Dynamic regulation of Drosophila nuclear receptor activity in vivo

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.

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.

A live zebrafish-based screening system for human nuclear receptor ligand and cofactor discovery

Nuclear receptors (NRs) belong to a superfamily of transcription factors that regulate numerous homeostatic, metabolic and reproductive processes. Taken together with their modulation by small lipophilic molecules, they also represent an important and successful class of drug targets. Although many NRs have been targeted successfully, the majority have not, and one third are still orphans. Here we report the development of an in vivo GFP-based reporter system suitable for monitoring NR activities in all cells and tissues using live zebrafish (Danio rerio). The human NR fusion proteins used also contain a new affinity tag cassette allowing the purification of receptors with bound molecules from responsive tissues. We show that these constructs 1) respond as expected to endogenous zebrafish hormones and cofactors, 2) facilitate efficient receptor and cofactor purification, 3) respond robustly to NR hormones and drugs and 4) yield readily quantifiable signals. Transgenic lines representing the majority of human NRs have been established and are available for the investigation of tissue- and isoform-specific ligands and cofactors.

Coordination of hepatocyte nuclear factor 4 and seven-up controls insect counter-defense cathepsin B expression

CmCatB, a cathepsin B-type cysteine protease, is insensitive to inhibition by the soybean cysteine protease inhibitor (scN). Cowpea bruchids dramatically induce CmCatB expression when major digestive proteases are inactivated by dietary scN, which is presumably an adaptive strategy that insects use to minimize effects of nutrient deficiency. In this study, we cloned the cowpea bruchid hepatocyte nuclear factor 4 (CmHNF-4) and demonstrated its involvement in transcriptional activation of CmCatB in the digestive tract of scN-adapted bruchids. Electrophoretic mobility shift assays demonstrated that CmHNF-4 binds to a CmCatB promoter region containing two tandem chicken ovalbumin upstream promoter (COUP) sites, which is also the cis-element for Seven-up (CmSvp), a previously identified transcriptional repressor of CmCatB. Although CmSvp is predominantly expressed in unadapted insect midgut, CmHNF-4 is more abundant in adapted bruchids. When transiently expressed in Drosophila S2 cells, CmHNF-4 substantially increased CmCatB expression through COUP binding. CmSvp inhibited CmHNF-4-mediated transcriptional activation even in the absence of its DNA-binding domain. Thus antagonism resulted, at least in part, from protein-protein interactions between CmSvp and CmHNF-4. Association of the two transcription factors was subsequently confirmed by glutathione S-transferase pulldown assays. Interestingly, anti-CmHNF-4 serum caused a supershift not only with nuclear extracts of scN-adapted insect midgut but with that of unadapted control insects as well. The presence of CmHNF-4 in unadapted insects further supported the idea that interplay between CmSvp and CmHNF-4 controls CmCatB transcription activation. Together, these results suggest that coordination between CmHNF-4 and CmSvp is important in counter-defense gene regulation in insects.

The unfulfilled gene is required for the development of mushroom body neuropil in Drosophila

BACKGROUND

The mushroom bodies (MBs) of Drosophila are required for complex behaviors and consist of three types of neurons, gamma, alpha'/beta' and alpha/beta. Previously, roles for transcription factors in MB neuronal differentiation have only been described for a subset of MB neurons. We are investigating the roles of unfulfilled (unf; HR51, CG16801) in MB development. unf encodes a nuclear receptor that is orthologous to the nuclear receptors fasciculation of axons defective 1 (FAX-1) of the nematode and photoreceptor specific nuclear receptor (PNR) of mammals. Based on our previous observations that unf transcripts accumulate in MB neurons at all developmental stages and the presence of axon pathfinding defects in fax-1 mutants, we hypothesized that unf regulates MB axon growth and pathfinding.

