Heavy chain dimers as well as complete antibodies are efficiently formed and secreted from Drosophila via a BiP-mediated pathway

Crystal Structure and Characterization of Human Heavy-Chain Only Antibodies Reveals a Novel, Stable Dimeric Structure Similar to Monoclonal Antibodies

We report the novel crystal structure and characterization of symmetrical, homodimeric humanized heavy-chain-only antibodies or dimers (HC2s). HC2s were found to be significantly coexpressed and secreted along with mAbs from transient CHO HC/LC cotransfection, resulting in an unacceptable mAb developability attribute. Expression of full-length HC2s in the absence of LC followed by purification resulted in HC2s with high purity and thermal stability similar to conventional mAbs. The V_H and C_H1 portion of the heavy chain (or Fd) was also efficiently expressed and yielded a stable, covalent, and reducible dimer (Fd2). Mutagenesis of all heavy chain cysteines involved in disulfide bond formation revealed that Fd2 intermolecular disulfide formation was similar to Fabs and elucidated requirements for Fd2 folding and expression. For one HC2, we solved the crystal structure of the Fd2 domain to 2.9 Å, revealing a highly symmetrical homodimer that is structurally similar to Fabs and is mediated by conserved (C_H1) and variable (V_H) contacts with all CDRs positioned outward for target binding. Interfacial dimer contacts revealed by the crystal structure were mutated for two HC2s and were found to dramatically affect HC2 formation while maintaining mAb bioactivity, offering a potential means to modulate novel HC2 formation through engineering. These findings indicate that human heavy-chain dimers can be secreted efficiently in the absence of light chains, may show good physicochemical properties and stability, are structurally similar to Fabs, offer insights into their mechanism of formation, and may be amenable as a novel therapeutic modality.

Functional analysis of RIP toxins from the Drosophila endosymbiont Spiroplasma poulsonii

Background

Insects frequently live in close relationship with symbiotic bacteria that carry out beneficial functions for their host, like protection against parasites and viruses. However, in some cases, the mutualistic nature of such associations is put into question because of detrimental phenotypes caused by the symbiont. One example is the association between the vertically transmitted facultative endosymbiont Spiroplasma poulsonii and its natural host Drosophila melanogaster. Whereas S. poulsonii protects its host against parasitoid wasps and nematodes by the action of toxins from the family of Ribosome Inactivating Proteins (RIPs), the presence of S. poulsonii has been reported to reduce host’s life span and to kill male embryos by a toxin called Spaid. In this work, we investigate the harmful effects of Spiroplasma RIPs on Drosophila in the absence of parasite infection.

Results

We show that only two Spiroplasma RIPs (SpRIP1 and SpRIP2) among the five RIP genes encoded in the S. poulsonii genome are significantly expressed during the whole Drosophila life cycle. Heterologous expression of SpRIP1 and 2 in uninfected flies confirms their toxicity, as indicated by a reduction of Drosophila lifespan and hemocyte number. We also show that RIPs can cause the death of some embryos, including females.

Conclusion

Our results indicate that RIPs released by S. poulsonii contribute to the reduction of host lifespan and embryo mortality. This suggests that SpRIPs may impact the insect-symbiont homeostasis beyond their protective function against parasites.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1410-1) contains supplementary material, which is available to authorized users.

Structure-Function Dissection of Pseudorabies Virus Glycoprotein B Fusion Loops

Conserved across the family Herpesviridae, glycoprotein B (gB) is responsible for driving fusion of the viral envelope with the host cell membrane for entry upon receptor binding and activation by the viral gH/gL complex. Although crystal structures of the gB ectodomains of several herpesviruses have been reported, the membrane fusion mechanism has remained elusive. Here, we report the X-ray structure of the pseudorabies virus (PrV) gB ectodomain, revealing a typical class III postfusion trimer that binds membranes via its fusion loops (FLs) in a cholesterol-dependent manner. Mutagenesis of FL residues allowed us to dissect those interacting with distinct subregions of the lipid bilayer and their roles in membrane interactions. We tested 15 gB variants for the ability to bind to liposomes and further investigated a subset of them in functional assays. We found that PrV gB FL residues Trp187, Tyr192, Phe275, and Tyr276, which were essential for liposome binding and for fusion in cellular and viral contexts, form a continuous hydrophobic patch at the gB trimer surface. Together with results reported for other alphaherpesvirus gBs, our data suggest a model in which Phe275 from the tip of FL2 protrudes deeper into the hydrocarbon core of the lipid bilayer, while the side chains of Trp187, Tyr192, and Tyr276 form a rim that inserts into the more superficial interfacial region of the membrane to catalyze the fusion process. Comparative analysis with gBs from beta- and gamma-herpesviruses suggests that this membrane interaction model is valid for gBs from all herpesviruses.IMPORTANCE Herpesviruses are common human and animal pathogens that infect cells by entering via fusion of viral and cellular membranes. Central to the membrane fusion event is glycoprotein B (gB), which is the most conserved envelope protein across the herpesvirus family. Like other viral fusion proteins, gB anchors itself in the target membrane via two polypeptide segments called fusion loops (FLs). The molecular details of how gB FLs insert into the lipid bilayer have not been described. Here, we provide structural and functional data regarding key FL residues of gB from pseudorabies virus, a porcine herpesvirus of veterinary concern, which allows us to propose, for the first time, a molecular model to understand how the initial interactions by gBs from all herpesviruses with target membranes are established.

