Ecdysone-mediated stimulation of DOPA decarboxylase activity and its relationship to ovarian development in Aedes aegypti

Effects of the compounds 2-methoxynaphthoquinone, 2-propoxynaphthoquinone, and 2-isopropoxynaphthoquinone on ecdysone 20-monooxygenase activity

The effects of the natural compound 2-methoxy-1,4-naphthoquinone, isolated from the leaves of Impatiens glandulifera and the synthetic compounds 2-propoxy-1,4-naphthoquinone and 2-isopropoxy-1,4-naphthoquinone on ecdysone 20-monooxygenase (E-20-M) activity were examined in three insect species. Homogenates of wandering stage third instar larvae of Drosophila melanogaster, or abdomens from adult female Aedes aegypti, or fat body or midgut from fifth instar larvae of Manduca sexta were incubated with radiolabelled ecdysone and increasing concentrations (from 1 x 10(-8) to 1 x 10(-3) M) of the three compounds. All three compounds were found to inhibit in a dose-dependent fashion the E-20-M activity in the three insect species. The concentration of these compounds required to elicit a 50% inhibition of this steroid hydroxylase activity in the three insect species examined ranged from approximately 3 x 10(-5) to 7 x 10(-4) M.

Effects of the neem tree compounds azadirachtin, salannin, nimbin, and 6-desacetylnimbin on ecdysone 20-monooxygenase activity

The effects of azadirachtin, salannin, nimbin, and 6-desacetylnimbin on ecdysone 20-monooxygenase (E-20-M) activity were examined in three insect species. Homogenates of wandering stage third instar larvae of Drosophila melanogaster, or abdomens from adult female Aedes aegypti, or fat body or midgut from fifth instar larvae of Manduca sexta were incubated with radiolabeled ecdysone and increasing concentrations (from 1 x 10(-8) to 1 x 10(-3) M) of the four compounds isolated from seed kernels of the neem tree, Azadirachta indica. All four neem tree compounds were found to inhibit, in a dose-dependent fashion, the E-20-M activity in three insect species. The concentration of these compounds required to elicit a 50% inhibition of this steroid hydroxylase activity in the three insect species examined ranged from approximately 2 x 10(-5) to 1 x 10(-3).

Effects of plant flavonoids and other allelochemicals on insect cytochrome P-450 dependent steroid hydroxylase activity

The plant flavonoids flavone, chrysin, apigenin, kaempferol, morin, quercetin, myricetin and phloretin were found to inhibit in a dose-dependent manner the cytochrome P-450 dependent ecdysone 20-monooxygenase activity associated with adult female Aedes aegypti, wandering stage larvae of Drosophila melanogaster, and fat body and midgut from prewandering and wandering stage last instar larvae of Manduca sexta. The concentrations of these flavonoids required to elicit a 50% inhibition of the steroid hydroxylase activity in all the insects ranged from ca 1 x 10(-5) to 1 x 10(-3) M. In addition, lower concentrations (1 x 10(-6) to 1 x 10(-5) M) of the flavonols kaempferol, morin, quercetin and myricetin significantly stimulated (50-100% above control) M. sexta fat body ecdysone 20-monooxygenase activity. Other plant allelochemicals examined and found to significantly inhibit insect ecdysone 20-monooxygenase activity include corynanthine, quinidine, and quinine; whereas, indican and mimosine were found to significantly stimulate M. sexta fat body steroid hydroxylase activity. Several allelochemicals were without effect at all concentrations tested. Although none of the compounds tested in this study elicited effects at very low concentrations (1 x 10(-9) to 1 x 10(-8) M), the in vitro monooxygenase radioassay does hold considerable promise as a screening tool for the detection and identification of plant allelochemicals which may function as biopesticides affecting insect ecdysteroidogenesis.

Effects of the chitin synthetase inhibitor plumbagin and its 2-demethyl derivative juglone on insect ecdysone 20-monooxygenase activity

The chitin synthetase inhibitor plumbagin and its 2-demethyl derivative juglone were found to inhibit in a dose-response fashion the cytochrome P-450 dependent ecdysone 20-monooxygenase activity associated with adult female Aedes aegypti, wandering stage larvae of Drosophila melanogaster, and fat body and midgut from last instar larvae of Manduca sexta. The concentration of these naphthoquinones required to elicit a 50% inhibition of the steroid hydroxylase activity in all the insects was approximately 1 x 10(-4) M.

