Inhibition of ecdysone biosynthesis in flies by a hexapeptide isolated from vitellogenic ovaries

The natural insect peptide Neb-colloostatin induces ovarian atresia and apoptosis in the mealworm Tenebrio molitor

Background

The injection of Neb-colloostatin into T. molitor females causes gonadoinhibitory effects on ovarian development. This peptide inhibits intercellular space formation (patency) in follicular epithelium and results in slowed vitellogenesis, delayed ovulation, reduced number of eggs laid and presumably cell death in the terminal follicles. However, as does the form of cell death in the terminal follicle, the mode of action of Neb-colloostatin remains unknown.

Results

We tested Neb-colloostatin for a sterilizing effect on females of Tenebrio molitor. We report that injection of nanomolar doses of Neb-colloostatin induce ovarian follicle atresia in 4-day old females during their first gonadotropic cycle. Light microscope observations revealed morphological changes in the ovary: after Neb-colloostatin injection the terminal oocytes are significantly smaller and elicit massive follicle resorption, but the control terminal follicles possess translucent ooplasm in oocytes at different stages of vitellogenesis. A patency is visible in follicular epithelium of the control vitellogenic oocytes, whereas peptide injection inhibits intercellular space formation and, in consequence, inhibits vitellogenesis. Confocal and electron microscope examination showed that peptide injection causes changes in the morphology indicating death of follicular cells. We observed F-actin cytoskeleton disorganization, induction of caspase activity, changes in chromatin organization and autophagic vacuole formation. Moreover, the apical cytoplasm of follicular cells is filled with numerous free ribosomes, probably indicating a higher demand for protein biosynthesis, especially in preparation for autophagic vacuole formation. On the other hand, the process of polyribosomes formation is inhibited, indicating the contributing effect of this hormone.

Conclusion

Neb-colloostatin induces atresia in the mealworm ovary. Degeneration of T. molitor follicles includes changes in morphology and viability of follicular cells, and oosorption as a consequence of these changes.

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.

Insect gonadotropic peptide hormones: some recent developments

Gonadotropic peptides are a new generation of peptide hormone regulators of insect reproduction. They have been isolated from ovaries, oviducts, or brains of insects. The subject of this paper is insect peptides that exert stimulatory or inhibitory effects on ovarian development and oocyte maturation. On the basis of the literature data and the results of our investigations, the structure and biological properties of different groups of peptides are presented.

Ovarian jelly-peptides (OJPs), a new family of regulatory peptides identified in the cephalopod Sepia officinalis

In marine invertebrates, numerous water-borne peptides involved in reproductive behavior have been characterized. In this study, we focused on three ovarian water-borne peptides, released by full-grown oocytes (FGO) in the genital coelom and in the lumen of the oviduct in the cuttlefish Sepia officinalis. The first one (DQVKIVL), was characterized by the monitoring of HPLC purified fraction using a myotropic bioassay. Subsequently, a peptidomic approach consisting of a mass spectrometry comparative screening performed between the peptide content of FGO with that of FGO-conditioned medium, led to the identification of two additional water-borne peptides. The second peptide identified (DEVKIVL) was characterized by MS/MS and the primary structure of the third one (DEVKIVLD) was elucidated by a combination of Edman degradation, acid hydrolysis and MS/MS analysis. Sequence homology, tissue mapping and bioactivity demonstrate that these peptides belong to the same family. DQVKIVL-related-peptides strictly localized in the female genital tract modulate the whole female genital tract and the main nidamental gland contractions. Furthermore, these peptides form a jelly, when resuspended in water. This particular property could play an important role in the kinetics of peptide diffusion in the external medium. Thus, these regulatory peptides were named ovarian jelly-peptides (OJPs).

