Nuclear receptors--a perspective from Drosophila

Nuclear receptor HR4 plays an essential role in the ecdysteroid-triggered gene cascade in the development of the hemimetabolous insect Blattella germanica

Despite the differences in the developmental strategies between hemimetabolous and holometabolous insects, a common feature between both types of development is that periodic pulses of the steroid hormone 20-hydroxyecdysone (20E) dictate each developmental transition. Although the molecular action of 20E has been extensively studied in holometabolous insects, data on hemimetabolous is scarce. To address this, we have used the German cockroach Blattella germanica to show that 20E signals through a transcriptional cascade of the nuclear hormone receptor-encoding genes BgE75, BgHR3 and BgFTZ-F1. Here, we report the isolation and functional characterization of BgHR4, another nuclear receptor involved in this cascade. Expression studies along with tissue incubations and RNAi experiments show that cross-regulation between BgE75 and BgHR3 directs the expression of BgHR4. Finally, we have also shown that BgHR4 is an essential gene required for successfully completing nymphal-nymphal and nymphal-adult transitions, by allowing the appropriate delay in the induction of BgFTZ-F1.

Peripheral regulation by ecdysteroids of olfactory responsiveness in male Egyptian cotton leaf worms, Spodoptera littoralis

Physiological and behavioral plasticity allows animals to adapt to changes in external (environmental) and internal (physiological) factors. In insects, the physiological state modulates adult behavior in response to different odorant stimuli. Hormones have the potential to play a major role in the plasticity of the olfactory responses. To explore if peripheral olfactory processing could be regulated by steroid hormones, we characterized the molecular, electrophysiological, and behavioral response to changes in endogenous hormone levels in adult male Spodoptera littoralis. The expression of the receptor complex (EcR/USP) was localized by in situ hybridization in the olfactory sensilla of antennae. Injections of 20-hydroxyecdysone (20E) induced an ecdysteroid signaling pathway in antennae and increased expression of the nuclear receptors EcR, USP and E75. Diacylglycerol kinase (DGK) and CaM expression were also up-regulated by 20E. Taken together, these molecular, electrophysiological, and behavioral results suggest a hormonal regulation of the peripheral olfactory processing in S. littoralis.

Modulation in the mRNA expression of ecdysone receptor gene in aquatic midge, Chironomus riparius upon exposure to nonylphenol and silver nanoparticles

Chironomus riparius, a non-biting midge (Chironomidae, Diptera), is extensively used as a model organism in aquatic ecotoxicological studies, although little is known about its genome sequences. Ecdysteroids are steroid hormones that play an important role in development, growth, moulting of larva, and reproduction in Chironomus spp. The effect of ecdysteroids is mediated by their binding to the ecdysteroid receptor (EcR). To study the effect of environmental stressors, nonylphenol and silver nanoparticles (AgNPs), on the modulation of EcR mRNA, in this study, full length cDNA of C. riparius ecdysone receptor (CrEcR) was identified from the Expressed Sequence Tags (ESTs) database and expression of the corresponding mRNA was analyzed following exposure to nonylphenol and AgNPs. The CrEcR cDNA was 2548 base pairs (bp) in length, with a 5' untranslated region (UTR) of 242 bp and a 3' UTR of 684 bp. The open reading frame contains 1617 nucleotides, encoding 539 amino acids with a predicted molecular weight of 61 kDa and pI of 5.89, and revealed the presence of several domains associated with DNA binding, dimerization, ligand binding and transcriptional activation characteristic of steroid receptor family members. It was found that the expression level of CrEcR was significantly up-regulated on exposure to nonylphenol and significant up or down regulation was observed on exposure to AgNPs. These finding shows that nonylphenol as well as AgNPs could modulate the ecdysone nuclear receptor and may have significant implications in different developmental stages in C. riparius.

Silencing of molt-regulating transcription factor gene, CiHR3, affects growth and development of sugarcane stem borer, Chilo infuscatellus

RNA interference (RNAi) is a technology for conducting functional genomic studies and a potential tool for crop protection against insect pests. Development of reliable methods for production and delivery of double-stranded RNA (dsRNA) is the major challenge for efficient pest control. In this study, Chilo infuscatellus Snellen (Crambidae: Lepidoptera) was fed with CiHR3 dsRNA expressed in bacteria or synthesized in vitro. The dsRNA ingested by C. infuscatellus successfully triggered silencing of the molt-regulating transcription factor CiHR3, an important gene for insect growth and development, and caused significant abnormalities and weight loss in insects within seven days of treatment. This study is an ideal example of feeding-based RNAi mediated by dsRNA expressed in bacteria or synthesized in vitro. The results also suggested that feeding-based RNA interference is a potential method for the management of C. infuscatellus.

Ecdysone- and NO-mediated gene regulation by competing EcR/Usp and E75A nuclear receptors during Drosophila development

The Drosophila ecdysone receptor (EcR/Usp) is thought to activate or repress gene transcription depending on the presence or absence, respectively, of the hormone ecdysone. Unexpectedly, we found an alternative mechanism at work in salivary glands during the ecdysone-dependent transition from larvae to pupae. In the absense of ecdysone, both ecdysone receptor subunits localize to the cytoplasm, and the heme-binding nuclear receptor E75A replaces EcR/Usp at common target sequences in several genes. During the larval-pupal transition, a switch from gene activation by EcR/Usp to gene repression by E75A is triggered by a decrease in ecdysone concentration and by direct repression of the EcR gene by E75A. Additional control is provided by developmentally timed modulation of E75A activity by NO, which inhibits recruitment of the corepressor SMRTER. These results suggest a mechanism for sequential modulation of gene expression during development by competing nuclear receptors and their effector molecules, ecdysone and NO.

Coordinated regulation of niche and stem cell precursors by hormonal signaling

In the developing Drosophila ovary, the ecdysone signaling pathway controls the differentiation of both niche and germ line stem cell precursors.