Successful synthesis of active human coagulation factor VII by co-expression of mammalian gamma-glutamyl carboxylase and modification of vit.K cycle in Drosophila Schneider S2 cells

Mammalian gamma-glutamyl carboxylase and reduced vitamin K are indispensable for synthesis of mature mammalian vitamin K dependent proteins including some of blood coagulation factors (factors II, VII, IX, and X). It was well known that Drosophila melanogaster expressed gamma-glutamyl carboxylase and possessed a vit.K cycle although native substrates for them have not been identified yet. Despite the potential capability of gamma carboxylation in D. melanogaster derived cells such as S2 cells, Drosophila gamma-glutamyl carboxylase failed to gamma carboxylate a peptide fused to the human coagulation factor IX propeptide. Thus, it had been believed that the Drosophila system was not adequate to synthesize mammalian vit.K dependent proteins. Indeed, we previously attempted to synthesize biologically active factor VII in S2 cells although we were not able to obtain it. However, recently, a successful transient expression of biologically active human factor IX from S2 cells was reported. In the present study, several expression vectors which enable expressing mammalian GGCX, VKORC1, and/or PDIA2 along with F7 were developed. S2 cells transfected with pMKA85, pMAK86, and pMAK219 successfully synthesized active FVII. Thus, mammalian GGCX was indispensable to synthesize active FVII while mammalian VKORC1 and PDIA2 were not critical but supportive factors for S2 cells.

Expression and immunogenicity of recombinant glycoprotein D of herpes simplex virus 1 in Drosophila S2 cells

Herpes simplex virus type 1 (HSV-1) is responsible for cold sores in the general population, but also contributes to the development of other more serious diseases in some circumstances. The viral glycoprotein D (gD) is essential for virus entry into host cells. In the present study, the Drosophila melanogaster Schneider 2 (S2) expression system (DES) was evaluated for the expression of recombinant gD1. The DNA sequences encoding the full-length gD1 (369aa, FLgD1) and a truncated gD1 form corresponding to the ectodomain (314aa, EgD1) were cloned into S2 expression vector pMT/BiP/V5-HisA to generate pMT-EgD1 and pMT-FLgD1, respectively. Two forms of gD1 gene were fitted with a hexahistidine tag to facilitate their purification. Cell populations expressing the highest gD1 levels were selected by using a limiting dilution assay. Western blot, flow cytometry (FACS), and confocal immunofluoresence assay demonstrated that the full-length form is restrained in the lipid membranes of the cell and the ectodomain form is secreted into the medium. Recombinant ectodomain gD1 was scaled up and purified from the culture medium using nickel nitrilotriacetic acid affinity chromatography, and a maximum production level of 56.8 mg/L of recombinant gD1 was obtained in a shake-flask culture of S2 cells after induction with 5 µM CdCl2 for 4 days. Mice were then immunized with recombinant purified gD1 and produced high titers of antibody measured by enzyme-linked immunosorbent assay (ELISA; 1:5,120,000) as well as high plaque neutralization titer (1:320). Overall, the data indicated that stable expression in S2 cells is a practical way of synthesizing gD1 for use in structural and functional studies in the further study.

TRP and Rhodopsin Transport Depends on Dual XPORT ER Chaperones Encoded by an Operon

TRP channels and G protein-coupled receptors (GPCRs) play critical roles in sensory reception. However, the identities of the chaperones that assist GPCRs in translocating from the endoplasmic reticulum (ER) are limited, and TRP ER chaperones are virtually unknown. The one exception for TRPs is Drosophila XPORT. Here, we show that the xport locus is bicistronic and encodes unrelated transmembrane proteins, which enable the signaling proteins that initiate and culminate phototransduction, rhodopsin 1 (Rh1) and TRP, to traffic to the plasma membrane. XPORT-A and XPORT-B are ER proteins, and loss of either has a profound impact on TRP and Rh1 targeting to the light-sensing compartment of photoreceptor cells. XPORT-B complexed in vivo with the Drosophila homolog of the mammalian HSP70 protein, GRP78/BiP, which, in turn, associated with Rh1. Our work highlights a coordinated network of chaperones required for the biosynthesis of the TRP channel and rhodopsin in Drosophila photoreceptor cells.

A Catalog of Proteins Expressed in the AG Secreted Fluid during the Mature Phase of the Chinese Mitten Crabs (Eriocheir sinensis)

The accessory gland (AG) is an important component of the male reproductive system of arthropods, its secretions enhance fertility, some AG proteins bind to the spermatozoa and affect its function and properties. Here we report the first comprehensive catalog of the AG secreted fluid during the mature phase of the Chinese mitten crab (Eriocheir sinensis). AG proteins were separated by one-dimensional gel electrophoresis and analyzed by reverse phase high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS). Altogether, the mass spectra of 1173 peptides were detected (1067 without decoy and contaminants) which allowed for the identification of 486 different proteins annotated upon the NCBI database (http://www.ncbi.nlm.nih.gov/) and our transcritptome dataset. The mass spectrometry proteomics data have been deposited at the ProteomeXchange with identifier PXD000700. An extensive description of the AG proteome will help provide the basis for a better understanding of a number of reproductive mechanisms, including potentially spermatophore breakdown, dynamic functional and morphological changes in sperm cells and sperm acrosin enzyme vitality. Thus, the comprehensive catalog of proteins presented here can serve as a valuable reference for future studies of sperm maturation and regulatory mechanisms involved in crustacean reproduction.

Development of a single-replicon miniBYV vector for co-expression of heterologous proteins

In planta production of recombinant proteins, including vaccine antigens and monoclonal antibodies, continues gaining acceptance. With the broadening range of target proteins, the need for vectors with higher performance is increasing. Here, we have developed a single-replicon vector based on beet yellows virus (BYV) that enables co-delivery of two target genes into the same host cell, resulting in transient expression of each target. This BYV vector maintained genetic stability during systemic spread throughout the host plant, Nicotiana benthamiana. Furthermore, we have engineered a miniBYV vector carrying the sequences encoding heavy and light chains of a monoclonal antibody (mAb) against protective antigen (PA) of Bacillius anthracis, and achieved the expression of the full-length functional anti-PA mAb at ~300 mg/kg of fresh leaf tissue. To demonstrate co-expression and functionality of two independent proteins, we cloned the sequences of the Pfs48/45 protein of Plasmodium falciparum and endoglycosidase F (PNGase F) from Flavobacterium meningosepticum into the miniBYV vector under the control of two subgenomic RNA promoters. Agroinfiltration of N. benthamiana with this miniBYV vector resulted in accumulation of biologically active Pfs48/45 that was devoid of N-linked glycosylation and had correct conformation and epitope display. Overall, our findings demonstrate that the new BYV-based vector is capable of co-expressing two functionally active recombinant proteins within the same host cell.

