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

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.

Lipophorin receptor is required for the accumulations of cuticular hydrocarbons and ovarian neutral lipids in Locusta migratoria

Lipophorin is the most abundant lipoprotein particle in insect hemolymph. Lipophorin receptor (LPR) is a glycoprotein that binds to the lipophorin and mediates cellular uptake and metabolism of lipids by endocytosis. However, the roles of LPR in uptake of lipids in the integument and ovary remain unknown in the migratory locust (Locusta migratoria). In present study, we characterized the molecular properties and biological roles of LmLPR in L. migratoria. The LmLPR transcript level was high in the first 2 days of the adults after eclosion, then gradually declined. LmLPR was predominately expressed in fat body, ovary and integument. Using immuno-detection methods, we revealed that LmLPR was mainly localized in the membrane of oenocytes, epidermal cells, fat body cells and follicular cells. RNAi-mediated silencing of LmLPR led to a slight decrease of the cuticle hydrocarbon contents but with little effect on the cuticular permeability. However, the neutral lipid content was significantly decreased in the ovary after RNAi against LmLPR, which led to a retarded ovarian development. Taken together, our results indicated that LmLPR is involved in the uptake and accumulation of lipids in the ovary and plays a crucial role in ovarian development in L. migratoria. Therefore, LmLPR could be a promising RNAi target for insect pest management by disrupting insect ovarian development.

Lipophorin receptor regulates the cuticular hydrocarbon accumulation and adult fecundity of the pea aphid Acyrthosiphon pisum

Cuticular hydrocarbons form a barrier that protects terrestrial insects from water loss via the epicuticle. Lipophorin loads and transports lipids, including hydrocarbons, from one tissue to another. In some insects, the lipophorin receptor (LpR), which binds to lipophorin and accepts its lipid cargo, is essential for female fecundity because it mediates the incorporation of lipophorin by developing oocytes. However, it is unclear whether LpR is involved in the accumulation of cuticular hydrocarbons and its precise role in aphid reproduction remains unknown. We herein present the results of our molecular characterization, phylogenetic analysis, and functional annotation of the pea aphid (Acyrthosiphon pisum) LpR gene (ApLpR). This gene was transcribed throughout the A. pisum life cycle, but especially during the embryonic stage and in the abdominal cuticle. Furthermore, we optimized the RHA interference (RNAi) parameters by determining the ideal dose and duration for gene silencing in the pea aphid. We observed that the RNAi-based ApLpR suppression significantly decreased the internal and cuticular hydrocarbon contents as well as adult fecundity. Additionally, a deficiency in cuticular hydrocarbons increased the susceptibility of aphids to desiccation stress, with decreased survival rates under simulated drought conditions. Moreover, ApLpR expression levels significantly increased in response to the desiccation treatment. These results confirm that ApLpR is involved in transporting hydrocarbons and protecting aphids from desiccation stress. Furthermore, this gene is vital for aphid reproduction. Therefore, the ApLpR gene of A. pisum may be a novel RNAi target relevant for insect pest management.

Lipid metabolism in insect disease vectors

More than a third of the world population is at constant risk of contracting some insect-transmitted disease, such as Dengue fever, Zika virus disease, malaria, Chagas' disease, African trypanosomiasis, and others. Independent of the life cycle of the pathogen causing the disease, the insect vector hematophagous habit is a common and crucial trait for the transmission of all these diseases. This lifestyle is unique, as hematophagous insects feed on blood, a diet that is rich in protein but relatively poor in lipids and carbohydrates, in huge amounts and low frequency. Another unique feature of these insects is that blood meal triggers essential metabolic processes, as molting and oogenesis and, in this way, regulates the expression of various genes that are involved in these events. In this paper, we review current knowledge of the physiology and biochemistry of lipid metabolism in insect disease vectors, comparing with classical models whenever possible. We address lipid digestion and absorption, hemolymphatic transport, and lipid storage by the fat body and ovary. In this context, both de novo fatty acid and triacylglycerol synthesis are discussed, including the related fatty acid activation process and the intracellular lipid binding proteins. As lipids are stored in order to be mobilized later on, e.g. for flight activity or survivorship, lipolysis and β-oxidation are also considered. All these events need to be finely regulated, and the role of hormones in this control is summarized. Finally, we also review information about infection, when vector insect physiology is affected, and there is a crosstalk between its immune system and lipid metabolism. There is not abundant information about lipid metabolism in vector insects, and significant current gaps in the field are indicated, as well as questions to be answered in the future.

