Dynamics of insect lipophorin metabolism

Foam fractionation of a recombinant biosurfactant apolipoprotein

Locusta migratoria apolipophorin III (apoLp-III) possesses the ability to exist as a water soluble amphipathic α-helix bundle and a lipid surface seeking apolipoprotein. The intrinsic ability of apoLp-III to transform phospholipid vesicles into reconstituted discoidal high-density lipoproteins (rHDL) has led to myriad applications. To improve the yield of recombinant apoLp-III, studies were performed in a bioreactor. Induction of apoLp-III expression generated a protein product that is secreted from E. coli into the culture medium. Interaction of apoLp-III with gas and liquid components in media produced large quantities of thick foam. A continuous foam fractionation process yielded a foamate containing apoLp-III as the sole major protein component. The yield of recombinant apoLp-III was ~0.2 g / liter bacterial culture. Mass spectrometry analysis verified the identity of the target protein and indicated no modifications or changes to apoLp-III occurred as a result of foam fractionation. The functional ability of apoLp-III to induce rHDL formation was evaluated by incubating foam fractionated apoLp-III with phosphatidylcholine vesicles. FPLC size exclusion chromatography revealed a single major population of particles in the size range of rHDL. The results described offer a novel approach to bioreactor-based apoLp-III production that takes advantage of its intrinsic biosurfactant properties.

Transcriptomic analysis reveals the inhibition of reproduction in rice brown planthopper, Nilaparvata lugens, after silencing the gene of MagR (IscA1)

MagR (IscA1) is a member of the iron-sulphur cluster assembly proteins, which plays vital roles in many physiological processes, such as energy metabolism, electron transfer, iron homeostasis, heme biosynthesis and physiologically magnetic response. Its deletion leads to the loss of mitochondrial DNA, inactivation of iron-sulphur proteins and abnormal embryonic development in organisms. However, the physiological roles of MagR in insects are unclear. This study characterized the effects and molecular regulatory mechanism of MagR gene silencing on the reproduction of brachypterous female adults of Nilaparvata lugens. After silencing the MagR gene using RNAi approach, the duration of reproductive period was shortened and the fecundity and hatchability reduced significantly. A total of 479 differentially expressed genes (DEGs) were identified for female adults after 2 days of dsRNA injection through RNA-sequencing technology, including 352 significantly upregulated DEGs and 127 significantly downregulated DEGs, among which 44 DEGs were considered the key genes involved in the effects of NlMagR silencing on the reproduction, revealing the regulatory mechanism of MagR at RNA transcription level and providing a new strategy for the control of N. lugens.

A journey into the world of insect lipid metabolism

Lipid metabolism is fundamental to life. In insects, it is critical, during reproduction, 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. Fat body contains various different cell types; however, adipocytes and oenocytes are the primary cells related to lipid metabolism. Lipid metabolism starts with the hydrolysis of dietary lipids, absorption of lipid monomers, followed by lipid transport from midgut to the fat body, lipogenesis or lipolysis in the fat body, and lipid transport from fat body to other sites demanding energy. Lipid metabolism is under the control of hormones, transcription factors, secondary messengers and posttranscriptional modifications. Primarily, lipogenesis is under the control of insulin-like peptides that activate lipogenic transcription factors, such as sterol regulatory element-binding proteins, whereas lipolysis is coordinated by the adipokinetic hormone that activates lipolytic transcription factors, such as forkhead box class O and cAMP-response element-binding protein. Calcium is the primary-secondary messenger affecting lipid metabolism and has different outcomes depending on the site of lipogenesis or lipolysis. Phosphorylation is central to lipid metabolism and multiple phosphorylases are involved in lipid accumulation or hydrolysis. Although most of the knowledge of insect lipid metabolism comes from the studies on the model Drosophila; other insects, in particular those with obligatory or facultative diapause, also have great potential to study lipid metabolism. The use of these models would significantly improve our knowledge of insect lipid metabolism.

Annelid Coelomic Fluid Proteins

The coelomic cavity is part of the main body plan of annelids. This fluid filled space takes up a considerable volume of the body and serves as an important site of exchange of both metabolites and proteins. In addition to low molecular substances such as amino acids and glucose and lactate, the coelomic fluid contains different proteins that can arise through release from adjacent tissues (intestine) or from secretion by coelomic cells. In this chapter, we will review the current knowledge about the proteins in the annelid coelomic fluid. Given the number of more than 20,000 extant annelid species, existing studies are confined to a relatively few species. Most studies on the oligochaetes are confined to the earthworms-clearly because of their important role in soil biology. In the polychaetes (which might represent a paraphyletic group) on the other hand, studies have focused on a few species of the Nereidid family. The proteins present in the coelomic fluid serve different functions and these have been studied in different taxonomic groups. In oligochaetes, proteins involved antibacterial defense such as lysenin and fetidin have received much attention in past and ongoing studies. In polychaetes, in contrast, proteins involved in vitellogenesis and reproduction, and the vitellogenic function of coelomic cells have been investigated in more detail. The metal binding metallothioneins as well as antimicrobial peptides, have been investigated in both oligochaetes and polychaetes. In the light of the literature available, this review will focus on lipoproteins, especially vitellogenin, and proteins involved in defense reactions. Other annelid groups such as the Pogonophora, Echiura, and Sipuncula (now considered polychaetes), have not received much attention and therefore, this overview is far from being complete.

