Glycoproteins from insect cells: sialylated or not?

New insights on the sialidase protein family revealed by a phylogenetic analysis in metazoa

Sialidases are glycohydrolytic enzymes present from virus to mammals that remove sialic acid from oligosaccharide chains. Four different sialidase forms are known in vertebrates: the lysosomal NEU1, the cytosolic NEU2 and the membrane-associated NEU3 and NEU4. These enzymes modulate the cell sialic acid content and are involved in several cellular processes and pathological conditions. Molecular defects in NEU1 are responsible for sialidosis, an inherited disease characterized by lysosomal storage disorder and neurodegeneration. The studies on the biology of sialic acids and sialyltransferases, the anabolic counterparts of sialidases, have revealed a complex picture with more than 50 sialic acid variants selectively present in the different branches of the tree of life. The gain/loss of specific sialoconjugates have been proposed as key events in the evolution of deuterostomes and Homo sapiens, as well as in the host-pathogen interactions. To date, less attention has been paid to the evolution of sialidases. Thus we have conducted a survey on the state of the sialidase family in metazoan. Using an in silico approach, we identified and characterized sialidase orthologs from 21 different organisms distributed among the evolutionary tree: Metazoa relative (Monosiga brevicollis), early Deuterostomia, precursor of Chordata and Vertebrata (teleost fishes, amphibians, reptiles, avians and early and recent mammals). We were able to reconstruct the evolution of the sialidase protein family from the ancestral sialidase NEU1 and identify a new form of the enzyme, NEU5, representing an intermediate step in the evolution leading to the modern NEU3, NEU4 and NEU2. Our study provides new insights on the mechanisms that shaped the substrate specificity and other peculiar properties of the modern mammalian sialidases. Moreover, we further confirm findings on the catalytic residues and identified enzyme loop portions that behave as rapidly diverging regions and may be involved in the evolution of specific properties of sialidases.

Differences in the fractional abundances of carbohydrates of natural and recombinant human tissue factor

BACKGROUND

Tissue factor (TF) is a single polypeptide integral membrane glycoprotein composed of 263 residues and is essential to life in its role as the initiator of blood coagulation. Previously we have shown that the activity of the natural placental TF (pTF) and the recombinant TF (rTF) from Sf9 insect cells is different (Krudysz-Amblo, J. et al (2010) J. Biol. Chem. 285, 3371-3382).

METHODS

In this study, using mass spectrometry, we show by quantitative analysis that the extent of glycosylation varies on each protein.

RESULTS AND CONCLUSIONS

Fractional abundance of each glycan composition at each of the three glycosylation sites reveals the most pronounced difference to be at asparagine (Asn) 11. This residue is located in the region of extensive TF-factor VIIa (FVIIa) interaction. Carbohydrate fractional abundance at Asn11 revealed that glycosylation in the natural placental TF is much more prevalent (~76%) than in the recombinant protein (~20%). The extent of glycosylation on Asn124 and Asn137 is similar in the two proteins, despite the pronounced differences in the carbohydrate composition. Additionally, 77% of rTF exists as TF des-1, 2 (missing the first two amino acids from the N-terminus). In contrast, only 31% of pTF is found in the des-1, 2 form.

CONCLUSION

These observations may attribute to the difference in the ability of TF-FVIIa complex to activate factor X (FX).

GENERAL SIGNIFICANCE

Structural and functional comparison of the recombinant and natural protein advances our understanding and knowledge on the biological activity of TF.

Molecular phylogeny and functional genomics of beta-galactoside alpha2,6-sialyltransferases that explain ubiquitous expression of st6gal1 gene in amniotes

Sialyltransferases are key enzymes in the biosynthesis of sialoglycoconjugates that catalyze the transfer of sialic residue from its activated form to an oligosaccharidic acceptor. β-Galactoside α2,6-sialyltransferases ST6Gal I and ST6Gal II are the two unique members of the ST6Gal family described in higher vertebrates. The availability of genome sequences enabled the identification of more distantly related invertebrates' st6gal gene sequences and allowed us to propose a scenario of their evolution. Using a phylogenomic approach, we present further evidence of an accelerated evolution of the st6gal1 genes both in their genomic regulatory sequences and in their coding sequence in reptiles, birds, and mammals known as amniotes, whereas st6gal2 genes conserve an ancestral profile of expression throughout vertebrate evolution.

Re-visiting the endogenous capacity for recombinant glycoprotein sialylation by baculovirus-infected Tn-4h and DpN1 cells

It was previously reported that Tn-4h and DpN1 cells have the endogenous capacity to efficiently sialylate secreted alkaline phosphatase (SEAP) when infected with a baculovirus expression vector. In contrast, it has been found that lepidopteran insect cell lines that are more widely used as hosts for baculovirus vectors typically fail to sialylate SEAP and other recombinant glycoproteins. Thus, the N-glycan processing capabilities of Tn-4h and DpN1 cells are of potential interest to investigators using the baculovirus expression system for recombinant glycoprotein production. In this study, we experimentally re-assessed the ability of Tn-4h and DpN1 cells to sialylate SEAP with Sf9 and glyco-engineered Sf9 cells (SfSWT-1) as negative and positive controls, respectively. Our results showed that the SEAP purified from SfSWT-1 cells was strongly sialylated and initially indicated that the SEAP purified from Tn-4h cells was weakly sialylated. However, further analyses suggested that the SEAP produced by Tn-4h cells only appeared to be sialylated because it was contaminated with an electrophoretically indistinguishable sialoglycoprotein derived from fetal bovine serum. We subsequently expressed, purified, and analyzed a second recombinant glycoprotein (GST-SfManI) from all four cell lines and found that only the SfSWT-1 cells were able to detectably sialylate this product. Together, these results showed that neither Tn-4h nor DpN1 cells efficiently sialylated SEAP or GST-SfManI when infected by baculovirus expression vectors. Furthermore, they suggested that previous reports of efficient SEAP sialylation by Tn-4h and DpN1 cells probably reflect contamination with a sialylated, co-migrating glycoprotein, perhaps bovine fetuin, derived from the serum used in the insect cell growth medium.

Sialyltransferase regulates nervous system function in Drosophila

In vertebrates, sialylated glycans participate in a wide range of biological processes and affect the development and function of the nervous system. While the complexity of glycosylation and the functional redundancy among sialyltransferases provide obstacles for revealing biological roles of sialylation in mammals, Drosophila possesses a sole vertebrate-type sialyltransferase, Drosophila sialyltransferase (DSiaT), with significant homology to its mammalian counterparts, suggesting that Drosophila could be a suitable model to investigate the function of sialylation. To explore this possibility and investigate the role of sialylation in Drosophila, we inactivated DSiaT in vivo by gene targeting and analyzed phenotypes of DSiaT mutants using a combination of behavioral, immunolabeling, electrophysiological, and pharmacological approaches. Our experiments demonstrated that DSiaT expression is restricted to a subset of CNS neurons throughout development. We found that DSiaT mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis, and defects of neuromuscular junctions. Our results indicate that DSiaT regulates neuronal excitability and affects the function of a voltage-gated sodium channel. Finally, we showed that sialyltransferase activity is required for DSiaT function in vivo, which suggests that DSiaT mutant phenotypes result from a defect in sialylation of N-glycans. This work provided the first evidence that sialylation has an important biological function in protostomes, while also revealing a novel, nervous system-specific function of alpha2,6-sialylation. Thus, our data shed light on one of the most ancient functions of sialic acids in metazoan organisms and suggest a possibility that this function is evolutionarily conserved between flies and mammals.

