Mutations in a steroid hormone-regulated gene disrupt the metamorphosis of internal tissues in Drosophila: salivary glands, muscle, and gut

Single-cell type analysis of wing premotor circuits in the ventral nerve cord of Drosophila melanogaster

To perform most behaviors, animals must send commands from higher-order processing centers in the brain to premotor circuits that reside in ganglia distinct from the brain, such as the mammalian spinal cord or insect ventral nerve cord. How these circuits are functionally organized to generate the great diversity of animal behavior remains unclear. An important first step in unraveling the organization of premotor circuits is to identify their constituent cell types and create tools to monitor and manipulate these with high specificity to assess their function. This is possible in the tractable ventral nerve cord of the fly. To generate such a toolkit, we used a combinatorial genetic technique (split-GAL4) to create 195 sparse driver lines targeting 198 individual cell types in the ventral nerve cord. These included wing and haltere motoneurons, modulatory neurons, and interneurons. Using a combination of behavioral, developmental, and anatomical analyses, we systematically characterized the cell types targeted in our collection. Taken together, the resources and results presented here form a powerful toolkit for future investigations of neural circuits and connectivity of premotor circuits while linking them to behavioral outputs.

U1 small nuclear ribonucleoprotein is essential for early larval development in silkworm, Bombyx mori

U1 small nuclear ribonucleoproteins (U1 snRNP) associates with 5' splice sites in the form of ribonucleoprotein particles and is highly conserved among species. The physiological functions of U1 snRNP in a lepidopteran model insect Bombyx mori is mostly unknown. Here, we showed that U1 snRNP plays an important role in the development of silkworm. Knockout of U1 snRNP in silkworm showed either delayed or stationary 1st instar larva development compared with the wild-type group. U1 snRNP deletion mutants exhibited abnormal cellular phenotypes with enlarged cell nucleus, scant cytoplasm and enlarged nuclei. RNA-seq analysis revealed that genes involved in metabolic pathway, biosynthesis of secondary metabolites and steroid hormone biosynthesis were significantly affected by U1 snRNP depletion. Taken together, our study suggests that U1 snRNP homeostasis plays an important role in silkworm development.

Drosophila E93 promotes adult development and suppresses larval responses to ecdysone during metamorphosis

Pulses of the steroid hormone ecdysone act through transcriptional cascades to direct the major developmental transitions during the Drosophila life cycle. These include the prepupal ecdysone pulse, which occurs 10 ​hours after pupariation and triggers the onset of adult morphogenesis and larval tissue destruction. E93 encodes a transcription factor that is specifically induced by the prepupal pulse of ecdysone, supporting a model proposed by earlier work that it specifies the onset of adult development. Although a number of studies have addressed these functions for E93, little is known about its roles in the salivary gland where the E93 locus was originally identified. Here we show that E93 is required for development through late pupal stages, with mutants displaying defects in adult differentiation and no detectable effect on the destruction of larval salivary glands. RNA-seq analysis demonstrates that E93 regulates genes involved in development and morphogenesis in the salivary glands, but has little effect on cell death gene expression. We also show that E93 is required to direct the proper timing of ecdysone-regulated gene expression in salivary glands, and that it suppresses earlier transcriptional programs that occur during larval and prepupal stages. These studies support the model that the stage-specific induction of E93 in late prepupae provides a critical signal that defines the end of larval development and the onset of adult differentiation.

Ecdysone Receptor Agonism Leading to Lethal Molting Disruption in Arthropods: Review and Adverse Outcome Pathway Development

Molting is critical for growth, development, reproduction, and survival in arthropods. Complex neuroendocrine pathways are involved in the regulation of molting and may potentially become targets of environmental endocrine disrupting chemicals (EDCs). Based on several known ED mechanisms, a wide range of pesticides has been developed to combat unwanted organisms in food production activities such as agriculture and aquaculture. Meanwhile, these chemicals may also pose hazards to nontarget species by causing molting defects, and thus potentially affecting the health of the ecosystems. The present review summarizes the available knowledge on molting-related endocrine regulation and chemically mediated disruption in arthropods (with special focus on insects and crustaceans), to identify research gaps and develop a mechanistic model for assessing environmental hazards of these compounds. Based on the review, multiple targets of EDCs in the molting processes were identified and the link between mode of action (MoA) and adverse effects characterized to inform future studies. An adverse outcome pathway (AOP) describing ecdysone receptor agonism leading to incomplete ecdysis associated mortality was developed according to the OECD guideline and subjected to weight of evidence considerations by evolved Bradford Hill Criteria. This review proposes the first invertebrate ED AOP and may serve as a knowledge foundation for future environmental studies and AOP development.

Broad relays hormone signals to regulate stem cell differentiation in Drosophila midgut during metamorphosis

Like the mammalian intestine, the Drosophila adult midgut is constantly replenished by multipotent intestinal stem cells (ISCs). Although it is well known that adult ISCs arise from adult midgut progenitors (AMPs), relatively little is known about the mechanisms that regulate AMP specification. Here, we demonstrate that Broad (Br)-mediated hormone signaling regulates AMP specification. Br is highly expressed in AMPs temporally during the larva-pupa transition stage, and br loss of function blocks AMP differentiation. Furthermore, Br is required for AMPs to develop into functional ISCs. Conversely, br overexpression drives AMPs toward premature differentiation. In addition, we found that Br and Notch (N) signaling function in parallel pathways to regulate AMP differentiation. Our results reveal a molecular mechanism whereby Br-mediated hormone signaling directly regulates stem cells to generate adult cells during metamorphosis.

