A role for juvenile hormone in the prepupal development of Drosophila melanogaster

Sesquiterpene action, and morphogenetic signaling through the ortholog of retinoid X receptor, in higher Diptera

Morphogenetic signaling by small terpenoid hormones is a common feature of both vertebrate and invertebrate development. Most attention on insect developmental signaling by small terpenoids has focused on signaling by juvenile hormone through bHLH-PAS proteins (e.g., the MET protein), especially as that signaling axis intersects with ecdysteroid action through the receptor EcR. However, a series of endocrine and pharmacological studies on pupariation in cyclorrhaphous Diptera have remained persistently refractory to explanation with the above two-axis model. Recently, the terpenoid compound methyl farnesoate has been physicochemically demonstrated to exist in circulation at physiological concentrations, in several mecopterid orders, including Diptera. In addition, it has also been recently demonstrated that the receptor to which methyl farnesoate binds with nanomolar affinity (ultraspiracle, an ortholog of retinoid X receptor) requires a functioning ligand binding pocket to sustain the morphogenetic transition to puparium formation. This review evaluates endocrine and pharmacological evidence for developmental pathways reached by methyl farnesoate action, and assesses the participation of the retinoid X receptor ligand pocket in signal transduction to those developmental endpoints.

Regulation of onset of female mating and sex pheromone production by juvenile hormone in Drosophila melanogaster

Juvenile hormone (JH) coordinates timing of female reproductive maturation in most insects. In Drosophila melanogaster, JH plays roles in both mating and egg maturation. However, very little is known about the molecular pathways associated with mating. Our behavioral analysis of females genetically lacking the corpora allata, the glands that produce JH, showed that they were courted less by males and mated later than control females. Application of the JH mimic, methoprene, to the allatectomized females just after eclosion rescued both the male courtship and the mating delay. Our studies of the null mutants of the JH receptors, Methoprene tolerant (Met) and germ cell-expressed (gce), showed that lack of Met in Met(27) females delayed the onset of mating, whereas lack of Gce had little effect. The Met(27) females were shown to be more attractive but less behaviorally receptive to copulation attempts. The behavioral but not the attractiveness phenotype was rescued by the Met genomic transgene. Analysis of the female cuticular hydrocarbon profiles showed that corpora allata ablation caused a delay in production of the major female-specific sex pheromones (the 7,11-C27 and -C29 dienes) and a change in the cuticular hydrocarbon blend. In the Met(27) null mutant, by 48 h, the major C27 diene was greatly increased relative to wild type. In contrast, the gce(2.5k) null mutant females were courted similarly to control females despite changes in certain cuticular hydrocarbons. Our findings indicate that JH acts primarily via Met to modulate the timing of onset of female sex pheromone production and mating.

Mechanisms regulating nutrition-dependent developmental plasticity through organ-specific effects in insects

Nutrition, via the insulin/insulin-like growth factor (IIS)/Target of Rapamycin (TOR) signaling pathway, can provide a strong molding force for determining animal size and shape. For instance, nutrition induces a disproportionate increase in the size of male horns in dung and rhinoceros beetles, or mandibles in staghorn or horned flour beetles, relative to body size. In these species, well-fed male larvae produce adults with greatly enlarged horns or mandibles, whereas males that are starved or poorly fed as larvae bear much more modest appendages. Changes in IIS/TOR signaling plays a key role in appendage development by regulating growth in the horn and mandible primordia. In contrast, changes in the IIS/TOR pathway produce minimal effects on the size of other adult structures, such as the male genitalia in fruit flies and dung beetles. The horn, mandible and genitalia illustrate that although all tissues are exposed to the same hormonal environment within the larval body, the extent to which insulin can induce growth is organ specific. In addition, the IIS/TOR pathway affects body size and shape by controlling production of metamorphic hormones important for regulating developmental timing, like the steroid molting hormone ecdysone and sesquiterpenoid hormone juvenile hormone. In this review, we discuss recent results from Drosophila and other insects that highlight mechanisms allowing tissues to differ in their sensitivity to IIS/TOR and the potential consequences of these differences on body size and shape.

Juvenile hormone regulation of Drosophila aging

Background

Juvenile hormone (JH) has been demonstrated to control adult lifespan in a number of non-model insects where surgical removal of the corpora allata eliminates the hormone’s source. In contrast, little is known about how juvenile hormone affects adult Drosophila melanogaster. Previous work suggests that insulin signaling may modulate Drosophila aging in part through its impact on juvenile hormone titer, but no data yet address whether reduction of juvenile hormone is sufficient to control Drosophila life span. Here we adapt a genetic approach to knock out the corpora allata in adult Drosophila melanogaster and characterize adult life history phenotypes produced by reduction of juvenile hormone. With this system we test potential explanations for how juvenile hormone modulates aging.

