Molecular characterization and developmental expression of a retinoid- and fatty acid-binding glycoprotein from Drosophila. A putative lipophorin

Lipophorin: The Lipid Shuttle

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

Two apolipoproteins in salmon louse (Lepeophtheirus salmonis), apolipoprotein 1 knock down reduces reproductive capacity

The salmon louse, Lepeophtheirus salmonis is an ectoparasite of salmonid fish in the Northern Hemisphere, causing large economical losses in the aquaculture industry and represent a threat to wild populations of salmonids. Like other oviparous animals, it is likely that female lice use lipoproteins for lipid transport to maturing oocytes and other organs of the body. As an important component of lipoproteins, apolipoproteins play a vital role in the transport of lipids through biosynthesis of lipoproteins. Apolipoproteins have been studied in detail in different organisms, but no studies have been done in salmon lice. Two apolipoprotein encoding genes (LsLp1 and LsLp2) were identified in the salmon lice genome. Transcriptional analysis revealed both genes to be expressed at all stages from larvae to adult with some variation, LsLp1 generally higher than LsLp2 and both at their highest levels in adult stages of the louse. In adult female louse, the LsLp1 and LsLp2 transcripts were found in the sub-epidermal tissue and the intestine. RNA interference-mediated knockdown of LsLp1 and LsLp2 in female lice resulted in reduced expression of both transcripts. LsLp1 knockdown female lice produced significantly less offspring than control lice, while knockdown of LsLp2 in female lice caused no reduction in the number of offspring. These results suggest that LsLp1 has an important role in reproduction in female salmon lice.

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Salmon lice are ectoparasites and a major threat to aquaculture industry and wild salmon.

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Two apolipoproteins in salmon louse (Lepeophtheirus salmonis).

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Expressed at all stages from larvae to adult, sub-epidermal tissue and the intestine .

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RNA interference-mediated knockdown of LsLp1 and LsLp2.

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LsLp1 knockdown female lice produced significantly less offspring than control lice.

Body-fat sensor triggers ribosome maturation in the steroidogenic gland to initiate sexual maturation in Drosophila

Fat stores are critical for reproductive success and may govern maturation initiation. Here, we report that signaling and sensing fat sufficiency for sexual maturation commitment requires the lipid carrier apolipophorin in fat cells and Sema1a in the neuroendocrine prothoracic gland (PG). Larvae lacking apolpp or Sema1a fail to initiate maturation despite accruing sufficient fat stores, and they continue gaining weight until death. Mechanistically, sensing peripheral body-fat levels via the apolipophorin/Sema1a axis regulates endocytosis, endoplasmic reticulum remodeling, and ribosomal maturation for the acquisition of the PG cells' high biosynthetic and secretory capacity. Downstream of apolipophorin/Sema1a, leptin-like upd2 triggers the cessation of feeding and initiates sexual maturation. Human Leptin in the insect PG substitutes for upd2, preventing obesity and triggering maturation downstream of Sema1a. These data show how peripheral fat levels regulate the control of the maturation decision-making process via remodeling of endomembranes and ribosomal biogenesis in gland cells.

Receptor-mediated yolk uptake is required for oskar mRNA localization and cortical anchorage of germ plasm components in the Drosophila oocyte

The Drosophila germ plasm is responsible for germ cell formation. Its assembly begins with localization of oskar mRNA to the posterior pole of the oocyte. The oskar translation produces 2 isoforms with distinct functions: short Oskar recruits germ plasm components, whereas long Oskar remodels actin to anchor the components to the cortex. The mechanism by which long Oskar anchors them remains elusive. Here, we report that Yolkless, which facilitates uptake of nutrient yolk proteins into the oocyte, is a key cofactor for long Oskar. Loss of Yolkless or depletion of yolk proteins disrupts the microtubule alignment and oskar mRNA localization at the posterior pole of the oocyte, whereas microtubule-dependent localization of bicoid mRNA to the anterior and gurken mRNA to the anterior-dorsal corner remains intact. Furthermore, these mutant oocytes do not properly respond to long Oskar, causing defects in the actin remodeling and germ plasm anchoring. Thus, the yolk uptake is not merely the process for nutrient incorporation, but also crucial for oskar mRNA localization and cortical anchorage of germ plasm components in the oocyte.

A study of the fruit fly Drosophila reveals that receptor-mediated yolk uptake is not merely a nutrient storage process for future embryogenesis, but is also required for localization of Oskar mRNA and cortical anchorage of germ plasm components in the oocyte during oogenesis.

Temporal specificity and heterogeneity of Drosophila immune cells

Immune cells provide defense against non-self and have recently been shown to also play key roles in diverse processes such as development, metabolism, and tumor progression. The heterogeneity of Drosophila immune cells (hemocytes) remains an open question. Using bulk RNA sequencing, we find that the hemocytes display distinct features in the embryo, a closed and rapidly developing system, compared to the larva, which is exposed to environmental and metabolic challenges. Through single-cell RNA sequencing, we identify fourteen hemocyte clusters present in unchallenged larvae and associated with distinct processes, e.g., proliferation, phagocytosis, metabolic homeostasis, and humoral response. Finally, we characterize the changes occurring in the hemocyte clusters upon wasp infestation, which triggers the differentiation of a novel hemocyte type, the lamellocyte. This first molecular atlas of hemocytes provides insights and paves the way to study the biology of the Drosophila immune cells in physiological and pathological conditions.

