Apolipophorin-III and adipokinetic hormone in lipid metabolism of larval Manduca sexta

Lipophorin: The Lipid Shuttle

Insects need to transport lipids through the aqueous medium of the hemolymph to the organs in demand, after they are absorbed by the intestine or mobilized from the lipid-producing organs. Lipophorin is a lipoprotein present in insect hemolymph, and is responsible for this function. A single gene encodes an apolipoprotein that is cleaved to generate apolipophorin I and II. These are the essential protein constituents of lipophorin. In some physiological conditions, a third apolipoprotein of different origin may be present. In most insects, lipophorin transports mainly diacylglycerol and hydrocarbons, in addition to phospholipids. The fat body synthesizes and secretes lipophorin into the hemolymph, and several signals, such as nutritional, endocrine, or external agents, can regulate this process. However, the main characteristic of lipophorin is the fact that it acts as a reusable shuttle, distributing lipids between organs without being endocytosed or degraded in this process. Lipophorin interacts with tissues through specific receptors of the LDL receptor superfamily, although more recent results have shown that other proteins may also be involved. In this chapter, we describe the lipophorin structure in terms of proteins and lipids, in addition to reviewing what is known about lipoprotein synthesis and regulation. In addition, we reviewed the results investigating lipophorin's function in the movement of lipids between organs and the function of lipophorin receptors in this process.

Clues on the function of Manduca sexta perilipin 2 inferred from developmental and nutrition-dependent changes in its expression

Cellular triglycerides (TG) are stored in cytosolic lipid droplets (LDs). Perilipins (PLIN) are a group of LD-proteins that play important roles in the assembly and transport of LDs and in TG metabolism. Two members of the PLIN family are found in insects (PLIN1 & 2 or Lsd1 & 2). We have cloned and expressed Manduca sexta PLIN2 (MsPLIN2), and studied developmental and nutritional changes in the expression of PLIN2. Nutritional changes induced fast alterations in PLIN2 mRNA and protein levels in fat body and midgut of the feeding larvae. The relationship observed between PLIN2 expression and TG synthesis in both larval fat body and midgut suggests that PLIN2 is needed when tissues are accumulating TG. However, when the fat body was storing TG at maximal capacity, MsPLIN2 levels declined. This unexpected finding suggests the occurrence of alternative mechanism/s to shield TG from the action of lipases in M. sexta LDs. In addition, it implies that the cellular level of lipid storage could be modulating MsPLIN2 expression and/or degradation. The study also confirmed that MsPLIN2 was most abundant in the adult fat body, which is characterized by a high rate of TG hydrolysis and lipid mobilization. Whether MsPLIN2 is directly involved in lipolysis and/or the secretion of lipids in the fat body of adult of M. sexta is unknown at this time. Nonetheless, the coexistence of high PLIN2 and lipolysis levels suggests a complex role for MsPLIN2. Altogether, we found that MsPLIN2 is needed when the synthesis of glycerides, DG and TG, is active even if the insect is accumulating or consuming TG.

Effect of starvation on lipophorin density in fifth instar larval Manduca sexta

Lipophorin (Lp) is a major insect lipoprotein and is responsible for lipid transport between organs. In this study, the effect of starvation on Lp properties was analyzed in larval Manduca sexta during the fifth instar. Lp hemolymph concentrations in larvae at days 1 and 2 were around 2-3 mg/ml and at day 3 it increased to 8 mg/ml. When larvae were starved for 24 h, they did not grow, but their body mass and hemolymph volume did not decrease significantly. Differences in Lp densities were observed. In fed larvae, from days 1 to 4, two major Lp populations were found with densities of 1.124 ± 0.002 (high density Lp-larval1 , HDLp-L1 ) and 1.141 ± 0.002 g/ml (HDLp-L2 ). When larvae were starved for 24 h, only one Lp population was present, with density 1.114 ± 0.001 g/ml (HDLp-Ls ). When larvae were abdominally ligated at day 1 or 2 of fifth instar, only HDLp-Ls was found after 24 h, indicating that the formation of this HDLp population was not dependent on any factor released by head. On the other hand, larvae that were ligated at day 3 showed the same Lp populations as the fed ones. In 24-h starved larvae, lipid load in Lp was higher as compared to the fed controls. In 24-h ligated larvae Lp lipid content increased when ligation was performed on day 1 or 2, but not on day 3. So, different responses to starvation can be observed depending on the developmental phase of the same larval instar.

