Stable expression of insect GABA receptors in insect cell lines. Promoters for efficient expression of Drosophila and mosquito Rdl GABA receptors in stably transformed mosquito cell lines

Agonist and antagonist properties of an insect GABA-gated chloride channel (RDL) are influenced by heterologous expression conditions

Pentameric ligand-gated ion channels (pLGICs) activated by the inhibitory neurotransmitter γ-aminobutyric acid (GABA) are expressed widely in both vertebrate and invertebrate species. One of the best characterised insect GABA-gated chloride channels is RDL, an abbreviation of ‘resistance to dieldrin’, that was originally identified by genetic screening in Drosophila melanogaster. Here we have cloned the analogous gene from the bumblebee Bombus terrestris audax (BtRDL) and examined its pharmacological properties by functional expression in Xenopus oocytes. Somewhat unexpectedly, the sensitivity of BtRDL to GABA, as measured by its apparent affinity (EC₅₀), was influenced by heterologous expression conditions. This phenomenon was observed in response to alterations in the amount of cRNA injected; the length of time that oocytes were incubated before functional analysis; and by the presence or absence of a 3’ untranslated region. In contrast, similar changes in expression conditions were not associated with changes in apparent affinity with RDL cloned from D. melanogaster (DmRDL). Changes in apparent affinity with BtRDL were also observed following co-expression of a chaperone protein (NACHO). Similar changes in apparent affinity were observed with an allosteric agonist (propofol) and a non-competitive antagonist (picrotoxinin), indicating that expression-depended changes are not restricted to the orthosteric agonist binding site. Interestingly, instances of expression-dependent changes in apparent affinity have been reported previously for vertebrate glycine receptors, which are also members of the pLGIC super-family. Our observations with BtRDL are consistent with previous data obtained with vertebrate glycine receptors and indicates that agonist and antagonist apparent affinity can be influenced by the level of functional expression in a variety of pLGICs.

Insights into the repression of fibroin modulator binding protein-1 on the transcription of fibroin H-chain during molting in Bombyx mori

Fibroin modulator binding protein-1 (FMBP-1) is a novel DNA-binding protein containing a conserved score and three amino acid peptide repeat (STPR) domain. The roles of factors containing STPR domain are less known. Although multiple transcription factors are involved in the transcriptional regulation of silk protein genes during the development of silkworm, the mechanism of transcriptional repression of silk protein genes during molting remains unclear. Here, we found that FMBP-1 expression was contrary to that of fibroin heavy chain (fib-H) during the fourth molting period of Bombyx mori. FMBP-1 repressed fib-H promoter activity by directly binding to the -130 element in the fib-H promoter region. We also identified two proteins, Bmsage and Bmdimm, that interacted with FMBP-1 in the posterior silk gland of silkworm larvae, and further verified these interactions by far western blotting and microscale thermophoresis in vitro, as well as co-immunoprecipitation and bimolecular fluorescence complementation at the cellular level. The luciferase reporter assay showed that the interaction between FMBP-1 and Bmdimm antagonized the activation of Bmdimm on fib-H transcription, but did not affect FMBP-1-mediated transcriptional repression on fib-H gene. Therefore, we proposed the following mechanism of fib-H transcriptional repression by FMBP-1 during the molting of silkworm larvae: 1) FMBP-1 directly binds to the -130 element in the fib-H promoter to repress fib-H transcription; 2) FMBP-1 interacts with Bmdimm to antagonize the activation of Bmdimm on fib-H transcription. Our findings promote a better understanding of fib-H transcriptional regulation and provide novel insights into the transcriptional repression of fib-H by FMBP-1 and basic helix-loop-helix factors Bmdimm during the molting of silkworm larvae. Our study also provides valuable information regarding the biological function of factors containing STPR domain.

A Baculovirus immediate-early gene, ie1, promoter drives efficient expression of a transgene in both Drosophila melanogaster and Bombyx mori

Many promoters have been used to drive expression of heterologous transgenes in insects. One major obstacle in the study of non-model insects is the dearth of useful promoters for analysis of gene function. Here, we investigated whether the promoter of the immediate-early gene, ie1, from the Bombyx mori nucleopolyhedrovirus (BmNPV) could be used to drive efficient transgene expression in a wide variety of insects. We used a piggyBac-based vector with a 3xP3-DsRed transformation marker to generate a reporter construct; this construct was used to determine the expression patterns driven by the BmNPV ie1 promoter; we performed a detailed investigation of the promoter in transgene expression pattern in Drosophila melanogaster and in B. mori. Drosophila and Bombyx belong to different insect orders (Diptera and Lepidoptera, respectively); however, and to our surprise, ie1 promoter-driven expression was evident in several tissues (e.g., prothoracic gland, midgut, and tracheole) in both insects. Furthermore, in both species, the ie1 promoter drove expression of the reporter gene from a relatively early embryonic stage, and strong ubiquitous ie1 promoter-driven expression continued throughout the larval, pupal, and adult stages by surface observation. Therefore, we suggest that the ie1 promoter can be used as an efficient expression driver in a diverse range of insect species.

