Early events in peptide-stimulated ecdysteroid secretion by the prothoracic glands of Manduca sexta

Responses of PTTH-producing neurosecretory neurons in Lymantria dispar caterpillars exposed to cadmium

Lymantria dispar, as most invasive insect species, is very adaptable and reacts quickly to changing environment. Neuroendocrine system first reacts to stress in insects, and specific neurohormonal reorganization may be used in early heavy metal risk assessment. Prothoracicotropic neurohormones (PTTH) control ecdysteroid synthesis (morphogenetic and stress hormones) in insects. In this article, we report the presence of PTTH immunoreactive molecules in L2' dorsolateral neurosecretory neurons (nsn) in caterpillar brains and changes after exposure to pollutant stress of different intensity. For 3 days, after molting into the 4th instar, caterpillars of Lymantria dispar were fed with a high wheat germ diet without (control) or with added cadmium (experimental groups: 10, 30, 100, 250 μg Cd/g dry food weight). Changes in PTTH producing L2' nsn and differences in the intensity of protein bands in the region of PTTH molecular mass (Mr 11-15 kDa) were analyzed. The number of L2' neurons tended to decrease except in the group given the highest cadmium concentration (250 μg). The neurons were enlarged after acute treatment especially in the group given the highest cadmium concentration. The size of L2' nsn nuclei was decreased only in the group fed with 30 μg Cd. Protein band intensity in the Mr region of PTTH remained unchanged in all groups except for the group given the diet with the highest Cd concentration.

Neuroendocrine regulation of Drosophila metamorphosis requires TGFbeta/Activin signaling

In insects, initiation of metamorphosis requires a surge in the production of the steroid hormone 20-hydroxyecdysone from the prothoracic gland, the primary endocrine organ of juvenile larvae. Here, we show that blocking TGFβ/Activin signaling, specifically in the Drosophila prothoracic gland, results in developmental arrest prior to metamorphosis. The terminal, giant third instar larval phenotype results from a failure to induce the large rise in ecdysteroid titer that triggers metamorphosis. We further demonstrate that activin signaling regulates competence of the prothoracic gland to receive PTTH and insulin signals, and that these two pathways act at the mRNA and post-transcriptional levels, respectively, to control ecdysone biosynthetic enzyme expression. This dual regulatory circuitry may provide a cross-check mechanism to ensure that both developmental and nutritional inputs are synchronized before initiating the final genetic program leading to reproductive adult development. As steroid hormone production in C. elegans and mammals is also influenced by TGFβ/Activin signaling, this family of secreted factors may play a general role in regulating developmental transitions across phyla.

Prothoracicotropic hormone regulates developmental timing and body size in Drosophila

In insects, control of body size is intimately linked to nutritional quality as well as environmental and genetic cues that regulate the timing of developmental transitions. Prothoracicotropic hormone (PTTH) has been proposed to play an essential role in regulating the production and/or release of ecdysone, a steroid hormone that stimulates molting and metamorphosis. In this report, we examine the consequences on Drosophila development of ablating the PTTH-producing neurons. Surprisingly, PTTH production is not essential for molting or metamorphosis. Instead, loss of PTTH results in delayed larval development and eclosion of larger flies with more cells. Prolonged feeding, without changing the rate of growth, causes the overgrowth and is a consequence of low ecdysteroid titers. These results indicate that final body size in insects is determined by a balance between growth-rate regulators such as insulin and developmental timing cues such as PTTH that set the duration of the feeding interval.

Altering the fate of stem cells from midgut of Heliothis virescens:the effect of calcium ions

Cultured stem cells from larval midgut tissue of the lepidopteran Heliothis virescens respond to alterations in external calcium ion concentration (Ca(2+) (out)) by changing the rate of stem cell proliferation and by differentiating to larval or non-larval phenotypes. Decreasing the external concentration of Ca(2+) with the Ca(2+) chelating agent EGTA increased proliferation of stem cells in culture, and doubled the proportion of cells differentiating to columnar and goblet cells typical of larval midgut compared to controls. In contrast, increasing inward transport of Ca(2+) into the cells by increasing the concentration of external calcium ion concentration, or by incubation with the Ca(2+) ionophore A23187 (which tends to open inward plasma membrane Ca(2+) channels), induced dose-dependent differentiation to non-midgut cell types such as squamous and scale-like cells. However, the latter treatments did not significantly alter stem cell proliferation or differentiation to normal larval midgut epithelium.

