Activation of triacylglycerol lipase in the fat body of a beetle by adipokinetic hormone

Comparative analysis of adipokinetic hormones and their receptors in Blattodea reveals novel patterns of gene evolution

Adipokinetic hormone (AKH) is a neuropeptide produced in the insect corpora cardiaca that plays an essential role in mobilising carbohydrates and lipids from the fat body to the haemolymph. AKH acts by binding to a rhodopsin-like G protein-coupled receptor (GPCR), the adipokinetic hormone receptor (AKHR). In this study, we tackle AKH ligand and receptor gene evolution as well as the evolutionary origins of AKH gene paralogues from the order Blattodea (termites and cockroaches). Phylogenetic analyses of AKH precursor sequences point to an ancient AKH gene duplication event in the common ancestor of Blaberoidea, yielding a new group of putative decapeptides. In total, 16 different AKH peptides from 90 species were obtained. Two octapeptides and seven putatively novel decapeptides are predicted for the first time. AKH receptor sequences from 18 species, spanning solitary cockroaches and subsocial wood roaches as well as lower and higher termites, were subsequently acquired using classical molecular methods and in silico approaches employing transcriptomic data. Aligned AKHR open reading frames revealed 7 highly conserved transmembrane regions, a typical arrangement for GPCRs. Phylogenetic analyses based on AKHR sequences support accepted relationships among termite, subsocial (Cryptocercus spp.) and solitary cockroach lineages to a large extent, while putative post-translational modification sites do not greatly differ between solitary and subsocial roaches and social termites. Our study provides important information not only for AKH and AKHR functional research but also for further analyses interested in their development as potential candidates for biorational pest control agents against invasive termites and cockroaches.

Biological Characteristics and Energy Metabolism of Migrating Insects

Through long-distance migration, insects not only find suitable breeding locations and increase the survival space and opportunities for the population but also facilitate large-scale material, energy, and information flow between regions, which is important in maintaining the stability of agricultural ecosystems and wider natural ecosystems. In this study, we summarize the changes in biological characteristics such as morphology, ovarian development, reproduction, and flight capability during the seasonal migration of the insect. In consideration of global research work, the interaction between flight and reproduction, the influence and regulation of the insulin-like and juvenile hormone on the flight and reproductive activities of migrating insects, and the types of energy substances, metabolic processes, and hormone regulation processes during insect flight are elaborated. This systematic review of the latest advances in the studies on insect migration biology and energy metabolism will help readers to better understand the biological behavior and regulation mechanism of the energy metabolism of insect migration.

Molecular identification and lipid mobilization role of adipokinetic hormone receptor in Spodoptera litura (F.)

Energy homeostasis is essential for organisms to maintain fluctuation in energy accumulation, mobilization. Lipids as the main energy reserve in insects, their metabolism is under the control of many physiological program. This study aimed to determine whether the adipokinetic hormone receptor (AKHR) was involved in the lipid mobilization in the Spodoptera litura. A full-length cDNA encoding AKHR was isolated from S. litura. The SlAKHR protein has a conserved seven-transmembrane domain which is the character of a putative G protein receptor. Expression profile investigation revealed that SlAKHR mRNA was highly expressed in immatural stage and abundant in fat body in newly emerged female adults. Knockdown of SlAKHR expression was achieved through RNAi by injecting double-stranded RNA (dsRNA) into the 6th instar larvae. The content of triacylgycerol (TAG) in the fat body increased significantly after the SlAKHR gene was knockdown. And decrease of TAG releasing to hemolymph with increase of free fatty acid (FFA) in hemolymph were observed when the SlAKHR gene was knowned-down. In addition, lipid droplets increased in fat body was also found. These results suggested that SlAKHR is critical for insects to regulate lipids metabolism.