Stem cells and their niches constitute units that act cooperatively to achieve adult body homeostasis. How such units form and whether stem cell and niche precursors might be coordinated already during organogenesis are unknown. In fruit flies, primordial germ cells (PGCs), the precursors of germ line stem cells (GSCs), and somatic niche precursors develop within the larval ovary. Together they form the 16–20 GSC units of the adult ovary. We show that ecdysone receptors are required to coordinate the development of niche and GSC precursors. At early third instar, ecdysone receptors repress precocious differentiation of both niches and PGCs. Early repression is required for correct morphogenesis of the ovary and for protecting future GSCs from differentiation. At mid-third instar, ecdysone signaling is required for niche formation. Finally, and concurrent with the initiation of wandering behavior, ecdysone signaling initiates PGC differentiation by allowing the expression of the differentiation gene bag of marbles in PGCs that are not protected by the newly formed niches. All the ovarian functions of ecdysone receptors are mediated through early repression, and late activation, of the ecdysone target gene broad. These results show that, similar to mammals, a brain-gland-gonad axis controls the initiation of oogenesis in insects. They further exemplify how a physiological cue coordinates the formation of a stem cell unit within an organ: it is required for niche establishment and to ensure that precursor cells to adult stem cells remain undifferentiated until the niches can accommodate them. Similar principles might govern the formation of additional stem cell units during organogenesis.

Tissue-specific stem cells reside in specialized microenvironments (niches). How the generation of niche cells and resident stem cells is coordinated, and how their correct numerical ratios are regulated, remains poorly understood. Here, we examine the potential mechanisms of this process by analyzing the formation of the fly ovary. Specifically, we uncover a role for ecdysone, which is a steroid hormone renowned for its role in insect molting but that also controls many aspects of larval development in flies and mammals. We find that ecdysone signaling in fly larvae coordinates the development of niche cells relative to their resident germ line stem cells (GSCs). Ecdysone receptors initially serve as repressors of differentiation, allowing precursor cells of both niches and GSCs time to proliferate and attain correct cell numbers. Later, ecdysone receptors allow differentiation of niches while simultaneously maintaining GSC precursors in an undifferentiated state, until the newly formed niches can accommodate them. Finally, ecdysone induces the differentiation of GSC precursors that are not incorporated in niches. Our work exemplifies one possible solution to coordinating stem cell and niche development: using a common signal to affect both cell types. A further understanding of these and other mechanisms will offer novel insights into regeneration and could help guide cell-based therapies for various diseases.

Conserved repressive function of Krüppel homolog 1 on insect metamorphosis in hemimetabolous and holometabolous species

Insect metamorphosis is regulated by ecdysteroids, which induce molts, and juvenile hormone (JH), which inhibits metamorphic changes. The molecular action of ecdysteroids has been thoroughly studied, but that of JH is poorly understood, with data currently only being available for holometabolous species, like Drosophila melanogaster and Tribolium castaneum. We studied the function of Krüppel homolog 1 (Kr-h1) in Blattella germanica, a hemimetabolous model. Kr-h1 is a Zn finger transcription factor whose function as transductor of the antimetamorphic action of JH has recently been demonstrated in D. melanogaster and T. castaneum. The RNAi experiments reported herein indicated that Kr-h1 transduces the antimetamorphic action of JH also in B. germanica, thereby suggesting that this role is an ancestral condition that has been conserved in insect evolution from hemimetabolous to holometabolous species.

Programmed autophagy in the fat body of Aedes aegypti is required to maintain egg maturation cycles

Autophagy plays a pivotal role by allowing cells to recycle cellular components under conditions of stress, starvation, development and cancer. In this work, we have demonstrated that programmed autophagy in the mosquito fat body plays a critical role in maintaining of developmental switches required for normal progression of gonadotrophic cycles. Mosquitoes must feed on vertebrate blood for their egg development, with each gonadotrophic cycle being tightly coupled to a separate blood meal. As a consequence, some mosquito species are vectors of pathogens that cause devastating diseases in humans and domestic animals, most importantly malaria and Dengue fever. Hence, deciphering mechanisms to control egg developmental cycles is of paramount importance for devising novel approaches for mosquito control. Central to egg development is vitellogenesis, the production of yolk protein precursors in the fat body, the tissue analogous to a vertebrate liver, and their subsequent specific accumulation in developing oocytes. During each egg developmental cycle, the fat body undergoes a developmental program that includes previtellogenic build-up of biosynthetic machinery, intense production of yolk protein precursors, and termination of vitellogenesis. The importance of autophagy for termination of vitellogenesis was confirmed by RNA interference (RNAi) depletions of several autophagic genes (ATGs), which inhibited autophagy and resulted in untimely hyper activation of TOR and prolonged production of the major yolk protein precursor, vitellogenin (Vg). RNAi depletion of the ecdysone receptor (EcR) demonstrated its activating role of autophagy. Depletion of the autophagic genes and of EcR led to inhibition of the competence factor, betaFTZ-F1, which is required for ecdysone-mediated developmental transitions. Moreover, autophagy-incompetent female mosquitoes were unable to complete the second reproductive cycle and exhibited retardation and abnormalities in egg maturation. Thus, our study has revealed a novel function of programmed autophagy in maintaining egg maturation cycles in mosquitoes.

