misexpression of BR-Z1 during either a larval or an adult molt of Drosophila suppressed stage-specific cuticle genes and activated pupal cuticle genes, showing that br is a major specifier of the pupal stage. Treatment with a JH mimic at the onset of the adult molt causes br re-expression and the formation of a second pupal cuticle in Manduca, but only in the abdomen of Drosophila. Expression of the BR isoforms during adult development of Drosophila suppressed bristle and hair formation when induced early or redirected cuticle production toward the pupal program when induced late. Expression of BR-Z1 at both of these times mimicked the effect of JH application but, unlike JH, it caused production of a new pupal cuticle on the head and thorax as well as on the abdomen. Consequently, the ‘status quo’ action of JH on the pupal-adult transformation is mediated by the JH-induced re-expression of BR.

Isolation and characterization of novel mutations of the Broad-Complex, a key regulatory gene of ecdysone induction in Drosophila melanogaster

Seven new alleles of the Broad-Complex gene of Drosophila melanogaster, which encodes a family of four zinc finger protein isoforms BR-C Z1, Z2, Z3 and Z4, were generated by transposase-induced mobilization of a P[Zw] element inserted in either the first intron downstream from the P165 promoter or the exon encoding the Z2-specific zinc finger domain. They were characterized by genetic complementation tests, molecular mapping and cytogenetic analysis of their effect on ecdysone-induced puffing and BR-C proteins binding to polytene chromosomes. Four mutations that correspond to three overlapping deletions and one tandem insertion of the P[Zw] element are located in the intron. They provide evidence that regulatory elements essential for a correct expression of the BR-C Z2 and BR-C Z3 transcripts are located within the intron downstream from the P165 promoter. Three mutations correspond to internal deletions of the locus and exhibit a complete loss of all BR-C(+) genetic functions in the complementation and cytogenetic tests. They thus provide well characterized new amorphic reference alleles of the BR-C gene. The precise cytogenetic location of more than 300 binding sites of BR-C proteins on larval salivary gland polytene chromosomes was determined by immunostaining using specific antibodies. Sites were found in big ecdysone inducible puffs, constitutively active small puffs as well as interbands. A complete list of the major sites on all four salivary gland polytene chromosomes of BR-C(+) larvae is presented.

Endocrine insights into the evolution of metamorphosis in insects

This review explores the roles of ecdysone and juvenile hormone (JH) in the evolution of complete metamorphosis and how metamorphosis, in turn, has impacted endocrine signaling. JH is a key player in the evolution of metamorphosis because it can act on embryos from more basal insect groups to suppress morphogenesis and cause premature differentiation, functions needed for transforming the transitional pronymphal stage of hemimetabolous insects into a functional larval stage. In the ancestral condition, imaginal-related growth is then delayed until JH finally disappears during the last larval instar. In the more derived groups of the Holometabola, selective tissues have escaped this JH suppression to form early-growing imaginal discs. We discuss how complete metamorphosis may have influenced the molecular aspects of both ecdysone and JH signaling.

The induction of DNA puff BhC4-1 gene is a late response to the increase in 20-hydroxyecdysone titers in last instar dipteran larvae

The characterization of DNA puff BhC4-1 expression was extended and its response to 20-hydroxyecdysone investigated in Bradysia hygida and in transgenic Drosophila carrying the BhC4-1 gene. In both organisms the activation of BhC4-1 in salivary glands occurs at the end of the larval stage coinciding with the peak in ecdysone titers which induces metamorphosis. Injections of 20-hydroxyecdysone into mid-fourth instar larvae of B. hygida show that the induction of BhC4-1 expression, as well as amplification and puff C4 expansion, are late events induced by the hormone. This late response of BhC4-1 expression was also observed in transgenic salivary glands cultivated in the presence of 20-hydroxyecdysone. In vitro studies using transgenic Drosophila indicate that both repressor and activator factors regulate the timing of BhC4-1 expression in salivary glands.