Effects of azadirachtin on insect cytochrome P-450 dependent ecdysone 20-monooxygenase activity

The effects of the insect growth and ecdysis inhibitor azadirachtin on ecdysone 20-monooxygenase activity were examined in three insect species. Homogenates of wandering stage third instar larvae of Drosophila melanogaster, or abdomens from adult female Aedes aegypti, or fat body or midgut from last instar larvae of Manduca sexta were incubated with radiolabelled ecdysone and increasing concentrations of azadirachtin and the ecdysone 20-monoxygenase activity quantified by radioassay. Azadirachtin was found to inhibit in a dose-response fashion the ecdysone 20-monooxygenase activity associated with all the insect preparations. The concentration of azadirachtin required to elicit approximately 50% inhibition of the ecdysone 20-monooxygenase activity ranged from a low of 1 x 10(-4) M for Drosophila to a high of 4 x 10(-4) M for Manduca midgut.

Multiple forms of cerebral peptides with steroidogenic functions in pupal and adult brains of the yellow fever mosquito, Aedes aegypti

In the adult mosquito, Aedes aegypti, the cerebral egg development neurosecretory hormone (EDNH) regulates reproduction by activating the ovaries to synthesize the steroid hormone ecdysone, while during postembryonic development the cerebral prothoracicotropic hormone (PTTH) activates the prothoracic glands to synthesize ecdysone. In each case, ecdysone than drives stage-specific physiological processes. Since EDNH and PTTH share a common steroidogenic function, it is conceivable that they are similar, if not the same, peptide(s). This investigation has begun to address this possibility by determining structural and functional relationships between these two families of neuropeptides. The peptides having EDNH activity in adult and pupal heads of Aedes aegypti were characterized on the basis of their molecular weights and their biological properties in in vitro and in vivo bioassays for EDNH. Gel filtration chromatography of extracts revealed fractions with EDNH activity having molecular weights of approximately 11 kDa and approximately 24 kDa. The similarities in the functional dynamics of these two activities in the different bioassays suggested that comparable moieties existed in pupal and adult brains. The sensitivity of these moieties to proteolytic hydrolysis indicated that they were proteins. Thus at least two EDNH-like peptides appear to be present in pupal and adult heads of Aedes aegypti. The apparent existence of more than one molecular weight form of EDNH has not been reported previously. Since these moieties are present in both pupal and adult heads, it is possible that they have the same steroidogenic function in the two different stages, i.e., to activate ecdysone synthesis, by prothoracic glands in larvae and pupae and by the ovaries in adults.

Hormonal regulation of ovarian ecdysteroid production in the autogenous mosquito, Aedes atropalpus

The effects of juvenile hormone (JH) and egg development neurosecretory hormone (EDNH) on ovarian ecdysteroid production during vitellogenesis in the autogenous mosquito, Aedes atropalpus, were investigated using in vitro techniques coupled with radioimmunoassay (RIA) and high-pressure liquid chromatography (HPLC). Normal females were characterized by quantitative, qualitative, and temporal patterns of in vitro ovarian ecdysteroid production. Females decapitated at emergence showed little ovarian ecdysteroid production and did not undergo vitellogenesis. A 500-ng dose of JH-I applied topically to decapitated females restored normal patterns of ecdysteroid production. In both normal and experimental females, ecdysone constituted the major portion of the ecdysteroids secreted by the ovaries in vitro. However, significant amounts of other RIA-active materials were detected, one of which was probably 20-OH-ecdysone. Fat body incubations indicated that these tissues produce little RIA-active material during the peak of vitellogenesis. During that period, the ovaries were the major source of ecdysteroid. Various doses of JH-I, applied to abdomens isolated at emergence, enhanced ovarian responsiveness to subsequent applications of head extracts containing EDNH both in vivo and in vitro. A 500-ng dose of JH-I, applied topically to blood-fed, decapitated Aedes aegypti, stimulated a significant increase in in vitro ovarian ecdysteroid production. Similarities between these data and those demonstrating prothoracicotropic effects of JH in the Lepidoptera are discussed.

Peritrophic membranes and protease activity in the midgut of the malaria mosquito, Anopheles stephensi (Liston) (Insecta: Diptera) under normal and experimental conditions

Formation and solubility of the peritrophic membranes (pm) in the midgut (stomach) of female Anopheles stephensi depended on salt concentration in the gut lumen. High calcium drastically reduced the solubility of the pm in vitro. Thin-layer chromatography revealed the presence of N-acetylgalactosamine and galactose in the pm. In contrast to Aedes aegypti no N-acetylglucosamine was present indicating significant differences between Anopheles and Aedes mosquitoes with respect to pm formation and composition. Experimental distension of the stomach epithelium did not trigger secretion of proteases, but membrane-bound granules were released from the stomach cells, and pm were formed. When females were fed blood 18 hr after an enema with salt solution, when practically no granules were present in the gut cells, protease activity was increased, and blood was digested normally, but no pm were formed. A functional correlation of the membrane-bound granules and pm formation is postulated. alpha-Amanitin prevented normal digestion, protease activity was only insignificantly increased, and no pm formed after blood intake.

