Lipid transfer particle catalyzes transfer of carotenoids between lipophorins of Bombyx mori

Role of Lipids in Phosphine Resistant Stored-Grain Insect Pests Tribolium castaneum and Rhyzopertha dominica

Insects rely on lipids as an energy source to perform various activities, such as growth, flight, diapause, and metamorphosis. This study evaluated the role of lipids in phosphine resistance by stored-grain insects. Phosphine resistant and susceptible strains of the two main stored-grain insects, Tribolium castaneum and Rhyzopertha dominica, were analyzed using liquid chromatography-mass spectroscopy (LC-MS) to determine their lipid contents. Phosphine resistant strains of both species had a higher amount of lipids than susceptible stains. Significant variance ratios between the resistant and susceptible strains of T. castaneum were observed for glycerolipids (1.13- to 53.10-fold) and phospholipids (1.05- to 20.00-fold). Significant variance ratios between the resistant and susceptible strains of R. dominica for glycerolipids were 1.04- to 31.50-fold and for phospholipids were 1.04- to 10.10-fold. Glycerolipids are reservoirs to face the long-term energy shortage. Phospholipids act as a barrier to isolate the cells from the surrounding environment and allow each cell to perform its specific function. Thus, lipids offer a consistent energy source for the resistant insect to survive under the stress of phosphine fumigation and provide a suitable environment to protect the mitochondria from phosphine. Hence, it was proposed through this study that the lipid content of phosphine-resistant and phosphine-susceptible strains of T. castaneum and R. dominica could play an important role in the resistance of phosphine.

Transcriptome Analysis Provides Insights into the Mechanism of Astaxanthin Enrichment in a Mutant of the Ridgetail White Prawn Exopalaemon carinicauda

A mutant of the ridgetail white prawn, which exhibited rare orange-red body color with a higher level of free astaxanthin (ASTX) concentration than that in the wild-type prawn, was obtained in our lab. In order to understand the underlying mechanism for the existence of a high level of free astaxanthin, transcriptome analysis was performed to identify the differentially expressed genes (DEGs) between the mutant and wild-type prawns. A total of 78,224 unigenes were obtained, and 1863 were identified as DEGs, in which 902 unigenes showed higher expression levels, while 961 unigenes presented lower expression levels in the mutant in comparison with the wild-type prawns. Based on Gene Ontology analysis and Kyoto Encyclopedia of Genes and Genomes analysis, as well as further investigation of annotated DEGs, we found that the biological processes related to astaxanthin binding, transport, and metabolism presented significant differences between the mutant and the wild-type prawns. Some genes related to these processes, including crustacyanin, apolipoprotein D (ApoD), cathepsin, and cuticle proteins, were identified as DEGs between the two types of prawns. These data may provide important information for us to understand the molecular mechanism of the existence of a high level of free astaxanthin in the prawn.

NMR resonance assignment and backbone dynamics of a C-terminal domain homolog of orange carotenoid protein

Photoprotection in cyanobacteria is mediated by the Orange Carotenoid Protein (OCP), a two-domain photoswitch which has multiple natural homologs of its N- and C-terminal domains. Recently, it was demonstrated that C-terminal domain homologs (CTDHs) of OCP are standalone carotenoproteins participating in multidirectional carotenoid transfer between membranes and proteins. Non-covalent embedment of a ketocarotenoid causes dimerization of the small 16-kDa water-soluble CTDH protein; however, dynamic interactions of CTDH with membranes and other proteins apparently require the monomeric state. Although crystallography recently provided static snapshots of the Anabaena CTDH (AnaCTDH) spatial structure in the apo-form, which predicted mobility of some putative functional segments, no crystallographic information on the holo-form of CTDH is presently available. In order to use NMR techniques to cope with the dynamics of the AnaCTDH protein, it was necessary to obtain ¹H, ¹³C and ¹⁵N resonance assignments. AnaCTDH samples enriched with ¹³C and ¹⁵N isotopes were prepared using recombinant protein expression, and NMR resonance assignment was achieved for more than 90% of the residues. The obtained results revealed that the structure of AnaCTDH in solution and in the crystal are largely equivalent. Together with ¹⁵N NMR relaxation experiments, our data shed light on the AnaCTDH dynamics and provide the platform for the subsequent analysis of the holo-CTDH structure in solution, for the better understanding of light-triggered protein-protein interactions and the development of antioxidant nanocarriers for biomedical applications in the future.