Engineering N-Glycosylation Pathway in Insect Cells: Suppression of β-N-Acetylglucosaminidase and Expression of β-1,4-Galactosyltransferase

Most insect cells have a simple N-glycosylation process and consequently paucimannosidic or simple core glycans predominate. It has been proposed that β-N-acetylglucosaminidase (GlcNAcase), a hexosaminidase in the Golgi membrane which removes a terminal N-acetylglucosamine (GlcNAc), might contribute to simple N-glycosylation profile in several insect cells including Drosophila S2. Here, we describe GlcNAcase suppression strategy using RNA interference (RNAi) to avoid the formation of paucimannosidic glycans in insect S2 cells. In addition, we describe coexpression of β(1,4)-galactosyltransferase (GalT) as a strategy to improve N-glycosylation pattern and enable recombinant therapeutic proteins to be produced in S2 cells with more complex N-glycans.

Differential centrifugation-based biochemical fractionation of the Drosophila adult CNS

Drosophila is widely used as a genetic model in questions of development, cellular function and disease. Genetic screens in flies have proven to be incredibly powerful in identifying crucial components for synapse formation and function, particularly in the case of the presynaptic release machinery. Although modern biochemical methods can identify individual proteins and lipids (and their binding partners), they have typically been excluded from use in Drosophila for technical reasons. To bridge this essential gap between genetics and biochemistry, we developed a fractionation method to isolate various parts of the synaptic machinery from Drosophila, thus allowing it to be studied in unprecedented biochemical detail. This is only possible because our protocol has unique advantages in terms of enriching and preserving endogenous protein complexes. The procedure involves decapitation of adult flies, homogenization and differential centrifugation of fly heads, which allow subsequent purification of presynaptic (and to a limited degree postsynaptic) components. It is designed to require only a rudimentary knowledge of biochemical fractionation, and it takes ∼3.5 h. The yield is typically 4 mg of synaptic membrane protein per gram of Drosophila heads.

Glycosylation: impact, control and improvement during therapeutic protein production

The emergence of the biopharmaceutical industry represented a major revolution for modern medicine, through the development of recombinant therapeutic proteins that brought new hope for many patients with previously untreatable diseases. There is a ever-growing demand for these therapeutics that forces a constant technological evolution to increase product yields while simultaneously reducing costs. However, the process changes made for this purpose may also affect the quality of the product, a factor that was initially overlooked but which is now a major focus of concern. Of the many properties determining product quality, glycosylation is regarded as one of the most important, influencing, for example, the biological activity, serum half-life and immunogenicity of the protein. Consequently, monitoring and control of glycosylation is now critical in biopharmaceutical manufacturing and a requirement of regulatory agencies. A rapid evolution is being observed in this context, concerning the influence of glycosylation in the efficacy of different therapeutic proteins, the impact on glycosylation of a diversity of parameters/processes involved in therapeutic protein production, the analytical methodologies employed for glycosylation monitoring and control, as well as strategies that are being explored to use this property to improve therapeutic protein efficacy (glycoengineering). This work reviews the main findings on these subjects, providing an up-to-date source of information to support further studies.

High yield of human monoclonal antibody produced by stably transfected Drosophila schneider 2 cells in perfusion culture using wave bioreactor

Since it was first introduced in late 1990s Wave bioreactor has been used for protein production by mammalian and insect cell lines. However, using Wave bioreactor to produce human monoclonal antibody by stable Drosophila Schneider 2 (S2) cell transfectants has not been reported before. In this study, S2 cells were co-transfected with an inducible vector expressing human monoclonal antibody heavy and light chains, respectively, specific for hemagglutinin (HA) of H5N1 influenza virus. Stable S2 transfectant clone was selected by limiting dilution assay. Stable S2 transfectant clone that produce the highest amount of human monoclonal antibody was inoculated into two 2-l disposable cellbags, where cell growth and antibody production were compared between batch and perfusion cultures using Wave bioreactor. Here, we report that maximum viable cell density reached 1.06 × 10(7) cells/ml in batch culture; whereas 1.04 × 10(8) cells/ml was achieved in perfusion culture. The maximum volumetric antibody productivity in batch culture was 52 mg/l/day; while perfusion culture yielded 1,437 mg/l/day. As a result, the total antibody production was 201 mg in batch culture and 8,212 mg in perfusion culture. The antibody produced by both cultures displays full neutralizing activity. Thus, our results provide strong support for using Wave bioreactor in perfusion culture for a large-scale production of human monoclonal antibody by stable S2 cell transfectants.

Production of recombinant antibodies in Drosophila melanogaster S2 cells

One of the major bottlenecks in antibody discovery for research and therapeutic applications is the need for large quantities of protein in a short amount of time. Here we describe an alternative method using the Drosophila melanogaster S2 expression system to produce high levels of antibodies (both IgG and Fab) with equivalent binding properties to antibodies produced in mammalian cell expression systems. Using the Drosophila S2 expression system for antibody production has many advantages over current mammalian systems making antibody expression, purification, and evaluation a much less time-consuming process.

Drosophila melanogaster S2 cells for expression of heterologous genes: From gene cloning to bioprocess development

In the present review we discuss strategies that have been used for heterologous gene expression in Drosophila melanogaster Schneider 2 (S2) cells using plasmid vectors. Since the growth of S2 cells is not dependent on anchorage to solid substrates, these cells can be easily cultured in suspension in large volumes. The factors that most affect the growth and gene expression of S2 cells, namely cell line, cell passage, inoculum concentration, culture medium, temperature, dissolved oxygen concentration, pH, hydrodynamic forces and toxic metabolites, are discussed by comparison with other insect and mammalian cells. Gene expression, cell metabolism, culture medium formulation and parameters involved in cellular respiration are particularly emphasized. The experience of the authors with the successful expression of a biologically functional protein, the rabies virus glycoprotein (RVGP), by recombinant S2 cells is presented in the topics covered.

