Inactivation of a tachykinin-related peptide: identification of four neuropeptide-degrading enzymes in neuronal membranes of insects from four different orders

Tachykinins: Neuropeptides That Are Ancient, Diverse, Widespread and Functionally Pleiotropic

Tachykinins (TKs) are ancient neuropeptides present throughout the bilaterians and are, with some exceptions, characterized by a conserved FX1GX2Ramide carboxy terminus among protostomes and FXGLMamide in deuterostomes. The best-known TK is the vertebrate substance P, which in mammals, together with other TKs, has been implicated in health and disease with important roles in pain, inflammation, cancer, depressive disorder, immune system, gut function, hematopoiesis, sensory processing, and hormone regulation. The invertebrate TKs are also known to have multiple functions in the central nervous system and intestine and these have been investigated in more detail in the fly Drosophila and some other arthropods. Here, we review the protostome and deuterostome organization and evolution of TK precursors, peptides and their receptors, as well as their functions, which appear to be partly conserved across Bilateria. We also outline the distribution of TKs in the brains of representative organisms. In Drosophila, recent studies have revealed roles of TKs in early olfactory processing, neuromodulation in circuits controlling locomotion and food search, nociception, aggression, metabolic stress, and hormone release. TK signaling also regulates lipid metabolism in the Drosophila intestine. In crustaceans, TK is an important neuromodulator in rhythm-generating motor circuits in the stomatogastric nervous system and a presynaptic modulator of photoreceptor cells. Several additional functional roles of invertebrate TKs can be inferred from their distribution in various brain circuits. In addition, there are a few interesting cases where invertebrate TKs are injected into prey animals as vasodilators from salivary glands or paralyzing agents from venom glands. In these cases, the peptides are produced in the glands of the predator with sequences mimicking the prey TKs. Lastly, the TK-signaling system appears to have duplicated in Panarthropoda (comprising arthropods, onychophores, and tardigrades) to give rise to a novel type of peptides, natalisins, with a distinct receptor. The distribution and functions of natalisins are distinct from the TKs. In general, it appears that TKs are widely distributed and act in circuits at short range as neuromodulators or cotransmitters.

Neprilysins: an evolutionarily conserved family of metalloproteases that play important roles in reproduction in Drosophila

Members of the M13 class of metalloproteases have been implicated in diseases and in reproductive fitness. Nevertheless, their physiological role remains poorly understood. To obtain a tractable model with which to analyze this protein family's function, we characterized the gene family in Drosophila melanogaster and focused on reproductive phenotypes. The D. melanogaster genome contains 24 M13 class protease homologs, some of which are orthologs of human proteases, including neprilysin. Many are expressed in the reproductive tracts of either sex. Using RNAi we individually targeted the five Nep genes most closely related to vertebrate neprilysin, Nep1-5, to investigate their roles in reproduction. A reduction in Nep1, Nep2, or Nep4 expression in females reduced egg laying. Nep1 and Nep2 are required in the CNS and the spermathecae for wild-type fecundity. Females that are null for Nep2 also show defects as hosts of sperm competition as well as an increased rate of depletion for stored sperm. Furthermore, eggs laid by Nep2 mutant females are fertilized normally, but arrest early in embryonic development. In the male, only Nep1 was required to induce normal patterns of female egg laying. Reduction in the expression of Nep2-5 in the male did not cause any dramatic effects on reproductive fitness, which suggests that these genes are either nonessential for male fertility or perform redundant functions. Our results suggest that, consistent with the functions of neprilysins in mammals, these proteins are also required for reproduction in Drosophila, opening up this model system for further functional analysis of this protein class and their substrates.

The host-seeking inhibitory peptide, Aea-HP-1, is made in the male accessory gland and transferred to the female during copulation

