E93 directs steroid-triggered programmed cell death in Drosophila

The Drosophila TIPE family member Sigmar interacts with the Ste20-like kinase Misshapen and modulates JNK signaling, cytoskeletal remodeling and autophagy

TNFAIP8 and other mammalian TIPE family proteins have attracted increased interest due to their associations with disease-related processes including oncogenic transformation, metastasis, and inflammation. The molecular and cellular functions of TIPE family proteins are still not well understood. Here we report the molecular and genetic characterization of the Drosophila TNFAIP8 homolog, CG4091/sigmar. Previous gene expression studies revealed dynamic expression of sigmar in larval salivary glands prior to histolysis. Here we demonstrate that in sigmar loss-of-function mutants, the salivary glands are morphologically abnormal with defects in the tubulin network and decreased autophagic flux. Sigmar localizes subcellularly to microtubule-containing projections in Drosophila S2 cells, and co-immunoprecipitates with the Ste20-like kinase Misshapen, a regulator of the JNK pathway. Further, the Drosophila TNF ligand Eiger can induce sigmar expression, and sigmar loss-of-function leads to altered localization of pDJNK in salivary glands. Together, these findings link Sigmar to the JNK pathway, cytoskeletal remodeling and autophagy activity during salivary gland development, and provide new insights into TIPE family member function.

Constraints, independence, and evolution of thermal plasticity: probing genetic architecture of long- and short-term thermal acclimation

Seasonal and daily thermal variation can limit species distributions because of physiological tolerances. Low temperatures are particularly challenging for ectotherms, which use both basal thermotolerance and acclimation, an adaptive plastic response, to mitigate thermal stress. Both basal thermotolerance and acclimation are thought to be important for local adaptation and persistence in the face of climate change. However, the evolutionary independence of basal and plastic tolerances remains unclear. Acclimation can occur over longer (seasonal) or shorter (hours to days) time scales, and the degree of mechanistic overlap is unresolved. Using a midlatitude population of Drosophila melanogaster, we show substantial heritable variation in both short- and long-term acclimation. Rapid cold hardening (short-term plasticity) and developmental acclimation (long-term plasticity) are positively correlated, suggesting shared mechanisms. However, there are independent components of these traits, because developmentally acclimated flies respond positively to short-term acclimation. A strong negative correlation between basal cold tolerance and developmental acclimation suggests that basal cold tolerance may constrain developmental acclimation, whereas a weaker negative correlation between basal cold tolerance and short-term acclimation suggests less constraint. Using genome-wide association mapping, we show the genetic architecture of rapid cold hardening and developmental acclimation responses are nonoverlapping at the SNP and corresponding gene level. However, genes associated with each trait share functional similarities, including genes involved in apoptosis and autophagy, cytoskeletal and membrane structural components, and ion binding and transport. These results indicate substantial opportunity for short-term and long-term acclimation responses to evolve separately from each other and for short-term acclimation to evolve separately from basal thermotolerance.

Splicing variants of NOL4 differentially regulate the transcription activity of Mlr1 and Mlr2 in cultured cells

Mlr1 (Mblk-1-related protein-1) and Mlr2 are mouse homologs of transcription factor Mblk-1 (Mushroom body large-type Kenyon cell-specific protein-1), which we originally identified from the honeybee brain. In the present study, aiming at identifying coregulator(s) of Mlr1 and Mlr2 from the mouse brain, we used yeast two-hybrid screening of mouse brain cDNA library to search for interaction partners of Mlr 1 and Mlr2, respectively. We identified nucleolar protein 4 (NOL4) splicing variants as major interaction partners for both Mlr1 and Mlr2. Among the three murine NOL4 splicing variants, we further characterized NOL4-S, which lacks an N-terminal part of NOL4-L, and NOL4-SΔ, which lacks nuclear localization signal (NLS)-containing domain of NOL4-S. A GST pull-down assay revealed that Mlr1 interacts with both NOL4-S and NOL4-SΔ, whereas Mlr2 interacts with NOL4-S, but not with NOL4-SΔ. These results indicate that the NLS-containing domain of NO4-S Is necessary for in vitro binding with Mlr2, but not for that with Mlr1. Furthermore, a luciferase assay using Schneider's Line 2 cells revealed that transactivation activity of Mlr1 was significantly suppressed by both NOL4-S and NOL4-SΔ, with almost complete suppression by NOL4-SΔ. In contrast, transactivation activity of Mlr2 was significantly suppressed by NOL4-S but rather activated by NOL4-SΔ. Our findings suggest that transactivation activities of Mlr1 and Mlr2 are differentially regulated by splicing variants of NOL4, which are expressed in a tissue-selective manner.

A systems-level interrogation identifies regulators of Drosophila blood cell number and survival

In multicellular organisms, cell number is typically determined by a balance of intracellular signals that positively and negatively regulate cell survival and proliferation. Dissecting these signaling networks facilitates the understanding of normal development and tumorigenesis. Here, we study signaling by the Drosophila PDGF/VEGF Receptor (Pvr) in embryonic blood cells (hemocytes) and in the related cell line Kc as a model for the requirement of PDGF/VEGF receptors in vertebrate cell survival and proliferation. The system allows the investigation of downstream and parallel signaling networks, based on the ability of Pvr to activate Ras/Erk, Akt/TOR, and yet-uncharacterized signaling pathway/s, which redundantly mediate cell survival and contribute to proliferation. Using Kc cells, we performed a genome wide RNAi screen for regulators of cell number in a sensitized, Pvr deficient background. We identified the receptor tyrosine kinase (RTK) Insulin-like receptor (InR) as a major Pvr Enhancer, and the nuclear hormone receptors Ecdysone receptor (EcR) and ultraspiracle (usp), corresponding to mammalian Retinoid X Receptor (RXR), as Pvr Suppressors. In vivo analysis in the Drosophila embryo revealed a previously unrecognized role for EcR to promote apoptotic death of embryonic blood cells, which is balanced with pro-survival signaling by Pvr and InR. Phosphoproteomic analysis demonstrates distinct modes of cell number regulation by EcR and RTK signaling. We define common phosphorylation targets of Pvr and InR that include regulators of cell survival, and unique targets responsible for specialized receptor functions. Interestingly, our analysis reveals that the selection of phosphorylation targets by signaling receptors shows qualitative changes depending on the signaling status of the cell, which may have wide-reaching implications for other cell regulatory systems.