Targeting antibodies to the cytoplasm

A growing number of research consortia are now focused on generating antibodies and recombinant antibody fragments that target the human proteome. A particularly valuable application for these binding molecules would be their use inside a living cell, e.g., for imaging or functional intervention. Animal-derived antibodies must be brought into the cell through the membrane, whereas the availability of the antibody genes from phage display systems allows intracellular expression. Here, the various technologies to target intracellular proteins with antibodies are reviewed.

Genetic encoding of non-natural amino acids in Drosophila melanogaster Schneider 2 cells

Insect cells are useful for the high-yield production of recombinant proteins including chemokines and membrane proteins. In this study, we developed an insect cell-based system for incorporating non-natural amino acids into proteins at specific sites. Three types of promoter systems were constructed, and their efficiencies were compared for the expression of the prokaryotic amber suppressor tRNA(Tyr) in Drosophila melanogaster Schneider 2 cells. When paired with a variant of Escherichia coli tyrosyl-tRNA synthetase specific for 3-iodo-L-tyrosine, the suppressor tRNA transcribed from the U6 promoter most efficiently incorporated the amino acid into proteins in the cells. The transient and stable introductions of these prokaryotic molecules into the insect cells were then compared in terms of the yield of proteins containing non-natural amino acids, and the "transient" method generated a sevenfold higher yield. By this method, 4-azido-L-phenylalanine was incorporated into human interleukin-8 at a specific site. The yield of the azido-containing IL-8 was 1 microg/1 mL cell culture, and the recombinant protein was successfully labeled with a fluorescent probe by the Staudinger-Bertozzi reaction.

Production of a recombinant mouse monoclonal antibody in transgenic silkworm cocoons

In the present study, we describe the production of transgenic silkworms expressing a recombinant mouse mAb in their cocoons. Two transgenic lines, L- and H-, were generated that carried cDNAs encoding the L- and H-chains of a mouse IgG mAb, respectively, under the control of the enhancer-linked sericin-1 promoter. Cocoon protein analysis indicated that the IgG L- or H-chain was secreted into the cocoons of each line. We also produced a transgenic line designated L/H, which carried both cDNAs, by crossing the L- and H-lines. This line efficiently produced the recombinant mAb as a fully assembled H(2)L(2) tetramer in its cocoons, with negligible L- or H-chain monomer and H-chain dimer production. Thus, the H(2)L(2) tetramer was synthesized in, and secreted from, the middle silk gland cells. Crossing of the L/H-line with a transgenic line expressing a baculovirus-derived trans-activator produced a 2.4-fold increase in mAb expression. The recombinant mAb was extracted from the cocoons with a buffer containing 3 m urea and purified by protein G affinity column chromatography. The antigen-binding affinity of the purified recombinant mAb was identical to that of the native mAb produced by a hybridoma. Analysis of the structure of the N-glycans attached to the recombinant mAb revealed that the mAb contained high mannose-, hybrid- and complex-type N-glycans. By contrast, insect-specific paucimannose-type glycans were not detected. Fucose residues alpha-1,3- and alpha-1,6-linked to the core N-acetylglucosamine residue, both of which are found in insect N-glycans, were not observed in the N-glycans of the mAb.

Rabies virus glycoprotein expression in Drosophila S2 cells. I: design of expression/selection vectors, subpopulations selection and influence of sodium butyrate and culture medium on protein expression

The cDNA encoding the rabies virus glycoprotein (RVGP) gene was cloned in expression plasmids under the control of the inductive metallothionein promoter. They were designed in order to bear or not a secretion signal (i) and a cDNA coding for the selection hygromycin. These vectors were transfected into S2 cells, cell populations selected and subpopulations were then obtained by reselection with hygromycin. Cell cultures were examined for kinetics of cell growth, detection of RVGP mRNA and expression of RVGP. All cell populations were shown to express the RVGP mRNA upon induction. S2MtRVGPHy cell population, transfected with one vector that contains RGPV gene and selection gene, was shown to express higher amounts of RVGP as evaluated by flow cytometry ( approximately 52%) and ELISA (0.64 microg/10(7)cells at day 7). Subpopulation selection allowed a higher RVGP expression, specially for the S2MtRVGPHy(+) (5.5 microg/10(7)cells at day 7). NaBu treatment leading to lower cell growth and higher RVGP expression allowed an even higher RVGP synthesis by S2MtRVGPHy(+) (8.4 microg/10(7)cells at day 7). SF900II medium leading to a higher S2MtRVGPHy(+)cell growth allowed a higher final RVGP synthesis in this cell culture. RVGP synthesis may be optimized by the expression/selection vectors design, cell subpopulations selection, chromatin exposure and culture medium employed.

Expression of the hepatitis B virus surface antigen in Drosophila S2 cells

Drosophila melanogaster S2 cells were transfected with a plasmid vector (pAcHBsAgHy) containing the S gene, coding for the hepatitis B virus surface antigen (HBsAg), under control of the constitutive drosophila actin promoter (pAc), and the hygromycin B (Hy) selection gene. The vector was introduced into Schneider 2 (S2) Drosophila cells by DNA transfection and a cell population (S2AcHBsAgHy) was selected by its resistance to hygromycin B. The pAcHBsAgHy vector integrated in transfected S2 cell genome and approximately 1,000 copies per cell were found in a higher HBsAg producer cell subpopulation. The HBsAg production varied in different subpopulations, but did not when a given subpopulation was cultivated in different culture flasks. Higher HBsAg expression was found in S2AcHBsAgHy cells cultivated in Insect Xpress medium (13.5 mug/1E7 cells) and SFX medium (7 mug/1E7 cells) in comparison to SF900II medium (0.6 mug/1E7 cells). An increase of HBsAg was observed in culture maintained under hygromycin selection pressure. Data presented in the paper show that S2AcHBsAgHy cells produce efficiently the HBsAg which is mainly found in the cell supernatant, suggesting that HBsAg is secreted from the cells. The data also show that our approach using the Drosophila expression system is suitable for the preparation of other viral protein preparation.