Results

A tissue specific driver inducing an inhibitor of a protein phosphatase was used to ablate the corpora allata while permitting normal development of adult flies. Corpora allata knockout adults had greatly reduced fecundity, inhibited oogenesis, impaired adult fat body development and extended lifespan. Treating these adults with the juvenile hormone analog methoprene restored all traits toward wildtype. Knockout females remained relatively long-lived even when crossed into a genotype that blocked all egg production. Dietary restriction further extended the lifespan of knockout females. In an analysis of expression profiles of knockout females in fertile and sterile backgrounds, about 100 genes changed in response to loss of juvenile hormone independent of reproductive state.

Conclusions

Reduced juvenile hormone alone is sufficient to extend the lifespan of Drosophila melanogaster. Reduced juvenile hormone limits reproduction by inhibiting the production of yolked eggs, and this may arise because juvenile hormone is required for the post-eclosion development of the vitellogenin-producing adult fat body. Our data do not support a mechanism for juvenile hormone control of longevity simply based on reducing the physiological costs of egg production. Nor does the longevity benefit appear to function through mechanisms by which dietary restriction extends longevity. We identify transcripts that change in response to juvenile hormone independent of reproductive state and suggest these represent somatically expressed genes that could modulate how juvenile hormone controls persistence and longevity.

The mode of action of juvenile hormone and ecdysone: towards an epi-endocrinological paradigm?

In some insect species, two sites of juvenile hormone (JH) synthesis have been reported: the very well documented corpora allata that secrete JH for "general use", and the reproductive system, in particular the male accessory glands, in which the function of the sometimes huge amounts of JH (e.g. in Hyalophora cecropia) remains to be clarified. A recent finding in Schistocerca gregaria, namely that suppression of the ecdysteroid peak preceding a molt by RNAi of the Halloween genes spook, phantom and shade does not impede normal molting, challenges the (never experimentally proven) classical concept that such a peak is causally linked to a molt. Recent developments in epigenetic control of gene expression in both the honey bee and in locusts suggest that, in addition to the classical scheme of hormone-receptor (membrane- and/or nuclear) mode of action, there may be a third way. Upon combining these and other orphan data that do not fit in the commonly accepted textbook schemes, we here advance the working hypothesis that both JH and ecdysone might be important but overlooked players in epigenetic control of gene expression, in particular at extreme concentrations (peak values or total absence). In this review, we put forward how epi-endocrinology can complement classical arthropod endocrinology.

Juvenile hormone and insulin suppress lipolysis between periods of lactation during tsetse fly pregnancy

Tsetse flies are viviparous insects that nurture a single intrauterine progeny per gonotrophic cycle. The developing larva is nourished by the lipid-rich, milk-like secretions from a modified female accessory gland (milk gland). An essential feature of the lactation process involves lipid mobilization for incorporation into the milk. In this study, we examined roles for juvenile hormone (JH) and insulin/IGF-like (IIS) signaling pathways during tsetse pregnancy. In particular, we examined the roles for these pathways in regulating lipid homeostasis during transitions between non-lactating (dry) and lactating periods. The dry period occurs over the course of oogenesis and embryogenesis, while the lactation period spans intrauterine larvigenesis. Genes involved in the JH and IIS pathways were upregulated during dry periods, correlating with lipid accumulation between bouts of lactation. RNAi suppression of Forkhead Box Sub Group O (FOXO) expression impaired lipolysis during tsetse lactation and reduced fecundity. Similar reduction of the JH receptor Methoprene tolerant (Met), but not its paralog germ cell expressed (gce), reduced lipid accumulation during dry periods, indicating functional divergence between Met and gce during tsetse reproduction. Reduced lipid levels following Met knockdown led to impaired fecundity due to inadequate fat reserves at the initiation of milk production. Both the application of the JH analog (JHA) methoprene and injection of insulin into lactating females increased stored lipids by suppressing lipolysis and reduced transcripts of lactation-specific genes, leading to elevated rates of larval abortion. To our knowledge, this study is the first to address the molecular physiology of JH and IIS in a viviparous insect, and specifically to provide a role for JH signaling through Met in the regulation of lipid metabolism during insect lactation.

Insect hormones: more than 50-years after the discovery of insect juvenile hormone analogues (JHA, juvenoids)

This review describes the over half-centennial history of research on insect juvenile hormone (JH) as well as its natural and synthetic bioanalogues (JHA or juvenoids).The leading theories of insect hormone action in growth and metamorphosis were created more than 50 years ago by the pioneers of insect endocrinology, V. B. Wigglesworth, C. M. Williams, V. J. A. Novák, H. Piepho, H. A. Schneiderman and L. I. Gilbert. There are two principal categories of hormones released from the central neuroendocrine system (neurosecretory cells of the brain, corpora cardiaca, corpora allata) that regulate insect growth and metamorphosis. The first is a complex set of neurohormones (neuropeptides) originating in the neurosecretory cells of the insect brain, which are released from the neurohaemal organs, the corpora cardiaca. These neuropeptides are responsible for stimulation of various developmental events, such as the release of the activation hormone, AH. The second category of centrally produced hormones in insects is the morphogenesis inhibiting hormone, or juvenile hormone (JH), produced by the associated endocrine glands, the corpora allata. JH is responsible for induction of the somatic larval growth in young instar larvae and stimulation of reproduction in the feeding adult stages.