RESULTS

We show that unf mutants exhibit a range of highly penetrant axon stalling phenotypes affecting all neurons of the larval and adult MBs. Phenotypic analysis of unfX1 mutants revealed that alpha'/beta' and alpha/beta neurons initially project axons but stall prior to the formation of medial or dorsal MB lobes. unfZ0001 mutants form medial lobes, although these axons fail to branch, which results in a failure to form the alpha or alpha' dorsal lobes. In either mutant background, gamma neurons fail to develop larval-specific dorsal projections. These mutant gamma neurons undergo normal pruning, but fail to re-extend axons medially during pupal development. unfRNAi animals displayed phenotypes similar to those seen in unfZ0001 mutants. Unique asymmetrical phenotypes were observed in unfX1/unfZ0001 compound heterozygotes. Expression of UAS-unf transgenes in MB neurons rescues the larval and adult unf mutant phenotypes.

CONCLUSIONS

These data support the hypothesis that unf plays a common role in the development of all types of MB neurons. Our data indicate that unf is necessary for MB axon extension and branching and that the formation of dorsal collaterals is more sensitive to the loss of unf function than medial projections. The asymmetrical phenotypes observed in compound heterozygotes support the hypothesis that the earliest MB axons may serve as pioneers for the later-born MB neurons, providing evidence for pioneer MB axon guidance in post-embryonic development.

Nuclear receptor unfulfilled regulates axonal guidance and cell identity of Drosophila mushroom body neurons

Nuclear receptors (NRs) comprise a family of ligand-regulated transcription factors that control diverse critical biological processes including various aspects of brain development. Eighteen NR genes exist in the Drosophila genome. To explore their roles in brain development, we knocked down individual NRs through the development of the mushroom bodies (MBs) by targeted RNAi. Besides recapitulating the known MB phenotypes for three NRs, we found that unfulfilled (unf), an ortholog of human photoreceptor specific nuclear receptor (PNR), regulates axonal morphogenesis and neuronal subtype identity. The adult MBs develop through remodeling of γ neurons plus de-novo elaboration of both α′/β′ and α/β neurons. Notably, unf is largely dispensable for the initial elaboration of γ neurons, but plays an essential role in their re-extension of axons after pruning during early metamorphosis. The subsequently derived MB neuron types also require unf for extension of axons beyond the terminus of the pruned bundle. Tracing single axons revealed misrouting rather than simple truncation. Further, silencing unf in single-cell clones elicited misguidance of axons in otherwise unperturbed MBs. Such axon guidance defects may occur as MB neurons partially lose their subtype identity, as evidenced by suppression of various MB subtype markers in unf knockdown MBs. In sum, unf governs axonal morphogenesis of multiple MB neuron types, possibly through regulating neuronal subtype identity.

The DHR96 nuclear receptor controls triacylglycerol homeostasis in Drosophila

Triacylglycerol (TAG) homeostasis is an integral part of normal physiology and essential for proper energy metabolism. Here we show that the single Drosophila ortholog of the PXR and CAR nuclear receptors, DHR96, plays an essential role in TAG homeostasis. DHR96 mutants are sensitive to starvation, have reduced levels of TAG in the fat body and midgut, and are resistant to diet-induced obesity, while DHR96 overexpression leads to starvation resistance and increased TAG levels. We show that DHR96 function is required in the midgut for the breakdown of dietary fat and that it exerts this effect through the CG5932 gastric lipase, which is essential for TAG homeostasis. This study provides insights into the regulation of dietary fat metabolism in Drosophila and demonstrates that the regulation of lipid metabolism is an ancestral function of the PXR/CAR/DHR96 nuclear receptor subfamily.

DNA-binding properties of Drosophila ecdysone receptor isoforms and their modification by the heterodimerization partner ultraspiracle