The lipid composition of Legionella dumoffii membrane modulates the interaction with Galleria mellonella apolipophorin III

Apolipophorin III (apoLp-III), an insect homologue of human apolipoprotein E (apoE), is a widely used model protein in studies on protein-lipid interactions, and anti-Legionella activity of Galleria mellonella apoLp-III has been documented. Interestingly, exogenous choline-cultured Legionella dumoffii cells are considerably more susceptible to apoLp-III than non-supplemented bacteria. In order to explain these differences, we performed, for the first time, a detailed analysis of L. dumoffii lipids and a comparative lipidomic analysis of membranes of bacteria grown without and in the presence of exogenous choline. (31)P NMR analysis of L. dumoffii phospholipids (PLs) revealed a considerable increase in the phosphatidylcholine (PC) content in bacteria cultured on choline medium and a decrease in the phosphatidylethanolamine (PE) content in approximately the same range. The interactions of G. mellonella apoLp-III with lipid bilayer membranes prepared from PLs extracted from non- and choline-supplemented L. dumoffii cells were examined in detail by means of attenuated total reflection- and linear dichroism-Fourier transform infrared spectroscopy. Furthermore, the kinetics of apoLp-III binding to liposomes formed from L. dumoffii PLs was analysed by fluorescence correlation spectroscopy and fluorescence lifetime imaging microscopy using fluorescently labelled G. mellonella apoLp-III. Our results indicated enhanced binding of apoLp-III to and deeper penetration into lipid membranes formed from PLs extracted from the choline-supplemented bacteria, i.e. characterized by an increased PC/PE ratio. This could explain, at least in part, the higher susceptibility of choline-cultured L. dumoffii to G. mellonella apoLp-III.

Locust flight activity as a model for hormonal regulation of lipid mobilization and transport

Flight activity of insects provides a fascinating yet relatively simple model system for studying the regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a long-standing favored model insect, which as one of the most infamous agricultural pests additionally has considerable economical importance. Remarkably many aspects and processes pivotal to our understanding of (neuro)hormonal regulation of lipid mobilization and transport during insect flight activity have been discovered in the locust; among which are the peptide adipokinetic hormones (AKHs), synthesized and stored by the neurosecretory cells of the corpus cardiacum, that regulate and integrate lipid (diacylglycerol) mobilization and transport, the functioning of the reversible conversions of lipoproteins (lipophorins) in the hemolymph during flight activity, revealing novel concepts for the transport of lipids in the circulatory system, and the structure and functioning of the exchangeable apolipopotein, apolipophorin III, which exhibits a dual capacity to exist in both lipid-bound and lipid-free states that is essential to these lipophorin conversions. Besides, the lipophorin receptor (LpR) was identified and characterized in the locust. In an integrative approach, this short review aims at highlighting the locust as an unrivalled model for studying (neuro)hormonal regulation of lipid mobilization and transport during insect flight activity, that additionally has offered a broad and profound research model for integrative physiology and biochemistry, and particularly focuses on recent developments in the concept of AKH-induced changes in the lipophorin system during locust flight, that deviates fundamentally from the lipoprotein-based transport of lipids in the circulation of mammals. Current studies in this field employing the locust as a model continue to attribute to its role as a favored model organism, but also reveal some disadvantages compared to model insects with a completely sequenced genome.

The helix bundle: a reversible lipid binding motif

Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.

Conditional animal models for the study of lipid metabolism and lipid disorders

The advent of technologies that allow conditional mutagenesis has revolutionized our ability to explore gene functions and to establish animal models of human diseases. Both aspects have proven to be of particular importance in the study of lipid-related disorders. Classical approaches to gene inactivation by conventional gene targeting strategies have been successfully applied to generate animal models like the LDL receptor- and the apolipoprotein E-knockout mice, which are still widely used to study diverse aspects of atherosclerosis, lipid transport, and neurodegenerative disease. In many cases, however, simply inactivating the gene of interest has resulted in early lethal or complex phenotypes which are difficult to interpret. In recent years, additional tools have therefore been developed that allow the spatiotemporally controlled manipulation of the genome, as described in detail in Part I of this volume. Our aim is to provide an exemplary survey of the application of different conditional mutagenesis techniques in lipid research in order to illustrate their potential to unravel physiological functions of a broad range of genes involved in lipid homeostasis.