Sequences that direct subcellular traffic of the Drosophila methoprene-tolerant protein (MET) are located predominantly in the PAS domains

Methoprene-tolerant protein (MET) is a key mediator of antimetamorphic signaling in insects. MET belongs to the family of bHLH-PAS transcription factors which regulate gene expression and determine essential physiological and developmental processes. The ability of many bHLH-PAS proteins to carry out their functions is related to the patterns of intracellular trafficking, which are determined by specific sequences and indicate that a nuclear localization signal (NLS) or a nuclear export signal (NES) is present and active. Therefore, the identification of NLS and NES signals is fundamental in order to understand the intracellular signaling role of MET. Nevertheless, data on the intracellular trafficking of MET are inconsistent, and until now there hasn't been any data on potential NLS and NES sequences. To analyze the trafficking of MET we designed a number of expression vectors encoding full-length MET, as well as various derivatives, that were fused to yellow fluorescent protein (YFP). Confocal microscopy analysis of the subcellular distribution of YFP-MET indicated that while this protein was localized mainly in the nucleus, it was also observed in the cytoplasm. This suggested the presence of both an NLS and NES in MET. Our work has shown that each of the two PAS domains of MET (PAS-A and PAS-B, respectively) contain one NLS and one NES sequence. Additional NES activity was present in the C-terminal fragment. The NLS activity located in PAS-B was dependent on the presence of juvenile hormone (JH), the potential ligand for MET. In contrast to this, JH didn't seem to be required for the NLS in PAS-A to be active. However, on the basis of current knowledge about the function of PAS-A in other bHLH-PAS proteins, we suggest there might be other proteins that control the activity of the NLS and possibly the NES located in the PAS-A of MET. Thus, the intracellular trafficking of MET seems to be regulated by a rather complicated network of different factors.

Sterol regulation of metabolism, homeostasis, and development

Sterol metabolites are critical signaling molecules that regulate metabolism, development, and homeostasis. Oxysterols, bile acids (BAs), and steroids work primarily through cognate sterol-responsive nuclear hormone receptors to control these processes through feed-forward and feedback mechanisms. These signaling pathways are conserved from simple invertebrates to mammals. Indeed, results from various model organisms have yielded fundamental insights into cholesterol and BA homeostasis, lipid and glucose metabolism, protective mechanisms, tissue differentiation, development, reproduction, and even aging. Here, we review how sterols act through evolutionarily ancient mechanisms to control these processes.

Insect nuclear receptors

The nuclear receptors (NRs) of metazoans are an ancient family of transcription factors defined by conserved DNA- and ligand-binding domains (DBDs and LBDs, respectively). The Drosophila melanogaster genome project revealed 18 canonical NRs (with DBDs and LBDs both present) and 3 receptors with the DBD only. Annotation of subsequently sequenced insect genomes revealed only minor deviations from this pattern. A renewed focus on functional analysis of the isoforms of insect NRs is therefore required to understand the diverse roles of these transcription factors in embryogenesis, metamorphosis, reproduction, and homeostasis. One insect NR, ecdysone receptor (EcR), functions as a receptor for the ecdysteroid molting hormones of insects. Researchers have developed nonsteroidal ecdysteroid agonists for EcR that disrupt molting and can be used as safe pesticides. An exciting new technology allows EcR to be used in chimeric, ligand-inducible gene-switch systems with applications in pest management and medicine.

Nuclear receptor DHR4 controls the timing of steroid hormone pulses during Drosophila development

Pulses of the steroid hormone ecdysone are turned off periodically through nucleo-cytoplasmic oscillations of a nuclear receptor that counteracts the neuropeptide signaling pathway responsible for activating hormone pulses in Drosophila melanogaster.

In insects, precisely timed periodic pulses of the molting hormone ecdysone control major developmental transitions such as molts and metamorphosis. The synthesis and release of ecdysone, a steroid hormone, is itself controlled by PTTH (prothoracicotopic hormone). PTTH transcript levels oscillate with an 8 h rhythm, but its significance regarding the timing of ecdysone pulses is unclear. PTTH acts on its target tissue, the prothoracic gland (PG), by activating the Ras/Raf/ERK pathway through its receptor Torso, however direct targets of this pathway have yet to be identified. Here, we demonstrate that Drosophila Hormone Receptor 4 (DHR4), a nuclear receptor, is a key target of the PTTH pathway and establishes temporal boundaries by terminating ecdysone pulses. Specifically, we show that DHR4 oscillates between the nucleus and cytoplasm of PG cells, and that the protein is absent from PG nuclei at developmental times when low titer ecdysone pulses occur. This oscillatory behavior is blocked when PTTH or torso function is abolished, resulting in nuclear accumulation of DHR4, while hyperactivating the PTTH pathway results in cytoplasmic retention of the protein. Increasing DHR4 levels in the PG can delay or arrest development. In contrast, reducing DHR4 function in the PG triggers accelerated development, which is caused by precocious ecdysone signaling due to a failure to repress ecdysone pulses. Finally, we show that DHR4 negatively regulates the expression of a hitherto uncharacterized cytochrome P450 gene, Cyp6t3. Disruption of Cyp6t3 function causes low ecdysteroid titers and results in heterochronic phenotypes and molting defects, indicating a novel role in the ecdysone biosynthesis pathway. We propose a model whereby nuclear DHR4 controls the duration of ecdysone pulses by negatively regulating ecdysone biosynthesis through repression of Cyp6t3, and that this repressive function is temporarily overturned via the PTTH pathway by removing DHR4 from the nuclear compartment.

Steroid hormones play fundamental roles in development and disease. They are often released as pulses, thereby orchestrating multiple physiological and developmental changes throughout the body. Hormone pulses must be regulated in a way so that they have a defined beginning, peak, and end. In Drosophila, pulses of the steroid hormone ecdysone govern all major developmental transitions, such as the molts or the transformation of a larva to a pupa. While we have a relatively good understanding of how an ecdysone pulse is initiated, little is known about how hormone production is turned off. In this study, we identify a critical regulator of this process, the nuclear receptor DHR4. When we interfere with the function of DHR4 specifically in the ecdysone-producing gland, we find that larvae develop much faster than normal, and that this is caused by the inability to turn off ecdysone production. We show that DHR4 oscillates between cytoplasm and nucleus of ecdysone-producing cells under the control of a neuropeptide that regulates ecdysone production. When the neuropeptide pathway is inactive, DHR4 enters the nucleus and represses another gene, Cyp6t3, for which we show a novel role in the production of ecdysone.