Protein expression in the Drosophila Schneider 2 cell system

The Schneider-2 (S2) Drosophila cell line is well suited for the stable overexpression of recombinant proteins using plasmid-based protein expression vectors. Following drug selection, a polyclonal S2 cell line can be induced to express on the order of 2 to 100 pmol/mg membrane protein for G-coupled protein receptors, 4000 to 100,000 sites/cell for other membrane receptors and 3 to 35 mg/liter for soluble and secreted proteins.

Kinetic response of a Drosophila melanogaster cell line to different medium formulations and culture conditions

In the past few years, Drosophila melanogaster cells have been employed for recombinant protein production purposes, and a comprehensive knowledge of their metabolism is essential for process optimization. In this work, the kinetic response of a Schneider S2 cell line, grown in shake flasks, in two different culture media, the serum-free SF900-II((R)) and the serum-supplemented TC-100, was evaluated. Cell growth, amino acids and glucose uptake, and lactate synthesis were measured allowing the calculation of kinetic parameters. The results show that S2 cells metabolism was able to adjust to different environmental situations, as determined by medium formulation, as well as by the particular situation resulting from the culture conditions. Cells attained a 163% higher final cell concentration (1.4 x 10(7) cells mL(-1)) in SF900 II((R)) medium, when compared to serum-supplemented TC-100 medium. Also, a maximum specific cell growth rate 52% higher in SF900 II((R) )medium, when compared to serum-supplemented TC-100 one, was observed. Glutamine was the growth limiting factor in SF900 II((R)) medium, while glucose, sometimes associated with glutamine, controlled growth in serum-supplemented TC-100 medium based formulation. The different pattern of lactate production is an example of the versatility of the metabolism of these cells. This by-product was produced only in glutamine limitation, but the amount synthesized depended not only on the excess glucose, but on other medium components. Therefore, in serum-supplemented TC-100 medium a much smaller lactate amount was generated. Besides, glucose was identified not only as a growth limiting factor, but also as a viability limiting factor, since its depletion accelerated cell death.

An entirely cell-based system to generate single-chain antibodies against cell surface receptors

The generation of recombinant antibodies (Abs) using phage display is a proven method to obtain a large variety of Abs that bind with high affinity to a given antigen. Traditionally, the generation of single-chain Abs depends on the use of recombinant proteins in several stages of the procedure. This can be a problem, especially in the case of cell-surface receptors, because Abs generated and selected against recombinant proteins may not bind the same protein expressed on a cell surface in its native form and because the expression of some receptors as recombinant proteins is problematic. To overcome these difficulties, we developed a strategy to generate single-chain Abs that does not require the use of recombinant protein at any stage of the procedure. In this strategy, stably transfected cells are used for the immunization of mice, measuring Ab responses to immunization, panning the phage library, high-throughput screening of arrayed phage clones, and characterization of recombinant single-chain variable regions. This strategy was used to generate a panel of single-chain Abs specific for the innate immunity receptor Toll-like receptor 2. Once generated, individual single-chain variable regions were subcloned into an expression vector allowing the production of recombinant Abs in insect cells, thus avoiding the contamination of recombinant Abs with microbial products. This cell-based system efficiently generates Abs that bind to native molecules on the cell surface, bypasses the requirement of recombinant protein production, and avoids risks of microbial component contamination.

Growth of recombinant Drosophila melanogaster Schneider 2 cells producing rabies virus glycoprotein in bioreactor employing serum-free medium

Drosophila melanogaster Schneider 2 (S2) cells have been increasingly used as a suitable expression system for the production of different recombinant proteins, and the employment of bioreactors for large-scale culture is an important tool for this purpose. In this work, Drosophila S2 cells producing the rabies virus glycoprotein RVGP were cultivated in bioreactor, employing a serum-free medium, aiming an improvement in cell growth and in glycoprotein production. To overcome cell growth limitation commonly observed in stirred flasks, different experiments in bioreactor were performed, in which some system modifications were carried out to attain the desired goal. The study showed that this cell line is considerably sensitive to hydrodynamic forces, and a high cell density (about 16.0 x 10(6) cells mL(-1)) was only obtained when Pluronic F68((R)) percentage was increased to 0.6% (w/v). Despite ammonium concentration affected RVGP production, and also cell growth, an elevated amount of the target protein was obtained, attaining 563 ng 10(-7) cells.

Culture of transgenic Drosophila melanogaster Schneider 2 cells in serum-free media based on TC100 basal medium

Requirements of eliminating animal proteins from cell culture have intensified in recent years, with the pressure of regulatory agencies related to biopharmaceuticals production. In this work, the substitution of fetal bovine serum by yeastolate and a soy hydrolysate (Hy Soy) for the culture of Drosophila melanogaster Schneider 2 cells transfected for the production of rabies virus G glycoprotein was evaluated. TC100 supplemented with glucose, glutamine, lipid emulsion and Pluronic F68 was employed as basal medium. Results show that yeastolate was more efficient on cell growth stimulation than Hy Soy. Cells adapted in medium formulation supplemented with 3 g/L yeastolate, 1% lipid emulsion, 10 g/L glucose, 3.5 g/L glutamine and 0.1% Pluronic F68 attained a maximum concentration of 10.7 x 10(6) cells/mL, with the expression of 9.4 ng/mL G glycoprotein.

Statistical determination of optimal baculovirus infection condition for recombinant protein production in Drosophila S2 cells

Insect Drosophila melanogaster S2 cell was developed as plasmid-based and, therefore, a nonlytic expression system for functional foreign proteins. To achieve multiple protein expressions, it was suggested that baculovirus be used on S2 cell system because baculovirus can infect S2 cells but cannot replicate inside the cells. Therefore, establishment of baculovirus infection conditions is the first important step and this should be properly optimized for production yield. We used statistical methodology to optimize the baculovirus infection conditions using green fluorescent protein (GFP) as a reporter protein. Consequently, we arrived at optimal infection conditions through a statistical regression method. The secreted GFP yield from vMT-GFP baculovirus-infected wild-type S2 cells under optimal infection conditions was >15-fold higher than that under nonoptimal conditions and comparable to that from stably transfected recombinant S2 cells.