Neuropeptides in the nervous system of Drosophila and other insects: multiple roles as neuromodulators and neurohormones

Neuropeptides in insects act as neuromodulators in the central and peripheral nervous system and as regulatory hormones released into the circulation. The functional roles of insect neuropeptides encompass regulation of homeostasis, organization of behaviors, initiation and coordination of developmental processes and modulation of neuronal and muscular activity. With the completion of the sequencing of the Drosophila genome we have obtained a fairly good estimate of the total number of genes encoding neuropeptide precursors and thus the total number of neuropeptides in an insect. At present there are 23 identified genes that encode predicted neuropeptides and an additional seven encoding insulin-like peptides in Drosophila. Since the number of G-protein-coupled neuropeptide receptors in Drosophila is estimated to be around 40, the total number of neuropeptide genes in this insect will probably not exceed three dozen. The neuropeptides can be grouped into families, and it is suggested here that related peptides encoded on a Drosophila gene constitute a family and that peptides from related genes (orthologs) in other species belong to the same family. Some peptides are encoded as multiple related isoforms on a precursor and it is possible that many of these isoforms are functionally redundant. The distribution and possible functions of members of the 23 neuropeptide families and the insulin-like peptides are discussed. It is clear that each of the distinct neuropeptides are present in specific small sets of neurons and/or neurosecretory cells and in some cases in cells of the intestine or certain peripheral sites. The distribution patterns vary extensively between types of neuropeptides. Another feature emerging for many insect neuropeptides is that they appear to be multifunctional. One and the same peptide may act both in the CNS and as a circulating hormone and play different functional roles at different central and peripheral targets. A neuropeptide can, for instance, act as a coreleased signal that modulates the action of a classical transmitter and the peptide action depends on the cotransmitter and the specific circuit where it is released. Some peptides, however, may work as molecular switches and trigger specific global responses at a given time. Drosophila, in spite of its small size, is now emerging as a very favorable organism for the studies of neuropeptide function due to the arsenal of molecular genetics methods available.

Proteolytic breakdown of the Neb-trypsin modulating oostatic factor (Neb-TMOF) in the hemolymph of different insects and its gut epithelial transport

The degradation of the unblocked hexapeptide, trypsin modulating oostatic factor of the flesh fly Neobellieria (Sarcophaga) bullata (Neb-TMOF) was studied in vitro in the hemolymph of the lepidopteran Spodoptera frugiperda, the orthopteran Schistocerca gregaria and the dictyopteran Leucophaea maderae. The half-life in the different species varied from approximately 3min in L. maderae to approximately 25min in S. gregaria. Purification of the degradation products and ESI-Qq-oa-Tof mass spectrometry revealed the fragments Asn-Pro-Thr-Asn, Leu-His and Asn-Pro, which were the same in the hemolymph of all species. Except in Leucophaea, Neb-TMOF was cleaved in dipeptides starting from the C-terminus and the reaction could be, at least partially, inhibited by captopril. These observations suggest that a dipeptidase, which has very similar enzymatic properties as mammalian angiotensin converting enzyme (ACE) and which circulates in the hemolymph, apparently is involved in the breakdown of Neb-TMOF and might be a common but not a universal enzyme in insect hemolymph.The introduction of Neb-TMOF into the gut of S. gregaria with the help of a capillary tube (intubation) demonstrated that the intact peptide is able to cross the gut epithelium and to appear in the hemolymph compartment. Since [3H]-inulin, which is too large to cross cell membranes, was found to penetrate the gut walls at a measurable rate, the paracellular pathway might be also permeable to smaller peptides. There was indeed a clear correlation between the molecular weight of inulin, Neb-TMOF, and inositol and the rate of penetration of these compounds through the gut epithelium to the hemolymph. These are promising findings in view of a potential use of such peptides for insect control purposes.

