Regulation of the ecdysteroid titer of Manduca sexta: reappraisal of the role of the prothoracic glands

Organs-specific metabolomics and anticholinesterase activity suggests a trade-off between metabolites for therapeutic advantages of Trillium govanianum Wall. ex D. Don

Trillium govanianum is traditionally used to treat innumerable alignments like sexual disorders, cancer, inflammation etc. Mainly rhizomes of T. govanianum have been explored for phytochemical profiling but comprehensive metabolomics of other parts has not been yet deeply investigated. Thus, current study was aimed for organs-specific (roots, rhizomes, rhizomatous buds, stems, leaves, and fruits) phytochemical profiling of T. govanianum via metabolomics approach. Targeted (steroidal saponins and free sugars) and non-targeted metabolomics were performed by UPLC-PDA/ELSD & UHPLC-Q-TOF-IMS. Among steroidal compounds, 20-hydroxyecdysone, pennogenin-3-O-β-chacotrioside, dioscin were found predominantly in all samples while diosgenin was identified only in rhizomes. Further, four free sugars viz. 2-deoxyribose (116.24 ± 1.26 mg/g: leaves), fructose (454.76 ± 12.14 mg/g: rhizomes), glucose (243.21 ± 7.53 mg/g: fruits), and galactose (69.06 ± 2.14 mg/g: fruits) were found significant in respective parts of T. govanianum. Elemental analysis of targeted samples was determined by atomic absorption spectrophotometer. Heavy metals (Cd, Hg, Pd, As) were absent while micro- (Mn, Na, Zn, Cu) and macro- (Ca, Fe, Mg, K) elements were found in all samples. Furthermore, UHPLC-Q-TOF-IMS had identified 103 metabolites based on their mass fragmentation patterns and 839 were tentatively predicted using METLIN database. The multivariate statistical analysis showed organs specific clustering and variance of metabolites. Apart from this, extracts were evaluated for in vitro anticholinesterase activity, and found potentials inhibitors with IC50 values 2.02 ± 0.15 to 27.65 ± 0.89 mg/mL and 3.58 ± 0.12 to 16.81 ± 2.48 mg/mL of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzyme, respectively. Thus, comprehensive metabolomics and anti-cholinesterase activity of different parts of T. govanianum would lay the foundation for improving medicinal importance and health benefits of T. govanianum.

Sterol Characteristics in Silkworm Brain and Various Tissues Characterized by Precise Sterol Profiling Using LC-MS/MS

Sterols, especially cholesterol (Chl), are fundamental for animal survival. Insects lacking the ability to synthesize Chl are sterol auxotrophic animals and utilize dietary Chl and phytosterols to survive. The sterols obtained from a diet are distributed to the tissues; however, sterol homeostasis in insect tissues remains to be elucidated. This study sought to understand the sterol characteristics of insect tissues through detailed sterol quantification and statistics. The combination of sterol quantification using liquid chromatography tandem mass spectrometry (LC-MS/MS) and principal component analysis (PCA) revealed tissue-specific sterol characteristics in the silkworm, Bombyx mori, a phytophagous insect. We found that insect tissues have tissue-intrinsic sterol profiles. The brain has a unique sterol composition as compared to other tissues—high concentration of Chl and less accumulation of phytosterols. Other tissues also have intrinsic sterol characteristics, which when defined by dietary sterols or Chl metabolites, indicate preference for a sterol and consistently manage their own sterol homeostasis. Though most tissues never change sterol profiles during development, the brain drastically changes its sterol profile at the wandering stage, indicating that it could alter sterol composition in preparation for metamorphosis. These results suggest the existence of tissue- and sterol-specific systems for sterol homeostasis in insects.

Ecdysone oxidase and 3-dehydroecdysone-3β-reductase contribute to the synthesis of ecdysone during early embryonic development of the silkworm

Maternal ecdysteroids regulate a variety of cellular processes during early embryonic development of insects, yet little is known about the genes involved in the biosynthesis of these hormones. In this study, we found that ecdysone oxidase (EO) gene, which encodes an enzyme to catalyze ecdysone (or 20-hydroxyecdysone, 20E) to 3-dehydroecdysone (3DE), was highly expressed in the mature ovaries of the domestic silkworm, Bombyx mori. B. mori EO (BmEO) was localized in the cytoplasm around the yolk granules of oocyte. Furthermore, the down-regulated expression of the BmEO gene using RNA interference could not affect normal development of the female silkworm, but lower the 20E titer and hatching rate of its offspring. Rescue experiments by injecting the product (3DE) of BmEO can significantly elevate the 20E level and hatching rate of the BmEO RNAi offspring. Meanwhile, during embryonic stage, the down-regulating expression of 3DE-3β-reductase, which can reduce 3DE into ecdysone, also lowered the 20E titer. Taken together, our results prove that 3DE can be synthesized from ecdysone in maternal ovary yolk granules, and then the maternal 3DE is converted into active ecdysone during the early embryonic development of offspring. Thus, our findings reveal a new pathway to explain the origin of high 20E level before the formation the prothoracic gland in the silkworm.

Circadian rhythms in insect disease vectors

Organisms from bacteria to humans have evolved under predictable daily environmental cycles owing to the Earth’s rotation. This strong selection pressure has generated endogenous circadian clocks that regulate many aspects of behaviour, physiology and metabolism, anticipating and synchronising internal time-keeping to changes in the cyclical environment. In haematophagous insect vectors the circadian clock coordinates feeding activity, which is important for the dynamics of pathogen transmission. We have recently witnessed a substantial advance in molecular studies of circadian clocks in insect vector species that has consolidated behavioural data collected over many years, which provided insights into the regulation of the clock in the wild. Next generation sequencing technologies will facilitate the study of vector genomes/transcriptomes both among and within species and illuminate some of the species-specific patterns of adaptive circadian phenotypes that are observed in the field and in the laboratory. In this review we will explore these recent findings and attempt to identify potential areas for further investigation.

The POU factor ventral veins lacking/Drifter directs the timing of metamorphosis through ecdysteroid and juvenile hormone signaling

Although endocrine changes are known to modulate the timing of major developmental transitions, the genetic mechanisms underlying these changes remain poorly understood. In insects, two developmental hormones, juvenile hormone (JH) and ecdysteroids, are coordinated with each other to induce developmental changes associated with metamorphosis. However, the regulation underlying the coordination of JH and ecdysteroid synthesis remains elusive. Here, we examined the function of a homolog of the vertebrate POU domain protein, Ventral veins lacking (Vvl)/Drifter, in regulating both of these hormonal pathways in the red flour beetle, Tribolium castaneum (Tenebrionidae). RNA interference-mediated silencing of vvl expression led to both precocious metamorphosis and inhibition of molting in the larva. Ectopic application of a JH analog on vvl knockdown larvae delayed the onset of metamorphosis and led to a prolonged larval stage, indicating that Vvl acts upstream of JH signaling. Accordingly, vvl knockdown also reduced the expression of a JH biosynthesis gene, JH acid methyltransferase 3 (jhamt3). In addition, ecdysone titer and the expression of the ecdysone response gene, hormone receptor 3 (HR3), were reduced in vvl knockdown larvae. The expression of the ecdysone biosynthesis gene phantom (phm) and spook (spo) were reduced in vvl knockdown larvae in the anterior and posterior halves, respectively, indicating that Vvl might influence ecdysone biosynthesis in both the prothoracic gland and additional endocrine sources. Injection of 20-hydroxyecdysone (20E) into vvl knockdown larvae could restore the expression of HR3 although molting was never restored. These findings suggest that Vvl coordinates both JH and ecdysteroid biosynthesis as well as molting behavior to influence molting and the timing of metamorphosis. Thus, in both vertebrates and insects, POU factors modulate the production of major neuroendocrine regulators during sexual maturation.