The Adipokinetic Peptides of Hemiptera: Structure, Function, and Evolutionary Trends

The Hemiptera comprise the most species-rich order of the hemimetabolous insects. Members of a number of superfamilies, most notably especially the more basal ones such as white flies, psyllids and aphids, belong to the most destructive agricultural insects known worldwide. At the other end of the phylogenetic tree are hemipterans that are notorious medical pests (e.g. kissing bugs). Most of the hemipteran species are good flyers, and lipid oxidation plays a pivotal role to power the contraction of flight muscles and, in aquatic water bugs, also deliver the ATP for the extensive swimming action of the leg muscles. Mobilization of stored lipids (mostly triacylglycerols in the fat body) to circulating diacylglycerols in the hemolymph is regulated by a set of small neuropeptides, the adipokinetic hormones (AKHs). We searched the literature and publicly available databases of transcriptomes and genomes to present here AKH sequences from 191 hemipteran species. Only few of these peptides were sequenced via Edman degradation or mass spectrometry, and even fewer were characterized with molecular biology methods; thus, the majority of the AKHs we have identified by bioinformatics are merely predicted sequences at this stage. Nonetheless, a total of 42 AKH primary sequences are assigned to Hemiptera. About 50% of these structures occur also in other insect orders, while the remaining 50% are currently unique for Hemiptera. We find 9 novel AKHs not shown to be synthesized before in any insect. Most of the hemipteran AKHs are octapeptides (28) but there is an impressive number of decapeptides (12) compared to other speciose orders such as Diptera and Lepidoptera. We attempt to construct a hypothetical molecular peptide evolution of hemipteran AKHs and find quite a bit of overlapping with current phylogenetic ideas of the Hemiptera. Lastly, we discuss the possibility to use the sequence of the aphid AKH as lead peptide for the research into a peptide mimetic fulfilling criteria of a green insecticide.

Krüppel homolog 1 regulates photoperiodic reproductive plasticity in the cabbage beetle Colaphellus bowringi

Many insects exhibit reproductive plasticity where the photoperiod determines whether the insect becomes reproductively active or enters diapause. Adult reproductive diapause is a strategy that allows insects to survive harsh environmental conditions. A deficiency in juvenile hormone (JH) leads to reproductive diapause. However, little is known about the molecular mechanisms by which JH signaling regulates reproductive diapause. In this study, we used the cabbage beetle Colaphellus bowringi, a serious pest, to investigate the role of Krüppel homolog 1 (Kr-h1) in controlling photoperiodic plasticity of female reproduction. We focused on Kr-h1, since it acts as a key mediator of JH signaling. We show here that JH-Methoprene-tolerant signaling upregulated the expression of Kr-h1 in reproductively active C. bowringi females when reared under short day conditions. In the long day-treated diapausing females, Kr-h1 transcripts decreased dramatically. Interfering with Kr-h1 function repressed reproductive development by blocking vitellogenesis and ovarian growth. Further, Kr-h1 depletion induced other diapause-like traits, including elevated lipid accumulation and high expression of diapause-related genes. RNA-Seq showed that Kr-h1 played both activating and repressive roles, depending on whether downstream genes were acting in reproduction- or diapause pathways, respectively. Finally, we identified the DNA replication gene mini-chromosome maintenance 4 and two triacylglycerol lipase genes as critical downstream factors of Kr-h1 that are critical for reproductive plasticity in C. bowringi. These results reveal that Kr-h1 is a key component of the regulatory pathway that coordinates reproduction and diapause in insects in response to photoperiodic input.

The Role of Peptide Hormones in Insect Lipid Metabolism

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

Adipokinetic Hormone Receptor Mediates Lipid Mobilization to Regulate Starvation Resistance in the Brown Planthopper, Nilaparvata lugens

Lipid storage must be efficiently mobilized to sustain the energy demands during processes of exercise or starvation. In insects, adipokinetic hormone (AKH) and brummer lipase are well-known regulators of lipid mobilization. We recently demonstrated that brummer-dependent lipolysis regulates starvation resistance in the brown planthopper, Nilaparvata lugens, one of the most destructive rice pests. The present work investigated the roles of the AKH signaling system in lipid mobilization during the starvation process in N. lugens. NlAKHR is a typical G protein-coupled receptor (GPCR) and possesses high structure and sequence similarity to other insect AKHRs. Spatial and developmental expression profiles suggested that NlAKH is released from the corpora cardiaca to activate NlAKHR mainly expressed in the fat body. Starvation significantly induced the expression of NlAKH and NlAKHR, indicating a potential role of the AKH signaling system in starvation resistance. To reveal the functions of the AKH signaling system, a double-stranded RNA (dsRNA)-mediated knockdown of NlAKHR and NlAKH peptide injection was performed. The results show NlAKHR silencing decreased the levels of 1,2-diacylglycerol (DAG) in the hemolymph and increased triacylglycerol (TAG) levels in the fat body, whereas NlAKH injection led to a critical accumulation of DAG in the hemolymph and a severe reduction of TAG content in the fat body. Knockdown of NlAKHR resulted in prolonged lifespan and high levels of whole-body TAG, indicating an inability to mobilize TAG reserves during starvation. Conversely, the NlAKH injection reduced the survival and accelerated TAG mobilization during starvation, which further confirms the role of NlAKH in lipolysis. Moreover, NlAKHR silencing caused obesity in N. lugens, whereas NlAKH injection depleted organismal TAG reserves in vivo and produced a slim phenotype. These results indicate that lipid mobilization is regulated by the AKH signaling system, which is essential for adjusting body lipid homeostasis and ensuring energy supplement during starvation in N. lugens.

