Is the titer of adipokinetic peptides in Leptinotarsa decemlineata fed on genetically modified potatoes increased by oxidative stress?

Functional homology of tachykinin signalling: The influence of human substance P on the immune system of the mealworm beetle, Tenebrio molitor L

Tachykinin-related peptides (TRPs) are one of the most prominent families of neuropeptides in the animal kingdom. Insect TRPs display strong structural and functional homology to vertebrate tachykinins (TKs). To study functional homologies between these two neuropeptide families, the influence of human substance P (SP, one of the essential vertebrate TKs) on the immune system of the mealworm beetle, Tenebrio molitor L., was analysed. Human SP influences the phagocytic abilities of T. molitor haemocytes. Peptide injection leads to an increase in the number of haemocytes participating in the phagocytosis of latex beads. In contrast, incubation of haemocytes from non-injected beetles in a solution of physiological saline and SP causes a decrease in phagocytic activity. Treatment with human SP also led to increased adhesion of haemocytes, but no changes in the arrangement of the F-actin cytoskeleton were observed. Interestingly, 6 h after human SP injection, increased DNA integrity in T. molitor haemocytes was reported. The opposite effects were observed 24 h after SP injection. Human SP caused the upregulation of humoral immune responses, such as phenoloxidase (PO) activity in the T. molitor haemolymph, and the downregulation of immune-related genes encoding coleoptericin A, tenecin 3 and Toll receptor. However, genes encoding attacin 2 and cecropin were upregulated. Despite these differences, the antimicrobial activity of T. molitor haemolymph was significantly lower in beetles injected with SP than in control beetles. Moreover, an analysis of the direct influence of SP on lysozyme activity was performed. Our results suggest that SP at a concentration of 10^-6 M can directly inhibit lysozyme activity. However, an opposite effect was reported after the application of SP at a concentration of 10^-4 M. The presented results suggest structural and functional homology between TK signalling in vertebrates and insects. Primarily, this was visible in the context of the humoral response and general antimicrobial activity of T. molitor haemolymph. However, some of the results related to haemocyte function may also indicate the importance of the TK and TRP sequences for evoking immunological effects.

Combined PacBio Iso-Seq and Illumina RNA-Seq Analysis of the Tuta absoluta (Meyrick) Transcriptome and Cytochrome P450 Genes

Tuta absoluta (Meyrick) is a devastating invasive pest worldwide. The abamectin and chlorantraniliprole complex have become an alternative option for chemical control because they can enhance insecticidal activity and delay increased drug resistance. Notably, pests are inevitably resistant to various types of insecticides, and compound insecticides are no exception. To identify potential genes involved in the detoxification of abamectin and chlorantraniliprole complex in T. absoluta, PacBio SMRT-seq transcriptome sequencing and Illumina RNA-seq analysis of abamectin and chlorantraniliprole complex-treated T. absoluta were performed. We obtained 80,492 non-redundant transcripts, 62,762 (77.97%) transcripts that were successfully annotated, and 15,524 differentially expressed transcripts (DETs). GO annotation results showed that most of these DETs were involved in the biological processes of life-sustaining activities, such as cellular, metabolic, and single-organism processes. The KEGG pathway enrichment results showed that the pathways related to glutathione metabolism, fatty acid and amino acid synthesis, and metabolism were related to the response to abamectin and chlorantraniliprole complex in T. absoluta. Among these, 21 P450s were differentially expressed (11 upregulated and 10 downregulated). The qRT-PCR results for the eight upregulated P450 genes after abamectin and chlorantraniliprole complex treatment were consistent with the RNA-Seq data. Our findings provide new full-length transcriptional data and information for further studies on detoxification-related genes in T. absoluta.

Role of adipokinetic hormone in the Colorado potato beetle, Leptinotarsa decemlineata infected with the entomopathogenic nematode Steinernema carpocapsae

The effects of the entomopathogenic nematode Steinernema carpocapsae on the Colorado potato beetle (CPB) Leptinotarsa decemlineata and the involvement of adipokinetic hormone (AKH) in the responsive reactions were examined in this study. It was observed that nematode application doubled the amount of AKH (Peram-CAH-I and Peram-CAH-II) in the central nervous system of L. decemlineata, indicating mobilization of anti-stress reactions in the body. Furthermore, the external co-application of Peram-CAH-II with the nematode significantly increased beetle mortality (5.6 and 1.8 times, 1 and 2 days after application, respectively). The mechanism underlying this phenomenon was investigated. As the effect on gut characteristics was equivocal, it was assumed that the nematodes profited from the observed mobilization of metabolites from the fat body into the Peram-CAH-II-induced hemolymph. This phenomenon supplied nematodes with a more nutrient-dense substrate on which they propagated. Furthermore, Peram-CAH-II lowered vitellogenin expression in the fat body, particularly in males, thus limiting the anti-pathogen defense capacity of the protein. However, there could be other possible mechanisms underpinning this chain of events. The findings could be theoretically intriguing but could also aid in developing real insect pest control methods in the future.

Insect Body Defence Reactions against Bee Venom: Do Adipokinetic Hormones Play a Role?

