Involvement of microtubules in prothoracicotropic hormone-stimulated ecdysteroidogenesis by insect (Manduca sexta) prothoracic glands

Lignoids in insects: chemical probes for the study of ecdysis, excretion and Trypanosoma cruzi-triatomine interactions

The effects of phytochemicals (lignans and neolignans) are reviewed in a variety of insect species with special focus on the recent advances on feeding, excretion and Trypanosoma cruzi interactions with Rhodnius prolixus. Burchellin, podophyllotoxin, pinoresinol, sesamin, licarin A, and nordihydroguaiaretic acid (NDGA) added to the diet of Rhodnius prolixus larvae induce antifeedant effects only in doses up to 100 microg/ml of blood meal. Additionally, pinoresinol and NDGA significantly inhibit ecdysis (ED(50)<20 microg/ml). Simultaneous application of ecdysone (1 microg/ml) counteracts ecdysial stasis as induced by NDGA in 5th-instar larvae. Experiments in vivo demonstrate that burchellin and podophyllotoxin (100 microg/ml) diminish excretion post-feeding. Simultaneous treatment with 5-hydroxytryptamine (1 mM, 5-HT), a diuretic hormone, partially reverses this effect of burchellin. Experiments in vitro, using isolated Malpighian tubules of R. prolixus, indicate that burchellin (i) decreases diuretic hormone levels in the hemolymph but not the amount of diuretic hormone stored in the thoracic ganglionic masses (including axons); (ii) reduces the volume of urine secreted by isolated Malpighian tubules; and (iii) 5-HT therapy cannot overcome the effect of burchellin on the Malpighian tubules. In R. prolixus fed on blood containing T. cruzi epimastigotes, the number of parasites in the digestive tract decreases drastically in the presence of burchellin and NDGA (10 microg/ml). When these phytochemicals are applied 20 days after T. cruzi infection, burchellin significantly reduces the gut infection, whereas NDGA does not. However, if the insects are pretreated with both compounds 20 days before subsequent infection with epimastigotes, the parasite infection is almost completely abolished. The same holds true when 5th-instar of R. prolixus are inoculated with 0.5 microg/microl/larva of both neolignans 1 day before infection. Taken together, these findings not only provide a better understanding of the lignoid function in insects, but also offer novel insights into basic physiological processes, which make lignoids interesting candidates for new types of insecticides.

The antimitotic effect of the neem terpenoid azadirachtin on cultured insect cells

When cultured insect cells (Sf9) were grown in the presence of 5 x 10(-6) M azadirachtin, there was a rapid increase in the mitotic index, with the appearance of many aberrant mitotic figures. Flow cytometry established that cells accumulated in the G2/M phase of the cell cycle, and that the effect was concentration-dependent. At 10(-8) M a period of 20 h was necessary to raise the proportion in G2/M to 42% above the control values, but at 5 x 10(-6) M more than 90% of the cells were in this phase. Azadirachtin had the same effect on C6/36 mosquito cells, but failed to affect L929 murine fibroblast cells even at a concentration of 10(-4) M over 72 h. Experiments with colchcine and taxol showed similarities of action between azadirachtin and colchicine, and azadirachtin was apparently able to displace colchicine-fluorescein from binding-sites in living insect cells. Another similarity between azdirachtin and colchicine was that both phytochemicals prevented the polymerisatrion in vitro of mammalian tubulin, although the azadirachtin was much less effective.

Control and biochemical nature of the ecdysteroidogenic pathway

Molting is elicited by a critical titer of ecdysteroids that includes the principal molting hormone, 20-hydroxyecdysone (20E), and ecdysone (E), which is the precursor of 20E but also has morphogenetic roles of its own. The prothoracic glands are the predominate source of ecdysteroids, and the rate of synthesis of these polyhydroxylated sterols is critical for molting and metamorphosis. This review concerns three aspects of ecdysteroidogenesis: (a) how the brain neuropeptide prothoracicotropic hormone (PTTH) initiates a transductory cascade in cells of the prothoracic gland, which results in an increased rate of ecdysteroid biosynthesis (upregulation); (b) how the concentrations of 20E in the hemolymph feed back on the prothoracic gland to decrease rates of ecdysteroidogenesis (downregulation); and (c) how the prothoracic gland cells convert cholesterol to the precursor of E and then 20E, a series of reactions only now being understood because of the use of a combination of classical biochemistry and molecular genetics.

