Molecular cloning and characterization ofBombyx moristerol carrier protein x/sterol carrier protein 2 (SCPx/SCP2) gene

Expression of honey bee (Apis mellifera) sterol homeostasis genes in food jelly producing glands of workers

Adult workers of Western honey bees (Apis mellifera L.) acquire sterols from their pollen diet. These food sterols are transported by the hemolymph to peripheral tissues such as the mandibular and the hypopharyngeal glands in the worker bees' heads that secrete food jelly which is fed to developing larvae. As sterols are obligatory components of biological membranes and essential precursors for molting hormone synthesis in insects, they are indispensable to normal larval development. Thus, the study of sterol delivery to larvae is important for a full understanding of honey bee larval nutrition and development. Whereas hypopharyngeal glands only require sterols for their membrane integrity, mandibular glands add sterols, primarily 24-methylenecholesterol, to its secretion. For this, sterols must be transported through the glandular epithelial cells. We have analyzed for the first time in A. mellifera the expression of genes which are involved in intracellular movement of sterols. Mandibular and hypopharyngeal glands were dissected from newly emerged bees, 6-day-old nurse bees that feed larvae and 26-day-old forager bees. The expression of seven genes involved in intracellular sterol metabolism was measured with quantitative real-time PCR. Relative transcript abundance of sterol metabolism genes was significantly influenced by the age of workers and specific genes but not by gland type. Newly emerged bees had significantly more transcripts for six out of seven genes than older bees indicating that the bulk of the proteins needed for sterol metabolism are produced directly after emergence.

The effects of glyphosate exposure on gene transcription and immune function of the silkworm, Bombyx mori

Glyphosate is a widely used herbicide and crop desiccant. However, whether its extensive use has any effect on the species diversity of nontarget organisms is still unclear. In this study, we used the silkworm, Bombyx mori, as the research subject, and performed RNA sequencing to analyze the transcriptional profile of silkworm midgut after exposure to glyphosate at 2975.20 mg/L (a concentration commonly used at mulberry fields). A total of 125 significantly differentially expressed genes (DEGs) were detected in the midgut of glyphosate-exposed silkworm (q < 0.05), of which 53 were upregulated and 72 were downregulated. Gene ontology enrichment analysis showed that the DEGs were mainly enriched in biological process, cellular component, and molecular function. Kyoto encyclopedia of genes and genomes analysis showed that the differential genes were mainly related to oxidative stress, nutrient metabolism, and immune defense pathways, including oxidative stress-related Cat and Jafrac1, nutrient metabolism-related Fatp and Scpx, and immune-related CYP6AN2, UGT40B4, CTL11, serpin-2, and so forth. Experimental verification showed that glyphosate exposure led to a 4.35-fold increase in the mortality of silkworm after Beauveria bassiana infection, which might be caused by the decreased PO (phenoloxidase) activity and impaired immunity. These results provide evidence for the potential effects of residue glyphosate on the physiological functions of silkworm, and also provide a reference for the biosafety evaluation of glyphosate.

The Potential of Biologically Active Brazilian Plant Species as a Strategy to Search for Molecular Models for Mosquito Control

Natural products are a valuable source of biologically active compounds and continue to play an important role in modern drug discovery due to their great structural diversity and unique biological properties. Brazilian biodiversity is one of the most extensive in the world and could be an effective source of new chemical entities for drug discovery. Mosquitoes are vectors for the transmission of dengue, Zika, chikungunya, yellow fever, and many other diseases of public health importance. These diseases have a major impact on tropical and subtropical countries, and their incidence has increased dramatically in recent decades, reaching billions of people at risk worldwide. The prevention of these diseases is mainly through vector control, which is becoming more difficult because of the emergence of resistant mosquito populations to the chemical insecticides. Strategies to provide efficient and safe vector control are needed, and secondary metabolites from plant species from the Brazilian biodiversity, especially Cerrado, that are biologically active for mosquito control are herein highlighted. Also, this is a literature revision of targets as insights to promote advances in the task of developing active compounds for vector control. In view of the expansion and occurrence of arboviruses diseases worldwide, scientific reviews on bioactive natural products are important to provide molecular models for vector control and contribute with effective measures to reduce their incidence.