Transcriptional activity of ecdysone receptor (EcR) isoforms varies considerably and is modified further by the heterodimerization partner and hormone treatment. To investigate whether differences in DNA binding of receptor complexes are responsible for these variations in transcriptional activity, interaction of Drosophila EcR isoforms, and variants of Ultraspiracle (Usp), the orthologue of RXR, with the ecdysone response elements (EcRE) hsp 27, PAL-1, and DR-1, were determined by electrophoretic mobility shift assays. Receptor proteins were expressed in vertebrate cells (CHO-K1) in order to rule out an influence of endogenous receptor proteins. In the absence of a heterodimerization partner, weak DNA binding of EcR was detected even without hormone with EcR-A and -B1, but not EcR-B2. In the presence of hormone, all three isoforms show increased binding to the hsp 27 EcRE. The heterodimerization partner Usp increased DNA binding considerably. The hormone effect of heterodimers is more pronounced with both EcR-B isoforms compared to EcR-A. Two specific bands were obtained for EcR-A and B1 but only one band is visible with EcR-B2. Deletion of the C-domain of Usp still allows basal DNA binding of the heterodimer, but in contrast to full-length Usp, addition of hormone decreases the intensity of the retarded receptor band of all EcR isoforms and the EcREs hsp27 and DR-1 considerably, whereas interaction with the EcRE PAL-1 is only slightly affected. Synergistic effects on transcriptional activity are associated with the formation of different receptor DNA-complexes observed with 1xhsp27 and 3xhsp27. Comparison of DNA-binding properties of EcR isoforms and EcR/Usp heterodimers revealed that binding of receptor complexes to hsp 27 EcRE is dependent on the AB domain of EcR and the AB-, C-, and D-domains of the heterodimerization partner. Interaction with the hsp 27 EcRE correlates neither with ligand binding nor with transcriptional activity of the various receptor complexes. We, therefore, conclude that the different receptor functions are regulated separately, for example, by interaction with co-modulators or post-transcriptional modifications.

Annotation, phylogenetics, and expression of the nuclear receptors in Daphnia pulex

BACKGROUND

The nuclear receptor superfamily currently consists of seven gene subfamilies that encompass over 80 distinct receptor proteins. These transcription factors typically share a common five-domain structure with a highly conserved DNA-binding domain. Some nuclear receptors are ubiquitous among the metazoans, while others are unique to specific phylogenetic groups. Crustaceans represent the second largest group of arthropods with insects being the largest. However, relative to insects, little is known about the nuclear receptors of crustaceans. The aim of this study was to identify putative nuclear receptors from the first assembled genome of a crustacean Daphnia pulex http://wFleaBase.org. Nuclear receptor expression was evaluated and receptors were subjected to phylogenetic analyses to gain insight into evolution and function.

RESULTS

Twenty-five putative nuclear receptors were identified in D. pulex based on the presence of a conserved DNA-binding domain. All of the nuclear receptor protein sequences contain a highly homologous DNA-binding domain and a less conserved ligand-binding domain with the exception of the NR0A group. These receptors lack a ligand-binding domain. Phylogenetic analysis revealed the presence of all seven receptor subfamilies. The D. pulex genome contains several nuclear receptors that have vertebrate orthologs. However, several nuclear receptor members that are represented in vertebrates are absent from D. pulex. Notable absences include receptors of the 1C group (peroxisome proliferators-activated receptors), the 3A group (estrogen receptor), and the 3C group (androgen, progestogen, mineralcorticoid, and glucocorticoid receptors). The D. pulex genome also contains nuclear receptor orthologs that are present in insects and nematodes but not vertebrates, including putative nuclear receptors within the NR0A group. A novel group of receptors, designated HR97, was identified in D. pulex that groups with the HR96/CeNHR8/48/DAF12 clade, but forms its own sub-clade. Gene products were detected in adult female D. pulex for 21 of the 25 receptors.

CONCLUSION

Nuclear receptors are ancient proteins with highly conserved DNA-binding domains. The DNA-binding domains of the nuclear receptors of D. pulex contain the same degree of conservation that is typically found within nuclear receptors of other species. Most of the receptors identified in D. pulex have orthologs within the vertebrate and invertebrate lineages examined with the exception of the novel HR97 group and the Dappu-HR10 and potentially the Dappu-HR11 receptors found in D. pulex. These groups of receptors may harbour functions that are intrinsic to crustacean physiology.