Male accessory glands (MAGs) of insects are responsible for the production of many of the seminal fluid proteins and peptides that elicit physiological and behavioral responses in the post-mated female. In the yellow fever mosquito, Aedes aegypti, seminal fluid components are responsible for stimulating egg production, changing female behavior away from host-seeking toward egg-laying and mating refractoriness, but hitherto no behavior-modifying molecule from the MAGs has been structurally characterized. We now show using mass spectrometry and HPLC/ELISA that the MAG is a major site of synthesis of the biologically active decapeptide, Aea-HP-1 (pERPhPSLKTRFamide) that was first characterized by Matsumoto and colleagues in 1989 from mosquito head extracts and shown to have host-seeking inhibitory properties. The peptide is localized to the anterior portion of the MAG, occurs at high concentrations in the gland and is transferred to the female reproductive tract on copulation. Aea-HP-1 has a pyroglutamic acid at the N-terminus, an amidated carboxyl at the C-terminus and an unusual 4-hydroxyproline in position 4 of the peptide. The structure of the peptide with its blocked N- and C-termini confers resistance to metabolic inactivation by MAG peptidases; however the peptide persists for less than 2h in the female reproductive tract after copulation. Aea-HP-1 is not a ligand for the mosquito sex peptide/myoinhibitory peptide receptor. A. aegypti often mate close to the host and therefore it is possible that male-derived Aea-HP-1 induces short-term changes to female host-seeking behavior to reduce potentially lethal encounters with hosts soon after insemination.

Discovery and functional study of a novel crustacean tachykinin neuropeptide

Tachykinin-related peptide (TRP) refers to a large and structurally diverse family of neuropeptides found in vertebrate and invertebrate nervous systems. These peptides have various important physiological functions, from regulating stress in mammals to exciting the pyloric (food filtering) rhythm in the stomatogastric nervous system (STNS) of decapod crustaceans. Here, a novel TRP, which we named CalsTRP (Callinectes sapidus TRP), YPSGFLGMRamide (m/z 1026.52), was identified and de novo sequenced using a multifaceted mass spectrometry-based platform in both the central nervous system (CNS) and STNS of C. sapidus. We also found, using isotopic formaldehyde labeling, that CalsTRP in the C. sapidus brain and commissural ganglion (CoG) was up-regulated after food-intake, suggesting that TRPs in the CNS and STNS are involved in regulating feeding in Callinectes. Using imaging mass spectrometry, we determined that the previously identified CabTRP Ia (APSGFLGMRamide) and CalsTRP were co-localized in the C. sapidus brain. Lastly, our electrophysiological studies show that bath-applied CalsTRP and CabTRP Ia each activates the pyloric and gastric mill rhythms in C. sapidus, as shown previously for pyloric rhythm activation by CabTRP Ia in the crab Cancer borealis. In summary, the newly identified CalsTRP joins CabTRP Ia as a TRP family member in the decapod crustacean nervous system, whose actions include regulating feeding behavior.

Transcriptome analysis of the desert locust central nervous system: production and annotation of a Schistocerca gregaria EST database

BACKGROUND

The desert locust (Schistocerca gregaria) displays a fascinating type of phenotypic plasticity, designated as 'phase polyphenism'. Depending on environmental conditions, one genome can be translated into two highly divergent phenotypes, termed the solitarious and gregarious (swarming) phase. Although many of the underlying molecular events remain elusive, the central nervous system (CNS) is expected to play a crucial role in the phase transition process. Locusts have also proven to be interesting model organisms in a physiological and neurobiological research context. However, molecular studies in locusts are hampered by the fact that genome/transcriptome sequence information available for this branch of insects is still limited.

METHODOLOGY

We have generated 34,672 raw expressed sequence tags (EST) from the CNS of desert locusts in both phases. These ESTs were assembled in 12,709 unique transcript sequences and nearly 4,000 sequences were functionally annotated. Moreover, the obtained S. gregaria EST information is highly complementary to the existing orthopteran transcriptomic data. Since many novel transcripts encode neuronal signaling and signal transduction components, this paper includes an overview of these sequences. Furthermore, several transcripts being differentially represented in solitarious and gregarious locusts were retrieved from this EST database. The findings highlight the involvement of the CNS in the phase transition process and indicate that this novel annotated database may also add to the emerging knowledge of concomitant neuronal signaling and neuroplasticity events.

CONCLUSIONS

In summary, we met the need for novel sequence data from desert locust CNS. To our knowledge, we hereby also present the first insect EST database that is derived from the complete CNS. The obtained S. gregaria EST data constitute an important new source of information that will be instrumental in further unraveling the molecular principles of phase polyphenism, in further establishing locusts as valuable research model organisms and in molecular evolutionary and comparative entomology.