Identification of genes encoding N-glycan processing beta-N-acetylglucosaminidases in Trichoplusia ni and Bombyx mori: Implications for glycoengineering of baculovirus expression systems

Glycoproteins produced by non-engineered insects or insect cell lines characteristically bear truncated, paucimannose N-glycans in place of the complex N-glycans produced by mammalian cells. A key reason for this difference is the presence of a highly specific N-glycan processing beta-N-acetylglucosaminidase in insect, but not in mammalian systems. Thus, reducing or abolishing this enzyme could enhance the ability of glycoengineered insects or insect cell lines to produce complex N-glycans. Of the three insect species routinely used for recombinant glycoprotein production, the processing beta-N-acetylglucosaminidase gene has been isolated only from Spodoptera frugiperda. Thus, the purpose of this study was to isolate and characterize the genes encoding this important processing enzyme from the other two species, Bombyx mori and Trichoplusia ni. Bioinformatic analyses of putative processing beta-N-acetylglucosaminidase genes isolated from these two species indicated that each encoded a product that was, indeed, more similar to processing beta-N-acetylglucosaminidases than degradative or chitinolytic beta-N-acetylglucosaminidases. In addition, over-expression of each of these genes induced an enzyme activity with the substrate specificity characteristic of processing, but not degradative or chitinolytic enzymes. Together, these results demonstrated that the processing beta-N-acetylglucosaminidase genes had been successfully isolated from Trichoplusia ni and Bombyx mori. The identification of these genes has the potential to facilitate further glycoengineering of baculovirus-insect cell expression systems for the production of glycosylated proteins.

Molecular cloning and expression profile analysis of a novel beta-D-N-acetylhexosaminidase of domestic silkworm (Bombyx mori)

Lepidoptera such as the domestic silkworm (Bombyx mori) produce proteins modified with unsialylated, mannose-rich moieties known as 'high mannose-type'N-glycans. However, we observed that, under intrinsic acetylglucosaminidase (GlcNAcase)-inhibited conditions, moth cells tend to synthesize different types of glycoform with sialic acid modification. To identify molecules essential to assemble Lepidoptera-specific N-glycans, we performed BLAST analysis on the silkworm genetic database and isolated the entire coding sequence of novel Bombyx GlcNAcase, BmGlcNAcase 2. This enzyme showed weak homology to currently known, lysosome-associated eukaryotic hexosaminidases, but it revealed remarkable similarity with recently reported glycosyl hydrolases of Spodoptera and Bombyx. Interestingly, BmGlcNAcase 2 was found to be expressed in embryos and in certain tissues of molting larvae (i.e. ovary, fat bodies, mid-intestine, skin), but not in pupae, suggesting its unique function in the carbohydrate metabolism of juvenile silkworm.

Restriction of rift valley Fever virus virulence in mosquito cells

Arboviruses are maintained in a natural cycle that requires blood-sucking arthropod and vertebrate hosts. Arboviruses are believed to persistently infect their arthropod host without overt pathology and cause acute infection with viremia in their vertebrate host. We have focused on elucidating how a specific arbovirus, Rift Valley fever (RVF) virus, causes cytopathic effect in cells derived from vertebrates and non-cytopathic infection in cells derived from arthropods. We demonstrate that the vertebrate virulence factor, NSs, is functional in arthropod cells but is expressed at significantly lower levels in infected arthropod versus infected vertebrate cells.

Posttranslational modifications and activity of natural and recombinant tissue factor

Tissue factor is a membrane protein, which in a complex with factor VIIa initiates in vivo blood coagulation. Due to the scarcity of natural tissue factor protein, most studies have relied upon recombinant tissue factor forms. However, there have been only cursory experimental comparisons of natural and recombinant tissue factor proteins. Our preliminary data suggested that placental tissue factor in a complex with factor VIIa was more efficient activator of factor X than the recombinant protein. After deglycosylation, both forms of tissue factor showed almost an identical activity in the extrinsic factor Xase. Analyses using tryptic digestion and mass-spectrometry revealed that the levels of glycosylation and the composition of carbohydrates present in natural placental tissue factor were different than those in its recombinant counterpart. These data indicate that natural and recombinant tissue factor proteins differ in their posttranslational modifications and that these differences translate into different cofactor activity. Thus the use of recombinant tissue factor proteins for the quantitation of natural tissue factor is misleading.

Carbohydrates and activity of natural and recombinant tissue factor

The effect of glycosylation on tissue factor (TF) activity was evaluated, and site-specific glycosylation of full-length recombinant TF (rTF) and that of natural TF from human placenta (pTF) were studied by liquid chromatography-tandem mass spectrometry. The amidolytic activity of the TF.factor VIIa (FVIIa) complex toward a fluorogenic substrate showed that the catalytic efficiency (V(max)) of the complex increased in the order rTF(1-243) (Escherichia coli) < rTF(1-263) (Sf9 insect cells) < pTF for the glycosylated and deglycosylated forms. Substrate hydrolysis was unaltered by deglycosylation. In FXase, the K(m) of FX for rTF(1-263)-FVIIa remained unchanged after deglycosylation, whereas the k(cat) decreased slightly. A pronounced decrease, 4-fold, in k(cat) was observed for pTF.FVIIa upon deglycosylation, whereas the K(m) was minimally altered. The parameters of FX activation by both rTF(1-263D)-FVIIa and pTF(D)-FVIIa were identical and similar to those for rTF(1-243)-FVIIa. In conclusion, carbohydrates significantly influence the activity of TF proteins. Carbohydrate analysis revealed glycosylation on asparagines 11, 124, and 137 in both rTF(1-263) and pTF. The carbohydrates of rTF(1-263) contain high mannose, hybrid, and fucosylated glycans. Natural pTF contains no high mannose glycans but is modified with hybrid, highly fucosylated, and sialylated sugars.

N-linked glycans on dengue viruses grown in mammalian and insect cells

This study compared the ability of mosquito and mammalian cell-derived dengue virus (DENV) to infect human dendritic cell-specific ICAM3-grabbing non-integrin (DC-SIGN)-expressing cells and characterized the structure of envelope (E) protein N-linked glycans on DENV derived from the two cell types. DENVs derived from both cell types were equally effective at infecting DC-SIGN-expressing human monocytes and dendritic cells. The N-linked glycans on mosquito cell-derived virus were a mix of high-mannose and paucimannose glycans. In virus derived from mammalian cells, the N-linked glycans were a mix of high-mannose and complex glycans. These results indicate that N-linked glycans are incompletely processed during DENV egress from cells, resulting in high-mannose glycans on viruses derived from both cell types. Studies with full-length and truncated E protein demonstrated that incomplete processing was most likely a result of the poor accessibility of glycans on the membrane-anchored protein.

Cryo-electron tomography of homophilic adhesion mediated by the neural cell adhesion molecule L1

The neural cell adhesion molecule L1 participates in homophilic interactions important for axon guidance and neuronal development. The structural details of homophilic adhesion mediated by L1 and other immunoglobulin superfamily members containing an N-terminal horseshoe arrangement of four immunoglobulin-like domains are unknown. Here we used cryo-electron tomography to study liposomes to which intact or truncated forms of the L1 ectodomain were attached. Tomographic reconstructions revealed an adhesion interface with a regular and repeating pattern consistent with interactions between paired horseshoes contributed by L1 proteins from neighboring liposomes. The characteristics of the pattern changed when N-linked carbohydrates were altered by removing sialic acids or converting from complex to high mannose or oligomannose glycans, suggesting a regulatory role for carbohydrates in L1-mediated homophilic adhesion. Using the results from tomograms and crystal structures of L1-related molecules, we present a structural model for L1-mediated homophilic adhesion that depends on protein-protein, protein-carbohydrate, and carbohydrate-carbohydrate interactions.

