Nuclear receptors are a superfamily of transcription factors restricted to animals. These transcription factors regulate a wide variety of genes with diverse roles in cellular homeostasis, development, and physiology. The origin and specificity of ligand binding within lineages of nuclear receptors (e.g., subfamilies) continues to be a focus of investigation geared toward understanding how the functions of these proteins were shaped over evolutionary history. Among early-diverging animal lineages, the retinoid X receptor (RXR) is first detected in the placozoan, Trichoplax adhaerens. To gain insight into RXR evolution, we characterized ligand- and DNA-binding activity of the RXR from T. adhaerens (TaRXR). Like bilaterian RXRs, TaRXR specifically bound 9-cis-retinoic acid, which is consistent with a recently published result and supports a conclusion that the ancestral RXR bound ligand. DNA binding site specificity of TaRXR was determined through protein binding microarrays (PBMs) and compared with human RXRɑ. The binding sites for these two RXR proteins were broadly conserved (∼85% shared high-affinity sequences within a targeted array), suggesting evolutionary constraint for the regulation of downstream genes. We searched for predicted binding motifs of the T. adhaerens genome within 1000 bases of annotated genes to identify potential regulatory targets. We identified 648 unique protein coding regions with predicted TaRXR binding sites that had diverse predicted functions, with enriched processes related to intracellular signal transduction and protein transport. Together, our data support hypotheses that the original RXR protein in animals bound a ligand with structural similarity to 9-cis-retinoic acid; the DNA motif recognized by RXR has changed little in more than 1 billion years of evolution; and the suite of processes regulated by this transcription factor diversified early in animal evolution.

Nuclear receptors are a superfamily of transcription factors restricted to animals. These transcription factors regulate a wide variety of genes with diverse roles in cellular homeostasis, development, and physiology. The origin and specificity of ligand binding within lineages of nuclear receptors (e.g., subfamilies) continues to be a focus of investigation geared toward understanding how the functions of these proteins were shaped over evolutionary history. Among early-diverging animal lineages, the retinoid X receptor (RXR) is first detected in the placozoan, Trichoplax adhaerens. To gain insight into RXR evolution, we characterized ligand- and DNA-binding activity of the RXR from T. adhaerens (TaRXR). Like bilaterian RXRs, TaRXR specifically bound 9-cis-retinoic acid, which is consistent with a recently published result and supports a conclusion that the ancestral RXR bound ligand. DNA binding site specificity of TaRXR was determined through protein binding microarrays (PBMs) and compared with human RXRɑ. The binding sites for these two RXR proteins were broadly conserved (∼85% shared high-affinity sequences within a targeted array), suggesting evolutionary constraint for the regulation of downstream genes. We searched for predicted binding motifs of the T. adhaerens genome within 1000 bases of annotated genes to identify potential regulatory targets. We identified 648 unique protein coding regions with predicted TaRXR binding sites that had diverse predicted functions, with enriched processes related to intracellular signal transduction and protein transport. Together, our data support hypotheses that the original RXR protein in animals bound a ligand with structural similarity to 9-cis-retinoic acid; the DNA motif recognized by RXR has changed little in more than 1 billion years of evolution; and the suite of processes regulated by this transcription factor diversified early in animal evolution.

The MYC oncoprotein regulates transcription of a large fraction of the genome as an obligatory heterodimer with the transcription factor MAX. The MYC:MAX heterodimer and MAX:MAX homodimer (hereafter MYC/MAX) bind Enhancer box (E-box) DNA elements (CANNTG) and have the greatest affinity for the canonical MYC E-box (CME) CACGTG. However, MYC:MAX also recognizes E-box variants and was reported to bind DNA in a “non-specific” fashion in vitro and in vivo. Here, in order to identify potential additional non-canonical binding sites for MYC/MAX, we employed high throughput in vitro protein-binding microarrays, along with electrophoretic mobility-shift assays and bioinformatic analyses of MYC-bound genomic loci in vivo. We identified all hexameric motifs preferentially bound by MYC/MAX in vitro, which include the low-affinity non-E-box sequence AACGTT, and found that the vast majority (87%) of MYC-bound genomic sites in a human B cell line contain at least one of the top 21 motifs bound by MYC:MAX in vitro. We further show that high MYC/MAX concentrations are needed for specific binding to the low-affinity sequence AACGTT in vitro and that elevated MYC levels in vivo more markedly increase the occupancy of AACGTT sites relative to CME sites, especially at distal intergenic and intragenic loci. Hence, MYC binds diverse DNA motifs with a broad range of affinities in a sequence-specific and dose-dependent manner, suggesting that MYC overexpression has more selective effects on the tumor transcriptome than previously thought.