Embryonic expression of juvenile hormone binding protein and its relationship to the toxic effects of juvenile hormone in Manduca sexta

The juvenile hormones (JHs) regulate a diverse array of insect developmental and reproductive processes. One molecular target of JH action is its transporter, hemolymph JH binding protein (hJHBP); in the larva of the tobacco hornworm, Manduca sexta, low doses of JH can immediately increase hJHBP gene expression. Less explored are the effects of JH on embryological development, where early hormonal treatment has been shown to affect embryonic development and pupation. This study examines the egg form of JHBP and its gene expression during embryogenesis of M. sexta, as well as the phenotypic effect JH treatment has on embryos and on JHBP gene expression. We here demonstrate that the preponderance of JHBP found in the egg is maternally derived and that the embryonic gene and protein appear identical to those found in the larva. Expression of the JHBP gene begins in both the embryo itself and extra-embryonic tissues 15 h after fertilization, long before emergence of a functional fat body and circulatory system. Topical application of low JH doses to early embryos resulted in larval abnormalities while high doses of the hormone induced embryonic mortality. These effects are not mediated through regulation of the JHBP gene, since embryonic expression appears invariant in response to JH challenge. The toxicity of JH is tightly correlated with the concentration of unbound hormone.

Wax moth, Galleria mellonella, high density lipophorin receptor: alternative splicing, tissue-specific expression, and developmental regulation

A lipophorin (Lp) receptor cDNA from the fat body of Galleria mellonella (Lepidoptera) was cloned and sequenced. This is the first result in this order, Lepidoptera. It showed the pattern of the VLDL receptor belonging to the LDL receptor family. Sequence homology with other Lp receptors in insects, Locusta migratoria and Aedes aegypti, was 70 and 61%, respectively and each domain was highly conserved. Polyclonal anti-Lp receptor antibody prepared against expressed Lp receptor fragment between ligand binding domain and EGF-precursor homology domain (R305-D549 of amino acid residues) specifically detected the Lp receptor. Through immuno-blotting, the Lp receptor of larval fat body has an approximate molecular mass of about 97 and 110 kDa under non-reducing and reducing conditions, respectively. This result was in agreement with that of the ligand-blotting. The variant Lp receptors were expressed in the fat body of G. mellonella; one is an Lp receptor which lacks 84 bp of O-linked sugar domain and the other is a full length form of the Lp receptor. Both forms were detected by the polyclonal anti-Lp receptor antibody. The Lp receptor from the fat body of G. mellonella was differently expressed depending on the tissue and the developmental stage with specific abundance in prepupal stage. A steroid hormone, 20-hydroxyecdysone (20-HE) plays a crucial role in insect development. With regards to this conception, day 1-2 last instar larvae were treated with 20-HE and drastic induction of the Lp receptor was observed 48 h after treatment. It was also observed that cholesterol caused an induction of the Lp receptor.

Interfacial exclusion pressure determines the ability of apolipoprotein A-IV truncation mutants to activate cholesterol ester transfer protein

We used a panel of recombinant human apolipoprotein (apo) A-IV truncation mutants, in which pairs of 22-mer alpha-helices were sequentially deleted along the primary sequence, to examine the impact of protein structure and interfacial activity on the ability of apoA-IV to activate cholesterol ester transfer protein. Circular dichroism and fluorescence spectroscopy revealed that the secondary structure, conformation, and molecular stability of recombinant human apoA-IV were identical to the native protein. However, deletion of any of the alpha-helical domains in apoA-IV disrupted its tertiary structure and impaired its molecular stability. Surprisingly, determination of the water/phospholipid interfacial exclusion pressure of the apoA-IV truncation mutants revealed that, for most, deletion of amphipathic alpha-helical domains increased their affinity for phospholipid monolayers. All of the truncation mutants activated the transfer of fluorescent-labeled cholesterol esters between high and low density lipoproteins at a rate higher than native apoA-IV. There was a strong positive correlation (r = 0.790, p = 0.002) between the rate constant for cholesterol ester transfer and interfacial exclusion pressure. We conclude that molecular interfacial exclusion pressure, rather than specific helical domains, determines the degree to which apoA-IV, and likely other apolipoproteins, facilitate cholesterol ester transfer protein-mediated lipid exchange.

Adipokinetic hormones of insect: release, signal transduction, and responses

Flight activity of insects provides an attractive yet relatively simple model system for regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a well-accepted model insect. Peptide adipokinetic hormones (AKHs) are synthesized and stored by neurosecretory cells of the corpus cardiacum, a neuroendocrine gland connected with the insect brain. The actions of these hormones on their fat body target cells trigger a number of coordinated signal transduction processes which culminate in the mobilization of both carbohydrate (trehalose) and lipid (diacylglycerol). These substrates fulfill differential roles in energy metabolism of the contracting flight muscles. The molecular mechanism of diacylglycerol transport in insect blood involving a reversible conversion of lipoproteins (lipophorins) has revealed a novel concept for lipid transport in the circulatory system. In an integrative approach, recent advances are reviewed on the consecutive topics of biosynthesis, storage, and release of insect AKHs, AKH signal transduction mechanisms and metabolic responses in fat body cells, and the dynamics of reversible lipophorin conversions in the insect blood.