Wigglesworth (1935) first described JH as an inhibitory hormone; Williams (1957) discovered its active extracts. Sláma (1961) discovered the hormonomimetic or pseudojuvenile effects of various lipid extracts and free fatty acids. In addition to lipid extracts with JH activity, a phenomenon found in various human organs, microorganisms and plants, JH-mimetic materials were found in American paper products in 1964. The source of the so-called “paper factor” was the wood of the Canadian balsam fir. The potential use of these and other analogues of JH as nontoxic, selectively acting “third generation pesticides” stimulated an enormous boom of activity among industrial and academic institutions all over the world, in the pursuit of synthetic JH analogues for replacement of toxic insecticides.

For practical reasons, in this review the chemical structures of the synthetic juvenoids have been divided into three categories: a) natural and synthetic, predominantly terpenoid juvenoids known before 1970; b) terpenoid and nonterpenoid juvenoids synthesized and tested before 1980, and; c) predominantly nonterpenoid, polycyclic juvenoids with relatively high JH activity, found and selected for practical use after 1980. Chemical structures of several juvenoids of theoretical or practical importance, together with the essential structure-activity relationships, are outlined in several figures and tables. The total number of all juvenoids reported active in one or more insects species has been estimated to be more than 4000 compounds. A juvenoid molecule has, more or less, a similar molecular size, roughly equivalent to a chain of 15 to 17 carbon atoms, with the presence of some slightly polar functional groups and a more or less lipophilic physico-chemical properties. Beyond these similarities, there are many variations in the structural types of juvenoids, including, derivatives of acyclic terpenoids, arylterpenoids, peptides, heterocyclic and polycyclic juvenoids, phenoxyphenyl juvenoids, juvenoid carbamates, and pyridyl-derivatives.

In addition to the generally known and intensively studied effects of juvenoids, such as inhibition of metamorphosis, inhibition of embryogenesis, and stimulation of ovarian growth, there are certain less remarkable and largely unexplored biological effects of juvenoids. Some of those phenomena, which are briefly described in this review, are: a) the effects of juvenoids on embryonic development (ovicidal effects); b) delayed effects of JH on metamorphosis from egg stage; c) sexually transmitted female sterility caused by juvenoid treatments of the males; d) the nonvolatile, biochemically activated juvenogen complexes, generating hormonally active juvenoids by enzymatic hydrolysis of the complex, and; e) antihormones with antijuvenile activity.

There are two basic hormonal theories on the regulation of insect metamorphosis by JH that have been proposed during the past 50 years. The first is the theory of Gilbert-Riddiford, which has been widely disseminated at universities worldwide, through textbooks on insect physiology, biochemistry and endocrinology. The second, less renowned, hormonal theory of insect development is that of Novák-Sláma. Briefly, the Gilbert-Riddiford theory is based on several fundamental principles. These are: a) the brain hormone-prothoracic gland (PG) concept created more than 50 years ago and later disproved by Williams; b) the conclusions of Piepho, who suggested that a large concentration of JH would cause a single epidermal cell to develop larval patterns, pupal patterns at medium concentrations, and adult epidermal patterns at zero concentration; c) small amounts of JH are necessary in the last larval instars of endopterygote insects for preventing precocious proliferation of imaginal discs; d) metamorphosis is stimulated by PG through a small endogenous peak of ecdysteroid preceding the large prepupal one; e) ecdysteroids are released from the PG in response to superimposed prothoracicotropic hormone (PTTH) from the brain; f) the true juvenile hormone of the corpora allata is a sesquiterpenoid compound known as epoxy homofarnesoate (JH-I), isolated from the adult male abdomens of the Cecropia silkmoths, and; g) physiological functions of JH and other hormones are regulated at the peripheral level by enzymes (esterase) or genes (methoprene tolerant,Metor a Broad complex gene).

The Novák-Sláma theory is based on completely different building blocks. Briefly, these are: a) the PG represent a peripheral organ which is not involved in the regulation of the moulting cycles, instead; b) the PG are a subordinated target of JH (not PTTH), they are inactive during the last larval instar and their removal does not abolish the cycles of moults; c) the PG are used to generate metabolic water during the growth of young larval instars by secreting of an adipokinetic superhormone, which stimulates total combustion of the dietary lipids; d) small, medium, or large concentrations of JH are unimportant, the hormone only needs to be present in the minimum, physiologically effective concentrations; e) an imperative condition for metamorphosis to occur is a virtual absence of JH starting from the second half of the penultimate larval instar; f) JH acts according to an “all-or-none” rule at the single cell level, and the temporal sensitivity to JH is strictly limited to a narrow period at the beginning of the moulting cycle, before the cells begin to divide; g) the corpus allatum never produces JH in a nonfeeding stage, and the sesquiterpenoid juvenoid JH-I cannot be the true JH of insects (it has very low JH activity, 100,000-fold smaller in comparison to human made peptidic juvenoids); h) the developmental cycles are stimulated exclusively by neuropeptides produced by the brain’s neurosecretory cells (AH); i) developmental stimulation by AH has nothing in common with the PTTH or PG; j) when environmental interventions in the hormonal system become obsolete, the regulation of moulting cycles becomes autonomic (hormone independent), supported by the stereotypic instructions coded on the genome; k) during the millions of years of insect evolution, the central neuroendocrine system acquired the superimposed, epigenetic ability to adapt gene functions and synchronize them with essential changes in the environment. A model based on the regulation of insect metamorphosis by simple combination of two hormones (AH, JH) of the central neuroendocrine system is outlined. A possibility that the 4000 known juvenoid molecules act as the feedback or homeostatic factors affecting permeability of the epidermal cell membranes has been suggested. Speculations about possible peptidic or proteinic nature of the corpus allatum hormone have been emphasized.