Serum-free transient protein production system based on adenoviral vector and PER.C6 technology: high yield and preserved bioactivity

Stable E1 transformed cells, like PER.C6, are able to grow at scale and to high cell densities. E1-deleted adenoviruses replicate to high titer in PER.C6 cells whereas subsequent deletion of E2A from the vector results in absence of replication in PER.C6 cells and drastically lowers the expression of adenovirus proteins in such cells. We therefore considered the use of an DeltaE1/DeltaE2 type 5 vector (Ad5) to deliver genes to PER.C6 cells growing in suspension with the aim to achieve high protein yield. To evaluate the utility of this system we constructed DeltaE1/DeltaE2 vector carrying different classes of protein, that is, the gene coding for spike protein derived from the Coronavirus causing severe acute respiratory syndrome (SARS-CoV), a gene coding for the SARS-CoV receptor or the genes coding for an antibody shown to bind and neutralize SARS-CoV (SARS-AB). The DeltaE1/DeltaE2A-vector backbones were rescued on a PER.C6 cell line engineered to constitutively over express the Ad5 E2A protein. Exposure of PER.C6 cells to low amounts (30 vp/cell) of DeltaE1/DeltaE2 vectors resulted in highly efficient (>80%) transduction of PER.C6 cells growing in suspension. The efficient cell transduction resulted in high protein yield (up to 60 picogram/cell/day) in a 4 day batch production protocol. FACS and ELISA assays demonstrated the biological activity of the transiently produced proteins. We therefore conclude that DeltaE1/DeltaE2 vectors in combination with the PER.C6 technology may provide a viable answer to the increasing demand for high quality, high yield recombinant protein.

Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context.

A fused lobes gene encodes the processing beta-N-acetylglucosaminidase in Sf9 cells

Manalpha6(Manalpha3)Manbeta4GlcNAcbeta4GlcNAc-R is the core structure of the major processed protein N-glycans produced by insect cells. Ultimately, this paucimannose type structure is produced by an unusual beta-N-acetylglucosaminidase, which removes the terminal N-acetylglucosamine residue from the upstream intermediate, Manalpha6(GlcNAcbeta2Manalpha3)Manbeta4GlcNAcbeta4GlcNAc-R. Because the N-glycan processing pathways leading to the production of this intermediate are probably identical in insects and higher eukaryotes, the presence or absence of this specific, processing beta-N-acetylglucosaminidase is a key factor distinguishing the processing pathways in these two different types of organisms. Recent studies have shown that the fused lobes (fdl) gene encodes the specific, processing beta-N-acetylglucosaminidase of Drosophila melanogaster. However, there are conflicting reports on the identity of the gene encoding this enzyme in the lepidopteran insect, Spodoptera frugiperda. One has suggested that a gene alternatively designated SfGlcNAcase-3 or SfHex encodes this function, whereas another has suggested that this gene encodes a broad-spectrum beta-N-acetylglucosaminidase that functions in glycan and chitin degradation. In this study we resolved this conflict by molecularly cloning an S. frugiperda fdl ortholog (Sf-fdl) and demonstrating that it encodes a product with the substrate specificity expected of the processing beta-N-acetylglucosaminidase. Moreover, we showed that the endogenous levels of specific, processing beta-N-acetylglucosaminidase activity were significantly reduced in S. frugiperda cells engineered to express a double-stranded RNA derived from the Sf-fdl gene. These results indicate that Sf-fdl encodes the specific, processing beta-N-acetylglucosaminidase of S. frugiperda and validate our previous suggestion that the broad-spectrum beta-N-acetylglucosaminidase encoded by the SfGlcNAcase-3/SfHex gene is more likely to be involved in N-glycan and/or chitin degradation.

Deciphering Dorin M glycosylation by mass spectrometry

The soft tick, Ornithodoros moubata, is a vector of several bacterial and viral pathogens including Borrelia duttoni, a causative agent of relapsing fever and African swine fever virus. Previously, a sialic acid-specific lectin Dorin M was isolated from its hemolymph. Here, we report on the complete characterization of the primary sequence of Dorin M. Using liquid chromatography coupled to mass spectrometry, we identified three different glycopeptides in the tryptic digest of Dorin M. The peptide, as well as the glycan part of all glycopeptides, were further fully sequenced by means of tandem mass spectrometry (MS2) and multiple-stage mass spectrometry (MS3). Two classical N-glycosylation sites were modified by high-mannose-type glycans containing up to nine mannose residues. The third site bore a glycan with four to five mannose residues and a deoxyhexose (fucose) attached to the proximal N-acetylglycosamine. The microheterogeneity at each site was estimated based on chromatographic behavior of different glycoforms. The fourth, a non-classical N-glycosylation site (Asn-Asn-Cys), was not glycosylated, probably due to the involvement of the cysteine residue in a disulfide bridge.

Gating of the shaker potassium channel is modulated differentially by N-glycosylation and sialic acids

N-linked glycans, including sialic acids, are integral components of ion channel complexes. To determine if N-linked sugars can modulate a rapidly inactivating K+ channel, the glycosylated Drosophila melanogaster Shaker K+ channel (ShB) and the N-glycosylation-deficient mutant (ShNQ), were studied under conditions of full and reduced sialylation. Through an apparent electrostatic mechanism, full sialylation induced uniform and significant hyperpolarizing shifts in all measured voltage-dependent ShB gating parameters compared to those measured under conditions of reduced sialylation. Steady-state gating of ShNQ was unaffected by changes in sialylation and was nearly identical to that observed for ShB under conditions of reduced sialylation, indicating that N-linked sialic acids were wholly responsible for the observed effects of sialic acid on ShB gating. Interestingly, the rates of transition among channel states and the voltage-independent rates of activation and inactivation were significantly slower for ShNQ compared to ShB. Both effects were independent of sialylation, indicating that N-linked sugars other than sialic acids alter ShB gating kinetics but have little to no effect on the steady-state distribution of channels among states. The effect of sialic acids on channel gating, particularly inactivation gating, and the impact of other N-linked sugars on channel gating kinetics are unique to the ShB isoform. Thus, ShB gating is modulated by two complementary but distinct sugar-dependent mechanisms, (1) an N-linked sialic acid-dependent surface charge effect and (2) a sialic acid-independent effect that is consistent with N-linked sugars affecting the stability of ShB among its functional states.

Classical swine fever virus E2 glycoprotein antigen produced in adenovirally transduced PK-15 cells confers complete protection in pigs upon viral challenge

E2 is the major envelope glycoprotein present on the outer surface of the classical swine fever virus (CSFV). It is exposed as a homodimer originated by disulfide linkage and represents an important target for the induction of neutralizing immune responses against the viral infection. The E2his glycoprotein nucleotide sequence used in this work contains the CSFV E2 extracellular domain preceded by the tissue plasminogen signal peptide and a hexa-histidine tag in the 3' terminus. The recombinant antigen was produced at a range of 120-150 microg/mL in the culture media of epithelial kidney pig cells, transduced with a replication defective adenoviral vector (Ad-E2his) generated by means of cloning the E2his sequence in the vector genome. The glycoprotein was obtained from clarified culture media as a homodimer of 110 kDa with purity over 95% after a single affinity chromatography step in Ni-NTA Agarose column. The E2his characterization by lectin-specific binding assay showed the presence of N-linked oligosaccharides of both hybrid and complex types. The protective capacity of E2his was demonstrated in two immunization and challenge experiments in pigs using doses of 15 or 30 microg of the glycoprotein, emulsified in Freund's adjuvant. The intramuscular immunization followed by a unique boost three weeks later, elicited high titers of neutralizing antibodies between the second and the fourth week after the primary vaccination. The immunized animals were fully protected from the viral infection after challenge with 10(5) PLD(50) of homologous CSFV "Margarita" strain administered by intramuscular injection. Consequently, no clinical signs of the disease or viral isolation from lymphocytes were detected in the vaccinated pigs. These results suggest that the E2his antigen produced in mammalian cells may be a feasible vaccine candidate for CSF prevention.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

Genetic determinants of Sindbis virus strain TR339 affecting midgut infection in the mosquito Aedes aegypti

Mosquito midgut epithelial cells (MEC) play a major role in determining whether an arbovirus can successfully infect and be transmitted by mosquitoes. The Sindbis virus (SINV) strain TR339 efficiently infects Aedes aegypti MEC but the SINV strain TE/5'2J poorly infects MEC. SINV determinants for MEC infection have been localized to the E2 glycoprotein. The E2 amino acid sequences of TR339 and TE/5'2J differ at two sites, E2-55 and E2-70. We have altered the TE/5'2J virus genome by site-directed mutagenesis to contain two TR339 residues, E2-55 H-->Q (histidine to glutamine) and E2-70 K-->E (lysine to glutamic acid). We have characterized the growth patterns of derived viruses in cell culture and determined the midgut infection rate (MIR) in A. aegypti mosquitoes. Our results clearly show that the E2-55 H-->Q and the E2-70 K-->E mutations in the TE/5'2J virus increase MIR both independently and in combination. TE/5'2J virus containing both TR339 E2 residues had MIRs similar to the parental TR339 virus. In addition, SINV propagated in a mammalian cell line had a significantly lower A. aegypti midgut 50 % infectious dose than virus propagated in a mosquito cell line.