Hormones play major roles in initiating major developmental transitions, such as puberty and metamorphosis. However, how organisms coordinate changes across multiple hormones remains unclear. In this study, we show that silencing the POU domain transcription factor Ventral veins lacking (Vvl)/Drifter in the red flour beetle Tribolium castaneum leads to precocious metamorphosis and an inability to molt. We show that Vvl regulates the biosynthesis and signaling of two key insect developmental hormones, juvenile hormone (JH) and ecdysteroids. Vvl therefore appears to act as a potential central regulator of developmental timing by influencing two major hormones. Because POU factors are known as a major regulator of the onset of puberty, POU factors play a major role during sexual maturation in both vertebrates and insects.

Amino acids and TOR signaling promote prothoracic gland growth and the initiation of larval molts in the tobacco hornworm Manduca sexta

Molting in arthropods is orchestrated by a series of endocrine changes that occur towards the end of an instar. However, little is understood about the mechanisms that trigger these endocrine changes. Here, nutritional inputs were manipulated to investigate the minimal nutritional inputs required for a Manduca sexta larva to initiate a molt. Amino acids were found to be necessary for a larva to molt, indicating the involvement of an amino acid sensitive pathway. Feeding rapamycin, an inhibitor of the target of rapamycin (TOR) signaling, delayed the onset of a molt and resulted in abnormally larger larvae. Rapamycin also suppressed the growth of the prothoracic glands relative to the whole body growth, and this was accompanied by suppression of ecdysone production and secretion. Higher doses of rapamycin also slowed the growth rate, indicating that TOR signaling also plays a role in systemic growth. TOR signaling therefore couples the nutritional status of the larva to the endocrine system to regulate the timing of a molt.

Molecular cloning and characterization of Ecdysone oxidase and 3-dehydroecdysone-3α-reductase involved in the ecdysone inactivation pathway of silkworm, Bombyx mori

Molting hormone (ecdysteroid) is one of the most important hormones in insects. The synthesis and inactivation of the ecdysteroid regulate the developmental process of insects. A major pathway of ecdysone inactivation is that ecdysone is converted to 3-dehydroecdysone, and then further to 3-epiecdysone in insects. Two enzymes (ecdysone oxidase: EO and 3DE-3α-reductase) participate in this pathway. In this study, based on the previously characterized cDNAs in Spodoptera littoralis, we cloned and characterized EO and 3DE-3α-reductase genes in the silkworm, Bombyx mori. The heterologously expressed proteins of the two genes in yeast showed the ecdysone oxidase and 3DE-3α-reductase activities, respectively. Expression of BmEO was only detected in the midgut at transcriptional and translational levels. We also localized EO within the midgut goblet cell cavities. For Bm3DE-3α-reductase gene, RT-PCR and western blot showed that it was expressed in the midgut and the Malpighian tubules. Moreover, we localized 3DE-3α-reductase within the midgut goblet cell cavities and the cytosol of principal cells of the Malpighian tubules. These two genes have similar expression profiles during different developmental stages. Both genes were highly expressed at the beginning of the 5^th instar, and remained a relative low level during the feeding stage, and then were highly expressed at the wandering stage. All these results showed that the profiles of the two genes were well correlated with the ecdysteroid titer. The functional characterization of the enzymes participating in ecdysone inactivation in the silkworm provides hints for the artificial regulation of the silkworm development and biological control of pests.

Cloning and characterization of the Bombyx mori ecdysone oxidase

The physiological titer of molting hormones in insects depends on relative activities of synthesis and degradation pathways. Ecdysone oxidase (EO) is a key enzyme in the inactivation of ecdysteroid. However, there are only a few reports on ecdysteroid inactivation and its enzymes in silkworm. In this study, we cloned and characterized the Bombyx mori EO (BmEO). The BmEO cDNA contains an ORF of 1,695 bp and the deduced protein sequence contains 564 amino acid residues. The deduced protein sequence contains two functional domains of glucose-methanol-choline oxidoreductase in N-terminal and C-terminal. Comparing the expression levels of BmEO in different tissues, high transcription was mainly present in hemocytes. Reduced expression of this enzyme is expected to lead to pathological accumulation of ecdysone in the hemolymph of silkworm larvae or pupae. Our data show that RNA inference of BmEO transcripts resulted in the accumulation of ecdysteroid and death of larvae or pupae. We infer that EO is a crucial element in the physiology of insect development.

Inhibitory effect of molt-inhibiting hormone on phantom expression in the Y-organ of the kuruma prawn, Marsupenaeus japonicus

Molting in crustaceans is induced by ecdysteroids as in insects. The ecdysteroid titre in hemolymph is negatively regulated by molt-inhibiting hormone (MIH) that inhibits the secretion of ecdysteroids from the Y-organ, an ecdysteroid-producing gland of crustaceans, whereas little is known about the molecular mechanism of inhibition by MIH. Recently, the Halloween genes encoding cytochrome P450 monooxygenases were characterized as the steroidogenic enzymes in insects. To elucidate whether the ecdysteroidogenesis in the Y-organ is regulated by molt-inhibiting hormone (MIH), we analyzed the expression level of an orthologue of a member of the Halloween genes, phantom (Cyp306a1, phm), in the Y-organ of a decapod crustacean, Marsupenaeus japonicus. A cDNA encoding phm (Mj-phm) was cloned by degenerate PCR and 5'- and 3'-RACEs. The deduced amino acid sequence of Mj-phm showed about 40% identity to those of insect phm. The six motif sequences and the four substrate recognition sites were well conserved between Mj-PHM and other PHM. RT-PCR showed the specific expression of Mj-phm mRNA in the Y-organ. In addition, quantitative real-time PCR verified that the expression level of Mj-phm was significantly increased at the pre-molt stage and decreased after ecdysis. Furthermore, exposure of the Y-organ to MIH significantly decreased the Mj-phm expression level in vitro. These results indicate that the transcription of Mj-phm in the Y-organ may be regulated by the inhibitory mechanism of MIH of M. japonicus, which involves the consequent negative regulation of ecdysteroidogenesis at the transcriptional level.

Studies on the Black Box: incorporation of 3-oxo-7-dehydrocholesterol into ecdysteroids by Drosophila melanogaster and Manduca sexta

It has long been hypothesized that the oxidation of 7-dehydrocholesterol (7dC), made from dietary cholesterol (C), to 3-oxo-7dC (3-oxo-Delta(5,7)C) immediately precedes the unknown "Black Box" oxidations that lead to the formation of the first up-stream intermediate exhibiting the highly characteristic ecdysteroid structure of the steroid molting hormone of insects, crustaceans and some other arthropods. Perhaps rate-limiting and under the control of the prothoracicotropic hormone (PTTH), the biosynthesis of 3-oxo-7dC and its subsequent oxidative modifications have been difficult to study because of their apparent instability, i.e. no intermediates between 7dC and the diketol (3-oxo-25,22,2-trideoxyecdysone) have ever been observed or identified in insect prothoracic gland incubations with radiolabelled precursors. However, we show that 3-oxo-7dC can be converted into lipophilic, photosensitive, ketone-blocked (PSKB) ketal derivatives which will release 3-oxo-7dC when and where desired following brief irradiation with innocuous long-wave (365 nm) UV-light both in vivo and in vitro. In this manner, 3-oxo-7dC is quickly and efficiently incorporated into ecdysteroids by adult male and female Drosophila raised on a diet containing the PSKB ketals and in prothoracic glands of Manduca sexta incubated with the ketals emulsified into media. The instability of 3-oxo-7dC and its spontaneous transformation into extensively electron-delocalized intermediates will be discussed in relation to a possible mechanism of the Black Box oxidations eventually leading to the production of the active molting hormone 20-hydroxyecdysone (20E).