Ligand binding pocket function of Drosophila USP is necessary for metamorphosis

The widely accepted paradigm that epoxidized methyl farnesoates ("juvenile hormones," JHs) are the principal sesquiterpenoid hormones regulating insect metamorphosis was assessed in Drosophila melanogaster. GC-MS analysis of circulating methyl farnesoids during the mid to late 3rd instar showed that methyl farnesoate is predominant over methyl epoxyfarnesoate (=JH III). The circulating concentration of methyl farnesoate (reaching nearly 500 nM), was easily high enough on a kinetic basis to load the Drosophila ortholog of the nuclear hormone receptor RXR (also known as "ultraspiracle," USP), whereas the circulating concentrations of JH III and methyl bisepoxyfarnesoate (bisepoxyJH III) were not. The hypothesis that the ligand pocket of USP necessarily binds an endogenous ligand for differentiation of the immature to the adult was tested with USP mutated at residue that normally extends a side chain into the ligand binding pocket. An equilibrium binding assay confirmed that the mutation (Q288A) strongly altered methyl farnesoate interaction with USP, while a heterologous cell-line transfection assay confirmed that the mutation did not allosterically alter the transcriptional response of the ultraspiracle/ecdysone receptor heterodimer to ecdysteroid signaling. Transgenic wildtype USP driven by the cognate natural promoter rescued null animals to develop to the adult inside a normally formed puparium, while in contrast animals transgenically expressing instead the ligand pocket mutant exhibited developmental derangement at the larval to pupal transition, including failure to form a properly shaped or sclerotized puparium. Other point mutations to the pocket strongly reducing affinity for methyl farnesoate similarly disrupted the larval to pupal metamorphosis. These results suggest that normal larval to pupal maturation in this mecopteran model insect requires the involvement of a distinct endocrine axis of USP binding to its own endogenous terpenoid ligand.

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.

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

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

Developmental checkpoints and feedback circuits time insect maturation

The transition from juvenile to adult is a fundamental process that allows animals to allocate resource toward reproduction after completing a certain amount of growth. In insects, growth to a species-specific target size induces pulses of the steroid hormone ecdysone that triggers metamorphosis and reproductive maturation. The past few years have seen significant progress in understanding the interplay of mechanisms that coordinate timing of ecdysone production and release. These studies show that the neuroendocrine system monitors complex size-related and nutritional signals, as well as external cues, to time production and release of ecdysone. Based on results discussed here, we suggest that developmental progression to adulthood is controlled by checkpoints that regulate the genetic timing program enabling it to adapt to different environmental conditions. These checkpoints utilize a number of signaling pathways to modulate ecdysone production in the prothoracic gland. Release of ecdysone activates an autonomous cascade of both feedforward and feedback signals that determine the duration of the ecdysone pulse at each developmental transitions. Conservation of the genetic mechanisms that coordinate the juvenile-adult transition suggests that insights from the fruit fly Drosophila will provide a framework for future investigation of developmental timing in metazoans.

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.

How does juvenile hormone control insect metamorphosis and reproduction?

In insects juvenile hormone (JH) regulates both metamorphosis and reproduction. This lecture focuses on our current understanding of JH action at the molecular level in both of these processes based primarily on studies in the tobacco hornworm Manduca sexta, the flour beetle Tribolium castaneum, the mosquito Aedes aegypti, and the fruit fly Drosophila melanogaster. The roles of the JH receptor complex and the transcription factors that it regulates during larval molting and metamorphosis are summarized. Also highlighted are the intriguing interactions of the JH and insulin signaling pathways in both imaginal disc development and vitellogenesis. Critical actions of JH and its receptor in the timing of maturation of the adult optic lobe and of female receptivity in Drosophila are also discussed.