Genome-wide identification and analysis of the thiolase family in insects

Thiolases are important enzymes involved in lipid metabolism in both prokaryotes and eukaryotes, and are essential for a range of metabolic pathways, while, little is known for this important family in insects. To shed light on the evolutionary models and functional diversities of the thiolase family, 137 thiolase genes were identified in 20 representative insect genomes. They were mainly classified into five classes, namely cytosolic thiolase (CT-thiolase), T1-thiolase, T2-thiolase, trifunctional enzyme thiolase (TFE-thiolase), and sterol carrier protein 2 thiolase (SCP2-thiolase). The intron number and exon/intron structures of the thiolase genes reserve large diversification. Subcellular localization prediction indicated that all the thiolase proteins were mitochondrial, cytosolic, or peroxisomal enzymes. Four highly conserved sequence fingerprints were found in the insect thiolase proteins, including CxS-, NEAF-, GHP-, and CxGGGxG-motifs. Homology modeling indicated that insect thiolases share similar 3D structures with mammals, fishes, and microorganisms. In Bombyx mori, microarray data and reverse transcription-polymerase chain reaction (RT-PCR) analysis suggested that some thiolases might be involved in steroid metabolism, juvenile hormone (JH), and sex pheromone biosynthesis pathways. In general, sequence and structural characteristics were relatively conserved among insects, bacteria and vertebrates, while different classes of thiolases might have differentiation in specific functions and physiological processes. These results will provide an important foundation for future functional validation of insect thiolases.

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.

Insect Sterol Nutrition: Physiological Mechanisms, Ecology, and Applications

Insects, like all eukaryotes, require sterols for structural and metabolic purposes. However, insects, like all arthropods, cannot make sterols. Cholesterol is the dominant tissue sterol for most insects; insect herbivores produce cholesterol by metabolizing phytosterols, but not always with high efficiency. Many insects grow on a mixed-sterol diet, but this ability varies depending on the types and ratio of dietary sterols. Dietary sterol uptake, transport, and metabolism are regulated by several proteins and processes that are relatively conserved across eukaryotes. Sterol requirements also impact insect ecology and behavior. There is potential to exploit insect sterol requirements to (a) control insect pests in agricultural systems and (b) better understand sterol biology, including in humans. We suggest that future studies focus on the genetic mechanism of sterol metabolism and reverse transportation, characterizing sterol distribution and function at the cellular level, the role of bacterial symbionts in sterol metabolism, and interrupting sterol trafficking for pest control.

Phylogeny and evolution of the cholesterol transporter NPC1 in insects

Sterols are essential nutrients for eukaryotes. Insects are obligate sterol auxotrophs and must acquire this key nutrient from their diets. The digestive tract is the organ for absorbing nutrients as well as sterols from food. In mice, the Niemann-Pick type C1 Like 1 (NPC1L1) gene is highly expressed in the intestine and is critical for cholesterol absorption. In contrast, the molecular mechanisms for the absorption of dietary sterols in insects have not been well studied. We annotated NPC1 genes in 39 insects from 10 orders using available genomic and transcriptomic information and inferred phylogenetic relationships. Insect NPC1 genes were grouped into two sister-clades, NPC1a and NPC1b, suggesting a likely duplication in the ancestor of insects. The former exhibited weaker gut-biased expression or a complete lack of tissue-biased expression, depending on the species, while the latter was highly enriched in the gut of three lepidopteran species. This result is similar to previous findings in Drosophila melanogaster. In insects, NPC1a accumulated non-synonymous substitutions at a lower rate than NPC1b. This pattern was consistent across orders, indicating that NPC1a evolved under stronger molecular constraint than NPC1b.

NMR structure and function of Helicoverpa armigera sterol carrier protein-2, an important insecticidal target from the cotton bollworm

The cotton bollworm, Helicoverpa armigera, has developed strong resistance to many insecticides. Sterol Carrier Protein-2 (SCP-2) is an important non-specific lipid transfer protein in insects and appears to be a potential new target. In order to elucidate the structure and function of Helicoverpa armigera SCP-2 (HaSCP-2), NMR spectroscopy, docking simulations, mutagenesis and bioassays were performed. HaSCP-2 composed of five α-helices and four stranded β-sheets. The folds of α-helices and β-sheets interacted together to form a hydrophobic cavity with putative entrance and exit openings, which served as a tunnel for accommodating and transporting of lipids. Several sterols and fatty acids could interact with HaSCP-2 via important hydrophobic sites, which could be potential targets for insecticides. Mutagenesis experiments indicated Y51, F53, F89, F110, I117 and Q131 may be the key functional sites. HaSCP-2 showed high cholesterol binding activity and SCP-2 inhibitors (SCPIs) could inhibit the biological activity of HaSCP-2. SCPI-treated larvae at young stage showed a significant decrease of cholesterol uptake in vivo. Our study describes for the first time a NMR structure of SCP-2 in lepidopteran H. armigera and reveals its important function in cholesterol uptake, which facilitates the screening of effective insecticides targeting the insect cholesterol metabolism.