Selective binding of Drosophila BR-C isoforms to a distal regulatory element in the hsp23 promoter

The Broad-Complex (BR-C) gene plays a key role in the ecdysone regulatory hierarchy. Together with other early ecdysone-inducible genes BR-C transmits the hormonal signal to a set of secondary response genes in a tissue-specific manner. Among its targets is the hsp23 gene. Previously we showed that expression of the hsp23 gene in late third instar is BR-C-dependent, and accompanied by the appearance of a BR-C-dependent DNase I hypersensitive site at position -1400 (DHS-1400). BR-C encodes a family of transcription factors, and we show here that at least three BR-C protein isoforms--Z1, Z2, and Z3--bind to the sequences around DHS-1400 in vitro. A DNase I footprinting assay reveals five protected regions, designated site 1 to site 5, each of which specifically associates with one or several BR-C protein isoforms. We also show that a 100 bp region overlapping site 5, which binds all three isoforms in vitro, is required for hsp23 activity in vivo. The deletion of binding site 5 in a reporter gene construct reproduced the effect of the npr class mutations, that is, hsp23 is no longer expressed in any tissue tested except brain. Thus, BR-C regulates hsp23 expression via direct interaction of the predominant isoform with the distal regulatory element.

Downregulation of the tissue-specific transcription factor Fork head by Broad-Complex mediates a stage-specific hormone response

Drosophila development is coordinated by pulses of the steroid hormone 20-hydroxyecdysone (20E). During metamorphosis, the 20E-inducible Broad-Complex (BR-C) gene plays a key role in the genetic hierarchies that transduce the hormone signal, being required for the destruction of larval tissues and numerous aspects of adult development. Most of the known BR-C target genes, including the salivary gland secretion protein (Sgs) genes, are terminal differentiation genes that are thought to be directly regulated by BR-C-encoded transcription factors. Here, we show that repression of Sgs expression is indirectly controlled by the BR-C through transcriptional down-regulation of fork head, a tissue-specific gene that plays a central role in salivary gland development and is required for Sgs expression. Our results demonstrate that integration of a tissue-specific regulatory gene into a 20E-controlled genetic hierarchy provides a mechanism for hormonal repression. Furthermore, they suggest that the BR-C is placed at a different position within the 20E-controlled hierarchies than previously assumed, and that at least part of its pleiotropic functions are mediated by tissue-specific regulators.

A UAS site substitution approach to the in vivo dissection of promoters: interplay between the GATAb activator and the AEF-1 repressor at aDrosophilaecdysone response unit

An ecdysone response unit (EcRU) directs the expression of the Fat body protein 1 (Fbp1) gene in the third instar larval Drosophila fat body. The tissue-specific activity of this regulatory element necessitates the binding of both the ligand-activated EcR/USP ecdysone receptor and GATAb. To analyze the role played by GATAb in the regulation of the Fbp1 EcRU activity, we have replaced the GATA-binding sites GBS1, GBS2 and GBS3 in the Fbp1 EcRU with UAS sites for the yeast GAL4 activator and tested the activity of the mutagenized Fbp1 EcRUs in transgenic lines, either in the presence or absence of ubiquitously expressed GAL4. Our results reveal that GATAb plays two distinguishable roles at the Fbp1 EcRU that contribute to the tissue-specific activity of this regulatory element. On the one hand, GATAb mediates a fat body-specific transcriptional activation. On the other hand, it antagonizes specifically in the fat body a ubiquitous repressor that maintains the Fbp1 EcRU in an inactive state, refractory to activation by GAL4. We identified this repressor as AEF-1, a factor previously shown to be involved in the regulation of the Drosophila Adh and yp1-yp2 genes. These results show that, for a functional dissection of complex promoter-dependent regulatory pathways, the replacement of specific regulatory target sites by UAS GAL4 binding sites is a powerful alternative to the widely used disruption approach.