Nuclear receptors (NRs) regulate gene expression by binding specific DNA sequences consisting of AG[G/T]TCA or AGAACA half site motifs in a variety of configurations. However, those motifs/configurations alone do not adequately explain the diversity of NR function in vivo. Here, a systematic examination of DNA binding specificity by protein-binding microarrays (PBMs) of three closely related human NRs—HNF4α, retinoid X receptor alpha (RXRα) and COUPTF2—reveals an HNF4-specific binding motif (H4-SBM), xxxxCAAAGTCCA, as well as a previously unrecognized polarity in the classical DR1 motif (AGGTCAxAGGTCA) for HNF4α, RXRα and COUPTF2 homodimers. ChIP-seq data indicate that the H4-SBM is uniquely bound by HNF4α but not 10 other NRs in vivo, while NRs PXR, FXRα, Rev-Erbα appear to bind adjacent to H4-SBMs. HNF4-specific DNA recognition and transactivation are mediated by residues Asp69 and Arg76 in the DNA-binding domain; this combination of amino acids is unique to HNF4 among all human NRs. Expression profiling and ChIP data predict ∼100 new human HNF4α target genes with an H4-SBM site, including several Co-enzyme A-related genes and genes with links to disease. These results provide important new insights into NR DNA binding.

The orphan nuclear receptor HR3 is essential for developmental switches during insect development and metamorphosis regulated by 20-hydroxyecdysone (20E). Reproduction of female mosquitoes of the major vector of Dengue fever, Aedes aegypti, is cyclic because of its dependence on blood feeding. 20E is an important hormone regulating vitellogenic events in this mosquito; however, any role for HR3 in 20E-driven reproductive events has not been known. Using RNA interference (RNAi) approach, we demonstrated that Aedes HR3 plays a critical role in a timely termination of expression of the vitellogenin (Vg) gene encoding the major yolk protein precursor. It is also important for downregulation of the Target-of-Rapamycin pathway and activation of programmed autophagy in the Aedes fat body at the end of vitellogenesis. HR3 is critical in activating betaFTZ-F1, EcRB and USPA, the expressions of which are highly elevated at the end of vitellogenesis. RNAi depletion of HR3 (iHR3) prior to the first gonadotrophic cycle affects a normal progression of the second gonadotrophic cycle. Most of ovaries 24 h post second blood meal from iHR3 females in the second cycle were small with follicles that were only slightly different in length from of those of resting stage. In addition, these iHR3 females laid a significantly reduced number of eggs per mosquito as compared to those of iMal and the wild type. Our results indicate an important role of HR3 in regulation of 20E-regulated developmental switches during reproductive cycles of A. aegypti females.

Mosquitoes are adapted to using vertebrate blood as a nutrient source to promote egg development and as a consequence serve as disease vectors. Blood-meal activated reproductive events in female mosquitoes are hormonally and nutritionally controlled with an insect steroid hormone 20-hydroxyecdysone (20E) playing a central role. The nuclear receptor E75 is an essential factor in the 20E genetic hierarchy, however functions of its three isoforms - E75A, E75B, and E75C - in mosquito reproduction are unclear. By means of specific RNA interference depletion of E75 isoforms, we identified their distinct roles in regulating the level and timing of expression of key genes involved in vitellogenesis in the fat body (an insect analog of vertebrate liver and adipose tissue) of the mosquito Aedes aegypti. Heme is required in a high level of expression of 20E-controlled genes in the fat body, and this heme action depends on E75. Thus, in mosquitoes, heme is an important signaling molecule, serving as a sensor of the availability of a protein meal for egg development. Disruption of this signaling pathway could be explored in the design of mosquito control approaches.

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.

Trabajo presentado en el 18th International Ecdysone Workshop, celebrado del 19 al 23 de julio en Ceske Budejovice (Republica Checa).

Metamorphosis in holometabolous insects is mainly based on the destruction of larval tissues. Intensive research in Drosophila melanogaster, a model of holometabolan metamorphosis, has shown that the steroid hormone 20-hydroxyecdysone (20E) signals cell death of larval tissues during metamorphosis. However, D. melanogaster shows a highly derived type of development and the mechanisms regulating apoptosis may not be representative in the insect class context. Unfortunately, no functional studies have been carried out to address whether the mechanisms controlling cell death are present in more basal hemimetabolous species. To address this, we have analyzed the apoptosis of the prothoracic gland of the cockroach Blattella germanica, which undergoes stage-specific degeneration just after the imaginal molt. Here, we first show that B. germanica has two inhibitor of apoptosis (IAP) proteins and that one of them, BgIAP1, is continuously required to ensure tissue viability, including that of the prothoracic gland, during nymphal development. Moreover, we demonstrate that the degeneration of the prothoracic gland is controlled by a complex 20E-triggered hierarchy of nuclear receptors converging in the strong activation of the death-inducer Fushi tarazu-factor 1 (BgFTZ-F1) during the nymphal–adult transition. Finally, we have also shown that prothoracic gland degeneration is effectively prevented by the presence of juvenile hormone (JH). Given the relevance of cell death in the metamorphic process, the characterization of the molecular mechanisms regulating apoptosis in hemimetabolous insects would allow to help elucidate how metamorphosis has evolved from less to more derived insect species.