Transcription factor CAAT/enhancer-binding protein is involved in regulation of expression of sterol carrier protein x in Spodoptera litura

The Spodoptera litura sterol carrier protein x (SlSCPx) gene is expressed in various tissues throughout the life cycle and plays important role in sterol absorption and transport. In this study, the effects of insect hormones (20-hydroexcdysone and juvenile hormone) and lipids (arachidonic acid, cholesterol) on the expression of SlSCPx was analysed by reverse-transcriptase PCR. The results showed that none of these substances significantly induced the expression of SlSCPx in Spodoptera litura-221 (Spli-221) cells. To identify the transcription factors responsible for regulation of SlSCPx expression, a 3311-bp promoter sequence of the gene was cloned. Transcriptional activity of the promoter was studied using an in vivo promoter/reporter system and a 29-bp sequence between -1000 and -1029 nucleotides (nt) upstream of this gene was found to be responsible for the up-regulation of the gene. Over-expression of CAAT/enhancer-binding protein (C/EBP) in Spli-221 cells increased the promoter activity 5.57-fold. An electrophoretic mobility shift assay showed that two nuclear proteins bound to this sequence. Recombinant C/EBP specifically bound with a putative cis-regulatory element (CRE). Mutation of the C/EBP CRE abolished the binding of the C/EBP with the CRE. These results suggest that the transcription factor C/EBP may regulate the expression of SlSCPx by binding to the CRE in the promoter of this gene.

Characterization of the sterol carrier protein-x/sterol carrier protein-2 gene in the cotton bollworm, Helicoverpa armigera

Cholesterol is a membrane component and the precursor of ecdysteroids in insects, but insects cannot synthesize cholesterol de novo. Therefore, cholesterol uptake and transportation during the feeding larval stages are critical processes in insects. The sterol carrier protein-2 domain (SCP-2) in sterol carrier proteins-x (SCP-x) has been speculated to be involved in intracellular cholesterol transfer and metabolism in vertebrates. However, a direct association between SCP-x gene expression, cholesterol absorption and development in lepidopteran insects is poorly understood. We identified the Helicoverpa armigera sterol carrier protein-x/2 (HaSCP-x/2) gene from the larval midgut cDNAs. The HaSCP-x/2 gene is well conserved during evolution and relatively divergent in heterogenetic species. Transcripts of HaSCP-x/2 were detected by qRT-PCR at the highest level in the midgut of H. armigera during the larval stages. Expression knockdown of HaSCP-x/2 transcripts via dsRNA interference resulted in delayed larval development and decreased adult fecundity. Sterol carrier protein-2 inhibitors were lethal to young larvae and decreased fertility in adults emerged from treated elder larvae in H. armigera. The results taken together suggest that HaSCPx/2 gene is important for normal development and fertility in H. armigera.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

The sterol carrier protein 2/3-oxoacyl-CoA thiolase (SCPx) is involved in cholesterol uptake in the midgut of Spodoptera litura: gene cloning, expression, localization and functional analyses

BACKGROUND

Sterol carrier protein-2/3-oxoacyl-CoA thiolase (SCPx) gene has been suggested to be involved in absorption and transport of cholesterol. Cholesterol is a membrane component and is a precursor of ecdysteroids, but cannot be synthesized de novo in insects. However, a direct association between SCPx gene expression, cholesterol absorption and development in lepidopteran insects remains to be experimentally demonstrated.

RESULTS

An SCPx cDNA (SlSCPx) cloned from the common cutworm, Spodoptera litura, was characterized. The SlSCPx cDNA encoded a 535-amino acid protein consisting of a 3-oxoacyl-CoA thiolase (SCPx-t) domain and a SCP-2 (SCPx-2) domain. SlSCPx mRNA was expressed predominately in the midgut, while SlSCPx-2 mRNA was detected in the midgut, fat body and epidermis and no SlSCPx-t mRNA was detected. A 58-kDa full-length SCPx protein and a 44-kDa SCPx-t protein were detected in the midgut of sixth instar larvae when the anti-SlSCPx-t antibody was used in western blotting analysis; a 16-kDa SCP-2 protein was detected when anti-SlSCPx-2 antibody was used. SlSCPx protein was post-translationally cleaved into two smaller proteins, SCPx-t and SCPx-2. The gene appeared to be expressed into two forms of mRNA transcripts, which were translated into the two proteins, respectively. SlSCPx-t and SlSCPx-2 proteins have distinct and different locations in the midgut of sixth instar larvae. SlSCPx and SlSCPx-t proteins were detected predominately in the cytoplasm, whereas SlSCPx-2 protein was detected in the cytoplasm and nuclei in the Spli-221 cells. Over-expression of SlSCPx and SlSCPx-2 proteins enhanced cholesterol uptake into the Spli-221 cells. Knocking-down SlSCPx transcripts by dsRNA interference resulted in a decrease in cholesterol level in the hemolymph and delayed the larval to pupal transition.

CONCLUSION

Spatial and temporal expression pattern of this SlSCPx gene during the larval developmental stages of S. litura showed its specific association with the midgut at the feeding stage. Over-expression of this gene increased cholesterol uptake and interference of its transcript decreased cholesterol uptake and delayed the larval to pupal metamorphosis. All of these results taken together suggest that this midgut-specific SlSCPx gene is important for cholesterol uptake and normal development in S. litura.