Bees originally developed their stinging apparatus and venom against members of their own species from other hives or against predatory insects. Nevertheless, the biological and biochemical response of arthropods to bee venom is not well studied. Thus, in this study, the physiological responses of a model insect species (American cockroach, Periplaneta americana) to honeybee venom were investigated. Bee venom toxins elicited severe stress (LD₅₀ = 1.063 uL venom) resulting in a significant increase in adipokinetic hormones (AKHs) in the cockroach central nervous system and haemolymph. Venom treatment induced a large destruction of muscle cell ultrastructure, especially myofibrils and sarcomeres. Interestingly, co-application of venom with cockroach Peram-CAH-II AKH eliminated this effect. Envenomation modulated the levels of carbohydrates, lipids, and proteins in the haemolymph and the activity of digestive amylases, lipases, and proteases in the midgut. Bee venom significantly reduced vitellogenin levels in females. Dopamine and glutathione (GSH and GSSG) insignificantly increased after venom treatment. However, dopamine levels significantly increased after Peram-CAH-II application and after co-application with bee venom, while GSH and GSSG levels immediately increased after co-application. The results suggest a general reaction of the cockroach body to bee venom and at least a partial involvement of AKHs.

The Intrinsic Nutrient Sensing Adipokinetic Hormone Producing Cells Function in Modulation of Metabolism, Activity, and Stress

All organisms confront the challenges of maintaining metabolic homeostasis in light of both variabilities in nutrient supplies and energetic costs of different physiologies and behaviors. While all cells are nutrient sensitive, only relative few cells within Metazoans are nutrient sensing cells. Nutrient sensing cells organize systemic behavioral and physiological responses to changing metabolic states. One group of cells present in the arthropods, is the adipokinetic hormone producing cells (APCs). APCs possess intrinsic nutrient sensors and receive contextual information regarding metabolic state through other endocrine connections. APCs express receptors for different hormones which modulate APC physiology and the secretion of the adipokinetic hormone (AKH). APCs are functionally similar to alpha cells in the mammalian pancreas and display a similar physiological organization. AKH release results in both hypertrehalosemia and hyperlipidemia through high affinity binding to the AKH receptor (AKHR). Another hallmark of AKH signaling is heightened locomotor activity, which accompanies starvation and is thought to enhance foraging. In this review, we discuss mechanisms of nutrient sensing and modulation of AKH release. Additionally, we compare the organization of AKH/AKHR signaling in different taxa. Lastly, we consider the signals that APCs integrate as well as recent experimental results that have expanded the functional repertoire of AKH signaling, further establishing this as both a metabolic and stress hormone.

Adipokinetic hormone enhances CarE-mediated chlorpyrifos resistance in the brown planthopper, Nilaparvata lugens

Adipokinetic hormone (AKH), the principal stress-responsive neurohormone in insects, has been implicated in insect responses to insecticides. However, the functionality of AKH and its mode of signalling in insecticide resistance are unknown. Herein, we demonstrated that the enhanced activity of carboxylesterases (CarEs) is involved in the chlorpyrifos resistance in Nilaparvata lugens [brown planthopper (BPH)]. Chlorpyrifos exposure significantly induced the expression of AKH and its receptor AKHR in the susceptible BPH (Sus), and these two AKH signalling genes were over-expressed in the chlorpyrifos-resistant strain (Res) compared to Sus. RNA interference (RNAi) against AKH or AKHR decreased the CarE activity and suppressed the BPH's resistance to chlorpyrifos in Res. Conversely, AKH peptide injection elevated the CarE activity and enhanced the BPH's survival against chlorpyrifos in Sus. Furthermore, five CarE genes were identified to be positively affected by the AKH pathway using RNAi and AKH injection. Among these CarE genes, CarE and Esterase E4-1 were found to be over-expressed in Res compared to Sus, and knockdown of either gene decreased the BPH's resistance to chlorpyrifos. In conclusion, AKH plays a role in enhancing chlorpyrifos resistance in the BPH through positive influence on the expression of CarE genes and CarE enzyme activity.

Adipokinetic Hormones Enhance the Efficacy of the Entomopathogenic Fungus Isaria fumosorosea in Model and Pest Insects

Insect adipokinetic hormones (AKHs) are neuropeptides with a wide range of actions, including the control of insect energy metabolism. These hormones are also known to be involved in the insect defence system against toxins and pathogens. In this study, our aim was to demonstrate whether the application of external AKHs significantly enhances the efficacy of the entomopathogenic fungus Isaria fumosorosea in a model species (firebug Pyrrhocoris apterus) and pest species (Egyptian cotton leafworm Spodoptera littoralis and pea aphid Acyrthosiphon pisum). It was found that the co-application of Isaria with AKHs significantly enhanced insect mortality in comparison to the application of Isaria alone. The mode of action probably involves an increase in metabolism that is caused by AKHs (evidenced by the production of carbon dioxide), which accelerates the turnover of Isaria toxins produced into the infected insects. However, several species-specific differences probably exist. Intoxication by Isaria elicited the stimulation of Akh gene expression and synthesis of AKHs. Therefore, all interactions between Isaria and AKH actions as well as their impact on insect physiology from a theoretical and practical point of view need to be discussed further.

The Effect of Genetically Modified Food on Infertility Indices: A Systematic Review Study

Background and Objectives. With the increase in the growth of genetically modified (GM) crops, concerns as to the adverse effects of GM crops have risen in the community. The present systematic review seeks to assess the GM plants' potential impacts on the sperm parameters, including sperm head, sperm motility, sperm abnormality, and fertility index. Materials and Methods. A structured literature search was independently performed by three authors on Scopus, Web of Science, PubMed, and Embase in January 2019. A total of 1467 publications were obtained by the search strategy after eliminating the duplicates. Moreover, the review only included articles written in English language. Other pertinent peer-reviewed publications were chosen (“snowballing”) from the reference lists in the selected publications. To assess the GM crop effects on infertility, experimental studies designed with the control group were selected. On the basis of abstract screening and full-text reviewing, 39 relevant publications were selected, seven of which were used in our review. To access the quality of articles, we used the Cochrane checklist. The collected articles were scored independently by three authors, and the publications with the eligibility criteria were included in our review. No article was excluded due to quality assessment. Results and Conclusion. Our findings indicated that GM products had no adverse effects on infertility indices such as the sperm head, sperm motility, sperm abnormality, and fertility indices. Long-term research still seems to be strongly necessary to ensure that the use of GM plants does not cause any harm to consumers, especially in infertility area.