Cyclodiene insecticide resistance: from molecular to population genetics

This review follows progress in the analysis of cyclodiene insecticide resistance from the initial isolation of the mutant, through cloning of the resistance gene, to an examination of the distribution of resistance alleles in natural populations. Emphasis is given to the use of a resistant Drosophila mutant as an entry point to cloning the associated gamma-aminobutyric acid (GABA) receptor subunit gene, Resistance to dieldrin. Resistance is associated with replacements of a single amino acid (alanine302) in the chloride ion channel pore of the protein. Replacements of alanine302 not only directly affect the drug binding site but also allosterically destabilize the drug preferred conformation of the receptor. Resistance is thus conferred by a unique dual mechanism associated with alanine302, which is the only residue replaced in a wide range of different resistant insects. The underlying mutations appear either to have arisen once, or multiply, depending on the population biology of the pest insect. Although resistance frequencies decline in the absence of selection, resistance alleles can persist at relatively high frequency and may cause problems for compounds to which cross-resistance is observed, such as the novel fipronils.

Construction of modular and versatile plasmid vectors for the high-level expression of single or multiple genes in insects and insect cell lines

We have constructed a series of plasmid vectors for the expression of foreign genes in insects or insect cell lines. We incorporated the Drosophila hsp70 and actin 5C promoters, as well as the hr5 enhancer-driven baculovirus ie1 promoter, into plasmids that allow convenient cloning of heterologous genes into multiple cloning sites. We combined these promoters with either a short, double poly-adenylation site derived from the Heliothis virescens p63 chaperonin gene, or with a fusion of the small t intron with the early 3' untranslated region and poly-adenylation sites of SV40. Unique eight base cutter restriction sites flanking the promoters and poly-adenylation sequences make it possible to transfer the entire transcription units into other sequence contexts, for example, into transposable elements or into other plasmids bearing selectable marker genes. It is also convenient to combine two of our transcription units on the same plasmid in order to express multiple genes simultaneously. To test the ability of our vectors to drive expression of reporter genes, luciferase derivatives were made of the expression plasmids and introduced into Aedes albopictus C6/36 cells by electroporation or into Anopheles gambiae embryos by biolistic particle bombardment. All three promoters directed high levels of luciferase expression. However, there were differences in their relative activities in the two experimental systems. In C6/36 cells, the actin 5C and hr5-ie1 promoters were significantly more active than the hsp70 promoter. In Anopheles embryos, hsp70 and actin 5C had maximal activities, while hr5-ie1 was weaker. We also found that the constructs containing the SV40 small t intron and early 3' untranslated region sequences had higher expression levels than their counterparts containing the Heliothis poly-adenylation sequence. Our most active construct combines the actin 5C promoter with the SV40 intron and 3' untranslated region sequences. This vector was also used to drive expression of a visible marker, the enhanced green fluorescent protein gene, resulting in readily visible green fluorescent protein expression in C6/36 cells.

Differences in the expression and localization of human melanotransferrin in lepidopteran and dipteran insect cell lines

The ability of several lepidopteran and dipteran insect cell lines to express human melanotransferrin (p97), a glycosyl phosphatidylinositol (GPI)-anchored, iron-binding sialoglycoprotein, was assessed. Spodoptera frugiperda-derived (Sf9) cell lines, transformed with the p97 gene under control of a baculovirus immediate-early promoter, were able to constitutively express the protein and correctly attach it to the outer cell membrane via a GPI anchor as demonstrated by PI-PLC treatment. In contrast, stable constitutive expression could not be demonstrated with cell lines derived from either Drosophila melanogaster (Kc1 or SL2) or Lymantria dispar (Ld652Y) despite the observation that p97 could be detected in transient expression assays. This may indicate that the long-term expression and accumulation of p97 is inhibitory to Drosophila cells, possibly due to improper localization of the protein and resultant competition for cellular iron. In stably transformed Sf9 cells, p97 was expressed on the cell at a maximal level of 0.18 microg/10(6) cells and was secreted at a maximal rate of 9.03 ng/10(6) cells/h. This level was comparable to the amount expressed with the baculovirus system (0.37 microg/10(6) cells and 31.2 ng/10(6) cells/h) and transformed CHO cells (0.88 microg/10(6) cells and 7.8 ng/10(6) cells/h). Deletion of the GPI cleavage/attachment site resulted in an eightfold increase in the secretion rate of p97, when compared to the intact construct suggesting that the rate-limiting step involves processing of the GPI anchor.