Stage dependent influences of polydnaviruses and the parasitoid larva on host ecdysteroids

In the solitary egg-larval parasitoid Chelonus inanitus (Braconidae) both polydnavirus and the parasitoid larva manipulate host development. Parasitization leads to a premature drop in juvenile hormone titre and a precocious onset of metamorphosis in the 5th larval instar. The C. inanitus bracovirus (CiBV) alone causes a reduction in host ecdysteroid titres at the pupal cell formation stage and prevents pupation. Here we report three new findings. (1) We show that parasitization causes a reduction in haemolymph ecdysteroid titre immediately after the moult to the 5th instar; similarly low values were seen in nonparasitized larvae after the moult to the 6th instar. These data along with parasitoid removal experiments indicate that the low ecdysteroid titre after the moult is a very early sign of the upcoming metamorphosis. (2) In vitro experiments with prothoracic glands and brain extracts showed that CiBV affects both prothoracic glands and prothoracicotropic hormone after the stage of pupal cell formation. (3) In the haemolymph of parasitized larvae the ecdysteroid titre increased in the late cell formation stage, i.e. immediately before egression of the parasitoid. In vitro experiments showed that late 2nd instar parasitoids release ecdysteroids and are thus very likely responsible for the rise in host ecdysteroids.

Hormone-dependent expression of fasciclin II during ganglionic migration and fusion in the ventral nerve cord of the moth Manduca sexta

The ventral nerve cord of holometabolous insects is reorganized during metamorphosis. A prominent feature of this reorganization is the migration of subsets of thoracic and abdominal larval ganglia to form fused compound ganglia. Studies in the hawkmoth Manduca sexta revealed that pulses of the steroid hormone 20-hydroxyecdysone (20E) regulate ganglionic fusion, but little is known about the cellular mechanisms that make migration and fusion possible. To test the hypothesis that modulation of cell adhesion molecules is an essential component of ventral nerve cord reorganization, we used antibodies selective for either the transmembrane isoform of the cell adhesion receptor fasciclin II (TM-MFas II) or the glycosyl phosphatidylinositol-linked isoform (GPI-MFas II) to study cell adhesion during ganglionic migration and fusion. Our observations show that expression of TM-MFas II is regulated temporally and spatially. GPI-MFas II was expressed on the surface of the segmental ganglia and the transverse nerve, but no evidence was obtained for regulation of GPI-MFas II expression during metamorphosis of the ventral nerve cord. Manipulation of 20E titers revealed that TM-MFas II expression on neurons in migrating ganglia is regulated by hormonal events previously shown to choreograph ganglionic migration and fusion. Injections of actinomycin D (an RNA synthesis inhibitor) or cycloheximide (a protein synthesis inhibitor) blocked ganglionic movement and the concomitant increase in TM-MFas II, suggesting that 20E regulates transcription of TM-MFas II. The few neurons that showed TM-MFas II immunoreactivity independent of endocrine milieu were immunoreactive to an antiserum specific for eclosion hormone (EH), a neuropeptide regulator of molting.

Drosophila Niemann-Pick type C-2 genes control sterol homeostasis and steroid biosynthesis: a model of human neurodegenerative disease

Mutations in either of the two human Niemann-Pick type C (NPC) genes, NPC1 and NPC2, cause a fatal neurodegenerative disease associated with abnormal cholesterol accumulation in cells. npc1a, the Drosophila NPC1 ortholog, regulates sterol homeostasis and is essential for molting hormone (20-hydroxyecdysone; 20E) biosynthesis. While only one npc2 gene is present in yeast, worm, mouse and human genomes, a family of eight npc2 genes (npc2a-h) exists in Drosophila. Among the encoded proteins, Npc2a has the broadest expression pattern and is most similar in sequence to vertebrate Npc2. Mutation of npc2a results in abnormal sterol distribution in many cells, as in Drosophila npc1a or mammalian NPC mutant cells. In contrast to the ecdysteroid-deficient, larval-lethal phenotype of npc1a mutants, npc2a mutants are viable and fertile with relatively normal ecdysteroid level. Mutants in npc2b, another npc2 gene, are also viable and fertile, with no significant sterol distribution abnormality. However, npc2a; npc2b double mutants are not viable but can be rescued by feeding the mutants with 20E or cholesterol, the basic precursor of 20E. We conclude that npc2a functions redundantly with npc2b in regulating sterol homeostasis and ecdysteroid biosynthesis, probably by controlling the availability of sterol substrate. Moreover, npc2a; npc2b double mutants undergo apoptotic neurodegeneration, thus constituting a new fly model of human neurodegenerative disease.

Gluttony and sex in female ixodid ticks: how do they compare to other blood-sucking arthropods?

The central issue dealt with here is the role of copulation in the control of feeding behaviour in ticks and some haematophagous insects. Female ticks of the family Ixodidae normally engorge to approximately 100 x their unfed body weight, and then drop from the host, produce and lay eggs, and die. Virgins, on the other hand, normally do not exceed 5-40% (depending on species) of the normal engorged body weight. But instead of detaching voluntarily at that point most virgins remain fixed to the host for extended periods, waiting for males to find them so they can complete engorgement. Virgin haematophagous insects, and virgin ticks of the family Argasidae display little, if any, reduction in blood meal size compared to mated females, at least not during the first ovarian cycle. During subsequent ovarian cycles, meal size in some virgin insects may be somewhat reduced depending on how many eggs are retained in the reproductive tract, but the reduction is not nearly to the same extent as that observed for virgin ixodid females. The stimulatory effect of copulation on engorgement in the latter is caused by a pair of proteins (voraxin alpha and beta) produced in the testis and transferred to the female with the spermatophore. Here, I propose why it might be adaptive for an ixodid female to remain small until mated. The hypothesis is suggested from the facts that ixodid ticks remain attached to the host for days (rather than minutes), and that virgin ticks, above a certain critical weight, lose all opportunity for producing viable offspring should they be groomed off the host prematurely, or should the host die while ticks are still attached.

Ecdysteroids and juvenile hormones of whiteflies, important insect vectors for plant viruses

Ecdysteroids and juvenile hormones (JHs) regulate many physiological events throughout the insect life cycle, including molting, metamorphosis, ecdysis, diapause, reproduction, and behavior. Fluctuation of whitefly ecdysteroid levels and the identity of the whitefly molting hormone (20-hydroxyecdysone) have only been reported within the last few years. An ecdysteroid commitment peak that is associated with the reprogramming of tissues for a metamorphic molt in many holometabolous and some hemimetabolous insect species was not observed in last nymphal instars of either the sweet potato whitefly, Bemisia tabaci (Biotype B), or the greenhouse whitefly, Trialeurodes vaporariorum. Ecdysteroids reach peak levels 1-2 days prior to the initiation of the nymphal-adult metamorphic molt. Adult eye and wing differentiation which signal the onset of this molt begin earlier in 4th instar T. vaporariorum (Stages 4 and 5, respectively) than in B. tabaci (Stage 6), and the premolt peak is 3-4 times greater in B. tabaci ( approximately 400 fg/microg protein) than in T. vaporariorum ( approximately 120 fg/microg protein). The JH of B. tabaci nymphs and eggs was found to be JH III, supporting the view that JHs I and II are, with rare exception, only present in lepidopteran insects. In B. tabaci eggs, JH levels were approximately 10 times greater on day 2/3 (0.44 fg/egg or 0.54 ng/g) than on day 5 (0.04 fg/egg or 0.054 ng/g) post-oviposition. Approximately, 1.4 fg/2nd-3rd instar nymph (0.36 ng/g) was detected. It is probable that the relatively high level of JH in day 2/3 eggs is associated with the differentiation of various whitefly tissues during embryonic development.

The Halloween genes code for cytochrome P450 enzymes mediating synthesis of the insect moulting hormone

The developmental events occurring during moulting and metamorphosis of insects are controlled by precisely timed changes in levels of ecdysteroids, the moulting hormones. The final four sequential hydroxylations of steroid precursors into the active ecdysteroid of insects, 20E (20-hydroxyecdysone), are mediated by four cytochrome P450 (P450) enzymes, encoded by genes in the Halloween family. Orthologues of the Drosophila Halloween genes phantom (phm; CYP306A1), disembodied (dib; CYP302A1), shadow (sad; CYP315A1) and shade (shd; CYP314A1) were obtained from the endocrinological model insect, the tobacco hornworm Manduca sexta. Expression of these genes was studied and compared with changes in the ecdysteroid titre that controls transition from the larval to pupal stage. phm, dib and sad, which encode P450s that mediate the final hydroxylations in the biosynthesis of ecdysone, were selectively expressed in the prothoracic gland, the primary source of ecdysone during larval and pupal development. Changes in their expression correlate with the haemolymph ecdysteroid titre during the fifth (final) larval instar. Shd, the 20-hydroxylase, which converts ecdysone into the more active 20E, is expressed in tissues peripheral to the prothoracic glands during the fifth instar. Transcript levels of shd in the fat body and midgut closely parallel the enzyme activity measured in vitro. The results indicate that these Halloween genes are transcriptionally regulated to support the high biosynthetic activity that produces the cyclic ecdysteroid pulses triggering moulting.