Adipokinetic hormone signaling determines dietary fatty acid preference through maintenance of hemolymph fatty acid composition in the cricket Gryllus bimaculatus

Adipokinetic hormone (AKH), an analog of mammalian glucagon, functions in supplying the required energy by releasing lipids and carbohydrates from the fat body into the hemolymph. Our previous study showed that AKH receptor (AKHR) knockdown in the two-spotted cricket Gryllus bimaculatus decreased hemolymph lipid levels and increased its feeding frequency. To reveal underlying mechanisms by which AKH signaling modulates lipid homeostasis, we analyzed the fatty acid composition as the lipid structure in the crickets. AKH administration significantly increased the proportion of unsaturated fatty acids (USFAs) to total fatty acids with decrease of the saturated fatty acids (SFAs) in hemolymph, while these proportions were inversely changed in RNA interference-mediated AKHR-knockdowned (AKHR^RNAi) crickets. Interestingly, knockdown of hormone-sensitive lipase (Hsl) by RNAi (Hsl^RNAi) affected the proportion of USFAs and SFAs in a similar manner to that observed in AKHR^RNAi crickets. AKH administration in Hsl^RNAi crickets did not change hemolymph fatty acid composition, indicating that AKH signaling critically altered fatty acid composition in the hemolymph through Hsl. In addition, a choice assay revealed that AKHR^RNAi significantly increases the preference of USFAs. These data indicate that hemolymph lipid level and composition were modulated by AKH signaling with a complementary feeding behavior toward USFAs.

Lipase Activity in the Larval Midgut of Rhynchophorus palmarum: Biochemical Characterization and the Effects of Reducing Agents

Lipases have key roles in insect lipid acquisition, storage, and mobilization and are also fundamental to many physiological processes in insects. Lipids are an important component of insect diets, where they are hydrolyzed in the midgut lumen, absorbed, and used for the synthesis of complex lipids. The South American palm weevil Rhynchophorus palmarum is one of the most important pests on commercial palm plantations. However, there are few studies about lipid digestion for this insect. In this work, we have described the biochemical characterization of the lipase activity in the posterior midgut of the R. palmarum palm weevil. Lipase activity was highest between the temperatures of 37 °C and 45 °C and at pH 6.5. Lipase activity was also sensitive to variations in salt and calcium concentrations. Lipases have been described structurally as enzymes with the Ser-His-Asp Catalytic Triad, containing an active serine. The serine protease inhibitor PMSF (phenylmethane sulfonyl fluoride) inhibited the lipases from R. palmarum, demonstrating the importance of a serine residue for this activity. The ability of the lipases to hydrolyze p-Nitrophenyl esters with different chain lengths has revealed the activities of a broad range of substrates. The lipase activities of R. palmarum increased in the presence of reduced glutathione (GSH) and dithiothreitol (DTT), while in the presence of oxidized glutathione (GSSG), activities were drastically reduced. To our knowledge, this study has provided the first information about lipase activity in the R. palmarum palm weevil.