Molt-inhibiting hormone mRNA levels and ecdysteroid titer during a molt cycle of the blue crab, Callinectes sapidus

Synthesis of ecdysteroid molting hormones by crustacean Y-organs is believed to be regulated by a neuropeptide, molt-inhibiting hormone (MIH), produced in eyestalk neural ganglia. In the present study, steady-state MIH mRNA and hemolymph ecdysteroid levels were determined by Northern blot and radioimmunoassay, respectively, during the molt cycle of the blue crab, Callinectes sapidus. The level of MIH mRNA dropped steadily during premolt (D1-D4), reaching a minimum in D3/D4, then increased by 10-fold in postmolt (A/B) and remained elevated during intermolt (C4). These stage-specific changes in MIH mRNA levels were accompanied by significant fluctuations in the hemolymph ecdysteroid titer. The ecdysteroid titer increased steadily to a peak of 377.0 ng/ml in D3 of premolt, then dropped to 120.0 ng/ml in D4 (just prior to molting), and was low during postmolt (A/B, 4.4 ng/ml) and intermolt (C4, 3.3 ng/ml). The results represent the first report of developmental changes in MIH gene expression and are generally consistent with the hypothesis that MIH negatively regulates ecdysteroid synthesis in crustaceans.

Cloning of a beta1 tubulin cDNA from an insect endocrine gland: developmental and hormone-induced changes in mRNA expression

A rapid increase in ecdysteroid hormone synthesis results when the insect prothoracic gland is stimulated with prothoracicotropic hormone (PTTH), a brain neuropeptide hormone. PTTH also stimulates the specific synthesis of several proteins, one of which is a beta tubulin. To further understand the possible roles of beta tubulin in the prothoracic gland, beta tubulin cDNA clones were isolated from a tobacco hornworm (Manduca sexta) gland cDNA library. Sequence analysis indicated that these clones were assignable to the beta1 tubulin isoform. Gland beta1 tubulin mRNA levels during the last larval instar and early pupal-adult development exhibited peaks that coincided with peaks in ecdysteroid synthesis. Manipulations of the glands hormonal milieu showed that beta1 tubulin mRNA levels respond to 20 hydroxyecdysone and PTTH. The data also support our earlier proposal that the prothoracic gland beta1 tubulin gene is ubiquitously expressed but exhibits tissue- and developmental-specific regulation of transcription and translation.

Control of ecdysteroidogenesis: activation and inhibition of prothoracic gland activity

The ecdysteroid hormones, mainly 20-hydroxyecdysone (20E), play a pivotal role in insect development by controlling gene expression involved in molting and metamorphosis. In the model insect Manduca sexta the production of ecdysteroids by the prothoracic gland is acutely controlled by a brain neurohormone, prothoracicotropic hormone (PTTH). PTTH initiates a cascade of events that progresses from the influx of Ca2+ and cAMP generation through phosphorylation of the ribosomal protein S6 and S6-dependent protein synthesis, and concludes with an increase in the synthesis and export of ecdysteroids from the gland. Recent studies indicate that S6 phosphorylation probably controls the steroidogenic effect of PTTH by gating the translation of selected mRNAs whose protein products are required for increased ecdysteroid synthesis. Inhibition of S6 phosphorylation prevents an increase in PTTH-stimulated protein synthesis and subsequent ecdysteroid synthesis. Two of the proteins whose translations are specifically stimulated by PTTH have been identified, one being a beta tubulin and the other a heat shock protein 70 family member. Current data suggest that these two proteins could be involved in supporting microtubule-dependent protein synthesis and ecdysone receptor assembly and/or function. Recent data also indicate that the 20E produced by the prothoracic gland feeds back upon the gland by increasing expression and phosphorylation of a specific USP isoform that is a constituent of the functional ecdysone receptor. Changes in the concentration and composition of the ecdysone receptor complex of the prothoracic gland could modulate the gland's potential for ecdysteroid synthesis (e.g. feedback inhibition) by controlling the levels of enzymes or other proteins in the ecdysteroid biosynthetic pathway.