Effect of natural toxins and adipokinetic hormones on the activity of digestive enzymes in the midgut of the cockroach Periplaneta americana

This study examined the effect of two natural toxins (a venom from the parasitic wasp Habrobracon hebetor and destruxin A from the entomopathogenic fungus Metarhizium anisopliae), and one pathogen (the entomopathogenic fungus Isaria fumosorosea) on the activity of basic digestive enzymes in the midgut of the cockroach Periplaneta americana. Simultaneously, the role of adipokinetic hormones (AKH) in the digestive processes was evaluated. The results showed that all tested toxins/pathogens elicited stress responses when applied into the cockroach body, as documented by an increase of AKH level in the central nervous system. The venom from H. hebetor showed no effect on digestive enzyme activities in the ceca and midgut in vitro. In addition, infection by I. fumosorosea caused a decrease in activity of all enzymes in the midgut and a variable decrease in activity in the ceca; application of AKHs did not reverse the inhibition. Destruxin A inhibited the activity of all enzymes in the midgut but none in the ceca in vitro; application of AKHs did reverse this inhibition, and no differences between both cockroach AKHs were found. Overall, the results demonstrated the variable effect of the tested toxins/pathogens on the digestive processes of cockroaches as well as the variable ability of AKH to counteract these effects.

Adipokinetic hormone and adenosine interfere with nematobacterial infection and locomotion in Drosophila melanogaster

This study examined how adipokinetic hormone (AKH) and adenosine affect defense responses in Drosophila melanogaster larvae infected with entomopathogenic nematodes (EPN, Steinernema carpocapsae and Heterorhabditis bacteriophora). Three loss-of-function mutant larvae were tested: Akh¹, AdoR¹ (adenosine receptor), and Akh¹ AdoR¹. Mortality decreased in all mutants post-EPN infection compared with the control (w¹¹¹⁸). Additionally, co-application of external AKH with EPN significantly increased mortality beyond rates observed in EPN-only treatment, while also elevating carbon dioxide production, a measure of metabolism. Furthermore trehalose levels increased in both w¹¹¹⁸ and Akh¹ larvae post-EPN infection, but the latter group exhibited a lower increase and total trehalose levels. Interestingly, baseline trehalose was relatively high in untreated AdoR¹ and Akh¹ AdoR¹ mutants, with levels remaining unaffected by infection. Infection also elevated haemolymph lipid content overall, but the different mutations did not substantially influence this change. In contrast, haemolymph protein content dropped after EPN infection in all tested groups, but this decline was more intense among Akh¹. In uninfected larvae mutations decreased antioxidative capacity in Akh¹ and increased in AdoR¹, however, its post-infection increases were similar in all mutants, suggesting that antioxidant response in Drosophila involves mechanisms also beyond AKH and adenosine. Furthermore, AKH application in w¹¹¹⁸ larvae significantly increased movement distance and percentage of larval activity, but reduced velocity. Mutations of Akh and AdoR did not strongly affect locomotion.

Analysis of Peptide Ligand Specificity of Different Insect Adipokinetic Hormone Receptors

Adipokinetic hormone (AKH) is a highly researched insect neuropeptide that induces the mobilization of carbohydrates and lipids from the fat body at times of high physical activity, such as flight and locomotion. As a naturally occurring ligand, AKH has undergone quite a number of amino acid changes throughout evolution, and in some insect species multiple AKHs are present. AKH acts by binding to a rhodopsin-like G protein-coupled receptor, which is related to the vertebrate gonadotropin-releasing hormone receptors. In the current study, we have cloned AKH receptors (AKHRs) from seven different species, covering a wide phylogenetic range of insect orders: the fruit fly, Drosophila melanogaster, and the yellow fever mosquito, Aedes aegypti (Diptera); the red flour beetle, Tribolium castaneum, and the large pine weevil, Hylobius abietis (Coleoptera); the honeybee, Apis mellifera (Hymenoptera); the pea aphid, Acyrthosiphon pisum (Hemiptera); and the desert locust, Schistocerca gregaria (Orthoptera). The agonistic activity of different insect AKHs, including the respective endogenous AKHs, at these receptors was tested with a bioluminescence-based assay in Chinese hamster ovary cells. All receptors were activated by their endogenous ligand in the nanomolar range. Based on our data, we can refute the previously formulated hypothesis that a functional AKH signaling system is absent in the beneficial species, Apis mellifera. Furthermore, our data also suggest that some of the investigated AKH receptors, such as the mosquito AKHR, are more selective for the endogenous (conspecific) ligand, while others, such as the locust AKHR, are more promiscuous and can be activated by AKHs from many other insects. This information will be of high importance when further analyzing the potential use of AKHRs as targets for developing novel pest control agents.