Biostable multi-Aib analogs of tachykinin-related peptides demonstrate potent oral aphicidal activity in the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae)

The tachykinin-related peptides (TRPs) are multifunctional neuropeptides found in a variety of arthropod species, including the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae). Two new biostable TRP analogs containing multiple, sterically hindered Aib residues were synthesized and found to exhibit significantly enhanced resistance to hydrolysis by angiotensin converting enzyme and neprilysin, membrane-bound enzymes that degrade and inactivate natural TRPs. The two biostable analogs were also found to retain significant myostimulatory activity in an isolated cockroach hindgut preparation, the bioassay used to isolate and identify the first members of the TRP family. Indeed one of the analogs (Leuma-TRP-Aib-1) matched the potency and efficacy of the natural, parent TRP peptide in this myotropic bioassay. The two biostable TRP analogs were further fed in solutions of artificial diet to the pea aphid over a period of 3 days and evaluated for antifeedant and aphicidal activity and compared with the effect of treatment with three natural, unmodified TRPs. The two biostable multi-Aib TRP analogs were observed to elicit aphicidal effects within the first 24 h. In contrast natural, unmodified TRPs, including two that are native to the pea aphid, demonstrated little or no activity. The most active analog, double-Aib analog Leuma-TRP-Aib-1 (pEA[Aib]SGFL[Aib]VR-NH(2)), featured aphicidal activity calculated at an LC(50) of 0.0083 nmol/μl (0.0087 μg/μl) and an LT(50) of 1.4 days, matching or exceeding the potency of commercially available aphicides. The mechanism of this activity has yet to be established. The aphicidal activity of the biostable TRP analogs may result from disruption of digestive processes by interfering with gut motility patterns and/or with fluid cycling in the gut; processes shown to be regulated by the TRPs in other insects. These active TRP analogs and/or second generation analogs offer potential as environmentally friendly pest aphid control agents.

Neprilysin 4, a novel endopeptidase from Drosophila melanogaster, displays distinct substrate specificities and exceptional solubility states

Proteins belonging to the family of neprilysins are typically membrane bound M13 endopeptidases responsible for the inactivation and/or activation of peptide signaling events on cell surfaces. Mammalian neprilysins are known to be involved in the metabolism of various regulatory peptides especially in the nervous, immune, cardiovascular and inflammatory systems. Although there is still much to learn about their participation in various diseases, they are potential therapeutic targets. Here we report on the identification and first characterization of neprilysin 4 (NEP4) from Drosophila melanogaster. Reporter lines as well as in situ hybridization combined with immunolocalization demonstrated NEP4 expression during embryogenesis in pericardial cells, muscle founder cells, glia cells and male gonads. Western blot analysis confirmed the prediction of one membrane bound and one soluble isoform, a finding quite unusual among neprilysins with presumably strong physiological relevance. At least one NEP4 isoform was found in every developmental stage indicating protein activities required throughout the whole life cycle of Drosophila. Heterologously expressed NEP4 exhibited substrate preferences comparable to human neprilysin 2 with distinct cleavage of substance P and angiotensin I.

Neuropeptidases and the metabolic inactivation of insect neuropeptides

Neuropeptidases play a key role in regulating neuropeptide signalling activity in the central nervous system of animals. They are oligopeptidases that are generally found on the surface of neuronal cells facing the synaptic and peri-synaptic space and therefore are ideally placed for the metabolic inactivation of neuropeptide transmitters/modulators. This review discusses the structure of insect neuropeptides in relation to their susceptibility to hydrolysis by peptidases and the need for specialist enzymes to degrade many neuropeptides. It focuses on five neuropeptidase families (neprilysin, dipeptidyl-peptidase IV, angiotensin-converting enzyme, aminopeptidase and dipeptidyl aminopeptidase III) that have been implicated in the metabolic inactivation of neuropeptides in the central nervous system of insects. Experimental evidence for the involvement of these peptidases in neuropeptide metabolism is reviewed and their properties are compared to similar neuropeptide inactivating peptidases of the mammalian brain. We also discuss how the sequencing of insect genomes has led to the molecular identification of candidate neuropeptidase genes.

