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

The many roles of fats in overwintering insects

Temperate, polar and alpine insects generally do not feed over winter and hence must manage their energy stores to fuel their metabolism over winter and to meet the energetic demands of development and reproduction in the spring. In this Review, we give an overview of the accumulation, use and conservation of fat reserves in overwintering insects and discuss the ways insects modify fats to facilitate their selective consumption or conservation. Many insects are in diapause and have depressed metabolic rates over winter; together with low temperatures, this means that lipid stores are likely to be consumed predominantly in the autumn and spring, when temperatures are higher but insects remain dormant. Although there is ample evidence for a shift towards less-saturated lipids in overwintering insects, switches between the use of carbohydrate and lipid stores during winter have not been well-explored. Insects usually accumulate cryoprotectants over winter, and the resulting increase in haemolymph viscosity is likely to reduce lipid transport. For freeze-tolerant insects (which withstand internal ice), we speculate that impaired oxygen delivery limits lipid oxidation when frozen. Acetylated triacylglycerols remain liquid at low temperatures and interact with water molecules, providing intriguing possibilities for a role in cryoprotection. Similarly, antifreeze glycolipids may play an important role in structuring water and ice during overwintering. We also touch on the uncertain role of non-esterified fatty acids in insect overwintering. In conclusion, lipids are an important component of insect overwintering energetics, but there remain many uncertainties ripe for detailed exploration.

Dynamic remodeling of lipids coincides with dengue virus replication in the midgut of Aedes aegypti mosquitoes

We describe the first comprehensive analysis of the midgut metabolome of Aedes aegypti, the primary mosquito vector for arboviruses such as dengue, Zika, chikungunya and yellow fever viruses. Transmission of these viruses depends on their ability to infect, replicate and disseminate from several tissues in the mosquito vector. The metabolic environments within these tissues play crucial roles in these processes. Since these viruses are enveloped, viral replication, assembly and release occur on cellular membranes primed through the manipulation of host metabolism. Interference with this virus infection-induced metabolic environment is detrimental to viral replication in human and mosquito cell culture models. Here we present the first insight into the metabolic environment induced during arbovirus replication in Aedes aegypti. Using high-resolution mass spectrometry, we have analyzed the temporal metabolic perturbations that occur following dengue virus infection of the midgut tissue. This is the primary site of infection and replication, preceding systemic viral dissemination and transmission. We identified metabolites that exhibited a dynamic-profile across early-, mid- and late-infection time points. We observed a marked increase in the lipid content. An increase in glycerophospholipids, sphingolipids and fatty acyls was coincident with the kinetics of viral replication. Elevation of glycerolipid levels suggested a diversion of resources during infection from energy storage to synthetic pathways. Elevated levels of acyl-carnitines were observed, signaling disruptions in mitochondrial function and possible diversion of energy production. A central hub in the sphingolipid pathway that influenced dihydroceramide to ceramide ratios was identified as critical for the virus life cycle. This study also resulted in the first reconstruction of the sphingolipid pathway in Aedes aegypti. Given conservation in the replication mechanisms of several flaviviruses transmitted by this vector, our results highlight biochemical choke points that could be targeted to disrupt transmission of multiple pathogens by these mosquitoes.

Ultrastructure of fat body cells and Malpighian tubule cells in overwintering Scoliopteryx libatrix (Noctuoidea)

The herald moths, Scoliopteryx libatrix, overwinter in hypogean habitats. The ultrastructure of their fat body (FB) cells and Malpighian tubule (MT) epithelial cells was studied by light microscopy and transmission electron microscopy, and essential biometric and biochemical measurements were performed. The FB was composed of adipocytes and sparse urocytes. The ultrastructure of both cells did not change considerably during this natural starvation period, except for rough endoplasmic reticulum (rER) which became more abundant in March females. In the cells, the reserve material consisted of numerous lipid droplets, glycogen rosettes, and protein granula. During overwintering, the lipid droplets diminished, and protein granula became laminated. The MTs consisted of a monolayer epithelium and individual muscle cells. The epithelial cells were attached to the basal lamina by numerous hemidesmosomes. The apical plasma membrane was differentiated into numerous microvilli, many of them containing mitochondria. Nuclei were surrounded by an abundant rER. There were numerous spherites in the perinuclear part of the cells. The basal plasma membrane formed infoldings with mitochondria in between. Nuclei were located either in the basal or in the central part of the cells. During overwintering, spherites were gradually exploited, and autophagic structures appeared: autophagosomes, autolysosomes, and residual bodies. There were no statistical differences between the sexes in any measured biometric and biochemical variables in the same time frames. The energy-supplying lipids and glycogen, and spherite stores were gradually spent during overwintering. In March, the augmented rER signified the intensification of synthetic processes prior to the epigean ecophase.

Modeling dietary influences on offspring metabolic programming in Drosophila melanogaster

The influence of nutrition on offspring metabolism has become a hot topic in recent years owing to the growing prevalence of maternal and childhood obesity. Studies in mammals have identified several factors correlating with parental and early offspring dietary influences on progeny health; however, the molecular mechanisms that underlie these factors remain undiscovered. Mammalian metabolic tissues and pathways are heavily conserved in Drosophila melanogaster, making the fly an invaluable genetic model organism for studying metabolism. In this review, we discuss the metabolic similarities between mammals and Drosophila and present evidence supporting its use as an emerging model of metabolic programming.

De novo transcriptome analysis of the excretory tubules of Carausius morosus (Phasmatodea) and possible functions of the midgut 'appendices'

The Malpighian tubules are the insect excretory organs, responsible for ion and water homeostasis and elimination of nitrogenous wastes. Post-genomic assays suggest they also metabolize and detoxify xenobiotic compounds and have antimicrobial properties. The Phasmatodea have an additional, unique set of excretory organs referred to predominantly as midgut appendices. Their function and how it compares to phasmid and other insect Malpighian tubules is unknown. Hypotheses include carbonic anhydrase activity, calcium and metal cation sequestration, and xenobiotic transport. This work presents the first comparative transcriptomic analysis of the Phasmatodean excretory organs, using the model insect Carausius morosus. I produced de novo transcriptomes of the midgut appendices, midgut wall, and Malpighian tubules, and looked for differentially expressed genes associated with putative organ functions. The appendices differentially and highly express lipid transport and metabolism proteins, and the biomineralization gene otopetrin. The Malpighian tubules differentially and highly express acid phosphatases and multiple transporter types, while appendices express fat-soluble vitamin and peptide transporters. Many defense proteins such as multidrug resistance proteins, ABC transporters, cytochrome P450's, and glutathione-S-transferases were differentially expressed in specific excretory organs. I hypothesize that the appendices and Malpighian tubules both have defensive / xenobiotic metabolism functions, but each likely target different substrates. Phasmid Malpighian tubules excrete as in other insects, while the appendices may predominantly regulate amino acids, fats, and fat-soluble compounds. Lipid metabolism in insects is poorly understood, and the Phasmatodea may thus serve as a model for studying this further.