Expression and functional characterization of classical swine fever virus E(rns) protein

E(rns) is an envelope glycoprotein of classical swine fever virus (CSFV) with unusual RNase activity. Recently, E(rns) was found to have a new function of counteracting the beta-interferon (IFN-beta) induction pathway. In this study, wildtype ErnsSM and two mutated E(rns) proteins ErnsH297k and ErnsH346k were expressed in insect cells and purified for RNase activity and function analysis. RNase activity assay in vitro demonstrated that only wildtype E(rns) protein had RNase activity. However, both wildtype ErnsSM and the two mutated E(rns)ErnsH297k and ErnsH346k as exogenous proteins had a block effect on Newcastle disease virus (NDV)-mediated IFN-beta promoter induction.

Production of an active feline interferon in the cocoon of transgenic silkworms using the fibroin H-chain expression system

We constructed the fibroin H-chain expression system to produce recombinant proteins in the cocoon of transgenic silkworms. Feline interferon (FeIFN) was used for production and to assess the quality of the product. Two types of FeIFN fusion protein, each with N- and C-terminal sequences of the fibroin H-chain, were designed to be secreted into the lumen of the posterior silk glands. The expression of the FeIFN/H-chain fusion gene was regulated by the fibroin H-chain promoter domain. The transgenic silkworms introduced these constructs with the piggyBac transposon-derived vector, which produced the normal sized cocoons containing each FeIFN/H-chain fusion protein. Although the native-protein produced by transgenic silkworms have almost no antiviral activity, the proteins after the treatment with PreScission protease to eliminate fibroin H-chain derived N- and C-terminal sequences from the products, had very high antiviral activity. This H-chain expression system, using transgenic silkworms, could be an alternative method to produce an active recombinant protein and silk-based biomaterials.

Dynamic developmental elaboration of N-linked glycan complexity in the Drosophila melanogaster embryo

The structural diversity of glycoprotein N-linked oligosaccharides is determined by the expression and regulation of glycosyltransferase activities and by the availability of the appropriate acceptor/donor substrates. Cells in different tissues and in different developmental stages utilize these control points to manifest unique glycan expression patterns in response to their surroundings. The activity of a Toll-like receptor, called Tollo/Toll-8, induces a pattern of incompletely defined, but neural specific, glycan expression in the Drosophila embryo. Understanding the full extent of the changes in glycan expression that result from altered Tollo/Toll-8 signaling requires characterization of the complete N-linked glycan profile of both wild-type and mutant embryos. N-Linked glycans harvested from wild-type or mutant embryos were subjected to direct structural analysis by analytic and preparative high pressure liquid chromatography, by multidimensional mass spectrometry, and by exoglycosidase digestion, revealing a predominance of high mannose and paucimannose glycans. Di-, mono-, and nonfucosylated forms of hybrid, complex biantennary, and triantennary glycans account for 12% of the total wild-type glycan profile. Two sialylated glycans bearing N-acetylneuraminic acid were detected, the first direct demonstration of this modification in Drosophila. Glycan profiles change during normal development consistent with increasing alpha-mannosidase II and core fucosyl-transferase enzyme activities, and with decreasing activity of the Fused lobes processing hexosaminidase. In tollo/toll-8 mutants, a dramatic, expected loss of difucosylated glycans is accompanied by unexpected decreases in monofucosylated and nonfucosylated hybrid glycans and increases in some nonfucosylated paucimannose and biantennary glycans. Therefore, tollo/toll-8 signaling influences flux through several processing steps that affect the maturation of N-linked glycans.

Surfactant protein A2 (SP-A2) variants expressed in CHO cells stimulate phagocytosis of Pseudomonas aeruginosa more than do SP-A1 variants

Surfactant protein A (SP-A) enhances phagocytosis of Pseudomonas aeruginosa. Two functional genes, SP-A1 and SP-A2, encode human SP-A. As we showed before, baculovirus-mediated insect cell-expressed SP-A2 enhances the association of P. aeruginosa with rat alveolar macrophages (rAMs) more than does SP-A1. However, true phagocytosis (internalization) was not shown, and insect cell derived proteins lack or are defective in certain mammalian posttranslational modifications that may be important for SP-A1 and SP-A2 activity and specificity. Here we used SP-A1 (6A(2), 6A(4)) and SP-A2 (1A(0), 1A(1)) allele variants expressed by CHO (Chinese hamster ovary) mammalian cells to study their effect on association and/or internalization of P. aeruginosa by rAMs and/or human AMs (hAMs) and to study if phagocytosis can be modulated differentially and/or more effectively by CHO cell-expressed SP-A variants than by insect-cell expressed SP-A variants. For cell association and internalization assessments, light microscopy and fluorescence-activated cell sorter analyses were used, respectively. We found the following for the first time. (i) SP-A2 variants enhanced phagocytosis (cell association and/or internalization) of P. aeruginosa more than SP-A1 variants did, and the cell association correlated with internalization. (ii) Differences in the activities of SP-A variants were observed in the following order: 1A(1)>1A(0)>6A(2)>6A(4). (iii) rAMs, although more active than hAMs, are an appropriate model, as SP-A2 variants exhibited activity higher than that seen for SP-A1 variants with either rAMs or hAMs. (iv) CHO cell-expressed SP-A was considerably more active than insect cell-expressed variants. We conclude that SP-A2 variants stimulate phagocytosis of P. aeruginosa more effectively than SP-A1 variants and that posttranslational modifications positively influence the phagocytic activity of SP-A.

Transforming lepidopteran insect cells for improved protein processing

The lepidopteran insect cells used with the baculovirus expression vector system (BEVS) are capable of synthesizing and accurately processing foreign proteins. However, proteins expressed in baculovirus-infected cells often fail to be completely processed, or are not processed in a manner that meets a researcher's needs. This chapter discusses a metabolic engineering approach that addresses this problem. Basically, this approach involves the addition of new or enhancement of existing protein processing functions in established lepidopteran insect cell lines. Methods for engineering these cell lines and assessing their properties as improved hosts for the BEVS are detailed. Examples of lepidopteran insect cell lines engineered for improved protein N-glycosylation and trafficking are described.

Secretion of the respiratory syncytial virus fusion protein from insect cells using the baculovirus expression system

Sequences derived from the respiratory syncytial virus (RSV) fusion (F) protein were expressed in insect cells as recombinant glutathione-S-transferase (GST)-tagged proteins. The sequence covering the F2 subunit (GST-F2), and a truncated form of the F protein in which the transmembrane domain was removed (GST-F2/F1), were cloned into the baculovirus pAcSecG2T secretory vector. These virus sequences also had the endogenous virus signal sequence removed and replaced with a signal sequence derived from the baculovirus gp67 glycoprotein, which was present in pAcSecG2T. The recombinant RSV glycoproteins were successfully detected in expressing cells by immunofluorescence assay and in the tissue culture medium by western blot analysis. The secreted recombinant GST-F2/F1 protein was further analysed using glycosidases. Our results showed that the GST-F2/F1 protein were sensitive to peptide:N-glycosidase F (PNGase F) treatment, but not to Endoglycosidase H (EndoH) treatment. This indicates that the secreted recombinant proteins were modified by the addition of mature N-linked glycan chains.