The genetic mechanism of selfishness and altruism in parent-offspring coadaptation

The social bond between parents and offspring is characterized by coadaptation and balance between altruistic and selfish tendencies. However, its underlying genetic mechanism remains poorly understood. Using transcriptomic screens in the subsocial European earwig, Forficula auricularia, we found the expression of more than 1600 genes associated with experimentally manipulated parenting. We identified two genes, Th and PebIII, each showing evidence of differential coexpression between treatments in mothers and their offspring. In vivo RNAi experiments confirmed direct and indirect genetic effects of Th and PebIII on behavior and fitness, including maternal food provisioning and reproduction, and offspring development and survival. The direction of the effects consistently indicated a reciprocally altruistic function for Th and a reciprocally selfish function for PebIII. Further metabolic pathway analyses suggested roles for Th-restricted endogenous dopaminergic reward, PebIII-mediated chemical communication and a link to insulin signaling, juvenile hormone, and vitellogenin in parent-offspring coadaptation and social evolution.

Different mechanisms for selective transport of fatty acids using a single class of lipoprotein in Drosophila

In mammals, lipids are selectively transported to specific sites using multiple classes of lipoproteins. However, in Drosophila, a single class of lipoproteins, lipophorin, carries more than 95% of the lipids in the hemolymph. Although a unique ability of the insect lipoprotein system for cargo transport has been demonstrated, it remains unclear how this single class of lipoproteins selectively transports lipids. In this study, we carried out a comparative analysis of the fatty-acid composition among lipophorin, the CNS, and CNS-derived cell lines and investigated the transport mechanism of fatty acids, particularly focusing on the transport of PUFAs in Drosophila We showed that PUFAs are selectively incorporated into the acyl chains of lipophorin phospholipids and effectively transported to CNS through lipophorin receptor-mediated endocytosis of lipophorin. In addition, we demonstrated that C14 fatty acids are selectively incorporated into the diacylglycerols (DAGs) of lipophorin and that C14 fatty-acid-containing DAGs are spontaneously transferred from lipophorin to the phospholipid bilayer. These results suggest that PUFA-containing phospholipids and C14 fatty-acid-containing DAGs in lipophorin could be transferred to different sites by different mechanisms to selectively transport fatty acids using a single class of lipoproteins.

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.

Metabolite transport across the mammalian and insect brain diffusion barriers

The nervous system in higher vertebrates is separated from the circulation by a layer of specialized endothelial cells. It protects the sensitive neurons from harmful blood-derived substances, high and fluctuating ion concentrations, xenobiotics or even pathogens. To this end, the brain endothelial cells and their interlinking tight junctions build an efficient diffusion barrier. A structurally analogous diffusion barrier exists in insects, where glial cell layers separate the hemolymph from the neural cells. Both types of diffusion barriers, of course, also prevent influx of metabolites from the circulation. Because neuronal function consumes vast amounts of energy and necessitates influx of diverse substrates and metabolites, tightly regulated transport systems must ensure a constant metabolite supply. Here, we review the current knowledge about transport systems that carry key metabolites, amino acids, lipids and carbohydrates into the vertebrate and Drosophila brain and how this transport is regulated. Blood-brain and hemolymph-brain transport functions are conserved and we can thus use a simple, genetically accessible model system to learn more about features and dynamics of metabolite transport into the brain.

Oral transfer of chemical cues, growth proteins and hormones in social insects

Social insects frequently engage in oral fluid exchange - trophallaxis - between adults, and between adults and larvae. Although trophallaxis is widely considered a food-sharing mechanism, we hypothesized that endogenous components of this fluid might underlie a novel means of chemical communication between colony members. Through protein and small-molecule mass spectrometry and RNA sequencing, we found that trophallactic fluid in the ant Camponotus floridanus contains a set of specific digestion- and non-digestion related proteins, as well as hydrocarbons, microRNAs, and a key developmental regulator, juvenile hormone. When C. floridanus workers' food was supplemented with this hormone, the larvae they reared via trophallaxis were twice as likely to complete metamorphosis and became larger workers. Comparison of trophallactic fluid proteins across social insect species revealed that many are regulators of growth, development and behavioral maturation. These results suggest that trophallaxis plays previously unsuspected roles in communication and enables communal control of colony phenotypes.

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.

Hedgehog and its circuitous journey from producing to target cells

The hedgehog (Hh) signaling protein has essential roles in the growth, development and regulation of many vertebrate and invertebrate organs. The processes that make Hh and prepare it for release from producing cells and that move it to target cells are both diverse and complex. This article reviews the essential features of these processes and highlights recent work that provides a novel framework to understand how these processes contribute to an integrated pathway.

The hemolymph proteome of fed and starved Drosophila larvae

The co-operation of specialized organ systems in complex multicellular organisms depends on effective chemical communication. Thus, body fluids (like blood, lymph or intraspinal fluid) contain myriads of signaling mediators apart from metabolites. Moreover, these fluids are also of crucial importance for immune and wound responses. Compositional analyses of human body fluids are therefore of paramount diagnostic importance. Further improving their comprehensiveness should increase our understanding of inter-organ communication. In arthropods, which have trachea for gas exchange and an open circulatory system, the single dominating interstitial fluid is the hemolymph. Accordingly, a detailed analysis of hemolymph composition should provide an especially comprehensive picture of chemical communication and defense in animals. Therefore we used an extensive protein fractionation workflow in combination with a discovery-driven proteomic approach to map out the detectable protein composition of hemolymph isolated from Drosophila larvae. Combined mass spectrometric analysis revealed more than 700 proteins extending far beyond the previously known Drosophila hemolymph proteome. Moreover, by comparing hemolymph isolated from either fed or starved larvae, we provide initial provisional insights concerning compositional changes in response to nutritional state. Storage proteins in particular were observed to be strongly reduced by starvation. Our hemolymph proteome catalog provides a rich basis for data mining, as exemplified by our identification of potential novel cytokines, as well as for future quantitative analyses by targeted proteomics.