Nitric oxide directly regulates gene expression during Drosophila development: need some gas to drive into metamorphosis?

Nitric oxide (NO) is an important second messenger involved in numerous biological processes, but how it regulates gene expression is not well understood. In this issue of Genes & Development, Cáceres and colleagues (pp. 1476-1485) report a critical requirement of NO as a direct regulator of gene expression through its binding to a heme-containing nuclear receptor in Drosophila. This may be an anciently evolved mechanism to coordinate behavior and metabolism during animal development.

Isoform-specific regulation of a steroid hormone nuclear receptor by an E3 ubiquitin ligase in Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone (20E) regulates gene transcription through the heterodimeric nuclear receptor composed of ecdysone receptor (EcR) and Ultraspiracle (USP). The EcR gene encodes three protein isoforms--A, B1, and B2--with variant N-terminal domains that mediate tissue and developmental stage-specific responses to 20E. Ariadne-1a is a conserved member of the RING finger family of ubiquitin ligases first identified in Drosophila melanogaster. Loss-of-function mutations at key cysteines in either of the two RING finger motifs, as well as general overexpression of this enzyme, cause lethality in pupae, which suggests a requirement in metamorphosis. Here, we show that Ariadne-1a binds specifically the isoform A of EcR and ubiquitylates it. Co-immunoprecipitation experiments indicate that the full sequence of EcRA is required for this binding. Protein levels of EcRA and USP change in opposite directions when those of ARI-1a are genetically altered. This is an isoform-specific, E3-dependent regulatory mechanism for a steroid nuclear receptor. Further, qRT-PCR experiments show that the ARI-1a levels lead to the transcriptional regulation of Eip78C, Eip74EF, Eip75B, and Br-C, as well as that of EcR and usp genes. Thus, the activity of this enzyme results in the regulation of dimerizing receptors at the protein and gene transcription levels. This fine-tuned orchestration by a conserved ubiquitin ligase is required during insect metamorphosis and, likely, in other steroid hormone-controlled processes across species.

NHR-23 dependent collagen and hedgehog-related genes required for molting

NHR-23, a conserved member of the nuclear receptor family of transcription factors, is required for normal development in Caenorhabditis elegans where it plays a critical role in growth and molting. In a search for NHR-23 dependent genes, we performed whole genome comparative expression microarrays on both control and nhr-23 inhibited synchronized larvae. Genes that decreased in response to nhr-23 RNAi included several collagen genes. Unexpectedly, several hedgehog-related genes were also down-regulated after nhr-23 RNAi. A homozygous nhr-23 deletion allele was used to confirm the RNAi knockdown phenotypes and the changes in gene expression. Our results indicate that NHR-23 is a critical co-regulator of functionally linked genes involved in growth and molting and reveal evolutionary parallels among the ecdysozoa.

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.

Ecdysone receptor expression and activity in adult Drosophila melanogaster

Disrupting components of the ecdysone/EcR/USP signaling pathway in insects leads to morphological defects and developmental arrest. In adult Drosophila melanogaster decreased EcR function affects fertility, lifespan, behavior, learning, and memory; however we lack a clear understanding of how EcR/USP expression and activity impacts these phenotypes. To shed light on this issue, we characterized the wild-type expression patterns and activity of EcR/USP in individual tissues during early adult life. EcR and usp were expressed in numerous adult tissues, but receptor activity varied depending on tissue type and adult age. Receptor activity did not detectably change in response to mating status, environmental stress, ecdysone treatment or gender but is reduced when a constitutively inactive ecdysone receptor is present. Since only a subset of adult tissues expressing EcR and usp contain active receptors, it appears that an important adult function of EcR/USP in some tissues may be repression of genes containing EcRE's.

Stage-specific regulation of juvenile hormone biosynthesis by ecdysteroid in Bombyx mori

In the penultimate (4th) instar larvae of Bombyx mori, juvenile hormone (JH) synthesis by corpora allata (CA) fluctuates. When diet containing 20-hydroxyecdysone (20E) was fed, JH synthetic activity of the CA was first stimulated as the ecdysteroid titer increased, then suppressed slightly by the higher molting concentration of ecdysteroids (>250 ng/ml). The overall JH biosynthetic activity was modulated by the expression of JH biosynthetic enzymes in the CA: primarily JH acid O-methyltransferase (JHAMT), isopentenyl diphosphate isomerase, and farnesyl diphosphate synthase 1. After the last (5th) larval ecdysis, the artificially increased high ecdysteroid level due to the 20E diet activated JH synthesis by the CA, which required intact nervous connections with the brain. A factor(s) from the 20E-activated brain controls mainly JHAMT and HMG Co-A reductase expression to stimulate the JH synthesis. In the normal last instar larvae, the ecdysteroid titer declines so that these activation mechanisms are absent; therefore the decline of the ecdysteroid titer after the final larval ecdysis is one of the factors which induces the cessation of the JH synthesis by CA.

Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

Nuclear hormone receptors (NHRs) are transcription factors that work in concert with co-activators and co-repressors to regulate gene expression. Some examples of ligands for NHRs include endogenous compounds such as bile acids, retinoids, steroid hormones, thyroid hormone, and vitamin D. This review describes the evolution of liver X receptors α and β (NR1H3 and 1H2, respectively), farnesoid X receptor (NR1H4), vitamin D receptor (NR1I1), pregnane X receptor (NR1I2), and constitutive androstane receptor (NR1I3). These NHRs participate in complex, overlapping transcriptional regulation networks involving cholesterol homeostasis and energy metabolism. Some of these receptors, particularly PXR and CAR, are promiscuous with respect to the structurally wide range of ligands that act as agonists. A combination of functional and computational analyses has shed light on the evolutionary changes of NR1H and NR1I receptors across vertebrates, and how these receptors may have diverged from ancestral receptors that first appeared in invertebrates.