Metabolic inactivation of the circadian transmitter, pigment dispersing factor (PDF), by neprilysin-like peptidases in Drosophila

Recent studies have firmly established pigment dispersing factor (PDF), a C-terminally amidated octodecapeptide, as a key neurotransmitter regulating rhythmic circadian locomotory behaviours in adult Drosophila melanogaster. The mechanisms by which PDF functions as a circadian peptide transmitter are not fully understood, however; in particular, nothing is known about the role of extracellular peptidases in terminating PDF signalling at synapses. In this study we show that PDF is susceptible to hydrolysis by neprilysin, an endopeptidase that is enriched in synaptic membranes of mammals and insects. Neprilysin cleaves PDF at the internal Ser7–Leu8 peptide bond to generate PDF1-7 and PDF8-18. Neither of these fragments were able to increase intracellular cAMP levels in HEK293 cells cotransfected with the Drosophila PDF receptor cDNA and a firefly luciferase reporter gene, confirming that such cleavage results in PDF inactivation. The Ser7–Leu8 peptide bond was also the principal cleavage site when PDF was incubated with membranes prepared from heads of adult Drosophila. This endopeptidase activity was inhibited by the neprilysin inhibitors phosphoramidon (IC50, 0.15 μmol l–1) and thiorphan (IC50, 1.2 μmol l–1). We propose that cleavage by a member of the Drosophila neprilysin family of endopeptidases is the most likely mechanism for inactivating synaptic PDF and that neprilysin might have an important role in regulating PDF signals within circadian neural circuits.

Angiotensin-converting enzyme as a target for the development of novel insect growth regulators

Insect angiotensin converting enzyme (ACE) is a zinc metallopeptidase capable of inactivating a variety of small to medium size peptide hormones by cleavage of C-terminal dipeptides and dipeptideamides. High levels of ACE activity are found in the hemolymph and in reproductive tissues of insects, where the enzyme is considered to have an important role in the metabolism of bioactive peptides. Therefore, inhibiting ACE activity is expected to interfere with the peptidergic endocrine system and to have detrimental effects on growth, development and reproduction. We will review the studies showing that ACE inhibitors do indeed disrupt growth and reproduction in various insect species. We will also present some new genetic and pharmacological data that strengthens our conclusion that ACE should be considered as a potential target for the development of new insect growth regulators.

New insights into the roles of metalloproteinases in neurodegeneration and neuroprotection

Proteolytic enzymes constitute around 2% of the human genome and are involved in many stages of cell development from fertilization to death (apoptosis). The identification of many novel proteases from genome-sequencing programs has suggested them as potential new therapeutic targets. In addition, several well-characterized metallopeptidases were recently shown to possess new biological roles in neuroinflammation and neurodegeneration. As a result of these studies, metabolism of the neurotoxic and inflammatory amyloid peptide (Abeta) is considered as a physiologically relevant process with several metallopeptidases being suggested for the role of amyloid-degrading enzymes. These include the neprilysin (NEP) family of metalloproteinases (including its homologue endothelin-converting enzyme), insulin-degrading enzyme, angiotensin-converting enzyme, plasmin, and, possibly, some other enzymes. NEP also has a role in metabolism of sensory and inflammatory neuropeptides such as tachykinins and neurokinins. The existence of natural enzymatic mechanisms for removal of amyloid peptides has extended the therapeutic avenues in Alzheimer's disease (AD) and neurodegeneration. The proteolytic events underlying AD are highly compartmentalized in the cell and formation of amyloid peptide from its precursor molecule APP (amyloid precursor protein) takes place both within intracellular compartments and in the plasma membrane, especially in lipid raft domains. Degradation of amyloid peptide by metallopeptidases can also be both intra- and extracellular depending on the activity of membrane-bound enzymes and their soluble partners. Soluble forms of proteases can be secreted or released from the cell surface through the activity of "sheddases"-another group of proteolytic enzymes involved in key cellular regulatory functions. The activity of proteases involved in amyloid metabolism depends on numerous factors (e.g., genetic, environmental, age), and some conditions (e.g., hypoxia and ischemia) shift the balance of amyloid metabolism toward accumulation of higher concentrations of Abeta. In this regard, regulation of the activity of amyloid-degrading enzymes should be considered as a viable strategy in neuroprotection.