Metabolic costs of capital energy storage in a small-bodied ectotherm

Reproduction is energetically financed using strategies that fall along a continuum from animals that rely on stored energy acquired prior to reproduction (i.e., capital breeders) to those that rely on energy acquired during reproduction (i.e., income breeders). Energy storage incurs a metabolic cost. However, previous studies suggest that this cost may be minimal for small‐bodied ectotherms. Here I test this assumption. I use a laboratory feeding experiment with the European green crab Carcinus maenas to establish individuals with different amounts of energy storage. I then demonstrate that differences in energy storage account for 26% of the variation in basal metabolic costs. The magnitudes of these costs for any individual crab vary through time depending on the amount of energy it has stored, as well as on temperature‐dependent metabolism. I use previously established relationships between temperature‐ and mass‐dependent metabolic rates, combined with a feasible annual pattern of energy storage in the Gulf of Maine and annual sea surface temperature patterns in this region, to estimate potential annual metabolic costs expected for mature female green crabs. Results indicate that energy storage should incur an ~8% increase in metabolic costs for female crabs, relative to a hypothetical crab that did not store any energy. Translated into feeding, for a medium‐sized mature female (45 mm carapace width), this requires the consumption of an additional ~156 mussels annually to support the metabolic cost of energy storage. These results indicate, contrary to previous assumptions, that the cost of energy storage for small‐bodied ectotherms may represent a considerable portion of their basic operating energy budget. An inability to meet these additional costs of energy storage may help explain the recent decline of green crabs in the Gulf of Maine where reduced prey availability and increased consumer competition have combined to hamper green crab foraging success in recent years.

Cardiomyocyte Regulation of Systemic Lipid Metabolism by the Apolipoprotein B-Containing Lipoproteins in Drosophila

The heart has emerged as an important organ in the regulation of systemic lipid homeostasis; however, the underlying mechanism remains poorly understood. Here, we show that Drosophila cardiomyocytes regulate systemic lipid metabolism by producing apolipoprotein B-containing lipoproteins (apoB-lipoproteins), essential lipid carriers that are so far known to be generated only in the fat body. In a Drosophila genetic screen, we discovered that when haplo-insufficient, microsomal triglyceride transfer protein (mtp), required for the biosynthesis of apoB-lipoproteins, suppressed the development of diet-induced obesity. Tissue-specific inhibition of Mtp revealed that whereas knockdown of mtp only in the fat body decreases systemic triglyceride (TG) content on normal food diet (NFD) as expected, knockdown of mtp only in the cardiomyocytes also equally decreases systemic TG content on NFD, suggesting that the cardiomyocyte- and fat body-derived apoB-lipoproteins serve similarly important roles in regulating whole-body lipid metabolism. Unexpectedly, on high fat diet (HFD), knockdown of mtp in the cardiomyocytes, but not in fat body, protects against the gain in systemic TG levels. We further showed that inhibition of the Drosophila apoB homologue, apolipophorin or apoLpp, another gene essential for apoB-lipoprotein biosynthesis, affects systemic TG levels similarly to that of Mtp inhibition in the cardiomyocytes on NFD or HFD. Finally, we determined that HFD differentially alters Mtp and apoLpp expression in the cardiomyocytes versus the fat body, culminating in higher Mtp and apoLpp levels in the cardiomyocytes than in fat body and possibly underlying the predominant role of cardiomyocyte-derived apoB-lipoproteins in lipid metabolic regulation. Our findings reveal a novel and significant function of heart-mediated apoB-lipoproteins in controlling lipid homeostasis.

The heart is increasingly recognized to serve an important role in the regulation of whole-body lipid homeostasis; however, the underlying mechanisms remained poorly understood. Here, our study in Drosophila reveals that cardiomyocytes regulate systemic lipid metabolism by producing apolipoprotein B-containing lipoproteins (apoB-lipoproteins), essential lipid carriers that are so far known to be generated only in the fat body (insect liver and adipose tissue). We found that apoB-lipoproteins generated by the Drosophila cardiomyocytes serve an equally significant role as their fat body-derived counterparts in maintaining systemic lipid homeostasis on normal food diet. Importantly, on high fat diet (HFD), the cardiomyocyte-derived apoB-lipoproteins are the major determinants of whole-body lipid metabolism, a role which could be attributed to the HFD-induced up-regulation of apoB-lipoprotein biosynthesis genes in the cardiomyocytes and their down-regulation in the fat body. Taken together, our results reveal that apoB-lipoproteins are new players in mediating the heart control of lipid metabolism, and provide first evidence supporting the notion that HFD-induced differential regulation of apoB-lipoprotein biosynthesis genes could alter the input of different tissue-derived apoB-lipoproteins in systemic lipid metabolic control.

The process of lipid storage in insect oocytes: The involvement of β-chain of ATP synthase in lipophorin-mediated lipid transfer in the chagas' disease vector Panstrongylus megistus (Hemiptera: Reduviidae)

Lipophorin is the main lipoprotein in the hemolymph of insects. During vitellogenesis, lipophorin delivers its hydrophobic cargo to developing oocytes by its binding to non-endocytic receptors at the plasma membrane of the cells. In some species however, lipophorin may also be internalized to some extent, thus maximizing the storage of lipid resources in growing oocytes. The ectopic β chain of ATP synthase (β-ATPase) was recently described as a putative non-endocytic lipophorin receptor in the anterior midgut of the hematophagous insect Panstrongylus megistus. In the present work, females of this species at the vitellogenic stage of the reproductive cycle were employed to investigate the role of β-ATPase in the transfer of lipids to the ovarian tissue. Subcellular fractionation and western blot revealed the presence of β-ATPase in the microsomal membranes of the ovarian tissue, suggesting its localization in the plasma membrane. Immunofluorescence assays showed partial co-localization of β-ATPase and lipophorin in the membrane of oocytes as well as in the basal domain of the follicular epithelial cells. Ligand blotting and co-immunoprecipitation approaches confirmed the interaction between lipophorin and β-ATPase. In vivo experiments with an anti-β-ATPase antibody injected to block such an interaction demonstrated that the antibody significantly impaired the transfer of fatty acids from lipophorin to the oocyte. However, the endocytic pathway of lipophorin was not affected. On the other hand, partial inhibition of ATP synthase activity did not modify the transfer of lipids from lipophorin to oocytes. When the assays were performed at 4°C to diminish endocytosis, the results showed that the antibody interfered with lipophorin binding to the oocyte plasma membrane as well as with the transfer of fatty acids from the lipoprotein to the oocyte. The findings strongly support that β-ATPase plays a role as a docking lipophorin receptor at the ovary of P. megistus, similarly to its function in the midgut of such a vector. In addition, the role of β-ATPase as a docking receptor seems to be independent of the enzymatic ATP synthase activity.

Lipid metabolism in Rhodnius prolixus: Lessons from the genome

The kissing bug Rhodnius prolixus is both an important vector of Chagas' disease and an interesting model for investigation into the field of physiology, including lipid metabolism. The publication of this insect genome will bring a huge amount of new molecular biology data to be used in future experiments. Although this work represents a promising scenario, a preliminary analysis of the sequence data is necessary to identify and annotate the genes involved in lipid metabolism. Here, we used bioinformatics tools and gene expression analysis to explore genes from different genes families and pathways, including genes for fat breakdown, as lipases and phospholipases, and enzymes from β-oxidation, fatty acid metabolism, and acyl-CoA and glycerolipid synthesis. The R. prolixus genome encodes 31 putative lipase genes, including 21 neutral lipases and 5 acid lipases. The expression profiles of some of these genes were analyzed. We were able to identify nine phospholipase A2 genes. A variety of gene families that participate in fatty acid synthesis and modification were studied, including fatty acid synthase, elongase, desaturase and reductase. Concerning the synthesis of glycerolipids, we found a second isoform of glycerol-3-phosphate acyltransferase that was ubiquitously expressed throughout the organs. Finally, all genes involved in fatty acid β-oxidation were identified, but not a long-chain acyl-CoA dehydrogenase. These results provide fundamental data to be used in future research on insect lipid metabolism and its possible relevance to Chagas' disease transmission.