20-hydroxyecdysone transcriptionally regulates humoral immunity in the fat body of Helicoverpa armigera

20-hydroxyecdysone (20E) increases its titre level during the wandering stage and influences innate immunity in many holometabolous insects. However, the function of 20E as an immune-activator or -suppressor needs to be determined. Here, the transcriptome of the peptidoglycan-challenged fat body of the cotton bollworm, Helicoverpa armigera, was analysed using Illumina sequencing technology. Overall, 32 073 unigenes were assembled with a mean length of 643 nucleotides. Gene expression dynamics in the fat body during the wandering stage and of peptidoglycan-challenged individuals were investigated by the digital gene expression system. Pattern recognition receptors [such as peptidoglycan recognition protein B (PGRP B), PGRP S2 precursor, C-type lectin 5, hemolin and β-1,3-glucan recognition protein 2a] and antimicrobial peptides (namely attacin, gloverin, gloverin precursor, gloverin-like, cecropin 2, cecropin D, cecropin D-like and i-type lysozyme) significantly increased their mRNA levels during the wandering stage. 20E treatment significantly induced the expression of these genes. Antibacterial activities were also enhanced during the wandering stage and after 20E injections. Bacillus subtilis peptidoglycan induced the expression of PGRP D, PGRP B, PGRP S2 precursor, gloverin, gloverin precursor, gloverin-like, cecropin 2, cecropin D and lebocin-like genes. These results demonstrate that 20E acts by enhancing humoral immunity in H. armigera.

The MEKRE93 (Methoprene tolerant-Krüppel homolog 1-E93) pathway in the regulation of insect metamorphosis, and the homology of the pupal stage

Recent studies on transcription factor E93 revealed that it triggers adult morphogenesis in Blattella germanica, Tribolium castaneum and Drosophila melanogaster. Moreover, we show here that Krüppel homolog 1 (Kr-h1), a transducer of the antimetamorphic action of juvenile hormone (JH), represses E93 expression. Kr-h1 is upstream of E93, and upstream of Kr-h1 is Methoprene-tolerant (Met), the latter being the JH receptor in hemimetabolan and holometabolan species. As such, the Met - Kr-h1 - E93 pathway (hereinafter named "MEKRE93 pathway") appears to be central to the status quo action of JH, which switch adult morphogenesis off and on in species ranging from cockroaches to flies. The decrease in Kr-h1 mRNA and the rise of E93 expression that triggers adult morphogenesis occur at the beginning of the last instar nymph or in the prepupae of hemimetabolan and holometabolan species, respectively. This suggests that the hemimetabolan last nymph (considering the entire stage, from the apolysis to the last instar until the next apolysis that gives rise to the adult) is ontogenetically homologous to the holometabolan pupa (also considered between two apolyses, thus comprising the prepupal stage).

UTX coordinates steroid hormone-mediated autophagy and cell death

Correct spatial and temporal induction of numerous cell type-specific genes during development requires regulated removal of the repressive histone H3 lysine 27 trimethylation (H3K27me3) modification. Here we show that the H3K27me3 demethylase dUTX is required for hormone-mediated transcriptional regulation of apoptosis and autophagy genes during ecdysone-regulated programmed cell death of Drosophila salivary glands. We demonstrate that dUTX binds to the nuclear hormone receptor complex Ecdysone Receptor/Ultraspiracle, and is recruited to the promoters of key apoptosis and autophagy genes. Salivary gland cell death is delayed in dUTX mutants, with reduced caspase activity and autophagy that coincides with decreased apoptosis and autophagy gene transcripts. We further show that salivary gland degradation requires dUTX catalytic activity. Our findings provide evidence for an unanticipated role for UTX demethylase activity in regulating hormone-dependent cell death and demonstrate how a single transcriptional regulator can modulate a specific complex functional outcome during animal development.

Dimorphic ovary differentiation in honeybee (Apis mellifera) larvae involves caste-specific expression of homologs of ark and buffy cell death genes

The establishment of the number of repeated structural units, the ovarioles, in the ovaries is one of the critical events that shape caste polyphenism in social insects. In early postembryonic development, honeybee (Apis mellifera) larvae have a pair of ovaries, each one consisting of almost two hundred ovariole primordia. While practically all these ovarioles continue developing in queen-destined larvae, they undergo massive programmed cell death (PCD) in worker-destined larvae. So as to gain insight into the molecular basis of this fundamental process in caste differentiation we used quantitative PCR (qPCR) and fluorescent in situ hybridization (FISH) to investigate the expression of the Amark and Ambuffy genes in the ovaries of the two honeybee castes throughout the fifth larval instar. These are the homologs of ark and buffy Drosophila melanogaster genes, respectively, involved in activating and inhibiting PCD. Caste-specific expression patterns were found during this time-window defining ovariole number. Amark transcript levels were increased when ovariole resorption was intensified in workers, but remained at low levels in queen ovaries. The transcripts were mainly localized at the apical end of all the worker ovarioles, but appeared in only a few queen ovarioles, thus strongly suggesting a function in mediating massive ovariolar cell death in worker larvae. Ambuffy was mainly expressed in the peritoneal sheath cells covering each ovariole. The levels of Ambuffy transcripts increased earlier in the developing ovaries of queens than in workers. Consistent with a protective role against cell death, Ambuffy transcripts were localized in practically all queen ovarioles, but only in few worker ovarioles. The results are indicative of a functional relationship between the expression of evolutionary conserved cell death genes and the morphological events leading to caste-specific ovary differentiation in a social insect.