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.

Steroid hormone control of cell death and cell survival: molecular insights using RNAi

The insect steroid hormone ecdysone triggers programmed cell death of obsolete larval tissues during metamorphosis and provides a model system for understanding steroid hormone control of cell death and cell survival. Previous genome-wide expression studies of Drosophila larval salivary glands resulted in the identification of many genes associated with ecdysone-induced cell death and cell survival, but functional verification was lacking. In this study, we test functionally 460 of these genes using RNA interference in ecdysone-treated Drosophila l(2)mbn cells. Cell viability, cell morphology, cell proliferation, and apoptosis assays confirmed the effects of known genes and additionally resulted in the identification of six new pro-death related genes, including sorting nexin-like gene SH3PX1 and Sox box protein Sox14, and 18 new pro-survival genes. Identified genes were further characterized to determine their ecdysone dependency and potential function in cell death regulation. We found that the pro-survival function of five genes (Ras85D, Cp1, CG13784, CG32016, and CG33087), was dependent on ecdysone signaling. The TUNEL assay revealed an additional two genes (Kap-alpha3 and Smr) with an ecdysone-dependent cell survival function that was associated with reduced cell death. In vitro, Sox14 RNAi reduced the percentage of TUNEL-positive l(2)mbn cells (p<0.05) following ecdysone treatment, and Sox14 overexpression was sufficient to induce apoptosis. In vivo analyses of Sox14-RNAi animals revealed multiple phenotypes characteristic of aberrant or reduced ecdysone signaling, including defects in larval midgut and salivary gland destruction. These studies identify Sox14 as a positive regulator of ecdysone-mediated cell death and provide new insights into the molecular mechanisms underlying the ecdysone signaling network governing cell death and cell survival.

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.

Transcription factor broad suppresses precocious development of adult structures during larval-pupal metamorphosis in the red flour beetle, Tribolium castaneum

Broad (br), a transcription factor containing the Broad-Tramtrack-Bric-a-brac (BTB) and zinc finger domains was shown to mediate 20-hydroxyecdysone (20E) action and pupal development in Drosophila melanogaster and Manduca sexta. We determined the key roles of br during larval-pupal metamorphosis using RNA interference (RNAi) in a coleopteran insect, Tribolium castaneum. Two major peaks of T. castaneum broad (Tcbr) mRNA, one peak at the end of feeding stage prior to the larvae entering the quiescent stage and another peak during the quiescent stage were detected in the whole body and midgut tissue dissected from staged insects. Expression of br during the final instar larval stage is essential for successful larval-pupal metamorphosis, because, RNAi-mediated knock-down of Tcbr during this stage derailed larval-pupal metamorphosis and produced insects that showed larval, pupal and adult structures. Tcbr dsRNA injected into the final instar larvae caused reduction in the mRNA levels of genes known to be involved in 20E action (EcRA, E74 and E75B). Tcbr dsRNA injected into the final instar larvae also caused an increase in the mRNA levels of JH-response genes (JHE and Kr-h1b). Knock-down of Tcbr expression also affected 20E-mediated remodeling of midgut during larval-pupal metamorphosis. These data suggest that the expression of Tcbr during the final instar larval stage promotes pupal program while suppressing the larval and adult programs ensuring a transitory pupal stage in holometabolous insects.

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.

Fork head controls the timing and tissue selectivity of steroid-induced developmental cell death

Cell death during Drosophila melanogaster metamorphosis is controlled by the steroid hormone 20-hydroxyecdysone (20E). Elements of the signaling pathway that triggers death are known, but it is not known why some tissues, and not others, die in response to a particular hormone pulse. We found that loss of the tissue-specific transcription factor Fork head (Fkh) is both required and sufficient to specify a death response to 20E in the larval salivary glands. Loss of fkh itself is a steroid-controlled event that is mediated by the 20E-induced BR-C gene, and that renders the key death regulators hid and reaper hormone responsive. These results implicate the D. melanogaster FOXA orthologue Fkh with a novel function as a competence factor for steroid-controlled cell death. They explain how a specific tissue is singled out for death, and why this tissue survives earlier hormone pulses. More generally, they suggest that cell identity factors like Fkh play a pivotal role in the normal control of developmental cell death.

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.

The ecdysone-induced DHR4 orphan nuclear receptor coordinates growth and maturation in Drosophila

A critical determinant of insect body size is the time at which the larva stops feeding and initiates wandering in preparation for metamorphosis. No genes have been identified that regulate growth by contributing to this key developmental decision to terminate feeding. We show here that mutations in the DHR4 orphan nuclear receptor result in larvae that precociously leave the food to form premature prepupae, resulting in abbreviated larval development that translates directly into smaller and lighter animals. In addition, we show that DHR4 plays a central role in the genetic cascades triggered by the steroid hormone ecdysone at the onset of metamorphosis, acting as both a repressor of the early ecdysone-induced regulatory genes and an inducer of the betaFTZ-F1 midprepupal competence factor. We propose that DHR4 coordinates growth and maturation in Drosophila by mediating endocrine responses to the attainment of critical weight during larval development.

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.