Characterization in relation to development of an ecdysteroid agonist-responsive cytochrome P450, CYP18A1, in Lepidoptera

Cytochrome P450 enzymes are involved in a number of steps in ecdysteroid (moulting hormone) homeostasis in insects. We report the cloning and characterization of an ecdysteroid agonist-responsive cytochrome P450, CYP18A1, from the cotton leafworm, Spodoptera littoralis. Northern blot analysis showed that the mRNA transcript was expressed at times of increasing ecdysteroid titre in final instar S. littoralis larvae and was induced by the ecdysteroid receptor agonist, RH-5992, in midgut and fat body. In addition, transcript expression was also detected in the prothoracic glands, a major ecdysteroid biosynthetic tissue, in both S. littoralis and the tobacco hornworm, Manduca sexta, at a time of increasing ecdysteroid titre. The exact significance of the temporal and spatial expression of CYP18A1 is unclear. The characterization of a P450 that is ecdysteroid agonist-responsive may provide a future target for exploitation in the development of novel insect control strategies.

Marine invertebrate cytochrome P450: emerging insights from vertebrate and insects analogies

Cytochrome P450 enzymes (P450s) are responsible for the oxidative metabolism of a plethora of endogenous and exogenous substrates. P450s and associated activities have been demonstrated in numerous marine invertebrates belonging to the phyla Cnidaria, Annelida (Polychaeta), Mollusca, Arthropoda (Crustacea) and Echinodermata. P450s of marine invertebrates and vertebrates show considerable sequence divergence and the few orthologs reveal the selective constraint on physiologically significant enzymes. P450s are present in virtually all tissues of marine invertebrates, although high levels usually are found in hepatic-like organs and steroidogenic tissues. High-throughput technologies result in the rapid acquisition of new marine invertebrate P450 sequences; however, the understanding of their function is poor. Based on analogy to vertebrates and insects, it is likely that P450s play a pivotal role in the physiology of marine invertebrates by catalyzing the biosynthesis of signal molecules including steroids such as 20-hydroxyecdysone (the molting hormone of crustaceans). The metabolism of many exogenous compounds including benzo(a)pyrene (BaP), pyrene, ethoxyresorufin, ethoxycoumarin and aniline is mediated by P450 enzymes in tissues of marine invertebrates. P450 gene expression, protein levels and P450 mediated metabolism of xenobiotics are induced by PAHs in some marine invertebrate species. Thus, regulation of P450 enzyme activity may play a central role in the adaptation of animals to environmental pollutants. Emphasis should be put on the elucidation of the function and regulation of the ever-increasing number of marine invertebrate P450s.

Circadian orchestration of developmental hormones in the insect, Rhodnius prolixus

This review presents a new perspective on the circadian regulation and functions of insect developmental hormones. In Rhodnius prolixus (Hemiptera), the brain neuropeptide prothoracicotropic hormone (PTTH) is released with a circadian rhythm that is controlled by paired photosensitive clocks in the brain. These clocks comprise the dorsal and lateral PER/TIM clock neurons known to regulate behavioral rhythms in Drosophila. Axons of PTTH and clock cells make close contact. Photosensitive PER/TIM clocks also reside in the paired prothoracic glands (PGs), which generate rhythmic synthesis and release of the ecdysteroid molting hormones. The PG clocks are entrained by both light and PTTH. These four clocks are coupled together by both nerves and hormones into a timing system whose primary regulated output is the circadian rhythm of ecdysteroids in the hemolymph. This complex timing system appears necessary to ensure circadian organization of the gene expression that is induced in target cells by ecdysteroids via circadian cycling of the nuclear ecdysteroid receptor (EcR). This multioscillator system serves to transduce 'the day outside' into endocrine rhythms that orchestrate 'the day inside'. It has many functional similarities with vertebrate circadian systems.

Discrete pulses of molting hormone, 20-hydroxyecdysone, during late larval development of Drosophila melanogaster: correlations with changes in gene activity

Periodic pulses of the insect steroid molting hormone 20-hydroxyecdysone (20E), acting via its nuclear receptor complex (EcR/USP), control gene expression at many stages throughout Drosophila development. However, during the last larval instar of some lepidopteran insects, subtle changes in titers of ecdysteroids have been documented, including the so-called "commitment peak." This small elevation of 20E reprograms the larva for metamorphosis to the pupa. Similar periods of ecdysteroid immunoreactivity have been observed during the last larval instar of Drosophila. However, due to low amplitude and short duration, along with small body size and staging difficulties, their timing and ecdysteroid composition have remained uncertain. Employing a rigorous regimen of Drosophila culture and a salivary gland reporter gene, Sgs3-GFP, we used RP-HPLC and differential ecdysteroid RIA analysis to determine whole body titers of 20E during the last larval instar. Three small peaks of 20E were observed at 8, 20, and 28 hr following ecdysis, prior to the well-characterized large peak around the time of pupariation. The possible regulation of 20E levels by biosynthetic P450 enzymes and the roles of these early peaks in coordinating gene expression and late larval development are discussed.

Developmental expression of Manduca shade, the P450 mediating the final step in molting hormone synthesis

The ecdysone 20-monooxygenase (E20MO; 20-hydroxylase) is the enzyme that mediates the conversion of ecdysone (E) to the active insect molting hormone, 20-hydroxyecdysone (20E), which coordinates developmental progression. We report the identification and developmental expression of the Halloween gene shade (shd; CYP314A1) that encodes the E20MO in the tobacco hornworm, Manduca sexta. Manduca Shd (MsShd) mediates the conversion of E to 20E when expressed in Drosophila S2 cells. In accord with the central dogma, the data show that Msshd is expressed mainly in the midgut, Malpighian tubules, fat body and epidermis with very low expression in the prothoracic gland and nervous system. Developmental variations in E20MO enzymatic activity are almost perfectly correlated with comparable changes in the gene expression of Msshd in the fat body and midgut during the fifth instar and the beginning of pupal-adult development. The results indicate three successive and overlapping peaks of expression in the fat body, midgut and Malpighian tubules, respectively, during the fifth larval instar. The data suggest that precise tissue-specific transcriptional regulation controls the levels, and thereby the activity, of the Manduca E20MO.

Identification, characterization and developmental expression of Halloween genes encoding P450 enzymes mediating ecdysone biosynthesis in the tobacco hornworm, Manduca sexta

The insect molting hormone 20-hydroxyecdysone (20E) plays a central role in regulating gene expression during development and metamorphosis. In many Lepidoptera, the pro-hormone 3-dehydroecdysone (3DE), synthesized from cholesterol in the prothoracic gland, is rapidly converted to ecdysone (E) by a hemolymph reductase, and E is subsequently converted to 20E in various peripheral target tissues. Recently, four Drosophila melanogaster P450 enzymes, encoded by specific Halloween genes, were cloned and functionally characterized as mediating the last hydroxylation steps leading to 20E. We extended this work to the tobacco hornworm Manduca sexta, an established model for endocrinological and developmental studies. cDNA clones were obtained for three Manduca orthologs of CYP306A1 (phantom; phm, the 25-hydroxylase), CYP302A1 (disembodied; dib, the 22-hydroxylase) and CYP315A1 (shadow; sad, the 2-hydroxylase), expressed predominantly in the prothoracic gland during the fifth (final) larval instar and during pupal-adult development, with fifth instar mRNA levels closely paralleling the hemolymph ecdysteroid titer. The data indicate that transcriptional regulation of phm, dib and sad plays a role in the developmentally varying steroidogenic capacities of the prothoracic glands during the fifth instar. The consistent expression of the Halloween genes confirms the importance of the prothoracic glands in pupal-adult development. These studies establish Manduca as an excellent model for examining the regulation of the Halloween genes.

molting defective is required for ecdysone biosynthesis

20-hydroxyecdysone was discovered as the major biologically active insect steroid hormone half a century ago, yet much remains to be learned about its biosynthesis and its activities. 20-hydroxyecdysone controls many biological processes, including progression between larval stages, entry to pupariation and metamorphosis. A number of genes required for 20-hydroxyecdysone production have been identified, including those encoding enzymes that mediate four of the late steps of biosynthesis. A second smaller group of low ecdysone mutants do not encode enzymes. Here, we report identification of one such gene, which we call molting defective, on the basis of its lethal phenotype. molting defective encodes a nuclear zinc finger protein required for ecdysone biosynthesis.