Adipokinetic hormone receptor gene identification and its role in triacylglycerol metabolism in the blood-sucking insect Rhodnius prolixus

Adipokinetic hormone (AKH) has been associated with the control of energy metabolism in a large number of arthropod species due to its role on the stimulation of lipid, carbohydrate and amino acid mobilization/release. In the insect Rhodnius prolixus, a vector of Chagas' disease, triacylglycerol (TAG) stores must be mobilized to sustain the metabolic requirements during moments of exercise or starvation. Besides the recent identification of the R. prolixus AKH peptide, other components required for the AKH signaling cascade and its mode of action remain uncharacterized in this insect. In the present study, we identified and investigated the expression profile of the gene encoding the AKH receptor of R. prolixus (RhoprAkhr). This gene is highly conserved in comparison to other sequences already described and its transcript is abundant in the fat body and the flight muscle of the kissing bug. Moreover, RhoprAkhr expression is induced in the fat body at moments of increased TAG mobilization; the knockdown of this gene resulted in TAG accumulation both in fat body and flight muscle after starvation. The inhibition of Rhopr-AKHR transcription as well as the treatment of insects with the peptide Rhopr-AKH in its synthetic form altered the transcript levels of two genes involved in lipid metabolism, the acyl-CoA-binding protein-1 (RhoprAcbp1) and the mitochondrial glycerol-3-phosphate acyltransferase-1 (RhoprGpat1). These results indicate that the AKH receptor is regulated at transcriptional level and is required for TAG mobilization under starvation. In addition to the classical view of AKH as a direct regulator of enzymatic activity, we propose here that AKH signaling may account for the regulation of nutrient metabolism by affecting the expression profile of target genes.

Gαq, Gγ1 and Plc21C control Drosophila body fat storage

Adaptive mobilization of body fat is essential for energy homeostasis in animals. In insects, the adipokinetic hormone (Akh) systemically controls body fat mobilization. Biochemical evidence supports that Akh signals via a G protein-coupled receptor (GPCR) called Akh receptor (AkhR) using cyclic-AMP (cAMP) and Ca(2+) second messengers to induce storage lipid release from fat body cells. Recently, we provided genetic evidence that the intracellular calcium (iCa(2+)) level in fat storage cells controls adiposity in the fruit fly Drosophila melanogaster. However, little is known about the genes, which mediate Akh signalling downstream of the AkhR to regulate changes in iCa(2+). Here, we used thermogenetics to provide in vivo evidence that the GPCR signal transducers G protein α q subunit (Gαq), G protein γ1 (Gγ1) and Phospholipase C at 21C (Plc21C) control cellular and organismal fat storage in Drosophila. Transgenic modulation of Gαq, Gγ1 and Plc21C affected the iCa(2+) of fat body cells and the expression profile of the lipid metabolism effector genes midway and brummer, which results in severely obese or lean flies. Moreover, functional impairment of Gαq, Gγ1 and Plc21C antagonised Akh-induced fat depletion. This study characterizes Gαq, Gγ1 and Plc21C as anti-obesity genes and supports the model that Akh employs the Gαq/Gγ1/Plc21C module of iCa(2+) control to regulate lipid mobilization in adult Drosophila.

TGL-mediated lipolysis in Manduca sexta fat body: possible roles for lipoamide-dehydrogenase (LipDH) and high-density lipophorin (HDLp)

Triglyceride-lipase (TGL) is a major fat body lipase in Manduca sexta. The knowledge of how TGL activity is regulated is very limited. A WWE domain, presumably involved in protein-protein interactions, has been previously identified in the N-terminal region of TGL. In this study, we searched for proteins partners that interact with the N-terminal region of TGL. Thirteen proteins were identified by mass spectrometry, and the interaction with four of these proteins was confirmed by immunoblot. The oxidoreductase lipoamide-dehydrogenase (LipDH) and the apolipoprotein components of the lipid transporter, HDLp, were among these proteins. LipDH is the common component of the mitochondrial α-keto acid dehydrogenase complexes whereas HDLp occurs in the hemolymph. However, subcellular fractionation demonstrated that these two proteins are relatively abundant in the soluble fraction of fat body adipocytes. The cofactor lipoate found in typical LipDH substrates was not detected in TGL. However, TGL proved to have critical thiol groups. Additional studies with inhibitors are consistent with the notion that LipDH acting as a diaphorase could preserve the activity of TGL by controlling the redox state of thiol groups. On the other hand, when TG hydrolase activity of TGL was assayed in the presence of HDLp, the production of diacylglycerol (DG) increased. TGL-HDLp interaction could drive the intracellular transport of DG. TGL may be directly involved in the lipoprotein assembly and loading with DG, a process that occurs in the fat body and is essential for insects to mobilize fatty acids. Overall the study suggests that TGL occurs as a multi-protein complex supported by interactions through the WWE domain.