Arthropod nuclear receptors and their role in molting

The molting process in arthropods is regulated by steroid hormones acting via nuclear receptor proteins. The most common molting hormone is the ecdysteroid, 20-hydroxyecdysone. The receptors of 20-hydroxyecdysone have also been identified in many arthropod species, and the amino acid sequences determined. The functional molting hormone receptors consist of two members of the nuclear receptor superfamily, namely the ecdysone receptor and the ultraspiracle, although the ecdysone receptor may be functional, in some instances, without the ultraspiracle. Generally, the ecdysone receptor/ultraspiracle heterodimer binds to a number of ecdysone response elements, sequence motifs that reside in the promoter of various ecdysteroid-responsive genes. In the ensuing transcriptional induction, the ecdysone receptor/ultraspiracle complex binds to 20-hydroxyecdysone or to a cognate ligand that, in turn, leads to the release of a corepressor and the recruitment of coactivators. 3D structures of the ligand-binding domains of the ecdysone receptor and the ultraspiracle have been solved for a few insect species. Ecdysone agonists bind to ecdysone receptors specifically, and ligand-ecdysone receptor binding is enhanced in the presence of the ultraspiracle in insects. The basic mode of ecdysteroid receptor action is highly conserved, but substantial functional differences exist among the receptors of individual species. Even though the transcriptional effects are apparently similar for ecdysteroids and nonsteroidal compounds such as diacylhydrazines, the binding shapes are different between them. The compounds having the strongest binding affinity to receptors ordinarily have strong molting hormone activity. The ability of the ecdysone receptor/ultraspiracle complex to manifest the effects of small lipophilic agonists has led to their use as gene switches for medical and agricultural applications.

Pb2+: an endocrine disruptor in Drosophila?

Environmental exposure to Pb(2+) affects hormone-mediated responses in vertebrates. To help establish the fruit fly, Drosophila melanogaster, as a model system for studying such disruption, we describe effects of Pb(2+) on hormonally regulated traits. These include duration of development, longevity, females' willingness to mate, fecundity and adult locomotor activity. Developmental Pb(2+) exposure has been shown to affect gene expression in a specific region of the Drosophila genome (approximately 122 genes) involved in lead-induced changes in adult locomotion and to affect regulation of intracellular calcium levels associated with neuronal activity at identified synapses in the larval neuromuscular junction. We suggest ways in which Drosophila could become a new model system for the study of endocrine disruptors at genetic, neural and behavioral levels of analysis, particularly by use of genomic methods. This will facilitate efforts to distinguish between behavioral effects of Pb(2+) caused by direct action on neural mechanisms versus effects of Pb(+2) on behavior mediated through endocrine disruption.

Drosophila HNF4 regulates lipid mobilization and beta-oxidation

Drosophila HNF4 (dHNF4) is the single ancestral ortholog of a highly conserved subfamily of nuclear receptors that includes two mammalian receptors, HNFalpha and HNFgamma, and 269 members in C. elegans. We show here that dHNF4 null mutant larvae are sensitive to starvation. Starved mutant larvae consume glycogen normally but retain lipids in their midgut and fat body and have increased levels of long-chain fatty acids, suggesting that they are unable to efficiently mobilize stored fat for energy. Microarray studies support this model, indicating reduced expression of genes that control lipid catabolism and beta-oxidation. A GAL4-dHNF4;UAS-lacZ ligand sensor can be activated by starvation or exogenous long-chain fatty acids, suggesting that dHNF4 is responsive to dietary signals. Taken together, our results support a feed-forward model for dHNF4, in which fatty acids released from triglycerides activate the receptor, inducing enzymes that drive fatty acid oxidation for energy production.

Endocrine regulation of aging and reproduction in Drosophila

Hormonal signals can modulate lifespan and reproductive capacity across the animal kingdom. The use of model organisms such as worms, flies and mice has been fundamentally important for aging research in the discovery of genetic alterations that can extend healthy lifespan. The effects of mutations in the insulin and insulin-like growth factor-like signaling (IIS) pathways are evolutionarily conserved in that they can increase lifespan in all three animal models. Additionally, steroids and other lipophilic signaling molecules modulate lifespan in diverse organisms. Here we shall review how major hormonal pathways in the fruit fly Drosophila melanogaster interact to influence reproductive capacity and aging.

