Release of Ecdysteroid-Phosphates from Egg Yolk Granules and Their Dephosphorylation during Early Embryonic Development in Silkworm, Bombyx mori

Lipids in Insect Reproduction: Where, How, and Why

Modern insects have inhabited the earth for hundreds of millions of years, and part of their successful adaptation lies in their many reproductive strategies. Insect reproduction is linked to a high metabolic rate that provides viable eggs in a relatively short time. In this context, an accurate interplay between the endocrine system and the nutrients synthetized and metabolized is essential to produce healthy offspring. Lipids guarantee the metabolic energy needed for egg formation and represent the main energy source consumed during embryogenesis. Lipids availability is tightly regulated by a complex network of endocrine signals primarily controlled by the central nervous system (CNS) and associated endocrine glands, the corpora allata (CA) and corpora cardiaca (CC). This endocrine axis provides hormones and neuropeptides that significatively affect tissues closely involved in successful reproduction: the fat body, which is the metabolic center supplying the lipid resources and energy demanded in egg formation, and the ovaries, where the developing oocytes recruit lipids that will be used for optimal embryogenesis. The post-genomic era and the availability of modern experimental approaches have advanced our understanding of many processes involved in lipid homeostasis; therefore, it is crucial to integrate the findings of recent years into the knowledge already acquired in the last decades. The present chapter is devoted to reviewing major recent contributions made in elucidating the impact of the CNS/CA/CC-fat body-ovary axis on lipid metabolism in the context of insect reproduction, highlighting areas of fruitful research.

Dynamic transcriptome analysis of Bombyx mori embryonic development

Bombyx mori has been extensively studied but the gene expression control of its embryonic development is unclear. In this study, we performed transcriptome profiling of six stages of B. mori embryonic development using RNA sequencing (RNA-seq). A total of 12 894 transcripts were obtained from the embryos. Of these, 12 456 transcripts were shared among the six stages, namely, fertilized egg, blastoderm, germ-band, organogenesis, reversal period, and youth period stages. There were 111, 48, 41, 54, 77, and 107 transcripts specifically expressed during the six stages, respectively. By analyzing weighted gene correlation networks and differently expressed genes, we found that during embryonic development, many genes related to DNA replication, transcription, protein synthesis, and epigenetic modifications were upregulated in the early embryos. Genes of cuticle proteins, chitin synthesis-related proteins, and neuropeptides were more abundant in the late embryos. Although pathways of juvenile hormone and the ecdysteroid 20-hydroxyecdysone, and transcription factors were expressed throughout the embryonic development stages, more regulatory pathways were highly expressed around the organogenesis stage, suggesting more gene expression for organogenesis. The results of RNA-seq were confirmed by quantitative real-time polymerase chain reaction of 16 genes of different pathways. Nucleic acid methylation and seven sites in histone H3 modifications were confirmed by dot blot and western blot. This study increases the understanding of the molecular mechanisms of the embryonic developmental process and information on the regulation of B. mori development.

Open questions on the functional biology of the yolk granules during embryo development

Biogenesis and consumption of the yolk are well-conserved aspects of the reproductive biology in oviparous species. Most egg-laying animals accumulate yolk proteins within the oocytes thus creating the source of nutrients and energy that will feed embryo development. Yolk accumulation drives the generation of a highly specialized oocyte cytoplasm with maternal mRNAs, ribosomes, mitochondria, and, mainly, a set of organelles collectively referred to as yolk granules (Ygs). Following fertilization, the Ygs are involved in regulated mechanisms of yolk degradation to fuel the anabolic metabolism of the growing embryo. Thus, yolk accumulation and degradation are essential processes that allow successful development in many species. Nevertheless, the molecular machinery and mechanisms dedicated to the programmed yolk mobilization throughout development are still enigmatic and remain mostly unexplored. Moreover, while the Ygs functional biology as a nutritional source for the embryo has been acknowledged, several reports have suggested that Ygs cargoes and functions go far beyond yolk storage. Evidence of the role of Ygs in gene expression, microbiota harboring, and paracrine signaling has been proposed. In this study, we summarize the current knowledge of the Ygs functional biology pointing to open questions and where further investigation is needed.

