Expression of the human mu opioid receptor in a stable Sf9 cell line

Evaluation of Combinatory Effects of Plasmodium Circumsporozoite Protein and Complement Regulatory Protein Expression of Recombinant Baculovirus Vectors

Baculovirus vectors (BVs) are safely able to transduce foreign genes and express them in mammalian cells. However, the transduction activity of BVs is strongly reduced by the attack of serum complement, which is one of the major obstacles in the use of BVs for in vivo gene transfer. One strategy to overcome this problem is the display of complement regulatory proteins (CRPs) on BV virions. We previously developed CD46-decay accelerating factor (DAF)-CD59 triple fusion type BV showing potent complement resistance. We also developed BVs expressing Plasmodium circumsporozoite protein (CSP) to enhance transduction efficacy in hepatic cells. In this study, we investigated the combination of CSP and CRPs in a BV system to evaluate transduction efficacy along with complement resistance. To accomplish the combination of CSP and CRPs, we generated insect Sf9 cells stably expressing CRPs, to which CSP type BV was infected. The BVs collected from these infected cells were confirmed to possess both CSP and CRPs in virions. We demonstrated that CSP-CD46-DAF-CD59 type BV, containing both CSP and CD46-DAF-CD59, showed a significant increase in transduction efficacy in human hepatoma HepG2 cells under intact serum exposure compared with control type BV or CSP type BV, retaining both advantages of CSP and CD46-DAF-CD59. Collectively, these results demonstrated that the utilization of stably expressing Sf9 cells to introduce the protein products of interest, e.g., CRPs into BVs, would be useful strategy to generate BVs with novel functions such as resistance against serum complement attack.

Advancements in molecular design and bioprocessing of recombinant adeno-associated virus gene delivery vectors using the insect-cell baculovirus expression platform

Despite rapid progress in the field, scalable high-yield production of adeno-associated virus (AAV) is still one of the critical bottlenecks the manufacturing sector is facing. The insect cell-baculovirus expression vector system (IC-BEVS) has emerged as a mainstream platform for the scalable production of recombinant proteins with clinically approved products for human use. In this review, we provide a detailed overview of the advancements in IC-BEVS for rAAV production. Since the first report of baculovirus-induced production of rAAV vector in insect cells in 2002, this platform has undergone significant improvements, including enhanced stability of Bac-vector expression and a reduced number of baculovirus-coinfections. The latter streamlining strategy led to the eventual development of the Two-Bac, One-Bac, and Mono-Bac systems. The one baculovirus system consisting of an inducible packaging insect cell line was further improved to enhance the AAV vector quality and potency. In parallel, the implementation of advanced manufacturing approaches and control of critical processing parameters have demonstrated promising results with process validation in large-scale bioreactor runs. Moreover, optimization of the molecular design of vectors to enable higher cell-specific yields of functional AAV particles combined with bioprocess intensification strategies may also contribute to addressing current and future manufacturing challenges.

Accelerating HIV-1 VLP production using stable High Five insect cell pools

Stable cell pools are receiving a renewed interest as a potential alternative system to clonal cell lines. The shorter development timelines and the capacity to achieve high product yields make them an interesting approach for recombinant protein production. In this study, stable High Five cell pools are assessed for the production of a simple protein, mCherry, and the more complex HIV-1 Gag-eGFP virus-like particles (VLPs). Random integration coupled to fluorescence-activated cell sorting (FACS) in suspension conditions is applied to accelerate the stable cell pool generation process and enrich it with high producer cells. This methodology is successfully transferred to a bioreactor for VLP production, resulting in a 2-fold increase in VLP yields with respect to shake flask cultures. In these conditions, maximum viable cell concentration improves by 1.5-fold, and by-product formation is significantly reduced. Remarkably, a global increase in the uptake of amino acids in the Gag-eGFP stable cell pool is observed when compared with parental High Five cells, reflecting the additional metabolic burden associated with VLP production. These results suggest that stable High Five cell pools are a robust and powerful approach to produce VLPs and other recombinant proteins, and put the basis for future studies aiming to scale up this system.