Hedgehog on the move: a precise spatial control of Hedgehog dispersion shapes the gradient

Hedgehog (Hh) as morphogen directs cell differentiation during development activating various target genes in a concentration dependent manner. The mechanisms that permit controlled Hh dispersion and gradient formation remain controversial. New research in the Drosophila wing disc epithelium has revealed a crucial role of Hh recycling for its release and transportation from source cells. Lipid modifications on Hh mediate key interactions with different elements of the pathway, which balance the retention and release of the molecule through the basolateral side of the epithelium, allowing its tight spatial control. Dispersion of Hh is also determined by its hydrophobic nature, and the mechanisms that include membrane-tethered transport of Hh are increasingly proposed.

Origin and evolution of yolk proteins: expansion and functional diversification of large lipid transfer protein superfamily

Vitellogenin (VTG) and apolipoprotein (APO) play a central role in animal reproduction and lipid circulation, respectively. Although previous studies have examined the structural and functional relationships of these large lipid transfer proteins (LLTPs) from an evolutionary perspective, the mechanism in generating these different families have not been addressed in invertebrates. In this study, the most comprehensive phylogenetic and genomic analysis of the LLTP superfamily genes is carried out. We propose the expansion and diversification of LLTPs in invertebrates are mediated via retrotransposon-mediated duplications, followed by either subfunctionalization or neofunctionalization in different lineages. In agreement with a previous hypothesis, our analysis suggests that all LLTPs originate from a series of duplications of a primitive yolk protein gene similar to VTG. Two early consecutive duplications of the yolk protein genes resulted in the formation of microsomal triglyceride transfer protein (MTP) and the APO gene ancestor. Gains and losses of domains and genes occurred in each of these families in different animal lineages, with MTP becoming truncated. MTP maintained only the components stabilizing the huge lipoprotein particle. Surprisingly, for the first time, two VTG-like protein families were found to independently arise in the lineages of insects. This work consolidates the reconstruction of the evolutionary roadmap of the LLTP superfamily and provides the first mechanistic explanation on the expansion of family members via retrotransposition in invertebrates.

Evolution of yolk protein genes in the Echinodermata

Vitellogenin genes (vtg) encode large lipid transfer proteins (LLTPs) that are typically female-specific, functioning as precursors to major yolk proteins (MYPs). Within the phylum Echinodermata, however, the MYP of the Echinozoa (Echinoidea + Holothuroidea) is expressed by an unrelated transferrin-like gene that has a reproductive function in both sexes. We investigated egg proteins in the Asterozoa (Asteroidea + Ophiuroidea), a sister clade to the Echinozoa, showing that eggs of the asteroid Parvulastra exigua contain a vitellogenin protein (Vtg). vtg is expressed by P. exigua, a species with large eggs and nonfeeding larvae, and by the related asterinid Patiriella regularis which has small eggs and feeding larvae. In the Asteroidea, therefore, the reproductive function of vtg is conserved despite significant life history evolution. Like the echinozoan MYP gene, asteroid vtg is expressed in both sexes and may play a role in the development of both ovaries and testes. Phylogenetic analysis indicated that a putative Vtg from the sea urchin genome, a likely pseudogene, does not clade with asteroid Vtg. We propose the following sequence as a potential pathway for the evolution of YP genes in the Echinodermata: (1) the ancestral echinoderm produced YPs derived from Vtg, (2) bisexual vtg expression subsequently evolved in the echinoderm lineage, (3) the reproductive function of vtg was assumed by a transferrin-like gene in the ancestral echinozoan, and (4) redundant echinozoan vtg was released from stabilizing selection.

Drosophila S2 cells secrete wingless on exosome-like vesicles but the wingless gradient forms independently of exosomes

Wingless acts as a morphogen in Drosophila wing discs, where it specifies cell fates and controls growth several cell diameters away from its site of expression. Thus, despite being acylated and membrane associated, Wingless spreads in the extracellular space. Recent studies have focussed on identifying the route that Wingless follows in the secretory pathway and determining how it is packaged for release. We have found that, in medium conditioned by Wingless-expressing Drosophila S2 cells, Wingless is present on exosome-like vesicles and that this fraction activates signal transduction. Proteomic analysis shows that Wingless-containing exosome-like structures contain many Drosophila proteins that are homologous to mammalian exosome proteins. In addition, Evi, a multipass transmembrane protein, is also present on exosome-like vesicles. Using these exosome markers and a cell-based RNAi assay, we found that the small GTPase Rab11 contributes significantly to exosome production. This finding allows us to conclude from in vivo Rab11 knockdown experiments, that exosomes are unlikely to contribute to Wingless secretion and gradient formation in wing discs. Consistent with this conclusion, extracellularly tagged Evi expressed from a Bacterial Artificial Chromosome is not released from imaginal disc Wingless-expressing cells.

Transcriptional characteristics of gene expression in the midgut of domestic silkworms (Bombyx mori) exposed to phoxim

Silkworm (Bombyx mori) is not only an economically important insect but also a model system for lepidoptera. As a vital organ of digestion and nutrient absorption, the midgut of insects also serves as the first physiological barrier to chemical pesticides. In this study, microarray was performed to profile the gene expression changes in the midgut of silkworms exposed to phoxim. After 24h of phoxim exposure (4.0μg/mL), 266 genes displayed at least 2.0-fold changes in expression levels. Among them, 192 genes were up-regulated, and 74 genes were down-regulated. The most significant changes were 14.88-fold up-regulation and 23.36-fold down-regulation. According to gene ontology annotation and pathway analysis, differentially expressed genes were mainly classified into different groups based on their potential involvements in detoxification, immunne response, stress response, energy metabolism and transport. Particularly, the transcription levels of detoxification-related genes were up-regulated, such as cytochrome P450s, esterases and glutathione-S-transferase (GST), indicating increased detoxification activity in the midgut. Our study provides new insights into the molecular mechanism of pesticide metabolism in the midgut of insects, which may promote the development of highly efficient insecticides.

