Accumulation of 3-hydroxykynurenine in ovarian follicles in relation to diapause in the silkworm, Bombyx mori L

Circadian Clock Gene Period Contributes to Diapause via GABAeric-Diapause Hormone Pathway in Bombyx mori

Diapause is a developmental transition in insects based on seasonal adaptation to adversity; it is regulated by a circadian clock system and the endocrine system. However, the molecular node and its mechanism underlying the effects of these systems are still unclear. Here, a mutant of Bombyx mori with the circadian clock gene Period (Per) knocked out was constructed, which dramatically changed the classic diapause-destined pathway. Per-knockout silkworms powerfully attenuated, but could not completely block, the predetermined effects of temperature and photoperiod on diapause determination, and this effect depended on the diapause hormone (DH) pathway. The impaired transcription-translation feedback loop of the circadian clock system lacking the Per gene caused direct up-regulation of the expression of GRD, a receptor of γ-aminobutyric acid (GABA), by changing expression level of Cycle. The synthesis of GABA in the tissue complex of brain-suboesophageal ganglion then increased and restricted the decomposition, which continuously promoted the GABAergic signal to play a role, and finally inhibiting (delaying) the release of DH to the hemolymph, and reducing the diapause-inducing effect of DH. The results provided an example to explain the regulatory mechanism of the circadian clock on endocrine hormones in the silkworm.

Bombyx mori kynurenine 3-monooxygenase gene editing and insect molecular breeding using the clustered regularly interspaced short palindromic repeat/CRISPR associated protein 9 system

Genome editing by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR associated protein (Cas)9, a third-generation gene scissors, and molecular breeding at the genome level are attracting considerable attention as future breeding techniques. In the present study, genetic and phenotypic analyses were conducted to examine the molecular breeding of Bombyx mori through CRISPR/Cas9-mediated editing of the kynurenine 3-monooxygenase (KMO) gene. The synthesized guide RNAs (gRNAs) were analyzed using T7 endonuclease I after introduction into the BM-N silkworm cell line. To edit the silkworm gene, K1P gRNA, and Cas9 complexes were microinjected into silkworm embryos. After microinjection, the hatching rate and the incidence of mutation were determined as 18.1% and 60%, respectively. Gene mutation was verified in the heterozygous G0 generation, but no phenotypic change was observed; however, certain embryos and moths produced through sib-mating had significant differences compared to the wild-type. In successive generations, a distinct phenotypic change was also observed by continuous mating. Thus, although there are limitations in the phenotypic expression in breeding through the induction of deletion mutations, as in the present study, the process is believed to yield successful results within a shorter period compared to traditional breeding and is safer than transgenic technology.

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.

Molecular properties and tissue distribution of 30K proteins as ommin-binding proteins from diapause eggs of the silkworm, Bombyx mori

We previously reported the purification of an ommin-binding protein (OMBP) from an acid-methanol extract of diapause eggs of the silkworm and that OMBP reacted with the anti-30K proteins antiserum. In order to clarify the relationship between OMBP and the 30K proteins, we attempted to determine the sequence of the N-terminal amino acid of OMBP, which was separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). We observed ten protein spots of various isoelectric points; the spots corresponded with 30 kDa. Based on the sequence of the N-terminal amino acid (20 residues), the spots belonged to two kinds of 30K proteins (6G1 and 19G1), which are known as the major plasma proteins in the larval hemolymph of the silkworm. The proteins are expected to attach to polysaccharide because they reacted with concanavalin A and elderberry bark lectin. Immunohistochemical observations clarified that the proteins were localized in yolk granules and serosa in the diapause egg. These results suggest that OMBP is composed of 30K proteins which were modified with polysaccharides. In addition, the expression of 30K proteins mRNA was observed at early embryonic stage in diapause eggs by RT-PCR analysis. The 30K proteins as OMBP may play an important role in the transport and accumulation of tryptophan metabolites and ommochrome during the formation of serosa.

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.