A balance between the diap1 death inhibitor and reaper and hid death inducers controls steroid-triggered cell death in Drosophila

The steroid hormone ecdysone directs the massive destruction of obsolete larval tissues during Drosophila metamorphosis, providing a model system for defining the molecular mechanisms of steroid-regulated programmed cell death. Although earlier studies have identified an ecdysone triggered genetic cascade that immediately precedes larval tissue cell death, no death regulatory genes have been functionally linked to this death response. We show here that ecdysone-induced expression of the death activator genes reaper (rpr) and head involution defective (hid) is required for destruction of the larval midgut and salivary glands during metamorphosis, with hid playing a primary role in the salivary glands and rpr and hid acting in a redundant manner in the midguts. We also identify the Drosophila inhibitor of apoptosis 1 as a survival factor in the larval cell death pathway, delaying death until its inhibitory effect is overcome by rpr and hid. This study reveals functional interactions between rpr and hid in Drosophila cell death responses and provides evidence that the precise timing of larval tissue cell death during metamorphosis is achieved through a steroid-triggered shift in the balance between the Drosophila inhibitor of apoptosis 1 and the rpr and hid death activators.

The molecular site of action of juvenile hormone and juvenile hormone insecticides during metamorphosis: how these compounds kill insects

Studies in a variety of insects during the past four decades has deepened our understanding of juvenile hormone (JH) physiology, but how this hormone works at the molecular level remains elusive. Similarly, the mechanism of toxicity of JH analogue insecticides is still in question. There is much evidence from laboratory usage that JHAs act as JH agonists and generally show the highest toxicity when applied at the onset of metamorphosis. A physiological basis for the toxicity and morphogenetic effects has been suggested by recent work linking these effects with interference with the expression or action of certain genes, particularly the Broad-Complex (BR-C) transcription factor gene, that direct metamorphic change. Misexpressed BR-C then leads to improper expression of one or more downstream effector genes controlled by BR-C gene products, resulting in abnormal developmental and physiological changes that disrupt metamorphosis. Therefore, JH is a necessary molecule at certain times in insect development but becomes toxic when present during metamorphosis.

Use of Sindbis virus-mediated RNA interference to demonstrate a conserved role of Broad-Complex in insect metamorphosis

The transcription factor Broad-Complex (BR-C) is required for differentiation of adult structures as well as for the programmed death of obsolete larval organs during metamorphosis of the fruit fly Drosophila melanogaster. Whether BR-C has a similar role in other holometabolous insects could not be proven without a loss-of-function genetic test, performed in a non-drosophilid species. Here we use a recombinant Sindbis virus as a tool to silence BR-C expression in the silkmoth Bombyx mori. The virus expressing a BR-C antisense RNA fragment reduced endogenous BR-C mRNA levels in infected tissues (adult wing and leg primordia) via RNA interference (RNAi). The RNAi knock-down of BR-C resulted in the failure of animals to complete the larval-pupal transition or in later morphogenetic defects, including differentiation of adult compound eyes, legs, and wings from their larval progenitors. BR-C RNAi also perturbed the programmed cell death of larval silk glands. These developmental defects correspond to loss-of-function phenotypes of BR-C Drosophila mutants in both the morphogenetic and degenerative aspects, suggesting that the critical role of BR-C in metamorphosis is evolutionarily conserved. We also demonstrate that the Sindbis virus is a useful vehicle for silencing of developmental genes in new insect models.

Caspases function in autophagic programmed cell death in Drosophila

Self-digestion of cytoplasmic components is the hallmark of autophagic programmed cell death. This auto-degradation appears to be distinct from what occurs in apoptotic cells that are engulfed and digested by phagocytes. Although much is known about apoptosis, far less is known about the mechanisms that regulate autophagic cell death. Here we show that autophagic cell death is regulated by steroid activation of caspases in Drosophila salivary glands. Salivary glands exhibit some morphological changes that are similar to apoptotic cells, including fragmentation of the cytoplasm, but do not appear to use phagocytes in their degradation. Changes in the levels and localization of filamentous Actin, alpha-Tubulin, alpha-Spectrin and nuclear Lamins precede salivary gland destruction, and coincide with increased levels of active Caspase 3 and a cleaved form of nuclear Lamin. Mutations in the steroid-regulated genes beta FTZ-F1, E93, BR-C and E74A that prevent salivary gland cell death possess altered levels and localization of filamentous Actin, alpha-Tubulin, alpha-Spectrin, nuclear Lamins and active Caspase 3. Inhibition of caspases, by expression of either the caspase inhibitor p35 or a dominant-negative form of the initiator caspase Dronc, is sufficient to inhibit salivary gland cell death, and prevent changes in nuclear Lamins and alpha-Tubulin, but not to prevent the reorganization of filamentous Actin. These studies suggest that aspects of the cytoskeleton may be required for changes in dying salivary glands. Furthermore, caspases are not only used during apoptosis, but also function in the regulation of autophagic cell death.

Distinct promoter regions regulate spatial and temporal expression of the Drosophila caspase dronc

DRONC is an apical Drosophila caspase essential for programmed cell death during fly development. During metamorphosis, dronc gene expression is regulated by the steroid hormone ecdysone, which also regulates the levels of a number of other critical cell death proteins. As DRONC protein levels are important in determining caspase activation and initiation of cell death, we have analyzed the regulation of the dronc promoter using transgenic flies expressing a LacZ reporter gene under the control of the dronc promoter. Our results indicate that dronc expression is highly dynamic during Drosophila development, and is controlled both spatially and temporally. We demonstrate that while a 2.3 kb dronc promoter region contains most of the information required for correct gene expression, a 1.1 kb promoter region is expressed in some tissues and not others. We further demonstrate that during larval-pupal metamorphosis, two ecdysone-induced transcription factors, Broad-Complex and E93, are required for correct dronc expression. Our data suggest that the dronc promoter is regulated in a highly complex manner, and provides an ideal system to explore the temporal and spatial regulation of gene expression driven by nuclear hormone receptors.