Expression of genes encoding proteins involved in ecdysteroidogenesis in the female mosquito, Aedes aegypti

A blood meal induces the ovaries of female Aedes aegypti mosquitoes to produce ecdysteroid hormones that regulate many processes required for egg maturation. Various proteins involved in the intracellular transport and biosynthesis of ecdysteroid precursors have been identified by analysis of Drosophila melanogaster mutants and by biochemical and molecular techniques in other insects. To begin examining these processes in mosquito ovaries, complete cDNAs were cloned for putative orthologs of diazepam-binding inhibitor (DBI), StAR-related lipid transfer domain containing protein (Start1), aldo/keto reductase (A/KR), adrenodoxin reductase (AR), and the cytochrome P450 enzymes, CYP302a1 (22-hydroxylase), CYP315a1 (2-hydroxylase) and CYP314a1 (20-hydroxylase). As shown by RT-PCR, transcripts for all seven genes were present in ovaries and other tissues both before and following a blood meal. Expression of these genes likely supports the low level of ecdysteroids produced in vitro (7-10 pg /tissue/6 h) by tissues other than ovaries. Ovaries from females not blood fed and up to 6 h post blood meal (PBM) also produced low amounts of ecdysteroids in vitro, but by 18 and 30 h PBM, ecdysteroid production was greatly increased (75-106 pg/ovary pair/6h) and thereafter (48 and 72 h PBM) returned to low levels. As determined by real-time PCR analysis, gene transcript abundance for AedaeCYP302 and AedaeCYP315a1 was significantly greater (9 and 12 fold, respectively) in ovaries during peak ecdysteroid production relative to that in ovaries from females not blood fed or 2 h PBM. AedaeStart1, AedaeA/KR and AedaeAR also had high transcript levels in ovaries during peak ecdysteroid production, and AedaeDBI transcripts had the greatest increase at 48 h PBM. In contrast, gene transcript abundance of AedaeCYP314a1 decreased PBM. This study shows for the first time that transcription of a few key genes for proteins involved in ecdysteroid biosynthesis is positively correlated with the rise in ecdysteroid production by ovaries of a female insect.

Interactions between Ca2+ and cAMP in ecdysteroid secretion from the prothoracic glands of Bombyx mori

The interaction between Ca2+ and cAMP in the mediation of ecdysteroid secretion from prothoracic glands (PGs) of Bombyx mori was investigated in vitro. Omission of Ca2+ from the PGs' incubation medium decreased basal ecdysteroid secretion from day 3 until day 6. On day 6, the ability of forskolin or 3-isobutyl-1-methylxanthine (IBMX) to stimulate ecdysteroid secretion was affected by the omission of Ca2+ from the medium. The cAMP agonist Sp-adenosine 3',5'-cyclic monophosphothioate (Sp-cAMPS) and the cAMP analogue dibutyryl cyclic AMP (dbcAMP) stimulated ecdysteroid secretion even in the absence of Ca2+ from the medium. The Sp-cAMPS-stimulated ecdysteroid secretion was inhibited by the cAMP antagonist Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) and the L-type Ca2+ channel blocker verapamil. Both the Ca2+ ionophore A23187 and the L-type Ca2+ channel agonist S(-) Bay K 8644 could stimulate ecdysteroid secretion. The A23187-induced ecdysteroid secretion was partially inhibited by Rp-cAMPS. The combined results indicate that Ca2+ and cAMP signaling pathways can cooperatively, as well as independently, stimulate ecdysteroid secretion from the PGs.

Intracellular calcium in prothoracic glands of Manduca sexta

Cytosolic free calcium was measured in individual prothoracic gland cells of Manduca larvae with Fura-2. During the last larval instar there was no correlation between intracellular calcium concentration and ecdysteroid secretion by the glands. The addition of prothoracicotropic hormone (PTTH) from brains of Manduca larvae to prothoracic glands in vitro resulted in a significant increase in the calcium concentration of the gland cells. The effect of PTTH was inhibited by the inorganic calcium channel antagonists, cadmium, lanthanum and nickel, and by the antagonist of T-type calcium channels, amiloride, whereas all the other antagonists tested failed to block the action of PTTH. TMB-8, an inhibitor of intracellular calcium mobilization, did not reduce the PTTH-induced rise in calcium, which suggests that IP(3)-dependent intracellular calcium stores are not involved in the calcium-mediated stimulation of ecdysteroid synthesis. Moreover, PTTH is thought to increase intracellular calcium in prothoracic glands of Manduca by influencing calcium channels in the plasma membrane.

