Juvenile hormone-dependent vitellogenin synthesis in Locusta migratoria fat body: inducibility related to sex and stage

Expression and function of the vitellogenin receptor in the hypopharyngeal glands of the honey bee Apis mellifera (Hymenoptera: Apidae) workers

The vitellogenin receptor (VgR) is essential for the uptake and transport of the yolk precursor, vitellogenin (Vg). Vg is synthesized in the fat body, released in the hemolymph, and absorbed in the ovaries, via receptor-mediated endocytosis. Besides its important role in the reproductive pathway, Vg occurs in nonreproductive worker honey bee, suggesting its participation in other pathways. The objective was to verify if the VgR occurs in the hypopharyngeal glands of Apis mellifera workers and how Vg is internalized by these cells. VgR occurrence in the hypopharyngeal glands was evaluated by qPCR analyses of VgR and immunohistochemistry in workers with different tasks. The VgR gene is expressed in the hypopharyngeal glands of workers with higher transcript levels in nurse honey bees. VgR is more expressed in 11-day-old workers from queenright colonies, compared to orphan ones. Nurse workers with developed hypopharyngeal glands present higher VgR transcripts than those with poorly developed glands. The immunohistochemistry results showed the co-localization of Vg, VgR and clathrin (protein that plays a major role in the formation of coated vesicles in endocytosis) in the hypopharyngeal glands, suggesting receptor-mediated endocytosis. The results demonstrate that VgR performs the transport of Vg to the hypopharyngeal glands, supporting the Ovary Ground Plan Hypothesis and contributing to the understanding of the role of this gland in the social context of honey bees.

Juvenile hormone receptor Methoprene tolerant: Functions and applications

During the past 15years, after confirming Methoprene tolerant (Met) as a juvenile hormone (JH) receptor, tremendous progress has been made in understanding the function of Met in supporting JH signal transduction. Met role in JH regulation of development, including metamorphosis, reproduction, diapause, cast differentiation, behavior, im`munity, sleep and epigenetic modifications, have been elucidated. Met's Heterodimeric partners involved in performing some of these functions were discovered. The availability of JH response elements (JHRE) and JH receptor allowed the development of screening assays in cell lines and yeast. These screening assays facilitated the identification of new chemicals that function as JH agonists and antagonists. These new chemicals and others that will likely be discovered in the near future by using JH receptor and JHRE will lead to highly effective species-specific environmentally friendly insecticides for controlling pests and disease vectors.

Sequential oogenesis is controlled by an oviduct factor in the locusts Locusta migratoria and Schistocerca gregaria: Overcoming the doctrine that patency in follicle cells is induced by juvenile hormone

In insects that lay eggs in large clutches, yolk accumulation in each of the many ovarioles is restricted to the basal (terminal) oocyte, the one closest to the lateral oviduct. All succeeding (subterminal) oocytes remain small until the terminal oocytes finished their development and were ovulated into the oviduct. The major step regulating yolk uptake by terminal oocytes is the formation of gaps between cells of the follicle layer, a process termed patency. In the migratory as well as in the desert locust, patency is induced by a Patency Inducing Factor (PIF) produced by the lateral oviducts. PIF is secreted in all regions of the lateral oviducts and interacts with the basal follicle cells via the pedicel, a fine duct that connects an ovariole with the oviduct. By this mechanism, patency is triggered in the follicle cells of the terminal oocyte only, restricting yolk accumulation to the oocytes next to ovulation. In contrast to the previous hypothesis, juvenile hormone (JH) is not necessary to induce patency, rather JH amplifies the effect of PIF.

Effects of a methanolic extract of the plant Haplophyllum tuberculatum and of teflubenzuron on female reproduction in the migratory locust, Locusta migratoria (Orthoptera: Oedipodinae)

The effects of a methanolic extract of the plant Haplophyllum tuberculatum (ME-Ht) and of teflubenzuron (TFB) were compared on several reproductive variables and ecdysteroid titers in the females of Locusta migratoria. The test products were administered orally to newly emerged females at doses of 1500 (ME-Ht) and 10μg/female (TFB). The methanolic extract and TFB had comparable effects on several of the variables examined. Both significantly delayed the first oviposition and reduced fecundity and fertility. ME-Ht and TFB also displayed similar effects on ovarian growth, vitellogenesis and ecdysteroid titers. Both treatments induced a drop in hemolymph protein levels as well as a reduction in vitellogenin uptake by oocytes. This delay in oogenesis was accompanied by a resorption of terminal oocytes. However, whereas TFB completely blocked egg hatch, ME-Ht only had a modest inhibitory effect on this variable. Hemolymph and ovarian ecdysteroid titers, as measured by radioimmunoassay, were similar and low in both control and treated females, except for a peak observed only in control females at the end of vitellogenesis. We discuss the functional significance of the observed effects in the context of the putative modes of action of the methanolic plant extract and TFB.