Site-specific biotinylation of human myeloid differentiation protein 88 in Drosophila melanogaster Schneider 2 cell cytoplasm

In this work we describe the production of site-specific biotinylated human myeloid differentiation factor 88 (MyD88). A vector containing a coding sequence for a peptide derived from the carboxyl terminus of the Klebsiella pneumoniae oxalacetate decarboxylase alpha subunit was used to allow expression and biotinylation of MyD88 in Drosophila melanogaster Schneider 2 cell cytoplasm. As estimated by a comparison of Schneider 2 lysate with standard protein, the maximum expression level was 1.3 mug 107 cells-1. About 4 mg of biotinylated protein was purified by affinity chromatography on monomeric avidin from a 1-L culture. Exogenous biotin added to the culture medium increased the biotinylation efficiency of the expressed protein. Biotinylated MyD88 produced in Drosophila cells was able to precipitate recombinant MyD88 expressed in human embryonic kidney cells. The stable expression of MyD88 in Drosophila Schneider 2 cells offers a convenient and attractive method for large-scale production, which may be required to clarify the role of MyD88 in the inflammatory response. Moreover, site-specific biotinylation of MyD88 provides a useful tag for interaction assays where high sensitivity is required.

Efficient expression of recombinant human monoclonal antibodies in Drosophila S2 cells

We have explored the Drosophila S2 cell line for expression of Ig molecules isolated as Fab or scFv cDNA from phage-displayed libraries. We present a series of vectors for inducible expression and secretion of human Ig heavy (HC) and light chains (LC), both on separate plasmids and in combination constructs. Both HC (tested as human gamma(1)) and LC (human kappa) could be expressed separately and were secreted into the medium, confirming previous reports. When the combination vector carrying both the HC and LC cDNA, as well as when the HC and LC vectors were co-transfected, complete IgG1 was found in the medium. Transient transfection resulted in production levels of 0.5-1 mg/l. Stable cell lines could be established within 2-3 weeks. After 10-12 days of expression from such cell lines, Ig molecules accumulated and the medium contained typically 5-35 mg/l of IgG1. The IgG in these preparations was purified to more than 90% purity on protein G columns. Binding characteristics for IgG of the same clone expressed in S2 cells or mammalian cells were indistinguishable. The main advantages with this system compared to mammalian expression were its robustness and the much faster establishment of stable, high level producing cell lines.

Proteomic identification of Drosophila melanogaster male accessory gland proteins, including a pro-cathepsin and a soluble gamma-glutamyl transpeptidase

BACKGROUND

In Drosophila melanogaster, the male seminal fluid contains proteins that are important for reproductive success. Many of these proteins are synthesised by the male accessory glands and are secreted into the accessory gland lumen, where they are stored until required. Previous studies on the identification of Drosophila accessory gland products have largely focused on characterisation of male-specific accessory gland cDNAs from D. melanogaster and, more recently, Drosophila simulans. In the present study, we have used a proteomics approach without any sex bias to identify proteins in D. melanogaster accessory gland secretions.

RESULTS

Thirteen secreted accessory gland proteins, including seven new accessory gland proteins, were identified by 2D-gel electrophoresis combined with mass spectrometry of tryptic fragments. They included protein-folding and stress-response proteins, a hormone, a lipase, a serpin, a cysteine-rich protein and two peptidases, a pro-enzyme form of a cathepsin K-like cysteine peptidase and a gamma-glutamyl transpeptidase. Enzymatic studies established that accessory gland secretions contain a cysteine peptidase zymogen that can be activated at low pH. This peptidase may have a role in the processing of female and other male-derived proteins, but is unlikely to be involved in the processing of the sex peptide. gamma-Glutamyl transpeptidases are type II integral membrane proteins; however, the identified AG gamma-glutamyl transpeptidase (GGT-1) is unusual in that it is predicted to be a soluble secreted protein, a prediction that is supported by biochemical evidence. GGT-1 is possibly involved in maintaining a protective redox environment for sperm. The strong gamma-glutamyl transpeptidase activity found in the secretions provides an explanation for the observation that glutamic acid is the most abundant free amino acid in accessory gland secretions of D. melanogaster.

CONCLUSION

We have applied biochemical approaches, not used previously, to characterise prominent D. melanogaster accessory gland products. Of the thirteen accessory gland secreted proteins reported in this study, six were represented in a D. simulans male accessory gland EST library that was biased for male-specific genes. Therefore, the present study has identified seven new secreted accessory gland proteins, including GGT-1, which was not recognised previously as a secreted accessory gland product.

Protein O-fucosyltransferase 2 adds O-fucose to thrombospondin type 1 repeats

O-Fucose is an unusual form of glycosylation found on epidermal growth factor-like (EGF) repeats and thrombospondin type 1 repeats (TSRs) in many secreted and transmembrane proteins. Recently O-fucose on EGF repeats was shown to play important roles in Notch signaling. In contrast, physiological roles for O-fucose on TSRs are unknown. In the accompanying paper (Luo, Y., Nita-Lazar, A., and Haltiwanger, R. S. (2006) J. Biol. Chem. 281, 9385-9392), we demonstrated that an enzyme distinct from protein O-fucosyltransferase 1 adds O-fucose to TSRs. A known homologue of O-fucosyltransferase 1 is putative protein O-fucosyltransferase 2. The cDNA sequence encoding O-fucosyltransferase 2 was originally identified during a data base search for fucosyltransferases in Drosophila. Like O-fucosyltransferase 1, O-fucosyltransferase 2 is conserved from Caenorhabditis elegans to humans. Although O-fucosyltransferase 2 was assumed to be another protein O-fucosyltransferase, no biochemical characterization existed supporting this contention. Here we show that RNAi-mediated reduction of the O-fucosyltransferase 2 message significantly decreased TSR-specific O-fucosyltransferase activity in Drosophila S2 cells. We also found that O-fucosyltransferase 2 is predominantly localized in the endoplasmic reticulum compartment of these cells. Furthermore, we expressed recombinant Drosophila O-fucosyltransferase 2 and showed that it O-fucosylates TSRs but not EGF repeats in vitro. These results demonstrate that O-fucosyltransferase 2 is in fact a TSR-specific O-fucosyltransferase.

