Temporal changes in DNA synthesis of prothoracic gland cells during larval development and their correlation with ecdysteroidogenic activity in the silkworm,Bombyx mori

Positive feedback regulation of prothoracicotropic hormone secretion by ecdysteroid--a mechanism that determines the timing of metamorphosis

When insect larvae have fully grown, prothoracicotropic hormone (PTTH) is released from the brain, triggering the initiation of metamorphic development through stimulation of ecdysteroid secretion by the prothoracic glands. The present study analyzes the mechanism that regulates the occurrence of this PTTH surge. In the silkworm Bombyx mori, the PTTH surge occurs on day 6 of the fifth instar and is preceded by a small rise in hemolymph ecdysteroid titer, which occurs late on day 5. We therefore hypothesized that this rise of ecdysteroid titer is involved in the induction of the PTTH surge. To test this hypothesis, two experiments were conducted. First, a small amount of 20-hydroxyecdysone was injected on day 4, two days before the expected day of the PTTH surge, to simulate the small rise in hemolymph ecdysteroid titer on day 5. This injection led to a precocious surge of PTTH the next day. Next, the hemolymph ecdysteroid titer on day 5 was artificially lowered by injecting ecdysteroid-22-oxidase, which inactivates 20-hydroxyecdysone. After this treatment, the PTTH surge did not occur on day 6 in 80% of the animals. These results indicate that a small rise of the hemolymph ecdysteroid titer plays a critical role in the induction of the PTTH surge. Since basal ecdysteroidogenic activity of the prothoracic glands increases with larval growth, a circulating level of ecdysteroids may convey information about larval maturity to the brain, to coordinate larval growth and metamorphosis. This is the first report in invertebrates to demonstrate positive feedback regulation of the surge of a tropic hormone by a downstream steroid hormone.

Insulin signaling pathways in lepidopteran ecdysone secretion

Molting and metamorphosis are stimulated by the secretion of ecdysteroid hormones from the prothoracic glands. Insulin-like hormones have been found to enhance prothoracic gland activity, providing a mechanism to link molting to nutritional state. In silk moths (Bombyx mori), the prothoracic glands are directly stimulated by insulin and the insulin-like hormone bombyxin. Further, in Bombyx, the neuropeptide prothoracicotropic hormone (PTTH) appears to act at least in part through the insulin-signaling pathway. In the prothoracic glands of Manduca sexta, while insulin stimulates the phosphorylation of the insulin receptor and Akt, neither insulin nor bombyxin II stimulate ecdysone secretion. Involvement of the insulin-signaling pathway in Manduca prothoracic glands was explored using two inhibitors of phosphatidylinositol-3-kinase (PI3K), LY294002 and wortmannin. PI3K inhibitors block the phosphorylation of Akt and 4EBP but have no effect on ecdysone secretion, or on the phosphorylation of the MAPkinase, ERK. Inhibitors that block phosphorylation of ERK, including the MEK inhibitor U0126, and high doses of the RSK inhibitor SL0101, effectively inhibit ecdysone secretion. The results highlight differences between the two lepidopteran insects most commonly used to directly study ecdysteroid secretion. In Bombyx, the PTTH and insulin-signaling pathways intersect; both insulin and PTTH enhance the phosphorylation of Akt and stimulate ecdysteroid secretion, and inhibition of PI3K reduces ecdysteroid secretion. By contrast, in Manduca, the action of PTTH is distinct from insulin. The results highlight species differences in the roles of translational regulators such as 4EBP, and members of the MAPkinase pathway such as ERK and RSK, in the regulation of insect ecdysone secretion, and in the impact of nutritionally-sensitive hormones such as insulin in the control of ecdysone secretion and molting.