Age-dependent changes of fat body stores and the regulation of fat body lipid synthesis and mobilisation by adipokinetic hormone in the last larval instar of the cricket, Gryllus bimaculatus

Data on the hormonal regulation of the formation and mobilisation of fat body stores are presented and discussed in relation to general parameters of last instar larval development such as growth, food intake, and moulting. Crickets feed voraciously during the first half of the last larval stage. With the onset of feeding, fat body lipid synthesis increases, leading to increasing lipid stores in the fat body with a maximum reached on day 5. Lipid (42% of fat body fresh mass) is the main constituent of the fat body stores, followed by protein (6%) and glycogen (2%). During the second half of the last larval stage, feeding activity dramatically decreases, the glycogen reserves are depleted but lipid and protein reserves in the fat body remain at a high level except for the last day of the last larval stage when lipid and protein in the fat body are also largely depleted. The process of moulting consumes almost three quarters of the caloric equivalents that were acquired during the last larval stage. Adipokinetic hormone (AKH) inhibits effectively the synthesis of lipids in the larval fat body. Furthermore, AKH stimulates lipid mobilisation by activating fat body triacylglycerol lipase (TGL) in last larval and adult crickets. Both effects of AKH are weaker in larvae than in adults. This is the first report on the age-dependent basal activity of TGL in larval and adult insects. In addition, for the first time, an activation of TGL by AKH in a larval insect is shown.

Illness-induced anorexia and its possible function in the caterpillar, Manduca sexta

Although many animals exhibit illness-induced anorexia when immune-challenged, the adaptive significance of this behavior remains unclear. Injecting Manduca sexta larvae (caterpillars) with live bacteria (Serratia marcescens), heat-killed bacteria or bacterial lipopolysaccharides resulted in a decline in feeding, demonstrating illness-induced anorexia in this species. We used M. sexta to test four commonly suggested adaptive functions for illness-induced anorexia. (1) Food deprivation did not reduce the iron content of the hemolymph. (2) Immune-challenged M. sexta were not more likely to move to a different part of the plant. Therefore, the decline in feeding is unlikely to be an adaptive response allowing the animal to move away from a patch of contaminated food. (3) M. sexta force-fed S. marcescens bacteria were not more susceptible to a S. marcescens systemic infection than were M. sexta force-fed nutrient broth. (4) Force-feeding infected M. sexta during illness-induced anorexia did not increase mortality and short-term food deprivation did not enhance survival. However, force-feeding M. sexta with a high lipid diet (linseed oil and water) resulted in an increase in mortality when challenged with S. marcescens. Force-feeding sucrose or water did not reduce resistance. Force-feeding a high lipid diet into healthy animals did not reduce weight gain, suggesting that it was not toxic. We hypothesize that there is a conflict between lipid metabolism and immune function, although whether this conflict has played a role in the evolution of illness-induced anorexia remains unknown. The adaptive function of illness-induced anorexia requires further study in both vertebrates and invertebrates.

The role of neuropeptides in caterpillar nutritional ecology

Plant diet strongly impacts the fitness of insect herbivores. Immediately, we think of plant defensive compounds that may act as feeding deterrents or toxins. We are, probably, less aware that plants also influence insect growth and fecundity through their nutritional quality. However, most herbivores respond to their environment and select the diet which optimizes their growth and development. This regulation of nutritional balance may occur on many levels: through selecting and ingesting appropriate plant tissue and nutrient digestion, absorption and utilization. Here, we review evidence of how nutritional requirements, particularly leaf protein to digestible carbohydrate ratios, affect caterpillar herbivores. We propose a model where midgut endocrine cells assess and integrate hemolymph nutritional status and gut content and release peptides which influence digestive processes. Understanding the effects of diet on the insect herbivore is essential for the rational design and implementation of sustainable pest management practices.