Captopril, a specific inhibitor of angiotensin converting enzyme, enhances both trypsin and vitellogenin titers in the grey fleshfly Neobellieria bullata

A strong and constitutive angiotensin converting enzyme- or ACE-like activity was demonstrated in the hemolymph of the adult grey fleshfly Neobellieria bullata. In a competition assay, the N. bullata trypsin modulating oostatic factor (Neb-TMOF) was confirmed to be an in vitro substrate for this circulating Neb-ACE. Oral uptake of captopril, a selective and specific inhibitor of ACE, resulted in a complete phenotypic knockout of circulating ACE activity. When compared with control animals, captopril-fed female flies showed an increase in the liver meal-induced trypsin peak in the midgut and elevated levels of protein meal-induced yolk polypeptides in the hemolymph. The latter effect was not due to a slower vitellogenin uptake by the ovaries, because oocyte growth was not affected by the captopril treatment. The apparent synergism between the demonstrated ACE functionality and the previously reported effects of the oostatic peptide Neb-TMOF are discussed in the context of our recent finding that Neb-TMOF represents a prime candidate for being the first known in vivo substrate for circulating insect ACE. Arch.

Action kinetics of a prothoracicostatic peptide from Bombyx mori and its possible signaling pathway

A prothoracicostatic peptide (PTSP), purified from the brains of Bombyx mori, was found to inhibit ecdysteroidogenesis in the prothoracic glands (PGs) of this insect. This peptide was active at inhibiting ecdysteroidogenesis in the PGs at concentrations higher than 23 nM and glands incubated in 230 nM PTSP in vitro exhibited maximum inhibition of ecdysteroid production. By incubating PGs in vitro at different incubation periods it was observed that the first statistically significant inhibitory effect occurred after 30 min incubation in the presence of PTSP. Transferral of PGs from a medium with PTSP to a medium without PTSP resulted in the resumption of ecdysteroid production. Statistically significant inhibition of ecdysteroid production by PTSP was observed only in day 6 and in day 3 PGs of the 5th instar. The extracts of day 6 glands incubated in the presence of PTSP did not contain elevated amounts of ecdysteroid relative to controls after the incubations, indicating that PTSP does not inhibit the secretion, rather the synthesis, of ecdysteroid in the PGs. The presence of PTSP completely blocked the increased ecdysteroid production via L-type Ca(2+) channel activation by S(-)*Bay K 8644. There was no inhibition of ecdysteroid production by PTSP with glands incubated in Ca(2+)-free medium. The combined results suggest that PTSP regulates ecdysteroid synthesis only during specific stages of the 5th instar through a mechanism that likely involves the blocking of Ca(2+) influx through voltage-sensitive Ca(2+) channels in the PG cells of B. mori.

In vitro degradation of the Neb-Trypsin modulating oostatic factor (Neb-TMOF) in gut luminal content and hemolymph of the grey fleshfly, Neobellieria bullata

The unblocked hexapeptidic Trypsin Modulating Oostatic Factor of the fleshfly, an inhibitor of both trypsin and ecdysone biosynthesis, resists very well proteolytic breakdown by enzymes present in the lumen of the gut of previtellogenic fleshflies. However, when incubated in hemolymph of adult flies, females and males, its half-life time is a mere 0.5 min. In hemolymph of last instar larvae, this value increases to about 1.5 min. Whereas PMSF, a potent inhibitor of serine proteases has no effect, captopril and lisinopril, both known to be specific inhibitors of mammalian angiotensin I converting enzyme (ACE), effectively inhibit TMOF breakdown in fly hemolymph. Digestion of Neb-TMOF by recombinant Drosophila AnCE on itself results in identical degradation products as with total hemolymph. In both cases ESI-Qq-oa-Tof mass spectrometry demonstrated the appearance of peptide fragments with the sequences NPTN, LH and NP. These observations not only confirm the reported presence of circulating ACE-like activity in flies but also strongly suggest that in flies this hemolymph ACE-like activity might be involved in the regulation of the oostatic activity as exerted by Neb-TMOF.