Lipoproteins in Drosophila melanogaster--assembly, function, and influence on tissue lipid composition

Interorgan lipid transport occurs via lipoproteins, and altered lipoprotein levels correlate with metabolic disease. However, precisely how lipoproteins affect tissue lipid composition has not been comprehensively analyzed. Here, we identify the major lipoproteins of Drosophila melanogaster and use genetics and mass spectrometry to study their assembly, interorgan trafficking, and influence on tissue lipids. The apoB-family lipoprotein Lipophorin (Lpp) is the major hemolymph lipid carrier. It is produced as a phospholipid-rich particle by the fat body, and its secretion requires Microsomal Triglyceride Transfer Protein (MTP). Lpp acquires sterols and most diacylglycerol (DAG) at the gut via Lipid Transfer Particle (LTP), another fat body-derived apoB-family lipoprotein. The gut, like the fat body, is a lipogenic organ, incorporating both de novo-synthesized and dietary fatty acids into DAG for export. We identify distinct requirements for LTP and Lpp-dependent lipid mobilization in contributing to the neutral and polar lipid composition of the brain and wing imaginal disc. These studies define major routes of interorgan lipid transport in Drosophila and uncover surprising tissue-specific differences in lipoprotein lipid utilization.

Silkworm apolipophorin protein inhibits Staphylococcus aureus virulence

Silkworm hemolymph inhibits hemolysin production by Staphylococcus aureus. We purified a factor in the silkworm hemolymph responsible for this inhibitory activity. The final fraction with the greatest specific activity contained 220- and 74-kDa proteins. Determination of the N-terminal amino acid sequence revealed that the 220- and 74-kDa proteins were apolipophorin I and apolipophorin II, respectively, indicating that the factor was apolipophorin (ApoLp). The purified ApoLp fraction showed decreased expression of S. aureus hla encoding α-hemolysin, hlb encoding β-hemolysin, saeRS, and RNAIII, which activate the expression of these hemolysin genes. Injection of an anti-ApoLp antibody into the hemolymph increased the sensitivity of silkworms to the lethal effect of S. aureus. Hog gastric mucin, a mammalian homologue of ApoLp, decreased the expression of S. aureus hla and hlb. These findings suggest that ApoLp in the silkworm hemolymph inhibits S. aureus virulence and contributes to defense against S. aureus infection and that its activity is conserved in mammalian mucin.

Drosophila lipophorin receptors mediate the uptake of neutral lipids in oocytes and imaginal disc cells by an endocytosis-independent mechanism

Lipids are constantly shuttled through the body to redistribute energy and metabolites between sites of absorption, storage, and catabolism in a complex homeostatic equilibrium. In Drosophila, lipids are transported through the hemolymph in the form of lipoprotein particles, known as lipophorins. The mechanisms by which cells interact with circulating lipophorins and acquire their lipidic cargo are poorly understood. We have found that lipophorin receptor 1 and 2 (lpr1 and lpr2), two partially redundant genes belonging to the Low Density Lipoprotein Receptor (LDLR) family, are essential for the efficient uptake and accumulation of neutral lipids by oocytes and cells of the imaginal discs. Females lacking the lpr2 gene lay eggs with low lipid content and have reduced fertility, revealing a central role for lpr2 in mediating Drosophila vitellogenesis. lpr1 and lpr2 are transcribed into multiple isoforms. Interestingly, only a subset of these isoforms containing a particular LDLR type A module mediate neutral lipid uptake. Expression of these isoforms induces the extracellular stabilization of lipophorins. Furthermore, our data indicate that endocytosis of the lipophorin receptors is not required to mediate the uptake of neutral lipids. These findings suggest a model where lipophorin receptors promote the extracellular lipolysis of lipophorins. This model is reminiscent of the lipolytic processing of triglyceride-rich lipoproteins that occurs at the mammalian capillary endothelium, suggesting an ancient role for LDLR–like proteins in this process.

Understanding the complex mechanisms that regulate the storage of caloric surpluses in the form of fat is critical in view of the public health problems caused by the continuous rise of obesity and diabetes. Important advances in the field have been obtained from studies using simple animal models like worms or flies. Here we focus on the molecular mechanisms involved in how cells capture neutral lipids from the extracellular milieu, using the fruit fly Drosophila melanogaster as a model organism. Lipids are transported through the blood or the insect hemolymph as small particles known as lipoproteins. We show that two Drosophila proteins related to the mammalian Low Density Lipoprotein Receptor, Lipophorin Receptor 1 and 2, are essential for the cellular acquisition of neutral lipids from extracellular lipoproteins. We have found that the endocytic uptake of the lipoprotein particles was not required for this process. Instead, we propose that lipophorin receptors favor the extracellular hydrolysis of lipids contained in lipoproteins, followed by uptake of the released free fatty acids. This process is similar to the extracellular processing of lipoproteins that takes place in the capillaries of mammals, suggesting an ancient role for LDLR–related proteins in the extracellular processing of lipoproteins.

Shaping morphogen gradients by proteoglycans

During development, secreted morphogens such as Wnt, Hedgehog (Hh), and BMP emit from their producing cells in a morphogenetic field, and specify different cell fates in a direct concentration-dependent manner. Understanding how morphogens form their concentration gradients to pattern tissues has been a central issue in developmental biology. Various experimental studies from Drosophila have led to several models to explain the formation of morphogen gradients. Over the past decade, one of the main findings in this field is the characterization of heparan sulfate proteoglycan (HSPG) as an essential regulator for morphogen gradient formation. Genetic and cell biological studies have showed that HSPGs can regulate morphogen activities at various steps including control of morphogen movement, signaling, and intracellular trafficking. Here, we review these data, highlighting recent findings that reveal mechanistic roles of HSPGs in controlling morphogen gradient formation.