FATMETABOLISM ININSECTS

The study of fat metabolism in insects has received considerable attention over the years. Although by no means complete, there is a growing body of information about dietary lipid requirements, and the absolute requirement for sterol is of particular note. In this review we (a) summarize the state of understanding of the dietary requirements for the major lipids and (b) describe in detail the insect lipid transport system. Insects digest and absorb lipids similarly to vertebrates, but with some important differences. The hallmark of fat metabolism in insects centers on the lipid transport system. The major lipid transported is diacylglycerol, and it is carried by a high-density lipoprotein called lipophorin. Lipophorin is a reusable shuttle that picks up lipid from the gut and delivers it to tissues for storage or utilization without using the endocytic processes common to vertebrate cells. The mechanisms by which this occurs are not completely understood and offer fruitful areas for future research.

Role of lipid transfer particle in delivery of diacylglycerol from midgut to lipophorin in larval Manduca sexta

The present work analyzed the function of lipid transfer particle (LTP) in the process of exporting diacylglycerol from larval Manduca sexta midgut cells to lipophorin. When midgut sacs, which had been prelabeled in vivo with [(3)H]oleic acid, were incubated in vitro with a lipophorin-containing medium, a significant amount of radiolabeled diacylglycerol was transferred to lipophorin. Negligible amounts of diacylglycerol were released into lipophorin-free medium. In contrast, lipid-labeled lipophorin did not transfer diacylglycerol to the midgut sacs. The transfer of diacylglycerol from the midgut sac to lipophorin was blocked by preincubation of midgut sacs with antibody against LTP. Diacylglycerol transfer was restored to control values by the addition of purified LTP to midgut sacs that had been treated with antibody against LTP. Under these conditions the amount of diacylglycerol transferred was a function of the LTP concentration. These are the first results showing that LTP is required to export diacylglycerol from the midgut to lipophorin.

Lipid storage and mobilization in insects: current status and future directions

In this paper we review the current status of research on fatty acid absorption and conversion to diacylglycerol in the midgut. We further discuss how diacylglycerol may leave the midgut and associate with lipophorin in hemolymph. We review the present understanding of the role of the lipid transfer particle and lipophorin receptors in lipid delivery between lipophorin and tissues. Finally, we discuss recent studies on the mobilization of diacylglycerol from the fat body in response to adipokinetic hormone. Several suggestions for exciting areas of future research are described.

Lipophorin as a yolk protein precursor in the mosquito, Aedes aegypti

We examined expression of the lipophorin (Lp) gene, lipophorin (Lp) synthesis and secretion in the mosquito fat body, as well as dynamic changes in levels of this lipoprotein in the hemolymph and ovaries, during the first vitellogenic cycle of females of the yellow fever mosquito, Aedes aegypti. Lipophorin was purified by potassium bromide (KBr) density gradient ultracentrifugation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Polyclonal antibodies were produced against individual Lp apoproteins, apolipoprotein-I (apoLp-I) and apolipoprotein-II (apoLp-II), with molecular weights of 240 and 75 kDa, respectively. We report here that in the mosquito A. aegypti, Lp was synthesized by the fat body, with a low level of the Lp gene expression and protein synthesis being maintained in pre- and postvitellogenic females. Following a blood meal, the Lp gene expression and protein synthesis were significantly upregulated. Our findings showed that the fat body levels of Lp mRNA and the rate of Lp secretion by this tissue reached their maximum at 18 h post-blood meal (PMB). 20-Hydroxyecdysone was responsible for an increase in the Lp gene expression and Lp protein synthesis in the mosquito fat body. Finally, the immunocytochemical localization of Lp showed that in vitellogenic female mosquitoes, this protein was accumulated by developing oocytes where it was deposited in yolk granules.

Mobilisation of triacylglycerol stores

Triacylglycerol (TAG) is an energy dense substance which is stored by several body tissues, principally adipose tissue and the liver. Utilisation of stored TAG as an energy source requires its mobilisation from these depots and transfer into the blood plasma. The means by which TAG is mobilised differs in adipose tissue and liver although the regulation of lipid metabolism in each of these organs is interdependent and synchronised in an integrated manner. This review deals principally with the mechanism of hepatic TAG mobilisation since this is a rapidly expanding area of research and may have important implications for the regulation of plasma very-low-density lipoprotein metabolism. TAG mobilisation plays an important role in fuel selection in non-hepatic tissues such as cardiac muscle and pancreatic islets and these aspects are also reviewed briefly. Finally, studies of certain rare inherited disorders of neutral lipid storage and mobilisation may provide useful information about the normal enzymology of TAG mobilisation in healthy tissues.