Ultrastructural evidence for the endocrine nature of the lateral organs of the cattle tick Boophilus microplus

The lateral organs of the tick Boophilus microplus were previously thought to have a neurohaemal function, but the present study shows that they consist of glandular cells which contain a rich system of smooth endoplasmic reticulum (SER) and Golgi but no indication of neurosecretory production or release. There is acid phosphatase activity throughout the SER as well as in Golgi and a major function of the latter may be the production of lysosomal enzymes. It is suggested that the organs are endocrine glands and that, in engorged females, may secrete a hormone involved in the control of vitellogenesis. The organs are more active in feeding than in unfed males and a related function could be in control of the development of genital organs or spermatogenesis. Also present in the cells are coated vesicles, lipid droplets and microtubules. Coated vesicles close to Golgi are probably primary lysosomes whereas those near the periphery are shown by ferritin tracer to arise from coated pits. Pinocytosis could be involved in membrane retrieval but, in the absence of evidence for exocytosis, this seems unlikely. It is tentatively proposed that, by analogy with vertebrate and insect endocrine glands, the lateral organs may take up hormone precursor via coated vesicles for storage in lipid droplets and conversion to hormone in the SER. As in other SER-rich endocrine glands, the release mechanism for the hormone or other secretory product of the lateral organs is uncertain. Both the steroid, ecdysone and the terpenoid, juvenile hormone, are discussed as possible candidates for the lateral organ hormones.

Induction of Follicle Separation in the Mosquito by Physiological Amounts of Ecdysterone

Physiological quantities of the molting hormone, ecdysterone, injected into female Aedes aegypti prematurely induced separation of incipient follicles in the ovarioles, an event that normally occurs only in blood-fed females. It was possible to stimulate this morphological event with physiological amounts of hormone by mimicking, with two injections, the timing of normal increases in endogenous hormone of blood-fed females.

Production of specific-protein secretion granules by fat body cells of the blowfly, Calliphora erythrocephala. Substitution of an ovarian key factor by beta-ecdysone

During the first four days of the imaginal stage the fat cells of ovariectomized females of Calliphora develop a protein synthetic apparatus, and produce dense bodies (lysosomes) as do the fat cells of normal females, but apparently they cannot synthesize the protein secretion granules that characterize the productive phase of the fat cells of normal females and that we believe to represent vitellogenin. Injection of ovariectomized females with beta-ecdysone restored the ability of the fat cells to produce the secretion granules. It is suggested that the ovary gives off a factor which induces the production of the protein secretion granules by the fat cells, and that the factor from the ovary can be substituted by beta-ecdysone. This, we believe, is the first ultrastructural evidence for an effect of the ovary and of beta-ecdysone on the synthesis of specific protein.

Studies on the mode of action of ecdysterone in adult female Aedes aegypti

Injection of ecdysterone into non-blood fed adult female Aedes aegypti results in a marked stimulation of aromatic-L-amino-acid decarboxylase (formerly DOPA-decarboxylase) activity (Schlaeger and Fuchs, 1974a). When the hormone and alpha-amanitin are injected either simultaneously or if the toxin is administered first no inhibition of subsequent enzymatic activity is observed and in fact substantial enhancement occurs. Cordycepin injection along with ecdysterone gives results similar to alpha-amanitin. The inhibitors by themselves elicit a very small increase in DOPA decarboxylase activity compared to saline-injected controls. Conversely, actinomycin D causes severe depression of ecdysterone-mediated DOPA decarboxylase activity as dose cycloheximide and puromycin. We interpret our data to mean that the mRNA for DOPA decarboxylase is already present prior to exposure to ecdysterone. We postulate that the function of the hormone would be to modulate translation of specific pre-formed mRNA's by an unknown mechanism or to induce transcription of specific tRNAs necessary for the initation of translation of selecting existing messengers.

The ovary as a source of alpha-ecdysone in an adult mosquito

The ovaries of the mosquito Aedes aegypti cultured in vitro secrete material that behaves like ecdysone in a radioimmunoassay. The material was identified as alpha-ecdysone by high-resolution liquid and gas-liquid chromatography. Secretion reached a maximum 16 hr after a blood meal as shown by bioassay and direct determination. Ovariectomy reduced the concentration of ecdysone in the adult after a blood meal. Qualitative analysis of whole-body extracts indicated beta-ecdysone to be the principal species present. Thus the ovaries appear to secrete a prohormone, alpha-ecdysone, which is converted to beta-ecdysone. Beta-ecdysone plays a significant role in stimulating egg development in the adult mosquito and may have reproductive roles in other insects.