Protein N-glycosylation in the baculovirus-insect cell expression system and engineering of insect cells to produce "mammalianized" recombinant glycoproteins

Baculovirus expression vectors are frequently used to express glycoproteins, a subclass of proteins that includes many products with therapeutic value. The insect cells that serve as hosts for baculovirus vector infection are capable of transferring oligosaccharide side chains (glycans) to the same sites in recombinant proteins as those that are used for native protein N-glycosylation in mammalian cells. However, while mammalian cells produce compositionally more complex N-glycans containing terminal sialic acids, insect cells mostly produce simpler N-glycans with terminal mannose residues. This structural difference between insect and mammalian N-glycans compromises the in vivo bioactivity of glycoproteins and can potentially induce allergenic reactions in humans. These features obviously compromise the biomedical value of recombinant glycoproteins produced in the baculovirus expression vector system. Thus, much effort has been expended to characterize the potential and limits of N-glycosylation in insect cell systems. Discoveries from this research have led to the engineering of insect N-glycosylation pathways for assembly of mammalian-style glycans on baculovirus-expressed glycoproteins. This chapter summarizes our knowledge of insect N-glycosylation pathways and describes efforts to engineer baculovirus vectors and insect cell lines to overcome the limits of insect cell glycosylation. In addition, we consider other possible strategies for improving glycosylation in insect cells.

Isolation and analysis of a baculovirus vector that supports recombinant glycoprotein sialylation by SfSWT-1 cells cultured in serum-free medium

The inability to sialylate recombinant glycoproteins is a critical limitation of the baculovirus-insect cell expression system. This limitation is due, at least in part, to the absence of detectable sialyltransferase activities and CMP-sialic acids in the insect cell lines routinely used as hosts in this system. SfSWT-1 is a transgenic insect cell line encoding five mammalian glycosyltransferases, including sialyltransferases, which can contribute to sialylation of recombinant glycoproteins expressed by baculovirus vectors. However, sialylation of recombinant glycoproteins requires culturing SfSWT-1 cells in the presence of fetal bovine serum or another exogenous source of sialic acid. To eliminate this requirement and extend the utility of SfSWT-1 cells, we have isolated a new baculovirus vector, AcSWT-7B, designed to express two mammalian enzymes that can convert N-acetylmannosamine to CMP-sialic acid during the early phase of infection. AcSWT-7B was also designed to express a model recombinant glycoprotein during the very late phase of infection. Characterization of this new baculovirus vector showed that it induced high levels of intracellular CMP-sialic acid and sialylation of the recombinant N-glycoprotein upon infection of SfSWT-1 cells cultured in serum-free medium supplemented with N-acetylmannosamine. In addition, co-infection of SfSWT-1 cells with AcSWT-7B plus a conventional baculovirus vector encoding human tissue plasminogen activator resulted in sialylation of this recombinant N-glycoprotein under the same culture conditions. These results demonstrate that AcSWT-7B can be used in two different ways to support recombinant N-glycoprotein sialylation by SfSWT-1 cells in serum-free medium. Thus, AcSWT-7B can be used to extend the utility of this previously described transgenic insect cell line for recombinant sialoglycoprotein production.

Recombinant human alpha-1 proteinase inhibitor: towards therapeutic use

Human alpha-1-proteinase inhibitor is a well-characterized protease inhibitor with a wide spectrum of anti-protease activity. Its major physiological role is inhibition of neutrophil elastase in the lungs, and its deficiency is associated with progressive ultimately fatal emphysema. Currently in the US, only plasma-derived human alpha-1-proteinase inhibitor is available for augmentation therapy, which appears to be insufficient to meet the anticipated clinical demand. Moreover, despite effective viral clearance steps in the manufacturing process, the potential risk of contamination with new and unknown pathogens still exists. In response, multiple efforts to develop recombinant versions of human alpha-1-proteinase inhibitor, as an alternative to the plasma-derived protein, have been reported. Over the last two decades, various systems have been used to express the human gene for alpha-1-proteinase inhibitor. This paper reviews the recombinant versions of human alpha-1-proteinase inhibitor produced in various hosts, considers current major safety and efficacy issues regarding recombinant glycoproteins as potential therapeutics, and the factors that are impeding progress in this area(1).

Null Mutations in Drosophila N-Acetylglucosaminyltransferase I Produce Defects in Locomotion and a Reduced Life Span

UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (encoded by Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans. Mice make hybrid and complex N-glycans but little or no paucimannose N-glycans. In contrast, Drosophila melanogaster and Caenorhabditis elegans make paucimannose N-glycans but little or no hybrid or complex N-glycans. To determine the functional requirement for beta1,2-N-acetylglucosaminyltransferase I in Drosophila, we generated null mutations by imprecise excision of a nearby transposable element. Extracts from Mgat1(1)/Mgat1(1) null mutants showed no beta1,2-N-acetylglucosaminyltransferase I enzyme activity. Moreover, mass spectrometric analysis of these extracts showed dramatic changes in N-glycans compatible with lack of beta1,2-N-acetylglucosaminyltransferase I activity. Interestingly, Mgat1(1)/Mgat1(1) null mutants are viable but exhibit pronounced defects in adult locomotory activity when compared with Mgat1(1)/CyO-GFP heterozygotes or wild type flies. In addition, in null mutants males are sterile and have a severely reduced mean and maximum life span. Microscopic examination of mutant adult fly brains showed the presence of fused beta lobes. The removal of both maternal and zygotic Mgat1 also gave rise to embryos that no longer express the horseradish peroxidase antigen within the central nervous system. Taken together, the data indicate that beta1,2-N-acetylglucosaminyltransferase I-dependent N-glycans are required for locomotory activity, life span, and brain development in Drosophila.

Transgenic plants in the biopharmaceutical market

Many of our 'small-molecule-drugs' are natural products from plants, or are synthetic compounds based on molecules found naturally in plants. However, the vast majority of the protein therapeutics (or biopharmaceuticals) we use are from animal or human sources, and are produced commercially in microbial or mammalian bioreactor systems. Over the last few years, it has become clear that plants have great potential for the production of human proteins and other protein-based therapeutic entities. Plants offer the prospect of inexpensive biopharmaceutical production without sacrificing product quality or safety, and following the success of several plant-derived technical proteins, the first therapeutic products are now approaching the market. In this review, the different plant-based production systems are discussed and the merits of transgenic plants are evaluated compared with other platforms. A detailed discussion is provided of the development issues that remain to be addressed before plants become an acceptable mainstream production technology. The many different proteins that have already been produced using plants are described, and a sketch of the current market and the activities of the key players is provided. Despite the currently unclear regulatory framework and general industry inertia, the benefits of plant-derived pharmaceuticals are now bringing the prospect of inexpensive veterinary and human medicines closer than ever before.