Lipid transport biochemistry and its role in energy production

Recent advances on the biochemistry of flight-related lipid mobilization, transport, and metabolism are reviewed. The synthesis and release of adipokinetic hormones and their function in activation of fat body triacylglycerol lipase to produce diacylglycerol is discussed. The dynamics of reversible lipoprotein conversions and the structural properties and role of the exchangeable apolipoprotein, apolipophorin III, in this process is presented. The nature and structure of hemolymph lipid transfer particle and the potential role of a recently discovered lipoprotein receptor of the low-density lipoprotein receptor family, in lipophorin metabolism and lipid transport is reviewed.

Acquisition of lipoproteins in the procyclic form of Trypanosoma brucei

The procyclic form of Trypanosoma brucei binds and internalizes bovine high density lipoprotein (HDL) particles in a saturable process; the binding and uptake of (125)I-labeled HDL are inhibited by excess unlabeled HDL. We calculated that each procyclic trypanosome exposes approximately 1.0 x 10(6) binding sites for bovine HDL, with an equilibrium dissociation constant (Kd) of approximately 1.26 x 10(-7) M. Uptake of HDL particles does not occur at 4 degrees C. At 28 degrees C, a significant amount of the internalized HDL particles were efficiently degraded through a process that is sensitive to the presence of 50 microM chloroquine. These results suggested that the uptake of HDL particles in procyclic T. brucei may occur via receptor mediated endocytosis, leading to proteolytic degradation of the particles in an acidic and endocytic compartment.

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.

Distribution of diacylglycerols among plasma lipoproteins in control subjects and in patients with non-insulin-dependent diabetes

BACKGROUND

Diacylglycerols (DAGs), which are well-known components of insect lipophorins, have been recently recognized as a major glyceride of human high-density lipoprotein (HDL). Moreover, DAGs are good substrates for hepatic lipase and for the phospholipid transfer protein (PLTP). The present work was undertaken to determine the lipoprotein concentrations of DAGs, in control subjects, in non-insulin-dependent diabetic (NIDD) patients and in patients with severe hypertriglyceridaemia.

MATERIALS AND METHODS

Lipoproteins were isolated from 11 control subjects, 17 diabetic patients and three hypertriglyceridaemic patients, using a combination of ultracentrifugation and precipitation. After lipid extraction, DAGs were separated by thin-layer chromatography and quantified by a glyceride assay.

RESULTS

DAGs were detectable in all lipoprotein fractions of the three groups of subjects. Total DAGs were correlated with total triglycerides (TGs) and even more strikingly with very low-density lipoprotein triglycerides. Although the majority of DAG was recovered in apo B-containing lipoproteins, the proportion of DAG with respect to TG was most elevated in HDL.

CONCLUSION

These findings indicate that DAGs are probably formed from TG during lipolysis and that they can be transported to HDL through the action of PLTP. This raises the question whether DAG might act as an inhibitor of phospholipid transfer by competition for binding to PLTP.

Biosynthesis of Aedes aegypti lipophorin and gene expression of its apolipoproteins

The biosynthesis of lipophorin of the yellow fever mosquito, Aedes aegypti, was investigated. Fat bodies were incubated in vitro with radiolabeled methionine and cysteine, and radiolabeled proteins secreted into the medium were analyzed by density gradient ultracentrifugation, SDS-PAGE and fluorography. Lipophorin was synthesized in the fat body and secreted into the medium. Its density was 1.114 g/ml, similar to that of lipophorin circulating in hemolymph. Three peptides of a tryptic digest of apolipophorin II were sequenced and degenerate oligonucleotide primers were designed based on the amino acid sequences. With these primers, a cDNA product of 1.2 kb was amplified by RT-PCR using as template RNA extracted from adult female mosquitoes 24 h after ingestion of a blood meal. This cDNA was cloned, sequenced and used as a probe for Northern blot analysis, which revealed that the apoproteins of lipophorin were coded for by a single mRNA of approximately 10 kb. The expression of the apolipophorins was induced by blood feeding. From the data presented we concluded that Aedes aegypti lipophorin is synthesized in the fat body and that the expression of its apolipophorins is induced by blood feeding.

Role of glycosylation in the lipid-binding activity of the exchangeable apolipoprotein, apolipophorin-III

Non-glycosylated recombinant Locusta migratoria apolipophorin-III, apoLp-III, was expressed in E. coli and its physical-chemical properties were compared to those of the glycosylated native apoLp-III. Fluorescence quantum yield and acrylamide quenching studies indicated a slightly higher accessibility of the Trp residues in the recombinant apoLp-III. Far-UV CD spectroscopy indicated that the recombinant apoLp-III has a lower alpha-helical content than the glycosylated apoLp-III. Both proteins spontaneously formed discoidal recombinant lipoprotein particles when incubated with dimyristoylphosphatidylcholine (DMPC). Interaction with lipid promotes an increase in alpha-helical content. CD and fluorescence studies indicate that both proteins adopt the same conformation in the lipid-bound state. However, the kinetics of association of the recombinant protein with DMPC is 5-fold faster than that of the native protein. The results suggest that glycosylation inhibits the lipid binding activity by preventing the exposure of hydrophobic domains and/or decreasing the conformational flexibility of the protein.