Evolution of the Drosophila broad locus: the Manduca sexta broad Z4 isoform has biological activity in Drosophila

The Drosophila melanogaster broad locus is essential for normal metamorphic development. Broad encodes three genetically distinct functions (rbp, br, and 2Bc) and a family of four zinc-finger DNA-binding proteins (Z1-Z4). The Z1, Z2, and Z3 protein isoforms are primarily associated with the rbp, br, and 2Bc genetic functions respectively. The Z4 protein isoform also provides some rbp genetic function, however an essential function for the Z4 isoform in metamorphosis has not been identified. To determine the degree of conservation of Z4 function between the tobacco hornworm Manduca sexta and Drosophila we generated transgenic Drosophila expressing the Manduca broad Z4 isoform and used this transgene to rescue rbp mutant lethality during Drosophila metamorphosis. We find that the Manduca Z4 protein has significant biological activity in Drosophila with respect to rescue of rbp-associated lethality. There was also some overlap in effects on cuticle gene expression between the Manduca Z4 and Drosophila Z1 isoforms that was not shared with the Drosophila Z4 isoform. Our findings show that Z4 function has been conserved over the 260-million-year period since the divergence of Diptera and Lepidoptera, and are consistent with the hypothesis that the Drosophila Z4 and Manduca Z4 isoforms have essential roles in metamorphosis.

Autophagic programmed cell death in Drosophila

Autophagic programmed cell death occurs during the development of diverse animal groups, but the mechanisms that control this genetically regulated form of cell killing are poorly understood. Genetic studies of bulk protein degradation in yeast have provided important advances in our understanding of autophagy, and recent investigations of Drosophila autophagic cell death suggest that some of these mechanisms may be conserved. In Drosophila, several steroid-regulated genes that encode transcription regulators are required for autophagic cell death. These transcription regulators appear to activate a large number of genes that play a more direct role in cell killing, including genes that function in apoptosis such as caspases. While caspase function is required for autophagic cell death during Drosophila development, genes encoding proteins that are similar to the yeast autophagy regulators are also induced in dying salivary glands. Furthermore, numerous noncaspase proteases, cytoplasmic organizing factors, signaling molecules, and unknown factors are expressed in interesting patterns during autophagic cell death. This article reviews the current knowledge of the regulation of autophagic programmed cell death during development of Drosophila.

Genetic mechanism for the stage- and tissue-specific regulation of steroid triggered programmed cell death in Drosophila

Steroid hormones trigger a wide variety of cell-specific responses during animal development, but the mechanisms by which these systemic signals specify either cell division, differentiation, morphogenesis or death remain uncertain. Here, we analyze the function of the steroid-regulated genes betaFTZ-F1, BR-C, E74A, and E93 during salivary gland programmed cell death. While mutations in the betaFTZ-F1, BR-C, E74A, and E93 genes prevent destruction of salivary glands, only betaFTZ-F1 is required for DNA fragmentation. Analyses of BR-C, E74A, and E93 loss-of-function mutants indicate that these genes regulate stage-specific transcription of the rpr, hid, ark, dronc, and crq cell death genes. Ectopic expression of betaFTZ-F1 is sufficient to trigger premature cell death of larval salivary glands and ectopic transcription of the rpr, dronc, and crq cell death genes that normally precedes salivary gland cell death. The E93 gene is necessary for ectopic salivary gland cell destruction, and ectopic rpr, dronc, and crq transcription, that is induced by expression of betaFTZ-F1. Together, these observations indicate that betaFTZ-F1 regulates the timing of hormone-induced cell responses, while E93 functions to specify programmed cell death.

Steroid regulation of midgut cell death during Drosophila development

Steroid hormones trigger dynamic tissue changes during animal development by activating cell proliferation, cell differentiation, and cell death. Here we characterize steroid regulation of changes in midgut structure during the onset of Drosophila metamorphosis. Following an increase in the steroid 20-hydroxyecdysone (ecdysone) at the end of larval development, future adult midgut epithelium is formed, and the larval midgut is rapidly destroyed. Mutations in the steroid-regulated genes BR-C and E93 differentially impact larval midgut cell death but do not affect the formation of adult midgut epithelia. In contrast, mutations in the ecdysone-regulated E74A and E74B genes do not appear to perturb midgut development during metamorphosis. Larval midgut cells possess vacuoles that contain cellular organelles, indicating that these cells die by autophagy. While mutations in the BR-C, E74, and E93 genes do not impact DNA degradation during this cell death, mutations in BR-C inhibit destruction of larval midgut structures, including the proventriculus and gastric caeca, and E93 mutants exhibit decreased formation of autophagic vacuoles. Dying midguts express the rpr, hid, ark, dronc, and crq cell death genes, suggesting that the core cell death machinery is involved in larval midgut cell death. The transcription of rpr, hid, and crq are altered in BR-C mutants, and E93 mutants possess altered transcription of the caspase dronc, providing a mechanism for the disruption of midgut cell death in these mutant animals. These studies indicate that ecdysone triggers a two-step hierarchy composed of steroid-induced regulatory genes and apoptosis genes that, in turn, regulate the autophagic death of midgut cells during development.