Localization of injected apolipophorin III in vivo - new insights into the immune activation process directed by this protein

A few years ago, it was shown that intrahemocoelic injection of the insect apolipoprotein apolipophorin III (apoLp-III) stimulates an immune response in larvae of the greater wax moth, Galleria mellonella. Since the mode of action of this activation process is unknown, we followed apoLp-III's pathway in the early phase of the immune-stimulating process, using biotin as a probe. Biotinylated apoLp-III was injected and localized using avidin-coupled horseradish peroxidase. The labeled protein was fully functional; the added amount of biotin per apoLp-III molecule used in this study only slightly decreased its ability to associate with phospholipase C-treated human low-density lipoprotein, as well as the immune-stimulating capability of apoLp-III.Gel electrophoresis with subsequent staining of biotin moieties and lipids revealed that apoLp-III undergoes lipid association in vivo within the first few minutes after injection. After two hours, no biotinylated apoLp-III was detectable in cell-free hemolymph. At this time, a subpopulation of hemocytes showed a distinct peroxidase staining. Control injections of biotinylated bovine serum albumin did not lead to similar results, giving evidence for the specificity of the phenomena observed. The results indicate that lipid association of apoLp-III occurs prior to endocytosis by immune-competent hemocytes, which is followed by the induction of a humoral immune response.

Carbohydrate metabolism during starvation in the silkworm Bombyx mori

The effect of starvation on carbohydrate metabolism in the last instar larvae of the silkworm Bombyx mori was examined. Trehalose concentration in the hemolymph increased slightly during the first 6 h of starvation and decreased thereafter, whereas glucose concentration decreased rapidly immediately after diet deprivation. Starvation-induced hypertrehalosemia was completely inhibited by neck ligation, suggesting that starvation stimulates the release of a hypertrehalosemic factor(s) from the head. The percentage of active glycogen phosphorylase in the fat body increased within 3 h of starvation and its glycogen content decreased gradually. These observations suggest that production of trehalose from glycogen is enhanced in starved larvae. However, hypertrehalosemia during starvation cannot be explained by the increased supply of trehalose into hemolymph alone, as similar changes in phosphorylase activity and glycogen content in the fat body were observed in neck-ligated larvae, in which hemolymph trehalose concentration did not increase but decreased gradually. When injected into larvae, trehalose disappeared from hemolymph at a rate about 40% lower in starved larvae than neck-ligated larvae. The hemolymph lipid concentration increased during starvation, suggesting that an increased supply of lipids to tissues suppresses the consumption of hemolymph trehalose and this is an important factor in hypertrehalosemia.

Characterization of lipophorin binding to the midgut of larval Manduca sexta

Lipophorin binding to the midgut of Manduca sexta larvae was characterized in a midgut membrane preparation, using iodinated larval high-density lipophorin ((125)I-HDLp-L). The iodination procedure did not change the affinity of the preparation for lipophorin. In the presence of increasing concentrations of membrane protein, corresponding increases in lipophorin binding were observed. The time-course of lipophorin binding to the membranes was affected by the lipophorin concentration in the medium, and at a low lipoprotein concentration, a longer time was required for equilibrium to be reached. The specific binding of lipophorin to the midgut membrane was a saturable process with a K(d) = 1.5+/-0.2x10(-7) M and a maximal binding capacity = 127+/-17 ng lipophorin/microg of membrane protein. Binding did not depend on calcium, was maximal around pH 5.5, was strongly inhibited by an increase in the ionic strength, and abolished by suramin. However, suramin did not completely displace lipophorin that was previously bound to the membrane preparation. The lipid content of the lipophorin did not significantly affect the affinity of the membrane preparation for lipoprotein.