Interhelical contacts are required for the helix bundle fold of apolipophorin III and its ability to interact with lipoproteins

Apolipophorin-III (apoLp-III) from the insect, Manduca sexta, is a 166-residue exchangeable apolipoprotein that plays a critical role in the dynamics of plasma lipoprotein interconversions. Our previous work indicated that a 36-residue C-terminal peptide fragment, generated by cyanogen bromide digestion of apoLp-III, was unable to bind to lipid surfaces (Narayanaswami V, Kay CM, Oikawa K, Ryan RO, 1994, Biochemistry 33:13312-13320), and showed no secondary structure in aqueous solution. In this paper, we have performed structural studies of this peptide (E131-Q166) complexed with SDS detergent micelles, or in the presence of the helix-inducing solvent trifluoroethanol (TFE), by two-dimensional 1H NMR spectroscopy. The peptide adopts an alpha-helical structure in the presence of both SDS and 50% TFE. The lipid-bound structure of the peptide, generated from the NMR NOE data, showed an elongated, slightly curved alpha-helix. Despite its high alpha-helix forming propensity, the peptide requires alpha helix-promoting environment to adopt an alpha-helical structure. This indicates the importance of the surrounding chemical environment and implies that, in the absence of lipid, tertiary contacts in the folded protein play a role in maintaining its structural integrity. Furthermore, the data suggest that the amphipathic helix bundle organization serves as a prerequisite structural motif for the reversible lipoprotein-binding activity of M. sexta apoLp-III.

Lipophorin levels in the yellow fever mosquito, Aedes aegypti, and the effect of feeding

High density lipophorin (HDLp) is the major lipid transport vehicle in insect hemolymph. Using an indirect ELISA, levels of HDLp were measured in the yellow fever mosquito, Aedes aegypti. The level of lipophorin, when normalized to the total weight of the insect, was similar in the different developmental stages. Starvation (access to water only) of adult females did not affect the level of HDLp nor its density when compared to sugar-fed females. On the other hand, blood feeding (of normally sugar-fed females) resulted in a three-fold increase of the HDLp level at 40 h after feeding. This increase was accompanied by a slight but significant increase in the density of HDLp at 24 h after feeding. Ingestion of a lipid-free protein meal or a lipid-supplemented protein meal induced changes in HDLp level and density that were comparable to those induced by ingestion of a blood meal. Ingestion of a blood meal, following starvation (access to water only) from the moment of adult emergence, did not induce an increase in HDLp level. The results presented indicate that, in contrast to other insect species, A. aegypti responds to an increased need for lipid transport in the hemolymph by increasing the amount of HDLp. Arch. Insect Biochem.

Protection against atherogenesis in mice mediated by human apolipoprotein A-IV

Apolipoproteins are protein constituents of plasma lipid transport particles. Human apolipoprotein A-IV (apoA-IV) was expressed in the liver of C57BL/6 mice and mice deficient in apoE, both of which are prone to atherosclerosis, to investigate whether apoA-IV protects against this disease. In transgenic C57BL/6 mice on an atherogenic diet, the serum concentration of high density lipoprotein (HDL) cholesterol increased by 35 percent, whereas the concentration of endogenous apoA-I decreased by 29 percent, relative to those in transgenic mice on a normal diet. Expression of human apoA-IV in apoE-deficient mice on a normal diet resulted in an even more severe atherogenic lipoprotein profile, without affecting the concentration of HDL cholesterol, than that in nontransgenic apoE-deficient mice. However, transgenic mice of both backgrounds showed a substantial reduction in the size of atherosclerotic lesions. Thus, apoA-IV appears to protect against atherosclerosis by a mechanism that does not involve an increase in HDL cholesterol concentration.

The amino terminus of apolipoprotein B is necessary but not sufficient for microsomal triglyceride transfer protein responsiveness

Human apolipoprotein (apo) B mediates the formation of neutral lipid-containing lipoproteins in the liver and intestine. The association of apoB with lipid is thought to be promoted by the microsomal triglyceride transfer protein complex. We have reconstituted lipoprotein assembly in an insect cell line that normally does not support this process. Expression of human microsomal triglyceride transfer protein (MTP) and apolipoprotein B48 (apoB48) together enabled Sf-21 insect cells to secrete approximately 60-fold more lipoprotein-associated triacylglycerol than control cells. This dramatic effect demonstrates that effective partitioning of triacylglycerol into the secretory pathway requires an endoplasmic reticulum-associated neutral lipid transporter (provided by MTP) and an apolipoprotein to shuttle the lipid through the pathway. Expression of the human apoB48 gene in insect cells resulted in secretion of the protein product. Including both MTP subunits with apoB48 and oleic acid specifically increased apoB48 secretion 8-fold over individual subunits alone. To assess whether specific regions of apoB are necessary for MTP responsiveness, nine apoB segments were expressed. These included NH2-terminal segments as well as internal and COOH-terminal regions of apoB fused with a heterologous signal sequence. ApoB segments containing the NH2-terminal 17% of the protein were secreted and responded to MTP activity; however, a segment containing only the NH2-terminal 17% of the protein was not significantly responsive to MTP. Segments lacking the NH2 terminus were not MTP-responsive, and five of six of these proteins were trapped intracellularly but, in certain cases, could be rescued by fusion to apoB17. These results suggest that the NH2 terminus of apoB is necessary but not sufficient for MTP responsiveness.

