A hypolipaemic factor from the corpus cardiacum of locusts

A Rhodnius prolixus Insulin Receptor and Its Conserved Intracellular Signaling Pathway and Regulation of Metabolism

The insulin signaling pathway is a modulator of metabolism in insects and can regulate functions associated with growth and development, as well as lipid and carbohydrate balance. We have previously reported the presence of an insulin-like peptide and an insulin-like growth factor in Rhodnius prolixus, which are involved in the homeostasis of lipids and carbohydrates in post-feeding and non-feeding periods. In the present study, we have characterized the first insulin receptor (IR) to be discovered in R. prolixus, Rhopr-IR, and investigated its intracellular signaling cascade and its role in nutrient control. We identified a candidate protein sequence within R. prolixus putative peptidome and predicted its conserved features using bioinformatics. Tissue-specific expression analyses indicated that the Rhopr-IR transcript is differentially-expressed in all tissues tested, with the highest values observed in the central nervous system (CNS). Treatment of insects with the IR kinase activator BpV(phen), glucose, or porcine insulin resulted in the activation of protein phosphorylation in the fat body, and stimulated the phosphorylation of protein kinase Akt, an evolutionarily conserved key regulator of the intracellular insulin signaling cascade. We also observed activation of Akt and phosphorylation of its downstream targets glycogen synthase kinase 3 β (GSK3β) and the transcription factor FOXO for several days following a blood meal. We used dsRNA to knockdown transcript expression and examined the resulting effects on metabolism and intracellular signaling. Furthermore, knockdown of the Rhopr-IR transcript increased lipid levels in the hemolymph, while reducing lipid content in the fat body. Interestingly, the levels of carbohydrates in the hemolymph and in the fat body did not show any alterations. The activation of Akt and phosphorylation of FOXO were also reduced in knockdown insects, while the phosphorylation pattern of GSK3β did not change. Our results support the identification of the first IR in R. prolixus and suggest that Rhopr-IR signaling is involved in hemolymph nutrient homeostasis and fat body storage both in post-feeding and in non-feeding stages. These metabolic effects are likely regulated by the activation of Akt and downstream cascades similar to mammalian insulin signaling pathways.

Identification of a glycogenolysis-inhibiting peptide from the corpora cardiaca of locusts

A mass spectrometric study of the peptidome of the neurohemal part of the corpora cardiaca of Locusta migratoria and Schistocerca gregaria shows that it contains several unknown peptides. We were able to identify the sequence of one of these peptides as pQSDLFLLSPK. This sequence is identical to the part of the Locusta insulin-related peptide (IRP) precursor that is situated between the signal peptide and the B-chain. We designated this peptide as IRP copeptide. This IRP copeptide is also present in the pars intercerebralis, which is likely to be the site of synthesis. It is identical in both L. migratoria and S. gregaria. It shows no effect on the hemolymph lipid concentration in vivo or muscle contraction in vitro. The IRP copeptide is able to cause a decreased phosphorylase activity in locust fat body in vitro, opposite to the effect of the adipokinetic hormones and therefore possibly represents a glycogenolysis-inhibiting peptide.

Insulin-like peptides are not involved in maturation or functional recovery of neural circuits in the locust flight system

We sought to manipulate maturation and functional recovery of locust flight circuitry by treating locusts with pharmacological doses of bovine anti-insulin and insulin. Anti-insulin treatment of maturing locusts caused reduced growth of the thoracic nervous system, lower body weight, and softer cuticles compared with control locusts. We were unable to block either maturation or recovery of flight circuitry with anti-insulin. We propose that insulin-related peptides are involved in growth and cuticular changes during adult maturation, but have no role in promoting neuronal sprouting during this period or as a result of injury.Key words: insulin, maturation, functional recovery, proprioceptors, flight.