Ecdysteroid hormones regulate key developmental processes throughout the life cycle of insects. 20-Hydroxyecdysone (20E) acts upon binding to a heterodimeric receptor formed by the nuclear receptors EcR and USP. The receptor, once 20E bounds to it, elicits cascades of gene expression that mediate and amplify the hormonal signal. The molecular characterization of the 20E-mediated hierarchy of transcription factors has been analyzed in detail in holometabolous insects, especially in Drosophila melanogaster, but rarely in more basal hemimetabolous species. Using the hemimetabolous species Blattella gemianica (German cockroach) as model, we have cloned and characterized five isoforms of B. germanica E75, a member of the nuclear receptor family participating in the 20E-triggered genetic hierarchy. The five isoforms present characteristic expression patterns during embryo and nymphal development, and experiments in vitro with fat body tissue have shown that the five isoforms display specific 20E responsiveness. RNAi experiments in vivo during the penultimate and last nymphal instars of B. germanica revealed that BgE75 is required for successfully complete nymphal-nymphal and nymphal-adult transitions. Detailed analysis of knockdown specimens during the last nymphal instar showed that BgE75 is required for the rise of circulating ecdysteroids that occurs towards the end of the instar. The main cause of ecdysteroid deficiency in BgE75 knockdowns is the premature stage-specific degeneration of the prothoracic gland. As a consequence, BgE75 knockdown nymphs do not molt, live for up to 90 days and start the adult developmental program properly, in spite of remaining as nymphs from a morphological point of view. Finally, RNAi of specific isoforms during the last nymphal instar of B. germanica has showed that they are functionally redundant. Furthermore, it also revealed the occurrence of a complex regulatory relationship among BgE75 isoforms, which is responsible of their sequential expression. (C) 2007 Elsevier Inc. All rights reserved.

Postembryonic development of holometabolous and hemimetabolous insects occurs through successive molts triggered by 20‐hydroxyecdysone (20E). The molecular action of 20E has been extensively studied in holometabolous insects, but data on hemimetabolous are scarce. We have demonstrated that during the nymphal development of the hemimetabolous insect Blattella germanica , 20E binds to the heterodimeric receptor formed by the nuclear receptors BgEcR‐A and BgRXR activating a cascade of gene expression, including the nuclear receptors BgE75 and BgHR3. Herein, we report the characterization of BgFTZ‐F1, another nuclear hormone receptor involved in 20E action. BgFTZ‐F1 is activated at the end of each instar, and RNAi has demonstrated that BgHR3 is needed for BgFTZ‐F1 activation, and that BgFTZ‐F1 has critical functions of during the last nymphal instar. Nymphs with silenced BgFTZ‐F1 cannot ecdyse, arrest development, and show structures of ectodermal origin duplicated. BgFTZ‐F1 also controls the timing of the ecdysteroid molting pulse. Developmental Dynamics 237:3179–3191, 2008. © 2008 Wiley‐Liss, Inc.

Trabajo presentado en el 17th International Ecdysone Workshop, celebrado en Ulm (Alemania) del 20 al 24 de junio de 2008.

Trabajo presentado en el 17th International Ecdysone Workshop, celebrado en Ulm (Alemania) del 20 al 24 de julio de 2008.

Trabajo presentado en la 9th International Conference on Juvenile Hormones, celebrada en York (UK) del 5 al 10 de agosto de 2007.

The molecular basis of ecdysteroid function during development has been analyzed in detail in holometabolous insects, especially in Drosophila melanogaster, but rarely in hemimetabolous. Using the hemimetabolous species Blattella germanica (German cockroach) as model, we show that the ecdysone receptor isoform-A (BgEcR-A) mRNA is present throughout the penultimate and last nymphal instars in all tissues analyzed (prothoracic gland, epidermis and fat body). To study the functions of BgEcR-A, we reduced its expression using systemic RNAi in vivo, and we obtained knockdown specimens. Examination of these specimens indicated that BgEcR-A during the last nymphal instar is required for nymphal survival, and that reduced expression is associated with molting defects, lower circulating ecdysteroid levels and defects in cell proliferation in the follicular epithelium. Some BgEcR-A knockdown nymphs survive to the adult stage. The features of these specimens indicate that BgEcR-A is required for adult-specific developmental processes, such as wing development, prothoracic gland degeneration and normal choriogenesis.

Ecdysteroids play a major role during developmental growth in insects. The more active form of these hormones, 20-hydroxyecdysone (20E), acts upon binding to its heterodimeric receptor, formed by the two nuclear receptors, EcR and RXR/USP. Functional characterization of USP has been exclusively conducted on the holometabolous insect Drosophila melanogaster. However, it has been impossible to extend such analysis to primitive-hemimetabolous insects since species of this group are not amenable to genetic analysis. The development of methodologies based on gene silencing using RNA interference (RNAi) after treatment with double-stranded RNA (dsRNA) in vivo has resolved such limitations. In this paper, we show that injection of dsRNA into the haemocoel of nymphs and adults of the cockroach Blattella germanica can be used to silence gene function in vivo. In our initial attempt to test RNAi techniques, we halted the expression of the adult-specific vitellogenin gene. We then used the same technique to silence the expression of the B. germanica RXR/USP (BgRXR) gene in vivo during the last nymphal instar. BgRXR knockdown nymphs progressed through the instar correctly but they arrested development at the end of the stage and were unable to molt into adults. The results described herein suggest that RXR/USP function, in relation to molting, is conserved across the insect Class.