Transcription factor E93 specifies adult metamorphosis in hemimetabolous and holometabolous insects

All immature animals undergo remarkable morphological and physiological changes to become mature adults. In winged insects, metamorphic changes either are limited to a few tissues (hemimetaboly) or involve a complete reorganization of most tissues and organs (holometaboly). Despite the differences, the genetic switch between immature and adult forms in both types of insects relies on the disappearance of the antimetamorphic juvenile hormone (JH) and the transcription factors Krüppel-homolog 1 (Kr-h1) and Broad-Complex (BR-C) during the last juvenile instar. Here, we show that the transcription factor E93 is the key determinant that promotes adult metamorphosis in both hemimetabolous and holometabolous insects, thus acting as the universal adult specifier. In the hemimetabolous insect Blattella germanica, BgE93 is highly expressed in metamorphic tissues, and RNA interference (RNAi)-mediated knockdown of BgE93 in the nymphal stage prevented the nymphal-adult transition, inducing endless reiteration of nymphal development, even in the absence of JH. We also find that BgE93 down-regulated BgKr-h1 and BgBR-C expression during the last nymphal instar of B. germanica, a key step necessary for proper adult differentiation. This essential role of E93 is conserved in holometabolous insects as TcE93 RNAi in Tribolium castaneum prevented pupal-adult transition and produced a supernumerary second pupa. In this beetle, TcE93 also represses expression of TcKr-h1 and TcBR-C during the pupal stage. Similar results were obtained in the more derived holometabolous insect Drosophila melanogaster, suggesting that winged insects use the same regulatory mechanism to promote adult metamorphosis. This study provides an important insight into the understanding of the molecular basis of adult metamorphosis.

E93 predominantly transduces 20-hydroxyecdysone signaling to induce autophagy and caspase activity in Drosophila fat body

During the larval-prepupal transition in Drosophila, a balancing crosstalk occurs between autophagy and caspase activity in the remodeling fat body: the inhibition of autophagy induces caspase activity and the inhibition of caspases induces autophagy. Both autophagy and caspase activity are induced by a pulse of molting hormone (20-hydroxyecdysone, 20E) via the 20E nuclear receptor complex, EcR-USP. We here demonstrate that E93, a 20E primary-response gene encoding an HTH transcription factor, predominantly transduces 20E signaling to induce autophagy and caspase activity in the remodeling fat body. RNAi knockdown or mutation of E93 blocks autophagy and caspase activity, E93 overexpression induces them both, while E93 overexpression has a better rescuing effect on the inhibition of autophagy than caspase activity caused by EcR(DN) overexpression. At the transcriptional level, E93 not only greatly impacts the 20E-triggered transcriptional cascade, but also upregulates essential autophagy and apoptosis genes. Meanwhile, at the phosphorylational level, E93 blocks the PI3K-TORC1 signaling to initiate autophagy. Taken together, we conclude that autophagy and caspase activity are induced by 20E and predominantly transduced by E93 in the remodeling fat body of Drosophila.

Genetic control of specificity to steroid-triggered responses in Drosophila

Steroid hormones trigger a wide variety of biological responses through stage- and tissue-specific activation of target gene expression. The mechanisms that provide specificity to systemically released pulses of steroids, however, remain poorly understood. We previously completed a forward genetic screen for mutations that disrupt the destruction of larval salivary glands during metamorphosis in Drosophila melanogaster, a process triggered by the steroid hormone 20-hydroxyecdysone (ecdysone). Here, we characterize 10 complementation groups mapped to genes from this screen. Most of these mutations disrupt the ecdysone-induced expression of death activators, thereby failing to initiate tissue destruction. However, other responses to ecdysone, even within salivary glands, occur normally in mutant animals. Many of these newly identified regulators of ecdysone signaling, including brwd3, med12, med24, pak, and psg2, represent novel components of the ecdysone-triggered transcriptional hierarchy. These genes function combinatorially to provide specificity to ecdysone pulses, amplifying the hormonal cue in a stage-, tissue-, and target gene-specific manner. Most of the ecdysone response genes identified in this screen encode homologs of mammalian nuclear receptor coregulators, demonstrating an unexpected degree of functional conservation in the mechanisms that regulate steroid signaling between insects and mammals.

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.

The role of autophagy in Drosophila metamorphosis

Macroautophagy (autophagy) is a conserved catabolic process that targets cytoplasmic components to lysosomes for degradation. Autophagy is required for cellular homeostasis and cell survival in response to starvation and stress, and paradoxically, it also plays a role in programmed cell death during development. The mechanisms that regulate the relationship between autophagy, cell survival, and cell death are poorly understood. Here we review research in Drosophila that has provided insights into the regulation of autophagy by steroid hormones and nutrient restriction and discuss how autophagy influences cell growth, nutrient utilization, cell survival, and cell death.

Ecdysone control of developmental transitions: lessons from Drosophila research

The steroid hormone ecdysone is the central regulator of insect developmental transitions. Recent new advances in our understanding of ecdysone action have relied heavily on the application of Drosophila melanogaster molecular genetic tools to study insect metamorphosis. In this review, we focus on three major aspects of Drosophila ecdysone biology: (a) factors that regulate the timing of ecdysone release, (b) molecular basis of stage- and tissue-specific responses to ecdysone, and (c) feedback regulation and coordination of ecdysone signaling.

MET is required for the maximal action of 20-hydroxyecdysone during Bombyx metamorphosis

Little is known about how the putative juvenile hormone (JH) receptor, the bHLH-PAS transcription factor MET, is involved in 20-hydroxyecdysone (20E; the molting hormone) action. Here we report that two MET proteins found in the silkworm, Bombyx mori, participate in 20E signal transduction. Met is 20E responsive and its expression peaks during molting and pupation, when the 20E titer is high. As found with results from RNAi knockdown of EcR-USP (the ecdysone receptor genes), RNAi knockdown of Met at the early wandering stage disrupts the 20E-triggered transcriptional cascade, preventing tissue remodeling (including autophagy, apoptosis and destruction of larval tissues and generation of adult structures) and causing lethality during the larval-pupal transition. MET physically interacts with EcR-USP. Moreover, MET, EcR-USP and the 20E-response element (EcRE) form a protein-DNA complex, implying that MET might modulate 20E-induced gene transcription by interacting with EcR-USP. In conclusion, the 20E induction of MET is required for the maximal action of 20E during Bombyx metamorphosis.