Expression of a functional Drosophila melanogaster CMP-sialic acid synthetase. Differential localization of the Drosophila and human enzymes

CMP-N-acetylneuraminic acid is a critical metabolite in the generation of glycoconjugates that play a role in development and other physiological processes. Whereas pathways for its generation are firmly established in vertebrates, the presence and function of the relevant synthetic enzyme in insects and other protostomes is unknown. In this study, we characterize the first functional CMP-sialic acid synthase (DmCSAS) from any protostome lineage expressed from a D. melanogaster cDNA clone. Homologous genes were subsequently identified in other insect species. The gene is developmentally regulated, with expression first appearing at 12-24 h of embryogenesis, low expression through larval and pupal stages, and greatly enriched expression in the adult head, suggesting a possible role in the central nervous system. Activity of the enzyme was verified by an increase in in vitro and in vivo CMP-N-acetylneuraminic acid levels when expressed in a heterologous host. Unlike all known vertebrate CMP-sialic acid synthetase (CSAS) proteins that localize to the nucleus, the D. melanogaster CSAS protein was targeted to the Golgi compartment when expressed in both heterologous mammalian and insect cell lines. Replacement of the N-terminal leader sequence of DmCSAS with the human CSAS N-terminal sequence resulted in the redirection of the chimeric CSAS protein to the nucleus but with a concomitant loss of enzymatic activity. The localization of CSAS orthologs to different intracellular organelles represents, to our knowledge, the first example of differential protein targeting of orthologs in eukaryotes and reveals how the sialylation pathway diverged during the evolution of protostomes and deuterostomes.

Changes in cellular secretory processing during baculovirus infection

In the baculovirus expression vector system (BEVS), secretory green fluorescent protein (sGFP) transcripts were expressed from day 2 to day 5 post-infection (p.i.), while transcripts of the endoplasmic reticulum (ER) molecular chaperone Bombyx mori protein disulfide isomerase (bPDI) were measurable in mock cells and in cells at day 1 p.i. The GFP was expressed from day 3 to day 5 p.i. whereas the levels of two ER chaperone proteins, bPDI and calnexin, decreased from day 3 p.i. and were not detected from day 4 p.i. These findings suggest that the rate-limited expression of ER molecular chaperones is strongly associated with the maximal expression of exogenous proteins in BEVS.

The Drosophila fused lobes Gene Encodes an N-Acetylglucosaminidase Involved in N-Glycan Processing

Most processed, e.g. fucosylated, N-glycans on insect glycoproteins terminate in mannose, yet the relevant modifying enzymes require the prior action of N-acetylglucosaminyltransferase I. This led to the hypothesis that a hexosaminidase acts during the course of N-glycan maturation. To determine whether the Drosophila melanogaster genome indeed encodes such an enzyme, a cDNA corresponding to fused lobes (fdl), a putative beta-N-acetylglucosaminidase with a potential transmembrane domain, was cloned. When expressed in Pichia pastoris, the enzyme exhibited a substrate specificity similar to that previously described for a hexosaminidase activity from Sf-9 cells, i.e. it hydrolyzed exclusively the GlcNAc residue attached to the alpha1,3-linked mannose of the core pentasaccharide of N-glycans. It also hydrolyzed p-nitrophenyl-N-acetyl-beta-glucosaminide, but not chitooligosaccharides; in contrast, Drosophila HEXO1 and HEXO2 expressed in Pichia cleaved both these substrates but not N-glycans. The localization of recombinant FDL tagged with green fluorescent protein in Drosophila S2 cells by immunoelectron microscopy showed that this enzyme transits through the Golgi, is present on the plasma membrane and in multivesicular bodies, and is secreted. Finally, the N-glycans of two lines of fdl mutant flies were analyzed by mass spectrometry and reversed-phase high-performance liquid chromatography. The ratio of structures with terminal GlcNAc over those without (i.e. paucimannosidic N-glycans) was drastically increased in the fdl-deficient flies. Therefore, we conclude that the fdl gene encodes a novel hexosaminidase responsible for the occurrence of paucimannosidic N-glycans in Drosophila.

Baculovirus as versatile vectors for protein expression in insect and mammalian cells

Today, many thousands of recombinant proteins, ranging from cytosolic enzymes to membrane-bound proteins, have been successfully produced in baculovirus-infected insect cells. Yet, in addition to its value in producing recombinant proteins in insect cells and larvae, this viral vector system continues to evolve in new and unexpected ways. This is exemplified by the development of engineered insect cell lines to mimic mammalian cell glycosylation of expressed proteins, baculovirus display strategies and the application of the virus as a mammalian-cell gene delivery vector. Novel vector design and cell engineering approaches will serve to further enhance the value of baculovirus technology.

Comparing N-glycan processing in mammalian cell lines to native and engineered lepidopteran insect cell lines

In the past decades, a large number of studies in mammalian cells have revealed that processing of glycoproteins is compartmentalized into several subcellular organelles that process N-glycans to generate complex-type oligosaccharides with terminal N -acetlyneuraminic acid. Recent studies also suggested that processing of N-glycans in insect cells appear to follow a similar initial pathway but diverge at subsequent processing steps. N-glycans from insect cell lines are not usually processed to terminally sialylated complex-type structures but are instead modified to paucimannosidic or oligomannose structures. These differences in processing between insect cells and mammalian cells are due to insufficient expression of multiple processing enzymes including glycosyltransferases responsible for generating complex-type structures and metabolic enzymes involved in generating appropriate sugar nucleotides. Recent genomics studies suggest that insects themselves may include many of these complex transferases and metabolic enzymes at certain developmental stages but expression is lost or limited in most lines derived for cell culture. In addition, insect cells include an N -acetylglucosaminidase that removes a terminal N -acetylglucosamine from the N-glycan. The innermost N -acetylglucosamine residue attached to asparagine residue is also modified with alpha(1,3)-linked fucose, a potential allergenic epitope, in some insect cells. In spite of these limitations in N-glycosylation, insect cells have been widely used to express various recombinant proteins with the baculovirus expression vector system, taking advantage of their safety, ease of use, and high productivity. Recently, genetic engineering techniques have been applied successfully to insect cells in order to enable them to produce glycoproteins which include complex-type N-glycans. Modifications to insect N-glycan processing include the expression of missing glycosyltransferases and inclusion of the metabolic enzymes responsible for generating the essential donor sugar nucleotide, CMP- N -acetylneuraminic acid, required for sialylation. Inhibition of N -acetylglucosaminidase has also been applied to alter N-glycan processing in insect cells. This review summarizes current knowledge on N-glycan processing in lepidopteran insect cell lines, and recent progress in glycoengineering lepidopteran insect cells to produce glycoproteins containing complex N-glycans.

Spiroplasma citri Spiralin Acts In Vitro as a Lectin Binding to Glycoproteins from Its Insect Vector Circulifer haematoceps

ABSTRACT In order to understand the molecular mechanisms underlying transmission of Spiroplasma citri by the leafhopper Circulifer haematoceps, we screened leafhopper proteins as putative S. citri-binding molecules using a spiroplasma overlay assay of protein blots (Far-western assay). Insect proteins were separated by one- or two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis, blotted, and probed with S. citri proteins. In this in vitro assay, we found that spiroplasma proteins exhibited affinity for seven leafhopper proteins. The interactions between S. citri proteins and insect proteins with molecular masses of 50 and 60 kDa were found to be sugar sensitive. These insect proteins were identified as high mannose N-glycoproteins, which support an interaction of glycoprotein-lectin type with S. citri proteins. Lectin detection in S. citri has revealed only one protein of 24 kDa. Using a leafhopper protein overlay assay on an S. citri protein blot, one spiroplasma protein with a similar molecular mass of 24 kDa was shown to display an insect protein-binding capacity. This protein was identified as the spiralin, which is the most abundant membrane protein of S. citri. Far-western experiments performed with purified spiralin and insect glycoproteins confirmed the binding of spiralin to the insect glycoproteins of 50 and 60 kDa. Thus, the spiralin could play a key role in the transmission of S. citri by mediating spiroplasma adherence to epithelial cells of insect vector gut or salivary gland.