Comparative and functional genomics of lipases in holometabolous insects

Lipases have key roles in insect lipid acquisition, storage and mobilisation and are also fundamental to many physiological processes underpinning insect reproduction, development, defence from pathogens and oxidative stress, and pheromone signalling. We have screened the recently sequenced genomes of five species from four orders of holometabolous insects, the dipterans Drosophila melanogaster and Anopheles gambiae, the hymenopteran Apis mellifera, the moth Bombyx mori and the beetle Tribolium castaneum, for the six major lipase families that are also found in other organisms. The two most numerous families in the insects, the neutral and acid lipases, are also the main families in mammals, albeit not in Caenorhabditis elegans, plants or microbes. Total numbers of the lipases vary two-fold across the five insect species, from numbers similar to those in mammals up to numbers comparable to those seen in C. elegans. Whilst there is a high degree of orthology with mammalian lipases in the other four families, the great majority of the insect neutral and acid lipases have arisen since the insect orders themselves diverged. Intriguingly, about 10% of the insect neutral and acid lipases have lost motifs critical for catalytic function. Examination of the length of lid and loop regions of the neutral lipase sequences suggest that most of the insect lipases lack triacylglycerol (TAG) hydrolysis activity, although the acid lipases all have intact cap domains required for TAG hydrolysis. We have also reviewed the sequence databases and scientific literature for insights into the expression profiles and functions of the insect neutral and acid lipases and the orthologues of the mammalian adipose triglyceride lipase which has a pivotal role in lipid mobilisation. These data suggest that some of the acid and neutral lipase diversity may be due to a requirement for rapid accumulation of dietary lipids. The different roles required of lipases at the four discrete life stages of holometabolous insects may also contribute to the diversity of lipases required by insects. In addition, insects use lipases to perform roles for which there are no correlates in mammals, including as yolk and male accessory gland proteins.

Circulatory lipid transport: lipoprotein assembly and function from an evolutionary perspective

Circulatory transport of neutral lipids (fat) 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). Latter proteins, which constitute the structural basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride transfer protein (MTP)--another LLTP family member--and bind them by means of amphipathic structures. Comparative research reveals that LLTPs have evolved from the earliest animals and additionally highlights the structural and functional adaptations in these lipid carriers. For instance, in contrast to mammalian apoB, the insect apoB homologue, apoLp-II/I, is post-translationally cleaved by a furin, resulting in their appearance of two non-exchangeable apolipoproteins in the insect low-density lipoprotein (LDL) homologue, high-density lipophorin (HDLp). An important difference between mammalian and insect lipoproteins relates to the mechanism of lipid delivery. Whereas in mammals, endocytic uptake of lipoprotein particles, mediated via members of the LDL receptor (LDLR) family, results in their degradation in lysosomes, the insect HDLp was shown to act as a reusable lipid shuttle which is capable of reloading lipid. Although the recent identification of a lipophorin receptor (LpR), a homologue of LDLR, reveals that endocytic uptake of HDLp may constitute an additional mechanism of lipid delivery, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. Binding studies indicate that the HDLp-LpR complex, in contrast to the LDL-LDLR complex, is resistant to dissociation at endosomal pH as well as by treatment with EDTA mimicking the drop in Ca(2+) concentration in the endosome. This remarkable stability of the ligand-receptor complex may provide a crucial key to the recycling mechanism. Based on the binding and dissociation capacities of mutant and hybrid receptors, the specific binding interaction of the ligand-binding domain of the receptor with HDLp was characterized. These structural similarities and functional adaptations of the lipid transport systems operative in mammals and insects are discussed from an evolutionary perspective.

Regulation of Wnt protein secretion and its role in gradient formation

In metazoans, many developmental and disease-related processes are mediated by Wnt proteins, which are secreted by specific cells to regulate cellular programmes in the surrounding tissue. Although the Wnt-induced signal-transduction cascades are well studied, little is known about how Wnts are secreted. The discovery of Porcupine, an endoplasmic-reticulum-resident acyltransferase, led to closer inspection of the secretory routes of Wnts, and the analysis of Wnt secretion has become an exciting new area of research. Wnt post-translational modifications, interaction partners and subcellular localizations now indicate that Wnt release is tightly regulated. In this review, we summarize recent advances in the field of Wnt secretion and discuss the possibility that separate pathways might regulate the release of lipid-linked morphogens for short-range and long-range signalling.

Multiple roles for lipids in the Hedgehog signalling pathway

The identification of endogenous sterol derivatives that modulate the Hedgehog (Hh) signalling pathway has begun to suggest testable hypotheses for the cellular biological functions of Patched, and for the lipoprotein association of Hh. Progress in the field of intracellular sterol trafficking has emphasized how tightly the distribution of intracellular sterol is controlled, and suggests that the synthesis of sterol derivatives can be influenced by specific sterol-delivery pathways. The combination of this field with Hh studies will rapidly give us a more sophisticated understanding of both the Hh signal-transduction pathway and the cell biology of sterol metabolism.