The juvenile hormone signaling pathway in insect development

The molecular action of juvenile hormone (JH), a regulator of vital importance to insects, was until recently regarded as a mystery. The past few years have seen an explosion of studies of JH signaling, sparked by a finding that a JH-resistance gene, Methoprene-tolerant (Met), plays a critical role in insect metamorphosis. Here, we summarize the recently acquired knowledge on the capacity of Met to bind JH, which has been mapped to a particular ligand-binding domain, thus establishing this bHLH-PAS protein as a novel type of an intracellular hormone receptor. Next, we consider the significance of JH-dependent interactions of Met with other transcription factors and signaling pathways. We examine the regulation and biological roles of genes acting downstream of JH and Met in insect metamorphosis. Finally, we discuss the current gaps in our understanding of JH action and outline directions for future research.

takeout-dependent longevity is associated with altered Juvenile Hormone signaling

In order to understand the molecular mechanisms of longevity regulation, we recently performed a screen designed to enrich for genes common to several longevity interventions. Using this approach, we identified the Drosophila melanogaster gene takeout. takeout is upregulated in a variety of long-lived flies, and extends life span when overexpressed. Here, we investigate the mechanisms of takeout-dependent longevity. takeout overexpression specifically in the fat body is sufficient to increase fly longevity and is additive to the longevity effects of Dietary Restriction. takeout long-lived flies do not show phenotypes often associated with increased longevity, such as enhanced stress resistance or major metabolic abnormalities. However, males exhibit greatly diminished courtship behavior, leading to a reduction in fertility. Interestingly, takeout contains a binding domain for Juvenile Hormone, a fly hormone that plays a role in the regulation of developmental transitions. Importantly, the longevity and courtship phenotypes of takeout overexpressing flies are reversed by treatment with the Juvenile Hormone analog methoprene. These data suggest that takeout is a key player in the tradeoff-switch between fertility and longevity. takeout may control fertility via modulation of courtship behavior. This regulation may occur through Juvenile Hormone binding to takeout and a subsequent reduction in Juvenile Hormone signaling activity.

Molecular Mechanisms of Transcription Activation by Juvenile Hormone: A Critical Role for bHLH-PAS and Nuclear Receptor Proteins

Juvenile hormone (JH) is responsible for controlling many biological processes. In several insect species JH has been implicated as a key regulator of developmental timing, preventing the premature onset of metamorphosis during larval growth periods. However, the molecular basis of JH action is not well-understood. In this review, we highlight recent advances which demonstrate the importance of transcription factors from the bHLH-PAS and nuclear receptor families in mediating the response to JH.

The Drosophila juvenile hormone receptor candidates methoprene-tolerant (MET) and germ cell-expressed (GCE) utilize a conserved LIXXL motif to bind the FTZ-F1 nuclear receptor

Juvenile hormone (JH) has been implicated in many developmental processes in holometabolous insects, but its mechanism of signaling remains controversial. We previously found that in Drosophila Schneider 2 cells, the nuclear receptor FTZ-F1 is required for activation of the E75A gene by JH. Here, we utilized insect two-hybrid assays to show that FTZ-F1 interacts with two JH receptor candidates, the bHLH-PAS paralogs MET and GCE, in a JH-dependent manner. These interactions are severely reduced when helix 12 of the FTZ-F1 activation function 2 (AF2) is removed, implicating AF2 as an interacting site. Through homology modeling, we found that MET and GCE possess a C-terminal α-helix featuring a conserved motif LIXXL that represents a novel nuclear receptor (NR) box. Docking simulations supported by two-hybrid experiments revealed that FTZ-F1·MET and FTZ-F1·GCE heterodimer formation involves a typical NR box-AF2 interaction but does not require the canonical charge clamp residues of FTZ-F1 and relies primarily on hydrophobic contacts, including a unique interaction with helix 4. Moreover, we identified paralog-specific features, including a secondary interaction site found only in MET. Our findings suggest that a novel NR box enables MET and GCE to interact JH-dependently with the AF2 of FTZ-F1.

Integrating body and organ size in Drosophila: recent advances and outstanding problems

OVER THE PAST TWO DECADES, FUNDAMENTAL STRIDES IN PHYSIOLOGY AND GENETICS HAVE ALLOWED US TO FINALLY GRASP THE DEVELOPMENTAL MECHANISMS REGULATING BODY SIZE, PRIMARILY IN ONE MODEL ORGANISM: the fruit fly Drosophila melanogaster. In Drosophila, as in all animals, final body size is regulated by the rate and duration of growth. These studies have identified important roles for the insulin and the target of rapamycin (TOR) signaling pathways in regulating the growth rate of the larva, the stage most important in determining final adult size. Furthermore, they have shown that the insulin/TOR pathway interacts with hormonal systems, like ecdysone and juvenile hormone, to regulate the timing of development and hence the duration of growth. This interaction allows the growing larvae to integrate cues from the environment with environmentally sensitive developmental windows to ensure that optimal size and proportions are reached given the larval rearing conditions. Results from this work have opened up new avenues of studies, including how environmental cues are integrated to regulate developmental time and how organs maintain proportional growth. Other researchers interested in the evolution of body size are beginning to apply these results to studies of body size evolution and the generation of allometry. With these new findings, and with the developments to come, the field of size control finds itself in the fortunate position of finally being able to tackle century old questions of how organisms achieve final adult size and proportions. This review discusses the state of the art of size control from a Drosophila perspective, and outlines an approach to resolving outstanding issues.