Beneficial effect of adipokinetic hormone on neuromuscular paralysis in insect body elicited by braconid wasp venom

The effect of Habrobracon hebetor venom and the role of the adipokinetic hormone (AKH) in poisoned adult females of the firebug Pyrrhocoris apterus were studied 24 and 48h after treatments. Venom application elicited total neuromuscular paralysis in firebugs, but the co-application of venom and Pyrap-AKH significantly reduced paralysis (up to 3.2 times) compared to the application of venom only. Although the mechanisms of their action are unknown, both agents might affect neuromuscular junctions. Venom application significantly increased the expression of both P. apterus Akh genes (Pyrap-Akh 5.4 times and Peram-Cah-II 3.6 times), as well as the level of AKHs in the central nervous system (2.5 times) and haemolymph (3.0 times). In the haemolymph, increased AKH levels might have led to the mobilization of stored lipids, which increased 1.9 times, while the level of free carbohydrates remained unchanged. Total metabolism, monitored by carbon dioxide production, significantly declined in paralysed P. apterus individuals (1.4 times and 1.9 times, 24 and 48h after the treatment, respectively), probably because of a malfunction of the muscular system. The results suggest an active role of AKH in the defence mechanism against the stress elicited by neuromuscular paralysis, and the possible involvement of this hormone in neuronal/neuromuscular signalling.

Adipokinetic hormones control amylase activity in the cockroach (Periplaneta americana) gut

This study examined the biochemical characteristics of α-amylase and hormonal (adipokinetic hormone: AKH) stimulation of α-amylase activity in the cockroach (Periplaneta americana) midgut. We applied two AKHs in vivo and in vitro, then measured resultant amylase activity and gene expression, as well as the expression of AKH receptor (AKHR). The results revealed that optimal amylase activity is characterized by the following: pH: 5.7, temperature: 38.4 °C, K_m (Michaelis-Menten constant): 2.54 mg starch/mL, and V_max (maximum reaction velocity): 0.185 μmol maltose/mL/min. In vivo application of AKHs resulted in significant increase of amylase activity: by two-fold in the gastric caeca and 4-7 fold in the rest of the midgut. In vitro experiments supported results seen in vivo: a 24-h incubation with the hormones resulted in the increase of amylase activity by 1.4 times in the caeca and 4-9 times in the midgut. Further, gene expression analyses reveal that AKHR is expressed in both the caeca and the rest of the midgut, although expression levels in the former were 23 times higher than levels in the latter. A similar pattern was found for the amylase (AMY) gene. Hormonal treatment did not affect the expression of either gene. This study is the first to provide evidence indicating direct AKH stimulation of digestive enzyme activity in the insect midgut, supported by specific AKHR gene expression in this organ.

Adipokinetic hormone activities in insect body infected by entomopathogenic nematode

The role of adipokinetic hormone (AKH) in the firebug Pyrrhocoris apterus adults infected by the entomopathogenic nematode (EPN) Steinernema carpocapsae was examined in this study. It was found that co-application of EPN and AKH enhanced firebug mortality about 2.5 times within 24h (from 20 to 51% in EPN vs. EPN+AKH treatments), and resulted in metabolism intensification, as carbon dioxide production in firebugs increased about 2.1 and 1.6times compared to control- and EPN-treated insects, respectively. Accordingly, firebugs with reduced expression of AKH receptors showed a significantly lower mortality (by 1.6 to 2.9-folds), and lower general metabolism after EPN+AKH treatments. In addition, EPN application increased Akh gene expression in the corpora cardiaca (1.6times), AKH level in the corpora cardiaca (1.3times) and haemolymph (1.7times), and lipid and carbohydrate amounts in the haemolymph. Thus, the outcomes of the present study demonstrate involvement of AKH into the anti-stress reaction elicited by the nematobacterial infection. The exact mechanism by which AKH acts is unknown, but results suggested that the increase of metabolism and nutrient amounts in haemolymph might play a role.

Role of adipokinetic hormone and adenosine in the anti-stress response in Drosophila melanogaster

The role of adipokinetic hormone (AKH) and adenosine in the anti-stress response was studied in Drosophila melanogaster larvae and adults carrying a mutation in the Akh gene (Akh(1)), the adenosine receptor gene (AdoR(1)), or in both of these genes (Akh(1) AdoR(1) double mutant). Stress was induced by starvation or by the addition of an oxidative stressor paraquat (PQ) to food. Mortality tests revealed that the Akh(1) mutant was the most resistant to starvation, while the AdoR(1) mutant was the most sensitive. Conversely, the Akh(1) AdoR(1) double mutant was more sensitive to PQ toxicity than either of the single mutants. Administration of PQ significantly increased the Drome-AKH level in w(1118) and AdoR(1) larvae; however, this was not accompanied by a simultaneous increase in Akh gene expression. In contrast, PQ significantly increased the expression of the glutathione S-transferase D1 (GstD1) gene. The presence of both a functional adenosine receptor and AKH seem to be important for the proper control of GstD1 gene expression under oxidative stress, however, the latter appears to play more dominant role. On the other hand, differences in glutathione S-transferase (GST) activity among the strains, and between untreated and PQ-treated groups were minimal. In addition, the glutathione level was significantly lower in all untreated AKH- or AdoR-deficient mutant flies as compared with the untreated control w(1118) flies and further declined following treatment with PQ. All oxidative stress characteristics modified by mutations in Akh gene were restored or even improved by 'rescue' mutation in flies which ectopically express Akh. Thus, the results of the present study demonstrate the important roles of AKH and adenosine in the anti-stress response elicited by PQ in a D. melanogaster model, and provide the first evidence for the involvement of adenosine in the anti-oxidative stress response in insects.