Knockdown of the adipokinetic hormone receptor increases feeding frequency in the two-spotted cricket Gryllus bimaculatus

Adipokinetic hormone (AKH) is a peptide hormone that regulates the nutritional state in insects by supporting the mobilization of lipids. In the present study, we manipulated AKH signaling to evaluate how metabolic state regulates feeding in an orthopteran insect, the two-spotted cricket, Gryllus bimaculatus. This was accomplished by RNA interference (RNAi) targeting the receptor gene for AKH [G. bimaculatus AKHR (GrybiAKHR)]. We found that the knockdown of GrybiAKHR by AKHR-double-stranded RNA treatment decreased the levels of 1,2-diacylglycerol and trehalose in the hemolymph, whereas it increased the level of triacylglycerol in the fat body. In addition, the knockdown of GrybiAKHR enhanced starvation resistance and increased food intake. Furthermore, direct observation of GrybiAKHR(RNAi) crickets revealed that the knockdown of GrybiAKHR increased feeding frequency but did not alter meal duration, whereas locomotor activity decreased. The increased frequency of feeding by GrybiAKHR(RNAi) crickets eventually resulted in an increase of food intake. These data demonstrate that the regulation of the metabolic state by AKH signaling affects feeding frequency, probably through nutritional control.

Triacylglycerol catabolism in the prawn Macrobrachium borellii (Crustacea: Palaemoniade)

While invertebrates store neutral lipids as their major energy source, little is known about triacylglycerol (TAG) class composition and their differential catabolism in aquatic arthropods. This study focuses on the composition of the main energy source and its catabolism by lipase from the midgut gland (hepatopancreas) of the crustacean Macrobrachium borellii. Silver-ion thin-layer chromatography of prawn large TAG deposit (80% of total lipids) and its subsequent fatty acid analysis by gas chromatography allowed the identification of 4 major fractions. These are composed of fatty acids of decreasing unsaturation and carbon chain length, the predominant being 18:1n-9. Fraction I, the most unsaturated one, contained mainly 20:5n-3; fraction II 18:2n-6; fraction III 18:1n-9 while the most saturated fraction contained mostly 16:0. Hepatopancreas main lipase (Mr 72 kDa) cross-reacted with polyclonal antibodies against insect lipase, was not dependent on the presence of Ca(2+) and had an optimum activity at 40°C and pH 8.0. Kinetic analysis showed a Michaelis-Menten behavior. A substrate competition assay evidenced lipase specificity following the order: 18:1n-9-TAG>PUFA-enriched-TAG>16:0-TAG different from that in vertebrates. These data indicate there is a reasonable correspondence between the fatty acid composition of TAG and the substrate specificity of lipase, which may be an important factor in determining which fatty acids are mobilized during lipolysis for oxidation in crustaceans.

Insect fat body: energy, metabolism, and regulation

The fat body plays major roles in the life of insects. It is a dynamic tissue involved in multiple metabolic functions. One of these functions is to store and release energy in response to the energy demands of the insect. Insects store energy reserves in the form of glycogen and triglycerides in the adipocytes, the main fat body cell. Insect adipocytes can store a great amount of lipid reserves as cytoplasmic lipid droplets. Lipid metabolism is essential for growth and reproduction and provides energy needed during extended nonfeeding periods. This review focuses on energy storage and release and summarizes current understanding of the mechanisms underlying these processes in insects.

Hormonal regulation of energy metabolism in insects as a driving force for performance