Identification of the first insulin-like peptide in the disease vector Rhodnius prolixus: Involvement in metabolic homeostasis of lipids and carbohydrates

Insulin-like peptides (ILPs) are functional analogs of insulin and have been identified in many insect species. The insulin cell signaling pathway is a conserved regulator of metabolism, and in insects, as well as in other animals, can modulate physiological functions associated with the metabolism of lipids and carbohydrates. In the present study, we have identified the first ILP from the Rhodnius prolixus genome (termed Rhopr-ILP) and investigated its involvement in energy metabolism of unfed and recently fed fifth instars. We have cloned the cDNA sequence and analyzed the expression profile of the transcript, which is predominantly present in neurosecretory cells in the brain, similar to other insect ILPs. Using RNAi, we have reduced the expression of this peptide transcript by 90% and subsequently measured the carbohydrate and lipid levels in the hemolymph, fat body and leg muscles. Reduced levels of Rhopr-ILP transcript induced increased carbohydrate and lipid levels in the hemolymph and increased lipid content in the fat body, in unfed insects and recently fed insects. Also their fat bodies displayed enlarged lipid droplets within the cells. On the other hand, the carbohydrate content in the fat body and in the leg muscles of unfed insects were decreased when compared to control insects. Our results indicate that Rhopr-ILP is a modulator of lipid and carbohydrate metabolism, probably through signaling the presence of available energy and nutrients in the hemolymph.

Cold adaptation mechanisms in the ghost moth Hepialus xiaojinensis: Metabolic regulation and thermal compensation

Ghost moths (Lepidoptera: Hepialidae) are cold-adapted stenothermal species inhabiting alpine meadows on the Tibetan Plateau. They have an optimal developmental temperature of 12-16 °C but can maintain feeding and growth at 0 °C. Their survival strategies have received little attention, but these insects are a promising model for environmental adaptation. Here, biochemical adaptations and energy metabolism in response to cold were investigated in larvae of the ghost moth Hepialus xiaojinensis. Metabolic rate and respiratory quotient decreased dramatically with decreasing temperature (15-4 °C), suggesting that the energy metabolism of ghost moths, especially glycometabolism, was sensitive to cold. However, the metabolic rate at 4 °C increased with the duration of cold exposure, indicating thermal compensation to sustain energy budgets under cold conditions. Underlying regulation strategies were studied by analyzing metabolic differences between cold-acclimated (4 °C for 48 h) and control larvae (15 °C). In cold-acclimated larvae, the energy generating pathways of carbohydrates, instead of the overall consumption of carbohydrates, was compensated in the fat body by improving the transcription of related enzymes. The mobilization of lipids was also promoted, with higher diacylglycerol, monoacylglycerol and free fatty acid content in hemolymph. These results indicated that cold acclimation induced a reorganization on metabolic structure to prioritise energy metabolism. Within the aerobic process, flux throughout the tricarboxylic acid (TCA) cycle was encouraged in the fat body, and the activity of α-ketoglutarate dehydrogenase was the likely compensation target. Increased mitochondrial cristae density was observed in the midgut of cold-acclimated larvae. The thermal compensation strategies in this ghost moth span the entire process of energy metabolism, including degration of metabolic substrate, TCA cycle and oxidative phosphorylation, and from an energy budget perspective explains how ghost moths sustain physiological activity in cold environments.

BmPLA2 containing conserved domain WD40 affects the metabolic functions of fat body tissue in silkworm, Bombyx mori

PLA2 enzyme hydrolyzes arachidonic acid, and other polyunsaturated fatty acids, from the sn-2 position to release free arachidonic acid and a lysophospholipid. Previous studies reported that the PLA2 in invertebrate organisms participates in lipid signaling molecules like arachidonic acid release in immune-associated tissues like hemocytes and fat bodies. In the present study, we cloned the BmPLA2 gene from fat body tissue of silkworm Bombyx mori, which has a total sequence of 1.031 kb with a 31.90 kDa protein. In silico results of BmPLA2 indicated that the protein has a putative WD40 conserved domain and its phylogeny tree clustered with Danaus plexippus species. We investigated the transcriptional expression in development stages and tissues. The highest expression of BmPLA2 was screened in fat body among the studied tissues of third day fifth instar larva, with a high expression on third day fifth instar larva followed by a depression of expression in the wandering stage of the fifth instar larva. The expression of BmPLA2 in female pupa was higher than that of male pupa. Our RNAi-mediated gene silencing results showed highest reduction of BmPLA2 expression in post-24 h followed by post-48 and post-72 h. The BmPLA2-RNAi larvae and pupa could be characterized by pharate adult lethality and underdevelopment. The phenotypic characters of fat body cells in RNAi-induced larva implied that BmPLA2 affects the metabolic functions of fat body tissue in silkworm Bombyx mori.

Drosophila Lipophorin Receptors Recruit the Lipoprotein LTP to the Plasma Membrane to Mediate Lipid Uptake

Lipophorin, the main Drosophila lipoprotein, circulates in the hemolymph transporting lipids between organs following routes that must adapt to changing physiological requirements. Lipophorin receptors expressed in developmentally dynamic patterns in tissues such as imaginal discs, oenocytes and ovaries control the timing and tissular distribution of lipid uptake. Using an affinity purification strategy, we identified a novel ligand for the lipophorin receptors, the circulating lipoprotein Lipid Transfer Particle (LTP). We show that specific isoforms of the lipophorin receptors mediate the extracellular accumulation of LTP in imaginal discs and ovaries. The interaction requires the LA-1 module in the lipophorin receptors and is strengthened by a contiguous region of 16 conserved amino acids. Lipophorin receptor variants that do not interact with LTP cannot mediate lipid uptake, revealing an essential role of LTP in the process. In addition, we show that lipophorin associates with the lipophorin receptors and with the extracellular matrix through weak interactions. However, during lipophorin receptor-mediated lipid uptake, LTP is required for a transient stabilization of lipophorin in the basolateral plasma membrane of imaginal disc cells. Together, our data suggests a molecular mechanism by which the lipophorin receptors tether LTP to the plasma membrane in lipid acceptor tissues. LTP would interact with lipophorin particles adsorbed to the extracellular matrix and with the plasma membrane, catalyzing the exchange of lipids between them.

Lipoprotein assembly and function in an evolutionary perspective

Circulatory fat transport in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). ApoB and apoLp-II/I, constituting the structural (non-exchangeable) basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride-transfer protein, another LLTP family member, and bind them by means of amphipathic α-helical and β-sheet structural motifs. Comparative research reveals that LLTPs evolved from the earliest animals and highlights the structural adaptations in these lipid-binding proteins. Thus, in contrast to apoB, apoLp-II/I is cleaved post-translationally by a furin, resulting in the appearance of two non-exchangeable apolipoproteins in the single circulatory lipoprotein in insects, high-density lipophorin (HDLp). The remarkable structural similarities between mammalian and insect lipoproteins notwithstanding important functional differences relate to the mechanism of lipid delivery. Whereas in mammals, partial delipidation of apoB-containing lipoproteins eventually results in endocytic uptake of their remnants, mediated by members of the low-density lipoprotein receptor (LDLR) family, and degradation in lysosomes, insect HDLp functions as a reusable lipid shuttle capable of alternate unloading and reloading of lipid. Also, during muscular efforts (flight activity), an HDLp-based lipoprotein shuttle provides for the transport of lipid for energy generation. Although a lipophorin receptor - a homolog of LDLR - was identified that mediates endocytic uptake of HDLp during specific developmental periods, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. These data highlight that the functional adaptations in the lipoprotein lipid carriers in mammals and insects also emerge with regard to the functioning of their cognate receptors.