Honey bee (Apis mellifera) drones survive oxidative stress due to increased tolerance instead of avoidance or repair of oxidative damage

Oxidative stress can lead to premature aging symptoms and cause acute mortality at higher doses in a range of organisms. Oxidative stress resistance and longevity are mechanistically and phenotypically linked; considerable variation in oxidative stress resistance exists among and within species and typically covaries with life expectancy. However, it is unclear whether stress-resistant, long-lived individuals avoid, repair, or tolerate molecular damage to survive longer than others. The honey bee (Apis mellifera L.) is an emerging model system that is well-suited to address this question. Furthermore, this species is the most economically important pollinator, whose health may be compromised by pesticide exposure, including oxidative stressors. Here, we develop a protocol for inducing oxidative stress in honey bee males (drones) via Paraquat injection. After injection, individuals from different colony sources were kept in common social conditions to monitor their survival compared to saline-injected controls. Oxidative stress was measured in susceptible and resistant individuals. Paraquat drastically reduced survival but individuals varied in their resistance to treatment within and among colony sources. Longer-lived individuals exhibited higher levels of lipid peroxidation than individuals dying early. In contrast, the level of protein carbonylation was not significantly different between the two groups. This first study of oxidative stress in male honey bees suggests that survival of an acute oxidative stressor is due to tolerance, not prevention or repair, of oxidative damage to lipids. It also demonstrates colony differences in oxidative stress resistance that might be useful for breeding stress-resistant honey bees.

Hormonal Regulation of Response to Oxidative Stress in Insects-An Update

Insects, like other organisms, must deal with a wide variety of potentially challenging environmental factors during the course of their life. An important example of such a challenge is the phenomenon of oxidative stress. This review summarizes the current knowledge on the role of adipokinetic hormones (AKH) as principal stress responsive hormones in insects involved in activation of anti-oxidative stress response pathways. Emphasis is placed on an analysis of oxidative stress experimentally induced by various stressors and monitored by suitable biomarkers, and on detailed characterization of AKH’s role in the anti-stress reactions. These reactions are characterized by a significant increase of AKH levels in the insect body, and by effective reversal of the markers—disturbed by the stressors—after co-application of the stressor with AKH. A plausible mechanism of AKH action in the anti-oxidative stress response is discussed as well: this probably involves simultaneous employment of both protein kinase C and cyclic adenosine 3′,5′-monophosphate pathways in the presence of extra and intra-cellular Ca²⁺ stores, with the possible involvement of the FoxO transcription factors. The role of other insect hormones in the anti-oxidative defense reactions is also discussed.

Energy Homeostasis Control in Drosophila Adipokinetic Hormone Mutants

Maintenance of biological functions under negative energy balance depends on mobilization of storage lipids and carbohydrates in animals. In mammals, glucagon and glucocorticoid signaling mobilizes energy reserves, whereas adipokinetic hormones (AKHs) play a homologous role in insects. Numerous studies based on AKH injections and correlative studies in a broad range of insect species established the view that AKH acts as master regulator of energy mobilization during development, reproduction, and stress. In contrast to AKH, the second peptide, which is processed from the Akh encoded prohormone [termed "adipokinetic hormone precursor-related peptide" (APRP)] is functionally orphan. APRP is discussed as ecdysiotropic hormone or as scaffold peptide during AKH prohormone processing. However, as in the case of AKH, final evidence for APRP functions requires genetic mutant analysis. Here we employed CRISPR/Cas9-mediated genome engineering to create AKH and AKH plus APRP-specific mutants in the model insect Drosophila melanogaster. Lack of APRP did not affect any of the tested steroid-dependent processes. Similarly, Drosophila AKH signaling is dispensable for ontogenesis, locomotion, oogenesis, and homeostasis of lipid or carbohydrate storage until up to the end of metamorphosis. During adulthood, however, AKH regulates body fat content and the hemolymph sugar level as well as nutritional and oxidative stress responses. Finally, we provide evidence for a negative autoregulatory loop in Akh gene regulation.

Knockdown of adipokinetic hormone synthesis increases susceptibility to oxidative stress in Drosophila--a role for dFoxO?

Insect adipokinetic hormones (AKHs) are pleiotropic hormones known to play a protective role in response to oxidative stress (OS). However, the precise signaling pathways are unclear. We present evidence that AKH may primarily employ the Forkhead box class O transcription factor (FoxO) to exert this effect. The impact of knocking down AKH synthesis or its over-expression in its response to OS was studied in Drosophila melanogaster. AKH knockdown (AKH-RNAi) as well as AKH overexpression (AKH-oex) was achieved using the Gal-4/UAS system and controls were w(1118) (+/+), AKH-Gal4/+, UAS-AKH/+ and UAS-AKH-RNAi/+. Exposure to 80 μM hydrogen peroxide (HP) revealed that AKH-RNAi flies showed significantly higher mortality than AKH-oex or the respective control lines. This susceptibility was evidenced by significantly enhanced levels of protein carbonyls - a biomarker of OS, in AKH-RNAi flies compared to controls and AKH-oex flies. Interestingly, AKH-oex flies had the least amount of protein carbonyls. AKH-RNAi flies had significantly less dFoxO transcript and translated protein compared to control and AKH-oex flies in un-challenged condition as well as when challenged with HP. Sestrin - a major antioxidant defense protein and one of the targets of dFoxO - was also significantly down-regulated (both at mRNA and protein level) in AKH-RNAi flies (both unchallenged and challenged with HP) compared to control flies and flies with over-expressed AKH. These findings imply that dFoxO may act downstream of AKH as a transcription factor to mediate response to OS in D. melanogaster.