Identification and characterization of highly active promoters from the fall armyworm, Spodoptera frugiperda

The cell lines derived from the fall armyworm (FAW), Spodoptera frugiperda, have been widely used for production of recombinant proteins for applications in both basic research and applications in medicine and agriculture. Promoters from the nucleopolyhedrovirus (NPV) are commonly used in these expression systems. These promoters have some limitations, which may be overcome by using promoters of genes from S. frugiperda. However, information on these promoters is not available. We identified several highly expressed genes from the transcriptomes of S. frugiperda midgut, fat body, epidermis, ovarian cell line (Sf9), and a midgut cell line (Sf17). The activity of potential promoters of 21 highly expressed genes was evaluated in Sf9 and Sf17 cells. Two of these promoters, SfHSC70-P1780 and SfPub-P2009, showed higher activity than commonly used hr5/ie1 (baculovirus enhancer element, hr5 and immediate early gene 1, ie1) promoter. Interestingly, the activity of these two promoters increased after adding hr5 enhancer element. The hr5/SfPub-P2009 promoter performance was evaluated by expressing an exogenous P450 protein in Sf9 cells using a plasmid-based expression system. The activity of this promoter was also evaluated in the FAW by expressing green fluorescence protein using the baculovirus expression system. In both cases, the hr5/SfPub-P2009 promoter performed better than the commonly used hr5/ie1 promoter. These strong endogenous promoters will be useful for studies in S. frugiperda and other lepidopteran insects for multiple applications, including protein expression, genome editing, and transgenic insects.

Infectious DNAs derived from insect-specific flavivirus genomes enable identification of pre- and post-entry host restrictions in vertebrate cells

Flaviviruses such as West Nile virus (WNV), dengue virus and Zika virus are mosquito-borne pathogens that cause significant human diseases. A novel group of insect-specific flaviviruses (ISFs), which only replicate in mosquitoes, have also been identified. However, little is known about the mechanisms of ISF host restriction. We report the generation of infectious cDNA from two Australian ISFs, Parramatta River virus (PaRV) and Palm Creek virus (PCV). Using circular polymerase extension cloning (CPEC) with a modified OpIE2 insect promoter, infectious cDNA was generated and transfected directly into mosquito cells to produce infectious virus indistinguishable from wild-type virus. When infectious PaRV cDNA under transcriptional control of a mammalian promoter was used to transfect mouse embryo fibroblasts, the virus failed to initiate replication even when cell entry steps were by-passed and the type I interferon response was lacking. We also used CPEC to generate viable chimeric viruses between PCV and WNV. Analysis of these hybrid viruses revealed that ISFs are also restricted from replication in vertebrate cells at the point of entry. The approaches described here to generate infectious ISF DNAs and chimeric viruses provide unique tools to further dissect the mechanisms of their host restriction.

Extensive use of FRET in biological imaging

Förster resonance energy transfer (FRET) is a phenomenon used for bioimaging ranging from single molecules to in vivo scale. A large variety of organic dyes and fluorescent proteins are available for FRET probes. In this review, we introduce the representative pairs of FRET probes developed thus far. The efficiency of FRET is depending on the spectral overlap of donor emission and acceptor absorption, the orientation of donor and acceptor and their distance. For FRET-based indicators composed of fluorescent proteins, their orientation and dimeric property of donor and acceptor largely affect the FRET efficiency, indicating the effect for the performance of indicators. In addition, three major applications of FRET, including genetically encoded indicators, single-molecule FRET, and enhancement of chemiluminescent proteins, have been introduced and their functions have also been discussed.