Bombyxin-related peptides: cDNA structure and expression in the brain of the hornworm Agrius convolvuli [corrected]

We have cloned three cDNAs from the sweet potato hornworm Agrius convolvuli that encode precursor molecules for peptides structurally related to bombyxin, an insulin-related brain secretory peptide in Bombyx mori. The Agrius bombyxin-related peptide (ABRP) cDNAs are classified into type A and B according to their sequence similarity. The prepro-ABRPs deduced from the cDNA sequences have the insulin-like domain organization of signal peptide/B chain/C peptide/A chain. The ABRP transcripts in Agrius brain were shown to locate in four pairs of medial neurosecretory cells, the homologous group of neurosecretory cells that produce bombyxins in Bombyx brain. Genomic Southern analysis indicated the presence of multiple copies of ABRP gene in the Agrius genome. Results showed that the ABRP genes are remarkably different from the vertebrate insulin genes in the number of copy and spatial localization of the transcripts.

Isolation and structural characterization of an insulin-related molecule, a predominant neuropeptide from Locusta migratoria

Neurohaemal lobes of corpora cardiaca of Locusta migratoria are an established storage site for neurohormones produced by the neurosecretory cells of the brain. As previously reported [Hietter, H., Van Dorsselaer, A., Green, B., Denoroy, L., Hoffmann, J.A. & Luu, B. (1990) Eur. J. Biochem. 187, 241-247], the isolation and characterization of a novel 5-kDa peptide from these lobes served as the basis for oligonucleotide screening of cDNA libraries prepared from poly(A) RNA from neurosecretory cells of the central nervous system. From subsequent cDNA cloning studies [Lagueux, M., Lwoff, L., Meister, M., Goltzené, F. & Hoffmann, J.A. (1990) Eur. J. Biochem. 187, 249-254], the existence of a 145-residue precursor protein was deduced, which contained, in addition to the 5-kDa peptide, amino-acid sequences with homology to the A and B chains of an insulin-related peptide. In the present study we have isolated the native molecule from corpora cardiaca of Locusta and characterized, by Edman degradation and plasma-desorption mass spectrometry, the two chains as follows: A chain, Gly-Val-Phe-Asp-Glu-Cys-Cys-Arg-Lys-Ser-Cys-Ser-Ile-Ser-Glu-Leu-Gln-Thr- Tyr-Cys - Gly (Ile, isoleucine); B chain, Ser-Gly-Ala-Pro-Gln-Pro-Val-Ala-Arg-Tyr-Cys-Gly-Glu-Lys-Leu-Ser-Asn-Ala- Leu-Lys - Leu-Val-Cys-Arg-Gly-Asn-Tyr-Asn-Thr-Met-Phe. Taken in conjunction with the previous cloning studies, our data lead to a clear picture of the processing of Locusta preproinsulin. They indicate that locusta corpora cardiaca contain remarkably large amounts of one single insulin form, in contrast to multiple insulin isoforms of Bombyx mori, the only other insect species from which insulin-related peptides have been isolated and characterized [Nagasawa, H., Kataoka, H., Isogai, A., Tamura, S., Suzuki, A., Mizoguchi, A., Fujiwara, Y., Suzuki, A., Takahashi, S. & Ishizaki, H. (1986) Proc. Natl Acad. Sci. USA 83, 5840-5843].

Immunocytochemical localization and immunochemical characterization of an insulin-related peptide in the insect Leucophaea maderae

Immunocytochemical tests with eight monoclonal antibodies against either bovine or human insulin and seven polyclonal antibodies against bovine insulin were carried out to determine the presence of insulin-like neuropeptides in the brain and affiliated neuroendocrine structures of the insect Leucophaea maderae. Reaction products identified in the brain, subesophageal ganglion, and corpus cardiacum-corpus allatum complex indicate the presence of materials resembling mammalian insulins in its antigenic properties. The immunostaining observed with monoclonal antibodies appears to indicate the occurrence of an insulin-related peptide that shows sequential similarities with parts of both the A- and B-chains of mammalian insulin molecules. These suppositions are supported by the results of dot-blot and two-site time-resolved immunofluorometric assay (TRI-IFMA) screenings of fractions of Leucophaea tissue extracts obtained by chromatography. The polyclonal antibodies yielded reaction products in some of the same areas and in additional parts of the neuroendocrine system not visualized by the monoclonal antibodies. Immunoreaction was observed in the following areas: the pars intercerebralis of the protocerebrum, the nervi corporis cardiaci I transporting insulin-like material to the corpus cardiacum, the dorsolateral protocerebral area and the optic lobes, the deutocerebrum, the tritocerebrum, and the subesophageal ganglion. In addition, smaller cell bodies with immunoreactive deposits occur at the border between proto- and deuto-cerebrum, and in the central area of the protocerebrum. The distribution of reactive material in the corpus cardiacum-corpus allatum complex after use of both groups of antibodies was the same.(ABSTRACT TRUNCATED AT 250 WORDS)