Molecular characterization of the lipophorin receptor in the crustacean ectoparasite Lepeophtheirus salmonis

The Salmon louse (Lepeophtheirus salmonis) is a marine ectoparasite of salmonid fish in the Northern Hemisphere and considered as a major challenge in aquaculture and a threat to wild populations of salmonids. Adult female lice produce a large number of lipid-rich eggs, however, the mechanism of maternal lipid transport into developing eggs during salmon louse reproduction has not been described. In the present study, a full-length L. salmonis lipophorin receptor (LsLpR) consisting of 16 exons was obtained by RACE and RT-PCR. The predicted ORF was 952 amino acids and structural analysis showed five functional domains that are similar to LpR of insects and decapods. Phylogenetic analysis placed the LsLpR together with LpRs from decapods and insects. Expression analysis revealed that the relative abundance of LsLpR transcripts was highest in the larvae and adult female lice. In adult females, the LsLpR transcripts and protein were found in the ovary and vitellogenic oocytes whereas, in larvae, the LsLpR transcripts were found in the neuronal somata of the brain and the intestine. Oil Red O stain results revealed that storage of neutral lipids was found in vitellogenic oocytes and ovaries of adult females, and in the yolk of larvae. Moreover, RNA interference (RNAi) was conducted to demonstrate the function of LsLpR in reproduction and lipid metabolism in L. salmonis. In larvae, the transcription of LsLpR was decreased by 44–54% while in an experiment LsLpR knockdown female lice produced 72% less offspring than control lice.

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.

Four cDNAs encoding lipoprotein receptors from shrimp (Pandalopsis japonica): structural characterization and expression analysis during maturation

As in all other oviparous animals, lipoprotein receptors play a critical role in lipid metabolism and reproduction in decapod crustaceans. Four full-length cDNAs encoding lipoprotein receptors (Paj-VgR, Paj-LpR1, Paj-LpR2A, and Paj-LpR2B) were identified from Pandalopsis japonica through a combination of EST screening and PCR-based cloning. Paj-LpR1 appears to be the first crustacean ortholog of insect lipophorin receptors, and its two paralogs, Paj-LpR2A and Paj-LpR2B, exhibited similar structural characteristics. Several transcriptional isoforms were also identified for all three Paj-LpRs. Each expression pattern was unique, suggesting different physiological roles for these proteins. Paj-VgR is an ortholog of vitellogenin (Vg) receptors from other decapod crustaceans. A phylogenetic analysis of lipoproteins and their receptors suggested that the nomenclature of Vgs from decapod crustaceans may need to be changed. A PCR-based transcriptional analysis showed that Paj-VgR and Paj-LpR2B are expressed almost exclusively in the ovary, whereas Paj-LpR1 and Paj-LpR2A are expressed in multiple tissues. The various transcriptional isoforms of the three Paj-LpRs exhibited unique tissue distribution profiles. A transcriptional analysis of each receptor using tissues with different GSI values showed that the change in transcription of Paj-VgRs, Paj-LpR2A and Paj-LpR1 was not as significant as that of Vgs during maturation. However, the transcriptional levels of Paj-LpR2B decreased in ovary at maturation, suggesting that their transcriptional regulation is involved in reproduction.

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.

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.