Hormonal enhancement of insecticide efficacy in Tribolium castaneum: oxidative stress and metabolic aspects

Insect anti-stress responses, including those induced by insecticides, are controlled by adipokinetic hormones (AKHs). We examined the physiological consequences of Pyrap-AKH application on Tribolium castaneum adults (AKH-normal and AKH-deficient prepared by the RNAi technique) treated by two insecticides, pirimiphos-methyl and deltamethrin. Co-application of pirimiphos-methyl and/or deltamethrin with AKH significantly increased beetle mortality compared with application of the insecticides alone. This co-treatment was accompanied by substantial stimulation of general metabolism, as monitored by carbon dioxide production. Further, the insecticide treatment alone affected some basic markers of oxidative stress: it lowered total antioxidative capacity as well as the activity of superoxide dismutase in the beetle body; in addition, it enhanced the activity of catalase and glutathione-S-transferase. However, these discrepancies in oxidative stress markers were eliminated/reduced by co-application with Pyrap-AKH. We suggest that the elevation of metabolism, which is probably accompanied with faster turnover of toxins, might be responsible for the higher mortality that results after AKH and insecticide co-application. Changes in oxidative stress markers are probably not included in the mechanisms responsible for increased mortality.

Molecular characterization, tissue distribution, and ultrastructural localization of adipokinetic hormones in the CNS of the firebug Pyrrhocoris apterus (Heteroptera, Insecta)

Adipokinetic hormones (AKHs) are a group of insect metabolic neurohormones, synthesized and released from an endocrine retrocerebral gland, the corpus cardiacum (CC). Small amounts of AKH have also been identified in the brain, although their role in this organ is not clear. To address this gap in the knowledge about insect brain biology, we studied the nucleotide sequence, tissue distribution, and subcellular localization of AKHs in the brain and CC of the firebug Pyrrhocoris apterus. This insect expresses two AKHs; the octapeptides Pyrap-AKH and Peram-CAH-II, the presence of which was documented in the both studied organs. In situ hybridization and quantitative reverse-transcription (q-RT)-PCR revealed the expression of the genes encoding for both AKHs not only in the CC, but also in brain. Electron microscopy analysis of the brain revealed the presence of these hormones in specialized secretory granules localized predominantly in the cellular bodies of neurons. The hormones might be transported from the granules into the axons, where they could play a role in neuronal signaling. Under acute stress induced by the injection of 3μmol KCl, the level of AKHs in the brain increased to a greater extent than that in the CC. These results might indicate an enhanced role of brain-derived AKHs in defence reaction under acute stress situations.

Role of adipokinetic hormone in stimulation of salivary gland activities: the fire bug Pyrrhocoris apterus L. (Heteroptera) as a model species

The effect of adipokinetic hormone (Pyrap-AKH) in stimulating the function of insect salivary glands (SGs) in extra-oral digestive processes was studied in the firebug, Pyrrhocoris apterus L. (Heteroptera). The analyses were performed on samples of SGs and extracts of linden seeds, a natural source of the bug's food. The SGs from 3-day old P. apterus females (when the food ingestion culminates), primarily contained polygalacturonase (PG) enzyme activity, whereas the level of lipase, peptidase, amylase and α-glucosidase was negligible. The transcription of PG mRNA and enzymatic activity were significantly increased in SGs after Pyrap-AKH treatment. The piercing and sucking of linden seeds by the bugs stimulated the intrinsic enzymatic cocktail of seeds (lipase, peptidase, amylase, glucosidase), and moreover the activity of these enzymes was significantly enhanced when the seeds were fed on by the Pyrap-AKH treated bugs. Similarly, a significant increase in PG activity was recorded in linden seeds fed on by hormonally-treated bugs or when injected by SG extract from hormonally treated ones as compared to untreated controls. The mechanism of AKH action in SGs is unknown, but likely involves cAMP (and excludes cGMP) as a second messenger, since the content of this compound doubled in SGs after Pyrap-AKH treatment. This new and as yet undescribed function of AKH in SGs is compared with the effect of this hormone on digestive processes in the midgut elucidated earlier.

Adipokinetic hormone exerts its anti-oxidative effects using a conserved signal-transduction mechanism involving both PKC and cAMP by mobilizing extra- and intracellular Ca2+ stores

The involvement of members of the adipokinetic hormone (AKH) family in regulation of response to oxidative stress (OS) has been reported recently. However, despite these neuropeptides being the best studied family of insect hormones, their precise signaling pathways in their OS responsive role remain to be elucidated. In this study, we have used an in vitro assay to determine the importance of extra and intra-cellular Ca(2+) stores as well as the involvement of protein kinase C (PKC) and cyclic adenosine 3',5'-monophosphate (cAMP) pathways by which AKH exerts its anti-oxidative effects. Lipid peroxidation product (4-HNE) was significantly enhanced and membrane fluidity reduced in microsomal fractions of isolated brains (CNS) of Pyrrhocoris apterus when treated with hydrogen peroxide (H2O2), whereas these biomarkers of OS were reduced to control levels when H2O2 was co-treated with Pyrap-AKH. The effects of mitigation of OS in isolated CNS by AKH were negated when these treatments were conducted in the presence of Ca(2+) channel inhibitors (CdCl2 and thapsigargin). Presence of either bisindolylmaliemide or chelyrythrine chloride (inhibitors of PKC) in the incubating medium also compromised the anti-oxidative function of AKH. However, supplementing the medium with either phorbol myristate acetate (PMA, an activator of PKC) or forskolin (an activator of cAMP) restored the protective effects of exogenous AKH treatment by reducing 4-HNE levels and increasing membrane fluidity to control levels. Taken together, our results strongly implicate the importance of both PKC and cAMP pathways in AKHs' anti-oxidative action by mobilizing both extra and intra-cellular stores of Ca(2+).