Role of nuclear receptors in hepatitis B and C infections

Nuclear receptors are key regulators of many cellular functions including energy supply by the direct control of the expression of target genes. They constitute a super-family of transcription factors activated by ligands, hormones or metabolites, and therefore, sensible to host metabolic stimuli. Viral replication and production requires energy and elementary building blocks from the infected cells. Hepatitis B and C virus replication is modulated in part by liver nuclear receptors that regulate the glucose and lipid metabolism. However, nuclear receptors control the two viruses' replication by different mechanisms. The expression of hepatitis B virus genes is directly under the control of nuclear receptors, which bind to the viral genome regulatory regions. Viral replication and production may, therefore, be optimal when cells receive the correct metabolic signals. Hepatitis C virus replication and production depend to a large extent on lipidogenesis and lipoprotein secretion. The role of nuclear receptors in controlling hepatitis C replication may be to turn on the cellular mode that would provide the appropriate metabolic environment for viral replication.

Hormone receptor-like in 96 and Broad-Complex modulate phenobarbital induced transcription of cytochrome P450 CYP6D1 in Drosophila S2 cells

Phenobarbital (PB) is a prototypical inducer for studies of xenobiotic responses in animals. In mammals, the nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR) have been identified as key transcription factors regulating PB induced transcription of xenobiotic responsive genes. In insects, much less is known about the transcription factors involved in regulating PB induced transcription, although CAR and PXR have a single orthologue hormone receptor-like in 96 (HR96) in Drosophila melanogaster. Using dual luciferase reporter assays in Drosophila Schneider (S2) cells, constructs containing variable lengths of the promoter of the PB inducible cytochrome P450 CYP6D1 were evaluated in the presence and absence of PB. The promoter region between -330 and -280 (relative to the position of transcription start site, +1) was found to be critical for PB induction. Putative binding sites for Drosophila Broad-Complex (BR-C) and deformed (Dfd) were identified within this promoter region using TFsearch. RNA interference (RNAi) treatment of S2 cells in conjunction with CYP6D1 promoter assays showed that suppression of Drosophila HR96 and BR-C transcription in S2 cells resulted in a significant decrease and increase, respectively, of PB induction. Effects of HR96 and BR-C in mediating PB induction were PB specific and PB dependent. This represents new functional evidence that Drosophila HR96 and BR-C can act as an activator and repressor, respectively, in regulating PB induced transcription in insects.

Steroid hormone inactivation is required during the juvenile-adult transition in Drosophila

Steroid hormones are systemic signaling molecules that regulate juvenile-adult transitions in both insects and mammals. In insects, pulses of the steroid hormone 20-hydroxyecdysone (20E) are generated by increased biosynthesis followed by inactivation/clearance. Although mechanisms that control 20E synthesis have received considerable recent attention, the physiological significance of 20E inactivation remains largely unknown. We show that the cytochrome P450 Cyp18a1 lowers 20E titer during the Drosophila prepupal to pupal transition. Furthermore, this reduction of 20E levels is a prerequisite to induce βFTZ-F1, a key factor in the genetic hierarchy that controls early metamorphosis. Resupplying βFTZ-F1 rescues Cyp18a1-deficient prepupae. Because Cyp18a1 is 20E-inducible, it appears that the increased production of steroid is responsible for its eventual decline, thereby generating the regulatory pulse required for proper temporal progression of metamorphosis. The coupling of hormone clearance to βFTZ-F1 expression suggests a general mechanism by which transient signaling drives unidirectional progression through a multistep process.

Nuclear Receptors: Small Molecule Sensors that Coordinate Growth, Metabolism and Reproduction

One of the largest groups of metazoan transcription factors (TFs), the Nuclear Receptor superfamily, regulates genes required for virtually all aspects of development, reproduction and metabolism. Together, these master regulators can be thought of as a fundamental operating system for metazoan life. Their most distinguishing feature is a structurally conserved domain that acts as a switch, powered by the presence of small diffusible ligands. This ligand-responsive regulation has allowed the Nuclear Receptors to help their hosts adapt to a wide variety of physiological niches and roles, making them one of the most evolutionarily successful TF families. Originally discovered as receptors for steroid hormones, the Nuclear Receptor field has grown to encompass much more than traditional endocrinology. For example, recent work has highlighted the role of Nuclear Receptors as major regulators of metabolism and biological clocks. By monitoring endogenous metabolites and absorbed xenobiotics, these receptors also coordinate rapid, system-wide responses to changing metabolic and environmental states. While many new Nuclear Receptor ligands have been discovered in the past couple of decades, approximately half of the 48 human receptors are still orphans, with a significantly higher percentage of orphans in other organisms. The discovery of new ligands has led to the elucidation of new regulatory mechanisms, target genes, pathways and functions. This review will highlight both the common as well as newly emerging traits and functions that characterize this particularly unique and important TF family.

A tale of tailless

Drosophila Tailless(Tll) and its vertebrate homologue Tlx are conserved orphan nuclear receptors specifically expressed in the eye and the forebrain. Tll and Tlx act primarily as transcriptional repressors through their interactions with transcriptional corepressors, Atrophin family proteins, and histone-tail/chromatin-modifying factors such as lysine-specific histone demethylase 1 and histone deacetylases. The functional importance of Tll and Tlx is made apparent by the recent discovery that they are expressed in neural stem cells (NSCs) and are required for self-renewal of these cells in both Drosophila and the mouse. This review provides a snapshot of current knowledge about Tll and Tlx and their transcriptional network, which maintains NSCs in developing and adult animals.