Coordinated responses to developmental hormones in the Kenyon cells of the adult worker honey bee brain (Apis mellifera L.)

The brains of experienced forager honey bees exhibit predictable changes in structure, including significant growth of the neuropil of the mushroom bodies. In vertebrates, members of the superfamily of nuclear receptors function as key regulators of neuronal structure. The adult insect brain expresses many members of the nuclear receptor superfamily, suggesting that insect neurons are also likely important targets of developmental hormones. The actions of developmental hormones (the ecdysteroids and the juvenile hormones) in insects have been primarily explored in the contexts of metamorphosis and vitellogenesis. The cascade of gene expression activated by 20-hydroxyecdysone and modulated by juvenile hormone is strikingly conserved in these different physiological contexts. We used quantitative RT-PCR to measure, in the mushroom bodies of the adult worker honey bee brain, relative mRNA abundances of key members of the nuclear receptor superfamily (EcR, USP, E75, Ftz-f1, and Hr3) that participate in the metamorphosis/vitellogenesis cascade. We measured responses to endogenous peaks of hormones experienced early in adult life and to exogenous hormones. Our studies demonstrate that a population of adult insect neurons is responsive to endocrine signals through the use of conserved portions of the canonical ecdysteroid transcriptional cascade previously defined for metamorphosis and vitellogenesis.

Unexpected novel relational links uncovered by extensive developmental profiling of nuclear receptor expression

Nuclear receptors (NRs) are transcription factors that are implicated in several biological processes such as embryonic development, homeostasis, and metabolic diseases. To study the role of NRs in development, it is critically important to know when and where individual genes are expressed. Although systematic expression studies using reverse transcriptase PCR and/or DNA microarrays have been performed in classical model systems such as Drosophila and mouse, no systematic atlas describing NR involvement during embryonic development on a global scale has been assembled. Adopting a systems biology approach, we conducted a systematic analysis of the dynamic spatiotemporal expression of all NR genes as well as their main transcriptional coregulators during zebrafish development (101 genes) using whole-mount in situ hybridization. This extensive dataset establishes overlapping expression patterns among NRs and coregulators, indicating hierarchical transcriptional networks. This complete developmental profiling provides an unprecedented examination of expression of NRs during embryogenesis, uncovering their potential function during central nervous system and retina formation. Moreover, our study reveals that tissue specificity of hormone action is conferred more by the receptors than by their coregulators. Finally, further evolutionary analyses of this global resource led us to propose that neofunctionalization of duplicated genes occurs at the levels of both protein sequence and RNA expression patterns. Altogether, this expression database of NRs provides novel routes for leading investigation into the biological function of each individual NR as well as for the study of their combinatorial regulatory circuitry within the superfamily.

NRs are key molecules controlling development, metabolism, and reproduction in metazoans. Since NRs are implicated in many human diseases such as cancer, metabolic syndrome, and hormone resistance, they are important pharmaceutical targets and are under intense scrutiny to better understand their biological functions. In the present study, we determined the expression patterns of all NR genes as well as their main transcriptional coregulators during zebrafish development. We used zebrafish because the transparency of its embryo allows us to perform whole-mount in situ hybridization from early development to late organogenesis. This complete developmental profiling offers an unprecedented view of NR expression during embryogenesis, uncovering their potential function during central nervous system and retina formation. We observed that in contrast to NR genes, only a few coregulators exhibit a restricted expression pattern, suggesting that tissue specificity of hormone action is conferred more by the receptors than by their coregulators. Lastly, by evolutionary analysis of expression pattern divergence of duplicated genes, we observed that neofunctionalization occurs at the levels of both protein sequence and mRNA expression patterns. Taken together, our data provide the starting point for functional analysis of an entire gene family during development and call for the study of the intersection between metabolism and development.