Amsacta moorei entomopoxvirus encodes a functional esterase (amv133) with protease activity

OBJECTIVES

Lipolytic genes have been investigated in several viral genomes, and some of them show enzyme activity which can be used for various functions including the production of DNA replication metabolites, rescue from endosomes, and membrane fusion. Amsacta moorei entomopoxvirus (AMEV) replicates in nearly the entire insect body, especially in the adipose tissue. One of the open reading frames (ORFs) in the AMEV genome, amv133, encodes a putative lipase enzyme. In this study, we therefore investigate the enzyme activity of amv133.

METHODS

amv133 was aligned with known lipase genes and their homologs in entomopoxviruses. Expressed proteins were partially purified and assayed for lipase, esterase and protease.

RESULTS

We found that amv133 contains all the domains required for a functional lipase enzyme and that it shows a significant similarity with homologs in other entomopoxviruses. Since there is a similarity of the catalytic triad between lipases and serine proteases, we also investigated the protease activity of amv133. Lipase, esterase and protease assays showed that amv133 encodes a functional esterase enzyme with protease activity.

CONCLUSION

The current data show that amv133 is a conserved gene in all entomopoxvirus genomes sequenced so far and might contribute greatly to degrading the lipids or proteins and hence improve the virus infection.

Recent progress in molecular genetic studies on the carotenoid transport system using cocoon-color mutants of the silkworm

The existence of tissue-specific delivery for certain carotenoids is supported by genetic evidence from the silkworm Bombyx mori and the identification of cocoon color mutant genes, such as Yellow blood (Y), Yellow cocoon (C), and Flesh cocoon (F). Mutants with white cocoons are defective in one of the steps involved in transporting carotenoids from the midgut lumen to the middle silk gland via the hemolymph lipoprotein, lipophorin, and the different colored cocoons are caused by the accumulation of specific carotenoids into the middle silk gland. The Y gene encodes carotenoid-binding protein (CBP), which is expected to function as the cytosolic transporter of carotenoids across the enterocyte and epithelium of the middle silk gland. The C and F genes encode the C locus-associated membrane protein, which is homologous to a mammalian high-density lipoprotein receptor-2 (Cameo2) and scavenger receptor class B member 15 (SCRB15), respectively; these membrane proteins are expected to function as non-internalizing lipophorin receptors in the middle silk gland. Cameo2 and SCRB15 belong to the cluster determinant 36 (CD36) family, with Cameo2 exhibiting specificity not only for lutein, but also for zeaxanthin and astaxanthin, while SCRB15 seems to have specificity toward carotene substrates such as α-carotene and β-carotene. These findings suggest that Cameo2 and SCRB15 can discriminate the chemical structure of lutein and β-carotene from circulating lipophorin during uptake. These data provide the first evidence that CD36 family proteins can discriminate individual carotenoid molecules in lipophorin.

Host regulation and nutritional exploitation by parasitic wasps

The physiological alterations observed in naturally parasitized hosts are characterized by a number of reproductive and developmental changes. Some of these changes are also associated with alterations in host physiology that benefit the nutrition and development of wasp offspring. Here we review the breadth of host-parasitoid nutritional interactions, and discuss current understanding of underlying mechanisms. We also discuss priorities for future studies that could enhance understanding of basic questions about the parasitoid lifestyle and provide insights of value for insect control.

Metabolite localization by atmospheric pressure high-resolution scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging in whole-body sections and individual organs of the rove beetle Paederus riparius

Mass spectrometry imaging provides for non-targeted, label-free chemical imaging. In this study, atmospheric pressure high-resolution scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI) was used for the first time to describe the chemical distribution of the defensive compounds pederin, pseudopederin, and pederon in tissue sections (16 μm thick) of the rove beetle Paederus riparius. The whole-insect tissue section was scanned with a 20-μm step size. Mass resolution of the orbital trapping mass spectrometer was set to 100,000 at m/z 200. Additionally, organ-specific compounds were identified for brain, nerve cord, eggs, gut, ovaries, and malpighian tubules. To confirm the distribution of the specific compounds, individual organs from the insect were dissected, and MSI experiments were performed on the dissected organs. Three ganglia of the nerve cord, with a dimension of 250–500 μm, were measured with 10-μm spatial resolution. High-quality m/z images, based on high spatial resolution and high mass accuracy were generated. These features helped to assign mass spectral peaks with high confidence. Mass accuracy of the imaging experiments was <3 ppm root mean square error, and mapping of different compound classes from a single experiment was possible. This approach improved the understanding of the biochemistry of P. riparius. Concentration differences and distributions of pederin and its analogues could be visualized in the whole-insect section. Without any labeling, we assigned key lipids for specific organs to describe their location in the body and to identify morphological structures with a specificity higher than with staining or immunohistology methods.

Ovarian nutritional resources during the reproductive cycle of the hematophagous Dipetalogaster maxima (Hemiptera: Reduviidae): focus on lipid metabolism

In this study, we have analyzed the changes of the ovarian nutritional resources in Dipetalogaster maxima at representative days of the reproductive cycle: previtellogenesis, vitellogenesis, as well as fasting-induced early and late atresia. As expected, the amounts of ovarian lipids, proteins, and glycogen increased significantly from previtellogenesis to vitellogenesis and then, diminished during atresia. However, lipids and protein stores found at the atretic stages were higher in comparison to those registered at previtellogenesis. Specific lipid staining of ovarian tissue sections evidenced remarkable changes in the shape, size, and distribution of lipid droplets throughout the reproductive cycle. The role of lipophorin (Lp) as a yolk protein precursor was analyzed by co-injecting Lp-OG (where OG is Oregon Green) and Lp-DiI (where DiI is 1,10-dioctadecyl-3,3,30,30-tetramethylindocarbocyanine) to follow the entire particle, demonstrating that both probes colocalized mainly in the yolk bodies of vitellogenic oocytes. Immunofluorescence assays also showed that Lp was associated to yolk bodies, supporting its endocytic pathway during vitellogenesis. The involvement of Lp in lipid delivery to oocytes was investigated in vivo by co-injecting fluorescent probes to follow the fate of the entire particle (Lp-DiI) and its lipid cargo (Lp-Bodipy-FA). Lp-DiI was readily incorporated by vitellogenic oocytes and no lipoprotein uptake was observed in terminal follicles of ovaries at atretic stages. Bodipy-FA was promptly transferred to vitellogenic oocytes and, to a much lesser extent, to previtellogenic follicles and to oocytes of ovarian tissue at atretic stages. Colocalization of Lp-DiI and Lp-Bodipy-FA inside yolk bodies indicated the relevance of Lp in the buildup of lipid and protein oocyte stores during vitellogenesis.

β-chain of ATP synthase as a lipophorin binding protein and its role in lipid transfer in the midgut of Panstrongylus megistus (Hemiptera: Reduviidae)

Lipophorin, the main lipoprotein in the circulation of the insects, cycles among peripheral tissues to exchange its lipid cargo at the plasma membrane of target cells, without synthesis or degradation of its apolipoprotein matrix. Currently, there are few characterized candidates supporting the functioning of the docking mechanism of lipophorin-mediated lipid transfer. In this work we combined ligand blotting assays and tandem mass spectrometry to characterize proteins with the property to bind lipophorin at the midgut membrane of Panstrongylus megistus, a vector of Chagas' disease. We further evaluated the role of lipophorin binding proteins in the transfer of lipids between the midgut and lipophorin. The β subunit of the ATP synthase complex (β-ATPase) was identified as a lipophorin binding protein. β-ATPase was detected in enriched midgut membrane preparations free of mitochondria. It was shown that β-ATPase partially co-localizes with lipophorin at the plasma membrane of isolated enterocytes and in the sub-epithelial region of the midgut tissue. The interaction of endogenous lipophorin and β-ATPase was also demonstrated by co-immunoprecipitation assays. Blocking of β-ATPase significantly diminished the binding of lipophorin to the isolated enterocytes and to the midgut tissue. In vivo assays injecting the β-ATPase antibody significantly reduced the transfer of [(3)H]-diacylglycerol from the midgut to the hemolymph in insects fed with [9,10-(3)H]-oleic acid, supporting the involvement of lipophorin-β-ATPase association in the transfer of lipids. In addition, the β-ATPase antibody partially impaired the transfer of fatty acids from lipophorin to the midgut, a less important route of lipid delivery to this tissue. Taken together, the findings strongly suggest that β-ATPase plays a role as a docking lipophorin receptor at the midgut of P. megistus.