Translational control by the DEAD Box RNA helicase belle regulates ecdysone-triggered transcriptional cascades

Steroid hormones act, through their respective nuclear receptors, to regulate target gene expression. Despite their critical role in development, physiology, and disease, however, it is still unclear how these systemic cues are refined into tissue-specific responses. We identified a mutation in the evolutionarily conserved DEAD box RNA helicase belle/DDX3 that disrupts a subset of responses to the steroid hormone ecdysone during Drosophila melanogaster metamorphosis. We demonstrate that belle directly regulates translation of E74A, an ets transcription factor and critical component of the ecdysone-induced transcriptional cascade. Although E74A mRNA accumulates to abnormally high levels in belle mutant tissues, no E74A protein is detectable, resulting in misregulation of E74A-dependent ecdysone response genes. The accumulation of E74A mRNA in belle mutant salivary glands is a result of auto-regulation, fulfilling a prediction made by Ashburner nearly 40 years ago. In this model, Ashburner postulates that, in addition to regulating secondary response genes, protein products of primary response genes like E74A also inhibit their own ecdysone-induced transcription. Moreover, although ecdysone-triggered transcription of E74A appears to be ubiquitous during metamorphosis, belle-dependent translation of E74A mRNA is spatially restricted. These results demonstrate that translational control plays a critical, and previously unknown, role in refining transcriptional responses to the steroid hormone ecdysone.

Pulses of steroid hormones regulate a variety of biological processes, but how these simple global cues are converted into specific local responses remains unclear. While steroid responses have traditionally been thought to be regulated at the transcriptional level, here we demonstrate that translational control plays a novel role in refining steroid signals. The DEAD box RNA helicase belle directly regulates the translation of E74A mRNA, which encodes a transcription factor that is induced by the fly steroid hormone ecdysone and then rapidly repressed. This process is disrupted in belle mutant tissues, where E74A mRNA accumulates to abnormally high levels but is not translated. We demonstrate that Belle-dependent translation of E74A is required to both repress its own transcription and to induce tissue-specific target genes. These findings confirm the prediction that auto-regulation is important for the self-limiting behavior of steroid responses and demonstrate a critical role for translational control in refining a global hormonal signal into specific biological responses.

Identification of factors that function in Drosophila salivary gland cell death during development using proteomics

Proteasome inhibitors induce cell death and are used in cancer therapy, but little is known about the relationship between proteasome impairment and cell death under normal physiological conditions. Here, we investigate the relationship between proteasome function and larval salivary gland cell death during development in Drosophila. Drosophila larval salivary gland cells undergo synchronized programmed cell death requiring both caspases and autophagy (Atg) genes during development. Here, we show that ubiquitin proteasome system (UPS) function is reduced during normal salivary gland cell death, and that ectopic proteasome impairment in salivary gland cells leads to early DNA fragmentation and salivary gland condensation in vivo. Shotgun proteomic analyses of purified dying salivary glands identified the UPS as the top category of proteins enriched, suggesting a possible compensatory induction of these factors to maintain proteolysis during cell death. We compared the proteome following ectopic proteasome impairment to the proteome during developmental cell death in salivary gland cells. Proteins that were enriched in both populations of cells were screened for their function in salivary gland degradation using RNAi knockdown. We identified several factors, including trol, a novel gene CG11880, and the cop9 signalsome component cop9 signalsome 6, as required for Drosophila larval salivary gland degradation.

Relationship between growth arrest and autophagy in midgut programmed cell death in Drosophila

Autophagy has been implicated in both cell survival and programmed cell death (PCD), and this may explain the apparently complex role of this catabolic process in tumourigenesis. Our previous studies have shown that caspases have little influence on Drosophila larval midgut PCD, whereas inhibition of autophagy severely delays midgut removal. To assess upstream signals that regulate autophagy and larval midgut degradation, we have examined the requirement of growth signalling pathways. Inhibition of the class I phosphoinositide-3-kinase (PI3K) pathway prevents midgut growth, whereas ectopic PI3K and Ras signalling results in larger cells with decreased autophagy and delayed midgut degradation. Furthermore, premature induction of autophagy is sufficient to induce early midgut degradation. These data indicate that autophagy and the growth regulatory pathways have an important relationship during midgut PCD. Despite the roles of autophagy in both survival and death, our findings suggest that autophagy induction occurs in response to similar signals in both scenarios.

High-throughput sequencing to reveal genes involved in reproduction and development in Bactrocera dorsalis (Diptera: Tephritidae)

BACKGROUND

Tephritid fruit flies in the genus Bactrocera are of major economic significance in agriculture causing considerable loss to the fruit and vegetable industry. Currently, there is no ideal control program. Molecular means is an effective method for pest control at present, but genomic or transcriptomic data for members of this genus remains limited. To facilitate molecular research into reproduction and development mechanisms, and finally effective control on these pests, an extensive transcriptome for the oriental fruit fly Bactrocera dorsalis was produced using the Roche 454-FLX platform.

RESULTS

We obtained over 350 million bases of cDNA derived from the whole body of B. dorsalis at different developmental stages. In a single run, 747,206 sequencing reads with a mean read length of 382 bp were obtained. These reads were assembled into 28,782 contigs and 169,966 singletons. The mean contig size was 750 bp and many nearly full-length transcripts were assembled. Additionally, we identified a great number of genes that are involved in reproduction and development as well as genes that represent nearly all major conserved metazoan signal transduction pathways, such as insulin signal transduction. Furthermore, transcriptome changes during development were analyzed. A total of 2,977 differentially expressed genes (DEGs) were detected between larvae and pupae libraries, while there were 1,621 DEGs between adults and larvae, and 2,002 between adults and pupae. These DEGs were functionally annotated with KEGG pathway annotation and 9 genes were validated by qRT-PCR.