TOR signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in the silkworm, Bombyx mori

The prothoracicotropic hormone (PTTH) is a stimulator of ecdysteroidogenesis in prothoracic gland of larval insects. Our recent studies showed that phosphoinositide 3-kinase (PI3K)/Akt signaling was involved in PTTH-stimulated ecdysteroidogenesis by Bombyx mori prothoracic glands. In the present study, downstream signaling of PI3K/Akt was further investigated. Results showed that PTTH rapidly enhanced the phosphorylation of translational repressor 4E-binding protein (4E-BP) and p70 ribosomal protein S6 kinase (S6K), two known downstream signaling targets of the target of rapamycin complex 1 (TORC1). PTTH stimulated 4E-BP phosphorylation in time- and dose-dependent manners. Injection of PTTH into day-6 last instar larvae greatly increased 4E-BP phosphorylation, verifying the in vitro effect. PTTH-stimulated 4E-BP phosphorylation was blocked by both LY294002 and wortmannin, indicating the involvement of PI3K. Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors (PD 98059 and U0126), did not inhibit PTTH-stimulated 4E-BP phosphorylation, implying that ERK signaling is not related to PTTH-stimulated 4E-BP phosphorylation. The phosphorylation of S6K was also stimulated by PTTH both in vitro and in vivo. PI3K signaling appears to be involved in PTTH-stimulated phosphorylation of S6K. Rapamycin, a specific inhibitor of mammalian TOR signaling attenuated PTTH-stimulated phosphorylation of 4E-BP and S6K of the glands, and greatly inhibited PTTH-stimulated ecdysteroidogenesis. Examination of gene expression levels of 4E-BP and S6K showed that PTTH inhibited mRNA levels of both 4E-BP and S6K, indicating that PTTH may exert its action at both the transcriptional and phosphorylation levels. These results suggest that PTTH/PI3K/TOR/4E-BP (S6K) signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in B. mori.

Autophagy, apoptosis, and ecdysis-related gene expression in the silk gland of the silkworm (Bombyx mori) during metamorphosis

Degeneration of larval-specific tissues during insect metamorphosis has been suggested to be the result of apoptosis and autophagy and is triggered by ecdysteroids. However, the relationship between autophagy and apoptosis pathways and the mechanism of regulation by ecdysteroids remain to be elucidated. This study examined the events of autophagy, apoptosis, and the expression of ecdysis-related genes in the silk gland of the silkworm ( Bombyx mori L., 1758) during the larval to pupal transformation. The results indicated that autophagic features appeared in the silk gland at the wandering and spinning stages of the larvae, whereas the apoptotic features such as apoptotic bodies and DNA fragmentation occurred at the prepupal or early-pupal stages. The autophagic granules fused with each other to form large vacuoles where the cytoplasmic material was degraded. Autophagosomes, autolysosomes, and apoptotic bodies were found later in the degenerating silk-gland cells. Expression of the ecdysone receptor gene BmEcR and the transcription factor genes BmE74A and BmBR-C preceded the onset of autophagy and apoptosis, indicating that they may be responsible for triggering these programmed cell death pathways in the silk gland. The results suggest that both autophagy and apoptosis occur in the silk-gland cells during degeneration, but autophagy precedes apoptosis.

Control of ecdysteroidogenesis in prothoracic glands of insects: a review

The very first step in the study of the endocrine control of insect molting was taken in 1922. Stefan Kopec characterized a factor in the brain of the gypsy moth, Lymantria dispar which appeared to be essential for metamorphosis. This factor was later identified as the neuropeptide prothoracicotropic hormone (PTTH), the first discovery of a series of factors involved in the regulation of ecdysteroid biosynthesis in insects. It is now accepted that PTTH is the most important regulator of prothoracic gland (PG) ecdysteroidogenesis. The periodic increases in ecdysteroid titer necessary for insect development can basically be explained by the episodic activation of the PGs by PTTH. However, since the characterization of the prothoracicostatic hormone (PTSH), it has become clear that in addition to 'tropic factors', also 'static factors', which are responsible for the 'fine-tuning' of the hemolymph ecdysteroid titer, are at play. Many of these regulatory factors are peptides originating from the brain, but also other, extracerebral factors both of peptidic and non-peptidic nature are able to affect PG ecdysteroidogenesis, such as the 'classic' insect hormones, juvenile hormone (JH) and the molting hormone (20E) itself. The complex secretory pattern of ecdysteroids as observed in vivo is the result of the delicate balance and interplay between these ecdysiotropic and ecdysiostatic factors.