Artificial parthenogenesis and control of voltinism to manage transgenic populations in Bombyx mori

In order to improve the management of transformed populations in a routine application of transgenesis technology in Bombyx mori, we modified its mode of reproduction and its voltinism. On one hand, after a stable integration of the gene of interest by transgenesis, it is preferable to maintain this gene in an identical genomic context through successive generations. This can be obtained by artificial parthenogenetic reproduction (ameiotic parthenogenesis) giving isogenic females identical to their transformed mother. On the other hand, it is essential to obtain continuous generations (polyvoltinism) after microinjection, in order to screen positive transgenic insects and study genetics and insertion of the transgene. Thereafter, it is more convenient to store these populations, as diapause eggs before their use in biotechnology application. We obtained such polyvoltine parthenoclones, first by selection for a parthenogenetic character in polyvoltine races, and second, by selection for a polyvoltine character in a parthenogenetic, but diapausing clone of B. mori. As diapause was directly under the control of diapause hormone (DH), we also tested direct injection of DH in female pupae of polyvoltine strains, as well as anti-DH antibody treatment to eliminate diapause in univoltine strains. We discussed the advantages and limitations of these methods and proved the feasibility in obtaining polyvoltine parthenoclones and determining the voltinism in B. mori. These methods would permit us to improve the management of populations used in transgenesis technology.

Neuroendocrine control of diapause hormone secretion in the silkworm, Bombyx mori

To clarify the control mechanism of diapause hormone (DH) secretion in the silkworm Bombyx mori a series of anatomical and pharmacological experiments were carried out. The arrangement of 'diapause' and 'non-diapause' eggs in the ovarioles of the moths was determined by the coloration method to estimate the accumulation of 3-hydroxykynurenine in the eggs. The females destined to lay non-diapause eggs (non-diapause producers) had diapause eggs in their ovaries if their subesophageal ganglions (Sg) had been surgically removed at 2days after larval-pupal ecdysis or later. In contrast when the surgical extirpation extended to the brain and the corpora cardiaca (CC)-corpora allata (CA) complex in addition to the Sg, the non-diapause producers had no diapause eggs. When the Sg was removed from the females destined to lay diapause eggs (diapause-producers), diapause eggs appeared in response to the treatment at 2days after larval-pupal ecdysis, but the appearance of diapause eggs was delayed by 2days when the brain-CC-CA complex was included among the organs removed. These observations suggested that DH is produced in Sg and transferred to the CC-CA complex, and that the secretion of DH from the complex is suppressed in non-diapause producers. The pattern of diapause and non-diapause eggs induced by the transection of the subesophageal connective in diapause and non-diapause producers suggested a regenerative and secretory capacity of the neurosecretory cells after the operation. The appearance of diapause eggs in non-diapause producers with transected protocerebrum of the brain confirmed that there was an inhibitory center in the protocerebrum. Changes in parts of the ovarioles containing diapause and non-diapause eggs with time of injection of gamma-aminobutyric acid (GABA) and picrotoxin suggested that a GABAergic inhibitory mechanism in DH secretion may be active in non-diapause producers but inactive in diapause producers throughout the pupal stage.

Genetic and biochemical studies on ommochrome genesis in an albino strain of a terrestrial isopod, Armadillidium vulgare

Genetic studies and quantitative determination of levels of 3-hydroxykynurenine and kynurenine were performed in an albino strain of a terrestrial isopod Armadillidium vulgare. From the results of matings between the albino and the albino, the red, the dark red, or the wild type individuals, the albino A. vulgare seems to be regulated by an autosomal gene(s) recessive to its wild allele. Litter mating of F1 progenies obtained by crossing the albino and the red mutant or the albino and the dark red mutant yielded progenies at a ratio of 3:6:3:4 for the red, the dark red, the wild, and the albino phenotypes, respectively. The albino gene(s) seems not to be allelic but to be epistatic to the red gene(s) with respect to ommochrome biosynthesis. Quantitative determination of 3-hydroxykynurenine carried out by high-performance liquid chromatography with electrochemical detection revealed that the 3-hydroxykynurenine content in the albino was significantly lower than that in the wild or the red type. The whole content of 3-hydroxykynurenine after enzymatic conversion of kynurenine to 3-hydroxykynurenine was still considerably lower than that found in the wild type, even though it increased after the conversion. The albino gene(s) seems to be associated with a blockage at distinct level(s) of ommochrome biosynthesis.

Silkworm Bombyx mori L.: Nature of Diapause Factor

The diapause factor which is responsible for the induction of embryonic diapause in the silkworm (Bombyx mori L.) was isolated from the heads of female moths. It was extracted with 80 percent ethanol or 80 percent methanol and was heat-stable and not dialyzable. Since the active material was precipitated with ammonium sulfate and lost when incubated with proteolytic enzymes, it is highly probable that the diapause factor is a protein or a complex molecule containing peptide bonds. Molecular sieve techniques revealed two species of the diapause factor. The molecular weight of the smaller species lies between 5,000 and 10,000.