Enhanced early expression of membrane receptors with the Rous sarcoma virus promoter in baculovirus-infected insect cells

The effect of the Rous sarcoma virus (RSV) long terminal repeat enhancer/promoter on expression levels of complementary DNAs (cDNAs) encoding seven transmembrane receptors was studied using the baculovirus expression vector system. Expression of the human α(2B)-adrenoceptor (AR) cDNA under the control of the polyhedrin (POL) promoter produced up to 7.6 pmol/mg protein at 28 H post infection (p.i.) in Sf9 cells. The addition of the RSV promoter increased the expression to 11.6 pmol/mg protein. Dramatic increases in expression levels at early times were also obtained with the α(2A)-AR, the M₁ and M₄ muscarinic receptors, and the orexin OX₁ receptor. Analysis of the time-dependent expression revealed that expression driven by the RSV promoter reaches almost maximum 24 H p.i. and that this promoter is superior to the often used POL promoter at early times p.i. when functional studies need to be performed. Functional enhancement of signaling as a result of early expression is demonstrated with the α(2B)-AR and the OX₁ receptor. Finally, enhanced green fluorescent protein fluorescence in living cells was used to monitor expression by various viral promoters. The results verified the early transcriptional activity of the RSV promoter, whereas the cytomegalovirus promoter was found to be poorly active in Sf9 cells.

The Reporter System for GPCR Assay with the Fission Yeast Schizosaccharomyces pombe

G protein-coupled receptors (GPCRs) are associated with a great variety of biological activities. Yeasts are often utilized as a host for heterologous GPCR assay. We engineered the intense reporter plasmids for fission yeast to produce green fluorescent protein (GFP) through its endogenous GPCR pathway. As a control region of GFP expression on the reporter plasmid, we focused on seven endogenous genes specifically activated through the pathway. When upstream regions of these genes were used as an inducible promoter in combination with LPI terminator, the mam2 upstream region produced GFP most rapidly and intensely despite the high background. Subsequently, LPI terminator was replaced with the corresponding downstream regions. The SPBC4.01 downstream region enhanced the response with the low background. Furthermore, combining SPBC4.01 downstream region with the sxa2 upstream region, the signal to noise ratio was obviously better than those of other regions. We also evaluated the time- and dose-dependent GFP productions of the strains transformed with the reporter plasmids. Finally, we exhibited a model of simplified GPCR assay with the reporter plasmid by expressing endogenous GPCR under the control of the foreign promoter.

Ice breaking in GPCR structural biology

G-protein-coupled receptors (GPCRs) are one of the most challenging targets in structural biology. To successfully solve a high-resolution GPCR structure, several experimental obstacles must be overcome, including expression, extraction, purification, and crystallization. As a result, there are only a handful of unique structures reported from this protein superfamily, which consists of over 800 members. In the past few years, however, there has been an increase in the amount of solved GPCR structures, and a few high-impact structures have been determined: the peptide receptor CXCR4, the agonist bound receptors, and the GPCR-G protein complex. The dramatic progress in GPCR structural studies is not due to the development of any single technique, but a combination of new techniques, new tools and new concepts. Here, we summarize the progress made for GPCR expression, purification, and crystallization, and we highlight the technical advances that will facilitate the future determination of GPCR structures.

Functional significance of cleavable signal peptides of G protein-coupled receptors

About 5-10% of the G protein-coupled receptors (GPCRs) contain N-terminal signal peptides that are cleaved off by the signal peptidases of the endoplasmic reticulum (ER) during the translocon-mediated receptor insertion into the ER membrane. The reason as to why only a subset of the GPCRs requires these additional signal peptides was addressed in the past decade only by a limited number of studies. Recent progress suggests that signal peptides of GPCRs do not only serve the classical ER targeting and translocon gating functions as described for the signal peptides of secretory proteins. In the case of GPCRs, uncleaved pseudo signal peptides may regulate receptor expression at the plasma membrane and may also influence G protein coupling. Moreover, signal peptides of GPCRs seem to match functionally with sequences of the mature N tails. In this review, we summarize the current knowledge about cleavable signal peptides of GPCRs and address the question whether these sequences may be future drug targets in pharmacology.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Heterologous expression and purification systems for structural proteomics of mammalian membrane proteins