The animal sialyltransferases and sialyltransferase-related genes: a phylogenetic approach

The animal sialyltransferases are Golgi type II transmembrane glycosyltransferases. Twenty distinct sialyltransferases have been identified in both human and murine genomes. These enzymes catalyze transfer of sialic acid from CMP-Neu5Ac to the glycan moiety of glycoconjugates. Despite low overall identities, they share four conserved peptide motifs [L (large), S (small), motif III, and motif VS (very small)] that are hallmarks for sialyltransferase identification. We have identified 155 new putative genes in 25 animal species, and we have exploited two lines of evidence: (1) sequence comparisons and (2) exon-intron organization of the genes. An ortholog to the ancestor present before the split of ST6Gal I and II subfamilies was detected in arthropods. An ortholog to the ancestor present before the split of ST6GalNAc III, IV, V, and VI subfamilies was detected in sea urchin. An ortholog to the ancestor present before the split of ST3Gal I and II subfamilies was detected in ciona, and an ortholog to the ancestor of all the ST8Sia was detected in amphioxus. Therefore, single examples of the four families (ST3Gal, ST6Gal, ST6GalNAc, and ST8Sia) have appeared in invertebrates, earlier than previously thought, whereas the four families were all detected in bony fishes, amphibians, birds, and mammals. As previously hypothesized, sequence similarities among sialyltransferases suggest a common genetic origin, by successive duplications of an ancestral gene, followed by divergent evolution. Finally, we propose predictions on these invertebrates sialyltransferase-related activities that have not previously been demonstrated and that will ultimately need to be substantiated by protein expression and enzymatic activity assays.

Mammalian-like nonsialyl complex-type N-glycosylation of equine gonadotropins in Mimic™ insect cells

Recombinant equine luteinizing hormone/chorionic gonadotropin (eLH/CG) was expressed in Mimic insect cells, that are commercial stably transformed Spodoptera frugiperda (Sf9) cells expressing five mammalian genes encoding glycosyltransferases involved in the synthesis of complex-type monosialylated N-glycans. We previously showed that it exhibited no in vivo bioactivity although expressing full in vitro bioactivity, and it was suspected that this was because of insufficient sialylation of eLH/CG N-glycans. Lectin binding analyses were performed with recombinant dimeric eLH/CG or its alpha subunit, secreted in the serum-containing supernatant of infected Sf9 and Mimic cells. Two types of specific lectin affinity assays (blot analyses and enzyme-linked immunosorbent assay) were used to compare the ability or inability of natural and recombinant gonadotropins to bind to various lectins. In natural equine chorionic gonadotropin (eCG), complex-type N-glycans terminating with both Siaalpha2,3Gal (based on Maackia amurensis agglutinin [MAA] binding) and Siaalpha2,6Gal (based on Sambucus nigra agglutinin [SNA] binding) were found, but in the alpha subunit dissociated from natural eCG, we only detected Siaalpha2-6Gal. In eLH/CG and its alpha subunit produced by Sf9 cells, N-glycans were found to be terminated by mannosyl residues (based on Galanthus nivalis agglutinin [GNA] binding), whereas those produced in Mimic cells were terminated by galactoses (based on binding to Ricinus communis agglutinin I [RCA I] , but not to SNA or MAA). This is in agreement with the fact that the nucleotide donor substrate of sialic acid is not naturally synthesized in insect cells. On the basis of binding to Arachis Hypogaea agglutinin [PNA], O-glycans exhibited the Galbeta1-3GalNAc structure in recombinant-free alpha and eLH/CG from both Sf9 and Mimic cell lines. Both N- and O-linked carbohydrate side chains synthesized in Mimic cells should thus be amenable to further acellular sialylation.

Recombinant HIV-1 Pr55gag virus-like particles: potent stimulators of innate and acquired immune responses

Several previous reports have clearly demonstrated the strong effectiveness of human immunodeficiency virus (HIV) Gag polyprotein-based virus-like particles (VLP) to stimulate humoral and cellular immune responses in complete absence of additional adjuvants. Yet, the mechanisms underlying the strong immunogenicity of these particulate antigens are still not very clear. However, current reports strongly indicate that these VLP act as "danger signals" to trigger the innate immune system and possess potent adjuvant activity to enhance the immunogenicity of per se only weakly immunogenic peptides and proteins. Here, we review the current understanding of how various particle-associated substances and other impurities may contribute to the observed immune-activating properties of these complex immunogens.

Factor H binds to washed human platelets

BACKGROUND

Factor H regulates the alternative pathway of complement. The protein has three heparin-binding sites, is synthesized primarily in the liver and copurifies from platelets with thrombospondin-1. Factor H mutations at the C-terminus are associated with atypical hemolytic uremic syndrome, a condition in which platelets are consumed. Objectives The aim of this study was to investigate if factor H interacts with platelets.

METHODS

Binding of factor H, recombinant C- or N-terminus constructs and a C-terminus mutant to washed (plasma and complement-free) platelets was analyzed by flow cytometry. Binding of factor H and constructs to thrombospondin-1 was measured by surface plasmon resonance.

RESULTS

Factor H bound to platelets in a dose-dependent manner. The major binding site was localized to the C-terminus. The interaction was partially blocked by heparin. Inhibition with anti-GPIIb/IIIa, or with fibrinogen, suggested that the platelet GPIIb/IIIa receptor is involved in factor H binding. Factor H binds to thrombospondin-1. Addition of thrombospondin-1 increased factor H binding to platelets. Factor H mutated at the C-terminus also bound to platelets, albeit to a significantly lesser degree.

CONCLUSIONS

This study reports a novel property of factor H, i.e. binding to platelets, either directly via the GPIIb/IIIa receptor or indirectly via thrombospondin-1, in the absence of complement. Binding to platelets was mostly mediated by the C-terminal region of factor H and factor H mutated at the C-terminus exhibited reduced binding.

Complement regulatory proteins are incorporated into lentiviral vectors and protect particles against complement inactivation

Lentiviral vectors pseudotyped with G glycoprotein from vesicular stomatitis virus (VSV-G) and baculovirus gp64 are inactivated by human complement. The extent of vector inactivation in serum from individual donors was examined and results showed wide donor-dependent variation in complement sensitivity for VSV-G-pseudotyped lentivectors. Amphotropic envelope (Ampho)-pseudotyped vectors were generally resistant to serum from all donors, while gp64-pseudotyped vectors were inactivated but showed less donor-to-donor variation than VSV-G. In animal sera, the vectors were mostly resistant to inactivation by rodent complement, whereas canine complement caused a moderate reduction in titer. In a novel advance for the lentiviral vector system, human complement-resistant-pseudotyped lentivector particles were produced through incorporation of complement regulatory proteins (CRPs). Decay accelerating factor (DAF)/CD55 provided the most effective protection using this method, while membrane cofactor protein (MCP)/CD46 showed donor-dependent protection and CD59 provided little or no protection against complement inactivation. Unlike previous approaches using CRPs to produce complement-resistant viral vectors, CRP-containing lentivectors particles were generated for this study without engineering the CRP molecules. Thus, through overexpression of native DAF/CD55 in the viral producer cell, an easy method was developed for generation of lentiviral vectors that are almost completely resistant to inactivation by human complement. Production of complement-resistant lentiviral particles is a critical step toward use of these vectors for in vivo gene therapy applications.

Glycosylation patterns are sexually dimorphic throughout development of the olfactory system in Manduca sexta

In the moth Manduca sexta, development of the adult olfactory system depends on complex interactions between olfactory receptor neurons in the antenna, antennal-lobe neurons in the brain, and several classes of glial cells. As one approach to characterizing molecules that may play roles in these interactions, we used lectins to screen antennae and antennal lobes at different stages of adult development. We find that each of the major neural cell types has a distinct pattern of labeling by lectins. Effects of enzymatic and other treatments on lectin labeling lead us to conclude that the predominant lectin ligands are: glycosphingolipids and an O-linked, fucose-containing glycoprotein on axons of olfactory receptor neurons, O-linked glycoproteins on antennal-lobe neurons, and N-linked glycoproteins on all classes of glial cells in the primary olfactory pathway. Wheat germ agglutinin labels all olfactory axons uniformly during much of development, but labeling becomes restricted to the pheromone-responsive olfactory receptor neurons in the adult male. Succinylated WGA reveals differences in these axon classes earlier, as glomerului develop from protoglomeruli. The adult female displays a less pronounced difference in labeling of axons targeting ordinary and sexually dimorphic glomeruli. Differences in labeling of receptor axons targeted to ordinary and sexually dimorphic glomeruli may be correlated with differences in function or connectivity in different regions of the antennal lobe.