Genomic and transcriptomic analyses in Drosophila suggest that the ecdysteroid kinase-like (EcKL) gene family encodes the 'detoxification-by-phosphorylation' enzymes of insects

Phosphorylation is a phase II detoxification reaction that, among animals, occurs near exclusively in insects, but the enzymes responsible have never been cloned or otherwise identified. We propose the hypothesis that members of the arthropod-specific ecdysteroid kinase-like (EcKL) gene family encode detoxicative kinases. To test this hypothesis, we annotated the EcKL gene family in 12 species of Drosophila and explored their evolution within the genus. Many ancestral EcKL clades are evolutionarily unstable and have experienced repeated gene gain and loss events, while others are conserved as single-copy orthologs. Leveraging multiple published gene expression datasets from D. melanogaster, and using the cytochrome P450s-a classical detoxification family-as a test case, we demonstrate relationships between xenobiotic induction, detoxification tissue-enriched expression and evolutionary instability in the EcKLs and the P450s. We devised a systematic method for identifying candidate detoxification genes in large gene families that is concordant with experimentally determined functions of P450 genes in D. melanogaster. Applying this method to the EcKLs suggested a significant proportion of these genes play roles in detoxification, and that the EcKLs may constitute a detoxification gene family in insects. Additionally, we estimate that between 11 and 16 uncharacterised D. melanogaster P450s are strong detoxification candidates. Lastly, we also found previously unreported genomic and transcriptomic variation in a number of EcKLs and P450s associated with toxic stress phenotypes using a targeted phenome-wide association study (PheWAS) approach in D. melanogaster, presenting multiple future avenues of research for detoxification genetics in this species.

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.

In vivo effect of Neuropeptide F on ecdysteroidogenesis in adult female desert locusts (Schistocerca gregaria)

Neuropeptides are important regulatory factors that mediate key life processes, both in vertebrates and invertebrates. Many insect neuropeptides display pleiotropic activities, which means that they can influence multiple aspects of insect physiology. In the fruit fly, Drosophila melanogaster, Neuropeptide F (NPF) mediates diverse physiological processes, such as learning, stress responses, feeding and male courtship behavior. In locusts, only a truncated form of the predicted "full-length" NPF, the nonapeptide "trNPF", has been isolated. This nonapeptide previously proved to be biologically active, since it was shown to influence food intake and weight increase, as well as oocyte growth in adult female desert locusts (Schistocerca gregaria [Forskål]). In the present study, we have further analyzed the effect of trNPF on female reproductive physiology in S. gregaria. We confirmed that daily trNPF injections in adult females elicit an increase of oocyte size. In addition, an RNAi-mediated knockdown of the Schgr-NPF precursor transcript in adult female locusts resulted in the opposite effect, i.e. significantly smaller oocytes. Moreover, we discovered that daily injections of trNPF in adult female S. gregaria, caused higher ecdysteroid titers in the ovaries and accelerated the appearance of ecdysteroid peaks in the hemolymph of these animals. The RNAi-based knockdown of the Schgr-NPF precursor transcript clearly resulted in reduction of both hemolymph and ovarian ecdysteroid concentrations, confirming the stimulatory effects of trNPF injections on adult female ecdysteroid levels. The observed results are discussed in relation to previous reports on NPF activities in locusts and other insects.

Diapause prevention effect of Bombyx mori by dimethyl sulfoxide

HCl treatment has been, for about 80 years, the primary method for the prevention of entry into embryonic diapauses of Bombyx mori. This is because no method is as effective as the HCl treatment. In this study, we discovered that dimethyl sulfoxide (DMSO) prevented entry into the diapause of the silkworm, Bombyx mori. The effect of diapause prevention was 78% as a result of treatment with 100% DMSO concentration, and the effect was comparable to that of the HCl treatment. In contrast, in the case of non-diapause eggs, hatchability was decreased by DMSO in a concentration-dependent manner. The effect of DMSO was restricted within 24 hours after oviposition of diapause eggs, and the critical period was slightly shorter than the effective period of the HCl treatment. DMSO analogs, such as dimethyl formamide (DMF) and dimethyl sulfide (DMS), did little preventive effect against the diapause. Furthermore, we also investigated the permeation effects of chemical compounds by DMSO. When treated with an inhibitor of protein kinase CK2 (CK2) dissolved in DMSO, the prevention rate of the diapause was less than 40%. This means that the inhibition effect by the CK2 inhibitor was the inhibition of embryonic development after diapause prevention by DMSO. These data suggest that DMSO has the effects of preventing from entering into the diapause and permeation of chemicals into diapause eggs.