A Molecular Genetic Approach to the Biosynthesis of the Insect Steroid Molting Hormone

Insect growth, development, and molting depend upon a critical titer of the principal molting hormone of arthropods, 20-hydroxyecdysone (20E). Although the structure of 20E as a polyhydroxylated steroid was determined more than five decades ago, the exact steps in its biosynthesis have eluded identification. Over the past several years, the use of the fly database and the techniques and paradigms of biochemistry, analytical chemistry, and molecular genetics have allowed the cloning and sequencing of four genes in the Halloween gene family of Drosophila melanogaster, all of them encoding cytochrome P450 (CYP) enzymes, each of which mediates one of the four terminal hydroxylation steps in 20E biosynthesis. Further, the sequence of these hydroxylations has been determined, and developmental alterations in the expression of each of these genes have been quantified during both embryonic and postembryonic life.

Phantom encodes the 25-hydroxylase of Drosophila melanogaster and Bombyx mori: a P450 enzyme critical in ecdysone biosynthesis

We have reported recently the identification and characterization of the last three mitochondrial cytochrome P450 enzymes (CYP) controlling the biosynthesis of 20-hydroxyecdysone, the molting hormone of insects. These are encoded by the following genes: disembodied (dib, Cyp302a1, the 22-hydroxylase); shadow (sad, Cyp315a1, the 2-hydroxylase); and shade (shd, Cyp314a1, the 20-hydroxylase). Employing similar gene identification and transfection techniques and subsequent biochemical analysis of the expressed enzymatic activity, we report the identity of the Drosophila gene phantom (phm), located at 17D1 of the X chromosome, as encoding the microsomal 25-hydroxylase (Cyp306a1). Similar analysis following differential display-based gene identification has also resulted in the characterization of the corresponding 25-hydroxylase gene in Bombyx mori. Confirmation of 2,22,25-trideoxyecdysone (3beta,5beta-ketodiol) conversion to 2,22-dideoxyecdysone (3beta,5beta-ketotriol) mediated by either Phm enzyme employed LC, MS and definitive NMR analysis. In situ developmental gene analysis, in addition to northern, western and RT-PCR techniques during Drosophila embryonic, larval and adult development, are consistent with this identification. That is, strong expression of phm is restricted to the prothoracic gland cells of the Drosophila larval ring gland, where it undergoes dramatic changes in expression, and in the adult ovary, but also in the embryonic epidermis. During the last larval-larval transition in Bombyx, a similar expression pattern in the prothoracic gland is observed, but as in Drosophila, slight expression is also present in other tissues, suggesting a possible additional role for the phantom enzyme.

A protein from the cabbage looper, Trichoplusia ni, regulated by a bacterial infection is homologous to 3-dehydroecdysone 3beta-reductase

During the screening of immune-regulated genes from the cabbage looper, Trichoplusia ni, a 3-dehydroecdysone 3beta-reductase homologue (DERH) was cloned. In the course of development, 3-dehydroecdysone 3beta-reductase mediates the conversion of 3-dehydroecdysone (3dE) secreted from the prothoracic glands to ecdysone (E), which is subsequently converted to 20-hydroxyecdysone (20E), the major insect molting hormone. The cloned gene is upregulated in fat body during development and is strongly induced after the larva is challenged with bacteria. The gene codes for a 308 amino acid residue protein which shows 42.5% identity to Spodoptera littoralis 3-dehydroecdysone 3beta-reductase. Using the baculovirus expression system, the recombinant DERH was expressed. The purified protein mediates the reduction of 3-dehydromakisterone A to makisterone A, and requires NADPH as a cofactor. Western blots using an antiserum to T. ni DERH revealed the presence of the protein in larval hemolymph and integument. The data indicate that the protein is regulated developmentally and is induced after a challenge with bacteria. Immunohistochemical studies localized the enzyme exclusively in the epidermis and the cuticle.

Molecular cloning and induction of nuclear receptors from insect cell lines

Fragments of EcR and USP were cloned from two insect cell lines, Sf21 and High Five cells (derived respectively from Spodoptera frugiperda and Trichoplusia ni), using a PCR-based approach employing degenerate primers designed on the basis of conserved regions of nuclear receptors, together with 5'- and 3'-RACE. An additional orphan nuclear receptor, HR4 fragment, was cloned from High Five cells. Comparison of these fragments with Manduca sexta counterparts showed that the cloned SfEcR [ecdysone receptor (EcR) from Sf21 cells] had high similarity to MsEcR-B1, whereas the cloned SfUSP [ultraspiracle (USP) from Sf21 cells] and TnUSP (USP from High Five cells) matched more closely to MsUSP-2 than to MsUSP-1. The TnHR4 showed most similarity to a recently cloned Bombyx mori GRF. While EcR and USP were constitutively expressed in both cell lines, HR4 was barely detectable by Northern blot analysis in High Five cells. Treatment with 20-hydroxyecdysone (20E) and agonist RH-5992 enhanced transcription of EcR in both cell lines, while the transcription of USP was suppressed in High Five cells. Such suppressed USP transcription was not observed in Sf21 cells. Transcription of TnEcR could also be enhanced by ecdysone and 3-dehydroecdysone, whereas transcription of SfEcR was unchanged with these two ecdysteroid compounds. Induction of HR4 transcription was also observed with 20E, RH-5992, ecdysone and 3-dehydroecdysone. The protein synthesis inhibitor, cycloheximide, superinduced expression of EcR and HR4 and restored the 20E/RH-5992-suppressed expression of TnUSP in the cells. Northern blot analysis also revealed that PCR, using degenerate USP primers, was able to amplify some other orphan nuclear receptors and their expression was inducible by 20E and RH-5992 and some of them were superinducible by cycloheximide.

Control and biochemical nature of the ecdysteroidogenic pathway

Molting is elicited by a critical titer of ecdysteroids that includes the principal molting hormone, 20-hydroxyecdysone (20E), and ecdysone (E), which is the precursor of 20E but also has morphogenetic roles of its own. The prothoracic glands are the predominate source of ecdysteroids, and the rate of synthesis of these polyhydroxylated sterols is critical for molting and metamorphosis. This review concerns three aspects of ecdysteroidogenesis: (a) how the brain neuropeptide prothoracicotropic hormone (PTTH) initiates a transductory cascade in cells of the prothoracic gland, which results in an increased rate of ecdysteroid biosynthesis (upregulation); (b) how the concentrations of 20E in the hemolymph feed back on the prothoracic gland to decrease rates of ecdysteroidogenesis (downregulation); and (c) how the prothoracic gland cells convert cholesterol to the precursor of E and then 20E, a series of reactions only now being understood because of the use of a combination of classical biochemistry and molecular genetics.