CONCLUSION

Our data represent the extensive sequence resources available for B. dorsalis and provide for the first time access to the genetic architecture of reproduction and development as well as major signal transduction pathways in the Tephritid fruit fly pests, allowing us to elucidate the molecular mechanisms underlying courtship, ovipositing, development and detailed analyses of the signal transduction pathways.

20-hydroxyecdysone upregulates apoptotic genes and induces apoptosis in the Bombyx fat body

During insect metamorphosis, obsolete larval tissues are removed by programed cell death (PCD), mainly apoptosis and autophagy, which is directed by the molting hormone, 20-hydroxyecdysone (20E) and the 20E-triggered transcriptional cascade. Here, we investigated how 20E regulates apoptosis at the transcriptional level in the fat body of the silkworm, Bombyx mori. As detected by TdT-mediated dUTP Nick-End Labeling (TUNEL), apoptosis weakly occurred during the fourth larval molting, decreased to undetected levels during the early fifth instar, and gradually increased from day 4 of fifth instar to the wandering stage to the prepupal stage. Meanwhile, as determined by quantitative real-time PCR, eight genes involved in apoptosis, including Apaf-1, Nedd2 like1, Nedd2 like2, ICE1, ICE3, ICE5, Arp, and IAP, were highly expressed during molting and pupation, when the 20E titer is high. Injection of 20E into day 2 of fifth instar larvae significantly induced apoptosis and upregulated apoptotic genes after 6 h of treatment, and in vitro treatment of larval fat body tissues with 20E upregulated all the eight apoptotic genes. Moreover, RNAi knockdown of USP, a component of the 20E receptor complex EcR-USP, at the early-wandering stage reduced apoptosis and downregulated apoptotic genes after 24 h of treatment. Taken together, we infer that 20E upregulates apoptotic genes and thus induces apoptosis in the Bombyx fat body during larval molting and the larval-pupal transition.

Control of target gene specificity during metamorphosis by the steroid response gene E93

Hormonal control of sexual maturation is a common feature in animal development. A particularly dramatic example is the metamorphosis of insects, in which pulses of the steroid hormone ecdysone drive the wholesale transformation of the larva into an adult. The mechanisms responsible for this transformation are not well understood. Work in Drosophila indicates that the larval and adult forms are patterned by the same underlying sets of developmental regulators, but it is not understood how the same regulators pattern two distinct forms. Recent studies indicate that this ability is facilitated by a global change in the responsiveness of target genes during metamorphosis. Here we show that this shift is controlled in part by the ecdysone-induced transcription factor E93. Although long considered a dedicated regulator of larval cell death, we find that E93 is expressed widely in adult cells at the pupal stage and is required for many patterning processes at this time. To understand the role of E93 in adult patterning, we focused on a simple E93-dependent process, the induction of the Dll gene within bract cells of the pupal leg by EGF receptor signaling. In this system, we show that E93 functions to cause Dll to become responsive to EGF receptor signaling. We demonstrate that E93 is both necessary and sufficient for directing this switch. E93 likely controls the responsiveness of many other target genes because it is required broadly for patterning during metamorphosis. The wide conservation of E93 orthologs suggests that similar mechanisms control life-cycle transitions in other organisms, including vertebrates.

The steroid hormone 20-hydroxyecdysone upregulated the protein phosphatase 6 for the programmed cell death in the insect midgut

Programmed cell death (PCD) plays an important role in insect midgut remodeling during metamorphosis. Insect midgut PCD is triggered by the steroid hormone 20-hydroxyecdysone (20E) and it is mediated by a series of genes. However, the mechanism by which 20E triggers midgut PCD is still unclear. Here, we report a protein phosphatase 6 (PP6) from Helicoverpa armigera playing roles in midgut PCD. PP6 was expressed in the midgut during larval growth and it is significantly increased during metamorphosis. The increase was proven to be regulated by 20E. The juvenile hormone analog methoprene has no effect on PP6 expression. RNA interference analysis suggests that 20E upregulated the PP6 transcript levels through the ecdysone receptor EcRB1. PP6 knockdown by larval feeding or PP6 dsRNA injection resulted in the repression of the midgut PCD during the metamorphic stage. The mechanism was demonstrated to be through the suppression of genes such as Broad (Br), E74a, E75b, HR3, E93, rpr, and caspase, which are involved in 20E signaling pathway or midgut PCD. These findings suggest that PP6 is involved in the 20E signal transduction pathway and participates in the PCD in midgut.

mir-11 limits the proapoptotic function of its host gene, dE2f1

The E2F family of transcription factors regulates the expression of both genes associated with cell proliferation and genes that regulate cell death. The net outcome is dependent on cellular context and tissue environment. The mir-11 gene is located in the last intron of the Drosophila E2F1 homolog gene dE2f1, and its expression parallels that of dE2f1. Here, we investigated the role of miR-11 and found that miR-11 specifically modulated the proapoptotic function of its host gene, dE2f1. A mir-11 mutant was highly sensitive to dE2F1-dependent, DNA damage-induced apoptosis. Consistently, coexpression of miR-11 in transgenic animals suppressed dE2F1-induced apoptosis in multiple tissues, while exerting no effect on dE2F1-driven cell proliferation. Importantly, miR-11 repressed the expression of the proapoptotic genes reaper (rpr) and head involution defective (hid), which are directly regulated by dE2F1 upon DNA damage. In addition to rpr and hid, we identified a novel set of cell death genes that was also directly regulated by dE2F1 and miR-11. Thus, our data support a model in which the coexpression of miR-11 limits the proapoptotic function of its host gene, dE2f1, upon DNA damage by directly modulating a dE2F1-dependent apoptotic transcriptional program.

