Association of birthweight with lung function and respiratory diseases: results from the GEIRD study

Background

Early life conditions are associated with lung function and the development of respiratory and non-respiratory illnesses. The relationship with birthweight (BW) is however conflicting. We examined associations of BW with lung function and respiratory diseases within the GEIRD (Gene-Environment Interaction in Respiratory Diseases) study.

Methods

GEIRD is an Italian multi-centre, multi-case control study of people aged 20-84 from the general population conducted from 2008 to 2014. The study included cases of COPD, asthma, allergic rhinitis and controls. Multinomial logistic regression was performed with case/control status (control/COPD/asthma/allergic rhinitis) as response variable, and BW as main determinant adjusting for sex, age and smoking status.

Results

Of 2,287 reporting BW, 6.4 % (n = 147) had low BW (<2500 gr), and this proportion was greater in women than men (7.8% vs. 5.1%; p = 0.006). Lung volumes were significantly lower in individuals with low than normal BW. Median FEV1 was 3.01 L (p25-p75=2.60-3.45 L) versus 3.16 L (2.65-3.86 L) (p = 0.019) and median FVC was 3.68 L (3.19-4.34 L) versus 3.91 (3.34-4.81 L) (p = 0.003). However, FEV1 and FVC were not affected by BW when expressed as percent predicted. Of note, both men and women with low BW were shorter than those with normal BW (mean±SD: 160.2±5.5 vs. 162.6±6.5 cm in women, p = 0.009; 172.4±6.1 vs. 174.8±7.2 cm in men, p < 0.001). FEV1/FVC expressed as absolute ratio or as percent predicted, was not affected by BW. In multinomial analysis, BW was not associated with respiratory diseases in adulthood. However, those with low BW had a higher risk of self-reported hospitalisation for lung disease before age 2 (10.3% vs. 4.1%) and severe respiratory infection before age 5 (16.9% vs. 8.8%) (p = 0.003).

Conclusions

BW was not associated with lung function in adulthood, when controlling for sex and height. Low BW was a risk factor for respiratory diseases in childhood, not in adulthood.

Key messages

• Low birthweight was associated with respiratory diseases in childhood but not in adulthood.

• Although spirometrically-assessed lung volumes were lower in adults with low birthweight, this is likely explained by associations of low birthweight with sex and height.

Clinical markers of asthma and IgE assessed in parents before conception predict asthma and hayfever in the offspring

BACKGROUND

Mice models suggest epigenetic inheritance induced by parental allergic disease activity. However, we know little of how parental disease activity before conception influences offspring's asthma and allergy in humans.

OBJECTIVE

We aimed to assess the associations of parental asthma severity, bronchial hyperresponsiveness (BHR), and total and specific IgEs, measured before conception vs. after birth, with offspring asthma and hayfever.

METHODS

The study included 4293 participants (mean age 34, 47% men) from the European Community Respiratory Health Survey (ECRHS) with information on asthma symptom severity, BHR, total and specific IgEs from 1991 to 1993, and data on 9100 offspring born 1972-2012. Adjusted relative risk ratios (aRRR) for associations of parental clinical outcome with offspring allergic disease were estimated with multinomial logistic regressions.

RESULTS

Offspring asthma with hayfever was more strongly associated with parental BHR and specific IgE measured before conception than after birth [BHR: aRRR = 2.96 (95% CI: 1.92, 4.57) and 1.40 (1.03, 1.91), respectively; specific IgEs: 3.08 (2.13, 4.45) and 1.83 (1.45, 2.31), respectively]. This was confirmed in a sensitivity analysis of a subgroup of offspring aged 11-22 years with information on parental disease activity both before and after birth.

CONCLUSION & CLINICAL RELEVANCE

Parental BHR and specific IgE were associated with offspring asthma and hayfever, with the strongest associations observed with clinical assessment before conception as compared to after birth of the child. If the hypothesis is confirmed in other studies, parental disease activity assessed before conception may prove useful for identifying children at risk for developing asthma with hayfever.

The locus C11orf30 increases susceptibility to poly-sensitization

A number of genetic variants have been associated with allergic sensitization, but whether these are allergen specific or increase susceptibility to poly-sensitization is unknown. Using data from the large multicentre population-based European Community Respiratory Health Survey, we assessed the association between 10 loci and specific IgE and skin prick tests to individual allergens and poly-sensitization. We found that the 10 loci associate with sensitization to different allergens in a nonspecific manner and that one in particular, C11orf30-rs2155219, doubles the risk of poly-sensitization (specific IgE/4 allergens: OR = 1.81, 95% CI 0.80-4.24; skin prick test/4+ allergens: OR = 2.27, 95% CI 1.34-3.95). The association of rs2155219 with higher levels of expression of C11orf30, which may be involved in transcription repression of interferon-stimulated genes, and its association with sensitization to multiple allergens suggest that this locus is highly relevant for atopy.