The SepOvotropin: a new ovarian peptide regulating oocyte transport in Sepia officinalis

In the cuttlefish Sepia officinalis, the successive steps of egg laying are controlled by multiple neuropeptides. Recent experiments led us to suppose that there was possible involvement of a second regulation pathway by the release of ovarian regulatory peptides in the genital tract. Using HPLC fractionation and an in vitro biological test, a C-terminal amidated peptide modulating the motility of the Sepia officinalis oviduct was isolated from an extract of vitellogenic ovarian follicles. The mass of this peptide as determined by MALDI-TOF (1501.8 Da) and analysis by Edman degradation led to the following sequence: Pro-Lys-Asp-Ser-Met-Leu-Leu-Leu-Gln-Val-Pro-Val-Tyr-amide. The peptide mapping performed by LC/MS revealed a distribution restricted to the follicles, the full grown oocytes and the eggs. This new peptide, called SepOvotropin, modulated contractions of the whole genital tract in physiological conditions from a threshold concentration between 10(-20) and 10(-19) M, demonstrating for the first time the occurrence of a specific peptidergic control of egg-laying in cephalopods.

Aedes aegypti TMOF modulates ecdysteroid production by prothoracic glands of the gypsy moth, Lymantria dispar

Trypsin modulating oostatic factor (TMOF) is a decapeptide that inhibits the biosynthesis of trypsin-like enzymes in the midgut of several insect species and, as such, serves as a dipteran oostatic hormone. In vitro incubation of lepidopteran prothoracic glands with Aedes aegypti TMOF revealed that this decapeptide, in the presence of brain extract, modulates ecdysteroid production. The modulatory effect was highly dependent on both the concentration of TMOF and brain extract. Typically, TMOF was stimulatory in the presence of lower concentrations of Lymantria dispar brain extract (0.01 and 0. 025 brain equivalent), and either neutral or inhibitory at higher concentrations (0.25, 0.5, and 1.0 brain equivalent) of extract. In the presence of European corn borer (Ostrinia nubilalis) brain extract, TMOF also exhibited modulatory effects, effects that again were dependent on the concentrations of both brain extract and TMOF present in the incubation medium. At 1.5 brain equivalents, TMOF was inhibitory at all but the highest concentration tested (5x10(-6) M), at 1.0 brain equivalent, TMOF was stimulatory at 10(-6) M and at 0. 5 brain equivalents, TMOF did not significantly affect PTG synthesis of ecdysteroids. Results suggest the presence of a modulatory peptide(s), which fine tunes the synthesis and release of ecdysteroids by PTGs in accordance with the insect's developmental/physiological requirements.

Identification of a prothoracicostatic peptide in the larval brain of the silkworm, Bombyx mori

Prothoracicotropic hormone (PTTH) stimulates ecdysteroid biosynthesis in the prothoracic gland (PG) of insects. A peptide inhibiting ecdysteroid biosynthesis in the PG was isolated from the extracts of 2,000 larval brains of the silkworm, Bombyx mori, using a protocol that included four reversed-phase high performance liquid chromatography procedures. The primary structure of this prothoracicostatic peptide (Bom-PTSP) was determined to be H-Ala-Trp-Gln-Asp-Leu-Asn-Ser-Ala-Trp-NH(2). This neuropeptide has the same sequence as Mas-MIP-I, a myoinhibitory peptide previously isolated from the ventral nerve cord of the tobacco hornworm, Manduca sexta, and is highly homologous with the N-terminal portion of vertebrate peptides of the galanin family. This peptide inhibited PTTH-stimulated ecdysteroidogenesis in the PG at both the spinning and feeding stages, which indicates that Bom-PTSP interferes with PTTH-stimulated ecdysteroidogenesis.

"Insects do not have sex hormones": a myth?

Mammals have two genes (SRY and DMT1) for testis formation-androgenesis, an anti-testis gene, DAX1, an anti-Müllerian duct hormone, and steroid sex hormones. Drosophila uses the sex-lethal, transformer, and doublesex genes for sexual differentation and is supposed to lack sex hormones. However, the statement that insects do not have sex hormones loses much of its credibility if one considers (1) the classical endocrinological work on sexual differentiation in the firefly Lampyris and in the hevea tussock moth Orgyia; (2) the recent identification of an androgenic hormone and its role in sex determination in the isopod Armadillidium; (3) the similarity between steroidogenic factor 1 (SF-1) of mammals and fushi tarazu factor 1 (FTZ-F1) of Drosophila; and (4) the steroidogenic effect of gonadotropins secreted by the brain of female locusts and mosquitoes and of male gypsy moth. In our model, based on data from the literature, ecdysone, when present in high concentrations, might function as an androgenic sex steroid. It is also the precursor of 20-OH-ecdysone, which is the moulting hormone of insects, and in vitellogenic females of many species, the counterpart of estrogens as well. Other gender-specific hormones are likely to exist in the brain-gonad axis.