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.

Loss of the ecdysteroid-inducible E75A orphan nuclear receptor uncouples molting from metamorphosis in Drosophila

Isoform-specific null mutations were used to define the functions of three orphan members of the nuclear receptor superfamily, E75A, E75B, and E75C, encoded by the E75 early ecdysteroid-inducible gene. E75B mutants are viable and fertile, while E75C mutants die as adults. In contrast, E75A mutants have a reduced ecdysteroid titer during larval development, resulting in developmental delays, developmental arrests, and molting defects. Remarkably, some E75A mutant second instar larvae display a heterochronic phenotype in which they induce genes specific to the third instar and pupariate without undergoing a molt. We propose that ecdysteroid-induced E75A expression defines a feed-forward pathway that amplifies or maintains the ecdysteroid titer during larval development, ensuring proper temporal progression through the life cycle.

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.

Molecular mechanisms of developmental timing in C. elegans and Drosophila

Characterization of the heterochronic genes has provided a strong foundation for understanding the molecular mechanisms of developmental timing in C. elegans. In apparent contrast, studies of developmental timing in Drosophila have demonstrated a central role for gene cascades triggered by the steroid hormone ecdysone. In this review, I survey the molecular mechanisms of developmental timing in C. elegans and Drosophila and outline how common regulatory pathways are beginning to emerge.

Steroid-triggered death by autophagy

Programmed cell death is a critical part of normal development, removing obsolete tissues or cells and sculpting body parts to assume their appropriate form and function. Most programmed cell death occurs by apoptosis of individual cells or autophagy of groups of cells. Although these pathways have distinct morphological characteristics, they also have a number of features in common, suggesting some overlap in their regulation. A recent paper by Lee and Baehrecke provides further support for this proposal.(1) These authors present, for the first time, a genetic analysis of autophagy, using the steroid-triggered metamorphosis of Drosophila as a model system. They demonstrate a remarkable degree of overlap between the control of apoptosis and autophagy as well as a key role for the steroid-inducible gene E93 in directing the autophagic death response. This paper also shows that E93 can direct cell death independently from the known death-inducer genes, defining a novel death pathway in Drosophila.

Bonus, a Drosophila homolog of TIF1 proteins, interacts with nuclear receptors and can inhibit betaFTZ-F1-dependent transcription

The Drosophila bonus (bon) gene encodes a homolog of the vertebrate TIF1 transcriptional cofactors. bon is required for male viability, molting, and numerous events in metamorphosis including leg elongation, bristle development, and pigmentation. Most of these processes are associated with genes that have been implicated in the ecdysone pathway, a nuclear hormone receptor pathway required throughout Drosophila development. Bon is associated with sites on the polytene chromosomes and can interact with numerous Drosophila nuclear receptor proteins. Bon binds via an LxxLL motif to the AF-2 activation domain present in the ligand binding domain of betaFTZ-F1 and behaves as a transcriptional inhibitor in vivo.

Insect metamorphosis: out with the old, in with the new

During insect metamorphosis, the steroid hormone ecdysone activates programmed cell death of larval tissues and the further development of adult tissues. Recent studies suggest that the E93 gene is both necessary and sufficient to target tissues for ecdysone-induced apoptosis.

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

Metamorphosis in Drosophila melanogaster is orchestrated by the steroid hormone ecdysone, which triggers a cascade of primary-response transcriptional regulators and secondary effector genes during the third larval instar and prepupal periods of development. The early ecdysone-response Broad-Complex (BR-C) gene, a key regulator of this cascade, is defined by three complementing functions (rbp, br, and 2Bc) and encodes several distinct zinc-finger-containing isoforms (Z1 to Z4). Using isoform-specific polyclonal antibodies we observe in the fat body a switch in BR-C isoform expression from the Z2 to the other three isoforms during the third instar. We show that the 2Bc(+) function that corresponds presumably to the Z3 isoform is required for the larval fat body-specific expression of a transgenic construct (AE) in which the lacZ gene is under the control of the ecdysone-regulated enhancer and minimal promoter of the fat body protein 1 (Fbp1) gene. Using hs(BR-C) transgenes, we demonstrate that overexpression of Z1, Z3, or Z4, but not Z2, is able to rescue AE activity with faithful tissue specificity in a BR-C null (npr1) genetic context, demonstrating a partial functional redundancy between Z1, Z3, and Z4 isoforms. We also show that continuous overexpression of Z2 during the third instar represses AE, while conversely, expression of Z3 earlier than its normal onset induces precocious expression of the construct. This finding establishes a tight correlation between the dynamic pattern of expression of the BR-C isoforms and their individual repressive or inductive roles in AE regulation. Altogether our results demonstrate that the balance between BR-C protein isoforms in the fat body mediates, in part, the precise timing of the ecdysone activation of the AE construct but does not modulate its tissue specificity.

E93 directs steroid-triggered programmed cell death in Drosophila

Steroid hormones coordinate multiple cellular changes, yet the mechanisms by which these systemic signals are refined into stage- and tissue-specific responses remain poorly understood. Here we show that the Drosophila E93 gene determines the nature of a steroid-induced biological response. E93 mutants possess larval salivary glands that fail to undergo steroid-triggered programmed cell death, and E93 is expressed in cells immediately before the onset of death. E93 protein is bound to the sites of steroid-regulated and cell death genes on polytene chromosomes, and the expression of these genes is defective in E93 mutants. Furthermore, expression of E93 is sufficient to induce programmed cell death. We propose that the steroid induction of E93 determines a programmed cell death response during development.