Unique roles of glucagon and glucagon-like peptides: Parallels in understanding the functions of adipokinetic hormones in stress responses in insects

Glucagon is conventionally regarded as a hormone, counter regulatory in function to insulin and plays a critical anti-hypoglycemic role by maintaining glucose homeostasis in both animals and humans. Glucagon performs this function by increasing hepatic glucose output to the blood by stimulating glycogenolysis and gluconeogenesis in response to starvation. Additionally it plays a homeostatic role by decreasing glycogenesis and glycolysis in tandem to try and maintain optimal glucose levels. To perform this action, it also increases energy expenditure which is contrary to what one would expect and has actions which are unique and not entirely in agreement with its role in protection from hypoglycemia. Interestingly, glucagon-like peptides (GLP-1 and GLP-2) from the major fragment of proglucagon (in non-mammalian vertebrates, as well as in mammals) may also modulate response to stress in addition to their other physiological actions. These unique modes of action occur in response to psychological, metabolic and other stress situations and mirror the role of adipokinetic hormones (AKHs) in insects which perform a similar function. The findings on the anti-stress roles of glucagon and glucagon-like peptides in mammalian and non-mammalian vertebrates may throw light on the multiple stress responsive mechanisms which operate in a concerted manner under regulation by AKH in insects thus functioning as a stress responsive hormone while also maintaining organismal homeostasis.

Structure-activity studies of Drosophila adipokinetic hormone (AKH) by a cellular expression system of dipteran AKH receptors

Structure-activity studies for the adipokinetic hormone receptor of insects were for the first time performed in a cellular expression system. A series of single amino acid replacement analogues for the endogenous adipokinetic hormone of Drosophila melanogaster (pGlu-Leu-Thr-Phe-Ser-Pro-Asp-Trp-NH(2)) were screened for activity with a bioluminescence cellular assay, expressing the G-protein coupled receptor. For this series of peptide analogues, one amino acid of the N-terminal tetrapeptide was successively replaced by alanine, while those of the C-terminal tetrapeptide were successively substituted by glycine; other modifications included the blocked N- and C-termini that were replaced by an acetylated alanine and a hydroxyl group, respectively. The analogue series was tested on the AKH receptors of two dipteran species, D. melanogaster and Anopheles gambiae. The blocked termini of the AKH peptide probably play a minor role in receptor interaction and activation, but are considered functionally important elements to protect the peptide against exopeptidases. In contrast, the amino acids at positions 2, 3, 4 and 5 from the N-terminus all seem to be crucial for receptor activation. This can be explained by the potential presence of a β-strand in this part of the peptide that interacts with the receptor. The inferred β-strand is probably followed by a β-turn in which the amino acids at positions 5-8 are involved. In this β-turn, the residues at positions 6 and 8 seem to be essential, as their substitutions induce only a very low degree of receptor activation. Replacement of Asp(7), by contrast, does not influence receptor activation at all. This implies that its side chain is folded inside the β-turn so that no interaction with the receptor occurs.

Adipokinetic hormone-induced antioxidant response in Spodoptera littoralis

The antioxidative potential of the Manduca sexta adipokinetic hormone (Manse-AKH) in the last instar larvae of Spodoptera littoralis (Noctuidae, Lepidoptera) was demonstrated after exposure to oxidative stress (OS) elicited by feeding on artificial diet containing tannic acid (TA). Determination of protein carbonyls (PCs) and reduced glutathione (GSH) levels, monitoring of activity of antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione-S-transferases (GSTs), as well as measuring of the mRNA expression of CAT and SOD were used as markers of the OS. Injection of the Manse-AKH (5 pmol per individual) reversed the OS status by mitigation of PCs formation and by stimulation of glutathione-S-transferases (GSTs) activity. The CAT and SOD mRNA expression was significantly suppressed after the Manse-AKH injection while activity of these enzymes was not affected. These results indicate that diminishing of OS after the AKH injection might be a result of activation of specific enzymatic pathway possibly at the post-translational level rather than a direct effect on regulation of antioxidant marker genes at the transcriptional level.

More than two decades of research on insect neuropeptide GPCRs: an overview

This review focuses on the state of the art on neuropeptide receptors in insects. Most of these receptors are G protein-coupled receptors (GPCRs) and are involved in the regulation of virtually all physiological processes during an insect's life. More than 20 years ago a milestone in invertebrate endocrinology was achieved with the characterization of the first insect neuropeptide receptor, i.e., the Drosophila tachykinin-like receptor. However, it took until the release of the Drosophila genome in 2000 that research on neuropeptide receptors boosted. In the last decade a plethora of genomic information of other insect species also became available, leading to a better insight in the functions and evolution of the neuropeptide signaling systems and their intracellular pathways. It became clear that some of these systems are conserved among all insect species, indicating that they fulfill crucial roles in their physiological processes. Meanwhile, other signaling systems seem to be lost in several insect orders or species, suggesting that their actions were superfluous in those insects, or that other neuropeptides have taken over their functions. It is striking that the deorphanization of neuropeptide GPCRs gets much attention, but the subsequent unraveling of the intracellular pathways they elicit, or their physiological functions are often hardly examined. Especially in insects besides Drosophila this information is scarce if not absent. And although great progress made in characterizing neuropeptide signaling systems, even in Drosophila several predicted neuropeptide receptors remain orphan, awaiting for their endogenous ligand to be determined. The present review gives a précis of the insect neuropeptide receptor research of the last two decades. But it has to be emphasized that the work done so far is only the tip of the iceberg and our comprehensive understanding of these important signaling systems will still increase substantially in the coming years.