Dietary and flight energetic adaptations in a salivary gland transcriptome of an insectivorous bat

We hypothesized that evolution of salivary gland secretory proteome has been important in adaptation to insectivory, the most common dietary strategy among Chiroptera. A submandibular salivary gland (SMG) transcriptome was sequenced for the little brown bat, Myotis lucifugus. The likely secretory proteome of 23 genes included seven (RETNLB, PSAP, CLU, APOE, LCN2, C3, CEL) related to M. lucifugus insectivorous diet and metabolism. Six of the secretory proteins probably are endocrine, whereas one (CEL) most likely is exocrine. The encoded proteins are associated with lipid hydrolysis, regulation of lipid metabolism, lipid transport, and insulin resistance. They are capable of processing exogenous lipids for flight metabolism while foraging. Salivary carboxyl ester lipase (CEL) is thought to hydrolyze insect lipophorins, which probably are absorbed across the gastric mucosa during feeding. The other six proteins are predicted either to maintain these lipids at high blood concentrations or to facilitate transport and uptake by flight muscles. Expression of these seven genes and coordinated secretion from a single organ is novel to this insectivorous bat, and apparently has evolved through instances of gene duplication, gene recruitment, and nucleotide selection. Four of the recruited genes are single-copy in the Myotis genome, whereas three have undergone duplication(s) with two of these genes exhibiting evolutionary 'bursts' of duplication resulting in multiple paralogs. Evidence for episodic directional selection was found for six of seven genes, reinforcing the conclusion that the recruited genes have important roles in adaptation to insectivory and the metabolic demands of flight. Intragenic frequencies of mobile- element-like sequences differed from frequencies in the whole M. lucifugus genome. Differences among recruited genes imply separate evolutionary trajectories and that adaptation was not a single, coordinated event.

Interaction of lipophorin with Rhodnius prolixus oocytes: biochemical properties and the importance of blood feeding

Lipophorin (Lp) is the main haemolymphatic lipoprotein in insects and transports lipids between different organs. In adult females, lipophorin delivers lipids to growing oocytes. In this study, the interaction of this lipoprotein with the ovaries of Rhodnius prolixus was characterised using an oocyte membrane preparation and purified radiolabelled Lp (125I-Lp). Lp-specific binding to the oocyte membrane reached equilibrium after 40-60 min and when 125I-Lp was incubated with increasing amounts of membrane protein, corresponding increases in Lp binding were observed. The specific binding of Lp to the membrane preparation was a saturable process, with a K(d) of 7.1 ± 0.9 x 10-8M and a maximal binding capacity of 430 ± 40 ng 125I-Lp/µg of membrane protein. The binding was calcium independent and pH sensitive, reaching its maximum at pH 5.2-5.7. Suramin inhibited the binding interaction between Lp and the oocyte membranes, which was completely abolished at 0.5 mM suramin. The oocyte membrane preparation from R. prolixus also showed binding to Lp from Manduca sexta. When Lp was fluorescently labelled and injected into vitellogenic females, the level of Lp-oocyte binding was much higher in females that were fed whole blood than in those fed blood plasma.

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.

Genetic studies of spectrin in the larval fat body of Drosophila melanogaster: evidence for a novel lipid uptake apparatus

Spectrin cytoskeleton defects produce a host of phenotypes affecting the plasma membrane, cell polarity, and secretory membrane traffic. However, many of the underlying molecular mechanisms remain unexplained by prevailing models. Here we used the larval fat body of Drosophila melanogaster as a genetic model system to further elucidate mechanisms of αβ-spectrin function. The results provide unexpected new insights into spectrin function as well as mechanisms of dietary fat uptake and storage. We show that loss of α- or β-spectrin in the fat body eliminated a population of small cortical lipid droplets and altered plasma membrane architecture, but did not affect viability of the organism. We present a novel model in which αβ-spectrin directly couples lipid uptake at the plasma membrane to lipid droplet growth in the cytoplasm. In contrast, strong overexpression of β-spectrin caused fat body atrophy and larval lethality. Overexpression of β-spectrin also perturbed transport of dietary fat from the midgut to the fat body. This hypermorphic phenotype appears to be the result of blocking secretion of the lipid carrier lipophorin from fat cells. However, this midgut phenotype was never seen with spectrin loss of function, suggesting that spectrin is not normally required for lipophorin secretion or function. The β-spectrin hypermorphic phenotype was ameliorated by co-overexpression of α-spectrin. Based on the overexpression results here, we propose that β-spectrin family members may be prone to hypermorphic effects (including effects on secretion) if their activity is not properly regulated.

The role of desaturases in the biosynthesis of marking pheromones in bumblebee males

Bumblebee males (Hymenoptera) produce species-specific labial gland secretions called marking pheromones (MPs). MPs generally consist of terpenoids and fatty-acid-derived aliphatic compounds with various chain lengths predominantly containing one or no double bonds. The unsaturated fatty-acid-derived MP components were hypothesized to be produced by fatty acid desaturases (FADs) that exhibit diverse substrate specificities. To address this hypothesis, we isolated and functionally characterized FADs from three bumblebee species: Bombus lucorum, Bombus terrestris, and Bombus lapidarius. By employing RNA sequencing of the male labial glands and fat bodies of B. lucorum and B. terrestris, we identified five paralogous FAD-like sequences but only two FAD lineages were abundant and differentially expressed in the labial glands. We found that abundant FAD lineages were also expressed in the labial gland and fat body of Bombus lapidarius. Functional characterization of FADs in a yeast expression system confirmed that Δ4-FADs exhibited a unique Δ4-desaturase activity exclusively on 14-carbon fatty acyls and Δ9-FADs displayed Δ9-desaturase activity on 14- to 18-carbon fatty acyls. These results indicate that Δ9-FADs are involved in the biosynthesis of major unsaturated components of MPs in B. lucorum and B. lapidarius despite the diverse MP composition of these bumblebee species. The contribution of lipases, acyltransferases, esterases, and fatty acid reductases to production of the species-specific MP composition is also discussed in light of the transcriptomic data obtained in this study.

Fatty acid modifications during autumnal cold-hardening in an obligatory ectoparasite, the deer ked (Lipoptena cervi)

Poikilothermic organisms often modify their tissue fatty acids (FA) in response to cold exposure by increased unsaturation. In insects, this has been found to be accompanied by increases in the activities or mRNA expression of desaturase enzymes. In the present study, the FA composition of an obligatory ectoparasite, the deer ked (Lipoptena cervi), was analyzed in August-November. In addition to studying the general FA profile of the species, the possible contribution of FA to autumnal cold-hardening was examined. The FA composition of the deer ked imago was relatively similar to previously studied dipteran species, with high percentages of monounsaturated FA (especially 18:1n-9 and 16:1n-7) and 16:0. The individuals caught later in autumn had significantly higher values for the ratio of unsaturated to saturated FA and, regarding individual FA, the percentages of 18:1n-9, 18:2n-6, 18:3n-3, 20:4n-6 and 22:6n-3 were higher but those of 16:0 and 16:1n-7 lower than in August. Potential selective use of particular FA for energy could not account for the large increase in the levels of polyunsaturated FA (PUFA). The observed increased degree of FA unsaturation may have resulted from cold-induced desaturation, as observed previously in other species, or increased survival of the keds with relatively large PUFA contents. The PUFA with low melting points probably allow lipid membranes to maintain sufficient fluidity required to maintain protein functions at low ambient temperatures.