A juvenile hormone agonist reveals distinct developmental pathways mediated by ecdysone-inducible broad complex transcription factors

Juvenile hormone (JH) is an important regulator of insect development that, by unknown mechanisms, modifies molecular, cellular, and organismal responses to the molting hormone, 20-hydroxyecdysone (20E). In dipteran insects such as Drosophila, JH or JH agonists, administered at times near the onset of metamorphosis, cause lethality. We tested the hypothesis that the JH agonist methoprene acts by interfering with function of the Broad Complex (BRC), a 20E-regulated locus encoding BTB/POZ-zinc finger transcription factors essential for metamorphosis of many tissues. We found that methoprene, administered by feeding or by topical application, disrupts the metamorphic reorganization of the central nervous system, salivary glands, and musculature in a dose-dependent manner. As we predicted, methoprene phenocopies a subset of previously described BRC defects; it also phenocopies Deformed and produces abnormalities not associated with known mutations. Interestingly, methoprene specifically disrupts those metamorphic events dependent on the combined action of all BRC isoforms, while sparing those that require specific isoform subsets. Thus, our data provide independent pharmacological evidence for the model, originally based on genetic studies, that BRC proteins function in two developmental pathways. Mutations of Methoprene-tolerant (Met), a gene involved in the action of JH, protect against all features of the "methoprene syndrome." These findings have allowed us to propose novel alternative models linking BRC, juvenile hormone, and MET.

A steroid-triggered transcriptional hierarchy controls salivary gland cell death during Drosophila metamorphosis

The steroid hormone ecdysone signals the stage-specific programmed cell death of the larval salivary glands during Drosophila metamorphosis. This response is preceded by an ecdysone-triggered switch in gene expression in which the diap2 death inhibitor is repressed and the reaper (rpr) and head involution defective (hid) death activators are induced. Here we show that rpr is induced directly by the ecdysone-receptor complex through an essential response element in the rpr promoter. The Broad-Complex (BR-C) is required for both rpr and hid transcription, while E74A is required for maximal levels of hid induction. diap2 induction is dependent on betaFTZ-F1, while E75A and E75B are each sufficient to repress diap2. This study identifies transcriptional regulators of programmed cell death in Drosophila and provides a direct link between a steroid signal and a programmed cell death response.

The timing of drosophila salivary gland apoptosis displays an l(2)gl-dose response

During Drosophila metamorphosis, larval tissues, such as the salivary glands, are histolysed whereas imaginal tissues differentiate into adult structures forming at eclosion a fly-shaped adult. Inactivation of the lethal(2)giant larvae (l(2)gl) gene encoding the cytoskeletal associated p127 protein, causes malignant transformation of brain neuroblasts and imaginal disc cells with developmental arrest at the larval-pupal transition phase. At this stage, p127 is expressed in wild-type salivary glands which become fully histolysed 12 - 13 h after pupariation. By contrast to wild-type, administration of 20-hydroxyecdsone to l(2)gl-deficient salivary glands is unable to induce histolysis, although it releases stored glue granules and gives rise to a nearly normal pupariation chromosome puffing, indicating that p127 is required for salivary gland apoptosis. To unravel the l(2)gl function in this tissue we used transgenic lines expressing reduced ( approximately 0.1) or increased levels of p127 (3.0). Here we show that the timing of salivary gland histolysis displays an l(2)gl-dose response. Reduced p127 expression delays histolysis whereas overexpression accelerates this process without affecting the duration of third larval instar, prepupal and pupal development. Similar l(2)gl-dependence is noticed in the timing of expression of the cell death genes reaper, head involution defective and grim, supporting the idea that p127 plays a critical role in the implementation of ecdysone-triggered apoptosis. These experiments show also that the timing of salivary gland apoptosis can be manipulated without affecting normal development and provide ways to investigate the nature of the components specifically involved in the apoptotic pathway of the salivary glands.

DHR3 is required for the prepupal-pupal transition and differentiation of adult structures during Drosophila metamorphosis

Pulses of the steroid hormone ecdysone activate genetic regulatory hierarchies that coordinate the developmental changes associated with Drosophila metamorphosis. A high-titer ecdysone pulse at the end of larval development triggers puparium formation and induces expression of the DHR3 orphan nuclear receptor. Here we use both a heat-inducible DHR3 rescue construct and clonal analysis to define DHR3 functions during metamorphosis. Clonal analysis reveals requirements for DHR3 in the development of adult bristles, wings, and cuticle, and no apparent function in eye or leg development. DHR3 mutants rescued to the third larval instar also reveal essential functions during the onset of metamorphosis, leading to lethality during prepupal and early pupal stages. The phenotypes associated with these lethal phases are consistent with the effects of DHR3 mutations on ecdysone-regulated gene expression. Although DHR3 has been shown to be sufficient for early gene repression at puparium formation, it is not necessary for this response, indicating that other negative regulators may contribute to this pathway. In contrast, DHR3 is required for maximal expression of the midprepupal regulatory genes, EcR, E74B, and betaFTZ-1. Reductions in EcR and betaFTZ-F1 expression, in turn, lead to submaximal early gene induction in response to the prepupal ecdysone pulse and corresponding defects in adult head eversion and salivary gland cell death. These studies demonstrate that DHR3 is an essential regulator of the betaFTZ-F1 midprepupal competence factor, providing a functional link between the late larval and prepupal responses to ecdysone. Induction of DHR3 in early prepupae ensures that responses to the prepupal ecdysone pulse will be distinct from responses to the late larval pulse and thus that the animal progresses in an appropriate manner through the early stages of metamorphosis.