Ligand-binding properties of a juvenile hormone receptor, Methoprene-tolerant

Juvenile hormone (JH) is a sesquiterpenoid of vital importance for insect development, yet the molecular basis of JH signaling remains obscure, mainly because a bona fide JH receptor has not been identified. Mounting evidence points to the basic helix-loop-helix (bHLH)/Per-Arnt-Sim (PAS) domain protein Methoprene-tolerant (Met) as the best JH receptor candidate. However, details of how Met transduces the hormonal signal are missing. Here, we demonstrate that Met specifically binds JH III and its biologically active mimics, methoprene and pyriproxyfen, through its C-terminal PAS domain. Substitution of individual amino acids, predicted to form a ligand-binding pocket, with residues possessing bulkier side chains reduces JH III binding likely because of steric hindrance. Although a mutation that abolishes JH III binding does not affect a Met-Met complex that forms in the absence of methoprene, it prevents both the ligand-dependent dissociation of the Met-Met dimer and the ligand-dependent interaction of Met with its partner bHLH-PAS protein Taiman. These results show that Met can sense the JH signal through direct, specific binding, thus establishing a unique class of intracellular hormone receptors.

Drosophila Met and Gce are partially redundant in transducing juvenile hormone action

The Drosophila Methoprene-tolerant (Met) and Germ cell-expressed (Gce) bHLH-PAS transcription factors are products of two paralogous genes. Both proteins potentially mediate the effect of juvenile hormone (JH) as candidate JH receptors. Here we report that Met and Gce are partially redundant in transducing JH action. Both Met and gce null single mutants are fully viable, but the Met gce double mutant, Met(27) gce(2.5k), dies during the larval-pupal transition. Precocious and enhanced caspase-dependent programmed cell death (PCD) appears in fat body cells of Met(27) gce(2.5k) during the early larval stages. Expression of Kr-h1, a JH response gene that inhibits 20-hydroxyecdysone (20E)-induced broad (br) expression, is abolished in Met(27) gce(2.5k) during larval molts. Consequently, expression of br occurs precociously in Met(27) gce(2.5k), which may cause precocious caspase-dependent PCD during the early larval stages. Defective phenotypes and gene expression changes in Met(27) gce(2.5k) double mutants are similar to those found in JH-deficient animals. Importantly, exogenous application of JH agonists rescued the JH-deficient animals but not the Met(27) gce(2.5k) mutants. Our data suggest a model in which Drosophila Met and Gce redundantly transduce JH action to prevent 20E-induced caspase-dependent PCD during larval molts by induction of Kr-h1 expression and inhibition of br expression.

Juvenile hormone action through a defined enhancer motif to modulate ecdysteroid-activation of natural core promoters

We have established a model system of hormone action, in an Sf9 cell transfection system, using defined enhancer motifs and natural core promoters of metamorphosis-associated genes. The DR1 enhancer, that is an established DNA binding site for the ecdysone receptor/ultraspiracle heterodimer, was necessary for transcriptional activation by 20-OH ecdysone. For this activated transcription, a natural sequence closely 5' to the TATA box is necessary. Cotreatment with juvenile hormone III strongly suppressed the steroid activation of transcription. However, in the absence of the sequence located closely 5' to the TATA box, cotreatment with juvenile hormone instead increased transcription over that occurring due to 20-hydroxy-ecdysone alone. This sensitivity to activation by cotreatment with juvenile hormone could be transferred to a related, but otherwise unresponsive, hexamerin core promoter simply by transferring to the unresponsive promoter the five base transcription start site (ACAGT) from the responsive hexamerin gene. These are the first reports that the direction of JH action on 20-OH ecdysone-activated transcription can be reversed by removal of a sequence at the core promoter, and that modulatory action of juvenile hormone can be transferred to a different gene by transferring the transcription start site motif.

Wnt signaling cross-talks with JH signaling by suppressing Met and gce expression

Juvenile hormone (JH) plays key roles in controlling insect growth and metamorphosis. However, relatively little is known about the JH signaling pathways. Until recent years, increasing evidence has suggested that JH modulates the action of 20-hydroxyecdysone (20E) by regulating expression of broad (br), a 20E early response gene, through Met/Gce and Kr-h1. To identify other genes involved in JH signaling, we designed a novel Drosophila genetic screen to isolate mutations that derepress JH-mediated br suppression at early larval stages. We found that mutations in three Wnt signaling negative regulators in Drosophila, Axin (Axn), supernumerary limbs (slmb), and naked cuticle (nkd), caused precocious br expression, which could not be blocked by exogenous JHA. A similar phenotype was observed when armadillo (arm), the mediator of Wnt signaling, was overexpressed. qRT-PCR revealed that Met, gce and Kr-h1expression was suppressed in the Axn, slmb and nkd mutants as well as in arm gain-of-function larvae. Furthermore, ectopic expression of gce restored Kr-h1 expression but not Met expression in the arm gain-of-function larvae. Taken together, we conclude that Wnt signaling cross-talks with JH signaling by suppressing transcription of Met and gce, genes that encode for putative JH receptors. The reduced JH activity further induces down-regulation of Kr-h1expression and eventually derepresses br expression in the Drosophila early larval stages.