Molecular cloning, developmental expression, and phosphorylation of ribosomal protein S6 in the endocrine gland responsible for insect molting

Phosphorylation of ribosomal protein S6 is requisite for prothoracicotropic hormone (PTTH)-stimulated specific protein synthesis and subsequent ecdysteroidogenesis in the prothoracic glands of the tobacco hornworm, Manduca sexta. To better understand the role of S6 in regulating ecdysteroidogenesis, S6 cDNA was isolated from a Manduca prothoracic gland cDNA library and sequenced. The deduced protein is comprised of 253 amino acids, has a molecular weight of 29,038, and contains four copies of a 10-amino acid motif defining potential DNA-binding sites. This Manduca S6 possesses a consensus recognition sequence for the p70(s6k) binding domain as well as six seryl residues at the carboxyl-terminal sequence of 17 amino acids. Phosphoamino acid analysis revealed that the phosphorylation of Manduca prothoracic gland S6 is limited exclusively to serine residues. Although alterations in the quantity of S6 mRNA throughout the last larval instar and early pupal-adult development were not well correlated with the hemolymph ecdysteroid titer, developmental expression and phosphorylation of S6 were temporally correlated with PTTH release and the hemolymph ecdysteroid titer. These data provide additional evidence that S6 phosphorylation is a critical element in the transduction pathway leading to PTTH-stimulated ecdysteroidogenesis.

Investigation of presumptive mobilization pathways for calcium in the steroidogenic action of big prothoracicotropic hormone

Ecdysteroidogenesis in the prothoracic glands of the tobacco hornworm Manduca sexta is stimulated by the cerebral neuropeptide prothoracicotropic hormone (PTTH). PTTH-stimulated cAMP synthesis and ecdysone secretion are dependent on the presence of extracellular calcium, suggesting that PTTH enhances calcium entry into the cytosol. Such entry into the cytosol might involve the opening of a plasma membrane calcium channel, or a mechanism dependent upon prior inositol triphosphate (IP3)-mediated release of intracellularly stored calcium. In pupal prothoracic glands, PTTH does not increase IP3 or other inositol phosphates over-times ranging from seconds up to 30 min, even in the presence of lithium. However, the L-type calcium channel antagonist nitrendipine completely prevents PTTH-stimulated ecdysone synthesis. A 41 kDa G-protein in prothoracic glands is ADP-ribosylated by pertussis toxin. However, PTTH-stimulated ecdysone synthesis is unaffected by prior exposure to pertussis toxin, indicating that the 41 kDa protein is not involved in the acute stimulation of steroidogenesis. By contrast, cholera toxin has a stimulatory effect on ecdysone secretion suggesting the involvement of a Gs-like protein. Based on the absence of PTTH-stimulated inositol phosphate formation in pupal prothoracic glands, it is suggested that calcium mobilization may occur through the opening of a calcium channel, possibly regulated by Gs.

Multiple phosphorylation of ribosomal protein S6 and specific protein synthesis are required for prothoracicotropic hormone-stimulated ecdysteroid biosynthesis in the prothoracic glands of Manduca sexta

Prothoracicotropic hormone (PTTH)-stimulated protein phosphorylation leads to ecdysteroidogenesis (molting hormone biosynthesis) in the prothoracic glands of the tobacco hornworm, Manduca sexta. The phosphorylation of 34 and 50 kDa peptides (p34 and p50) paralleled the increase in ecdysteroidogenesis, and the dephosphorylation of p34 and p50 preceded a decrease in ecdysteroidogenesis. Inhibition by rapamycin of p34, but not p50, phosphorylation prevented PTTH-stimulated ecdysteroidogenesis in a dose-dependent manner, suggesting that p34 phosphorylation is requisite for PTTH-stimulated ecdysteroidogenesis. Two proteins whose synthesis was rapidly stimulated by PTTH were p50 and p70. The time-course of PTTH-stimulated synthesis of p50 paralleled that of p34 phosphorylation and that of ecdysteroidogenesis. Rapamycin inhibited PTTH-stimulated synthesis of p50 and p70, suggesting that specific protein synthesis is also required for PTTH-stimulated ecdysteroidogenesis, confirming the results of Rybczynski and Gilbert [(1994) Insect Biochem. Molec. Biol. 24, 175-189], and that p34 phosphorylation may regulate the downstream synthesis of p50 and p70, possible key regulatory proteins leading to ecdysteroidogenesis. Results from two-dimensional (2D)-PAGE analysis of the ribosomal proteins purified from prothoracic glands, demonstrated that p34 is indeed ribosomal S6, and is phosphorylated at up to five sites (P1-5) upon PTTH stimulation. The multiple phosphorylation of S6 was inhibited completely by rapamycin as shown in 2D gel maps, further confirming that p34 is ribosomal protein S6. Temporal analysis of PTTH-stimulated S6 phosphorylation by 2D-PAGE revealed that phosphorylation of S6 at the P1 site was temporally correlated with the initiation of ecdysteroidogenesis, and that multiple phosphorylation at all five sites (P1-5) was correlated with the maximal synthesis of ecdysteroids. Dephosphorylation of S6 was accompanied by a decrease in ecdysteroidogenesis. These data demonstrate that p34 is ribosomal protein S6 and that both the phosphorylation of S6 and specific protein synthesis are required for PTTH-stimulated ecdysteroidogenesis in the prothoracic gland.