Membrane proteins (MPs) are responsible for the interface between the exterior and the interior of the cell. These proteins are implicated in numerous diseases, such as cancer, cystic fibrosis, epilepsy, hyperinsulinism, heart failure, hypertension and Alzheimer's disease. However, studies on these disorders are hampered by a lack of structural information about the proteins involved. Structural analysis requires large quantities of pure and active proteins. The majority of medically and pharmaceutically relevant MPs are present in tissues at very low concentration, which makes heterologous expression in large-scale production-adapted cells a prerequisite for structural studies. Obtaining mammalian MP structural data depends on the development of methods that allow the production of large quantities of MPs. This review focuses on the different heterologous expression systems, and the purification strategies, used to produce large amounts of pure mammalian MPs for structural proteomics.

The sequence after the signal peptide of the G protein-coupled endothelin B receptor is required for efficient translocon gating at the endoplasmic reticulum membrane

The heptahelical G protein-coupled receptors (GPCRs) must reach their correct subcellular location to exert their function. Receptor domains relevant for receptor trafficking include signal sequences mediating receptor integration into the membrane of the endoplasmic reticulum (ER) and anterograde or retrograde transport signals promoting receptor sorting into the vesicles of the secretory pathway. In addition, receptors must be correctly folded to pass the quality control system of the early secretory pathway. Taking the endothelin B receptor as a model, we describe a new type of a transport-relevant GPCR domain. Deletion of this domain (residues Glu(28) to Trp(54)) leads to a fully functional receptor protein that is expressed at a lower level than the wild-type receptor. Subcellular localization experiments and glycosylation state analyses demonstrate that the mutant receptor is neither misfolded, retained intracellularly, nor misrouted. Fluorescence recovery after photobleaching analyses demonstrate that constitutive internalization is also not affected. By using an in vitro prion protein targeting assay, we show that this domain is necessary for efficient translocon gating at the ER membrane during early receptor biogenesis. Taken together, we identified a novel transport-relevant domain in the GPCR protein family. Our data may also be relevant for other GPCRs and unrelated integral membrane proteins.

Production and characterization of the recombinant human mu-opioid receptor from transgenic silkworms

The production of useful quantities of G protein-coupled receptors is a major problem not only for screening of various drug compounds but also in performing structural biology studies. To solve this problem, we investigated the possibility of using transgenic silkworms for the production of these receptors. Using the human mu-opioid receptor gene, we constructed three transgenic silkworm strains that produced mu-opioid receptors. The silkworms expressed significant amounts of the receptor in the fat body and silk gland. The product was evaluated using a saturation ligand-binding assay. The expressed receptor exhibited ligand affinity similar to that of an authentic sample, and the yield from the transgenic silkworm was comparable to that obtained using an Sf9-baculovirus expression system. As the mass rearing of transgenic silkworms has already been established, the silkworms can be adapted for production of large quantities of receptors.

Using EGFP fusions to monitor the functional expression of GPCRs in the Drosophila Schneider 2 cells

In combining fluorescence measurements with ligand binding assays, the versatility of the EGFP C-terminally fused to the human mu opioid receptor (EGFP-hMOR) has been exploited to notably improve the expression level of functional G protein-coupled receptors in Drosophila S2 cells. A selected array of efficient optimization approaches is presented herein, ranging from a cell-sorting method, allowing for a substantial enrichment in EGFP-hMOR expressing cells, to the addition of chemical and pharmacological chaperones, significantly enhancing the yield and the activity of the expressed receptors. Consistent with previous studies, significant discrepancies were observed between the total amounts of fluorescent receptors over a limited subpopulation capable of ligand binding, even after expression optimization. Subsequently, membrane isopycnic centrifugation experiments allowed to separate the ligand binding active from the non-active membrane fraction, the latter most probably containing misfolded receptors. Taken together, these results illustrate a coherent set of advantageous productive and preparative methods for the production of GPCRs in the highly valuable Drosophila S2 expression system.