A journey into the world of insect lipid metabolism

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

Genetic Basis of Body Color and Spotting Pattern in Redheaded Pine Sawfly Larvae (Neodiprion lecontei)

Pigmentation has emerged as a premier model for understanding the genetic basis of phenotypic evolution, and a growing catalog of color loci is starting to reveal biases in the mutations, genes, and genetic architectures underlying color variation in the wild. However, existing studies have sampled a limited subset of taxa, color traits, and developmental stages. To expand the existing sample of color loci, we performed QTL mapping analyses on two types of larval pigmentation traits that vary among populations of the redheaded pine sawfly (Neodiprion lecontei): carotenoid-based yellow body color and melanin-based spotting pattern. For both traits, our QTL models explained a substantial proportion of phenotypic variation and suggested a genetic architecture that is neither monogenic nor highly polygenic. Additionally, we used our linkage map to anchor the current N. lecontei genome assembly. With these data, we identified promising candidate genes underlying (1) a loss of yellow pigmentation in populations in the mid-Atlantic/northeastern United States [C locus-associated membrane protein homologous to a mammalian HDL receptor-2 gene (Cameo2) and lipid transfer particle apolipoproteins II and I gene (apoLTP-II/I)], and (2) a pronounced reduction in black spotting in Great Lakes populations [members of the yellow gene family, tyrosine hydroxylase gene (pale), and dopamine N-acetyltransferase gene (Dat)]. Several of these genes also contribute to color variation in other wild and domesticated taxa. Overall, our findings are consistent with the hypothesis that predictable genes of large effect contribute to color evolution in nature.

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

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

Identification and analysis of the pigment composition and sources in the colored cocoon of the silkworm, Bombyx mori, by HPLC-DAD

This study used the larval tissues and colored cocoons of silkworms, Bombyx mori L. (Lepidoptera: Bombycidae), that were fed leaves of cultivated mulberry, Husang 32, as experimental material. The pigment composition and content in colored cocoons and tissues of the 5th instar larvae and the mulberry leaves were rapidly detected using organic solvent extraction and reverse phase high-performance liquid chromatography with diode array detection. It was found that the mulberry leaf mainly contained four types of pigment: lutein (30.86%), β-carotene (26.3%), chlorophyll a (24.62%), and chlorophyll b (18.21%). The silk glands, blood, and cocoon shells of six yellow-red cocoons were used as the experimental materials. The results showed that there were generally two kinds of carotenoids (lutein and β-carotene) in the silk gland and cocoon shell, a little violaxanthin was detected in silk gland, and the pigment found in the blood was mainly lutein in all varieties of silkworm tested. Chlorophyll a and b had not been digested and utilized in the yellow-red series of silkworm. The method used to detect visible pigments reported here could be used to breed new colors of cocoons and to develop and utilize the pigments found in mulberry.

Combined effect of Cameo2 and CBP on the cellular uptake of lutein in the silkworm, Bombyx mori

Formation of yellow-red color cocoons in the silkworm, Bombyx mori, occurs as the result of the selective delivery of carotenoids from the midgut to the silk gland via the hemolymph. This process of pigment transport is thought to be mediated by specific cellular carotenoids carrier proteins. Previous studies indicated that two proteins, Cameo2 and CBP, are associated with the selective transport of lutein from the midgut into the silk gland in Bombyx mori. However, the exact roles of Cameo2 and CBP during the uptake and transport of carotenoids are still unknown. In this study, we investigated the respective contributions of these two proteins to lutein and β-carotene transport in Bombyx mori as well as commercial cell-line. We found that tissues, expressed both Cameo2 and CBP, accumulate lutein. Cells, co-expressed Cameo2 and CBP, absorb 2 fold more lutein (P<0.01) than any other transfected cells, and the rate of cellular uptake of lutein was concentration-dependent and reached saturation. From immunofluorescence staining, confocal microscopy observation and western blot analysis, Cameo2 was localized at the membrane and CBP was expressed in the cytosol. What’s more, bimolecular fluorescence complementation analysis showed that these two proteins directly interacted at cellular level. Therefore, Cameo2 and CBP are necessarily expressed in midguts and silk glands for lutein uptake in Bombyx mori. Cameo2 and CBP, as the membrane protein and the cytosol protein, respectively, have the combined effect to facilitate the cellular uptake of lutein.