Merck Frosst award lecture 1995. La conference Merck Frosst 1995. Structural studies of lipoproteins and their apolipoprotein components

Lipid transport processes via the circulatory system of animals are a vital function that utilizes highly specialized lipoprotein complexes. These complexes of protein and lipid impart solubility to otherwise insoluble lipids. The apoprotein components of lipoprotein complexes serve to stabilize the lipid components and modulate particle metabolism and function as ligands for receptor-mediated endocytosis of lipoproteins. We have used an insect (Manduca sexta) model system for studies of lipid transport. In this system, flight activity elicits a dramatic increase in the demand for glycerolipid fuel molecules by flight muscle tissue. These lipids are mobilized from a storage organ and transported through the hemolymph (blood) to the flight muscle by the lipoprotein, lipophorin. This system possesses the unique property that lipids are loaded onto pre-existing high density lipophorin through the action of a lipid transfer particle (LTP). LTP is a high molecular weight hemolymph component that facilitates net vectorial lipid transfer from fat body tissue to lipophorin. The increase in lipid content of the lipoprotein induces association of a low molecular weight amphipathic exchangeable apolipoprotein, apolipophorin III (apoLp-III). ApoLp-III is a 18 kDa protein that normally exists as a water-soluble monomeric hemolymph protein. The structural properties of apoLp-III have been investigated by X-ray crystallography. ApoLp-III from Locusta migratoria adopts a five helix bundle conformation wherein each of the amphipathic helices orients with its hydrophobic face directed toward the interior of the bundle. It has been hypothesized that lipid association requires a dramatic conformational change wherein the helix bundle opens about putative hinge domains located in the loops between helices. The data accumulated support the concept that apoLp-III is a member of the broad class of exchangeable apolipoproteins and structural information learned from this system is directly applicable to analogous proteins in higher organisms.

Manduca sexta lipid transfer particle: synthesis by fat body and occurrence in hemolymph

Lipid transfer particle (LTP) is present in hemolymph of the tobacco hornworm Manduca sexta. Biosynthesis of LTP, occurrence in hemolymph, and the role of LTP-apoproteins in the lipid transfer reaction were investigated using antibodies specific for LTP or for each of the apoproteins. In vitro protein synthesis followed by immunoprecipitation demonstrated that LTP is synthesized by the fat body and secreted into the medium. In contrast to apolipophorin III, an exchangeable apoprotein of lipophorin (the major lipid transport protein in hemolymph), apoLTP-III could not be detected free in hemolymph. LTP concentrations in the hemolymph were measured by a sandwich ELISA using a mouse monoclonal antibody against apoLTP-III as capturing antibody and rabbit polyclonal antibody against apoLTP-I as detecting antibody. LTP concentration increased during the late fifth instar larval stage, followed by a decrease in the wandering stage. Subsequently, LTP concentrations were strongly increased in hemolymph of adult moths. The role of the three apoproteins of LTP in the lipid transfer reaction was analyzed using apoprotein-specific antibodies. All three, apoLTP-I, -II, and -III, appeared to be important for lipid transfer activity, as shown by inhibition of lipid transfer by antibodies specific for each of the three apoproteins.

Isolation and characterization of apolipophorin-III from the giant water bug (Lethocerus medius)

Upon injection of synthetic adipokinetic hormone, lipophorin from Lethocerus medius decreased in density and became associated with apolipophorin-III (apoLp-III). ApoLp-III isolated from hemolymph of Lethocerus medius had a M(r) = 19,000 and an amino acid composition high in methionine, in comparison with other apoLp-IIIs. Its circular dichroism spectrum was consistent with a protein with secondary structure of predominantly alpha-helix. NH2-terminal sequence alignment with apoLp-III sequences from other species showed a conservation of the hydrophobic or hydrophilic properties of residues at each position rather than of specific amino acids. ApoLp-III from Lethocerus medius has the potential to form amphipathic alpha-helices, similar to those found in the three-dimensional structure of Locusta migratoria apoLp-III. A portion of the apoLp-III molecules that are not associated with lipophorin contained the blue chromophore, biliverdin.