Patched, the receptor of Hedgehog, is a lipoprotein receptor

The Hedgehog (Hh) family of secreted signaling proteins has a broad variety of functions during metazoan development and implications in human disease. Despite Hh being modified by two lipophilic adducts, Hh migrates far from its site of synthesis and programs cellular outcomes depending on its local concentrations. Recently, lipoproteins were suggested to act as carriers to mediate Hh transport in Drosophila. Here, we examine the role of lipophorins (Lp), the Drosophila lipoproteins, in Hh signaling in the wing imaginal disk, a tissue that does not express Lp but obtains it through the hemolymph. We use the up-regulation of the Lp receptor 2 (LpR2), the main Lp receptor expressed in the imaginal disk cells, to increase Lp endocytosis and locally reduce the amount of available free extracellular Lp in the wing disk epithelium. Under this condition, secreted Hh is not stabilized in the extracellular matrix. We obtain similar results after a generalized knock-down of hemolymph Lp levels. These data suggest that Hh must be packaged with Lp in the producing cells for proper spreading. Interestingly, we also show that Patched (Ptc), the Hh receptor, is a lipoprotein receptor; Ptc actively internalizes Lp into the endocytic compartment in a Hh-independent manner and physically interacts with Lp. Ptc, as a lipoprotein receptor, can affect intracellular lipid homeostasis in imaginal disk cells. However, by using different Ptc mutants, we show that Lp internalization does not play a major role in Hh signal transduction but does in Hh gradient formation.

Malaria vector control in the third millennium: progress and perspectives of molecular approaches

Remarkable progress has been made towards a deeper understanding of mosquito biology since the completion of the Anopheles gambiae Giles genome project. Combined with the development of efficient transgenic technologies for genetic modification of major vector species and the availability of powerful molecular, genetic and bioinformatics tools, this is allowing the identification of genes involved in mosquito biological functions crucial to malaria transmission, ranging from host-seeking behaviour and innate immunity to insecticide resistance. Moreover, population genetic studies are beginning to elucidate the complex structure of vector populations. Finally, novel methods for malaria control are emerging that are based on the use of genetically modified mosquitoes either to interrupt the journey of the Plasmodium parasite within its insect host or to suppress those mosquito species that function as vectors for parasite transmission.

The Spn4 gene from Drosophila melanogaster is a multipurpose defence tool directed against proteases from three different peptidase families

By alternative use of four RSL (reactive site loop) coding exon cassettes, the serpin (serine protease inhibitor) gene Spn4 from Drosophila melanogaster was proposed to enable the synthesis of multiple protease inhibitor isoforms, one of which has been shown to be a potent inhibitor of human furin. Here, we have investigated the inhibitory spectrum of all Spn4 RSL variants. The analyses indicate that the Spn4 gene encodes inhibitors that may inhibit serine proteases of the subtilase family (S8), the chymotrypsin family (S1), and the papain-like cysteine protease family (C1), most of them at high rates. Thus a cohort of different protease inhibitors is generated simply by grafting enzyme-adapted RSL sequences on to a single serpin scaffold, even though the target proteases contain different types and/or a varying order of catalytic residues and are descendents of different phylogenetic lineages. Since all of the Spn4 RSL isoforms are produced as intracellular residents and additionally as variants destined for export or associated with the secretory pathway, the Spn4 gene represents a versatile defence tool kit that may provide multiple antiproteolytic functions.

Structure and expression of the lipophorin-encoding gene of the malaria vector, Anopheles gambiae

Lipophorin is the major hemolymph protein responsible for lipid transport among tissues of insects. This protein may be a lipid source for the development and reproduction of human malaria parasites in mosquitoes, and therefore could be a target to disrupt malaria parasite development in the vector. The lipophorin of Anopheles gambiae was purified by KBr gradient ultracentrifugation and showed variation in density from 1.111 to 1.143 g/ml during development. The amount and density of lipophorin increase in blood-fed females, indicating an adaptation of vitellogenic mosquitoes to an elevated rate of lipid transport to the developing eggs. The A. gambiae lipophorin gene is composed of eight exons and transcribes an mRNA that is 10,516 nucleotides in length. The predicted initial translation product is a preproapoliphorin consisting of 3332 amino acids, which is processed by proteolysis to generate two mature apolipophorins: apolipophorin-I (Mr = 280,000) and apolipophorin-II (Mr = 81,000). The gene is expressed in the fat body tissues throughout development. An elevated transcriptional activity of the lipophorin gene during vitellogenesis is consistent with the presence of putative cis-regulatory elements (GATA and ecdysone responsive elements) in its 3'-end flanking DNA sequence.

Regulation of lipid metabolism genes, lipid carrier protein lipophorin, and its receptor during immune challenge in the mosquito Aedes aegypti

In the mosquito Aedes aegypti, the expression of two fat body genes involved in lipid metabolism, a lipid carrier protein lipophorin (Lp) and its lipophorin receptor (LpRfb), was significantly increased after infections with Gram (+) bacteria and fungi, but not with Gram (-) bacteria. The expression of these genes was enhanced after the infection with Plasmodium gallinaceum. RNA interference (RNAi) knockdown of Lp strongly restricted the development of Plasmodium oocysts, reducing their number by 90%. In Vg-DeltaREL1-A transgenic mosquitoes, with gain-of-function phenotype of Toll/REL1 immune pathway activated after blood feeding, both the Lp and LpRfb genes were overexpressed independently of septic injury. The same phenotype was observed in the mosquitoes with RNAi knockdown of Cactus, an IkappaB inhibitor in the Toll/REL1 pathway. These results showed that, in the mosquito fat body, both Lp and LpRfb gene expression were regulated by the Toll/REL1 pathway during immune induction by pathogen and parasite infections. Indeed, the proximal region of the LpRfb promoter contained closely linked binding motifs for GATA and NF-kappaB transcription factors. Transfection and in vivo RNAi knockdown experiments showed that the bindings of both GATA and NF-kappaB transcription factors to the corresponding motif were required for the induction of the LpRfb gene. These findings suggest that lipid metabolism is involved in the mosquito systemic immune responses to pathogens and parasites.