Juvenile hormone regulates vitellogenin gene expression through insulin-like peptide signaling pathway in the red flour beetle, Tribolium castaneum

Our recent studies identified juvenile hormone (JH) and nutrition as the two key signals that regulate vitellogenin (Vg) gene expression in the red flour beetle, Tribolium castaneum. Juvenile hormone regulation of Vg synthesis has been known for a long time in several insects, but the mechanism of JH action is not known. Experiments were conducted to determine the mechanism of action of these two signals in regulation of Vg gene expression. Injection of bovine insulin or FOXO double-stranded RNA into the previtellogenic, starved, or JH-deficient female adults increased Vg mRNA and protein levels, thereby implicating the pivotal role for insulin-like peptide signaling in the regulation of Vg gene expression and possible cross-talk between JH and insulin-like peptide signaling pathways. Reduction in JH synthesis or its action by RNAi-mediated silencing of genes coding for acid methyltransferase or methoprene-tolerant decreased expression of genes coding for insulin-like peptides (ILPs) and influenced FOXO subcellular localization, resulting in the down-regulation of Vg gene expression. Furthermore, JH application to previtellogenic female beetles induced the expression of genes coding for ILP2 and ILP3, and induced Vg gene expression. FOXO protein expressed in baculovirus system binds to FOXO response element present in the Vg gene promoter. These data suggest that JH functions through insulin-like peptide signaling pathway to regulate Vg gene expression.

RNA interference of insulin-related peptide and neuroparsins affects vitellogenesis in the desert locust Schistocerca gregaria

The 'classic' insect hormones, juvenile hormone and 20-hydroxyecdysone, can stimulate vitellogenesis and/or ovarian development in adult females of several insect species. Accumulating evidence also indicates a crucial role in female reproductive physiology for peptide hormones, such as insulin-related peptides (IRPs) and neuroparsins (NPs). Especially in dipteran species, IRP signaling has been shown to regulate female reproductive events. The first NP was originally identified from the migratory locust (Locusta migratoria) as an antigonadotropic factor that delayed vitellogenesis. Moreover, NP family members display sequence similarities with the N-terminal domain of vertebrate insulin-like growth factor binding proteins (IGFBPs). In the current study, RNA interference (RNAi) was employed to investigate the possible involvement of IRP and NPs in the control of the female desert locust (Schistocerca gregaria) reproductive system. The cDNAs encoding an IRP (Scg-IRP) and four NPs (Scg-NPs) had previously been cloned from S. gregaria. An RNAi-mediated knock-down of either Scg-NP or Scg-IRP transcript levels was induced in adult female desert locusts and the subsequent effects were analyzed. Knock-down of the Scg-NPs or Scg-IRP affected vitellogenin transcript levels and oocyte growth in a positive and negative way, respectively. The current findings are indicative for a role of Scg-NPs and Scg-IRP in the control of vitellogenin synthesis. A plausible hypothesis is that Scg-IRP may act as a sensor of the nutritional and metabolic status that determines whether vitellogenesis can occur. That the same processes were affected in opposite ways in both RNAi experiments offers an extra argument for antagonizing roles of Scg-NPs and Scg-IRP.

JH III production, titers and degradation in relation to reproduction in male and female Anthonomus grandis

Juvenile hormone (JH) is necessary for the production of vitellogenin (Vg) in the boll weevil, Anthonomus grandis. Occurrence of Vg in this species is typically restricted to reproductively competent females, and is not detected in untreated males. However, the JH analog, methoprene stimulates Vg production in intact males and in the isolated abdomens of both male and female boll weevils (where in each case no Vg is detected without treatment), suggesting that males are competent to produce Vg but are normally not stimulated to do so. Preliminary work indicating that male boll weevil corpora allata (CA) produced little or no JH in vitro suggested that failure of males to produce Vg might be due to very low JH levels compared to females. This study re-examines the question of JH in male boll weevils by determining in vitro production of JH III by male CA during the first 10 days after adult emergence, determining hemolymph JH esterase activity during this same time period and hemolymph JH III titers in adults of both sexes. We also re-examine the ability of isolated male abdomens to produce Vg in response to hormonal stimulation, analyzing the effect of a wide range of methoprene and JH III dosages. Results indicate that male A. grandis have circulating JH titers and JH production similar to females. JH esterase activity is slightly but significantly higher in males than females. Vg production by isolated abdomens of both sexes after stimulation with methoprene or JH III was confirmed. Dose response studies indicated that high levels of methoprene were less effective than intermediate doses in stimulating Vg synthesis in both sexes. We conclude that the sexually dimorphic effect of JH on Vg synthesis is not due to differences in JH production or differences in JH titer between the sexes.