Characterization of lipophorin receptor (LpR) mediating the binding of high density lipophorin (HDLp) in the silkworm, Bombyx mori

In an earlier report, we described the gene encoding a lipophorin receptor (LpR) of the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae), and recombinant expression of the protein. The present study was performed to characterize the corresponding native BmLpR and its binding characteristics. Polyclonal anti-LpR antibody prepared against the cloned receptor fragment from the cytoplasmic domain specifically detected the receptor. Through immunoblotting, ovary and brain membrane protein samples of BmLpR have shown an apparent molecular mass of 105 kDa and 120 kDa under nonreducing and reducing conditions, respectively. Ligand binding of LpR supported the immunoblot results. It bound to high density lipophorin (HDLp) and has shown requirement of Ca(2+) in binding. Further, a dose-dependent inhibition by EDTA was observed in receptor ligand binding. The characteristics of the BmLpR protein confirm the properties of a ligand-receptor interaction similar to that of vertebrate low density lipoprotein receptor (LDLR).

Molecular cloning, characterization, expression pattern and cellular distribution of an ovarian lipophorin receptor in the cockroach, Leucophaea maderae

A cDNA that encodes a lipophorin receptor (LpR) with a predicted structure similar to that of the low density lipoprotein receptor (LDLR) gene superfamily was cloned from ovaries of the cockroach, Leucophaea maderae (Lem) and characterized. This is the first LpR sequenced from the order Dictyoptera. The cDNA has a length of 3362 bp coding for an 888-residue mature protein with a predicted molecular mass of ~99.14 kDa and a pI value of 4.68. The deduced amino acid sequence showed that the LemLpR harbours eight ligand-binding repeats (LBRs) at the N-terminus similar to the other insect LpRs, and thus resembles vertebrate VLDLRs. In addition to eight tandemly arranged LBRs, the five-domain receptor contains an O-linked sugar region and the classic LDLR internalization signal, FDNPVY. Northern blot analysis revealed the presence of ~4.0 kb ovarian mRNA that was transcribed throughout oogenesis with its peak especially during late previtellogenic and vitellogenic periods (from days 3 to 11). LpR transcript(s) or homologues of LDLRs were also detected in the head, midgut, Malpighian tubules, muscles and in the fat body. RNA in situ hybridization and immunocytochemistry localized the LpR mRNA and protein to germ line-derived cells, the oocytes, and revealed that LpR gene transcription and translation starts very early during oocyte differentiation in the germarium. LpR protein was evenly distributed throughout the cytoplasm during previtellogenic periods of oogenesis. However, during vitellogenic stages, the receptor was accumulated mainly in the cortex of the oocyte. Immunoblot analysis probed an ovarian LpR protein of ~115 and 97 kDa under reducing and nonreducing conditions, respectively. The protein signal appeared on day 2, increased every day and was high during vitellogenic periods from day 4 to day 7. Southern blot analysis suggested the presence of a single copy of the LpR gene in the genome of Le. maderae.

Biochemical and cellular characterization of lipophorin-midgut interaction in the hematophagous Panstrongylus megistus (Hemiptera: Reduviidae)

In order to better understand the metabolism of dietary lipids in hematophagous insects, we have performed a biochemical and cellular characterization of lipophorin (Lp)-midgut interaction in Panstrongylus megistus, a vector of Chagas' disease. The study was accomplished by solid-phase binding assays or with iodinated Lp ((125)I-Lp), using midgut membranes from fifth instar nymphs after ecdysis and after insects received a blood meal. Results obtained from both physiological conditions indicated that Lp interacted specifically with the midgut, implying the participation of receptors. Binding capacity of lipophorin to membranes was dependent on the amount of membranes added in the system, reaching saturation at 0.1 microg/ml. However, membranes obtained after a blood meal exhibited higher binding activity. Saturation kinetics results using (125)I-Lp suggested a single binding site with high affinity for Lp in the midgut membranes (K(d) = 5.1 +/- 3.6 x 10(-8) M). The unrelated lipoprotein, human LDL, did not compete with Lp for its binding site in the midgut. The binding was dependent on pH and the treatment of membranes with trypsin or heat causes a significant inhibition of the binding. Midgut-Lp interaction was affected by changes in ionic strength and by suramin, but showed no requirement of calcium. Ligand blotting assays revealed two membrane proteins that specifically bound Lp (61 and 45 kDa). At cellular level, Lp binding sites were located mainly at the basal plasma membrane of isolated enterocytes. Labeled Lp with fluorescent probes directed to its proteins or its phospholipids fraction co-localized mainly at the basement membrane of the midgut. In addition, no intracellular Lp was observed at any condition. The lack of an endocytic pathway for Lp in the midgut of P. megistus is analyzed in the context of insect physiology.