Functional Genomic Analysis of Midgut Epithelial Responses in Anopheles during Plasmodium Invasion

BACKGROUND

The malaria parasite Plasmodium must complete a complex developmental life cycle within Anopheles mosquitoes before it can be transmitted into the human host. One day after mosquito infection, motile ookinetes traverse the midgut epithelium and, after exiting to its basal site facing the hemolymph, develop into oocysts. Previously, we have identified hemolymph factors that can antagonize or promote parasite development.

RESULTS

We profiled on a genomic scale the transcriptional responses of the A. gambiae midgut to P. berghei and showed that more than 7% of the assessed mosquito transcriptome is differentially regulated during invasion. The profiles suggested that actin- and microtubule-cytoskeleton remodeling is a major response of the epithelium to ookinete penetration. Other responses encompass components of innate immunity, extracellular-matrix remodeling, and apoptosis. RNAi-dependent gene silencing identified both parasite antagonists and agonists among regulators of actin dynamics and revealed that actin polymerization is inhibitory to the invading parasite. Combined transcriptional and reverse-genetic analysis further identified an unexpected dual role of the lipid-trafficking machinery of the hemolymph for both parasite and mosquito-egg development.

CONCLUSIONS

We conclude that the determinants of malaria-parasite development in Anopheles include components not only of systemic humoral immunity but also of intracellular, local epithelial reactions. These results provide novel mechanistic insights for understanding malaria transmission in the mosquito vector.

Lipoprotein particles are required for Hedgehog and Wingless signalling

Wnt and Hedgehog family proteins are secreted signalling molecules (morphogens) that act at both long and short range to control growth and patterning during development. Both proteins are covalently modified by lipid, and the mechanism by which such hydrophobic molecules might spread over long distances is unknown. Here we show that Wingless, Hedgehog and glycophosphatidylinositol-linked proteins copurify with lipoprotein particles, and co-localize with them in the developing wing epithelium of Drosophila. In larvae with reduced lipoprotein levels, Hedgehog accumulates near its site of production, and fails to signal over its normal range. Similarly, the range of Wingless signalling is narrowed. We propose a novel function for lipoprotein particles, in which they act as vehicles for the movement of lipid-linked morphogens and glycophosphatidylinositol-linked proteins.

Biosynthesis and secretion of insect lipoprotein: involvement of furin in cleavage of the apoB homolog, apolipophorin-II/I

The biosynthesis of neutral fat-transporting lipoproteins involves the lipidation of their nonexchangeable apolipoprotein. In contrast to its mammalian homolog apolipoprotein B, however, insect apolipophorin-II/I (apoLp-II/I) is cleaved posttranslationally at a consensus substrate sequence for furin, resulting in the appearance of two apolipoproteins in insect lipoprotein. To characterize the cleavage process, a truncated cDNA encoding the N-terminal 38% of Locusta migratoria apoLp-II/I, including the cleavage site, was expressed in insect Sf9 cells. This resulted in the secretion of correctly processed apoLp-II and truncated apoLp-I. The cleavage could be impaired by the furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (decRVKRcmk) as well as by mutagenesis of the consensus substrate sequence for furin, as indicated by the secretion of uncleaved apoLp-II/I-38. Treatment of L. migratoria fat body, the physiological site of lipoprotein biosynthesis, with decRVKRcmk similarly resulted in the secretion of uncleaved apoLp-II/I, which was integrated in lipoprotein particles of buoyant density identical to wild-type high density lipophorin (HDLp). These results show that apoLp-II/I is posttranslationally cleaved by an insect furin and that biosynthesis and secretion of HDLp can occur independent of this processing step. Structure modeling indicates that the cleavage of apoLp-II/I represents a molecular adaptation in homologous apolipoprotein structures. We propose that cleavage enables specific features of insect lipoproteins, such as low density lipoprotein formation, endocytic recycling, and involvement in coagulation.

Molecular characterization of a cDNA encoding vitellogenin in the coonstriped shrimp, Pandalus hypsinotus and site of vitellogenin mRNA expression

In order to determine the primary structure of vitellogenin in a protandric species, the coonstriped shrimp Pandalus hypsinotus, we previously purified four vitellin components (designated as VnA, VnB, VnC, and VnD, respectively), and chemically analyzed their partial amino acid sequences. In this study, we subsequently cloned a cDNA encoding vitellogenin in this species based on the N-terminal and internal amino acid sequences of VnA, as well as the N-terminal sequence of VnC. The open reading frame of this cDNA encoded a pro-vitellogenin in which vitellins were arranged as follows: NH2-VnA-VnB-VnC/D-COOH. The deduced amino acid sequence possessed a single consensus cleavage sequence, R-X-K/R-R, along the lines of vitellogenins reported in other crustaceans and insects, and the N-terminal sequence of VnB was immediately preceded by this sequence. The comparison of primary structures revealed the existence of a basic and characteristic structure for the vitellogenin molecule in decapod crustacean species, and phylogenetic analysis reflected the current taxonomic classifications of Crustacea. An approximately 8 kb-long transcript of the vitellogenin gene was detected in the hepatopancreas of female shrimps having a gonadosomatic index higher than 1.0 by Northern blot analysis, but was not observed in the hepatopancreas and gonads of male shrimps and the hepatopancreas of female shrimps having a gonadosomatic index lower than 1.0. These results indicate that the hepatopancreas is responsible for vitellogenin synthesis.

Utilization of a new biotinylation reagent in the development of a nondiscriminatory investigative approach for the study of cell surface proteins

In order to circumvent the various problems encountered during the study of membrane-bound proteins, we designed and synthesized a novel membrane-impermeable biotinylation reagent incorporating chemical properties compatible with this goal. We then developed a nondiscriminatory analytical procedure for such studies which overcomes possible selectivity, contamination and solubility problems. The necessary steps (labeling, limited in situ proteolysis, affinity purification) are all conducted in mild or near native conditions. This versatile method could provide an accurate picture of the cell surface proteome.