The homeodomain protein PBX participates in JH-related suppressive regulation on the expression of major plasma protein genes in the silkworm, Bombyx mori

In the silkworm, Bombyx mori, major plasma proteins referred to as 30K proteins are the most abundant proteins in the hemolymph of final (fifth) instar larvae. Surgical extirpation of corpora allata, the source of a juvenile hormone (JH), causes rapid accumulation of 30K proteins in the hemolymph of fourth instar larvae. The 30K protein 6G1 (30K6G1) gene was repressed in primary cultured fat body cells treated with a JH analog (JHA), methoprene. To identify the JH response element present in the promoter region of the 30K6G1 gene, we performed transfection analyses of the 5'-deletion mutants of the 30K6G1 gene using primary cultured fat body cells, gel retardation assays and in vivo footprinting analysis. The results from those analyses revealed that a JH response element exists in the sequence between positions -147 and -140. When the promoter construct mutated at positions -143, -142, and -141 was transfected to fat body primary cultured cells, the suppression effect on the reporter gene expression caused by JHA was reduced. Gel retardation assay using specific antibody revealed that a PBX protein binds to the JH response element. Northern blot analysis revealed that the gene expression of Bombyx PBX is enhanced in the fat body cells by JHA treatment. These results indicate that PBX proteins are involved in the JH signaling pathway and play an important role in suppressing 30K protein gene expression in the fat body of B. mori.

Juvenile hormone-dependent motor activation in the adult locust Locusta migratoria

Abdominal motoneurones of the locust Locusta migratoria were investigated in immature, mature and allatectomised females to compare their response characteristics during reproductive development. These motoneurones were chosen because they control muscles which are involved in extreme lengthening during egg-laying behaviour. The study focused on changes in motoneurone firing activity and its possible regulation by juvenile hormone. In isolated nerve-muscle preparations, increased resting motor activity was found in mature (>14 days) but not in immature females (<5 days). Removing the corpora allata, the gland producing juvenile hormone in insects, prevented increased motor activity. Stimulus evoked activation of the motor system led to a characteristic burst of action potentials which lasted for a few seconds. The time-course and amount of activation changed significantly during reproductive development. Mature females displayed longer lasting and higher activity than immature or allatectomised females, but only those segments involved in egg-laying were found to express the altered firing properties. Single cell analysis of motoneurone dendritic morphology or membrane properties revealed no evidence that could be causative for the activity changes seen during reproductive development. The results suggest that altered motoneurone activity serves to adapt females to the neuromuscular requirements of egg-laying behaviour.

Maturation of muscle properties and its hormonal control in an adult insect

The oviposition of female locusts requires longitudinal muscles to tolerate remarkable lengthening. Whether this ability together with concomitant properties develops during maturation or is present throughout life was investigated. The properties of the locust abdominal muscles involved in oviposition behaviour were investigated with respect to their maturation, segment- and gender-specificity and regulation by juvenile hormone (JH). Muscles from the sixth abdominal segment (an oviposition segment) of mature females (&gt;18 days old) were able to tolerate large extensions (&gt;8 mm). At this length, muscles were still able to generate considerable neurally evoked twitch tension. In contrast, muscle fibres from females less than 5 days old did not tolerate extension of more than 4 mm. At this length, tension generation was negligible. The maximum tension generated at different stimulus frequencies was significantly higher in muscles of females more than 18 days old than in females less than 5 days old. Furthermore, the cross-sectional area of muscle fibres increased significantly during reproductive development. Current-clamp recordings from denervated muscle fibres of females more than 18 days old revealed their ability to generate overshooting action potentials. The potentials were tetrodotoxin (TTX)-insensitive (0.5 μmol l–1 TTX), but were blocked by Cd2+ (50 μmol l–1) or nifedipine (50 μmol l–1), which suggests the involvement of L-type Ca2+ channels. Action potentials recorded from females less than 5 days old differed considerably in amplitude and shape from those recorded from females more than 18 days old, suggesting their maturation during the first 2 weeks of adult life. Inactivation of the corpora allata (CA) by precocene inhibited the maturation of these muscle properties, whereas injection of JH into precocene-treated females reversed this effect. Homologous muscles from the third abdominal segment (a non-oviposition segment, M169) and muscles from males (M214) revealed no comparable changes, although some minor changes occurred during reproductive development. The results suggest a gender- and segment-specific maturation of muscle properties that is related to reproductive behaviour and controlled by JH.