Native functions of the androgen receptor are essential to pathogenesis in a Drosophila model of spinobulbar muscular atrophy

Spinobulbar muscular atrophy (SBMA) is a neurodegenerative disease caused by expansion of a polyglutamine tract in the androgen receptor (AR). This mutation confers toxic function to AR through unknown mechanisms. Mutant AR toxicity requires binding of its hormone ligand, suggesting that pathogenesis involves ligand-induced changes in AR. However, whether toxicity is mediated by native AR function or a novel AR function is unknown. We systematically investigated events downstream of ligand-dependent AR activation in a Drosophila model of SBMA. We show that nuclear translocation of AR is necessary, but not sufficient, for toxicity and that DNA binding by AR is necessary for toxicity. Mutagenesis studies demonstrated that a functional AF-2 domain is essential for toxicity, a finding corroborated by a genetic screen that identified AF-2 interactors as dominant modifiers of degeneration. These findings indicate that SBMA pathogenesis is mediated by misappropriation of native protein function, a mechanism that may apply broadly to polyglutamine diseases.

The function of nuclear receptors in regulation of female reproduction and embryogenesis in the red flour beetle, Tribolium castaneum

Nineteen canonical and two Knirps-like family nuclear receptors (NRs) were identified in the genome of Tribolium castaneum. The current study was conducted to determine the function of these NRs in regulation of female reproduction and embryogenesis. RNA interference (RNAi)-aided knock-down in the expression of genes coding for 21 NRs showed that seven NRs E75, hormone receptor 3 (HR3), ecdysone receptor (EcR), ultraspiracle (USP), seven-up (SVP), FTZ transcription factor 1 (FTZ-F1) and hormone receptor 4 (HR4) are required for successful vitellogenesis and oogenesis. Knocking down the expression of genes coding for these seven NRs affected egg production by reducing the levels of vitellogenin mRNAs as well as by affecting the oocyte maturation. Expression of seven additional NRs hormone receptor 96 (HR96), hormone receptor 51 (HR51), hormone receptor 38 (HR38), hormone receptor 39 (HR39), Tailless (Tll), Dissatisfaction (Dsf) and Knirps-like is required for successful embryogenesis. The knock-down in the expression of genes coding for three other NRs (E78, hepatocyte nuclear factor 4, HNF4 and Eagle) partially blocked embryogenesis. This study showed that at least 17 out of the 21 NRs identified in T. castaneum play key roles in female reproduction and embryogenesis.

Developmental expression of mRNAs for epidermal and fat body proteins and hormonally regulated transcription factors in the tobacco hornworm, Manduca sexta

This paper provides a compilation of diagrammatic representations of the expression profiles of epidermal and fat body mRNAs during the last two larval instars and metamorphosis of the tobacco hornworm, Manduca sexta. Included are those encoding insecticyanin, three larval cuticular proteins, dopa decarboxylase, moling, and the juvenile hormone-binding protein JP29 produced by the dorsal abdominal epidermis, and arylphorin and the methionine-rich storage proteins made by the fat body. The mRNA profiles of the ecdysteroid-regulated cascade of transcription factors in the epidermis during the larval molt and the onset of metamorphosis and in the pupal wing during the onset of adult development are also shown. These profiles are accompanied by a brief summary of the current knowledge about the regulation of these mRNAs by ecdysteroids and juvenile hormone based on experimental manipulations, both in vivo and in vitro.

Role of epithelial sodium channels and their regulators in hypertension

The kidney has a central role in the regulation of blood pressure, in large part through its role in the regulated reabsorption of filtered Na(+). Epithelial Na(+) channels (ENaCs) are expressed in the most distal segments of the nephron and are a target of volume regulatory hormones. A variety of factors regulate ENaC activity, including several aldosterone-induced proteins that are present within an ENaC regulatory complex. Proteases also regulate ENaC by cleaving the channel and releasing intrinsic inhibitory tracts. Polymorphisms or mutations within channel subunits or regulatory pathways that enhance channel activity may contribute to an increase in blood pressure in individuals with essential hypertension.

dDOR is an EcR coactivator that forms a feed-forward loop connecting insulin and ecdysone signaling

BACKGROUND

Mammalian DOR was discovered as a gene whose expression is misregulated in muscle of Zucker diabetic rats. Because no DOR loss-of-function mammalian models are available, we analyze here the in vivo function of DOR by studying flies mutant for Drosophila DOR (dDOR).

RESULTS

We show that dDOR is a novel coactivator of ecdysone receptor (EcR) that is needed during metamorphosis. dDOR binds EcR and is required for maximal EcR transcriptional activity. In the absence of dDOR, flies display a number of ecdysone loss-of-function phenotypes such as impaired spiracle eversion, impaired salivary gland degradation, and pupal lethality. Furthermore, dDOR knockout flies are lean. We find that dDOR expression is inhibited by insulin signaling via FOXO.

CONCLUSION

This work uncovers dDOR as a novel EcR coactivator. It also establishes a mutual antagonistic relationship between ecdysone and insulin signaling in the fly fat body. Furthermore, because ecdysone signaling inhibits insulin signaling in the fat body, this also uncovers a feed-forward mechanism whereby ecdysone potentiates its own signaling via dDOR.

Steroid hormone regulation of C. elegans and Drosophila aging and life history

In the last two decades it has become clear that hormones and gene mutations in endocrine signaling pathways can exert major effects on lifespan and related life history traits in worms, flies, mice, and other organisms. While most of this research has focused on insulin/insulin-like growth factor-1 signaling, a peptide hormone pathway, recent work has shown that also lipophilic hormones play an important role in modulating lifespan and other life history traits. Here we review how steroid hormones, a particular group of lipophilic hormones, affect life history traits in the nematode worm (Caenorhabditis elegans) and the fruit fly (Drosophila melanogaster), with a particular focus on longevity. Interestingly, a comparison suggests that parallel endocrine principles might be at work in worms and flies in these species and that steroid hormones interact with the gonad to affect lifespan.