Oxidative stress elicited by insecticides: a role for the adipokinetic hormone

Adipokinetic hormones (AKHs) are insect neuropeptides responding to stress situations including oxidative stress. Two insecticides - endosulfan and malathion - were used to elicit oxidative stress conditions in the firebug Pyrrhocoris apterus, and the physiological functions of AKHs and their ability to activate protective antioxidative reactions were studied. The insecticide treatments elicited only a slight increase of the AKH level in CNS, but more intensive increase in haemolymph, which indicates an immediate involvement of AKH in the stress response. The treatment also resulted in a significant increase of catalase activity in the bug's body and depletion of the reduced glutathione pool in the haemolymph, however, co-application of the insecticides with the AKH (80 pmol) reduced the effect. It has also been found that co-application of the insecticides with AKH increased significantly the bug mortality compared to that induced by the insecticides alone. This enhanced effect of the insecticides probably resulted from the stimulatory role of AKH on bug metabolism: the carbon dioxide production was increased significantly after the co-treatment by AKH with insecticides compared to insecticide treatment alone. It was hypothesized that the increased metabolic rate could intensify the insecticide action by an accelerated rate of exchange of metabolites accompanied by faster penetration of insecticides into tissues.

Functional Characterization of Hypertrehalosemic Hormone Receptor in Relation to Hemolymph Trehalose and to Oxidative Stress in the Cockroach Blattella germanica

Hypertrehalosemic hormone (HTH) is a peptide hormone that belongs to the adipokinetic hormone/red pigment concentrating hormone (AKH/RPCH) family, which exerts pleiotropic actions related to catabolic reaction and stress response. AKH peptides have been demonstrated to participate in stress response including oxidative stress in several insects. In order to study the signaling pathway of HTH involved in anti-oxidative stress, we have characterized a HIH receptor cDNA in Blattella germanica (Blage-HTHR) in structural and in functional terms using RNA interference (RNAi). Blage-HTHR is expressed in various female adult tissues (brain-CC-CA, ventral nerve cord, midgut, fat body, oviduct), but maximal expression is observed in the fat body. RNAi-mediated knockdown of Blage-HTHR expression results in a significantly lower level of hemolymph trehalose, even though HTH is exogenously administered. Paraquat elicits lethal oxidative stress in B. germanica, and co-injection of paraquat and HTH reduces this detrimental effect and extends the median survival time. Interestingly, the "rescue" effect of HTH on mortality caused by paraquat is diminished in specimens with depleted expression of Blage-HTH and Blage-HTHR. Finally, lipid peroxidation in the hemolymph increases 4 h after paraquat treatment, in comparison with control specimens or with HTH-treated specimens. However, lipid peroxidation induced by paraquat was not "rescued" by HTH in Blage-HTH and Blage-HTHR knockdown specimens. Our results demonstrate that HTH acts as a stress hormone mediating anti-oxidative protection in B. germanica, and that its receptor, Blage-HTHR, is essential for this action.

Adipokinetic hormone (Pyrap-AKH) enhances the effect of a pyrethroid insecticide against the firebug Pyrrhocoris apterus

BACKGROUND

Adipokinetic hormones (AKHs) are insect neuropetides controlling stress situations including those elicited by insecticide treatment. The effect of Pyrap-AKH on the mortality of the firebug Pyrrhocoris apterus (L.) treated with the insecticide permethrin (Ambush 25 EC) was studied.

RESULTS

Coinjection of 50 ng permethrin with 80 pmol Pyrap-AKH induced a significant 2.3-fold increase in bug mortality compared with the insecticide alone. The results were confirmed by topical coapplication of both agents (400 ng and 80 pmol respectively). Injections of 50 and 100 ng permethrin elicited a significant increase in the AKH level in CNS and the haemolymph. The results indicate an involvement of AKH in stress response to permethrin. The enhanced effect of insecticide by AKH treatments probably results from the stimulatory role in bug metabolism: carbon dioxide production was increased 3.5- and 2.5-fold respectively 1 and 3 h after permethrin treatment, and 4.3- and 3.4-fold after the permethrin plus AKH cotreatment, compared with the control.

CONCLUSION

The elevation of metabolism could intensify the permethrin action by its faster penetration into tissues and by stimulation of biochemically active cells, and could be a reason for enhanced action of permethrin after its cotreatment with Pyrap-AKH.

Adipokinetic hormone-induced enhancement of antioxidant capacity of Pyrrhocoris apterus hemolymph in response to oxidative stress

The in vivo effects of oxidative stress on adipokinetic hormone (AKH) titer in short-winged (brachypterous) males of the firebug Pyrrhocoris apterus were tested using paraquat (PQ), a bipyridilium herbicide. PQ undergoes a cyclic redox reaction with oxygen during microsomal and electron transfer reactions forming free radicals in the insect body. Oxidative insult (40 pmol PQ) resulted in enhanced protein carbonylation (a biomarker for oxidative stress) and a depletion of glutathione (GSH) pool in the hemolymph. Interestingly, AKH titer was significantly enhanced in hemolymph at 4 h post inoculation of PQ, while its content in CNS (brain with corpora cardiaca) showed non-specific changes in comparable period. Co-injection of AKH with PQ (40 pmol each) reversed these effects by decreasing protein carbonyl formation, increasing reduced GSH levels, and enhancing the total antioxidant capacity of cell free plasma. Our results indicate that there is a positive feedback regulation between an oxidative stressor action and the level of AKH in insect body, and that AKHs might be involved in the activation of antioxidant protection mechanism.