cDNAs from neurosecretory cells of brains of Locusta migratoria (Insecta, Orthoptera) encoding a novel member of the superfamily of insulins

From neurohaemal lobes of corpora cardiaca of Locusta migratoria a 5-kDa peptide has been isolated and its sequence established [see the accompanying paper, by Hietter et al. (1990) Eur. J. Biochem. 187, 241-247]. We have designed oligonucleotide probes from the peptide sequence of this molecule and screened a library prepared from mRNA of the neurosecretory cell region of the brain of this insect. Several positive cDNAs were isolated, the combined nucleotide sequences of which predict a large precursor of 145 residues (15770 Da) containing the newly isolated 5-kDa peptide. The peptide is flanked by regions homologous to the A and B chains of the superfamily of insulins. The overall organization of the precursor is as follows: signal peptide/domain homologous to the B chain of insulins/C (connecting)-peptide (corresponding to the newly isolated 5-kDa peptide)/domain homologous to the A chain of insulins. The numbers and relative positions of the cysteines of the Locusta peptide are equivalent to those of the other members of the insulin superfamily and most of the hydrophobic core residues are conserved.

A substance resembling somatomedin C in the American cockroach

Material antigenically resembling somatomedin C (type I insulin-like growth factor, IGF-I) is demonstrated in the American cockroach Periplaneta americana by means of a monoclonal antibody immunoperoxidase technique. It was localized histochemically in neuronal cell somata and axonal fibers (probably interneurons) of the central nervous/neuroendocrine system and in 'endocrine-type' cells lining the midgut epithelium. The IGF-I-like substance is different from vertebrate insulin and also distinct from materials immunostained by different insulin antibodies in the brain and neurohaemal complex of this insect species. These findings are viewed in the light of recent reports on the presence and action of insulin-like chemicals in insects, and with respect to the existence of an insect brain-midgut system similar to the mammalian brain-gastroenteropancreatic system.

Production and characterization of antibodies to neuroparsins A and B isolated from the corpora cardiaca of the locust

Antisera were raised in rabbits against neuroparsins A and B which were purified to near homogeneity using electroelution from 7.5% polyacrylamide electrophoresis gels. They were characterized using immunohistochemical, enzyme-linked immunosorbent assay (ELISA), and protein-blotting methods. The antineuroparsin A and the antineuroparsin B sera have different titers and sensitivities (higher titer for antineuroparsin A, higher sensitivity for antineuroparsin B). They exhibit very good specificity. The immunohistochemical study of the entire central nervous system using either antineuroparsin A or antineuroparsin B sera shows that only the A1 type of the protocerebral median neurosecretory cells are immunostained. Moreover, among the numerous proteins of the median region of the brain and of the corpora cardiaca, each immune serum recognized only neuroparsin A or neuroparsin B. Displacement curves obtained for each immune serum by competition between either neuroparsin A or neuroparsin B demonstrate that the neuroparsin A is recognized as well as neuroparsin B, with both antisera supporting the concept that these two proteins are chemically related. The nonspecific binding of neuroparsins to an antisomatostatin immune serum used at 1/100 dilution indicates that any cross-reactivities of invertebrate molecules obtained with very low dilutions of antisera to vertebrate molecules must be considered carefully before concluding any immunological relation between invertebrate and vertebrate products.

