The in vitro synthesis and secretion of alpha-ecdysone by the ring glands of the fly, Sarcophaga bullata

Comprehensive physiology and toxicology of ecdysogens--The metabolically activated porphyrin-ecdysteroid complexes in insects

The polyhydroxylated derivatives of 6-keto,7-dehydrocholesterol (ecdysone, ecdysteroids, Ecd) are natural compounds widely distributed in plants. They exhibit strong anabolic, vitamin D-like, pharmacological effects in vertebrate animals and in the human body. In the larval stages of insects, injections of pure Ecd cause serious pathophysiological, "hyperecdysonic" syndromes associated with neuromuscular paralysis, premature cuticular apolysis and complete inhibition of ecdysis. Ecds do not penetrate insect cuticle. For this reason, all previous attempts to induce ecdysone responses by topical applications of Ecd failed. In this work, we tried to induce the topical effects of Ecd by preparation of more lipophilic complexes, with 2 or 4 molecules of 20-hydroxyecdysone (E20) attached to a relatively large nucleus of the porphyrin. The resulting porphyrin-E20 complexes (ecdysogens) have been subjected to standardised assays for ecdysone activity in the ligatured larvae ("dauerlarvae") of the greater waxmoth (Galleria mellonella). Similarly like the free E20 alone, porphyrin-E20 complexes had no effect when applied on the body surface or administered in the larval diet. When injected, however, they exhibited delayed effects, but the adverse ("hyperecdysonic") pathophysiological syndromes were reduced or abolished. It is concluded, therefore, that the replacement of pathophysiological, precocious or "hyperecdysonic" moults by the larval-pupal transformation, was due to successive metabolic liberation of the biologically active, free E20 from the porphyrin-E20 complex. The biological status of Ecd does not agree with their definition as the prothoracic gland (PG) hormone of insects, nor with the assumptions about a growth hormone of plants. A possibility that the most important status of Ecd may depend on the pharmacological properties of a sterolic D6 vitamin has been discussed.

Evidence for the presence of makisterone A in Drosophila larvae and the secretion of 20-deoxymakisterone A by the ring gland

Ring glands or brain-ring gland complexes from third-instar Drosophila melanogaster larvae secreted ecdysone [(22R)-2beta,3beta,14alpha,22,25-pentahydroxy-5beta-cholest-7-en-6-one] and two less polar ecdysteroids (LP1 and LP2) in vitro. Radioimmunoassay with different antisera indicated that LP1 and LP2 are side-chain-modified analogues of ecdysone. In high-performance liquid chromatography, the retention time of LP2 was equivalent to that of a precursor of makisterone A [(20R,22R)-2beta,3beta,14alpha,20,22,25-hexahydroxy-24-methyl-5beta-cholest-7-en-6-one] secreted by Dysdercus fasciatus prothoracic glands in vitro. LP2 was metabolized in vitro by the fat body of Drosophila larvae to a product with the characteristics of makisterone A when analyzed by gas chromatography/mass spectrometry (selected ion monitoring). Evidence was obtained for the presence of makisterone A in Drosophila larvae. These data suggest that LP2 is 20-deoxymakisterone A (24-methylecdysone) and that makisterone A could function as an additional moulting hormone in Drosophila, although 20-deoxymakisterone A production is apparently dependent on the sterol composition of the diet. LP1 has not been identified.

Larval feeding duration affects ecdysteroid levels and nutritional reserves regulating pupal commitment in the yellow fever mosquito Aedes aegypti (Diptera: Culicidae)

What little is known about the endocrine regulation of mosquito development suggests that models based on Lepidoptera and Drosophila may not apply. We report on basic parameters of larval development and the commitment to metamorphosis in the yellow fever mosquito Aedes aegypti that are affected by varying the length of feeding time for last instar larvae. A critical mass for pupal commitment was achieved after 24 h of feeding by last instars, also the age at which tissue production and hemolymph titers of ecdysteroids are increasing. A greater proportion of last instars successfully pupated and eclosed as adults as the length of their feeding time increased. Less than 24 h of feeding time resulted in last instars that were developmentally arrested; these larvae tolerated starvation conditions for up to 2 weeks and retained the capacity to pupate if re-fed. Starvation tolerance may be a common trait among container-inhabiting species, and this period is an important factor to be considered for vectorial capacity and control measures. To distinguish cues for metamorphosis related to a larva's nutritional status versus its age, newly molted last instars were fed for different periods of time but sampled at the same age; ecdysteroid levels, body mass and nutrient reserves were then measured for each group. Our data suggest that metamorphic capacity is dependent on a larva's nutritional condition and not just the age at which ecdysteroid titers increase. Last instars that have fed for a particular length of time may initiate their metamorphic molt when both threshold levels of nutrient reserves and ecdysteroid titer have been met. Future studies will lead to a conceptual model specific for the nutritional and hormonal regulation of mosquito post-embryonic development. This model should facilitate the exploitation of current and novel insect growth regulators that are among favored strategies for vector population suppression.