Lipid transfer particle from the silkworm, Bombyx mori, is a novel member of the apoB/large lipid transfer protein family

Lipid transfer particle (LTP) is a high-molecular-weight, very high-density lipoprotein known to catalyze the transfer of lipids between a variety of lipoproteins, including both insects and vertebrates. Studying the biosynthesis and regulation pathways of LTP in detail has not been possible due to a lack of information regarding the apoproteins. Here, we sequenced the cDNA and deduced amino acid sequences for three apoproteins of LTP from the silkworm (Bombyx mori). The three subunit proteins of the LTP are coded by two genes, apoLTP-II/I and apoLTP-III. ApoLTP-I and apoLTP-II are predicted to be generated by posttranslational cleavage of the precursor protein, apoLTP-II/I. Clusters of amphipathic secondary structure within apoLTP-II/I are similar to Homo sapiens apolipoprotein B (apoB) and insect lipophorins. The apoLTP-II/I gene is a novel member of the apoB/large lipid transfer protein gene family. ApoLTP-III has a putative conserved juvenile hormone-binding protein superfamily domain. Expression of apoLTP-II/I and apoLTP-III genes was synchronized and both genes were primarily expressed in the fat body at the stage corresponding to increased lipid transport needs. We are now in a position to study in detail the physiological role of LTP and its biosynthesis and assembly.

CD36 homolog divergence is responsible for the selectivity of carotenoid species migration to the silk gland of the silkworm Bombyx mori

Dietary carotenoids are absorbed in the intestine and delivered to various tissues by circulating lipoproteins; however, the mechanism underlying selective delivery of different carotenoid species to individual tissues remains elusive. The products of the Yellow cocoon (C) gene and the Flesh (F) gene of the silkworm Bombyx mori determine the selectivity for transport of lutein and β-carotene, respectively, to the silk gland. We previously showed that the C gene encodes Cameo2, a CD36 family member, which is thought to function as a transmembrane lipoprotein receptor. Here, we elucidated the molecular identity of the F gene product by positional cloning, as SCRB15, a paralog of Cameo2 with 26% amino acid identity. In the F mutant, SCRB15 mRNA structure was severely disrupted, due to a 1.4 kb genomic insertion in a coding exon. Transgenic expression of SCRB15 in the middle silk gland using the binary GAL4-UAS expression system enhanced selective β-carotene uptake by the middle silk gland, while transgenic expression of Cameo2 enhanced selective lutein uptake under the same GAL4 driver. Our findings indicate that divergence of genes in the CD36 family determines the selectivity of carotenoid species uptake by silk gland tissue and that CD36-homologous proteins can discriminate among carotenoid species.

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.

Housefly maggots (Musca domestica) protein-enriched fraction/extracts (PE) inhibit lipopolysaccharide-induced atherosclerosis pro-inflammatory responses

AIM

To investigate the effects of housefly maggot (Musca domestica) protein-enriched fraction/extracts (PE) on lipopolysaccharide (LPS)-induced atherosclerosis (AS) pro-inflammatory responses in mice and macrophages.

METHODS

The mouse model of AS was established by feeding a cholesterol-enriched diet and inducing by LPS. Changes in the levels of blood lipids (total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL) and high-density lipoprotein cholesterol (HDL)) and pro-inflammatory cytokines (interferon-gamma (IFNγ), tumor necrosis factor alpha (TNFα) and interleukin-1alpha (IL-1α)) were determined. Histomorphometric analysis of the pathological condition of the artery was also carried out. The macrophages were stimulated by LPS in the presence or absence of PE, and then the levels of TNFα, IL-1α and monocyte chemotactic protein 1 (MCP-1) in cell culture supernatant were measured.