Peptidomic and proteomic analyses of the systemic immune response of Drosophila

Insects have developed an efficient host defense against microorganisms, which involves humoral and cellular mechanisms. Numerous data highlight similarities between defense responses of insects and innate immunity of mammals. The fruit fly, Drosophila melanogaster, is a favorable model system for the analysis of the first line defense against microorganisms. Taking advantages of improvements in mass spectrometry (MS), two-dimensional (2D) gel electrophoresis and bioinformatics, differential analyses of blood content (hemolymph) from immune-challenged versus control Drosophila were performed. Two strategies were developed: (i) peptidomic analyses through matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS and high performance liquid chromatography for molecules below 15 kDa, and (ii) proteomic studies based on 2D gel electrophoresis, MALDI-TOF fingerprinting and database searches, for compounds of greater molecular masses. The peptidomic strategy led to the detection of a large number of peptides induced in the hemolymph of challenged flies as compared to controls. Of these, 28 were characterized, amongst which were antimicrobial peptides. The 2D gel electrophoresis strategy led to the detection of 70 spots differentially regulated by at least fivefold after microbial infection. This approach yielded the identity of a series of proteins that were related to the Drosophila immune response, such as proteases, protease inhibitors, prophenoloxydase-activating enzymes, serpins and a Gram-negative binding protein-like protein. This strategy also brought to light new candidates with a potential function in the immune response (odorant-binding protein, peptidylglycine alpha-hydroxylating monooxygenase and transferrin). Interestingly, several molecules resulting from the cleavage of proteins were detected after a fungal infection. Together, peptidomic and proteomic analyses represent new tools to characterize molecules involved in the innate immune reactions of Drosophila.

Gene expression changes presage neurodegeneration in a Drosophila model of Parkinson's disease

Transgenic Drosophila expressing human alpha-synuclein faithfully replicate essential features of human Parkinson's disease, including age-dependent loss of dopaminergic neurons, Lewy-body-like inclusions and locomotor impairment. To define the transcriptional program encoding molecular machinery involved in alpha-synuclein pathology, we characterized expression of the entire Drosophila genome at pre-symptomatic, early and advanced disease stages. Fifty-one signature transcripts, including lipid, energy and membrane transport mRNAs, were tightly associated with alpha-synuclein expression. Most importantly, at the pre-symptomatic stage, when the potential for neuroprotection is greatest, expression changes revealed specific pathology. In age-matched tau transgenic Drosophila, the transcription of alpha-synuclein associated genes was normal, suggesting highly distinct pathways of neurodegeneration. Temporal profiling of progressive gene expression changes in neurodegenerative disease models provides unbiased starting points for defining disease mechanisms and for identifying potential targets for neuroprotective drugs at pre-clinical stages.

A Drosophila microsomal triglyceride transfer protein homolog promotes the assembly and secretion of human apolipoprotein B. Implications for human and insect transport and metabolism

The assembly and secretion of triglyceride-rich lipoproteins in vertebrates requires apolipoprotein B (apoB) and the endoplasmic reticulum-localized cofactor, microsomal triglyceride transfer protein (MTP). Invertebrates, particularly insects, transport the majority of their neutral and polar lipids in lipophorins; however, the assembly of lipophorin precursor particles was presumed to be MTP-independent. A Drosophila melanogaster expressed gene sequence (CG9342), displaying 23% identity with human MTP, was recently identified. When coexpressed in COS cells, CG9342 promoted the assembly and secretion of apoB34 and apoB41 (N-terminal 34 and 41% of human apoB). The apoB34-containing particles assembled by human MTP and CG9342 displayed similar peak densities of approximately 1.169 g/ml and similar lipid compositions. However, CG9342 displayed differential sensitivities to two inhibitors of human MTP and low vesicle-based lipid transfer activity, in vitro. In addition, important predicted structural distinctions exist between the human and Drosophila proteins suggesting overlapping but not identical functional roles. We conclude that CG9342 and human MTP are orthologs that share only a subset of functions, consistent with known differences in intracellular and extracellular aspects of vertebrate and invertebrate lipid transport and metabolism.

Instability of crab vitellogenin and its immunological relatedness with mammalian atherogenic lipoproteins

Vitellogenesis is the process of accumulation of vitellogenin (Vg) in rapidly growing oocytes of oviparous animals and its' subsequent transformation into lipovitellin (Lv). Lipovitellin, which forms the major yolk protein, serves as a principal nutrient reserve for the developing embryo. In the present study, Vg and Lv were purified from the hemolymph and ovary, respectively of the crab Scylla serrata by gel filtration followed by preparative gel electrophoresis. It was observed that purified Vg, but not Lv, possessed an intrinsic protease activity with which it underwent autoproteolysis giving rise to several smaller proteins. Furthermore, urea-mediated unfolding studies by UV-spectral analysis revealed clearly that Vg was easily disrupted by urea whereas Lv was resistant. Taken together, these results suggest that although Lv had a stable conformation, its precursor Vg was labile and highly sensitive to degradation. Another aspect that was investigated in the present study was the immunological kinship of crab Vg and Lv to mammalian atherogenic lipoproteins, the low density lipoprotein (LDL), very low density lipoprotein (VLDL), and apolipoprotein B (apoB). By Western blot analysis, it was demonstrated that crab Vg and Lv were immunoreactive to antibodies to human LDL, VLDL, and apoB. These observations suggest the existence of common epitope recognition sites in crab Vg and mammalian lipid transferring proteins. This corroborates well with our earlier study on the recognition of crab Vg receptor by mammalian lipoproteins.

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.