Since all life processes depend on energy, the endocrine control of energy metabolism is one of the driving forces for the performance of an individual. Here, we review the literature on the key players in the endocrine regulation of energy homeostasis in insects, the adipokinetic hormones. These pleiotropic peptides not only control dynamic performance traits (flight, swimming, walking) but also regulatory performance traits (egg production, larval growth, and molting). Adipokinetic hormone is released into the hemolymph during intense muscular activity (flight) and also during apparently less energy-demanding locomotory activities, such as swimming and even walking, and, finally, activates the catabolic enzymes phosphorylase and/or triacylglycerol lipase that mobilize carbohydrates and/or lipids and proline, respectively. At the same time, anabolic processes such as the synthesis of protein, lipid, and glycogen are inhibited. Furthermore, adipokinetic hormones affect locomotory activity via neuromodulatory mechanisms that apparently employ biogenic amines. During oogenesis, it is thought that adipokinetic hormone performs similar tasks, because energetic substrates have to be mobilized and transported from the fat body to the ovaries in order to support oocyte growth. Inhibition of anabolic processes by exogenous adipokinetic hormone results in females that lay fewer and smaller eggs. Much less is known about the role of adipokinetic hormones during larval development and during molting but in this case energy homeostasis has to be tightly regulated as well: in general, during the early phase of a larval instar intake of food prevails and the energy stores of the fat body are established, whereas, prior to the molt, insects stop feeding and mobilize energy stores in the fat body, thereby fueling energy-demanding processes such as the formation of the new cuticle and the emergence from the old one. From the few data available to date, it is clear that adipokinetic hormones are involved in the regulation of these events in larvae.

Peptides of the adipokinetic hormone/red pigment-concentrating hormone family with special emphasis on Caelifera: primary sequences and functional considerations contrasting grasshoppers and locusts

The presented work is a hybrid of an overview and an original research paper. First, we review briefly the structure, biosynthesis, release, mode of action and function of those peptides that constitute the adipokinetic/red pigment-concentrating family. Second, we collate the data on primary sequences available for caeliferan orthoptera, i.e. grasshoppers and locusts, and add a number of new data from previously unpublished work. The data are interpreted in conjunction with morphological and molecular biology data with respect to phylogenetic relationships of these various taxa. Finally, we discuss the differences between the adipokinetic response of grasshoppers and locusts to corpus cardiacum extract or synthetic adipokinetic hormone with regard to flight ability, phase polymorphism, age, presence of adipokinetic hormones, lipophorin system and other parameters. It appears that the higher hyperlipaemic response is always correlated with pronounced flight ability.

A chitin deacetylase and putative insect intestinal lipases are components of the Mamestra configurata (Lepidoptera: Noctuidae) peritrophic matrix

One- and two-dimensional gel electrophoresis coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to identify cDNA encoding a chitin deacetylase (McCDA1) and three insect intestinal lipases (McIIL1, McIIL2 and McIIL3) associated with the Mamestra configurata (bertha armyworm) peritrophic matrix. Recombinant McCDA1 was active and chitin deacetylase activities were detected in the midgut. McCDA1 and the McIIL genes were expressed exclusively in the midgut; however, McCDA1 and McIIL2 were expressed in all larval stages, whereas McIIL1 was expressed mainly in feeding larvae and McIIL3 primarily during the moult.

Lipid metabolism in Rhodnius prolixus (Hemiptera: Reduviidae): role of a midgut triacylglycerol-lipase

The utilization of dietary lipids was studied in adult females of Rhodnius prolixus with the use of radiolabeled triacylglycerol (TAG). It was shown that (3)H-triolein, when added to blood meal, was hydrolyzed to free fatty acids in the posterior midgut lumen. Subsequently, free fatty acids were absorbed by posterior midgut epithelium and used in the synthesis of phospholipids, diacylglycerol (DAG) and TAG. Phospholipids, DAG and free fatty acids were then found in hemolymph, from where they were rapidly cleared, and label was found in the fat body, mainly associated with TAG. Radioactive lipids, especially TAG and phospholipids, also accumulated in the ovaries. The TAG-lipase activities of posterior midgut luminal content and tissue were characterized by incubation of these samples with (3)H-triolein in the presence of the detergent Triton X-100 and determination of the amounts of released radioactive free fatty acids. Under the conditions employed here, the release of free fatty acids was proportional to the incubation time and to the amount of sample obtained from insect midgut (enzyme source) that was added. TAG-lipase activities were affected by pH and posterior midgut tissue showed optimum activity around pH 7.0-7.5, but the luminal content had the highest activities as pH decreased. Differences in activities were observed according to calcium concentration in the medium. TAG-lipase activities were also affected by the concentration of NaCl and were activated in the presence of increasing salt concentrations. These activities were inhibited by phenylmethylsulphonyl fluoride (PMSF). On the second day after blood meal, when digestion is very intense, TAG-lipase activities were maximal and then gradually decreased.