Reproduction and love: strategies of the organism's cellular defense system?

A novel view is presented which states that primordial germ cells and their descendants can be regarded as 'cancerous cells' which emit signals that activate a whole array of cellular defensive mechanisms by the somatoplasm. These cells have become unrestrained in response to the lack of typical cell adhesion properties of epithelial cells. From this point of view: (1) the encapsulation of oocytes by follicle cells, vitelline membrane and egg shell; (2) the suppression of gonadal development in larval life; (3) the production of sex steroid hormones and of vitellogenin; and (4) the expulsion of the gametes from the body fit into a general framework for a defense strategy of the somatoplasm against germ line cells. Accordingly, the origin of sexual reproduction appears to be a story of failure and intercellular hostility rather than a 'romantic' and altruistic event. Yet, it has resulted in evolutionary success for the system in which it has evolved; probably through realizing feelings of 'pleasure' associated with reproduction.

Proline-specific dipeptidyl peptidase from the blue blowfly Calliphora vicina hydrolyzes in vitro the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF)

To elucidate the mechanisms of inactivation of the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF) in the blue blowfly Calliphora vicina, we investigated its proteolytic degradation. In homogenates and membrane and soluble fractions, this hexapeptide (sequence: NPTNLH) was hydrolyzed into two fragments, NP and TNLH, suggesting the involvement of a proline-specific dipeptidyl peptidase. The dipeptidyl peptidase activity was highest in the late larval stage. It was purified 240-fold from soluble fractions of pupae of mixed age and classified on the basis of several catalytic properties as an invertebrate homologue of mammalian dipeptidyl peptidase IV (EC 3.4.14.5). Fly dipeptidyl peptidase IV has a molecular mass of 200 kDa, showed a pH optimum of 7.5-8.0 with the chromogenic substrate Gly-Pro-4-nitroanilide, and cleaved other chromogenic substrates with penultimate Pro or, with lower activity, Ala. It liberated Xaa-Pro dipeptides from the N-terminus of several bioactive peptides including substance P, neuropeptide Y, and peptide YY but not from bradykinin, indicating that the peptide bond between the two proline residues was resistant to cleavage. Fly dipeptidyl peptidase belongs to the serine class of proteases as the mammalian enzyme does; the fly enzyme, however, is not inhibited by several selective or nonselective inhibitors of its mammalian counterpart. It is suggested that dipeptidyl peptidases exert a regulatory role for the clearance not only of TMOF in files but for other bioactive peptides in various invertebrates.

Ecdysteroidostatin from the house fly, Musca domestica

Ovaries from house flies maintained on sucrose secrete large amounts of ecdysteroid when they are cultured with ovarian ecdysteroidogenic hormone, OEH. However, ovarian ecdysteroid secretion is reduced by incubation with both OEH and the ovarian ecdysteroidostatin (OES). A partially purified OES fraction from a semi-preparative reverse phase HPLC C18 column caused a 98% inhibition of ovarian ecdysteroid secretion in vitro at a concentration of 0.8 equivalents per microliter. Ovaries can be activated to produce ecdysteroid in vivo by feeding diet containing protein to flies maintained on sucrose. Ecdysteroid secretion was inhibited when the in vivo stimulated ovaries were cultured with OES. This suggests that OES does not interfere with the OEH activation mechanism, but blocks ovarian ecdysteroid synthesis or release. Furthermore, OES inhibition is reversible and ecdysteroid secretion resumes when OES is removed. Musca OES could explain the decrease in ecdysteroid levels found in flies after mid-vitellogenesis. Both adult male and female abdomens contain OES, but OES was not transferred to females during mating. Evidence is presented that OES is not a trypsin modulating oostatic factor.