Identification and characterization of nuclear receptors from the red flour beetle, Tribolium castaneum

Nuclear receptors (NRs) are classified by the presence of a well-conserved DNA-binding domain and a less conserved ligand-binding domain and function as key control points in diverse signaling and metabolic pathways. NRs are switched on and off by small molecule ligands with properties similar to insecticides. Therefore, NRs are attractive targets for developing new insecticides. Nineteen canonical and two Knirps family NRs were identified in the genome of Tribolium castaneum. RNAi analysis showed that 10 out of the 19 canonical NRs, TcE75, TcHR3, TcHR4, TcEcR, TcUSP, TcFTZ-F1, TcHR51, SVP, TcHR38, TcHR39 are important for metamorphosis. Knocking down the expression of five NRs, TcTll, TcDsf, TcHNF4 and TcHR78 caused defects in production of offspring. TcHNF4, TcHR78, TCHR51 and TcDsf affected egg production and TcTll affected embryonic development. Knocking down the expression of non-canonical NR Knirps-like affected adults and caused reduction in egg production. The other Knirps family member, Eagle, and five canonical NRs, TcE78, TcHR83, TcHR96, TcPNR-like and TcERR did not show much effect on metamorphosis or production of offspring. Quantitative real-time reverse transcriptase analysis showed that the mRNA levels of all NRs tested were reduced in DsRNA injected larvae when compared to their levels in control larvae injected with bacterial malE DsRNA suggesting that the RNAi worked well but reduction in expression levels of some of the NRs did not affect metamorphosis or production of offspring.

Annotation of Tribolium nuclear receptors reveals an increase in evolutionary rate of a network controlling the ecdysone cascade

The Tribolium genome contains 21 nuclear receptors, representing all of the six known subfamilies. This first complete set for a coleopteran species reveals a strong conservation of the number and identity of nuclear receptors in holometabolous insects. Two novelties are observed: the atypical NR0 gene knirps is present only in brachyceran flies, while the NR2E6 gene is found only in Tribolium and in Apis. Using a quantitative analysis of the evolutionary rate, we discovered that nuclear receptors could be divided into two groups. In one group of 13 proteins, the rates follow the trend of the Mecopterida genome-wide acceleration. In a second group of five nuclear receptors, all acting early during the ecdysone cascade, we observed an even higher increase of the evolutionary rate during the early divergence of Mecopterida. We thus extended our analysis to the 12 classic ecdysone transcriptional regulators and found that six of them (ECR, USP, HR3, E75, HR4 and Kr-h1) underwent an increase in evolutionary rate at the base of the Mecopterida lineage. By contrast, E74, E93, BR, HR39, FTZ-F1 and E78 do not show this divergence. We suggest that coevolution occurred within a network of regulators that control the ecdysone cascade. The advent of Tribolium as a powerful model should allow a better understanding of this evolutionary event.

Spatial and temporal control of gene expression in Drosophila using the inducible GeneSwitch GAL4 system. I. Screen for larval nervous system drivers

There is a critical need for genetic methods for the inducible expression of transgenes in specific cells during development. A promising approach for this is the GeneSwitch GAL4 system of Drosophila. With GeneSwitch GAL4 the expression of upstream activating sequence (UAS) effector lines is controlled by a chimeric GAL4 protein that becomes active in the presence of the steroid RU486 (mifepristone). To improve the utility of this expression system, we performed a large-scale enhancer-trap screen for insertions that yielded nervous system expression. A total of 204 GeneSwitch GAL4 lines with various larval expression patterns in neurons, glia, and/or muscle fibers were identified for chromosomes I-III. All of the retained lines show increased activity when induced with RU486. Many of the lines reveal novel patterns of sensory neurons, interneurons, and glia. There were some tissue-specific differences in background expression, with muscles and glia being more likely to show activity in the absence of the inducing agent. However, >90% of the neuron-specific driver lines showed little or no background activity, making them particularly useful for inducible expression studies.