Insect immune activation by recombinant Galleria mellonella apolipophorin III(1)

Apolipophorin III (apoLp-III) is an exchangeable insect apolipoprotein. Its function, as currently understood, lies in the stabilization of low-density lipophorin particles (LDLp) crossing the hemocoel in phases of high energy consumption to deliver lipids from the fat body to the flight muscle cells. Recent studies with native Galleria mellonella-apoLp-III gave first indications of an unexpected role of that protein in insect immune activation. Here we report the immune activation by the recombinant protein, documenting a newly discovered correlation between lipid physiology and immune defense in insects. The complete cDNA sequence of G. mellonella-apoLp-III was identified by mixed oligonucleotide-primed amplification of cDNA (MOPAC), 3'-RACE-PCR, and cRACE-PCR. The sequence coding for the native protein was ligated into a pET-vector; this construct was transfected into Escherichia coli and overexpressed in the bacteria. Photometric turbidity assays with human low density lipoprotein (LDL) and transmission electron microscopy studies on apoLp-III-stabilized lipid discs revealed the full functionality of the isolated recombinant apoLp-III with regard to its lipid-association ability. For proving its immune-stimulating capacity, apoLp-III was injected into the hemocoel of last instar G. mellonella larvae and the antibacterial activity in cell-free hemolymph was determined 24 h later. As a result, the hemolymph samples of injected insects contained strongly increased antibacterial activities against E. coli as well as clearly enhanced lysozyme-like activities. From Northern blot analysis of total RNA from insects injected with apoLp-III or the bacterial immune provocator lipopolysaccharide, it could be concluded that the transcription rate of apoLp-III mRNA does not vary in comparison to untreated last instar larvae.

A novel lipoprotein from the hemolymph of the cochineal insect, Dactylopius confusus

A new type of insect lipoprotein was isolated from the hemolymph of the female cochineal insect Dactylopius confusus. The lipoprotein from the cochineal insect hemolymph was found to have a relative molecular mass of 450 000. It contains 48% lipid, mostly diacylglycerol, phospholipids and hydrocarbons. The protein moiety of the lipoprotein consists of two apoproteins of approximately 25 and 22 kDa, both of which are glycosylated. Both apolipoproteins are also found free in the hemolymph, unassociated with any lipid. Purified cochineal apolipoproteins can combine with Manduca sexta lipophorin, if injected together with adipokinetic hormone into M. sexta. This could indicate that the cochineal lipoprotein can function as a lipid shuttle similar to lipophorins of other insects, and that the cochineal insect apolipoproteins have an overall structure similar to insect apolipophorin-III.

Analogs of Manduca adipokinetic hormone tested in a bioassay and in a receptor-binding assay

Single amino acid replacement analogs of Manduca adipokinetic hormone (M-AKH) pGlu-Leu-Thr-Phe-Thr-Ser-Ser-Trp-GlyNH2 were tested for activity in bioassays as well as receptor binding assays. Amino acids were replaced by Ala and by D-analogs. In addition an extended M-AKH and analogs containing photo affinity labels were tested. All analogs had reduced activity. All the peptides which had enough activity to allow a full dose response curve reached the same maximal activity as native M-AKH. The use of analogs, in which L-Phe4 was replaced by Ala or by D-Phe and of L-Thr3 replaced by D-Thr, as competitors led to improved binding of M-AKH in our competitive receptor binding assay. In the bioassay an inactive concentration of Ala4 M-AKH increased the activity of a half optimal concentration of native M-AKH.

Hemagglutinating properties of apolipophorin III from the hemolymph of Galleria mellonella larvae

In search for factors that cause encapsulation of foreign bodies in insect hemolymph we discovered that larval hemolymph of Galleria mellonella caused aggregation of mammalian erythrocytes. The hemagglutinating agent was identified as an 18-kDa protein that did not react with lectins. The sequence of 81 amino acids in three protein fragments and the properties of the protein revealed that it was Galleria homologue of apolipophorin III (apoLp-III). ApoLp-III was found in high amounts in the hemolymph of Galleria larvae, pupae, and adults, as well as in the molting fluid. The hemagglutinating action of the whole hemolymph or the purified apoLp-III was independent of the presence of sugars in the medium. This indicated that it was not mediated by carbohydrates on the erythrocyte surface. The hemagglutination was inhibited at low pH (3.0), in the absence of calcium ions, and in the presence of certain bacterial lipopolysaccharides or their essential component, the 2-keto-3-deoxyoctonate-3-deoxyoctulosonic acid (KDO). It is suggested that interaction of apoLp-III with lipopolysaccharides in bacterial cell walls may play a role in insect immune reactions.