Biological activity of structural analogs and effect of oil as a carrier of trypsin modulating oostatic factor of the gray fleshfly Neobellieria bullata

The trypsin modulating oostatic factor from the gray fleshfly Neobellieria bullata (Neb-TMOF) is released from the ovary at the end of vitellogenesis and inhibits trypsin biosynthesis in the midgut. This inhibition indirectly results in an arrest of oocyte growth. Additional experiments with N. bullata were performed to characterize its trypsin modulating and oostatic properties in more detail. After suspending the peptide in wheat germ oil, the threshold dose for oostatic activity was lowered one thousand times (2.10(-5) in oil versus 2.10(-2) pmoles per fly in Ringer). By use of the Neobellieria trypsin biosynthesis assay, 17 analogs of the hexapeptide were tested for inhibitory activity. The following structural elements were demonstrated to be critical for biological activity: the alcohol function at position 3 (Thr residue); a positively charged basic group at the C terminus (His residue); and the Asn side chain at positions 1 and 4.

Hormones and the cytoskeleton of animals and plants

It is often overlooked that a cell can exert its specific functions only after it has acquired a specific morphology: function follows form. The cytoskeleton plays an important role in establishing this form, and a variety of hormones can influence it. The cytoskeletal framework has also been shown to function in a variety of cellular processes, such as cell motility (important for behavior), migration (important for the interrelationship between the endocrine and immune systems, e.g., chemotaxis), intracellular transport of particles, mitosis and meiosis, maintenance of cellular morphology, spatial distribution of cell organelles (e.g., nucleus and Golgi system), cellular responses to membrane events (e.g., endocytosis and exocytosis), intracellular communication including conductance of electrical signals, localization of mRNA, protein synthesis, and--more specifically in plants--ordered cell wall deposition, cytoplasmic streaming, and spindle function followed by phragmoplast function. All classes of hormones seem to make use of the cytoskeleton, either during their synthesis, transport, secretion, degradation, or when influencing their target cells. In this review special attention is paid to cytoskeleton-mediated effects of selected hormones related to growth, transepithelial transport, steroidogenesis, thyroid and parathyroid functioning, motility, oocyte maturation, and cell elongation in plants.

An ecdysiostatin from flies

A bioassay for compounds with ecdysiostatic activity ('ecdysiostatins') was developed in order to prove the existence of an ecdysiostatin in blowfly larvae. The factor eluted by HPLC like Neb-TMOF (trypsin modulating oostatic factor), a hexapeptide that inhibits ecdysone biosynthesis. The ecdysiostatic activity of Neb-TMOF is specific, related peptides were less active or inactive. TMOF inhibits ecdysone biosynthesis rather than its secretion. It increases the cAMP level of ring glands. Effectors that elevate the intracellular cAMP can mimic the inhibitory effect of the ecdysiostatin. This suggests that cAMP may control steroidogenesis in fly larvae by shutting ecdysone biosynthesis. Thus, the hexapeptide acts as a prothoracicostatic hormone (PTSH) that controls ecdysone biosynthesis.

Neb-colloostatin, a second folliculostatin of the grey fleshfly, Neobellieria bullata

During the purification of trypsin-modulating oostatic factor (TMOF) of the grey fleshfly Neobellieria bullata, a new factor with oostatic activity was discovered. We report herein its purification, primary structure and effects on oocyte development. Its amino acid sequence was determined as H-SIVPLGLPVPIGPIVVGPR-OH. Due to structural sequence similarities with parts of several known collagens and its oostatic activity, we named it Neb-colloostatin. The synthetic peptide inhibits yolk uptake by previtellogenic oocytes and might have a role in the absence of yolk deposition in penultimate oocytes. Neb-colloostatin does not inhibit trypsin biosynthesis in the gut or ecdysone biosynthesis by larval ring glands. It decreases vitellogenin concentrations in the hemolymph by an unknown mode of action. The role of extracellular matrix proteins in the feedback control of growth is discussed.