Human apolipoprotein E mediates processive buoyant lipoprotein formation in insect larvae

The expression of human apolipoprotein E in tobacco hornworm larvae causes a dramatic change in the buoyant density of the insect's endogenous lipoproteins. Larvae without apoE have lipoproteins that are found exclusively in the high-density range. Baculovirus-mediated apoE expression results in the conversion of approximately one-fourth of the endogenous lipoproteins to low-density species. This density conversion is progressive and parallels a similar change in apoE density distribution. ApoE is secreted from the lipoprotein producing fat body tissue in a lipid-poor form, but readily associates with circulating insect lipoproteins in the hemolymph where the density conversion takes place. Analysis of the buoyant lipoprotein particles indicates that they contain apoE and insect apolipophorins I and II with few or no other proteins present. Immunoprecipitation of apolipophorins I and II results in coprecipitation of apoE. This association is disrupted by detergent, consistent with the three proteins sharing the same lipoprotein particles. The ability of apoE to influence buoyant lipoprotein formation in an invertebrate system leads us to suggest that small apolipoproteins such as apoE may play a role in buoyant lipoprotein production in mammals.

Antioxidant systems in insects

Insects possess a suite of antioxidant enzymes and small molecular weight antioxidants that may form a concatenated response to an onslaught of dietary and endogenously produced oxidants. Antioxidant enzymes such as superoxide dismutase, catalase, glutathione transferase, and glutathione reductase have been characterized in insects. Water-soluble and lipid-soluble antioxidants such as ascorbate, glutathione, tocopherols, and carotenoids have not been well studied in insects but may play very important antioxidant roles. Additionally, the peritrophic matrix and trehalose may possess important antioxidant functions in insects. The enzymatic recycling of ascorbate, first noted in green plants, may also exist in insects. A greater understanding of these antioxidant systems may provide greater understanding about the ecological relationships of insects with their hosts.

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

The hemolymph lipid levels were measured and the density of lipophorin was determined during late larval development in Manduca sexta. During the feeding phase of the 4th and 5th instar larvae the lipid level in hemolymph remained largely unchanged at less than 2 mg/ml. During the molt from 4th to 5th instar, the hemolymph lipid level increased, but decreased after feeding restarted in the 5th instar. In wandering larvae and prepupae the hemolymph lipid level increased from about 2 to nearly 10 mg/ml. The density of lipophorin from feeding larvae was found to be 1.148 g/ml with minor amounts of lipophorin having a lower density of about 1.128 g/ml and sometimes a small amount with a density of 1.174 g/ml. In molting larvae, however, the density was clearly lower, 1.116 g/ml. In wandering larvae of all ages, two predominant forms of lipophorin were observed; the density of these forms was 1.132 g/ml and 1.177 g/ml. Rarely, one or three different forms of lipophorin were observed. While the lipophorin of feeding larvae contains only apoLp-I and II (and lipids), the lipophorin of molting larvae contains in addition apoLp-III. ApoLp-III is seldom present in lipophorin from wandering larvae. According to our current models, lipophorin can take up only a certain amount of diacylglycerol before it needs apoLp-III for surface stabilization. Injection of 1 pmol of M. sexta AKH into feeding larvae increased the hemolymph lipid level, decreased the density of lipophorin to 1.125 g/ml and resulted in the association of apoLp-III with lipophorin. Cardiacectomy did not prevent feeding larvae from developing to wandering larvae.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and binding characteristics of the carboxyl terminal fragment of apolipophorin III from Manduca sexta

The molecular basis of the interaction of apolipophorin III (apoLp-III), an exchangeable apolipoprotein from hemolymph of the sphinx moth. Manduca sexta, with lipoprotein surfaces and phospholipids was studied by investigating the structural and binding properties of the C-terminal fragment of the native protein. A 4K peptide, corresponding to the terminal helical segment of the native protein, was generated by cyanogen bromide treatment, purified by gel filtration and reverse-phase HPLC, and characterized by N-terminal sequencing and amino acid and mass spectrometric analysis. Circular dichroism (CD) spectroscopy of the peptide in buffer indicated a predominantly unstructured state while addition of trifluoroethanol (TFE), a helix-inducing agent, resulted in an alpha-helical structure. Sedimentation equilibrium studies revealed that the 4K peptide was monomeric in buffer. The 4K peptide assumed an alpha-helical conformation in the presence of sodium dodecyl sulfate (SDS) and lysolecithin, but was unstructured in the presence of dimyristoylphosphatidylcholine, either when added to preformed vesicles or upon cosonication, indicating an ability to bind to detergent micelles but not to phospholipid bilayers. Unlike native apoLp-III, the 4K peptide did not confer protection against turbidity development to human low density lipoprotein upon incubation with phospholipase C, indicating an inability to interact with the surface of lipoproteins. Upon interaction with SDS micelles, both the 4K peptide and apoLp-III were resistant to urea-induced denaturation when compared to free apoLp-III, as evaluated by CD spectroscopy. The structural stability conferred upon interaction with detergents was similar for both the peptide and the native protein.(ABSTRACT TRUNCATED AT 250 WORDS)