RESULTS

Compared with the negative control group, the levels of three pro-inflammatory cytokines were significantly enhanced in the PE treatment group (p< 0.01). The concentrations of TC, TG and LDL were lower in the PE treatment group than in the negative control group (p< 0.01). HDL concentration in the PE treatment group was higher than in the negative control group (p< 0.01). Histomorphometric analysis showed that the thickness of the intima and media area, as well as the area ratio of the intima to media in the PE treatment group were lower than in the negative control group (p< 0.01). The expression of TNFα, IL-1α and MCP-1 in LPS-induced macrophages was inhibited by different concentrations of PE (p< 0.01).

CONCLUSION

These results indicate that PE potently inhibited multiple pro-inflammatory responses in experimental atherosclerosis lesions in vivo, and possessed anti-pro-inflammatory properties in vitro.

A CD36-related transmembrane protein is coordinated with an intracellular lipid-binding protein in selective carotenoid transport for cocoon coloration

The transport pathway of specific dietary carotenoids from the midgut lumen to the silk gland in the silkworm, Bombyx mori, is a model system for selective carotenoid transport because several genetic mutants with defects in parts of this pathway have been identified that manifest altered cocoon pigmentation. In the wild-type silkworm, which has both genes, Yellow blood (Y) and Yellow cocoon (C), lutein is transferred selectively from the hemolymph lipoprotein to the silk gland cells where it is accumulated into the cocoon. The Y gene encodes an intracellular carotenoid-binding protein (CBP) containing a lipid-binding domain known as the steroidogenic acute regulatory protein-related lipid transfer domain. Positional cloning and transgenic rescue experiments revealed that the C gene encodes Cameo2, a transmembrane protein gene belonging to the CD36 family genes, some of which, such as the mammalian SR-BI and the fruit fly ninaD, are reported as lipoprotein receptors or implicated in carotenoid transport for visual system. In C mutant larvae, Cameo2 expression was strongly repressed in the silk gland in a specific manner, resulting in colorless silk glands and white cocoons. The developmental profile of Cameo2 expression, CBP expression, and lutein pigmentation in the silk gland of the yellow cocoon strain were correlated. We hypothesize that selective delivery of lutein to specific tissue requires the combination of two components: 1) CBP as a carotenoid transporter in cytosol and 2) Cameo2 as a transmembrane receptor on the surface of the cells.

Purification and expression analysis of imaginal disc growth factor in the silkworm, Bombyx mori

In the present study, we purified and sequenced a homolog of the Drosophila imaginal disc growth factor (IDGF) from the hemolymph of Bombyx mori (BmIDGF). Antibodies against BmIDGF were produced and subsequently used in immunoblotting analyses. The immunoblotting analyses demonstrated an extremely high level of BmIDGF in the hemolymph throughout the period of rapid growth of the organs of B. mori. The results of RT-PCR showed that BmIDGF was predominantly expressed in fat bodies. Real-time RT-PCR revealed that BmIDGF transcripts in fat bodies were highly expressed during the feeding stage but significantly suppressed during the molting, wandering, and pupal stages. Starvation brought about a significant decline of BmIDGF mRNAs in the fat bodies and BmIDGF proteins in the hemolymph. After re-feeding, the BmIDGF transcripts in fat bodies and BmIDGF proteins in the hemolymph increased again. In addition, an immunocytochemical study revealed BmIDGF proteins on the surface of wing discs. The present findings suggest that the level of BmIDGF in the hemolymph was modulated by the fat body in response to nutritional conditions and that BmIDGF was transported to target organs through the hemolymph.