Two-photon excitation fluorescence cross-correlation assay for ligand-receptor binding: cell membrane nanopatches containing the human micro-opioid receptor

Current ligand-receptor binding assays for G-protein coupled receptors cannot directly measure the system's dissociation constant, Kd, without purification of the receptor protein. Accurately measured Kd's are essential in the development of a molecular level understanding of ligand-receptor interactions critical in rational drug design. Here we report the introduction of two-photon excitation fluorescence cross-correlation spectroscopy (TPE-FCCS) to the direct analysis of ligand-receptor interactions of the human micro opioid receptor (hMOR) for both agonists and antagonists. We have developed the use of fluorescently distinct, dye-labeled hMOR-containing cell membrane nanopatches ( approximately 100-nm radius) and ligands, respectively, for this assay. We show that the output from TPE-FCCS data sets can be converted to the conventional Hill format, which provides Kd and the number of active receptors per nanopatch. When ligands are labeled with quantum dots, this assay can detect binding with ligand concentrations in the subnanomolar regime. Interestingly, conjugation to a bulky quantum dot did not adversely affect the binding propensity of the hMOR pentapeptide ligand, Leu-enkephalin.

Heterologous expression and comparative characterization of the human neuromedin U subtype II receptor using the methylotrophic yeast Pichia pastoris and mammalian cells

Neuromedin U (a neuropeptide) plays regulatory roles in feeding, anxiety, smooth muscle contraction, blood flow and pain. The physiological actions of NmU are mediated via two recently identified G protein-coupled receptors namely the neuromedin U type 1 receptor (NmU(1)R) and the neuromedin U type 2 receptor (NmU(2)R). Despite their crucial roles in cell physiology, structural information on these receptors is limited, mainly due to their low expression levels in native tissues. Here, we report the overexpression of the human NmU(2)R in the methylotrophic yeast Pichia pastoris and baby hamster kidney (BHK) cells using the Semliki Forest virus (SFV) system. The recombinant receptor was expressed as a fusion protein with three different affinity tags namely, the Flag tag, the histidine 10 tag and the biotinylation domain of Propionobacterium shermanii. Expression level of the recombinant receptor was 6-9pmol/mg under optimized conditions, which is significantly higher than the expression level in the native tissues. The recombinant receptor binds to its endogenous ligand neuromedin U with high affinity (Kd=0.8-1.0nM) and the binding constant for the recombinant receptor is similar to that of the wild type NmU(2)R. Enzymatic deglycosylation suggested that the recombinant NmU(2)R was glycosylated in P. pastoris, but not in BHK cells. Confocal laser scanning microscopy and immunogold labelling experiment revealed that the recombinant receptor was predominantly localized in the intracellular membranes. To our knowledge, this is the first report of heterologous overexpression of an affinity tagged recombinant NmU(2)R and it should facilitate further characterization of this receptor.

Comparative analysis of the human angiotensin II type 1a receptor heterologously produced in insect cells and mammalian cells

Angiotensin II type 1a receptor (AT1aR) is a member of GPCR superfamily and it plays crucial roles in the regulation of blood pressure, hormone secretion and renal functions. Here, we report functional overexpression and characterization of the human AT1aR in insect cells using the baculovirus system and in mammalian cells using the Semliki Forest virus system. The recombinant receptor was expressed at a level of 29-32 pmol/mg and it binds to angiontensin II with high affinity (Kd=0.98-1.1 nM). Angiotensin II stimulated accumulation of inositol phosphate and phosphorylation of MAP kinase was also observed, which indicated that the recombinant AT1aR could couple to endogenous Galphaq protein. Confocal laser scanning microscopy revealed that the recombinant receptor was predominantly localized in the plasma membrane and agonist induced internalization of the recombinant AT1aR was also observed. The recombinant AT1aR was expressed in glycosylated form and in vivo inhibition of glycosylation suppressed its surface expression.