Isolated Apolipophorin III from Galleria mellonella Stimulates the Immune Reactions of This Insect

Apolipophorin III (apoLp-III) was isolated from the haemolymph of last instar larvae of Galleria mellonella. The ultraviolet (u.v.) spectrum and the N-terminal amino acid sequence reveal high similarities with the apoLp-III from Manduca sexta. The protein is heat-stable. The molecular mass of apoLp-III was determined to be 18 077 Da using mass spectrometry. The heat treatment (90 degrees C, 30 min) resulted in a pI shift from 6.6 for the non-heated to 6.1 for the heat-treated apoLp-III without change in the molecular mass, indicating that a conformational change might have been caused by the heat treatment, rather than covalent alterations. Intrahaemocoelic injection of pure apoLp-III into last instar G. mellonella larvae is followed by a dose-dependent increase of antibacterial activity in cell-free haemolymph of treated larvae 24 h after injection. Furthermore, pure apoLp-III enhances the phagocytic activity of isolated haemocytes in vitro. The newly discovered role of apoLp-III in inducing immune-related functions in insects is discussed in regard to the known features of this molecule in lipid metabolism. Arylphorin, another heat-stable protein in G. mellonella haemolymph, was likewise isolated in this study. The protein was identified by N-terminal protein sequencing, the sequence obtained exactly matches the known sequence data for this protein. Copyright 1997 Elsevier Science Ltd. All rights reserved

Variations in the density of lipophorins during late larval and early pupal development of Manduca sexta

The density of lipophorin was determined in individual Manduca sexta during development from the second day of the fifth larval instar to the second day of the pupal stage. Lipophorin formed defined bands when subjected to density gradient ultracentrifugation. All lipophorin observed was high density lipophorin; however, the densities varied from 1.100 to 1.184 g/ml, and 40% of the animals had more than one density form of lipophorin. The lipophorins were divided into five density classes: class 1 from 1.100 to 1.113 g/ml, class 2 from 1.114 to 1.132 g/ml, class 3 from 1.133 to 1.145 g/ml, class 4 from 1.146 to 1.162 g/ml, and class 5 from 1.163 to 1.184 g/ml. In feeding larvae, classes 2 and 3 were the most abundant. Larvae of the first day of wandering had either lipophorin in class 2 or in classes 2 and 5. Later during wandering the variation increased, but on the third day most of the lipophorin was in class 2. In first day pupae, only lipophorins of classes 4 and 5 were detected, while on the second day of the pupal stage, classes 2 and 3 were predominant. Class 1 lipophorin was abundant in larvae injected with Manduca adipokinetic hormone (M-AKH), and rare in young feeding larvae. In no other stage was class 1 lipophorin observed. Our results show that the density of lipophorin is much more variable than previously reported which makes it difficult to ascribe any lipophorin density to a developmental stage. These results also show that adipokinetic hormone decreases the density of lipophorin in larvae.

Characterization of the adipokinetic hormone receptor form the fat body of Manduca sexta

A tritium labeled Manduca sexta adipokinetic hormone (M-AKH) was synthesized (pE-L-T-[p3H]F-T-S-S-W-G-NH2) (specific activity 27 Ci/mmol) which was fully active in a bioassay. It was used in a filtration based binding assay to characterize the M-AKH receptor from the fat body of M. sexta. Membrane fractions were prepared from fat body and optimal binding conditions were determined. A Kd of 7.10(-10) M was determined and the receptor concentration estimated to be 0.5 pmol/mg membrane protein. No receptor binding was found when membranes were prepared from brain, heart or flight muscle of M. sexta or from fat body of the cockroach Blaberus discoidalis. However, specific binding was found with membrane preparations from the pterothoracic ganglion of M. sexta. The membranes from the ganglion had a much smaller number of binding sites than the fat body membranes, however, the binding was specific and observed in each experiment.