Fatty acid solubilizer from the oral disk of the blowfly

Background

Blowflies are economic pests of the wool industry and potential vectors for epidemics. The establishment of a pesticide-free, environmentally friendly blowfly control strategy is necessary. Blowflies must feed on meat in order to initiate the cascade of events that are involved in reproduction including juvenile hormone synthesis, vitellogenesis, and mating. During feeding blowflies regurgitate salivary lipase, which may play a role in releasing fatty acids from triglycerides that are found in food. However, long-chain fatty acids show low solubility in aqueous solutions. In order to solubilize and ingest the released hydrophobic fatty acids, the blowflies must use a solubilizer.

Methodology

We applied native PAGE, Edman degradation, cDNA cloning, and RT-PCR to characterize a protein that accumulated in the oral disk of the black blowfly, Phormia regina. In situ hybridization was carried out to localize the expression at the cellular level. A fluorescence competitive binding assay was used to identify potential ligands of this protein.

Conclusion

A protein newly identified from P. regina (PregOBP56a) belonged to the classic odorant-binding protein (OBP) family. This gene was expressed in a cluster of cells that was localized between pseudotracheae on the oral disk, which are not accessory cells of the taste peg chemosensory sensilla that normally synthesize OBPs. At pH 7 and pH 6, PregOBP56a bound palmitic, stearic, oleic, and linoleic acids, that are mainly found in chicken meat. The binding affinity of PregOBP56a decreased at pH 5. We propose that PregOBP56a is a protein that solubilizes fatty acids during feeding and subsequently helps to deliver the fatty acids to the midgut where it may help in the process of reproduction. As such, PregOBP56a is a potential molecular target for controlling the blowfly.

Developmental changes in the protein composition of Manduca sexta lipid droplets

The lipid droplets (LDs) are intracellular organelles mainly dedicated to the storage and provision of fatty acids. To accomplish these functions the LDs interact with other organelles and cytosolic proteins. In order to explore possible correlations between the physiological states of cells and the protein composition of LDs we have determined and compared the proteomic profiles of lipid droplets isolated from the fat bodies of 5th-instar larvae and adult Manduca sexta insects and from ovaries. These LD-rich tissues represent three clearly distinct metabolic states in regard to lipid metabolism: 1) Larval fat body synthesizes fatty acids (FA) and accumulates large amounts as triglyceride (TG); 2) Fat body from adult insects provides FA to support reproduction and flight; 3) Ovaries do not synthesize FA, but accumulate considerable amounts of TG in LDs. Major qualitative and semi-quantitative variations in the protein compositions of the LDs isolated from these three tissues were observed by MS/MS and partially validated by immuno-blotting. The differences observed included changes in the abundance of lipid droplet specific proteins, cytosolic proteins, mitochondrial proteins and also proteins associated with the machinery of protein synthesis. These results suggest that changes in the interaction of LDs with other organelles and cytosolic proteins are tightly related to the physiological state of cells. Herein, we summarize and compare the protein compositions of three subtypes of LDs and also describe for the first time the proteomic profile of LDs from an insect ovary. The compositions and compositional differences found among the LDs are discussed to provide a platform for future studies on the role of LDs, and their associated proteins, in cellular metabolism.

Functional analysis of a fatty acid binding protein produced by Aphidius ervi teratocytes

Aphidius ervi (Hymenoptera, Braconidae) is an endophagous parasitoid of various aphid species, including Acyrthosiphon pisum (Homoptera, Aphididae), the model host used in the present study. Parasitized hosts show a marked increase of their nutritional suitability for the developing parasitoid larvae. This alteration of the biochemical and metabolic profile is due to a castration process mediated by the combined action of the venom, injected at the oviposition, and of the teratocytes, cells deriving from the dissociation of the embryonic membrane. Teratocytes produce and release in the host haemocoel two parasitism-specific proteins, which are of crucial importance for the development of their sister larvae. One of the proteins is a fatty acid binding protein (Ae-FABP), which shows a high affinity for C14-C18 saturated fatty acids (FAs) and for oleic and arachidonic acids. To better define the possible nutritional role of this protein, we have studied its immunolocalization profile in vivo and the impact on FA uptake by the epidermal and midgut epithelia of A. ervi larvae. During the exponential growth of A. ervi larvae, Ae-FABP is distributed around discrete lipid particles, which are abundantly present in the haemocoel of parasitized host aphids and in the midgut lumen of parasitoid larvae. Moreover, a strong immunodetection signal is evident on the surface of the two larval epithelia involved in nutrient absorption: the parasitoid midgut epithelium and the external epidermal layer. These two epithelia can effectively absorb radiolabelled myristic acid, but the FA transport rates are not affected by the presence in the medium of Ae-FABP. The protein appears to act essentially as a vector in the host haemolymph, transferring FAs from the digestion sites of host lipids to the growing parasitoid larvae. These data indicate that the proteins produced by A. ervi teratocytes may play complementary roles in the nutritional exploitation of the host.

Lipophorin acts as a shuttle of lipids to the milk gland during tsetse fly pregnancy

During pregnancy in the viviparous tsetse fly, lipid mobilization is essential for the production of milk to feed the developing intrauterine larva. Lipophorin (Lp) functions as the major lipid transport protein in insects and closely-related arthropods. In this study, we assessed the role of Lp and the lipophorin receptor (LpR) in the lipid mobilization process during tsetse reproduction. We identified single gene sequences for GmmLp and GmmLpR from the genome of Glossinamorsitansmorsitans, and measured spatial and temporal expression of gmmlp and gmmlpr during the female reproductive cycle. Our results show that expression of gmmlp is specific to the adult fat body and larvae. In the adult female, gmmlp expression is constitutive. However transcript levels increase in the larva as it matures within the mother's uterus, reaching peak expression just prior to parturition. GmmLp was detected in the hemolymph of pregnant females and larvae, but not in the uterine fluid or larval gut contents ruling out the possibility of direct transfer of GmmLp from mother to offspring. Transcripts for gmmlpr were detected in the head, ovaries, midgut, milk gland/fat body, ovaries and developing larva. Levels of gmmlpr remain stable throughout the first and second gonotrophic cycles with a slight dip observed during the first gonotrophic cycle. GmmLpR was detected in multiple tissues, including the midgut, fat body, milk gland, spermatheca and head. Knockdown of gmmlp by RNA interference resulted in reduced hemolymph lipid levels, delayed oocyte development and extended larval gestation. Similar suppresion of gmmlpr did not significantly reduce hemolymph lipid levels or oogenesis duration, but did extend the duration of larval development. Thus, GmmLp function as the primary shuttle for lipids originating from the midgut and fat body to the ovaries and milk gland to supply resources for developing oocytes and larval nourishment, respectively. Once in the milk gland however, lipids are apparently transferred into the developing larva not by lipophorin but by another carrier lipoprotein.