High level expression of monomeric and dimeric human α1,3-fucosyltransferase V

alpha3/4-Fucosyltransferases play a crucial role in inflammatory processes and tumor metastasis. While several human fucosyltransferases (FucTs) with different acceptor substrate specificities have been identified, the design of specific inhibitors for therapeutic approaches is hampered by the lack of structural information. In this study, we evaluated the expression of different constructs of human fucosyltransferase V to generate the large amounts required for structural studies. The truncated constructs lacking the transmembrane region and the cytosolic N-terminus, were expressed in baculovirus-infected Trichoplusia ni (Tn) insect cells and in two non-lytic expression systems, stably transfected human HEK 293 and T. ni cells. Since secretion of some glycosyltransferases is controlled by formation of dimeric molecules via disulfide bonds, one of the fucosyltransferase V constructs contained the N-terminal cysteine residue 64 for dimerization, whereas this residue was replaced in the other construct by serine. In both human and insect cells dimerization did not prove to be essential for efficient expression and secretion. On the basis of enzymatic activity, the yield of secreted fucosyltransferase V was approximately 10-fold higher in stably transfected insect cells than in HEK 293 cells. In particular the monomeric form of the enzyme provides a valuable tool for structural analyses to elucidate the fine specifity of fucosyltransferase V-mediated fucosylation of Lewis type glycans.

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.

Production and N-glycan analysis of secreted human erythropoietin glycoprotein in stably transfected Drosophila S2 cells

Schneider 2 (S2) cells from Drosophila melanogaster have been used as a plasmid-based, non-lytic expression system for foreign proteins. Here, a plasmid encoding the human erythropoietin (hEPO) gene fused with a hexahistidine (His(6)) tag under the control of the Drosophila metallothionein (MT) promoter was stably transfected into Drosophila S2 cells. After copper sulfate induction, transfected S2 cells were found to secrete hEPO with a maximum expression level of 18 mg/L and a secretion efficiency near 98%. The secreted hEPO from Drosophila S2 had an apparent molecular weight of about 23-27 kDa which was significantly lower than a recombinant hEPO expressed in Chinese hamster ovary (CHO) cells (about 36 kDa). N-glycosidase F digestion almost completely eliminated the difference and resulted in the same molecular weight ( approximately 20 kDa) of de-N-glycosylated hEPO proteins. These data suggest that recombinant hEPO from S2 cells was modified with smaller N-glycans. Subsequently, the major N-glycans were identified following glycoamidase A digestion, labeling with 2-aminopyridine (PA), and two-dimensional high-performance liquid chromatography (HPLC) analysis in concert with exoglycosidase digestion. This analysis of N-glycans revealed that hEPO was modified to include paucimannosidic glycans containing two or three mannose residues with or without core fucose. A similar glycosylation pattern was observed on a recombinant human transferrin expressed in S2 cells. These results provide a detailed analysis of multiple N-glycan structures produced in a Drosophila cell line that will be useful in the subsequent application of these cells for the generation of heterologous glycoproteins.

Protease domain of human ADAM33 produced by Drosophila S2 cells

Human ADAM33 is a multiple-domain, type-I transmembrane zinc metalloprotease recently implicated in asthma susceptibility [Nature 418 (2002) 426]. To provide an active protease for functional studies, expression of a recombinant ADAM33 zymogen (pro-catalytic domains, pro-CAT) was attempted in several insect cells. The pro-CAT was cloned into baculovirus under the regulation of the polyhedron promoter and using either the honeybee mellitin or ADAM33 signal sequence. Sf9 or Hi5 cells infected with these recombinant viruses expressed the majority of the protein unprocessed and as inclusion bodies ( approximately 10 mg/L). On the other hand, similar constructs could be expressed, processed, and secreted by Drosophila S2 cells using a variety of constitutive (actin, pAc5.1) or inducible (metallothionein, PMT) promoters and leader sequences (e.g., native and BiP). Higher expression level of 10-fold was observed for the inducible system resulting in an average yield of 20 mg/L after purification. The majority of the catalytic domain purified from the Drosophila conditioned media remained associated with the pro-domain after several chromatography steps. An induction cocktail containing cadmium chloride and zinc chloride was subsequently developed for the PMT system as an alternative to using cupric sulfate or cadmium chloride as single inducers. The novel induction cocktail resulted in an increased ratio of secreted catalytic to pro-domain, and yielded milligram amounts of highly purified protease. The availability of this modified expression system facilitated purification of the wild type and several glycosylation mutants, one of which (N231Q) crystallized recently for X-ray structure determination [J. Mol. Biol. 335 (2003) 129].

Irreversible engineering of the multielement-binding antibody 2D12.5 and its complementary ligands

Engineering the permanent formation of a receptor-ligand complex has a number of potential applications in chemistry and biology, including targeted medical imaging and therapy. Starting from the crystal structure of the rare-earth-DOTA binding antibody 2D12.5 (Corneillie, T. M., Fisher, A. J., and Meares, C. F. (2003) J. Am. Chem. Soc. 125, 15039-15048), we used the site-directed incorporation of cysteine nucleophiles at the periphery of the antibody's binding site, paired with the chemical design of a weakly electrophilic ligand, to produce a receptor-ligand pair that associates efficiently and permanently. Protein residues proximal to the ligand's side chain were identified for engineering cysteine mutants. Fab fragments incorporating a cysteine at position 54, 55, or 56 of the heavy chain (complementarity determining region 2) were designed from the structure and then cloned, expressed in Drosophila S2 cells, and tested for reactivity with mildly electrophilic DOTA-yttrium ligands. All showed permanent binding activity, indicating that there is some tolerance for the location of the reactive mutant on the protein surface near the binding site. The G54C Fab mutant displayed the highest expression levels and permanent binding activity in initial experiments and was produced in high yield for further study. Upon examining the behavior of the G54C mutant with a small set of electrophilic ligands, differences in reactivity were observed which indicated that the substituents near the electrophilic atom can be important determinants of permanent binding. The G54C mutant permanently attaches to Y(3+) complexes of (S)-2-(4-acrylamidobenzyl)-DOTA with a half-time of approximately 13 min at 37 degrees C, making it potentially useful for in vivo pretargeting applications.