A hypoglycemic factor from the corpus cardiacum of the American cockroach Periplaneta americana

Thin-layer chromatography of cockroach corpora cardiaca (cc) extracts revealed a fraction (F1 in solvent system 1) which had hyperlipemic activity in locusts and which differed in mobility from synthetic locust adipokinetic hormone I (AKH I). Fractions 4 and 5 exhibited hypolipemic activity when injected into locusts. Pooled samples of these fractions caused a diminution of carbohydrate levels in the cockroach. Bovine insulin mimicked both these effects. No change in hemolymph lipid was evident in the cockroach after injection of either CC fractions 4 and 5 or bovine insulin. Both the active fraction from the cockroach CC and bovine insulin caused a decrease in hemolymph carbohydrate in neck-ligated locusts. Injection of locust hypolipemic hormone, separated from locust CC storage lobes by TLC, into neck-ligated locusts also caused a decrease in hemolymph carbohydrate concentration. Normal locusts showed no change in hemolymph carbohydrate.

Immunocytochemical localization of peptidergic neurons and neurosecretory cells in the neuro-endocrine system of the Colorado potato beetle with antisera to vertebrate regulatory peptides

A large number of antisera to regulatory vertebrate peptides was tested immunocytochemically on the nervous system of the Colorado potato beetle to further characterize the peptidergic cells of the neuro-endocrine system and to reveal cells participating in endocrine control mechanisms. Neurons, neurosecretory cells, axons and axon terminals were revealed by antisera to ACTH, gastrin, CCK, alpha-endorphin, beta-endorphin, gamma 1-MSH, insulin, motilin, human calcitonin, growth hormone, somatostatin, CRF, ovine prolactin and rat prolactin. Together with previously described results these findings demonstrate that at least 19 different peptidergic cell types are present in the Colorado potato beetle. Several of these cell types are identical with the known neurosecretory cells, while others have not been identified before. The functions of the immunoreactive neurons are as yet unclear, although in two cases the localization of these cells gives some clues. Thus the lateral neurosecretory cells, which are immunoreactive with antisera to beta-endorphin and ovine prolactin, may regulate corpus allatum activity, whereas a CRF immunoreactive substance seems to be used as neurotransmitter by antennal receptors. These immunocytochemical findings do not imply that the immunoreactive substances are evolutionarily related to the vertebrate peptides to which the antisera were raised. It is postulated that if the part of the substance recognized by a certain antiserum is functionally important for the insect, which should be so if the insect peptide is evolutionarily related to its vertebrate homologue, the antiserum should reveal homologous cells in different insect species. The consequence of this hypothesis is, that if an antiserum does not reveal homologous neurons in different insect species, the immunologically demonstrated substance is probably of little physiological importance, and will not be related evolutionarily to the vertebrate analogue. The positive immunocytochemical results in the Colorado potato beetle are discussed in relation to these considerations.

Is octopamine a transmitter mediating hormone release in insects?

The release of hyperlipemic hormone from the glandular cells of the corpus cardiacum (CC) of Locusta migratoria is under the synaptic control of axons in nervus corpus cardiacum II (NCC II). The effects of aminergic agonists and antagonists on the release of the hyperlipemic hormone induced by electrical stimulation of NCC II have been examined. CC isolated from reserpine-injected locusts did not release hormone when subjected to electrical stimulation of NCC II but continued to release hormone in response to high-potassium saline. The electrically stimulated release of hormone from isolated CC was abolished by the alpha-adrenergic blocking agent, phenoxybenzamine, but potentiated by the beta-adrenergic blocking agent, propranolol. Phenoxybenzamine did not interfere with release induced by high-potassium saline. It is suggested that the postsynaptic receptors on the glandular cells are similar to the alpha-adrenergic receptors of vertebrates. Octopamine was found to be present in the glandular lobe of the CC at concentrations of 0.62 pmole per gland pair. Reserpine depleted the content to 0.3 pmole per pair. Bathing the CC in 10(-7) M octopamine resulted in the release of hyperlipemic hormone, and this release was blocked by phenoxybenzamine. It is concluded that the neurotransmitter involved in the synapse between axons of NCC II and the cells releasing hyperlipemic hormone is aminergic, possibly octopaminergic. Octopamine may well be a transmitter mediating hormone release in insects.