An ecdysteroid-inducible insulin-like growth factor-like peptide regulates adult development of the silkmoth Bombyx mori

Insulin-like growth factors (IGFs) play essential roles in fetal and postnatal growth and development of mammals. They are secreted by a wide variety of tissues, with the liver being the major source of circulating IGFs, and regulate cell growth, differentiation and survival. IGFs share some biological activities with insulin but are secreted in distinct physiological and developmental contexts, having specific functions. Although recent analyses of invertebrate genomes have revealed the presence of multiple insulin family peptide genes in each genome, little is known about functional diversification of the gene products. Here we show that a novel insulin family peptide of the silkmoth Bombyx mori, which was purified and sequenced from the hemolymph, is more like IGFs than like insulin, in contrast to bombyxins, which are previously identified insulin-like peptides in B. mori. Expression analysis reveals that this IGF-like peptide is predominantly produced by the fat body, a functional equivalent of the vertebrate liver and adipocytes, and is massively released during pupa-adult development. Studies using in vitro tissue culture systems show that secretion of the peptide is stimulated by ecdysteroid and that the secreted peptide promotes the growth of adult-specific tissues. These observations suggest that this peptide is a Bombyx counterpart of vertebrate IGFs and that functionally IGF-like peptides may be more ubiquitous in the animal kingdom than previously thought. Our results also suggest that the known effects of ecdysteroid on insect adult development may be in part mediated by IGF-like peptides.

Effects of RH-5992 on ecdysteroidogenesis of the prothoracic glands during the fourth larval instar of the silkworm, Bombyx mori

Stage-dependent effects of RH-5992 on ecdysteroidogenesis of the prothoracic glands during the fourth larval instar of the silkworm, Bombyx mori, were studied in the present report. When larvae were treated with RH-5992 during the early stages of the fourth larval instar (between day 0 and day 1), initially ecdysteroid levels in the hemolymph were inhibited. However, 24 h after RH-5992 application, ecdysteroid levels were greatly increased as compared with those treated with acetone. The examination of the in vitro prothoracic gland activity upon RH-5992 application during the early stages of the fourth larval instar confirmed a short-term inhibitory effect. When RH-5992 was applied to the later stages of the fourth larval instar, no effects on both hemolymph ecdysteroid levels and prothoracic gland activity were observed. Addition of RH-5992 to incubation medium strongly inhibited ecdysteroid secretion by the prothoracic glands from the early fourth instar, indicating direct action of RH-5992 on ecdysteroidogenesis by prothoracic glands. Four hours after application with RH-5992 on day 1.5, prothoracic glands still showed an activated response to PTTH in both PTTH-cAMP signaling and the extracellular signal-regulated kinase (ERK) signaling. Moreover, addition of RH-5992 to incubation medium did not interfere with the stimulatory effect of the glands to PTTH in ecdysteroidogenesis. These results indicated that both PTTH-cAMP signaling and PTTH-ERK signaling may not be involved in short-term inhibitory regulation by RH-5992.

Autocrine activation of ecdysteroidogenesis in the prothoracic glands of the silkworm, Bombyx mori

Ecdysteroidogenesis in the prothoracic glands is activated by the neuropeptide, prothoracicotropic hormone (PTTH). The present study demonstrates autocrine activation of ecdysteroidogenesis in prothoracic glands of the silkworm, Bombyx mori. Using both a long-term in vitro organ culture system and an ecdysteroid radioimmunoassay, it was found that either decreasing the incubation volume, from 100 to 5 microl, or increasing the number of glands incubated per drop (50 microl) from 1 to 5 significantly increased ecdysteroid secretion. Prothoracic gland-conditioned medium was used to clarify the autocrine factor. The results showed that activation of ecdysteroidogenesis by the prothoracic gland-conditioned medium appeared to be dose dependent and a dramatic increase in ecdysteroid secretion was observed after 6h of incubation in the conditioned medium. Moreover, it appeared that autocrine activation occurred when glands were incubated in large volumes of incubation medium and during a short incubation period, indicating that the factor may exert its action in situ at some specific developmental stages. This tropic factor was further characterized, and it was found that the factor seemed to be heat-stable, with a molecular weight estimated to be between 1000 and 3000 Da. Injection of the concentrated putative autocrine factor into day 5 last instar larvae greatly increased ecdysteroidogenic activity of the prothoracic glands compared to those injected with saline, indicating the possible in vivo function of the present factor.