Angiotensin converting enzyme has an inhibitory role in CGRP metabolism in human skin

The neutral endopeptidase (NEP) is important for calcitonin gene related peptide (CGRP) degradation, while the role of angiotensin converting enzyme (ACE) remains unclear. By using dermal microdialysis we explored the effect of phosphoramidon (NEP blocker), captopril (ACE blocker) and a mixture of both drugs on the intensity of electrically-induced CGRP-mediated neurogenic flare. The results reveal that phosphoramidon elevated flare intensity, but that this was not further increased by adding captopril. In contrast, neurogenic flare was decreased when the drug mixture was applied in compared to NEP only. Electrically released CGRP levels could be measured directly in perfusates containing phosphoramidon and the mixture. Again, CGRP levels were elevated in phosphoramidon treated sites, and significantly reduced upon adding captopril. These findings suggest that NEP and ACE do not have additive effects regarding neuropeptide degradation. In contrast, inhibition of ACE seems to augment CGRP catabolism.

Inhibition of neutral endopeptidase (NEP) facilitates neurogenic inflammation

Neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE) are involved in neuropeptide degradation and may modulate neurogenic inflammation. We therefore explored the effect of specific blockers of NEP and ACE on the intensity of neurogenic inflammation. We investigated eight subjects on three occasions. Two pairs of microdialysis fibers equipped with intraluminal wires were inserted intracutaneously into the volar forearms and electrical stimuli were delivered via the intraluminal electrodes. The microdialysis fibers were perfused either with normal saline, phosphoramidon (NEP inhibitor), or captopril (ACE inhibitor). CGRP release was assessed in the microdialysis eluate via a specific EIA and by evaluating the extent and intensity of the neurogenic flare via a laser Doppler imager. The area of hyperalgesia and allodynia was assessed during electrical stimulation. Inhibition of NEP with phosphoramidon increased flare intensity (P < 0.002) and size (P < 0.01), while blocking ACE had no effect on neurogenic vasodilation. CGRP release could be measured in microdialysis samples after phosphoramidon perfusion only (P < 0.03), not in samples with captopril or saline perfusion. No effect on the areas of hyperalgesia and allodynia could be detected. Our findings suggest that NEP but not ACE is most important for CGRP degradation in human skin. This may be of particular importance for the understanding of pain disorders like migraine or complex regional pain syndrome.

Drosophila melanogaster NEP2 is a new soluble member of the neprilysin family of endopeptidases with implications for reproduction and renal function

The mammalian neprilysin (NEP) family members are typically type II membrane endopeptidases responsible for the activation/inactivation of neuropeptides and peptide hormones. Differences in substrate specificity and subcellular localization of the seven mammalian NEPs contribute to their functional diversity. The sequencing of the Drosophila melanogaster genome has revealed a large expansion of this gene family, resulting in over 20 fly NEP-like genes, suggesting even greater diversity in structure and function than seen in mammals. We now report that one of these genes (Nep2) codes for a secreted endopeptidase with a highly restricted pattern of expression. D. melanogaster NEP2 is expressed in the specialized stellate cells of the renal tubules and in the cyst cells that surround the elongating spermatid bundles in adult testis, suggesting roles for the peptidase in renal function and in spermatogenesis. D. melanogaster NEP2 was found in vesicle-like structures in the syncytial cytoplasm of the spermatid bundles, suggesting that the protein was acquired by endocytosis of protein secreted from the cyst cells. Expression of NEP2 cDNA in D. melanogaster S2 cells confirmed that the peptidase is secreted and is only weakly inhibited by thiorphan, a potent inhibitor of human NEP. D. melanogaster NEP2 also differs from human NEP in the manner in which the peptidase cleaves the tachykinin, GPSGFYGVR-amide. Molecular modelling suggests that there are important structural differences between D. melanogaster NEP2 and human NEP in the S1' and S2' ligand-binding subsites, which might explain the observed differences in inhibitor and substrate specificities. A soluble isoform of a mouse NEP-like peptidase is strongly expressed in spermatids, suggesting an evolutionarily conserved role for a soluble endopeptidase in spermatogenesis.