Transcriptome analysis of a spontaneous mutant in sweet orange [Citrus sinensis (L.) Osbeck] during fruit development

Bud mutations often arise in citrus. The selection of mutants is one of the most important breeding channels in citrus. However, the molecular basis of bud mutation has rarely been studied. To identify differentially expressed genes in a spontaneous sweet orange [C. sinensis (L.) Osbeck] bud mutation which causes lycopene accumulation, low citric acid, and high sucrose in fruit, suppression subtractive hybridization and microarray analysis were performed to decipher this bud mutation during fruit development. After sequencing of the differentially expressed clones, a total of 267 non-redundant transcripts were obtained and 182 (68.2%) of them shared homology (E-value < or = 1x10(-10)) with known gene products. Few genes were constitutively up- or down-regulated (fold change > or = 2) in the bud mutation during fruit development. Self-organizing tree algorithm analysis results showed that 95.1% of the differentially expressed genes were extensively coordinated with the initiation of lycopene accumulation. Metabolic process, cellular process, establishment of localization, response to stimulus, and biological regulation-related transcripts were among the most regulated genes. These genes were involved in many biological processes such as organic acid metabolism, lipid metabolism, transport, and pyruvate metabolism, etc. Moreover, 13 genes which were differentially regulated at 170 d after flowering shared homology with previously described signal transduction or transcription factors. The information generated in this study provides new clues to aid in the understanding of bud mutation in citrus.

Surface functionalized amorphous nanosilica and microsilica with nanopores as promising tools in biomedicine

Cellular interactions with engineered nanoparticles (NPs) are dependent on many properties, inherent to the nanoparticle (viz. size, shape, surface characteristics, degradation, agglomeration/dispersal, and charge, etc.). Modification of the surface reactivity via surface functionalization of the nanoparticles to be targeted seems to be important. Utilization of different surface functionalization methods of nanoparticles is an emerging field of basic and applied nanotechnology. It is well known that many disease-causing organisms induce host lipids and if deprived, their growth is inhibited in vivo. Amorphous nanosilica (ANS) and amorphous microsilica with nanopores (AMS) were prepared by a combination of wet chemistry and high-energy ball milling. Lipophilic moieties were attached to both ANS and AMS via chemical surface functionalization method. Lipophilic ANS and AMS were found to inhibit the growth of Bombyx mori nuclear polyhedrosis virus (BmNPV) and chicken malarial parasites via absorption of silkworm hemolymph and chicken serum lipids/lipoproteins, respectively, in vivo. Therefore, intelligent surface functionalization of NP is an important concept, and its application in curing chicken malaria and BmNPV is presented here. Surface functionalization method reported in this paper might serve as a valuable technology for treating many diseases where pathogens induce host lipid.

Vertebrate and invertebrate carotenoid-binding proteins

In invertebrates and vertebrates, carotenoids are ubiquitous colorants, antioxidants, and provitamin A compounds that must be absorbed from dietary sources and transported to target tissues where they are taken up and stabilized to perform their physiological functions. These processes occur in a specific and regulated manner mediated by high-affinity carotenoid-binding proteins. In this mini-review, we examine the published literature on carotenoid-binding proteins in vertebrate and invertebrate systems, and we report our initial purification and characterization of a novel lutein-binding protein isolated from liver of Japanese quail (Coturnix japonica).

Further Studies of Lipid Droplets in the Bombykol-Producing Pheromone Gland ofBombyx mori

Lipid droplets are abundant in the pheromone-producing cells of B. mori at adult eclosion, followed by daily fluctuations in both their size and number. Their dynamics are related to PBAN-stimulated de novo bombykol production. To elucidate associated events, we performed the following: (1) extraction, purification, and partial characterization of lipid droplet-associated proteins found on their surface since their function could possibly be to transport and/or dock putative lipases that are responsible for the lipolysis of triglycerides in them; (2) separation, purification, and initial analysis of lipids carried by lipophorins and lipid transfer particles originating from pupal and adult hemolymph because of their role in the formation and accumulation of lipid droplets.