"Insects do not have sex hormones": a myth?

Mammals have two genes (SRY and DMT1) for testis formation-androgenesis, an anti-testis gene, DAX1, an anti-Müllerian duct hormone, and steroid sex hormones. Drosophila uses the sex-lethal, transformer, and doublesex genes for sexual differentation and is supposed to lack sex hormones. However, the statement that insects do not have sex hormones loses much of its credibility if one considers (1) the classical endocrinological work on sexual differentiation in the firefly Lampyris and in the hevea tussock moth Orgyia; (2) the recent identification of an androgenic hormone and its role in sex determination in the isopod Armadillidium; (3) the similarity between steroidogenic factor 1 (SF-1) of mammals and fushi tarazu factor 1 (FTZ-F1) of Drosophila; and (4) the steroidogenic effect of gonadotropins secreted by the brain of female locusts and mosquitoes and of male gypsy moth. In our model, based on data from the literature, ecdysone, when present in high concentrations, might function as an androgenic sex steroid. It is also the precursor of 20-OH-ecdysone, which is the moulting hormone of insects, and in vitellogenic females of many species, the counterpart of estrogens as well. Other gender-specific hormones are likely to exist in the brain-gonad axis.

New insecticides with ecdysteroidal and juvenile hormone activity

Agrochemical research over the last two decades has resulted in the discovery of chemically novel insecticides that mimic the action of the two insect growth and developmental hormones, the steroidal 20-hydroxyecdysone (20E) and the sesquiterpenoid juvenile hormone (JH). Bisacylhydrazines are non-steroidal agonists of 20E and exhibit their insecticidal activity via interaction with the ecdysteroid receptor proteins. Interestingly, two of the bisacylhydrazine (tebufenozide and RH-2485) insecticides are very selectively toxic to lepidopteran pests. These insecticides are safe to beneficial insects and have a benign ecotoxicological profile. Aromatic non-terpenoidal insecticides (fenoxycarb and pyriproxyfen) mimic the action of JHs. However, like the JHs, their exact mode of action is not well understood. These insecticides are toxic to a broad spectrum of insects during their embryonic, last larval, or reproductive stages. The insecticidal, ecotoxicological properties and the mode of action of the two groups of insecticides are reviewed in this article.

The biosynthesis and processing of vitellogenin in the fat bodies of females and males of the cockroach Leucophaea maderae

The juvenile hormone analog (JHA) methoprene was used to induce the synthesis of the yolk protein precursor vitellogenin (Vg) in adult females and males of the cockroach Leucophaea maderae. The female- and male-produced vitellogenin (VgF and VgM, respectively) contained polypeptides of 112, 95, 92, and 54 kDa. Also present in the secreted vitellogenins was a soffmall quantity of a short-lived transitional 155 kDa Vg polypeptide, and a variable amount of an 85 dDa species. Quantitatively, the VgF and VgM were significantly different in the Vg112 and Vg95 units (VgF > VgM), and in the Vg85 polypeptide (VgF < VgM). In the present study, the biosynthesis of Vg precursors in the fat bodies of females and males was examined using a short radiopulse with 35S-methionine/cysteine and 32P-orthophosphate. The glycosylation of the Vg precursors was examined by digestion with endoglycosidase H and by the inhibition of N-linked glycosylation with tunicamycin. The data showed that in both females and males, the synthesis of the vitellogenin precursor occurred in a stepwise fashion: (1) the co-translational glycosylation of Vg203; (2) the post-translational phosphorylation of Vg203 to form Vg220; (3) the proteolytic processing of Vg220 to form the constituent Vg polypeptides. The 203 and 220 kDa Vg precursors of females and males appeared to be similarly glycosylated and phosphorylated. The additional processing of Vg112 to Vg85 was more pronounced in the fat bodies of males than in females, and appears to account for the quantitative difference in the distribution of these polypeptides in VgF and VgM. Finally, the major oligosaccharides of VgF and VgM appear to be those of N-linked mannose residues. The treatment of females and males with tunicamycin indicated that the co-translational glycosylation of Vg precursors was required for the phosphorylation of the Vg precursor, as well as the secretion of Vg from the fat body.