Stably Transformed Insect Cell Lines: Tools for Expression of Secreted and Membrane‐anchored Proteins and High‐throughput Screening Platforms for Drug and Insecticide Discovery

Insect cell-based expression systems are prominent amongst current expression platforms for their ability to express virtually all types of heterologous recombinant proteins. Stably transformed insect cell lines represent an attractive alternative to the baculovirus expression system, particularly for the production of secreted and membrane-anchored proteins. For this reason, transformed insect cell systems are receiving increased attention from the research community and the biotechnology industry. In this article, we review recent developments in the field of insect cell-based expression from two main perspectives, the production of secreted and membrane-anchored proteins and the establishment of novel methodological tools for the identification of bioactive compounds that can be used as research reagents and leads for new pharmaceuticals and insecticides.

Bioprocess parameters of cell growth and human μ opioid receptor expression in recombinant Drosophila S2 cell cultures in a bioreactor

The paper describes a recombinant Schneider 2 (rS2) cell culture and protein expression in a bioreactor. S2 cells were transfected with a plasmid containing a fusion protein (human mu opioid receptor, hMOR, and green fluorescent protein, EGFP) under the control of inducible metallothionein promoter. A bioprocess in a bioreactor with 5% dissolved oxygen, 27 degrees C and 120 rpm enabled the cell culture to attain 5.3x10(7 )viable cells/mL at 96 h. The induction decreased the cell multiplication (2.5x10(7) viable cells/mL at 72 h). Glutamine and glucose and low levels of lactate were consumed. A fast recombinant protein synthesis took place and, at 6 h of induction, 2x10(4) receptors/cell could be detected by a functional binding assay. Fluorescence measurements showed a progressive increase of recombinant protein expression with a maximal value of 1.26x10(5) fluo counts/s at 24 h of induction. The data shown in this paper indicate a practical and scaleable cell culture bioprocess procedure for the preparation of recombinant proteins expressed in S2 cells.

The signal peptide of the rat corticotropin-releasing factor receptor 1 promotes receptor expression but is not essential for establishing a functional receptor

Approximately 5-10% of the GPCRs (G-protein-coupled receptors) contain N-terminal signal peptides that are cleaved off during receptor insertion into the ER (endoplasmic reticulum) membrane by the signal peptidases of the ER. The reason as to why only a subset of GPCRs requires these additional signal peptides is not known. We have recently shown that the signal peptide of the human ET(B)-R (endothelin B receptor) does not influence receptor expression but is necessary for the translocation of the receptor's N-tail across the ER membrane and thus for the establishment of a functional receptor [Köchl, Alken, Rutz, Krause, Oksche, Rosenthal and Schülein (2002) J. Biol. Chem. 277, 16131-16138]. In the present study, we show that the signal peptide of the rat CRF-R1 (corticotropin-releasing factor receptor 1) has a different function: a mutant of the CRF-R1 lacking the signal peptide was functional and displayed wild-type properties with respect to ligand binding and activation of adenylate cyclase. However, immunoblot analysis and confocal laser scanning microscopy revealed that the mutant receptor was expressed at 10-fold lower levels than the wild-type receptor. Northern-blot and in vitro transcription translation analyses precluded the possibility that the reduced receptor expression is due to decreased transcription or translation levels. Thus the signal peptide of the CRF-R1 promotes an early step of receptor biogenesis, such as targeting of the nascent chain to the ER membrane and/or the gating of the protein-conducting translocon of the ER membrane.