Insect vitellogenin/lipophorin receptors: molecular structures, role in oogenesis, and regulatory mechanisms

Insect vitellogenin and lipophorin receptors (VgRs/LpRs) belong to the low-density lipoprotein receptor (LDLR) gene superfamily and play a critical role in oocyte development by mediating endocytosis of the major yolk protein precursors Vg and Lp, respectively. Precursor Vg and Lp are synthesized, in the majority of insects, extraovarially in the fat body and are internalized by competent oocytes through membrane-bound receptors (i.e., VgRs and LpRs, respectively). Structural analysis reveals that insect VgRs/LpRs and all other LDLR family receptors share a group of five structural domains: clusters of cysteine-rich repeats constituting the ligand-binding domain (LBD), epidermal growth factor (EGF)-precursor homology domain that mediates the acid-dependent dissociation of ligands, an O-linked sugar domain of unknown function, a transmembrane domain anchoring the receptor in the plasma membrane, and a cytoplasmic domain that mediates the clustering of the receptor into the coated pits. The sequence analysis indicates that insect VgRs harbor two LBDs with five repeats in the first and eight repeats in the second domain as compared to LpRs which have a single 8-repeat LBD. Moreover, the cytoplasmic domain of all insect VgRs contains a LI internalization signal instead of the NPXY motif found in LpRs and in the majority of other LDLR family receptors. The exception is that of Solenopsis invicta VgR, which also contains an NPXY motif in addition to LI signal. Cockroach VgRs still harbor another motif, NPTF, which is also believed to be a functional internalization signal. The expression studies clearly demonstrate that insect VgRs are ovary-bound receptors of the LDLR family as compared to LpRs, which are transcribed in a wide range of tissues including ovary, fat body, midgut, brain, testis, Malpighian tubules, and muscles. VgR/LpR mRNA and the protein were detected in the germarium, suggesting that the genes involved in receptor-endocytotic machinery are specifically expressed long before they are functionally required.

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.

Expression analysis of putative vitellogenin and lipophorin receptors in honey bee (Apis mellifera L.) queens and workers

Two members of the low density lipoprotein receptor (LDLR) family were identified as putative orthologs for a vitellogenin receptor (Amvgr) and a lipophorin receptor (Amlpr) in the Apis mellifera genome. Both receptor sequences have the structural motifs characteristic of LDLR family members and show a high degree of similarity with sequences of other insects. RT-PCR analysis of Amvgr and Amlpr expression detected the presence of both transcripts in different tissues of adult female (ovary, fat body, midgut, head and specifically hypopharyngeal gland), as well as in embryos. In the head RNA samples we found two variant forms of AmLpR: a full length one and a shorter one lacking 29 amino acids in the O-linked sugar domain. In ovaries the expression levels of the two honey bee LDLR members showed opposing trends: whereas Amvgr expression was upregulated as the ovaries became activated, Amlpr transcript levels gradually declined. In situ hybridization analysis performed on ovaries detected Amvgr mRNA exclusively in germ line cells and corroborated the qPCR results showing an increase in Amvgr gene expression concomitant with follicle growth.