Muscle pattern diversification in Drosophila: the story of imaginal myogenesis

There are two phases of somatic muscle formation in Drosophila. During embryonic development, one phase of myogenesis generates larval muscle elements that mediate the relatively simple behavioural repertoire of the larva. During pupal metamorphosis, a diverse pattern of muscle fibres are assembled, and these facilitate the more elaborate behavioural patterns of the adult fly. In this review, we discuss the current status of understanding of the cellular, genetic, and molecular mechanisms of pattern formation during the second phase, imaginal muscle development. We briefly compare aspects of embryonic and adult myogenesis in Drosophila and muscle development in vertebrates and highlight conserved themes and disparities between these diverse myogenic programmes.

The Drosophila beta FTZ-F1 orphan nuclear receptor provides competence for stage-specific responses to the steroid hormone ecdysone

The acquisition of competence is a key mechanism for refining global signals to distinct spatial and temporal responses. The molecular basis of competence, however, remains poorly understood. Here, we show that the beta FTZ-F1 orphan nuclear receptor functions as a competence factor for stage-specific responses to the steroid hormone ecdysone during Drosophila metamorphosis. beta FTZ-F1 mutants pupariate normally in response to the late larval pulse of ecdysone but display defects in stage-specific responses to the subsequent ecdysone pulse in prepupae. The ecdysone-triggered genetic hierarchy that directs these developmental responses is severely attenuated in beta FTZ-F1 mutants, although ecdysone receptor expression is unaffected. This study define beta FTZ-F1 as an essential competence factor for stage-specific responses to a steroid signal and implicates interplay among nuclear receptors as a mechanism for achieving hormonal competence.

Juvenile hormone prevents ecdysteroid-induced expression of broad complex RNAs in the epidermis of the tobacco hornworm, Manduca sexta

A cDNA homolog of the Drosophila melanogaster Broad Complex (BRC) gene was isolated from the tobacco hornworm, Manduca sexta, which shows a predicted 88% amino acid identity with Drosophila BRC in the N-terminal BTB domain. Three zinc finger domains encoding homologs of the Drosophila Z2, Z3, and Z4 domains (93, 100, and 85% identity, respectively) were obtained by RT-PCR. In Manduca dorsal abdominal epidermis, BRC RNAs were not observed during the larval molt. Three BRC transcripts-6.0, 7.0, and 9.0 kb-first appeared at the end of the feeding stage of the fifth (final) instar when the epidermis is exposed to ecdysteroids in the absence of juvenile hormone (JH) and becomes committed to pupal differentiation. These RNAs were induced in day 2 fifth larval epidermis in vitro by 20-hydroxyecdysone (20E) in the absence of JH with dose-response and time courses similar to the induction of pupal commitment. This induction by 20E in vitro was prevented by the presence of JH I at levels seen in vivo during the larval molt. In the wing discs, the BRC RNAs appeared shortly after ecdysis to the fifth instar and coincided with the onset of metamorphic competence of these discs. Application of a JH analogue pyriproxifen during the fourth instar molt delayed and reduced the levels of BRC mRNAs seen in the wing discs in the early fifth instar, but did not completely prevent their appearance in this tissue that first differentiates at metamorphosis. The expression of the BRC transcription factors thus appears to be one of the first molecular indications of the genetic reprogramming of the epidermis necessary for insect metamorphosis. How JH prevents BRC expression in this epidermis may provide the key to understanding how this hormone controls metamorphosis.

Parallel molecular genetic pathways operate during CNS metamorphosis in Drosophila

Insect metamorphosis provides a valuable model for studying mechanisms of steroid hormone action on the nervous system during a dynamic phase of functional remodeling. The Drosophila Broad Complex (BRC) holds a pivotal position in the gene expression cascade triggered by the molting hormone 20-hydroxyecdysone (20E) at the onset of metamorphosis. We previously demonstrated that the BRC, which encodes a family of zinc-finger transcription factors, is essential for transducing 20E signals into the morphogenetic movements and cellular assembly that alter the CNS from juvenile to adult form and function. We set out to examine the relationship of BRC to two other genes, IMP-E1 and Deformed (Dfd), involved in the metamorphic transition of the CNS. Representatives of the whole family of BRC transcript isoforms accumulate in the CNS during the larval-to-pupal transition and respond directly to 20E in vitro. IMP-E1 is also directly regulated by 20E, but its induction is independent of BRC, revealing that 20E works through at least two pathways in the CNS. DFD expression is also independent of BRC function. Surprisingly, BRC and DFD proteins are expressed in distinct, nonoverlapping subsets of neuronal nuclei of the subesophageal ganglion even though both are required for its migration into the head capsule. This suggests that the segment identity and ecdysone cascades operate in parallel to control region-specific reorganization during metamorphosis.