The Drosophila FTZ-F1 nuclear receptor mediates juvenile hormone activation of E75A gene expression through an intracellular pathway

Juvenile hormone (JH) regulates a wide variety of biological activities in holometabolous insects, ranging from vitellogenesis and caste determination in adults to the timing of metamorphosis in larvae. The mechanism of JH signaling in such a diverse array of processes remains either unknown or contentious. We previously found that the nuclear receptor gene E75A is activated in S2 cells as a primary response to JH. Here, by expressing an intracellular form of JH esterase, we demonstrate that JH must enter the cell in order to activate E75A. To find intracellular receptors involved in the JH response, we performed an RNAi screen against nuclear receptor genes expressed in this cell line and identified the orphan receptor FTZ-F1. Removal of FTZ-F1 prevents JH activation of E75A, whereas overexpression enhances activation, implicating FTZ-F1 as a critical component of the JH response. FTZ-F1 is bound in vivo to multiple enhancers upstream of E75A, suggesting that it participates in direct JH-mediated gene activation. To better define the role of FTZ-F1 in JH signaling, we investigated interactions with candidate JH receptors and found that the bHLH-PAS proteins MET and GCE both interact with FTZ-F1 and can activate transcription through the FTZ-F1 response element. Removal of endogenous GCE, but not MET, prevents JH activation of E75A. We propose that FTZ-F1 functions as a competence factor by loading JH signaling components to the promoter, thus facilitating the direct regulation of E75A gene expression by JH.

Influence of helix 12 of Ultraspiracle on Drosophila melanogaster ecdysone receptor function

Although it has no ligand, helix 12 in the ligand binding domain of Ultraspiracle (USP) is locked in an antagonistic position. To investigate whether this position is of functional importance, we enhanced the flexibility of helix 12 by mutating two amino acids (259, located in L1-3 and F491 in helix 12). Mutated USP reduces the stability of USP and all isoforms of the ecdysone receptor (EcR) and impairs nuclear localization and DNA binding of EcR/USP(L259A/F491/A), resulting in lower levels of basal transcriptional activity. Although the affinity of the ligand ponasterone A to EcR/USP(L259/F491) is moderately diminished, hormone-induced stimulation of transcriptional activity is normal. Potentiation of the ecdysone response by juvenile hormone (JH) is selectively increased in mutated heterodimers with EcR-B1, demonstrating that the antagonistic position impairs functional interaction of the EcR complex with JHIII.

The hormonal and circadian basis for insect photoperiodic timing

Daylength perception in temperate zones is a critical feature of insect life histories, and leads to developmental changes for resisting unfavourable seasons. The role of the neuroendocrine axis in the photoperiodic response of insects is discussed in relation to the key organs and molecules that are involved. We also discuss the controversial issue of the possible involvement of the circadian clock in photoperiodicity. Drosophila melanogaster has a shallow photoperiodic response that leads to reproductive arrest in adults, yet the unrivalled molecular genetic toolkit available for this model insect should allow the systematic molecular and neurobiological dissection of this complex phenotype.

RNAi knockdown of dRNaseZ, the Drosophila homolog of ELAC2, impairs growth of mitotic and endoreplicating tissues

The Drosophila RNase Z(L) (dRNaseZ) gene encodes a member of the ELAC1/ELAC2 protein family with homologs in every living organism. All RNase Z proteins tested so far were found to possess endoribonuclease activity, which is responsible for the removal of a 3' trailer from a primary tRNA transcript. Given that tRNA 3'-end processing has been delineated using in vitro, bacterial and cell culture models, its relevance to RNase Z functions in vivo has yet to be established. In this study, we employed heritable RNA interference (RNAi) in combination with the GAL4/UAS system to spatiotemporally knockdown the dRNaseZ gene and study its biological role in cells of a developing fruit fly. We found that dRNaseZ is an essential gene, as ubiquitous knockdown caused growth arrest and early larval lethality. Molecular analysis confirmed that dRNaseZ is necessary for 3'-end processing of nuclear and mitochondrial tRNAs: knockdown larvae displayed significant accumulation of both types of processing intermediates with extensions at the 3' end. This is the first in vivo demonstration of RNase Z(L) dependent tRNA processing in the context of a metazoan model organism. Using tissue-specific GAL4 drivers, we also showed that in mitotically growing imaginal discs dRNaseZ is required for cell proliferation and/or viability, but not for the maintenance of their differentiated progeny. In endoreplicating salivary glands, dRNaseZ controls organ size by supporting cell growth but not DNA replication. Although the mechanisms remain unclear, our results support the notion that RNase Z(L) is involved in biological pathways regulating cell growth and proliferation.