Steroid-triggered, cell-autonomous death of a Drosophila motoneuron during metamorphosis

BACKGROUND

The metamorphosis of Drosophila melanogaster is accompanied by elimination of obsolete neurons via programmed cell death (PCD). Metamorphosis is regulated by ecdysteroids, including 20-hydroxyecdysone (20E), but the roles and modes of action of hormones in regulating neuronal PCD are incompletely understood.

RESULTS

We used targeted expression of GFP to track the fate of a larval motoneuron, RP2, in ventral ganglia. RP2s in abdominal neuromeres two through seven (A2 to A7) exhibited fragmented DNA by 15 hours after puparium formation (h-APF) and were missing by 20 h-APF. RP2 death began shortly after the 'prepupal pulse' of ecdysteroids, during which time RP2s expressed ecdysteroid receptors (EcRs). Genetic manipulations showed that RP2 death required the function of EcR-B isoforms, the death-activating gene, reaper (but not hid), and the apoptosome component, Dark. PCD was blocked by expression of the caspase inhibitor p35 but unaffected by manipulating Diap1. In contrast, aCC motoneurons in neuromeres A2 to A7, and RP2s in neuromere A1, expressed EcRs during the prepupal pulse but survived into the pupal stage under all conditions tested. To test the hypothesis that ecdysteroids trigger RP2's death directly, we placed abdominal GFP-expressing neurons in cell culture immediately prior to the prepupal pulse, with or without 20E. 20E induced significant PCD in putative RP2s, but not in control neurons, as assessed by morphological criteria and propidium iodide staining.

CONCLUSIONS

These findings suggest that the rise of ecdysteroids during the prepupal pulse acts directly, via EcR-B isoforms, to activate PCD in RP2 motoneurons in abdominal neuromeres A2 to A7, while sparing RP2s in A1. Genetic manipulations suggest that RP2's death requires Reaper function, apoptosome assembly and Diap1-independent caspase activation. RP2s offer a valuable 'single cell' approach to the molecular understanding of neuronal death during insect metamorphosis and, potentially, of neurodegeneration in other contexts.

Med24 and Mdh2 are required for Drosophila larval salivary gland cell death

The steroid hormone ecdysone triggers the rapid destruction of larval tissues through transcriptional cascades that culminate in rpr and hid expression and caspase activation. Here, we show that mutations in Mdh2 and Med24 block caspase cleavage and larval salivary gland cell death. Mdh2 encodes a predicted malate dehydrogenase that localizes to mitochondria. Consistent with this proposed function, Mdh2 mutants have significantly lower levels of ATP and accumulate late-stage citric acid cycle intermediates, suggesting that the cell death defects arise from a deficit in energy production. Med24 encodes a component of the Mediator transcriptional coactivator complex. Unexpectedly, however, expression of the key death regulator genes is normal in Med24 mutant salivary glands. This study identifies novel mechanisms for controlling the destruction of larval tissues during Drosophila metamorphosis and provides new directions for our understanding of steroid-triggered programmed cell death.

Precise temporal regulation of roughest is required for correct salivary gland autophagic cell death in Drosophila

The Drosophila roughest (rst) locus encodes an immunoglobulin superfamily transmembrane glycoprotein implicated in a variety of embryonic and postembryonic developmental processes. Here we demonstrate a previously unnoticed role for this gene in the autophagic elimination of larval salivary glands during early pupal stages by showing that overexpression of the Rst protein ectodomain in early pupa leads to persistence of salivary glands up to at least 12 hours after head eversion, although with variable penetrance. The same phenotype is observed in individuals carrying the dominant regulatory allele rst(D), but not in loss of function alleles. Analysis of persistent glands at the ultrastructural level showed that programmed cell death starts at the right time but is arrested at an early stage of the process. Finally we describe the expression pattern and intracellular distribution of Rst in wild type and rst(D) mutants, showing that its downregulation in salivary glands at the beginning of pupal stage is an important factor in the correct implementation of the autophagic program of this tissue in space and time.

Juvenile hormone counteracts the bHLH-PAS transcription factors MET and GCE to prevent caspase-dependent programmed cell death in Drosophila

Juvenile hormone (JH) regulates many developmental and physiological events in insects, but its molecular mechanism remains conjectural. Here we report that genetic ablation of the corpus allatum cells of the Drosophilaring gland (the JH source) resulted in JH deficiency, pupal lethality and precocious and enhanced programmed cell death (PCD) of the larval fat body. In the fat body of the JH-deficient animals, Dronc and Drice,two caspase genes that are crucial for PCD induced by the molting hormone 20-hydroxyecdysone (20E), were significantly upregulated. These results demonstrated that JH antagonizes 20E-induced PCD by restricting the mRNA levels of Dronc and Drice. The antagonizing effect of JH on 20E-induced PCD in the fat body was further confirmed in the JH-deficient animals by 20E treatment and RNA interference of the 20E receptor EcR. Moreover, MET and GCE, the bHLH-PAS transcription factors involved in JH action, were shown to induce PCD by upregulating Droncand Drice. In the Met- and gce-deficient animals, Dronc and Drice were downregulated, whereas in the Met-overexpression fat body, Dronc and Drice were significantly upregulated leading to precocious and enhanced PCD, and this upregulation could be suppressed by application of the JH agonist methoprene. For the first time, we demonstrate that JH counteracts MET and GCE to prevent caspase-dependent PCD in controlling fat body remodeling and larval-pupal metamorphosis in Drosophila.

The molecular mechanisms of cuticular melanization: the ecdysone cascade leading to dopa decarboxylase expression in Manduca sexta

Many insect developmental color changes are known to be regulated by both ecdysone and juvenile hormone. Yet the molecular mechanisms underlying this regulation have not been well understood. This review highlights the hormonal mechanisms involved in the regulation of two key enzymes [dopa decarboxylase (DDC) and phenoloxidase] necessary for insect cuticular melanization, and the molecular action of 20-hydroxyecdysone on various transcription factors leading to DDC expression at the end of a larval molt in Manduca sexta. In addition, the ecdysone cascade found in M. sexta is compared with that of other organisms.