The DHR78 nuclear receptor is required for ecdysteroid signaling during the onset of Drosophila metamorphosis

Pulses of ecdysteroids direct Drosophila through its life cycle by activating stage- and tissue-specific genetic regulatory hierarchies. Here we show that an orphan nuclear receptor, DHR78, functions at the top of the ecdysteroid regulatory hierarchies. Null mutations in DHR78 lead to lethality during the third larval instar with defects in ecdysteroid-triggered developmental responses. Consistent with these phenotypes, DHR78 mutants fail to activate the mid-third instar regulatory hierarchy that prepares the animal for metamorphosis. DHR78 protein is bound to many ecdysteroid-regulated puff loci, suggesting that DHR78 directly regulates puff gene expression. In addition, ectopic expression of DHR78 has no effects on development, indicating that its activity is regulated post-translationally. We propose that DHR78 is a ligand-activated receptor that plays a central role in directing the onset of Drosophila metamorphosis.

Identification of a broad complex-regulated enhancer in the developing visual system of Drosophila

During metamorphosis, the central nervous system (CNS) is reconstructed through the concerted action of cell birth, death, and remodeling, so that it can serve the novel and complex behavioral needs of the adult insect. In Drosophila, Broad Complex (BRC) zinc-finger transcription factors are essential for many aspects of metamorphosis, including reorganization of the CNS. In particular, we showed previously that some mutant alleles disrupt the assembly of visual system synaptic neuropils. Using an enhancer-detector screen, we have now identified a candidate BRC target gene, H217, that is normally expressed in visual system neural precursor cells of the inner proliferative center. Moreover, the P-element insertion in the H217 line has caused a hypomorphic mutation in an essential gene, with an optic lobe disorganization phenotype very similar to that seen in BRC mutants. In BRC mutants of the br complementation group (but not in rbp or 2Bc mutants), the H217 enhancer is ectopically expressed in lamina precursor cells (LPCs) whose proliferation is regulated by signals from photoreceptor axons. As predicted by the current model of BRC structure-function relationships, we demonstrated that BRC-Z2 isoforms, when induced during the third larval instar, can repress H217 enhancer activity in the LPCs, whereas BRC-Z3 cannot. Taken together, the data suggest that the H217 P element has tagged an essential gene repressed by BRC-Z2 in LPCs and required for the normal architecture of the retinotopically connected visual system neuropils.

Relationships between protein isoforms and genetic functions demonstrate functional redundancy at the Broad-Complex during Drosophila metamorphosis

Metamorphosis in holometabolous insects is an ecdysone-dependent process by which the larval form is replaced by a reproductive, adult form. At the onset of metamorphosis ecdysone induces a set of early genes which coordinate tissue-specific responses to hormone. The Broad-Complex (BR-C) early gene, which acts as a global regulator of tissue-specific responses to ecdysone, encodes a family of zinc-finger DNA binding proteins known as Z1, Z2, Z3, and Z4. Genetically the BR-C encodes three complementing functions, br, rbp, and 2Bc, and a class of npr1 alleles that fail to complement any of the other genetic functions. The effects of BR-C mutations on metamorphic development are highly pleiotropic, yet little is known about the roles of individual BR-C proteins in directing the required responses to ecdysone. Because the BR-C is a vital regulator of metamorphosis it is essential to establish the relationships between BR-C genetic functions and protein products. We present here the first general and definitive study of these relationships. Using heat-inducible transgenes we have rescued lethality associated with each of the complementing genetic functions and have restored transcriptional activity of tissue-specific BR-C(+)-dependent target genes. Our data lead us to conclude that br+ function is only provided by the Z2 isoform. We find that Z1 transgenes provide full rbp+ function, while Z4 provides partial function. Likewise, while Z3 provides full 2Bc+ function, Z2 also provides partial function. These results indicate possible functional redundancy or regulatory dependence (via autoregulation) associated with the rbp+ and 2Bc+ functions. The establishment of these relationships between BR-C genetic functions and protein isoforms is an important step toward understanding the roles of BR-C proteins in directing metamorphic responses to ecdysone.

Flies on steroids--Drosophila metamorphosis and the mechanisms of steroid hormone action

Recent studies have provided new insights into the molecular mechanisms by which the steroid hormone ecdysone triggers the larval-to-adult metamorphosis of Drosophila. Ecdysone-induced transcription factors activate large sets of secondary-response genes and provide the competence for subsequent regulatory responses to the hormone. It seems likely that similar hormone-triggered regulatory hierarchies exist in other higher organisms and that Drosophila is providing our first glimpses of the complexities of these gene networks.

Ecdysone signaling cascade and regulation of Drosophila metamorphosis

Pulses of the steroid hormone 20-hydroxyecdysone (ecdysone) regulate diverse biological responses during the life history of insects. Studies of the fruit fly, Drosophila melanogaster, have provided significant insights into the mechanisms underlying ecdysone mediated regulation of development. During the dramatic metamorphosis of Drosophila, ecdysone induces the histolysis of nearly all of the larval tissues and differentiation and morphogenesis of the structures composing the adult fly. These changes are mediated by a genetic signaling cascade that was first recognized as puffs in the giant polytene chromosomes of the salivary gland. This genetic regulatory cascade is composed of early and late genes that are intricately coordinated by changes in hormone titer. Early genes encode regulatory proteins that are involved in the proper regulation of late genes, which are thought to play a more direct role in development. The regulation and function of these genes is discussed in the context of the cell- and tissue-specific changes required for the reorganization of a larva to form an adult fly.