Structure and function of recombinant cobra venom factor

Cobra venom factor (CVF) is the complement-activating protein from cobra venom. It is a structural and functional analog of complement component C3. CVF functionally resembles C3b, the activated form of C3. Like C3b, CVF binds factor B, which is subsequently cleaved by factor D to form the bimolecular complex CVF,Bb. CVF,Bb is a C3/C5 convertase that cleaves both complement components C3 and C5. CVF is a three-chain protein that structurally resembles the C3b degradation product C3c, which is unable to form a C3/C5 convertase. Both C3 and CVF are synthesized as single-chain prepro-proteins. This study reports the recombinant expression of pro-CVF in two insect cell expression systems (baculovirus-infected Sf9 Spodoptera frugiperda cells and stably transfected S2 Drosophila melanogaster cells). In both expression systems pro-CVF is synthesized initially as a single-chain pro-CVF molecule that is subsequently proteolytically processed into a two-chain form of pro-CVF that structurally resembles C3. The C3-like form of pro-CVF can be further proteolytically processed into another two-chain form of pro-CVF that structurally resembles C3b. Unexpectedly, all three forms of pro-CVF exhibit functional activity of mature, natural CVF. Recombinant pro-CVF supports the activation of factor B in the presence of factor D and Mg2+ and depletes serum complement activity like natural CVF. The bimolecular convertase pro-CVF,Bb exhibits both C3 cleaving and C5 cleaving activity. The activity of pro-CVF and the resulting C3/C5 convertase is indistinguishable from CVF and the CVF,Bb convertase. The ability to produce active forms of pro-CVF recombinantly ensures the continued availability of an important research reagent for complement depletion because cobra venom as the source for natural CVF will be increasingly difficult to obtain as the Indian cobra is on the list of endangered species. Experimental systems to express pro-CVF recombinantly will also be invaluable for studies to delineate the structure and function relationship of CVF and its differences from C3 as well as to generate human C3 derivatives with CVF-like function for therapeutic complement depletion ("humanized CVF").

Functional characterization of Drosophila sialyltransferase

Sialylation is an important carbohydrate modification of glycoconjugates in the deuterostome lineage of animals. By contrast, the evidence for sialylation in protostomes has been scarce and somewhat controversial. In the present study, we characterize a Drosophila sialyltransferase gene, thus providing experimental evidence for the presence of sialylation in protostomes. This gene encodes a functional alpha2-6-sialyltransferase (SiaT) that is closely related to the vertebrate ST6Gal sialyltransferase family, indicating an ancient evolutionary origin for this family. Characterization of recombinant, purified Drosophila SiaT revealed a novel acceptor specificity as it exhibits highest activity toward GalNAcbeta1-4GlcNAc carbohydrate structures at the non-reducing termini of oligosaccharides and glycoprotein glycans. Oligosaccharides are preferred over glycoproteins as acceptors, and no activity toward glycolipid acceptors was detected. Recombinant Drosophila SiaT expressed in cultured insect cells possesses in vivo and in vitro autosialylation activity toward beta-linked GalNAc termini of its own N-linked glycans, thus representing the first example of a sialylated insect glycoconjugate. In situ hybridization revealed that Drosophila SiaT is expressed during embryonic development in a tissue- and stage-specific fashion, with elevated expression in a subset of cells within the central nervous system. The identification of a SiaT in Drosophila provides a new evolutionary perspective for considering the diverse functions of sialylation and, through the powerful genetic tools available in this system, a means of elucidating functions for sialylation in protostomes.

Engineering a novel secretion signal for cross-host recombinant protein expression

Protein secretion is conferred by a hydrophobic secretion signal usually located at the N-terminal of the polypeptide. We report here, the identification of a novel secretion signal (SS) that is capable of directing the secretion of recombinant proteins from both prokaryotes and eukaryotes. Secretion of fusion reporter proteins was demonstrated in Escherichia coli, Saccharomyces cerevisiae and six different eukaryotic cells. Estrogen-inducibility and secretion of fusion reporter protein was demonstrated in six common eukaryotic cell lines. The rate of protein secretion is rapid and its expression profile closely reflects its intracellular concentration of mRNA. In bacteria and yeast, protein secretion directed by SS is dependent on the growth culture condition and rate of induction. This secretion signal allows a flexible strategy for the production and secretion of recombinant proteins in numerous hosts, and to conveniently and rapidly study protein expression.

Surrogate light chain-mediated interaction of a soluble pre-B cell receptor with adherent cell lines

Signals initiated by the precursor B cell receptor (pre-BCR) are critical for B cell progenitors to mature into precursor B cells. The pre-BCR consists of a homodimer of microH chains, the covalently associated surrogate L (SL) chain composed of VpreB and lambda5, and the transmembrane signal molecules Ig(alpha) and Igbeta. One way to explain how maturation signals are initiated in late progenitor B cells is that the pre-BCR is transported to the cell surface and interacts from there with a ligand on stroma cells. To address this hypothesis, we first produced soluble Fab-like pre-BCR and BCR fragments, as well as SL chain, in baculovirus-infected insect cells. Flow cytometry revealed that, in contrast to Fab-like BCR fragments, the soluble pre-BCR binds to the surface of stroma and several other adherent cell lines, but not to B and T lymphoid suspension cells. The specific binding of the soluble pre-BCR to stroma cells is saturable, sensitive to trypsin digestion, and not dependent on bivalent cations. The binding of pre-BCR seems to be independent of the H chain of IgM (microH chain), because SL chain alone was able to interact with stroma cells. Finally, soluble pre-BCR specifically precipitated a 135-kDa protein from ST2 cells. These findings not only demonstrate for the first time the capacity of a pre-BCR to specifically bind to a structure on the surface of adherent cells, but also suggest that the pre-BCR interacts via its SL chain with a putative ligand on stroma cells.