Both prothoracicotropic hormone and an autocrine factor are involved in control of prothoracic gland ecdysteroidogenesis in Locusta migratoria and Schistocerca gregaria

In Bombyx mori, ecdysteroidogenesis by the prothoracic glands (PGs) is controlled by both prothoracicotropic hormone (PTTH) and a factor secreted by the glands themselves. This factor, which is active both in vitro and in vivo, has been named 'autocrine factor' (AF). To find out whether or not this dual control also exists in other species, in particular in hemimetabolous ones, we applied similar methods as were used to discover AF in Bombyx to the locusts Locusta migratoria and Schistocerca gregaria. Our results unequivocally show that locust PGs also secrete an as yet unidentified autocrine factor. Possible roles of AF are discussed.

Stage-dependent effects of starvation on the growth, metamorphosis, and ecdysteroidogenesis by the prothoracic glands during the last larval instar of the silkworm, Bombyx mori

The stage-dependent effects of starvation on the growth, metamorphosis, and ecdysteroidogenesis of the prothoracic glands during the last larval instar of the silkworm, Bombyx mori, were studied in the present study. When last instar larvae were starved beginning on day 1 of that instar, all larvae died between days 5 and 7 of the instar. Although the prothoracicotropic hormone (PTTH) release from the brain-corpus cardiacum-corpus allatum (BR-CC-CA) did not significantly change during starvation, a deficiency in PTTH signal transduction was maintained, which led to very low levels of hemolymph ecdysteroids after the beginning of starvation. However, when starvation began on day 3 of the last larval instar, the major hemolymph ecdysteroid peak, preceding larval-pupal transformation, occurred 1 day earlier than that in control larvae. Protein content of the prothoracic glands in day 3-starved larvae was maintained at a low level as compared to that of control larvae. The secretory activity of the prothoracic glands in day 3-starved larvae was maintained at a level similar to that of control larvae. However, the rate of ecdysteroidogenesis, expressed per microgram of glandular protein, was greatly enhanced in these starved larvae, indicating that upon starvation, larvae increased the ecdysteroid production rate to enhance the rate of survival.

Autocrine activation of DNA synthesis in prothoracic gland cells of the silkworm, Bombyx mori

Autocrine activation of DNA synthesis in prothoracic gland cells in last instar larvae of the silkworm, Bombyx mori, was studied using both a long-term in vitro organ culture system and immunocytochemical labeling with 5-bromo-2'-deoxyuridine (BrdU). When prothoracic glands were incubated in a small volume of culture medium (10 microl/gland), the numbers of DNA-synthesizing cells per gland increased significantly, and DNA synthesis was stimulated less by hemolymph, as compared with glands incubated in a large volume (50 microl/gland). Moreover, glands cultured in groups (6 glands per group in a 50-microl drop) also resulted in much higher levels of DNA synthesis than those cultured individually in a 50-microl drop. The mechanism by which alternation of the volume of the incubation medium results in changes in the levels of DNA synthesis was further examined. When prothoracic glands were incubated in medium (50-microl drop per gland) that was preconditioned with glands (in a 10-microl drop individually), a dramatic increase in DNA synthesis activity was also observed, indicating that prothoracic glands may release a factor that stimulates their own DNA synthesis. The growth-promoting factor was further characterized and it was found that the factor is heat stable, and its molecular weight was estimated to be between 1,000 and 3,000 Da. Moreover, the factor also stimulated corpus allatum cell DNA synthesis in vitro. Injection of concentrated putative growth-promoting factor into day 4 last instar-ligated larvae greatly increased cell DNA synthesis of the prothoracic glands, indicating the in vivo function of the present autocrine factor.