Changes in general and specific protein synthesis that accompany ecdysteroid synthesis in stimulated prothoracic glands of Manduca sexta

The prothoracic glands of fifth instar Manduca sexta larvae respond to stimulation by the brain neuropeptide, prothoracicotropic hormone (PTTH), with a several-fold increase in the rate of ecdysteroid synthesis. Previous studies have shown that this response requires protein synthesis and that the action of PTTH can be mimicked by dibutyryl cAMP (dbcAMP) and the Ca2+ ionophore, A23187. To further understand the role of protein synthesis in the response of prothoracic glands to PTTH, patterns of protein synthesis in stimulated glands were examined using glands incubated in vitro with [35S]methionine. All three agents caused an increase in the rate of ecdysteroid synthesis as well as an increase of up to 300% in the synthesis and/or accumulation of three proteins (p100, p70, and p"50") within 2 h of stimulation. Changes in these three proteins were specific to the prothoracic gland, were not elicited by non-brain peptides and were not simply a result of increased general protein synthesis in the gland. Exposure of the glands to A23187 alone, or concurrently with dbcAMP, resulted in increased synthesis of p100, p70, p"50" and ecdysteroids but decreased general protein synthesis. Increased synthesis of these proteins could be detected within 15 min after initiating PTTH stimulation. The behavior of these three proteins makes them candidates for modulators of ecdysteroid synthesis in the prothoracic gland. The results suggest also that PTTH may activate two biochemical pathways in the gland: one path leading to increased synthesis of the p100, p70, and p"50" proteins and increased ecdysteroid synthesis, and the second leading to increased general protein synthesis. This second trophic effect is vulnerable to intracellular Ca2+ changes that do not inhibit the first pathway.

Immunoaffinity purification of the neuropeptide prothoracicotropic hormone from Manduca sexta

The prothoracicotropic hormones (PTTH) are cerebral peptides that control insect postembryonic development by stimulating the prothoracic glands to synthesize ecdysteroids. Using immunoaffinity chromatography and SDS-PAGE, a 25.5 kDa big PTTH has been purified from Manduca sexta. Based upon SDS-PAGE and Western blot analysis, the native form of big PTTH appears to be a dimer with monomers of 16.5 kDa. Four HPLC-separated fragments of this acidic peptide were sequenced and exhibited no sequence similarity with Bombyx mori PTTH. In agreement with this finding, the basic Bombyx PTTH had no PTTH bioactivity in Manduca. One sequenced fragment of the Manduca PTTH is approximately 70% similar to the vertebrate cellular retinoid binding proteins, suggesting these binding proteins may be present in insects.

Stimulation of pheromone biosynthesis in the moth Helicoverpa zea: action of a brain hormone on pheromone glands involves Ca2+ and cAMP as second messengers

Isolated abdomen and pheromone gland bioassays were utilized to determine the physiological action of the pheromone-biosynthesis-activating neuropeptide (PBAN) in the corn earworm moth Helicoverpa (= Heliothis) zea. An isolated pheromone gland bioassay showed that synthetic PBAN was active at 0.02 pmol, with maximal activity occurring at 0.5 pmol and 60 min of incubation. Second-messenger studies demonstrated that extracellular Ca2+ is necessary for PBAN activity on isolated pheromone glands. The Ca2+ ionophore A23187 stimulated pheromone biosynthesis alone, whereas the Ca2+ channel blockers La3+ and Mn2+ inhibited PBAN activity. However, the organic Ca2+ channel blockers verapamil and nifedipine did not inhibit PBAN activity. Both forskolin and two cAMP analogues stimulated pheromone biosynthesis in the absence of extracellular Ca2+, indicating that Ca2+ may activate an adenylate cyclase. The biogenic amine octopamine did not elicit pheromone production in isolated gland or abdomen bioassays or when injected into intact female moths. Removal of the ventral nerve chord, including the terminal abdominal ganglia in isolated abdomens, did not affect PBAN stimulation of pheromone production. Similar levels of stimulation were found when isolated abdomens were treated with PBAN in scotophase or photophase.