Dynamics of lipid accumulation by the fat body of Rhodnius prolixus: the involvement of lipophorin binding sites

In insects, lipids are stored in the fat body, mainly as triacylglycerol (TAG). In Rhodnius prolixus, a hematophagous hemipteran, lipids are accumulated after blood meal to be used later on. In adult females, at the second day after feeding, the amount of TAG was 57+/-17 microg/fat body, it increased almost five times and at fourth day it was 244+/-35 microg/fat body. TAG content remained constant until day 13, but it then decreased and, at day 20th it was very low (31+/-4.9 microg/fat body). Radiolabeled free fatty acid was used to follow lipid accumulation by the fat body, as it was previously shown that, in R. prolixus, injected free fatty acids associate with lipophorin, a major hemolymphatic lipoprotein. (3)H-palmitic acid was injected into the hemocoel of R. prolixus females. It disappeared from the hemolymph very rapidly, and radioactivity was incorporated by the fat body. Sixty minutes after injection, radioactivity in the fat body was found mainly in TAGs. The capacity of the fat body to incorporate fatty acids from the hemolymph varied according to the days after blood meal, and it was maximal around the fourth day. Lipophorin binding to specific sites in fat body membrane preparations also showed variation at different days. When membranes obtained from insects at the second, fifth and tenth days were compared, binding was highest at fifth day after feeding.

Structural and RNAi characterization of the German cockroach lipophorin receptor, and the evolutionary relationships of lipoprotein receptors

BACKGROUND

Lipophorin receptors (LpRs) have been described in a number of insects, but functional studies have been reported only in locusts and mosquitoes. The aim of the present work was to characterize the LpR of the cockroach Blattella germanica, not only molecularly but also functionally using RNAi techniques, and to place LpRs in a phylogenetical context among lipoprotein receptors.

RESULTS

We cloned a putative LpR from B. germanica (BgLpR) using RT-PCR methods. Two isoforms of BgLpR that differ from each other by an insertion/deletion of 24 amino acids were obtained from the fat body and the ovary. A phylogenetical analysis of lipoprotein receptors showed that BgLpR grouped with other sequences annotated as LpR in a cluster placed as a sister group of vertebrate low density lipoprotein receptors (LDLR) + lipoprotein receptor-related proteins 8 (LPR8) + vitellogenin receptors (VgR) + very low density lipoprotein receptors (VLDLR). The two BgLpR isoforms are expressed in different adult female tissues (fat body, ovary, brain, midgut, muscle) and in embryos. In ovaries and fat body, the two isoforms are similarly expressed during the first gonadotrophic cycle. mRNA levels in the fat body increase in parallel to vitellogenesis, whereas they decrease in the ovaries. BgLpR protein levels increase in parallel to vitellogenesis in both organs. Treatment with juvenile hormone increases BgLpR protein. RNAi experiments show that females with lower BgLpR expression have less lipophorin in the growing oocytes with respect to controls.

CONCLUSION

The two isoforms of BgLpR are structurally similar to other LpRs. Phylogenetical analyses show that LpRs and the group formed by vertebrate LDLR + LPR8 + VgR + VLDLR, diverged from a common ancestor and diversified in parallel. The different expression patterns in the fat body and the ovary, comparing mRNA and protein, indicate that the corresponding mechanisms regulating BgLpR expression are different. Experiments with JH III suggest that such a hormone regulates the expression of BgLpR posttranscriptionally. RNAi experiments indicate that BgLpR is a functional lipophorin receptor.

Specialized hepatocyte-like cells regulate Drosophila lipid metabolism

Lipid metabolism is essential for growth and generates much of the energy needed during periods of starvation. In Drosophila, fasting larvae release large quantities of lipid from the fat body but it is unclear how and where this is processed. Here we identify the oenocyte as the principal cell type accumulating lipid droplets during starvation. Tissue-specific manipulations of the Slimfast amino-acid channel, the Lsd2 fat-storage regulator and the Brummer lipase indicate that oenocytes act downstream of the fat body. In turn, oenocytes are required for depleting stored lipid from the fat body during fasting. Hence, lipid-metabolic coupling between the fat body and oenocytes is bidirectional. When food is plentiful, oenocytes have critical roles in regulating growth, development and feeding behaviour. In addition, they specifically express many different lipid-metabolizing proteins, including Cyp4g1, an omega-hydroxylase regulating triacylglycerol composition. These findings provide evidence that some lipid-processing functions of the mammalian liver are performed in insects by oenocytes.