Flight fuel and neuropeptidergic control of fuel mobilisation in the twig wilter, Holopterna alata (Hemiptera, Coreidae)

The corpus cardiacum of the twig wilter Holopterna alata contains a factor that elicits increases in the concentration of lipids in the haemolymph of twig wilters and migratory locusts and causes hypertrehalosaemia in American cockroaches. A hyperlipaemic neuropeptide was isolated from corpora cardiaca of H. alata in a single high-performance liquid chromatography step. The primary sequence of this peptide was assigned by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, biological assay and co-elution with the synthetic peptide. The adipokinetic peptide of H. alata is an octapeptide with the sequence pGlu-Leu-Asn-Phe-Ser-Thr-Gly-Trp amide denoted Schgr-AKH-II which was sequenced previously from the corpora cardiaca of a number of Caelifera, Ensifera and some Hymenoptera. A dose of 1pmol of synthetic Schgr-AKH-II causes a pronounced hyperlipaemic effect in the twig wilter. Physiological experiments with the twig wilter reveal that during flight periods of 3 min, the normally low carbohydrate concentration in the haemolymph is significantly diminished, whereas the lipid concentration stays constant in most cases. During a subsequent rest period of 60 min after a 3 min flight episode, however, the concentration of lipids in the haemolymph increases substantially and significantly, indicating that lipids, too, are a major fuel during flight of twig wilters. This is corroborated by the activation of the enzyme triacylglycerol (TAG) lipase in the fat body, but not in the flight muscles, by injection of 5 pmol of synthetic Schgr-AKH-II, the endogenous adipokinetic hormone that is thought to be released during flight. Moreover, in the thorax there is a significant decrease in the concentration of glycogen and lipids measured after flight plus 60 min of rest compared to non-flown twig wilters, whereas no significant changes were monitored for these substrates stored in the abdomen. When the change in lipid class composition was analysed during flight plus 60 min of rest, TAG which comprised the major class in all compartments analysed (thorax, abdomen, haemolymph) was significantly reduced in abdomen and thorax, and diacylglycerol was significantly increased in all three compartments. From all the data collected, it is concluded that lipids are the major fuel class for flight in H. alata and that the contribution of carbohydrates is minimal.

Unique translational modification of an invertebrate neuropeptide: a phosphorylated member of the adipokinetic hormone peptide family

Separation of an extract of corpora cardiaca from the protea beetle, Trichostetha fascicularis, by single-step RP (reverse-phase)-HPLC and monitoring of tryptophan fluorescence resulted in two distinctive peaks, the material of which mobilized proline and carbohydrates in a bioassay performed using the beetle. Material from one of these peaks was; however, inactive in the classical bioassays of locusts and cockroaches that are used for detecting peptides belonging to the AKH (adipokinetic hormone) family. After enzymatically deblocking the N-terminal pyroglutamic acid (pGlu) residue in the peptide material and sequencing by Edman degradation, a partial sequence was obtained: (pGlu)-Ile-Asn-Met-Thr-Xaa-Gly-Trp. The complete sequence was deduced from ESI-MS(n) (electrospray ionization multi-stage-MS); position six was identified as a phosphothreonine residue and the C-terminus is amidated. The peptide, code-named Trifa-CC, was chemically synthesized and used in confirmatory experiments to show that the primary structure had been correctly assigned. To our knowledge, this is the first report of a phosphorylated invertebrate neuropeptide. Synthetic Trifa-CC co-elutes with the natural peptide, found in the gland of the protea beetle, after RP-HPLC. Moreover, the natural peptide can be dephosphorylated by alkaline phosphatase and the product of that reaction has the same retention time as a synthetic nonphosphorylated octapeptide which has the same sequence as Trifa-CC. Finally, synthetic Trifa-CC has hypertrehalosaemic and hyperprolinaemic biological activity in the protea beetle, but even high concentrations of synthetic Trifa-CC are inactive in locusts and cockroaches. Hence, the correct peptide structure has been assigned. Trifa-CC of the protea beetle is an unusual member of the AKH family that is unique in its post-translational modification. Since it increases the concentration of carbohydrates and proline in the haemolymph when injected into the protea beetle, and since these substrates are also used during flight, we hypothesize that Trifa-CC controls the mobilization of these metabolites in the protea beetle.