Lessons from "lower" organisms: what worms, flies, and zebrafish can teach us about human energy metabolism

A pandemic of metabolic diseases (atherosclerosis, diabetes mellitus, and obesity), unleashed by multiple social and economic factors beyond the control of most individuals, threatens to diminish human life span for the first time in the modern era. Given the redundancy and inherent complexity of processes regulating the uptake, transport, catabolism, and synthesis of nutrients, magic bullets to target these diseases will be hard to find. Recent studies using the worm Caenorhabditis elegans, the fly Drosophila melanogaster, and the zebrafish Danio rerio indicate that these “lower” metazoans possess unique attributes that should help in identifying, investigating, and even validating new pharmaceutical targets for these diseases. We summarize findings in these organisms that shed light on highly conserved pathways of energy homeostasis.

Structural and functional characterization of a novel type of ligand-independent RXR-USP receptor

Retinoid X receptor (RXR) and Ultraspiracle (USP) play a central role as ubiquitous heterodimerization partners of many nuclear receptors. While it has long been accepted that a wide range of ligands can activate vertebrate/mollusc RXRs, the existence and necessity of specific endogenous ligands activating RXR-USP in vivo is still matter of intense debate. Here we report the existence of a novel type of RXR-USP with a ligand-independent functional conformation. Our studies involved Tribolium USP (TcUSP) as representative of most arthropod RXR-USPs, with high sequence homology to vertebrate/mollusc RXRs. The crystal structure of the ligand-binding domain of TcUSP was solved in the context of the functional heterodimer with the ecdysone receptor (EcR). While EcR exhibits a canonical ligand-bound conformation, USP adopts an original apo structure. Our functional data demonstrate that TcUSP is a constitutively silent partner of EcR, and that none of the RXR ligands can bind and activate TcUSP. These findings together with a phylogenetic analysis suggest that RXR-USPs have undergone remarkable functional shifts during evolution and give insight into receptor-ligand binding evolution and dynamics.

Specific transcriptional responses to juvenile hormone and ecdysone in Drosophila

Previous studies have shown that ecdysone (E), and its immediate downstream product 20-hydroxyecdysone (20E), can have different biological functions in insects, suggesting that E acts as a distinct hormone. Here, we use Drosophila larval organ culture in combination with microarray technology to identify genes that are transcriptionally regulated by E, but which show little or no response to 20E. These genes are coordinately expressed for a brief temporal interval at the onset of metamorphosis, suggesting that E acts together with 20E to direct puparium formation. We also show that E74B, pepck, and CG14949 can be induced by juvenile hormone III (JH III) in organ culture, and that CG14949 can be induced by JH independently of protein synthesis. In contrast, E74A and E75A show no response to JH in this system. These studies demonstrate that larval organ culture can be used to identify Drosophila genes that are regulated by hormones other than 20E, and provide a basis for studying crosstalk between multiple hormone signaling pathways.

Functional interactions between the Moses corepressor and DHR78 nuclear receptor regulate growth in Drosophila

Expression of the Drosophila orphan nuclear receptor DHR78 is regulated by the steroid hormone ecdysone and is required for growth and viability during larval stages. In contrast to our understanding of its biological functions, however, relatively little is known about how DHR78 acts as a transcription factor. Here we show that DHR78 is an obligate partner for Moses (Middleman of seventy-eight signaling), a SAM (sterile alpha motif) domain-containing cofactor that requires DHR78 for its stability. Unlike other nuclear receptor cofactors, Moses has no obvious interaction domains and displays a unique binding specificity for DHR78. Moses acts as a corepressor, inhibiting DHR78 transcriptional activity independently of histone deacetylation. Consistent with their close association, DHR78 and Moses proteins are coexpressed during development and colocalize to specific genomic targets in chromatin. Moses mutants progress normally through early larval stages, like DHR78 mutants, but display an opposite overgrowth phenotype, with hypertrophy of adult tissues. Genetic interactions between DHR78 and moses result in a similar phenotype, suggesting that the relative dose of Moses and DHR78 regulates growth and prevents cancer. The tight functional association between DHR78 and Moses provides a new paradigm for understanding the molecular mechanisms by which cofactors modulate nuclear receptor signaling pathways.