Activation of an extracellular signal-regulated kinase (ERK) by the insect prothoracicotropic hormone

Ecdysteroid hormones are crucial in controlling the growth, molting and metamorphosis of insects. The predominant source of ecdysteroids in pre-adult insects is the prothoracic gland, which is under the acute control of the neuropeptide hormone prothoracicotropic hormone (PTTH). Previous studies using the tobacco hornworm, Manduca sexta, have shown that PTTH stimulates ecdysteroid synthesis via a series of events, including the activation of protein kinase A and the 70 kDa S6 kinase (p70(S6k)). In this study, PTTH was shown to stimulate also mitogen-activated protein kinase (MAPK) phosphorylation and activity in the Manduca prothoracic gland. The MAPK involved appears to be an extracellular signal-regulated kinase (ERK) homologue. The ERK phosphorylation inhibitors PD 98059 and UO 126 blocked basal and PTTH-stimulated ERK phosphorylation and ecdysteroid synthesis. PTTH-stimulated ERK activity may be important for both rapid regulation of ecdysteroid synthesis and for longer-term changes in the size and function of prothoracic gland cells.

Woc (without children) gene control of ecdysone biosynthesis in Drosophila melanogaster

The first step in ecdysteroidogenesis, i.e. the 7,8-dehydrogenation of dietary cholesterol (C) to 7-dehydrocholesterol (7dC), is blocked in Drosophila melanogaster homozygous woc (without children) third instar larval ring glands (source of ecdysone). Unlike ring glands from wild-type D. melanogaster larvae, glands from woc mutants cannot convert radiolabelled C or 25-hydroxycholesterol (25C) to 7dC or 7-dehydro-25-hydroxycholesterol (7d25C) in vitro, nor to ecdysone (E). Yet, when these same glands are incubated with synthetic tracer 7d25C, the rate of metabolism of this polar Delta(5,7)-sterol into E is identical to that observed with glands from comparably staged wild-type larvae. The absence of this enzymatic activity in vivo is probably the direct cause of the observed low whole-body ecdysteroid titers in late third instar homozygous mutant larvae, the low ecdysteroid secretory activity in vitro of brain-ring gland complexes from these animals, and the failure of the larvae to pupariate (undergo metamorphosis). Oral administration of 7dC, but not C, results in a dramatic increase in ecdysteroid production both in vivo and in vitro by the woc mutant brain-ring gland complexes and affects a partial rescue to the beginning of pupal-adult development, but no further, despite elevated whole-body ecdysteroid titers. Data previously reported (Wismar et al., 2000) indicate that the woc gene encodes a zinc-finger protein that apparently modulates the activity of the 7,8-dehydrogenase.

Regulation of Ecdysteroid Signaling: Cloning and Characterization of Ecdysone Oxidase

One route of inactivation of ecdysteroids in insects involves ecdysone oxidase-catalyzed conversion into 3-dehydroecdysteroid followed by irreversible reduction by 3-dehydroecdysone 3alpha-reductase to 3-epiecdysone. We have purified from Spodoptera littoralis the first ecdysone oxidase and subjected it to limited amino acid sequencing. A reverse-transcriptase polymerase chain reaction-based approach has been used to clone the cDNA (2.8 kilobases) encoding this 65-kDa protein. Northern blotting showed that the mRNA transcript was expressed in midgut during the prepupal stage of the last larval instar at a time corresponding to an ecdysteroid titer peak. Conceptual translation of the ecdysone oxidase cDNA and data base searching revealed that the enzyme is an FAD flavoprotein that belongs to the glucose-methanol-choline oxidoreductase superfamily. Ecdysone oxidase represents the only oxidase in eukaryotic animals known to catalyze oxygen-dependent oxidation of steroids; by contrast, oxidation of steroids in vertebrates occurs via NAD(P)(+)-linked dehydrogenases. The injection of RH-5992, an ecdysteroid agonist, induced the transcription of ecdysone oxidase, suggesting that ecdysone oxidase is an ecdysteroid-responsive gene. The gene encoding this enzyme, consisting of five exons, has also been isolated. Sequences similar to the binding motifs for Broad-Complex and FTZ-F1 have been found in the 5'-flanking region. Southern blotting indicated that ecdysone oxidase is encoded by a single-copy gene. We have determined the kinetic characteristics of this novel recombinant ecdysone oxidase produced using a baculovirus expression system.

Molecular cloning of a novel crustacean member of the aldoketoreductase superfamily, differentially expressed in the antennal glands

Biochemical studies on ecdysteroid metabolism in arthropods suggest that aldoketoreductase enzymes (AKRs) may be involved in this pathway, but very few molecular data are available on these oxidoreductases in invertebrates. Looking for such enzymes in the crayfish Orconectes limosus, we have used a PCR strategy with primers deduced from a recent insect 3beta-reductase sequence, and from mammalian 5beta-reductase sequences. A full-length cDNA, corresponding to a putative AKR, was isolated from crayfish antennal gland. This cDNA contains an open-reading frame of 1008 bp, encoding a predicted protein of 336 amino acids. Northern blots indicated a restricted expression of the transcript in the antennal glands, quite constant during the molting cycle, and in situ hybridization demonstrated a strong expression of the transcript in the labyrinth. This is to date the first member of the AKRs superfamily characterized in a crustacean species, and the putative function of the corresponding enzyme is discussed.

Aedes aegypti TMOF modulates ecdysteroid production by prothoracic glands of the gypsy moth, Lymantria dispar

Trypsin modulating oostatic factor (TMOF) is a decapeptide that inhibits the biosynthesis of trypsin-like enzymes in the midgut of several insect species and, as such, serves as a dipteran oostatic hormone. In vitro incubation of lepidopteran prothoracic glands with Aedes aegypti TMOF revealed that this decapeptide, in the presence of brain extract, modulates ecdysteroid production. The modulatory effect was highly dependent on both the concentration of TMOF and brain extract. Typically, TMOF was stimulatory in the presence of lower concentrations of Lymantria dispar brain extract (0.01 and 0. 025 brain equivalent), and either neutral or inhibitory at higher concentrations (0.25, 0.5, and 1.0 brain equivalent) of extract. In the presence of European corn borer (Ostrinia nubilalis) brain extract, TMOF also exhibited modulatory effects, effects that again were dependent on the concentrations of both brain extract and TMOF present in the incubation medium. At 1.5 brain equivalents, TMOF was inhibitory at all but the highest concentration tested (5x10(-6) M), at 1.0 brain equivalent, TMOF was stimulatory at 10(-6) M and at 0. 5 brain equivalents, TMOF did not significantly affect PTG synthesis of ecdysteroids. Results suggest the presence of a modulatory peptide(s), which fine tunes the synthesis and release of ecdysteroids by PTGs in accordance with the insect's developmental/physiological requirements.

Regulation of ecdysteroid signalling: molecular cloning, characterization and expression of 3-dehydroecdysone 3 alpha-reductase, a novel eukaryotic member of the short-chain dehydrogenases/reductases superfamily from the cotton leafworm, Spodoptera littoralis

One route of inactivation of ecdysteroids in insects involves ecdysone oxidase-catalysed conversion into 3-dehydroecdysone (3DE), followed by irreversible reduction by 3DE 3 alpha-reductase to 3-epiecdysone. The 3DE 3 alpha-reductase has been purified and subjected to limited amino acid sequencing. It occurs as two distinct forms, including a probable trimer of subunit molecular mass of approx. 26 kDa. A reverse-transcriptase PCR-based approach has been used to clone the cDNA (1.2 kb) encoding the 26 kDa protein. Northern blotting showed that the mRNA transcript was expressed in Malpighian tubules during the early stage of the last larval instar. Conceptual translation of the 3DE 3 alpha-reductase cDNA and database searching revealed that the enzyme belongs to the short-chain dehydrogenases/reductases superfamily. Furthermore, the enzyme is a novel eukaryotic 3-dehydrosteroid 3 alpha-reductase member of that family, whereas vertebrate 3-dehydrosteroid 3 alpha-reductases belong to the aldo-keto reductase (AKR) superfamily. Enzymically active recombinant 3DE 3 alpha-reductase has been produced using a baculovirus expression system. Surprisingly, we observed no similarity between this 3DE 3 alpha-reductase and a previously reported 3DE 3 beta-reductase, which acts on the same substrate and belongs to the AKR family.