Halloween genes encode P450 enzymes that mediate steroid hormone biosynthesis in Drosophila melanogaster

Mutation of members of the Halloween gene family results in embryonic lethality. We have shown that two of these genes code for enzymes responsible for specific steps in the synthesis of ecdysone, a polyhydroxylated sterol that is the precursor of the major molting hormone of all arthropods, 20-hydroxyecdysone. These two mitochondrial P450 enzymes, coded for by disembodied (dib) (CYP302A1) and shadow (sad) (CYP315A1), are the C22 and C2 hydroxylases, respectively, as shown by transfection of the gene into S2 cells and subsequent biochemical analysis. These are the last two enzymes in the ecdysone biosynthetic pathway. A third enzyme, necessary for the critical conversion of ecdysone to 20-hydroxyecdysone, the 20-monooxygenase, is encoded by shade (shd) (CYP314A1). All three enzymes are mitochondrial although shade has motifs suggesting both mitochondrial and microsomal locations. By tagging these enzymes, their subcellular location has been confirmed by confocal microscopy. Shade is present in several tissues as expected while disembodied and shadow are restricted to the ring gland. The paradigm used should allow us to define the enzymes mediating the entire ecdysteroid biosynthetic pathway.

Shade is the Drosophila P450 enzyme that mediates the hydroxylation of ecdysone to the steroid insect molting hormone 20-hydroxyecdysone

The steroid 20-hydroxyecdysone (20E) is the primary regulatory hormone that mediates developmental transitions in insects and other arthropods. 20E is produced from ecdysone (E) by the action of a P450 monooxygenase that hydroxylates E at carbon 20. The gene coding for this key enzyme of ecdysteroidogenesis has not been identified definitively in any insect. We show here that the Drosophila E-20-monooxygenase (E20MO) is the product of the shade (shd) locus (cytochrome p450, CYP314a1). When shd is transfected into Drosophila S2 cells, extensive conversion of E to 20E is observed, whereas in sorted homozygous shd embryos, no E20MO activity is apparent either in vivo or in vitro. Mutations in shd lead to severe disruptions in late embryonic morphogenesis and exhibit phenotypes identical to those seen in disembodied (dib) and shadow (sad) mutants, two other genes of the Halloween class that code for P450 enzymes that catalyze the final two steps in the synthesis of E from 2,22-dideoxyecdysone. Unlike dib and sad, shd is not expressed in the ring gland but is expressed in peripheral tissues such as the epidermis, midgut, Malpighian tubules, and fat body, i.e., tissues known to be major sites of E20MO activity in a variety of insects. However, the tissue in which shd is expressed does not appear to be important for developmental function because misexpression of shd in the embryonic mesoderm instead of the epidermis, the normal embryonic tissue in which shd is expressed, rescues embryonic lethality.

The ecdysoneless (ecd1ts) mutation disrupts ecdysteroid synthesis autonomously in the ring gland of Drosophila melanogaster

Ring glands dissected from homozygous l(3)ecd1ts wandering larvae and upshifted in vitro to the restrictive temperature, 29 degrees C, synthesize abnormally low quantities of ecdysteroid. Nevertheless, ecd1 ring glands retain the ability to respond at 29 degrees C to an extract prepared from wild-type larval neural tissues that presumably contain prothoracicotropic hormone (PTTH), although both basal and stimulated levels of synthesis are lower than those in wild-type ring glands. Extracts prepared from ecd1 neural tissue exhibit an unusually high level of PTTH activity. Mutant ring glands downshifted in vitro to the permissive temperature after removal from larvae maintained at 29 degrees C regain the ability to produce normal basal and stimulated ecdysteroid levels. Collectively, these experiments demonstrate that the ecd1 mutation disrupts the physiology of the ring gland at 29 degrees C autonomously and may also interfere with PTTH release.

Cyclic AMP mediates the negative regulation of Y-organ ecdysteroid production

The putative neuropeptide, molt-inhibiting hormone (MIH), regulates crustacean growth by periodically suppressing the secretion of ecdysteroid molting hormone from peripheral glands (Y-organs). A mediating role for cyclic AMP (cAMP) in MIH action was evaluated with isolated Y-organs of the crab, Cancer antennarius. MIH activity in eyestalk extracts inhibited ecdysteroid secretion but increased cAMP levels dose-dependently in 24-h incubations. The cAMP rise preceded the onset of ecdysteroid suppression. Dibutyryl cAMP, activators of adenylate cyclase (forskolin, choleragen), and an inhibitor of phosphodiesterase (IBMX), but not AMP or cGMP, mimicked the inhibitory action of MIH.