Disturbance of perfluorooctanoic acid on development and behavior in Drosophila larvae

Perfluorooctanoic acid (PFOA) is a well-known perfluorinated compound (PFC), and its toxicological impact is currently of worldwide concern. In this study, we sought to evaluate the potential biological effects and modes of action of PFOA in a range of physiologically and developmentally related phenotypes in the fruit fly Drosophila melanogaster. The results clearly indicated that the toxic effects of PFOA at the organismal level were associated with the developmental status of the organism, with larvae being most sensitive to this chemical. Except for the decreased weight of both sexes and the reduced longevity of male adults, PFOA had a robust effect on larval development, as determined by reduced body volume, aberrant foraging behavior, molting arrest, and polyphasic lethality. Remarkably, nutrient supplementation of the diet efficiently rescued the lethal effect of high PFOA concentrations on larval development. This result indicated that PFOA probably competed with nutritional components, leading to a disruption of the metabolic pathways responsible for larval development.

The steroid hormone ecdysone controls systemic growth by repressing dMyc function in Drosophila fat cells

How steroid hormones shape animal growth remains poorly understood. In Drosophila, the main steroid hormone, ecdysone, limits systemic growth during juvenile development. Here we show that ecdysone controls animal growth rate by specifically acting on the fat body, an organ that retains endocrine and storage functions of the vertebrate liver and fat. We demonstrate that fat body-targeted loss of function of the Ecdysone receptor (EcR) increases dMyc expression and its cellular functions such as ribosome biogenesis. Moreover, changing dMyc levels in this tissue is sufficient to affect animal growth rate. Finally, the growth increase induced by silencing EcR in the fat body is suppressed by cosilencing dMyc. In conclusion, the present work reveals an unexpected function of dMyc in the systemic control of growth in response to steroid hormone signaling.

Genetic manipulation of AML1-ETO-induced expansion of hematopoietic precursors in a Drosophila model

Among mutations in human Runx1/AML1 transcription factors, the t(8;21)(q22;q22) genomic translocation that creates an AML1-ETO fusion protein is implicated in etiology of the acute myeloid leukemia. To identify genes and components associated with this oncogene we used Drosophila as a genetic model. Expression of AML1-ETO caused an expansion of hematopoietic precursors in Drosophila, which expressed high levels of reactive oxygen species (ROS). Mutations in functional domains of the fusion protein suppress the proliferative phenotype. In a genetic screen, we found that inactivation of EcRB1 or activation of Foxo and superoxide dismutase-2 (SOD2) suppress the AML1-ETO-induced phenotype by reducing ROS expression in the precursor cells. Our studies indicate that ROS is a signaling factor promoting maintenance of normal as well as the aberrant myeloid precursors and suggests the importance of antioxidant enzymes and their regulators as targets for further study in the context of leukemia.

Steroids make you bigger? Fat chance says Myc

In flies, ecdysone integrates growth with developmental transitions by antagonizing insulin signaling, which links growth with nutritional status. Work in Developmental Cell (Delanoue et. al, 2010) finds that ecdysone represses the transcription factor Myc in the larval fat body to inhibit systemic growth, revealing a mechanism for such coordination.

Perfluorooctane sulfonate accumulation and parasite infestation in a field population of the amphipod Monoporeia affinis after microcosm exposure

Perfluorooctane sulfonate (PFOS) is the focus of intense toxicity research due to its persistence and widespread occurrence in biota. Studies on benthic invertebrates have shown them to be subjects of high PFOS exposure. However, effects on benthic invertebrates exposed to PFOS in the field are still far from elucidated. To fill a knowledge gap on concentrations and effects in benthic invertebrates, a microcosm study on the benthic amphipod Monoporeia affinis was performed. Field collected M. affinis were analysed for PFOS and showed average background concentrations 39 and 58 ng/g (wet weight) in two different samplings. The field collected animals were exposed to three concentrations of PFOS (50, 200 and 5000 microg PFOS/L water) for 3 weeks during gonad development. Body burdens of PFOS were determined after experiment termination. Results showed negative effects on survival and reproduction effects such as decreased sexual maturation and decreased oocyte viability caused by PFOS exposure. Additionally, a follow-up experiment demonstrated a significant increase in the infection incidence by a microsporidian muscle parasite in animals exposed to PFOS at tissue concentrations in the range of concentrations found in field collected benthic amphipods. This is the first study to demonstrate increased microsporidian infection with pollutant exposure and it suggests that ecologically relevant PFOS concentrations could be sufficient to elicit these effects.

Interactions of the crustacean nuclear receptors HR3 and E75 in the regulation of gene transcription

Endocrine signal transduction occurs through cascades that involve the action of both ligand-dependent and ligand-independent nuclear receptors. In insects, two such nuclear receptors are HR3 and E75 that interact to transduce signals initiated by ecdysteroids. We have cloned these nuclear receptors from the crustacean Daphnia pulex to assess their function as regulators of gene transcription in this ecologically and economically important group of organisms. Both nuclear receptors from D. pulex (DappuHR3 (group NR1F) and DappuE75 (group NR1D)) exhibit a high degree of sequence similarity to other NR1F and NR1D group members that is indicative of monomeric binding to the RORE (retinoid orphan receptor element). DappuE75 possesses key amino acid residues required for heme binding to the ligand-binding domain. Next, we developed a gene transcription reporter assay containing a luciferase reporter gene driven by the RORE. DappuHR3, but not DappuE75, activated transcription of the luciferase gene in this system. Co-transfection experiments revealed that DappuE75 suppressed DappuHR3-dependent luciferase transcription in a dose-dependent manner. Electrophoretic mobility shift assays confirmed that DappuHR3 bound to the RORE. However, we found no evidence that DappuE75 similarly bound to the response element. These experiments further demonstrated that DappuE75 prevented DappuHR3 from binding to the response element. In conclusion, DappuHR3 functions as a transcriptional activator of genes regulated by the RORE and DappuE75 is a negative regulator of this activity. DappuE75 does not suppress the action of DappuHR3 by occupying the response element but presumably interacts directly with the DappuHR3 protein. Taken together with the previous demonstration that daphnid HR3 is highly induced by 20-hydroxyecdysone, these results support the premise that HR3 is a major component of ecdysteroid signaling in some crustaceans and is under the negative regulatory control of E75.