Structure and expression of mRNA for vitellogenin in Bombyx mori

Vitellogenin, a precursor of major yolk protein of the silkworm, Bombyx mori is a tetramer composed of each two molecules of heavy and light subunits. We cloned mRNA sequence for the B. mori vitellogenin and analyzed its structure. Sequence alignment of several overlapping cDNA clones indicated that the vitellogenin mRNA is approx. 5.7 kb, containing an open reading frame for a peptide with 1782 amino acid residues. By comparing the deduced amino acid sequence with the amino-terminal primary structures of vitellogenin subunits, it is suggested that the heavy and light subunits of the B. mori vitellogenin are encoded by a single contiguous mRNA. The primary translation product of the vitellogenin mRNA was detected in the microsomal fraction prepared from the fat body of vitellogenic females. Northern blot analysis of the fat body RNA demonstrated that the biosynthesis of vitellogenin in B. mori is regulated in a tissue-, sex- and stage-specific manner at the level of mRNA. Possible cause for discrepancy between the present results and our previous proposal (Izumi, S. and Tomino, S. (1983) Insect Biochem. 13, 81-85) on the biosynthesis of B. mori vitellogenin is also discussed.

Pregnenolone, testosterone, and estradiol in the migratory locust Locusta migratoria; a gas chromatographical-mass spectrometrical study

Homogenates of ovaria as well as testes from the locust Locusta migratoria were analyzed for the presence of pregnenolone, testosterone, and estradiol by gas chromatography followed by mass spectrometry. Selected ion monitoring analyses revealed that the molecular ion and two characteristic fragment ions of pregnenolone, the molecular ion with its isotopes and two characteristic mass fragments of testosterone, and the molecular ion, its isotopes and four characteristic fragment ions of estradiol, were present at the proper retention times and with the correct abundance ratios.

Vitellogenin mRNA in locust fat body: coordinate induction of two genes by a juvenile hormone analog

Levels of vitellogenin (Vg) mRNA in Locusta migratoria fat body were determined as indicators of gene expression induced by the juvenile hormone analog methoprene. After injection of methoprene into juvenile hormone-deprived locusts, excised fat bodies were cultured with [3H]leucine for immunochemical assay of Vg synthesis, and RNA was assayed for Vg mRNA content by hybridization with probes from the previously cloned locust Vg genes A and B. In general, the rise in Vg mRNA paralleled the rise in Vg synthesis. During the primary response to methoprene (in female locusts in which the corpora allata had been destroyed immediately after emergence), Vg mRNA was first detected after 18-24 hr and accumulated rapidly between 36 and 48 hr. The secondary response (in locusts allatectomized during vitellogenesis and kept until Vg disappeared) was accelerated, as Vg mRNA was detectable at 12 hr and titers rose steeply after 18 hr. When Vg synthesis was prematurely induced by injection of methoprene into fifth-stage female larvae, the kinetics of mRNA accumulation were similar to those of primary stimulation in the adult. After allatectomy of vitellogenic females, fat body Vg mRNA decayed with a half-life of about 24 hr, roughly paralleling the decline in Vg synthesis. Assays with the two Vg probes showed coordinate accumulation of gene A and gene B messages under all conditions tested: during primary and secondary stimulation in adult females and in the low-level response obtained by treating male larvae with methoprene.

Nuclear development in locust fat body: the influence of juvenile hormone on inclusion bodies and the nuclear matrix

The hormonal induction of vitellogenesis in insects and in oviparous vertebrates are prime models of gene regulation in eukaryotes. In vertebrates the process is under estrogenic control and normally confined to females, although males can be artificially induced. In locust in contrast, juvenile hormone (JH) is central to fat body development in both males and females, yet the response is strongly sex limited not only for vitellogenin production but also in terms of total protein, DNA and RNA synthesis and nuclear ploidy levels. To differentiate further possible sex and/or JH related developmental aspects in locusts, large-scale nuclear events were examined during normal adult maturation and in animals treated with antiallatropins and JH analogs. Fat body nuclei undergo extensive restructuring during normal development in both sexes. This included progressive nuclear enlargement, accompanied by extensive proliferation of nuclear matrix components and elaboration of complex inclusion bodies (NB). The isolated protein matrix was unusually complex relative to similar structures from vertebrates and the NB were firmly anchored to it. Although matrix proteins were qualitatively similar to those from other sources, as assessed by SDS polyacrylamide gel electrophoresis, several major matrix polypeptides, including lamins A and B, and components greater than 150 kD, fluctuated quantitatively during development and in concert with nuclear enlargement. The number and morphology of the NB were unrelated to sex, but increased in direct proportion to absolute nuclear volumes. All changes were more pronounced in females, where higher ploidy levels, larger nuclei and correspondingly more internal matrix elements occurred. Suppression of JH production by precocene prevented all foregoing nuclear changes, but re-exposure to methoprene rapidly induced normal development. The results are compared to analogous nuclear changes in steroid responsive vertebrate tissues.