A eukaryotic initiation factor 5C is upregulated during metamorphosis in the cotton bollworm, Helicoverpa armigera

Background

The orthologs of eukaryotic initiation factor 5C (eIF5C) are essential to the initiation of protein translation, and their regulation during development is not well known.

Results

A cDNA encoding a polypeptide of 419 amino acids containing an N-terminal leucine zipper motif and a C-terminal eIF5C domain was cloned from metamorphic larvae of Helicoverpa armigera. It was subsequently named Ha-eIF5C. Quantitative real-time PCR (QRT-PCR) revealed a high expression of the mRNA of Ha-eIF5C in the head-thorax, integument, midgut, and fat body during metamorphosis. Immunohistochemistry suggested that Ha-eIF5C was distributed into both the cytoplasm and the nucleus in the midgut, fat body and integument. Ha-eIF5C expression was upregulated by 20-hydroxyecdysone (20E). Furthermore, the transcription of Ha-eIF5C was down regulated after silencing of ecdysteroid receptor (EcR) or Ultraspiracle protein (USP) by RNAi.

Conclusion

These results suggested that during metamorphosis of the cotton bollworm, Ha-eIF5C was upregulated by 20E through the EcR and USP transcription factors.

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.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

A matter of timing: microRNA-controlled temporal identities in worms and flies

The first microRNAs were identified in Caenorhabditis elegans based on their functions in the temporal regulation of stage-specific cell fate decisions. Until now, it was not known whether the so-called heterochronic genes that encode miRNAs are also involved in controlling developmental transitions in other organisms. New findings by Sokol et al. (this issue of Genes & Development, pp. 1591-1596) demonstrate that the Drosophila counterpart of a heterochronic miRNA gene from C. elegans, let-7, does indeed play a role in promoting stage-specific developmental events in neuromuscular tissues during the transition from larval to adult stages, thus pointing to a more widespread utilization of miRNAs in temporal regulation of animal development.

Local initiation of caspase activation in Drosophila salivary gland programmed cell death in vivo

Programmed cell death, or apoptosis, is an essential event in animal development. Spatiotemporal analysis of caspase activation in vivo could provide new insights into programmed cell death occurring during development. Here, using the FRET-based caspase-3 indicator, SCAT3, we report the results of live-imaging analysis of caspase activation in developing Drosophila in vivo. In Drosophila, the salivary gland is sculpted by caspase-mediated programmed cell death initiated by the steroid hormone 20-hydroxyecdysone (ecdysone). Using a SCAT3 probe, we observed that caspase activation in the salivary glands begins in the anterior cells and is then propagated to the posterior cells in vivo. In vitro salivary gland culture experiments indicated that local exposure of ecdysone to the anterior salivary gland reproduces the caspase activation gradient as observed in vivo. In betaFTZ-F1 mutants, caspase activation was delayed and occurred in a random pattern in vivo. In contrast to the in vivo response, the salivary glands from betaFTZ-F1 mutants showed a normal in vitro response to ecdysone, suggesting that betaFTZ-F1 may be involved in ecdysteroid biosynthesis and secretion of ecdysone from the ring gland for local initiation of programmed cell death. These results imply a role of betaFTZ-F1 in coordinating the initiation of salivary gland apoptosis in development.

Identification of genes differentially expressed during larval molting and metamorphosis of Helicoverpa armigera

BACKGROUND

Larval molting and metamorphosis are important physiological processes in the life cycle of the holometabolous insect. We used suppression subtractive hybridization (SSH) to identify genes differentially expressed during larval molting and metamorphosis.

RESULTS

We performed SSH between tissues from a variety of developmental stages, including molting 5th and feeding 6th instar larvae, metamorphically committed and feeding 5th instar larvae, and feeding 5th instar and metamorphically committed larvae. One hundred expressed sequence tags (ESTs) were identified and included 73 putative genes with similarity to known genes, and 27 unknown ESTs. SSH results were further characterized by dot blot, Northern blot, and RT-PCR. The expression levels of eleven genes were found to change during larval molting or metamorphosis, suggesting a functional role during these processes.

CONCLUSION

These results provide a new set of genes expressed specifically during larval molt or metamorphosis that are candidates for further studies into the regulatory mechanisms of those stage-specific genes during larval molt and metamorphosis.

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.

To die or not to die--a role for Fork head

The precise determination of when and where cells undergo programmed cell death is critical for normal development and tissue homeostasis. Cao et al. (2007; see p. 843 of this issue) report that the Fork head (Fkh) transcription factor, which is essential for the early development and function of the larval salivary glands in Drosophila melanogaster, also contributes to its demise. These authors show that fkh expression in the salivary glands is normally lost at puparium formation, which is ∼12 h before they undergo massive cell death triggered by the steroid hormone ecdysone, making room for their developing adult counterparts. The loss of Fkh eliminates its role in blocking cell death, allowing for subsequent ecdysone-induced reaper and head involution defective death activator expression and tissue destruction. This study provides new insights into the transcriptional regulation of programmed cell death and the mechanisms that underlie the precise spatial and temporal control of hormone responses during development.

Proteomic analysis of steroid-triggered autophagic programmed cell death during Drosophila development

Two morphological forms of programmed cell death, apoptosis and autophagic cell death, remove unneeded or damaged cells during animal development. Although the mechanisms that regulate apoptosis are well studied, little is known about autophagic cell death. A shotgun proteome analysis of purified dying larval salivary glands in Drosophila was used to identify proteins that are expressed during autophagic programmed cell death. A total of 5661 proteins were identified from stages before and after the onset of cell death. Analyses of these data enabled us to identify proteins from a number of interesting categories including regulators of transcription, the apoptosis, autophagy, lysosomal, and ubiquitin proteasome degradation pathways, and proteins involved in growth control. Several of the identified proteins, including the serine/threonine kinase warts (Wts), were not detected using whole-genome DNA microarrays, providing support for the importance of such high-throughput proteomic technology. Wts regulates cell-cycle arrest and apoptosis, and significantly, mutations in wts prevent destruction of salivary glands.