Tachykinin-related peptide precursors in two cockroach species

Tachykinins and tachykinin-related peptides (TKRPs) play major roles in signaling in the nervous system and intestine of both invertebrates and vertebrates. Here we have identified cDNAs encoding precursors of multiple TKRPs from the cockroaches Leucophaea maderae and Periplaneta americana. All nine LemTKRPs that had been chemically isolated in earlier experiments could be identified on the precursor of L. maderae. Four previously unidentified LemTKRPs were found in addition on the precursor. The P. americana cDNA displayed an open reading frame very similar to that of L. maderae with 13 different TKRPs. MALDI-TOF mass spectra from tissues of both species confirms the presence of all the TKRPs encoded on the precursor plus two additional peptides that are cleavage products of the N-terminally extended TKRPs. A tissue-specific distribution of TKRPs was observed in earlier experiments at isolation from brain and midgut of L. maderae. Our data do not suggest a differential gene expression but a different efficacy in processing of LemTKRP-2 and Lem/PeaTKRP-3 in the brain and intestine, respectively. This results in a gut-specific accumulation of these extended peptides, whereas in the brain their cleavage products, LemTKRP-1 and LemTKRP-3(11-19), are most abundant. Mass spectrometric analysis demonstrated the occurrence of the different TKRPs in single glomeruli of the tritocerebrum and in cells of the optical lobe.

Zinc-metalloproteases in insects: ACE and ECE

Research on the angiotensin-converting enzyme (ACE) in insects has substantially advanced during the recent decade. The cloning of this enzyme in many insect species, the determination of the 3D-structure and several molecular and physiological studies have contributed to the characterization of insect ACE as we know it today: a functional enzyme with a putative role in reproduction, development and defense. The discovery of the endothelin-converting enzyme in insects occurred more recently and cloning of the corresponding cDNA has been carried out in only one insect species so far. However, activity studies and analysis of insect genomes indicate that this enzyme is also widely distributed among insect species. Making hypotheses about its putative function would be preliminary, but its wide tissue distribution suggests a major and diverse biological role.

Structure, evolutionary conservation, and functions of angiotensin- and endothelin-converting enzymes

Angiotensin-converting enzyme, a member of the M2 metalloprotease family, and endothelin-converting enzyme, a member of the M13 family, are key components in the regulation of blood pressure and electrolyte balance in mammals. From this point of view, they serve as important drug targets. Recently, the involvement of these enzymes in the development of Alzheimer's disease was discovered. The existence of homologs of these enzymes in invertebrates indicates that these enzyme systems are highly conserved during evolution. Most invertebrates lack a closed circulatory system, which excludes the need for blood pressure regulators. Therefore, these organisms represent excellent targets for gaining new insights and revealing additional physiological roles of these important enzymes. This chapter reviews the structural and functional aspects of ACE and ECE and will particularly focus on these enzyme homologues in invertebrates.

Angiotensin I-converting enzyme (ACE) activity of the tomato moth, Lacanobia oleracea: changes in levels of activity during development and after copulation suggest roles during metamorphosis and reproduction

Angiotensin I-converting enzyme (ACE) is a dipeptidyl carboxypeptidase that removes C-terminal dipeptides from relatively short oligopeptides, usually smaller than 15 amino acids. In mammals, the enzyme has several important roles in the metabolism of vasoactive peptides, but its physiological role in insects is not fully understood. We now report the properties of an ACE in a lepidopteran species (the tomato moth, Lacanobia oleracea) and suggest new physiological roles for the enzyme in this insect. ACE activity increases four-fold during the last stadium and in early pupae, a rise which, in its timing, is similar to what has been observed previously in the transition of larva to pupa in Drosophila melanogaster. This suggests that the increase in ACE activity might be of general importance for peptide metabolism during metamorphosis in holometabolous insects. High levels of ACE activity were found in the haemolymph of sixth stadium larvae and adult insects, and in the reproductive tissues of both male and female adults. Almost all of the ACE activity in the reproductive tissues was found in the accessory glands of the male and the spermatheca and bursa copulatrix of the female. The decline in accessory gland ACE in mated males and the concomitant rise in ACE activity in the spermatheca and bursa copulatrix of the female suggested the transfer of ACE from the male to the female during copulation. Using several convenient peptides as substrates, we have shown that the spermatophore/bursa copulatrix taken from mated female insects possess an aminopeptidase, a carboxypeptidase and a dipeptidase, in addition to high levels of ACE. These peptidases might be involved in the breakdown of proteins to peptides and eventually to amino acids in the spermatophore. Evidence for such a proteolytic pathway and its role in providing substrates for the TCA cycle has been obtained previously in a study of reproduction in Bombyx mori.