Anopheles gambiae lipophorin: characterization and role in lipid transport to developing oocyte

Lipid transport in arthropods is achieved by highly specialized lipoproteins, which resemble those described in vertebrate blood. Here, we describe purification and characterization of the lipid-apolipoprotein complex, lipophorin (Lp), in the malaria vector mosquito Anopheles gambiae. We also describe the Lp-mediated lipid transfer to developing eggs and the distribution of the imported lipid in developing embryos. The density of the Lp complex was 1.135 g/ml with an apparent molecular weight of 630 kDa. It is composed of two major polypeptides, apoLp I (260 kDa) and apoLp II (74 kDa) and composed of 50% protein, 48% lipid and 2% carbohydrate (w/w). Hydrocarbon, cholesterol, phosphatidyl choline, phosphatidyl ethanolamine, cholesteryl ester and diacylglyceride were the major Lp-associated lipids. Using fluorescently tagged lipids, we observed patterns that suggest that in live developing oocytes, the Lp was taken up by a receptor-mediated endocytic process. Such process was blocked at low temperature and in the presence of excess unlabeled Lp, but not by bovine serum albumin. Imported Lp was segregated in the spherical yolk bodies (mean size 1.8 microm) and distributed evenly in the cortex of the oocyte. In embryonic larvae, before hatching, a portion of the fatty acid in vesicles was found evenly distributed along the body, whereas portion of phospholipids was accumulated in the intestine.

Lipophorin receptor of Bombyx mori: cDNA cloning, genomic structure, alternative splicing, and isolation of a new isoform

The cDNA and genomic structure of a putative lipophorin receptor from the silkworm, Bombyx mori (BmLpR), indicated the presence of four isoforms, designated LpR1, LpR2, LpR3, and LpR4. The deduced amino acid sequence of each isoform showed five functional domains that are homologous to vertebrate very low density lipoprotein receptor (VLDLR). All four isoforms seem to have originated from a single gene by alternative splicing and were differentially expressed in a tissue- and stage-specific manner. BmLpR1 harbored an additional 27 amino acids in the O-linked sugar domain, resulting in an extra exon. The silkworm BmLpR gene consisted of 16 exons separated by 15 introns spanning >122 kb and was at least three times larger than the human VLDLR gene. Surprisingly, one of the isoforms, LpR4, was expressed specifically in the brain and central nervous system. Additionally, it had a unique cytoplasmic tail, leading to the proposition that it represents a new candidate LpR for possible brain-related function(s). This is the first report on the genomic characterization of an arthropod lipoprotein receptor gene and the identification of a brain-specific receptor variant from a core member of the low density lipoprotein receptor family in invertebrates.

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.

Intracellular fate of LDL receptor family members depends on the cooperation between their ligand-binding and EGF domains

The insect low-density lipoprotein (LDL) receptor (LDLR) homologue LpR mediates endocytosis of an insect lipoprotein (lipophorin) that is structurally related to LDL. Despite these similarities, lipophorin and LDL follow distinct intracellular routes upon endocytosis by their receptors. Whereas LDL is degraded in lysosomes, lipophorin is recycled in a transferrin-like manner. We constructed several hybrid receptors composed of Locusta migratoria LpR and human LDLR regions to identify the domains implicated in LpR-mediated ligand recycling. Additionally, the triadic His562 residue of LDLR, which is putatively involved in ligand uncoupling, was mutated to Asn, corresponding to Asn643 in LpR, to analyse the role of the His triad in receptor functioning. The familial hypercholesterolaemia (FH) class 5 mutants LDLRH562Y and LDLRH190Y were also analysed in vitro. Fluorescence microscopic investigation and quantification suggest that LpR-mediated ligand recycling involves cooperation between the ligand-binding domain and epidermal growth factor (EGF) domain of LpR, whereas its cytosolic tail does not harbour motifs that affect this process. LDLR residue His562 appears to be essential for LDLR recycling after ligand endocytosis but not for constitutive receptor recycling. Like LDLRH562N, LDLRH562Y did not recycle bound ligand; moreover, the intracellular distribution of both mutant receptors after ligand incubation coincides with that of a lysosomal marker. The LDLR mutant characterization in vitro suggests that LDLR FH class 5 mutations might be divided into two subclasses.