Comparative analysis of promoters for transient gene expression in cultured mosquito cells

Three heterologous promoters (hsp70 and actin 5C from Drosophila melanogaster and IE1 from the immediate early gene of the Bombyx mori baculovirus) were assessed for their ability to drive transient luciferase expression in mosquito cells. Overall, the actin 5C promoter was considerably more effective at driving luciferase expression than either hsp70 or IE1 in cell lines derived from Anopheles, Aedes and Culex species. hsp70 functioned well when induced by heat shock and was also induced to a lesser extent by chemicals such as sodium arsenite. IE1 was also an effective initiator of transcription, particularly in two Anopheles cell lines, but generally it performed less well than the actin 5C promoter and was also outperformed by hsp70 in Anopheles gambiae cells.

Insect cells as hosts for the expression of recombinant glycoproteins

Baculovirus-mediated expression in insect cells has become well-established for the production of recombinant glycoproteins. Its frequent use arises from the relative ease and speed with which a heterologous protein can be expressed on the laboratory scale and the high chance of obtaining a biologically active protein. In addition to Spodoptera frugiperda Sf9 cells, which are probably the most widely used insect cell line, other mainly lepidopteran cell lines are exploited for protein expression. Recombinant baculovirus is the usual vector for the expression of foreign genes but stable transfection of - especially dipteran - insect cells presents an interesting alternative. Insect cells can be grown on serum free media which is an advantage in terms of costs as well as of biosafety. For large scale culture, conditions have been developed which meet the special requirements of insect cells. With regard to protein folding and post-translational processing, insect cells are second only to mammalian cell lines. Evidence is presented that many processing events known in mammalian systems do also occur in insects. In this review, emphasis is laid, however, on protein glycosylation, particularly N-glycosylation, which in insects differs in many respects from that in mammals. For instance, truncated oligosaccharides containing just three or even only two mannose residues and sometimes fucose have been found on expressed proteins. These small structures can be explained by post-synthetic trimming reactions. Indeed, cell lines having a low level of N-acetyl-beta-glucosaminidase, e.g. Estigmene acrea cells, produce N- glycans with non-reducing terminal N-acetylglucosamine residues. The Trichoplusia ni cell line TN-5B1-4 was even found to produce small amounts of galactose terminated N-glycans. However, there appears to be no significant sialylation of N-glycans in insect cells. Insect cells expressed glycoproteins may, though, be alpha1,3-fucosylated on the reducing-terminal GlcNAc residue. This type of fucosylation renders the N-glycans on one hand resistant to hydrolysis with PNGase F and on the other immunogenic. Even in the absence of alpha1,3-fucosylation, the truncated N-glycans of glycoproteins produced in insect cells constitute a barrier to their use as therapeutics. Attempts and strategies to "mammalianise" the N-glycosylation capacity of insect cells are discussed.

A series of broad host range shuttle vectors for constitutive and inducible expression of heterologous proteins in insect cell lines

A series of shuttle vectors have been constructed that allow expression of heterologous proteins in either dipteran or lepidopteran insect cell lines. Constitutive expression in a broad range of host cells is mediated by the Orgyia pseudotsugata multicapsid nucleopolyhedrosis virus (OpMNPV) immediate-early 2 (ie2) promoter. Alternatively, if inducible expression is required, for example to express cytotoxic proteins, a vector has been constructed that uses the Drosophila metallothionein (Mtn) promoter for metal-inducible protein expression in dipteran cell lines. A chimeric synthetic bacterial-OpMNPV ie promoter-Zeocin resistance gene cassette has been included to facilitate cloning in E. coli as well as the generation of stably transformed insect cell lines. The utility of the system is demonstrated by the constitutive and inducible expression of the highly processed glycosylphosphatidylinositol-anchored glycoprotein, human melanotransferrin, in transformed insect cell lines.

Stable insect cell cultures for recombinant protein production

Insect cells are relatively cheap to maintain and are capable of producing accurately translated and correctly processed heterologous proteins. Recent research has focused on the development of improved expression vectors for continuous, high-level production of foreign proteins, including a number of membrane-targeted receptors, in Drosophila and lepidopteran insect cells. Mosquito cells have also been employed for studies on the control of vector-borne diseases, such as malaria.