Altered juvenile hormone metabolism, reproduction and stress response in Drosophila adults with genetic ablation of the corpus allatum cells

Juvenile hormone (JH), which controls many developmental and physiological processes in Drosophila melanogaster, is synthesized de novo in the specialized endocrine glands, corpus allatum (CA). The present study concerns JH metabolism, reproduction and stress resistance in Drosophila with genetic ablation of a part of CA cells. The correlated regulation of JH biosynthesis and degradation in Drosophila adults has been found: ablation of CA cells led to (1) a dramatic decrease in activity of the key regulatory enzyme of JH biosynthesis, juvenile hormone acid methyl transferase and (2) a considerable increase in JH-hydrolyzing activity. It has been also shown that ablation of CA cells caused three significant physiological changes: (1) an increase in the intensity of response of JH degradation system to heat stress; (2) a disturbance of reproduction; (3) a decrease in stress resistance. Pharmacological rise of JH level rescued JH-hydrolyzing activity, fecundity and stress resistance in CA-ablated females. Pronouncedly, all the physiological effects caused by CA ablation were significant in females but not in males indicating a sexual dimorphism of JH physiological roles in Drosophila adults.

Activities of natural methyl farnesoids on pupariation and metamorphosis of Drosophila melanogaster

Methyl farnesoate (MF) and juvenile hormone (JH III), which bind with high affinity to the receptors USP and MET, respectively, and bisepoxy JH III (bisJH III) were assessed for several activities during Drosophila larval development, and during prepupal development to eclosed adults. Dietary MF and JH III were similarly active, and more active than bisJH III, in lengthening larval development prior to pupariation. However, the order of activity was changed (JH III>bisJH III>MF) with respect to preventing prepupae from eclosing as normal adults, whether administered in the larval diet or as topically applied at the white puparium stage. If endogenous production of all three larval methyl farnesoids was suppressed by a strongly driven RNAi against HMGCR in the corpora allata cells, most larvae did not attain pupariation. Farnesol (which has no demonstrated life-necessary function in larval life except in corpora allata cells as a precursor to methyl farnesoid biosynthesis) when incorporated into the diet rescued attainment of pupariation in a dose-dependent manner, presumably by rescuing endogenous production of all three hormones. A more mild suppression of endogenous methyl farnesoid production enabled larval attainment of pupariation. However, in this background dietary MF had increased activity in preventing puparia from attaining normal adult eclosion. The physiological relevance of using exogenous methyl farnesoids to block prepupal development to normally eclosed adults was tested by, instead, protecting in prepupae the endogenous titer of methyl farnesoids. JH esterase normally increases during the mid-late prepupal stage, presumably to clear endogenous methyl farnesoids. When JH esterase was inhibited with an RNAi, it prevented attainment of adult eclosion. Cultured adult corpora allata from male and female Aedes aegypti released both MF and JH III, and the A. aegypti nuclear receptor USP bound MF with nanomolar affinity. These A. aegypti data support the use of Drosophila as a model for mosquitoes of the binding of secreted MF to USP.

Evolutionary divergence of the paralogs Methoprene tolerant (Met) and germ cell expressed (gce) within the genus Drosophila

Juvenile hormone (JH) signaling underpins both regulatory and developmental pathways in insects. However, the JH receptor is poorly understood. Methoprene tolerant (Met) and germ cell expressed (gce) have been implicated in JH signaling in Drosophila. We investigated the evolution of Met and gce across 12 Drosophila species and found that these paralogs are conserved across at least 63 million years of dipteran evolution. Distinct patterns of selection found using estimates of dN/dS ratios across Drosophila Met and gce coding sequences, along with their incongruent temporal expression profiles in embryonic Drosophila melanogaster, illustrate avenues through which these genes have diverged within the Diptera. Additionally, we demonstrate that the annotated gene CG15032 is the 5' terminus of gce. In mosquitoes and beetles, a single Met-like homolog displays structural similarity to both Met and gce, and the intron locations are conserved with those of gce. We found that Tribolium and mosquito Met orthologs are assembled from Met- and gce-specific domains in a modular fashion. Our results suggest that Drosophila Met and gce experienced divergent evolutionary pressures following the duplication of an ancestral gce-like gene found in less derived holometabolous insects.

Paralogous genes involved in juvenile hormone action in Drosophila melanogaster

Juvenile hormone (JH) is critical for multiple aspects of insect development and physiology. Although roles for the hormone have received considerable study, an understanding of the molecules necessary for JH action in insects has been frustratingly slow to evolve. Methoprene-tolerant (Met) in Drosophila melanogaster fulfills many of the requirements for a hormone receptor gene. A paralogous gene, germ-cell expressed (gce), possesses homology and is a candidate as a Met partner in JH action. Expression of gce was found to occur at multiple times and in multiple tissues during development, similar to that previously found for Met. To probe roles of this gene in JH action, we carried out in vivo gce over- and underexpression studies. We show by overexpression studies that gce can substitute in vivo for Met, alleviating preadult but not adult phenotypic characters. We also demonstrate that RNA interference-driven knockdown of gce expression in transgenic flies results in preadult lethality in the absence of MET. These results show that (1) unlike Met, gce is a vital gene and shows functional flexibility and (2) both gene products appear to promote JH action in preadult but not adult development.