Molecular determinants of differential ligand sensitivities of insect ecdysteroid receptors

The functional receptor for insect ecdysteroid hormones is a heterodimer consisting of two nuclear hormone receptors, ecdysteroid receptor (EcR) and the retinoid X receptor homologue Ultraspiracle (USP). Although ecdysone is commonly thought to be a hormone precursor and 20-hydroxyecdysone (20E), the physiologically active steroid, little is known about the relative activity of ecdysteroids in various arthropods. As a step toward characterization of potential differential ligand recognition, we have analyzed the activities of various ecdysteroids using gel mobility shift assays and transfection assays in Schneider-2 (S2) cells. Ecdysone showed little activation of the Drosophila melanogaster receptor complex (DmEcR-USP). In contrast, this steroid functioned as a potent ligand for the mosquito Aedes aegypti receptor complex (AaEcR-USP), significantly enhancing DNA binding and transactivating a reporter gene in S2 cells. The mosquito receptor also displayed higher hormone-independent DNA binding activity than the Drosophila receptor. Subunit-swapping experiments indicated that the EcR protein, not the USP protein, was responsible for ligand specificity. Using domain-swapping techniques, we made a series of Aedes and Drosophila EcR chimeric constructs. Differential ligand responsiveness was mapped near the C terminus of the ligand binding domain, within the identity box previously implicated in the dimerization specificity of nuclear receptors. This region includes helices 9 and 10, as determined by comparison with available crystal structures obtained from other nuclear receptors. Site-directed mutagenesis revealed that Phe529 in Aedes EcR, corresponding to Tyr611 in Drosophila EcR, was most critical for ligand specificity and hormone-independent DNA binding activity. These results demonstrated that ecdysone could function as a bona fide ligand in a species-specific manner.

Molecular cloning and characterization of hemolymph 3-dehydroecdysone 3beta-reductase from the cotton leafworm, Spodoptera littoralis. A new member of the third superfamily of oxidoreductases

The primary product of the prothoracic glands of last instar larvae of Spodoptera littoralis is 3-dehydroecdysone (3DE). After secretion, 3DE is reduced to ecdysone by 3DE 3beta-reductase in the hemolymph. We have previously purified and characterized 3DE 3beta-reductase from the hemolymph of S. littoralis. In this study, cDNA clones encoding the enzyme were obtained by reverse transcription-polymerase chain reaction, employing primers based on the amino acid sequences, in conjunction with 5'- and 3'-rapid amplification of cDNA ends. Multiple polyadenylation signals and AT-rich elements were found in the 3'-untranslated region, suggesting that this region may have a role in regulation of expression of the gene. Conceptual translation and amino acid sequence analysis suggest that 3DE 3beta-reductase from S. littoralis is a new member of the third superfamily of oxidoreductases. Northern analysis shows that 3DE 3beta-reductase mRNA transcripts are widely distributed, but are differentially expressed, in some tissues. The developmental profile of the mRNA revealed that the gene encoding 3DE 3beta-reductase is only transcribed in the second half of the last larval instar and that this fluctuation in expression accounts for the change in the enzyme activity during the instar. Southern analysis indicates that the 3DE 3beta-reductase is encoded by a single gene, which probably contains at least one intron.

Ecdysteroid release by the prothoracic gland of Gryllus bimaculatus (Ensifera: Gryllidae) during larval-adult development

The in vitro secretion of ecdysteroids from the prothoracic glands of larvae of Gryllus bimaculatus was analysed by HPLC-RIA. The primary product was identified as 3-dehydroecdysone (65-93%), with lesser amounts of ecdysone (7-35%). Production and release of ecdysteroids from the prothoracic glands are calcium-dependent. The rate of ecdysteroid release was low during the beginning and the end of the last two larval stages and high in between. Prothoracic glands from young adult females produced only minor amounts of ecdysteroids and ceased hormone production around day 4 after the moult.

Ecdysteroid regulation of salivary gland degeneration in the ixodid tick, Amblyomma hebraeum: a reconciliation of in vivo and in vitro observations

Salivary gland degeneration in the female tick Amblyomma hebraeum Koch is triggered by an ecdysteroid (ES) hormone. Under both in vivo and in vitro conditions, degeneration requires 4 days for completion. In partially fed females that have fed beyond a "critical weight," the commitment period for salivary gland degeneration occurs between 24 and 48 h after removal from the host. Although tissue degeneration begins within 24 h postengorgement, ES titer as measured by radioimmunoassay (RIA) does not rise to threshold levels until 48 h postengorgement. To explain this anomaly we examined two hypotheses: (1) there is an early hormonal signal (e.g., 3-dehydroecdysone; 3DE) that is an ES not detectable by the antibody used in our RIA; and (2) the low hemolymph titer during the first 2 days postengorgement is not an accurate reflection of the ES concentration within the tissue itself. 3-Oxoecdysteroid 3 beta-reductase (ketoreductase) was present in salivary glands, but neither ketoreductase nor 3DE was detected in hemolymph. The ES concentration of salivary gland homogenates was similar to that of hemolymph, while that of saliva was undetectable. Together, these results support our second hypothesis that the metabolically active tissue of the salivary gland experiences a suprathreshold concentration of hormone even though the concentration in hemolymph is below threshold levels.

An immunophilin is a component of the insect ecdysone receptor (EcR) complex

The ecdysone receptor (EcR) complex has been identified in the prothoracic gland of Manduca sexta by specific immunoprecipitation and Western blot analyses, and includes EcR, ultraspiracle (USP) and FKBP46. The EcR complex binds ponasterone A in a dose-dependent manner with a Kd of 7.04 x 10(-9) M. Immunocytochemistry revealed that EcR, USP and FKBP46 were localized within the nucleus of the prothoracic gland cells, and suggested that the developmental expression patterns of EcR and USP changed in concert with the hemolymph ecdysteroid titer whereas that of FKBP46 did not. The composite results suggest that the hemolymph ecdysteroid titer, of which 20 hydroxyecdysone is the major component, modulates the expression of both EcR and USP in the prothoracic gland to achieve feedback regulation.

2-Deoxyecdysone is a circulating ecdysteroid in the beetle Zophobas atratus

A qualitative analysis of ecdysteroids has been performed during the post-embryonic development of the tenebrionid beetle, Zophobas atratus, by high performance liquid chromatography (HPLC) combined with enzyme immunoassay (EIA) using two different antibodies. Three HPLC peaks were found to be immunoreactive, in hemolymph extracts of both sexes. Moreover, these peaks had ecdysteroid-like UV spectra, determined using a photodiode array detector. The use of two different HPLC systems (reverse and normal phases), in combination with two different EIA antibodies, allowed us to identify 20-hydroxyecdysone (20E) and ecdysone (E), as the two main ecdysteroids, but also suggested the presence of 2-deoxyecdysone (2dE) as the third hemolymph component. Secretion of putative 2dE, together with E (but not 20E) was also demonstrated in vitro from incubations of prothoracic glands and of tegumental explants. In these experiments, either in vivo or in vitro, 3-dehydroecdysone was never observed. Our observations thus strongly suggest that 2dE is a circulating ecdysteroid in Z. atratus and may function as a prohormone during the development of some insects.

A novel neuropeptide-endocrine interaction controlling ecdysteroid production in ixodid ticks

Ixodid (hard) ticks are blood-feeding arthropods that require a blood meal to complete each stage of development. However, the hormonal events coordinating aspects of feeding and development are only poorly understood. We have delineated a new neuropeptide-endocrine interaction in the adult tick, Amblyomma hebraeum, that stimulates the synthesis of the moulting hormones, the ecdysteroids. In adult female ticks, ecdysteroid synthesis could be demonstrated in integumental tissue incubated in vitro with a synganglial (central nervous system) extract, but not in its absence. Stimulation by the synganglial extract is both time- and dose-dependent, but is completely abolished by trypsin treatment, suggesting that the activity is due to a peptide/protein. Integumental tissue ecdysteroidogenesis is also stimulated by elevation of the cAMP concentration using forskolin and 3-isobutyl-l-methyl-xanthine, or by 8-bromo-cAMP. This suggests the involvement of at least a cAMP second messenger system in the neuropeptide-ecdysteroidogenesis axis, without precluding a role for other second messengers as well. Despite involving a quite different steroidogenic tissue, the foregoing system has some parallels with the known prothoracicotropic hormone (neuropeptide)-prothoracic gland endocrine axis of insects.