Unscrambling butterfly oogenesis

Background

Butterflies are popular model organisms to study physiological mechanisms underlying variability in oogenesis and egg provisioning in response to environmental conditions. Nothing is known, however, about; the developmental mechanisms governing butterfly oogenesis, how polarity in the oocyte is established, or which particular maternal effect genes regulate early embryogenesis. To gain insights into these developmental mechanisms and to identify the conserved and divergent aspects of butterfly oogenesis, we analysed a de novo ovarian transcriptome of the Speckled Wood butterfly Pararge aegeria (L.), and compared the results with known model organisms such as Drosophila melanogaster and Bombyx mori.

Results

A total of 17306 contigs were annotated, with 30% possibly novel or highly divergent sequences observed. Pararge aegeria females expressed 74.5% of the genes that are known to be essential for D. melanogaster oogenesis. We discuss the genes involved in all aspects of oogenesis, including vitellogenesis and choriogenesis, plus those implicated in hormonal control of oogenesis and transgenerational hormonal effects in great detail. Compared to other insects, a number of significant differences were observed in; the genes involved in stem cell maintenance and differentiation in the germarium, establishment of oocyte polarity, and in several aspects of maternal regulation of zygotic development.

Conclusions

This study provides valuable resources to investigate a number of divergent aspects of butterfly oogenesis requiring further research. In order to fully unscramble butterfly oogenesis, we also now also have the resources to investigate expression patterns of oogenesis genes under a range of environmental conditions, and to establish their function.

CRF-like diuretic hormone negatively affects both feeding and reproduction in the desert locust, Schistocerca gregaria

Diuretic hormones (DH) related to the vertebrate Corticotropin Releasing Factor (CRF) have been identified in diverse insect species. In the migratory locust, Locusta migratoria, the CRF-like DH (CRF/DH) is localized in the same neurosecretory cells as the Ovary Maturating Parsin (OMP), a neurohormone that stimulates oocyte growth, vitellogenesis and hemolymph ecdysteroid levels in adult female locusts. In this study, we investigated whether CRF-like DH can influence feeding and reproduction in the desert locust, Schistocerca gregaria. We identified two highly similar S. gregaria CRF-like DH precursor cDNAs, each of which also encodes an OMP isoform. Alignment with other insect CRF-like DH precursors shows relatively high conservation of the CRF/DH sequence while the precursor region corresponding to OMP is not well conserved. Quantitative real-time RT-PCR revealed that the precursor transcripts mainly occur in the central nervous system and their highest expression level was observed in the brain. Injection of locust CRF/DH caused a significantly reduced food intake, while RNAi knockdown stimulated food intake. Therefore, our data indicate that CRF-like DH induces satiety. Furthermore, injection of CRF/DH in adult females retarded oocyte growth and caused lower ecdysteroid titers in hemolymph and ovaries, while RNAi knockdown resulted in opposite effects. The observed effects of CRF/DH may be part of a wider repertoire of neurohormonal activities, constituting an integrating control system that affects food intake and excretion, as well as anabolic processes like oocyte growth and ecdysteroidogenesis, following a meal. Our discussion about the functional relationship between CRF/DH and OMP led to the hypothesis that OMP may possibly act as a monitoring peptide that can elicit negative feedback effects.

Developmental changes in the localization of protein kinase CK2 in non-diapause and diapause eggs of the silkworm, Bombyx mori

To analyze the role of protein kinase CK2 (CK2) during early embryogenesis in non-diapause and diapause of the silkworm, the distribution and localization of Bombyx mori CK2 (BmCK2) were investigated by an immunohistochemical technique using antibodies against the α- and β-subunits of BmCK2. Both were localized in blastoderm cells of non-diapause and diapause eggs until 24 h after oviposition. More than 24 h after oviposition, however, the distribution of BmCK2 was different in non-diapause and diapause eggs. In non-diapause eggs, BmCK2 was mainly localized in yolk cells. In contrast, in diapause eggs, the localization was mainly observed in germ-band cells. Furthermore, we confirmed that the RNA helicase-like protein that was localized together with BmCK2 in non-diapause eggs was phosphorylated by BmCK2 in vitro. These data suggest that the role of BmCK2 is different in non-diapause and diapause eggs.