In vitro activity of dipteran ring glands and activation by the prothoracicotropic hormone

The relationship between hemolymph ecdysteroid titer, ring gland (RG) activity, and prothoracicotropic hormone (PTTH) activation of RG in vitro has been examined during the postfeeding larval, prepupal, and pupal stages of Sarcophaga bullata. Using the ecdysteroid radioimmunoassay (RIA), two significant peaks were recorded during the red spiracular stage and during the first few hours after the formation of the white prepupa and a third large peak 9 hr later. It is postulated that these increases in ecdysteroid titer are involved in the processes of pupariation, puparial tanning, and pupation, respectively. Ring glands isolated from Sarcophaga of known ages were incubated in vitro and the secreted ecdysone was quantified by RIA. Ring glands from early red spiracular stage larvae proved to be the most active and subsequent secretory activity of the RG oscillated every 4 hr with the oscillations gradually decreasing in amplitude. RG activity returned to a basal level 24 hr after formation of the white prepupa, about the time that the hemolymph ecdysteroid titer fell to its basal level. To demonstrate PTTH activity in vitro, brains from 3- to 4-hr prepupae were chosen to activate ring glands from postfeeding larvae. Using a graded series of dilutions of PTTH extract it was shown that a dose-response relationship could be obtained for Sarcophaga similar to that demonstrated or the Manduca sexta PTTH-prothoracic gland system. In Sarcophaga maximal activation resulted in a 10-fold increase in ecdysone synthesis and secretion by ring glands stimulated with 0.5 brain eq. Half-maximal stimulation was attained with 0.2 brain eq of PTTH extract.

Ecdysone 20-monooxygenase: characterization of an insect cytochrome p-450 dependent steroid hydroxylase

Ecdysone 20-monooxygenase, the enzyme system that hydroxylates ecdysone at C-20 of the side-chain to form ecdysterone, has been characterized in the fat body of early last instar larvae of the tobacco hornworm, Manduca sexta, using a radioenzymological assay. Ecdysterone was demonstrated to be the product of the enzyme system by high-pressure liquid chromatography, gas-liquid chromatography and mass spectrometry. Differential centrifugation, sucrose-gradient centrifugation, electron microscopy and organelle-marker enzyme analysis revealed that ecdysone 20-monooxygenase activity is associated with the mitochondria. The enzymatic properties of ecdysone 20-monooxygenase are that it is most active in a 0.05 M phosphate buffer, is inhibited by Mg2+ and exhibits pH and temperature optima at 7.5 and 30 degrees C, respectively. The enzyme complex has an apparent Km for ecdysone of 1.60 x 10(-7) M and is competitively inhibited by its product, ecdysterone, with an apparent Ki of 2.72 x 10(-5) M. The cytochrome P-450 nature of this insect steroid hydroxylase was initially suggested by its obligate requirement for NADPH and its inhibition by carbon monoxide, p-chloromercuribenzoate, metyrapone and p-aminoglutethimide but not by cyanide. Difference spectroscopy revealed the presence of cytochrome P-450 in the fat-body mitochondrial fraction. A photochemical action spectrum of ecdysone 20-monooxygenase activity confirmed the involvement of cytochrome P-450 in this monooxygenase system.

Ecdysone oxidase: reaction and specificity

Ecdysone oxidase oxidizes 3-hydroxysteroids of the excysteroid type irreversibly to 3-dehydro derivatives. The hydrogen of the steroid is transferred by the enzyme to oxygen which is reduced to hydrogen peroxide. Ecdysone oxidase is relatively specific for ecdysone and ecdysterone. Apparent Michaelis constants for these two physiological substrates are 98 and 31 micron, respectively. Cholesterol oxidase and 3alpha-hydroxysteroid and 3beta-hydroxysteroid dehydrogenases which catalyze similar reactions are unable to oxidize ecdysteroids.

In vitro metabolism of 3beta-hydroxy-, and 3beta,14alpha-dihydroxy-[3alpha-3H]-5beta-cholest-7-en-6-one by the prothoracic glands of Manduca sexta

alpha-Ecdysone (2beta,3beta,14alpha,22R,25-pentahydroxy-5beta-cholest-7-en-6-one) has been identified as the metabolism product of 3beta,14alpha-dihydroxy-5beta-cholest-7-en-6-one in isolated prothoracic glands of the tobacco hornworm, Manduca sexta. In contrast, 3beta-hydroxy-5beta-cholest-7-en-6-one is metabolized to 14-deoxy-alpha-ecdysone and a variety of intermediates all lacking the 14-hydroxy group. The results suggest that either the normal precursor for the synthesis of alpha-ecdysone by prothoracic glands is a sterol more highly oxygenated than cholesterol or that hydroxylation of a minimally oxygenated precursor at C-14 must precede introduction of the C-6 ketone and/or delta7 bond. The data further suggest that several alternative hydroxylation routes may exist for the latter steps of alpha-ecdysone biosynthesis.