Involvement of a 3beta-hydroxysteroid dehydrogenase activity in ecdysteroid biosynthesis

Ecdysteroid biosynthesis was analyzed in vitro using dissociated Y-organ cells from the shore crab Carcinus maenas. 3-Dehydroecdysone (3DE) was detected as a minor secretory product, in addition to the formerly identified end-products 25-deoxyecdysone and ecdysone (E). In conversion studies, 3DE was formed from tritiated 5beta-ketodiol (2,22,25-trideoxyecdysone), 2,22-deoxyecdysone and 2-deoxyecdysone but not from E. Further experiments were performed in order to understand the interconversions between 3-oxo and 3beta-OH compounds in the crab Y-organ. The enzyme involved in 3beta-dehydrogenation was not ecdysone oxidase, a soluble enzyme found in peripheral tissues of many arthropods but it presented strong similarities with 3beta-hydroxysteroid dehydrogenase enzymes from vertebrates: it was membrane-bound and NAD+-dependent. Moreover, a NADH-dependent 3beta-reduction of several 3-oxo-ecdysteroids was obtained using the same microsomal fraction (100,000 x g pellet) of Y-organs, indicating that the reaction might be reversible. As this activity was specific of molting glands, we hypothesize that there is at least one 3beta-hydroxysteroid dehydrogenase enzyme involved in the biosynthetic pathway of ecdysteroids.

Purification and characterisation of haemolymph 3-dehydroecdysone 3 beta-reductase in relation to ecdysteroid biosynthesis in the cotton leafworm Spodoptera littoralis

The in vitro secretion of ecdysteroids from the prothoracic glands of last instar larvae of Spodoptera littoralis was detected and analysed by HPLC-RIA. The primary product was identified as 3-dehydroecdysone (approximately 82%), with lesser amounts of ecdysone (approximately 18%). Interconversion of ecdysone and 3-dehydroecdysone by prothoracic glands was not detectable. 3-Dehydroecdysone 3 beta-reductase activity was demonstrated in the haemolymph. Ecdysone, the endproduct, was characterised by reverse-phase and adsorption HPLC, chemical transformation into ecdysone 2, 3-acetonide, and mass spectrometry. The conditions for optimal activity were determined. The enzyme requires NADPH or NADH as cofactor and Km values for NADPH and NADH were determined to be 0.94 microM, and 22.8 microM, respectively. Investigation of the kinetic properties of the enzyme, using either NADPH or NADH as cofactor, revealed that it exhibits maximal activity at low 3-dehydroecdysone substrate concentrations, with a drastic inhibition of activity at higher concentrations (> 5 microM). The results suggest that the 3-dehydroecdysone 3 beta-reductase has a high-affinity (low Km) binding site for 3-dehydroecdysone substrate, together with a lower-affinity inhibition site. The 3 beta-reductase enzyme was purified to homogeneity using a combination of poly(ethylene glycol) 6000 precipitation and successive FPLC fractionation on Mono-Q, phenyl Superose (twice), and hydroxyapatite columns. The native enzyme was shown to be a monomer with molecular mass of 36 kDa by SDS/PAGE and gel-filtration chromatography. Furthermore, the activity of the enzyme during the last larval instar was found to reach a peak prior to that of the haemolymph ecdysteroid titre, supporting a role for the enzyme in development.

Enzymes of ecdysteroid transformation and inactivation in the midgut of the cotton leafworm, Spodoptera littoralis: properties and developmental profiles

In the midgut cytosol of Lepidoptera, ecdysteroids undergo inactivation by transformation via the 3-dehydro derivative to the corresponding 3-epiecdysteroid (3 alpha-hydroxy) and by phosphate conjugation. The oxygen-dependent oxidase catalyses formation of 3-dehydroecdysteroid, which can be reduced either irreversibly by 3-dehydroecdysone 3 alpha-reductase to 3-epiecdysteroid, or by 3-dehydroecdysone 3 beta-reductase back to the initial ecdysteroid. Furthermore, these ecdysteroids undergo further inactivation by phosphorylation. These ecdysteroid transformations have been investigated in last instar larvae of the cotton leafworm, Spodoptera littoralis. The products of the phosphorylation have been characterized as predominantly ecdysteroid 2-phosphate accompanied by smaller amounts of the corresponding 22-phosphate. The phosphotransferases require Mg2+ and ATP. Whereas the 3-dehydroecdysone 3 alpha-reductase has a clear preference for NADPH rather than NADH, the corresponding 3 beta-reductase markedly favours NADH. The physiological significance of the latter enzyme is unclear. The profiles of the various enzymic activities in dialysed midgut cytosol supplemented with appropriate cofactors were determined throughout the last larval instar. All activities were detectable throughout the instar, but the respective enzymes exhibited maxima at different times. Ecdysone oxidase showed a peak early in the instar, with 3-dehydroecdysone 3 alpha-reductase increasing to a peak as the former activity declined. The 3-dehydroecdysone 3 beta-reductase exhibited peak activity late in the instar, a profile similar to that observed for the corresponding haemolymph enzyme involved in reduction of the 3-dehydroecdysone product of the prothoracic glands to ecdysone. Thus, the significance of the midgut 3 beta-reductase may be related to production of active hormone. Both ecydsteroid 22- and 2-phosphotransferases showed high activities early in the instar and then declined. The physiological significance of the profiles for the ecdysone oxidase, the 3-dehydroecdysone 3 alpha-reductase and phosphotransferases is unclear.

Regulation and consequences of cellular changes in the prothoracic glands of Manduca sexta during the last larval instar: a review

The prothoracic glands of the tobacco hornworm, Manduca sexta, respond to prothoracicotropic hormone (PTTH) by a regulatory pathway involving cAMP, protein phosphorylation, protein synthesis, and enhanced secretion of ecdysteroids including ecdysone and 3-dehydroecdysone. Recent investigations have revealed that PTTH acts by this general mechanism throughout the fifth larval instar, i.e., during the transition from larva to pupa. However, the glands undergo developmental changes in size, steroidogenic capacity, and in elements of the signalling pathway associated with synthesis, degradation, and intracellular action of cAMP. The present review describes such changes, and their possible regulation and consequences, in the general context of endocrine events underlying larval-pupal metamorphosis during the fifth larval stage.

Comparison of ecdysteroid production in Drosophila and Manduca: pharmacology and cross-species neural reactivity

In both Manduca sexta and Drosophila melanogaster, metamorphic events are driven by ecdysteroids whose production in prothoracic gland (PGs) is stimulated periodically by neural factors. Differences in the life cycle of moths and flies have made it difficult to compare the regulation of ecdysteroid biosynthesis in these two species. As in Manduca, at least two neural factors in the larval Drosophila BVG complex were separable by molecular weight, and they stimulated increased ecdysteroid biosynthesis from the ring gland, a composite organ that includes PG cells. Drosophila neural extracts accelerated ecdysteroid biosynthesis in Manduca PGs and, conversely, partially purified Manduca PTTH preparations elevated ecdysteroid biosynthesis in Drosophila ring glands, suggesting that the two species may share structurally similar prothoracicotropic factors. Drosophila ring glands required the presence of calcium ions to respond to neural extracts, but the phosphodiesterase inhibitor MIX and cAMP analogues exerted little, if any, positive effect on production. Mean ecdysteroid production rates of BVG-ring gland complexes taken from Drosophila larvae during various phases of the wandering period were often submaximal and highly variable, suggesting that they fluctuate widely prior to pupariation. Based on available data in Drosophila and the Manduca model for the control of ecdysteroid biosynthesis, a developmental scheme for neuroendocrine control in Drosophila is proposed.