Deamidase inactivates a D-amino acid-containing Aplysia neuropeptide

Membrane-catalyzed degradation of the cardioexcitatory peptide, Asn-D-Trp-Phe-NH(2) (N(d)WF-NH(2)), which was previously isolated from Aplysia, was investigated in relation to its inactivation mechanism. The principal degradation was deamidation of the C-terminal amide, producing biologically inert Asn-D-Trp-Phe-OH (N(d)WF-OH). Among membrane fractions prepared from different organs, the fraction from the ganglia showed the highest specific activity of the deamidation reaction. The deamidase activity was inhibited by Ebelactone B and the serine protease inhibitor, phenylmethanesulfonyl fluoride (PMSF), while the degradation of the synthetic stereoisomer, Asn-Trp-Phe-NH(2) (N(l)WF-NH(2)), was sensitive to the divalent cation-chelating agent, o-phenanthroline, and aminopeptidase inhibitors, amastatin and bestatin. The presence of D-Trp residue in the second position of N(d)WF-NH(2) endows this peptide not only with stereospecific bioactivity but also peptidase stability. The deamidation reaction seems to be the major inactivation mechanism for this peptide.

Extracellular peptidases of imaginal discs of Drosophila melanogaster

The imaginal discs of Drosophila melanogaster give rise to the adult epidermis during metamorphosis. During this developmental period several peptidase genes are expressed in disc cells, but there is a paucity of biochemical information regarding substrate specificity. We have used peptides and peptidyl 7-amino-4-methylcoumarin (AMC) substrates to detect several peptidases either positioned on the surface of wing discs or secreted by the imaginal cells. Using [Leu(5)]enkephalin as a substrate, a captopril sensitive dipeptidyl carboxypeptidase (angiotensin I-converting enzyme) and an amastatin-sensitive aminopeptidase were detected as prominent activities associated with intact discs. The formation of [Leu(5)]enkephalin-derived Phe was attributed to the concerted action of the D. melanogaster angiotensin I-converting enzyme (Ance) and a dipeptidase. The disc Ance also showed endopeptidic activity towards locust tachykinin-1 (LomTK-I) by cleaving the Gly-Val peptide bond, but this enzyme was not the sole endopeptidase activity associated with discs. Complete inhibition of the endopeptidic hydrolysis of the LomTK-1 by a disc homogenate required a combination of captopril and the neprilysin inhibitor, phosphoramidon, providing biochemical evidence for a neprilysin-like peptidase, in addition to Ance, in imaginal discs of D. melanogaster. Peptidyl AMC substrates for furin, prohormone convertase and tryptase provided evidence for trypsin-like serine endopeptidases in addition to the metalloendopeptidases. We conclude that imaginal discs are endowed with a variety of peptidases from different families that together are capable of hydrolyzing a broad range of peptides and proteins. Some of these peptidases might be responsible for the metabolic activation/inactivation of signaling peptides, as well as being involved in the production of dipeptides and free amino acids required for protein synthesis and osmotic balance during adult morphogenesis.

Peptidyl dipeptidases (Ance and Acer) of Drosophila melanogaster: major differences in the substrate specificity of two homologs of human angiotensin I-converting enzyme

Drosophila melanogaster angiotensin converting enzyme (Ance) and angiotensin converting enzyme related (Acer) are single domain homologs of mammalian peptidyl dipeptidase A (angiotensin I-converting enzyme) whose physiological substrates have not as yet been identified. We have investigated the in vitro substrate specificities of the two peptidases towards a variety of insect and mammalian peptides. Ance was generally much better than Acer at hydrolyzing peptides of 5-13 amino acids in length. Only two of the peptides, [Leu(5)]enkephalinamide and leucokinin-I were cleaved faster by Acer. Increasing NaCl concentration had opposite affects on the cleavage of [Leu(5)]enkephalin and [Leu(5)]enkephalinamide by Acer, decreasing the activity towards [Leu(5)]enkephalin but increasing the activity towards [Leu(5)]enkephalinamide. Of the insect peptides tested, the tachykinin-related peptide, Lom TK-1, proved to be the best substrate for Ance with a k(cat)/K(m) ratio of 0.122s(-1) microM(-1). However, in comparison, the D. melanogaster tachykinins, DTK-1, DTK-2, DTK-3 and DTK-4 were poor Ance substrates. DTK-5 was the best substrate of this family, but the apparent high K(m) for hydrolysis by Ance suggested that this peptide would not be a natural Ance substrate. This low affinity for DTK-5 is the likely reason why the peptide was not rapidly degraded in D. melanogaster hemolymph, where Ance was shown to be a major peptide-degrading activity.

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.