In vitro lipid transfer between lipoproteins and midgut-diverticula in the spider Polybetes pythagoricus

It has been already reported that most hemolymphatic lipids in the spider Polybetes pythagoricus are transported by HDL1 and VHDL lipoproteins. We studied in vitro the lipid transfer among midgut-diverticula (M-diverticula), and either hemolymph or purified lipoproteins as well as between hemolymphatic lipoproteins. M-diverticula and hemolymph were labeled by in vivo (14)C-palmitic acid injection. In vitro incubations were performed between M-diverticula and either hemolymph or isolated lipoproteins. Hemolymph lipid uptake was associated to HDL1 (67%) and VHDL (32%). Release from hemolymph towards M-diverticula showed the opposite trend, VHDL 75% and HDL1 45%. Isolated lipoproteins showed a similar behavior to that observed with whole hemolymph. Lipid transfer between lipoproteins showed that HDL1 transfer more (14)C-lipids to VHDL than vice versa. Only 38% FFA and 18% TAG were transferred from M-diverticula to lipoproteins, while on the contrary 75% and 73% of these lipids, respectively, were taken up from hemolymph. A similar trend was observed regarding lipoprotein phospholipids. This study supports the hypothesis that HDL1 and hemocyanin-containing VHDL are involved in the uptake and release of FFA, phospholipids and triacylglycerols in the spider P. pythagoricus. The data support a directional flow of lipids from HDL1 and VHDL suggesting a mode of lipid transport between lipoproteins and M-diverticula.

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.

Digestive enzyme activity of two stonefly species (Insecta, Plecoptera) and their feeding habits

The digestive enzymes of two stoneflies species, Hemimelaena flaviventris and Isoperla morenica, were studied for the first time. These species are temporary water inhabitants and exhibit great feeding plasticity. Although they are traditionally referred to as predators, a previous study revealed that H. flaviventris incorporates some diatoms into its diet in addition to feeding usually on several prey, and I. morenica (in that study under the name of I. curtata) only feeds on animals occasionally. The enzymatic activities of digestive amylase, lipase, protease, trypsin and chymotrypsin were determined for each species at the same developmental stage. The results show that H. flaviventris has a greater digestive enzymatic pool and higher relative and absolute protease, lipase and trypsin activities than I. morenica. The latter has a relative higher amylase activity. As higher amylase activity is typical of phytophagous species and higher protease activity typical of carnivorous species; these results reveal that H. flaviventris is a more efficient zoophagous species than I. morenica. The ecological implications of these findings, including the higher secondary production of H. flaviventris in its habitat, are discussed.

Evolutionary conservation of metabolism explains howDrosophila nutrigenomics can help us understand human nutrigenomics

While large populations in the third world are enduring famine, much of the developed world is undergoing an obesity epidemic. In addition to reflecting an unbalanced distribution of food, the "epidemic of overabundance" is ironically leading to a decrease in the health and longevity of the obese and improperly nourished in the first world. International consortia, such as the European Nutrigenomics Organization (NuGO), are increasing our knowledge of nutrientgene interactions and the effects of diet and obesity on human health. In this review, we summarize both previous and ongoing nutrigenomics studies in Drosophila and we explain how these studies can be used to provide insights into molecular mechanisms underlying nutrigenomics in humans. We will discuss how quantitative trait locus (QTL) experiments have identified genes that affect triglyceride levels in Drosophila, and how microarray analyses show that hundreds of genes have altered gene expression under different dietary conditions. Finally, we will discuss ongoing combined microarray-QTL studies, termed "genetical genomics," that promise to identify "master modulatory loci" that regulate global responses of potentially hundreds of genes under different dietary conditions. When "master modulatory loci" are identified in Drosophila, then experiments in mammalian models can be used to determine the relevance of these genes to human nutrition and health.

Changes in the composition of triacylglycerols in the fat bodies of bumblebee males during their lifetime

The age-dependent changes in the composition of triacylglycerols (TAG) in the fat bodies of bumblebee males were studied using HPLC/MS. Two related species (Bombus terrestris and B. lucorum) were compared, with the age of the males being 0-30 days. The total amount of TAG in B. lucorum was about 2.7 times higher than that in B. terrestris for all of the ages studied. One to three-day-old males had the highest content of TAG in their fat bodies (1.6-2.3 mg/individual in B. terrestris and 3.8-4.2 mg/individual in B. lucorum). The analytical data show different patterns in both species. The qualitative composition of fatty acids in TAG was similar, but the mean relative abundance between B. terrestris and B. lucorum differed: 14:0, 7 and 14%; 16:0, 20 and 44%; 18:3, 62 and 23%; 18:1, 3 and 8%, respectively (the data is based on a GC/MS integration). A statistical evaluation of the dynamic changes in the TAG composition revealed that in B. terrestris different age classes were well separated according to their TAG composition while in B. lucorum the TAG did not change substantially during the male's life. The TAG analyses provide more precise information on the differences between the classes studied than the FA composition alone.

The storage of nutritional resources during vitellogenesis of Panstrongylus megistus (Hemiptera: Reduviidae): the pathways of lipophorin in lipid delivery to developing oocytes

In this work, we have analyzed the pathways by which lipophorin (Lp) delivers its lipid cargo to developing oocytes of Panstrongylus megistus, a hematophagous vector of Chagas' disease. Lp, vitellin, total lipids and proteins were measured in ovarian tissues at different stages of the reproductive cycle. Localization of Lp in developing oocytes, mainly at their cortical area, was demonstrated by immunofluorescence assays using an anti-Lp antibody labeled with FITC. In vivo approaches injecting fluorescently labeled Lp to follow the course of the entire particle (Lp-DiI or Lp-Oregon Green) or its lipid cargo (Lp-Bodipy-FA) were monitored by laser scanning confocal microscopy. Significant increases in the amounts of lipids, proteins and vitellin were observed in ovarian tissue with the progress of vitellogenesis. Unexpectedly, an increase in the amount of Lp was also observed. The experiments in vivo demonstrated that the uptake of fluorescent Lp labeled on its protein or lipid moiety by developing oocytes occurred very fast, being impaired at low temperatures. The co-injection of fluorescent Lp and vitellogenin (Vg) showed that both particles co-localized inside yolk bodies, confirming the endocytic pathway for Lp. When the fate of lipids transferred to oocytes was evaluated in vitellogenic females by co-injecting Lp-Bodipy-FA and Lp-DiI, the signal for Bodipy-FA was found in both lipid droplets and yolk bodies. In contrast, in injected females kept at 4°C the fluorescence was reduced, being observed exclusively in lipid droplets, implying that lipid transfer to the oocyte was diminished but not abolished. Taken together, the results demonstrate that in the hematophagous P. megistus, the storage of lipid resources by developing oocytes occurs by the convergence of different pathways by which Lp maximizes the delivery of its lipid cargo. In addition, it was also shown that, to some extent, lipids stored in the oocyte lipid droplets can also originate from endocytosed Vg. The relevance of these events in the context of the physiology of reproduction in P. megistus is discussed.

Role of the fat body in the cave crickets Troglophilus cavicola and Troglophilus neglectus (Rhaphidophoridae, Saltatoria) during overwintering

The cave crickets Troglophilus cavicola and Troglophilus neglectus are the most widely distributed European species of the family Rhaphidophoridae. Their life cycles span two years. They overwinter twice in caves in 4-6 months lasting diapause, T. cavicola in warmer microhabitats. In caves, older T. cavicola undergo sexual maturation, while T. neglectus do not. We hypothesized that the use of energy-supplying compounds and reserve proteins in the fat body is more extensive in T. cavicola than in T. neglectus. We analyzed the contents and morphology of lipid droplets, glycogen rosettes and protein granula at the beginning, the middle and the end of overwintering applying optic, TEM and biochemical methods. In all individuals, the fat body is composed of about 40 oval ribbons consisted of gradually changing adipocytes and urocytes. T. cavicola use glycogen continuously, and stop using lipids in the middle of overwintering, while this is inverse in T. neglectus. Till the middle of overwintering, all individuals exploit proteins, afterwards they are unevenly exploited. We found that the fat body is differently engaged in metabolism of both cave crickets during overwintering, supporting a more glycogen-dependent metabolism in T. cavicola, and a more lipid-dependent one in T. neglectus.