Molecular cloning and functional characterization of a Lepidopteran insect beta4-N-acetylgalactosaminyltransferase with broad substrate specificity, a functional role in glycoprotein biosynthesis, and a potential functional role in glycolipid biosynthesis

A degenerate PCR approach was used to isolate a lepidopteran insect cDNA encoding a beta4-galactosyl-transferase family member. The isolation and initial identification of this cDNA was based on bioinformatics, but its identification as a beta4-galactosyltransferase family member was experimentally confirmed. The newly identified beta4-galactosyltransferase family member had unusually broad donor and acceptor substrate specificities in vitro, as transferred galactose, N-acetylglucosamine, and N-acetylgalactosamine to carbohydrate, glycoprotein, and glycolipid acceptors. However, the enzyme preferentially utilized N-acetylgalactosamine as the donor for all three acceptors, and its derived amino acid sequence was closely related to a known N-acetylgalactosaminyltransferase. These data suggested that the newly isolated cDNA encodes a beta4-N-acetylgalactosaminyltransferase that functions in insect cell glycoprotein biosynthesis, glycolipid biosynthesis, or both. The remainder of this study focused on the role of this enzyme in N-glycoprotein biosynthesis. The results showed that the purified enzyme transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to a synthetic N-glycan in vitro. The structure of the reaction product was confirmed by chromatographic, mass spectroscopic, and nuclear magnetic resonance analyses. Co-expression of the new cDNA product in insect cells with an N-glycoprotein reporter showed that it transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to this N-glycoprotein in vivo. Confocal microscopy showed that a GFP-tagged version of the enzyme was localized in the insect cell Golgi apparatus. In summary, this study demonstrated that lepidopteran insect cells encode and express a beta4-N-acetylgalactosaminyltransferase that functions in N-glycoprotein biosynthesis and perhaps in glycolipid biosynthesis, as well. The isolation and characterization of this gene and its product contribute to our basic understanding of insect protein N-glycosylation pathways and to the growing body of evidence that insects can produce glycoproteins with complex N-glycans.

A Naturally Occurring Soluble Form of Vascular Endothelial Growth Factor Receptor 2 Detected in Mouse and Human Plasma

Angiogenesis and vasculogenesis are regulated in large part by several different growth factors and their associated receptor tyrosine kinases (RTKs). Foremost among these is the vascular endothelial growth factor (VEGF) family including VEGF receptor (VEGFR)-2 and -1. VEGFR ligand binding and biological activity are regulated at many levels, one of which is by a soluble, circulating form of VEGFR-1 (sVEGFR-1). This sVEGFR-1 can act as a competitive inhibitor of its ligand, serve as a possible biomarker, and play important roles in cancer and other diseases such as preeclampsia. Recombinant forms of sVEGFR-2 have been shown to have antiangiogenic activity, but a naturally occurring sVEGFR-2 has not been described previously. Here, we report such an entity. Having a molecular weight of ∼160 kDa, sVEGFR-2 can be detected in mouse and human plasma with several different monoclonal and polyclonal anti-VEGFR-2 antibodies using both ELISA and immunoprecipitation techniques. In vitro studies have determined that the sVEGFR-2 fragment can be found in the conditioned media of mouse and human endothelial cells, thus suggesting that it may be secreted, similar to sVEGFR-1, or proteolytically cleaved from the cell. Potential biological activity of this protein was inferred from experiments in which mouse sVEGFR-2 could bind to VEGF-coated plates. Similar to sVEGFR-1 and other soluble circulating RTKs, sVEGFR-2 may have regulatory consequences with respect to VEGF-mediated angiogenesis as well as potential to serve as a quantitative biomarker of angiogenesis and antiangiogenic drug activity, particularly for drugs that target VEGF or VEGFR-2.

Recombinant replication-restricted VSV as an expression vector for murine cytokines

Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus that rapidly and efficiently shuts down the production of host cell-encoded proteins and utilizes the cell's protein production machinery to express high levels of virally encoded proteins. In an effort to take advantage of this characteristic of VSV, we have employed a reverse genetics system to create recombinant forms of VSV encoding a variety of murine cytokines. Previous studies have revealed that cells infected with recombinant VSV that lack expression of the surface glycoprotein (G protein), designated deltaG-VSV, more efficiently express and secrete recombinant proteins than do recombinant "wild-type" VSV. Therefore, murine cytokine-expressing recombinants were produced as deltaG viruses. Propagation of these deltaG viruses in cells that transiently express G protein in vitro results in G-complemented virions that can infect cells, shut down host protein synthesis, and express at high levels each virally encoded protein (including the designated cytokine). We assessed the ability of each deltaG-VSV construct to express recombinant cytokine by infecting BHK cells and then monitoring/measuring the production of the desired cytokine. When possible, the bioactivity of the cytokine products was also measured. The results presented here reveal that large quantities of bioactive cytokines can be produced rapidly and inexpensively using deltaG-VSV as a protein expression system.

The baculovirus GP64 protein mediates highly stable infectivity of a human respiratory syncytial virus lacking its homologous transmembrane glycoproteins

Baculovirus GP64 is a low-pH-dependent membrane fusion protein required for virus entry and cell-to-cell transmission. Recently, GP64 has generated interest for practical applications in mammalian systems. Here we examined the membrane fusion function of GP64 from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) expressed in mammalian cells, as well as its capacity to functionally complement a mammalian virus, human respiratory syncytial virus (HRSV). Both authentic GP64 and GP(64/F), a chimeric protein in which the GP64 cytoplasmic tail domain was replaced with the 12 C-terminal amino acids of the HRSV fusion (F) protein, induced low-pH-dependent cell-cell fusion when expressed transiently in HEp-2 (human) cells. Levels of surface expression and syncytium formation were substantially higher at 33 degrees C than at 37 degrees C. The open reading frames (ORFs) encoding GP64 or GP(64/F), along with two marker ORFs encoding green fluorescent protein (GFP) and beta-glucuronidase (GUS), were used to replace all three homologous transmembrane glycoprotein ORFs (small hydrophobic SH, attachment G, and F) in a cDNA of HRSV. Infectious viruses were recovered that lacked the HRSV SH, G, and F proteins and expressed instead the GP64 or GP(64/F) protein and the two marker proteins GFP and GUS. The properties of these viruses, designated RSDeltaSH,G,F/GP64 or RSDeltaSH,G,F/GP(64/F), respectively, were compared to a previously described HRSV expressing GFP in place of SH but still containing the wild-type HRSV G and F proteins (RSDeltaSH [A. G. Oomens, A. G. Megaw, and G. W. Wertz, J. Virol., 77:3785-3798, 2003]). By immunoelectron microscopy, the GP64 and GP(64/F) proteins were shown to incorporate into HRSV-induced filaments at the cell surface. Antibody neutralization, ammonium chloride inhibition, and replication levels in cell culture showed that both GP64 proteins efficiently mediated infectivity of the respective viruses in a temperature-sensitive, low-pH-dependent manner. Furthermore, RSDeltaSH,G,F/GP64 and RSDeltaSH,G,F/GP(64/F) replicated to higher levels and had significantly higher stability of infectivity than HRSVs containing the homologous HRSV G and F proteins. Thus, GP64 and a GP64/HRSV F chimeric protein were functional and efficiently complemented an unrelated human virus in mammalian cells, producing stable, infectious virus stocks. These results demonstrate the potential of GP64 for both practical applications requiring stable pseudotypes in mammalian systems and for studies of viral glycoprotein requirements in assembly and pathogenesis.