Ecdysone directly and indirectly regulates a cuticle protein gene, BMWCP10, in the wing disc of Bombyx mori

The hormonal regulation of cuticle protein genes is a good model to study the molecular mechanism of signaling by ecdysteroids, which initiates each of the major developmental transitions in insects. This study was conducted to clarify the regulation of the expression of an ecdysone-inducible cuticle protein gene, BMWCP10. Induction of the BMWCP10 transcript by ecdysone was partly inhibited in the presence of cycloheximide, which implies that the BMWCP10 promoter is directly and indirectly activated by ecdysone. Using electrophoretic mobility shift analysis and a competition experiment, we identified a putative ecdysone response element (EcRE1) located at positions -93 to -81 relative to the transcription start site. Site-directed mutagenesis of this site, followed by introduction into wing discs, dramatically abolished the reporter activity. This EcRE1 is necessary for the activation of the promoter by 20-hydroxyecdysone (20E) in the wing disc, since the mutation of EcRE1 caused loss of responsiveness to 20E. Collectively, the data obtained in our current and previous work indicate that ecdysone receptor and Broad-Complex Z2 (BR-Z2) are required for maximal BMWCP10 expression in wing disc.

Regulation of cytochrome P450 expression in Drosophila: Genomic insights

Genomic tools such as the availability of the Drosophila genome sequence, the relative ease of stable transformation, and DNA microarrays have made the fruit fly a powerful model in insecticide toxicology research. We have used transgenic promoter-GFP constructs to document the detailed pattern of induced Cyp6a2 gene expression in larval and adult Drosophila tissues. We also compared various insecticides and xenobiotics for their ability to induce this cytochrome P450 gene, and show that the pattern of Cyp6a2 inducibility is comparable to that of vertebrate CYP2B genes, and different from that of vertebrate CYP1A genes, suggesting a degree of evolutionary conservation for the "phenobarbital-type" induction mechanism. Our results are compared to the increasingly diverse reports on P450 induction that can be gleaned from whole genome or from "detox" microarray experiments in Drosophila. These suggest that only a third of the genomic repertoire of CYP genes is inducible by xenobiotics, and that there are distinct subsets of inducers / induced genes, suggesting multiple xenobiotic transduction mechanisms. A relationship between induction and resistance is not supported by expression data from the literature. The relative abundance of expression data now available is in contrast to the paucity of studies on functional expression of P450 enzymes, and this remains a challenge for our understanding of the toxicokinetic aspects of insecticide action.

Further understanding of fat biology: lessons from a fat fly

Obesity is a leading risk factor for insulin resistance, hypertension, hyperlipidemia, and cardiovascular complications, collectively referred to as metabolic diseases. Given the prevalence of obesity and its associated medical problems, new strategies are required to prevent or treat obesity and obesity-related metabolic effects. Here we summarize contributors of obesity, and molecular mechanisms controlling adipogenesis from studies in mammalian systems. We also discuss the possibilities of using Drosophila as a genetic model system to advance our understanding of players in fat biology.

Diversification of fasting regulated transcription in a cluster of duplicated nuclear hormone receptors in C. elegans

The genome of Caenorhabditis elegans encodes more than 280 nuclear hormone receptors (NHRs) in contrast to the 48 NHRs in humans and 18 NHRs in Drosophila. The majority of the C. elegans NHRs are categorized as supplementary nuclear receptors (supnrs) that evolved by successive duplications of a single ancestral gene. The evolutionary pressures that lead to the expansion of NHRs in nematodes, as well as the function of the majority of supnrs, are not known. Here, we have studied the expression of seven genes organized in a cluster on chromosome V: nhr-206, nhr-208, nhr-207, nhr-209, nhr-154, nhr-153 and nhr-136. Reverse transcription-quantitative PCR and analyses using transgenic lines carrying GFP fusion genes with their putative promoters revealed that all seven genes of this cluster are expressed and five have partially overlapping expression patterns including in the pharynx, intestine, certain neurons, the anal sphincter muscle, and male specific cells. Four genes in this cluster are conserved between C. elegans and Caenorhabditis briggsae whereas three genes are present only in C. elegans, the apparent result of a relatively recent expansion. Interestingly, we find that a subset of the conserved and non-conserved genes in this cluster respond transcriptionally to fasting in tissue-specific patterns. Our results reveal the diversification of the temporal, spatial, and metabolic gene expression patterns coupled with evolutionary drift within supnr family members.

The nuclear receptor DHR3 modulates dS6 kinase-dependent growth in Drosophila

S6 kinases (S6Ks) act to integrate nutrient and insulin signaling pathways and, as such, function as positive effectors in cell growth and organismal development. However, they also have been shown to play a key role in limiting insulin signaling and in mediating the autophagic response. To identify novel regulators of S6K signaling, we have used a Drosophila-based, sensitized, gain-of-function genetic screen. Unexpectedly, one of the strongest enhancers to emerge from this screen was the nuclear receptor (NR), Drosophila hormone receptor 3 (DHR3), a critical constituent in the coordination of Drosophila metamorphosis. Here we demonstrate that DHR3, through dS6K, also acts to regulate cell-autonomous growth. Moreover, we show that the ligand-binding domain (LBD) of DHR3 is essential for mediating this response. Consistent with these findings, we have identified an endogenous DHR3 isoform that lacks the DBD. These results provide the first molecular link between the dS6K pathway, critical in controlling nutrient-dependent growth, and that of DHR3, a major mediator of ecdysone signaling, which, acting together, coordinate metamorphosis.