Phosphoconjugation and dephosphorylation reactions of steroid hormone in insects

In insects, the major products of phase II metabolism of ecdysteroids, which include the molting hormone, are phosphate esters. The phosphoconjugation pathway is a reversible process, comprising two enzyme systems: ecdysteroid 22-kinase (EcKinase) and ecdysteroid-phosphate phosphatase (EPPase). We report here that: (1) the biochemical characteristics of EcKinase and EPPase, (2) the physiological significance of the reciprocal conversion of ecdysteroids and ecdysteroid phosphates in the ovary-egg system in insects, (3) the biochemical mechanism by which ecdysteroid phosphates are synthesized in the ovary, transferred to eggs, and finally dephosphorylated in eggs, and (4) the possible catalytic steps of EcKinase and EPPase on the basis of the data obtained by an in silico study. From these studies, it is obvious that ecdysteroid phosphates as well as steroid sulfates, which are major products of phase II metabolism in mammals, function as precursors for the formation of biologically active hormones.

In-vitro phosphorylation activity by recombinant alpha and beta subunits of Bombyx mori casein kinase 2

To clarify the control mechanism of the catalytic activity of casein kinase 2 (CK2) during early embryonic development in the silkworm, Bombyx mori, we attempted an in-vitro functional analysis by using the recombinant alpha and beta subunits of B. mori CK2 (rBmCK2alpha and rBmCK2beta) produced in a bacterial system. The renatured rBmCK2alpha possessed protein kinase activity. When rBmCK2alpha and rBmCK2beta were reconstituted in an approximate 1:1 molar ratio, the catalytic activity was almost the same as that of rBmCK2alpha alone. The catalytic activity of rBmCK2alpha was inhibited by polylysine, which is one of the activators of CK2 activity. However, when using the reconstituted rBmCK2alpha and rBmCK2beta (rBmCK2), activation by polylysine was observed. We examined the influence of sorbitol and 3-hydroxykynurenine (3-OHK), which are contained mainly in diapause eggs, on the phosphorylation activity of rBmCK2. Three-OHK inhibited rBmCK2 activity, but sorbitol had no effect on it. Furthermore, a functional analysis using rBmCK2alpha and beta subunits of Drosophila melanogaster CK2 revealed that a difference in the C-terminal amino acid of the CK2beta subunit influenced the phosphorylation activity of rBmCK2alpha. These results may provide new insights for clarifying the control mechanism of B. mori casein kinase 2 in eggs.

A novel RNA helicase-like protein during early embryonic development in silkworm Bombyx mori: molecular characterization and intracellular localization

In order to understand the molecular mechanism of development during early embryogenesis in diapause and non-diapause of the silkworm, mRNA from diapause and non-diapause eggs was compared using the differential display technique. We cloned the full length of a cDNA encoding a novel RNA helicase-like (RHL) protein by the RACE method using a cDNA fragment which was one of the specific cDNAs in the non-diapause eggs. A BLAST search using the predicted amino acid sequence of RHL revealed a low homology (21-25% identity of its partial length) with that of the DEAD-box RNA helicase. Gene expression of the RHL gene of the diapause and non-diapause eggs was investigated by RT-PCR until 60h after oviposition. Amplified RHL cDNA was observed through all the stages in the non-diapause eggs, while in the diapause eggs, cDNA was found in eggs 0-12h after oviposition but disappeared 24-60h after oviposition. When the diapause eggs were activated by HCl treatment after chilling at 4 degrees C for 6 days from 48h after oviposition (artificial diapause termination), cDNA was observed from 12h after HCl treatment. We also investigated the immunohistochemical distribution and localization of RHL in non-diapause eggs using anti-recombinant His-tag RHL antiserum. RHL was distributed in blastoderm cells and yolk cells and was localized in the nucleus and the cytosol of yolk cells. These data suggest that RHL has an important role in the early embryo of the silkworm.

Characterization of a tyrosine phosphatase activity in the oogenesis of Periplaneta americana

In this work, phosphatase activity was characterized in the ovary and the haemolymph of Periplaneta americana. The optimum pH for these activities was 4.0, and a temperature of 44 degrees C was ideal for the maximal enzyme activity. The phosphatase activities were inhibited by NaF, sodium tartrate, Pi, sodium orthovanadate, and ammonium molybdate. The ovarian phosphatase activity at pH 4.0 was almost exclusive against phosphotyrosine, with little or no effect on the residues of phosphoserine or phosphothreonine. These results indicate that this phosphatase activity is due to the presence of an acid tyrosine phosphatase. The phosphatase activities of acid extracts from P. americana ovaries (OEX) and an acid extract from P. americana haemolymph (HEX) were analyzed in non-denaturant gel electrophoresis using an analog substrate beta-naphtyl phosphate. The gel revealed two bands with phosphatase activity in the ovary and one band in the haemolymph; these bands were excised and submitted to a 10% SDS-PAGE showing a single 70-kDa polypeptide in both samples. Histochemistry of the ovary with alpha-naphtyl phosphate for localization of acid phosphatase activity showed mainly labeling associated to the oocyte peripheral vesicles, basal lamina, and between follicle cells. Electron microscopy analysis showed that acid phosphatase was localized in small peripheral vesicles in the oocyte, but not inside yolk granules. The possible role of this phosphatase during oogenesis and embryogenesis is also discussed in this article.