Receptor-mediated endocytosis and intracellular trafficking of lipoproteins and transferrin in insect cells

While the intracellular pathways of ligands after receptor-mediated endocytosis have been studied extensively in mammalian cells, in insect cells these pathways are largely unknown. We transfected Drosophila Schneider line 2 (S2) cells with the human low-density lipoprotein (LDL) receptor (LDLR) and transferrin (Tf) receptor (TfR), and used endocytosis of LDL and Tf as markers. After endocytosis in mammalian cells, LDL is degraded in lysosomes, whereas Tf is recycled. Fluorescence microscopy analysis revealed that LDL and Tf are internalized by S2 cells transfected with LDLR or TfR, respectively. In transfectants simultaneously expressing LDLR and TfR, both ligands colocalize in endosomes immediately after endocytic uptake, and their location remained unchanged after a chase. Similar results were obtained with Spodoptera frugiperda Sf9 cells that were transfected with TfR, suggesting that Tf is retained intracellularly by both cell lines. The insect lipoprotein, lipophorin, is recycled upon lipophorin receptor (LpR)-mediated endocytosis by mammalian cells, however, not after endocytosis by LpR-expressing S2 transfectants, suggesting that this recycling mechanism is cell-type specific. LpR is endogenously expressed by fat body tissue of Locusta migratoria for a limited period after an ecdysis. A chase following endocytosis of labeled lipophorin by isolated fat body tissue at this developmental stage resulted in a significant decrease of lipophorin-containing vesicles, indicative of recycling of the ligand.

Tissue- and stage-specific expression of two lipophorin receptor variants with seven and eight ligand-binding repeats in the adult mosquito

We identified two splice variants of lipophorin receptor (LpR) gene products specific to the mosquito fat body (AaLpRfb) and ovary (AaLpRov) with respective molecular masses of 99.3 and 128.9 kDa. Each LpR variant encodes a member of the low density lipoprotein receptor family with five characteristic domains: 1) ligand recognition, 2) epidermal growth factor precursor, 3) putative O-linked sugar, 4) single membrane-spanning domains, and 5) the cytoplasmic tail with a highly conserved internalization signal FDNPVY. Proposed phylogenetic relationships among low density lipoprotein receptor superfamily members suggest that the LpRs of insects are more closely related to vertebrate low density lipoprotein receptors and very low density lipoprotein receptor/vitellogenin receptor than to insect vitellogenin receptor/yolk protein receptors. Two mosquito LpR isoforms differ in their amino termini, the ligand-binding domains, and O-linked sugar domains, which are generated by differential splicing. Polymerase chain reaction and Southern blot hybridization analyses show that these two transcripts originated from a single gene. Significantly, the putative ligand-binding domain consists of seven and eight complement-type, cysteine-rich repeats in AaLpRfb and AaLRov, respectively. Seven cysteine-rich repeats in AaLpRfb are identical to the second through eighth repeats of AaLpRov. Previous analyses (1) have indicated that the AaLpRov transcript is present exclusively in ovarian germ-line cells, nurse cells, and oocytes throughout the previtellogenic and vitellogenic stages, with the peak at 24-30 h after blood meal, coincident with the peak of yolk protein uptake. In contrast, the fat body-specific AaLpRfb transcript expression is restricted to the postvitellogenic period, during which yolk protein production is terminated and the fat body is transformed to a storage depot of lipid, carbohydrate, and protein.