Nongenomic action of an insect steroid hormone in steroid-induced programmed cell death

Programmed cell death (PCD) of the silkworm silk glands is triggered by the insect steroid hormone, 20-hydroxyecdysone (20E), and proceeds sequentially through cell shrinkage, nuclear condensation, DNA fragmentation, nuclear fragmentation and apoptotic body formation. A protein synthesis inhibitor, cycloheximide (CHX, 2 mM) induced a cell death that exhibited only nuclear and DNA fragmentation. A concentration of 0.2 mM CHX was ineffective at inducing the cell death when added alone, but in the presence of 20E, a cell death similar to that induced by 2 mM CHX was resulted with accompanying nuclear condensation. Since 2 and 0.2 mM CHX inhibited protein synthesis equally, the DNA and nuclear fragmentation appear to be mediated by a nongenomic action of 20E. In addition, we show a possible involvement of Ca2+-PKC-caspase-3 like protease pathway in the nongenomic action. The data suggest that 20E-induced PCD is accomplished through the integration of genomic and nongenomic actions.

Expression of Two Ecdysteroid-Regulated Genes,Broad-ComplexandE75, in the Brain and Ovary of the Honeybee (Apis mellifera L.)

We previously demonstrated that two ecdysteroid-regulated genes, Mblk-1/E93 and E74, are expressed selectively in Kenyon cell subtypes in the mushroom bodies of the honeybee (Apis mellifera L.) brain. To further examine the possible involvement of ecdysteroid-regulated genes in brain function as well as in oogenesis in the honeybee, we isolated cDNAs for two other ecdysteroid-regulated genes, Broad-Complex (BR-C) and E75, and analyzed their expression in the worker brain as well as in the queen abdomen. In situ hybridization revealed that BR-C, like Mblk-1/ E93, is expressed selectively in the large-type Kenyon cells of the mushroom bodies in the worker brain, whereas E75 is expressed in all mushroom body neuron subtypes, suggesting a difference in the mode of response to ecdysteroid among Kenyon cell subtypes. In the queen ovary, both BR-C and E75 are expressed preferentially in the follicle cells that surround egg cells at the late stage, suggesting their role in oogenesis. These results suggest that BR-C and E75 are involved in the regulation of brain function as well as in reproductive physiology in the adult honeybee.

Coordinate responses of transcription factors to ecdysone during programmed cell death in the anterior silk gland of the silkworm, Bombyx mori

Programmed cell death (PCD) in Bombyx mori anterior silk glands (ASGs) is triggered by 20-hydroxyecdysone (20E). We examined the expression profiles and effects of 20E on 11 transcription factor genes in the fifth instar to determine whether they demonstrate the hierarchical control seen in Drosophila PCD. Results indicate that EcR-A and usp-2, but not EcR-B1 or usp-1, may be components of the ecdysone receptor complex. Up-regulation of E75A, BHR3, and three BR-C isoforms, but not E75B, appeared to be associated with the induction of PCD. betaFTZ-F1 was not expressed during PCD execution. Thus, gene control in B. mori ASGs differs from that in Drosophila salivary glands, despite both tissues undergoing PCD in response to 20E at pupal metamorphosis.

Mechanisms of midgut remodeling: juvenile hormone analog methoprene blocks midgut metamorphosis by modulating ecdysone action

In holometabolous insects such as mosquito, Aedes aegypti, midgut undergoes remodeling during metamorphosis. Insect metamorphosis is regulated by several hormones including juvenile hormone (JH) and 20-hydroxyecdysone (20E). The cellular and molecular events that occur during midgut remodeling were investigated by studying nuclear stained whole mounts and cross-sections of midguts and by monitoring the mRNA levels of genes involved in 20E action in methoprene-treated and untreated Ae. aegypti. We used JH analog, methoprene, to mimic JH action. In Ae. aegypti larvae, the programmed cell death (PCD) of larval midgut cells and the proliferation and differentiation of imaginal cells were initiated at about 36h after ecdysis to the 4th instar larval stage (AEFL) and were completed by 12h after ecdysis to the pupal stage (AEPS). In methoprene-treated larvae, the proliferation and differentiation of imaginal cells was initiated at 36h AEFL, but the PCD was initiated only after ecdysis to the pupal stage. However, the terminal events that occur for completion of PCD during pupal stage were blocked. As a result, the pupae developed from methoprene-treated larvae contained two midgut epithelial layers until they died during the pupal stage. Quantitative PCR analyses showed that methoprene affected midgut remodeling by modulating the expression of ecdysone receptor B, ultraspiracle A, broad complex, E93, ftz-f1, dronc and drice, the genes that are shown to play key roles in 20E action and PCD. Thus, JH analog, methoprene acts on Ae. aegypti by interfering with the expression of genes involved in 20E action resulting in a block in midgut remodeling and death during pupal stage.

MBR-1, a novel helix-turn-helix transcription factor, is required for pruning excessive neurites in Caenorhabditis elegans

In the developing brain, excessive neurites are actively pruned in the construction and remodeling of neural circuits. We demonstrate for the first time that the pruning of neurites occurs in the simple neural circuit of Caenorhabditis elegans and that a novel transcription factor, MBR-1, is involved in this process. We identified MBR-1 as a C. elegans ortholog of Mblk-1, a transcription factor that is expressed preferentially in the mushroom bodies of the honeybee brain. Although Mblk-1 homologs are conserved among animal species, their roles in the nervous system have never been analyzed. We used C. elegans as an ideal model animal for analysis of neuronal development. mbr-1 is expressed in various neurons in the head and tail ganglia. A comparison of the morphology of mbr-1-expressing neurons revealed that excessive neurites connecting the left and right AIM interneurons are eliminated during larval stages in wild-type but are sustained through the adult stage in the mbr-1 mutant. In addition, mbr-1 expression is regulated by UNC-86, a POU domain transcription factor, and the pruning of the excessive AIM connection is impaired in the unc-86 mutant. These findings provide an important clue for further genetic dissection of neurite pruning.