Purification, kinetic characterization, and molecular cloning of a novel enzyme, ecdysteroid 22-kinase

This is the first report succeeding in the isolation and characterization of an enzyme and its gene involved in the phosphorylation of a steroid hormone. It has been demonstrated that ecdysteroid 22-phosphates in insect ovaries, which are physiologically inactive, serve as a "reservoir" that supplies active free ecdysteroids during early embryonic development and that their dephosphorylation is catalyzed by a specific enzyme, ecdysteroid-phosphate phosphatase (Yamada, R., and Sonobe, H. (2003), J. Biol. Chem. 278, 26365-26373). In this study, ecdysteroid 22-kinase (EcKinase) was purified from the cytosol of the silkworm Bombyx mori ovaries to about 1,800-fold homogeneity in six steps of column chromatography and biochemically characterized. Results obtained indicated that the reciprocal conversion of free ecdysteroids and ecdysteroid 22-phosphates by two enzymes, EcKinase and ecdysteroid-phosphate phosphatase, plays an important role in ecdysteroid economy of the ovary-egg system of B. mori. On the basis of the partial amino acid sequence obtained from purified EcKinase, the nucleotide sequence of the cDNA encoding EcKinase was determined. The full-length cDNA of EcKinase was composed of 1,850 bp with an open reading frame encoding a protein of 386 amino acid residues. The cloned cDNA was confirmed to encode the functional EcKinase using the transformant harboring the open reading frame of EcKinase. A data base search showed that EcKinase has an amino acid sequence characteristic of phosphotransferases, in that it harbors Brenner's motif and putative ATP binding sites, but there are no functional proteins that share high identity with the amino acid sequence of EcKinase.

ERK/MAPK regulates ecdysteroid and sorbitol metabolism for embryonic diapause termination in the silkworm, Bombyx mori

The eggs of the silkworm Bombyx mori undergo a state of suspended overt development and diminished metabolism called "diapause" to escape adverse environmental conditions. Termination of Bombyx embryonic diapause requires 2-3 months of low temperature (5 degrees C), but the molecular mechanisms underlying diapause termination are unknown. Diapause termination requires a decrease in the sorbitol concentration, which arrests embryonic development, and the secretion factors from yolk cells that promote embryonic development. In the present study, we report that 20-hydroxyecdysone promoted the development of denuded embryos and that ecdysteroid-phosphate phosphatase (EPPase), which is a key enzyme for active ecdysteroid production, was induced by incubation of diapausing eggs at 5 degrees C. In dechorionated egg cultures, extracellular signal-regulated kinase (ERK), which is activated by incubating diapausing eggs at 5 degrees C, regulated sorbitol-glycogen conversion, ecdysteroid secretion via gene transcription of key enzymes, sorbitol dehydrogenase-2, and EPPase, suggesting that ERK has a key role in diapause termination.

Effects of juvenile hormone analogs and 20-hydroxyecdysone on diapause termination in eggs of Locusta migratoria and Oxya yezoensis

To understand the hormonal control of embryonic diapause, juvenile hormone analogs (JHAs), methoprene and hydroprene, and 20-hydroxyecdysone (20E) were applied onto diapause eggs of Locusta migratoria and Oxya yezoensis. These insects enter diapause at the mid-stage of embryogenesis prior to blastokinesis. Topical application of JHAs significantly facilitated diapause termination in both species but JHA-treated embryos underwent abnormal morphogenesis, pigmentation and sclerotization without dorsal closure. The Locusta eggs immersed in the 20E solution for 24h terminated diapause in a dose-dependent manner. We also investigated phosphorylation of extracellular signal-regulated kinase (ERK), a member of mitogen-activated protein kinase (MAPK), during diapause-terminating process of Locusta migratoria and found that ERK was activated either by cold exposure or JHA treatment. The possible involvement of the hormones and ERK in embryonic diapause and the possibility of ecdysteroids synthesis by prothoracic glands of diapause embryo were proposed.