The basis for colorless hemolymph and cocoons in the Y-gene recessive Bombyx mori mutants: a defect in the cellular uptake of carotenoids

Bombyx mori is an excellent model for the study of carotenoid-binding proteins (CBP). In previous papers, we identified and molecularly characterized a CBP from the Y-gene dominant mutants. In the present study, we attempted to correlate and establish lipid metabolism and distribution in these mutants. When [3H]-triolein was fed to the mutants, typical patterns of uptake of labeled fatty acids from midgut to hemolymph and subsequent delivery to fat body and silk glands were obtained in all mutants. Further analysis of lipid and carotenoid profiles revealed that the yellow coloration in the hemolymph associated with lipophorin is not attributed to a difference in lipophorin concentrations among the mutants, nor to its lipid composition, but rather to its carotenoid content. Lipophorin of the Y+I mutant exhibited the highest concentration of total carotenoids of 55.8 microg/mg lipophorin compared to 3.1 microg/mg in the +Y+I mutant, 1.2 microg/mg in the YI mutant and 0.5 microg/mg in the +YI mutant. Characteristic retention time in HPLC of the different classes of carotenoids of lipophorin identified the presence of lutein as the major chromophore (62-77%), followed by beta-carotenes (22-38%). Although lutein and beta-carotene content of mutants' lipophorin differed significantly, the ratio of lutein to beta-carotene of 3:1 was not different among mutants. Similarly, lipid compositions of mutant silk glands were not significantly different, but carotenoid contents were. The significantly high concentration of lutein in the Y+I mutant silk gland represented more than 160-fold increase compared to +Y+I mutant (p<0.001). In this report, we conclude that lipid metabolism in the mutants is not defected and that the molecular basis for colorless hemolymph and cocoons is a defect in the cellular uptake of lutein associated with the Y-gene recessive mutants.

A carotenoid-binding protein (CBP) plays a crucial role in cocoon pigmentation of silkworm (Bombyx mori) larvae

We examined the role of carotenoid-binding protein (CBP) in yellow cocoon pigmentation. First, using yellow or white cocoon races, we investigated the linkage between the yellow pigmentation and CBP expression. CBP was expressed only in the silk gland of the yellow cocoon races, which utilize carotenoids for cocoon pigmentation. Furthermore, CBP expression in the silk glands of day 1-7 fifth instar larvae matched the period of carotenoid uptake into the silk gland. Finally, we gave double-stranded CBP RNA to Bombyx mori (B. mori) larvae to induce RNA interference. The significantly reduced expression of CBP in the silk gland of fifth instar larva was confirmed on day 4 and a decrease in yellow pigmentation was observed in the cocoon. We showed that CBP plays a key role in the yellow cocoon pigmentation caused by carotenoids.

Characterization of the carotenoid-binding protein of the Y-gene dominant mutants of Bombyx mori

Carotenoids play important and diverse roles in insects. Recently, we purified and partially characterized a carotenoid-binding protein (CBP) from the wild type of Bombyx mori. In this report, we utilized immunoblotting, ELISA and immunocytochemistry to further characterize and localize the expression of CBP in the larval midgut and silk gland obtained from the wild type and four naturally occurring mutants linked to carotenoids transport. CBP was expressed throughout the 5th stadium, with highest expressions on days 4-5 in the silk gland and days 3-5 in the midgut. Immunoblotting analyses demonstrated the presence of CBP along the middle part of the midgut. Microscopic immunocytochemistry demonstrated that the 33 kDa CBP was uniformly expressed along the brush border of columnar cells in the epithelium of the midgut typifying its function in aiding absorption of dietary carotenoids. Similarly, CBP was highly expressed along the distal membrane of the middle part of the silk gland demonstrating its function in uptake of carotenoids from lipophorin. When the middle silk glands and midguts of the four mutants were incubated with rabbit anti-CBP antibody, only proteins of the Y-gene dominant mutants cross reacted with the antibody further accentuating the hypothesis that the CBP is a Y-gene dependent protein.