Dietary effects of harmine, a β-carboline alkaloid, on development, energy reserves and α-amylase activity of Plodia interpunctella Hübner (Lepidoptera: Pyralidae)

The physiological and developmental effects of harmine, a β-carboline alkaloid, on the insect pest Plodia interpunctella have been analyzed. When added at the larval diet, harmine induced a strong reduction of larvae weight, cannibalism between larvae, in addition to significant mortality. On the other hand, it caused a remarkable development disruption, manifested by both delay and reduction of pupation and adult emergence. Using spectrophotometric assays, we have shown that harmine ingestion provoked a severe reduction in protein, glycogen and lipid contents. Beside, when larvae fed harmine, the activity of the digestive enzyme α-amylase was strongly reduced. In conclusion, our experiments clearly show the susceptibility of P. interpunctella to harmine ingestion revealing the potent bioinsecticidal effect of harmine.

Up-regulation of lipophorin (Lp) and lipophorin receptor (LpR) gene in the mosquito, Culex quinquefasciatus (Diptera: Culicidae), infected with the filarial parasite, Wuchereria bancrofti (Spirurida: Onchocercidae)

In mosquitoes, including Culex quinquefasciatus, immune molecules are known to be upregulated or produced de novo upon exposure to parasites or pathogens. These molecules are regulatory in nature acting against parasite or pathogen infection and development. Similarly, there are molecules that are upregulated to facilitate parasite development in the vector mosquitoes. Lipophorin, a major lipid transporting lipoprotein in the hemolymph of insects, is implicated as a helper molecule in the clotting mechanism and facilitator of parasite and pathogen development in mosquitoes. In the present study, upregulation of a 240 kDa protein was detected in C. quinquefasciatus infected with the human lymphatic filarial parasite, Wuchereria bancrofti. It was identified as a lipophorin through nano-Lc-MS/MS analysis. Transcription of the lipophorin receptor gene also was identified through RACE-PCR. C. quinquefasciatus is the vector of W. bancrofti, and it allows successful development of the parasite. The role of upregulated lipophorin and transcription of its receptor gene in this mosquito could be implicated as a facilitator for the parasite development.

The major yolk protein vitellogenin interferes with the anti-plasmodium response in the malaria mosquito Anopheles gambiae

Functional gene analysis in malaria mosquitoes reveals molecules underpinning the trade-off between efficient reproduction and the antiparasitic response.

When taking a blood meal on a person infected with malaria, female Anopheles gambiae mosquitoes, the major vector of human malaria, acquire nutrients that will activate egg development (oogenesis) in their ovaries. Simultaneously, they infect themselves with the malaria parasite. On traversing the mosquito midgut epithelium, invading Plasmodium ookinetes are met with a potent innate immune response predominantly controlled by mosquito blood cells. Whether the concomitant processes of mosquito reproduction and immunity affect each other remains controversial. Here, we show that proteins that deliver nutrients to maturing mosquito oocytes interfere with the antiparasitic response. Lipophorin (Lp) and vitellogenin (Vg), two nutrient transport proteins, reduce the parasite-killing efficiency of the antiparasitic factor TEP1. In the absence of either nutrient transport protein, TEP1 binding to the ookinete surface becomes more efficient. We also show that Lp is required for the normal expression of Vg, and for later Plasmodium development at the oocyst stage. Furthermore, our results uncover an inhibitory role of the Cactus/REL1/REL2 signaling cassette in the expression of Vg, but not of Lp. We reveal molecular links that connect reproduction and immunity at several levels and provide a molecular basis for a long-suspected trade-off between these two processes.

Malaria annually claims the lives of almost 1 million infants and imposes a major socio-economic burden on Africa and other tropical regions. Meanwhile, the detailed biological interactions between the malaria parasite and its Anopheles mosquito vector remain largely enigmatic. What we do know is that the majority of malaria parasites are normally eliminated by the mosquito's immune response. Mosquitoes accidentally acquire an infection by sucking parasite-laden blood, but this belies the primary function of the blood in the provisioning of nutrients for egg development in the insect's ovaries. We have found that the molecular processes involved in delivering blood-acquired nutrients to maturing eggs diminish the efficiency of parasite killing by the mosquito immune system. Conversely, molecular pathways that set the immune system on its maximal capacity for parasite killing preclude the efficient development of the mosquito's eggs. Our results reveal some of the molecules that underpin this example of the trade-offs between reproduction and immunity, a concept that has long intrigued biologists.

Sterol carrier protein-x gene and effects of sterol carrier protein-2 inhibitors on lipid uptake in Manduca sexta

BACKGROUND

Cholesterol uptake and transportation during the feeding larval stages are critical processes in insects because they are auxotrophic for exogenous (dietary) cholesterol. The midgut is the main site for cholesterol uptake in many insects. However, the molecular mechanism by which dietary cholesterol is digested and absorbed within the midgut and then released into the hemolymph for transportation to utilization or storage sites is poorly understood. Sterol carrier proteins (SCP), non-specific lipid transfer proteins, have been speculated to be involved in intracellular cholesterol transfer and metabolism in vertebrates. Based on the high degree of homology in the conserved sterol transfer domain to rat and human SCP-2, it is supposed that insect SCP-2 has a parallel function to vertebrate SCP-2.

RESULTS

We identified the Manduca sexta sterol carrier protein-x and the sterol carrier protein-2 (MsSCP-x/SCP-2) gene from the larval fat body and the midgut cDNAs. The MsSCP-x/SCP-2 protein has a high degree of homology in the SCP-2 domain to other insects' SCP-2. Transcripts of MsSCP-2 were detected at high levels in the midgut and the fat body of M. sexta during the larval stages. Recombinant MsSCP-2 bound to NBD-cholesterol with high affinity, which was suppressed by sterol carrier protein-2 inhibitors.

CONCLUSIONS

The results suggest that MsSCP-2 may function as a lipid carrier protein in vivo, and targeting insect SCP-2 may be a viable approach for the development of new insecticides.

A nutritional profile of the social wasp Polistes metricus: differences in nutrient levels between castes and changes within castes during the annual life cycle

In wasps, nutrition plays a vital role for colony cohesion and caste determination. However, there is no baseline data set for the nutritional levels of wasps during the different stages of the colony cycle. Here we examined the levels of carbohydrates, lipids, protein, Ca, Cu, Fe, K, Mg, Mn, Na, and Zn in the wasp Polistes metricus at different stages of the wasp's lifecycle. Individuals were collected at the following stages (1) spring gynes, (2) foundress colonies, (3) early worker colonies, (4) late worker colonies, (5) emerging reproductives (gynes and males), (6) early fall reproductives, and (7) late fall reproductives. All eggs, larvae, pupae and adults were analyzed for their nutritional content to determine if there were any differences between the nutrient levels in the different castes and how these nutrients changed within a caste during its lifetime. The results show there are differences in macro and micronutrient levels between the reproductive females and workers during development. Gynes showed changes in nutrient levels during their lifetime especially as they changed roles from a solitary individual to a nesting queen. Males also showed distinct nutritional changes during their lifetime. The implications for these nutritional differences are discussed.