Expression and spectroscopic characterization of a large fragment of the μ-opioid receptor

We report here a procedure for the production in Escherichia coli and subsequent purification and characterization of an 80-residue fragment of the human mu-opioid receptor. The fragment ('TM2-3'), which comprises the second and third transmembrane segments as well as the first extracellular loop of the receptor, was expressed as a fusion with glutathione-S-transferase. The fusion protein, which accumulated in insoluble inclusion bodies, was solubilized with N-lauroylsarcosine, and TM2-3 was obtained by thrombin cleavage of the fusion protein followed by reversed-phase HPLC purification. CD spectroscopy of TM2-3 in lysophosphatidylcholine micelles showed that TM2-3 adopts approximately 50% alpha-helical structure in this environment, with the remainder consisting of disordered and/or beta-structure. This is consistent with the assumption of an alpha-helical structure by the two membrane-spanning regions and a nonhelical structure in the loop region of TM2-3. Fluorescence spectroscopy and fluorescence quenching experiments suggested that the extracellular loop lies near the surface of the lysophosphatidylcholine micelle. Our work shows that the study of large receptor fragments is a technically accessible approach to the study of the structural properties of the mu-opioid receptor and, possibly, other G-protein-coupled receptors as well.

Stable expression of recombinant human α3/4 fucosyltransferase III in Spodoptera frugiperda Sf9 cells

Human alpha3/4 fucosyltransferase III (FT3; EC 2.4.1.65) synthesizes fucosylated glycoconjugates, namely the Lewis (Le) determinants. FT3 is detected in milk, gastric mucosa, kidney and other organs, but is found in very low amounts in these native tissues. In this work, we describe the expression of a soluble secretory form of FT3 (SFT3) in Spodoptera frugiperda (Sf9) insect cells using a non-lytic vector system. The coding sequence was cloned into the expression vector pIB/V5-His-TOPO which contains the transcriptional control of the Orgyia pseudotsugata multicapsid nucleopolyhedrosis virus immediate-early 2 (OpIE2) promoter. Transfected cells were selected using blasticidin-HCl. It was observed that the secreted activity SFT3 increased until the sixth day of culture when it reached the value 1.9 mU x 10(-6) cells and 13.4 mg/l, whereas only 5% of activity was retained inside the cells. Western blot analysis of secreted and intracellularly retained SFT3 had a similar variation. Comparison of the stable with the lytic baculovirus expression system showed that the former yielded approx. 13-fold more active SFT3, which was possibly due to a lower accumulation of intracellular SFT3.

Expression of EGFP-amino-tagged human mu opioid receptor in Drosophila Schneider 2 cells: a potential expression system for large-scale production of G-protein coupled receptors

The G-protein coupled receptor (GPCR) human mu opioid receptor (hMOR) fused to the carboxy-terminus of the enhanced green fluorescent protein (EGFP) has been successfully and stably expressed in Drosophila Schneider 2 cells under the control of an inducible metallothionein promoter. Polyclonal cells expressing EGFPhMOR display high-affinity, saturable, and specific binding sites for the opioid antagonist diprenorphine. Competition studies with opioid agonists and antagonists defined the pharmacological profile of a mu opioid receptor similar to that observed in mammalian cells, suggesting proper folding of EGFPhMOR in a high-affinity state in Drosophila cells. The functionality of the fusion protein was demonstrated by the ability of agonist to reduce forskolin-stimulated cyclic AMP production and to induce [35S]GTPgammaS incorporation. The EGFPhMOR protein had the expected molecular weight (70kDa), as demonstrated by protein immunoblotting with anti-EGFP and anti-C-terminus hMOR antibodies. However, quantitative EGFP fluorescence intensity analysis revealed that the total level of expressed EGFPhMOR is 8-fold higher than the level of diprenorphine binding sites, indicating that part of the receptor is not in a high-affinity state. This may in part be due to a population of receptors localized in intracellular compartments, as shown by the distribution of fluorescence between the plasma membrane and the cell interior. This study shows that EGFP is a valuable and versatile tool for monitoring and quantifying expression levels as well as for optimizing and characterizing an expression system. Optimization of the Drosophila Schneider 2 cell expression system will allow large-scale purification of GPCRs, thus enabling structural studies to be undertaken.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.