Lipid transfer particle mediates the delivery of diacylglycerol from lipophorin to fat body in larval Manduca sexta

This work analyzed the process of lipid storage in fat body of larval Manduca sexta, focusing on the role of lipid transfer particle (LTP). Incubation of fat bodies with [(3)H]diacylglycerol-labeled lipophorin resulted in a significant accumulation of diacylglycerol (DAG) and triacylglycerol (TAG) in the tissue. Transfer of DAG to fat body and its storage as TAG was significantly inhibited (60%) by preincubating the tissue with anti-LTP antibody. Lipid transfer was restored to control values by adding LTP to fat body. Incubation of fat body with dual-labeled DAG lipophorin or its treatment with ammonium chloride showed that neither a membrane-bound lipoprotein lipase nor lipophorin endocytosis is a relevant pathway to transfer or to storage lipids into fat body, respectively. Treatment of fat body with suramin caused a 50% inhibition in [(3)H]DAG transfer from lipophorin. Treatment of [(3)H]DAG-labeled fat body with lipase significantly reduced the amount of [(3)H]DAG associated with the tissue, suggesting that the lipid is still on the external surface of the membrane. Whether this lipid represents irreversibly adsorbed lipophorin or a DAG lipase-sensitive pool is unknown. Nevertheless, these results indicate that the main pathway for DAG transfer from lipophorin to fat body is via LTP and receptor-mediated processes.

Isolation, characterization, and cDNA sequence of a carotenoid binding protein from the silk gland of Bombyx mori larvae

A carotenoid binding protein (CBP) has been isolated from the silk glands of Bombyx mori larvae. The protein has an apparent molecular mass of 33 kDa and binds carotenoids in a 1:1 molar ratio. Lutein accounts for 90% of the bound carotenoids, whereas alpha-carotene and beta-carotene are minor components. Immunological analysis demonstrated the presence of CBP only in the yellow-colored tissues of the silk gland, midgut, testis, and ovary. Several phenotypes of B. mori mutants linked to carotenoid transport have been utilized to characterize CBP. The Y (yellow hemolymph) gene controls uptake of carotenoids from the midgut lumen into the midgut epithelium, and larvae with the +(Y) gene lack this property. Immunoblotting analysis confirmed the presence of CBP in mutants with the dominant Y gene only. Immunohistochemistry verified the localization of CBP in the villi of the midgut epithelium, indicating that CBP might be involved in absorption of carotenoids. A cDNA clone for CBP encoding a protein of 297 amino acids has been isolated from the B. mori silk gland cDNA library. The deduced amino acid sequence revealed that CBP is a novel member of the steroidogenic acute regulatory (StAR) protein family with its unique structural feature of a StAR-related lipid transfer domain, known to aid in lipid transfer and recognition. Lutein-binding capacity of the recombinant CBP (rCBP) determined by incubating rCBP with lutein followed by immunoprecipitation using anti-CBP IgG conjugated to protein A-Sepharose, demonstrated the formation of a lutein-rCBP complex. Sequence analyses coupled with binding specificity suggest that CBP is a new member of the StAR protein family that binds carotenoids rather than cholesterol.

A class B scavenger receptor mediates the cellular uptake of carotenoids in Drosophila

Carotenoids are currently being intensely investigated regarding their potential to lower the risk of chronic disease and vitamin A deficiency. Invertebrate models in which vitamin A deficiency is not lethal allow the isolation of blind but viable mutants affected in the pathway leading from dietary carotenoids to vitamin A. Using a mutant in one of these model systems, Drosophila, the vitamin A-forming enzyme has recently been molecularly identified. We now show that the molecular basis for the blindness of a different Drosophila mutant, ninaD, is a defect in the cellular uptake of carotenoids. The ninaD gene encodes a class B scavenger receptor essential for the formation of the visual chromophore. A loss of this function results in a carotenoid-free and thus vitamin A-deficient phenotype. Our investigations provide molecular insight into how carotenoids may be distributed into cells of target tissues in animals and indicate a crucial role of class B scavenger receptors rendering dietary carotenoids available for subsequent cell metabolism, as needed for their various physiological functions.

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.

Sequestration of host plant carotenoids in the larval tissues of Helicoverpa zea

To determine the cause of the unique yellow coloration in mandibular glands of soybean-fed Helicoverpa zea larvae, the accumulation of carotenoids in various tissues of last instar larvae fed soybean, cotton and tomato foliage was quantified. Five carotenoids were detected in the foliage of all host plants but at significantly different concentrations. Xanthophylls rather than carotenes were most likely to accumulate in larval tissues. Carotenoids accumulated at different rates and some were significantly affected by larval diet. Highest levels of carotenoid accumulation, notably lutein, were detected in the testes, followed by midgut epithelium, fat body and integument. The midgut epithelium contained the greatest and the testes the least diversity of carotenoid types. Low levels of lutein were detected in both labial and mandibular glands. Tomato foliage had the highest carotenoid content and caterpillar tissues fed these leaves often had the highest amounts of carotenoid. However, the accumulation of carotenoids did not protect larvae from antibiotic effects of tomato foliage because these caterpillars had the highest mortality and slowest growth rates of all the three host plants. Transport and absorption of lipid and oxidative stress may be some reasons for differential carotenoid accumulation.

Purification and sequence determination of a yellow protein from sexually mature males of the desert locust, Schistocerca gregaria

A yellow protein from abdominal cuticle of the desert locust, Schistocerca gregaria, has been purified and its amino acid sequence determined. The yellow color comes from bound carotene, the protein is only deposited in the epidermis and cuticle of male locusts during their sexual maturation, and the deposition is dependent upon a sufficiently high titer of juvenile hormone. The sequence of the protein is atypical for a cuticular protein, but it has some similarity to a putative juvenile hormone binding protein from Manduca sexta. It is suggested that the protein is involved in the transport of carotenes from internal tissues to epidermis and cuticle of the locust.

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

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

Purification and characterization of a lipid transfer particle in Rhodnius prolixus: phospholipid transfer

In this study we report the purification and characterization of a lipid transfer particle (LTP) from Rhodnius prolixus hemolymph, and its participation in phospholipid and diacylglycerol transfer processes. (3)H-diacylglycerol labeled low density lipophorin from Manduca sexta ((3)H-LDLp) was incubated with R. prolixus lipophorin (Lp) in the presence of Rhodnius hemolymph. Following incubation and isolation, both lipoproteins showed equivalent amounts of (3)H-labeled lipids. Hemolymph was subjected to KBr gradient ultracentrifugation. SDS-PAGE analysis of gradient fractions showed the enrichment of bands with molecular masses similar to the M. sexta LTP standard. LTP containing fractions were assayed and lipid transfer activity was observed. Purification of LTP was accomplished by (i) KBr density gradient ultracentrifugation, (ii) size exclusion, (iii) Cu(++) affinity and (iv) ion exchange chromatographies. LTP molecular mass was estimated approximately 770 kDa, comprising three apoproteins, apoLTP-I (315 kDa), apoLTP-II (85 kDa) and apoLTP-III (58 kDa). Phospolipid content of (32)P-LTP was determined after two-dimensional TLC. (32)P-phospholipid-labeled and unlabeled lipophorins, purified from R. prolixus were incubated in the presence of LTP resulting in the time-dependent transfer of phospholipids. LTP-mediated phospholipid transfer was not a selective process.

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

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

Growth-promoting effects of ecdysteroids and juvenile hormone on in vitro development of the larval endoparasitoid, Venturia canescens (Hymenoptera: Ichneumonidae)

We previously reported that lipophorin, fetal bovine serum (FBS), and 20-hydroxyecdysone (20-HE) are essential for the development of the larval endoparasitoid Venturia canescens larvae in vitro. The present study was undertaken to determine the optimal concentrations of those three substances in the MGM-450 medium, and to examine the hormonal effects of ecdysteroids and juvenile hormone (JH) on the development of the parasitoid larvae in vitro. When the culture was started with embryos at the post-germband stage, concentrations of 3 mg/ml of lipophorin and 20% of FBS were most suitable for the development of the parasitoid. The growth-promoting effect of 20-HE increased in a concentration-dependent manner and peaked at a concentration of 1 &mgr;g/ml. Excess concentration led to malformations of the larvae. Three other ecdysteroids, ecdysone, 2-deoxy-20-hydroxyecdysone, and polypodine B had the same effect, although their activity was lower than that of 20-HE. Cholesterol had no effect; most larvae failed to develop. When the medium was supplemented with JH, the duration of the developmental period was significantly shortened, but this hormone was not found to be essential.

Lipid transport biochemistry and its role in energy production

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