Region-specific deletions of RIM1 reproduce a subset of global RIM1α(-/-) phenotypes

The presynaptic protein RIM1α mediates multiple forms of presynaptic plasticity at both excitatory and inhibitory synapses. Previous studies of mice lacking RIM1α (RIM1α(-/-) throughout the brain showed that deletion of RIM1α results in multiple behavioral abnormalities. In an effort to begin to delineate the brain regions in which RIM1 deletion mediates these abnormal behaviors, we used conditional (floxed) RIM1 knockout mice (fRIM1). By crossing these fRIM1 mice to previously characterized transgenic cre lines, we aimed to delete RIM1 selectively in the dentate gyrus (DG), using a specific preproopiomelanocortin promoter driving cre recombinase (POMC-cre) line , and in pyramidal neurons of the CA3 region of hippocampus, using the kainate receptor subunit 1 promoter driving cre recombinase (KA-cre). Neither of these cre driver lines was uniquely selective to the targeted regions. In spite of this, we were able to reproduce a subset of the global RIM1α(-/-) behavioral abnormalities, thereby narrowing the brain regions in which loss of RIM1 is sufficient to produce these behavioral differences. Most interestingly, hypersensitivity to the pyschotomimetic MK-801 was shown in mice lacking RIM1 selectively in the DG, arcuate nucleus of the hypothalamus and select cerebellar neurons, implicating novel brain regions and neuronal subtypes in this behavior.

The occupational contribution to severe exacerbation of asthma

The goal of this study was to identify occupational risk factors for severe exacerbation of asthma and estimate the extent to which occupation contributes to these events. The 966 participants were working adults with current asthma who participated in the follow-up phase of the European Community Respiratory Health Survey. Severe exacerbation of asthma was defined as self-reported unplanned care for asthma in the past 12 months. Occupations held in the same period were combined with a general population job-exposure matrix to assess occupational exposures. 74 participants reported having had at least one severe exacerbation event, for a 1-yr cumulative incidence of 7.7%. From regression models that controlled for confounders, the relative risk (RR) was statistically significant for low (RR 1.7, 95% CI 1.1-2.6) and high (RR 3.6, 95% CI 2.2-5.8) biological dust exposure, high mineral dust exposure (RR 1.8, 95% CI 1.02-3.2), and high gas and fumes exposure (RR 2.5, 95% CI 1.2-5.5). The summary category of high dust, gas, or fumes exposure had RR 3.1 (95% CI 1.9-5.1). Based on this RR, the population attributable risk was 14.7% among workers with current asthma. These results suggest occupation contributes to approximately one in seven cases of severe exacerbation of asthma in a working population, and various agents play a role.

Indoor nitrous acid and respiratory symptoms and lung function in adults

BACKGROUND

Nitrogen dioxide (NO2) is an important pollutant of indoor and outdoor air, but epidemiological studies show inconsistent health effects. These inconsistencies may be due to failure to account for the health effects of nitrous acid (HONO) which is generated directly from gas combustion and indirectly from NO2.

METHODS

Two hundred and seventy six adults provided information on respiratory symptoms and lung function and had home levels of NO2 and HONO measured as well as outdoor levels of NO2. The association of indoor HONO levels with symptoms and lung function was examined.

RESULTS

The median indoor HONO level was 3.10 ppb (IQR 2.05-5.09), with higher levels in homes with gas hobs, gas ovens, and in those measured during the winter months. Non-significant increases in respiratory symptoms were observed in those living in homes with higher HONO levels. An increase of 1 ppb in indoor HONO was associated with a decrease in forced expiratory volume in 1 second (FEV1) percentage predicted (-0.96%; 95% CI -0.09 to -1.82) and a decrease in percentage FEV1/forced vital capacity (FVC) (-0.45%; 95% CI -0.06 to -0.83) after adjustment for relevant confounders. Measures of indoor NO2 were correlated with HONO (r = 0.77), but no significant association of indoor NO2 with symptoms or lung function was observed. After adjustment for NO2 measures, the association of HONO with low lung function persisted.

CONCLUSION

Indoor HONO levels are associated with decrements in lung function and possibly with more respiratory symptoms. Inconsistencies between studies examining health effects of NO2 and use of gas appliances may be related to failure to account for this association.

Mammalian glycosyltransferase expression allows sialoglycoprotein production by baculovirus-infected insect cells

The baculovirus-insect cell expression system is widely used to produce recombinant mammalian glycoproteins, but the glycosylated end products are rarely authentic. This is because insect cells are typically unable to produce glycoprotein glycans containing terminal sialic acid residues. In this study, we examined the influence of two mammalian glycosyltransferases on N-glycoprotein sialylation by the baculovirus-insect cell system. This was accomplished by using a novel baculovirus vector designed to express a mammalian alpha2,6-sialyltransferase early in infection and a new insect cell line stably transformed to constitutively express a mammalian beta1,4-galactosyltransferase. Various biochemical assays showed that a foreign glycoprotein was sialylated by this virus-host combination, but not by a control virus-host combination, which lacked the mammalian glycosyltransferase genes. Thus, this study demonstrates that the baculovirus-insect cell expression system can be metabolically engineered for N-glycoprotein sialylation by the addition of two mammalian glycosyltransferase genes.

Novel baculovirus expression vectors that provide sialylation of recombinant glycoproteins in lepidopteran insect cells

This report describes novel baculovirus vectors designed to express mammalian beta1,4-galactosyltransferase and alpha2,6-sialyltransferase genes at early times after infection. Sf9 cells infected with these viral vectors, unlike cells infected with a wild-type baculovirus, produced a sialylated viral glycoprotein during the late phase of infection. Thus, the two mammalian glycosyltransferases encoded by these viral vectors are necessary and sufficient for sialylation of a foreign glycoprotein in insect cells under the conditions used in this study. While some of the new baculovirus vectors described in this study produced less, one produced wild-type levels of infectious budded virus progeny.

Glycoproteins from insect cells: sialylated or not?

Our growing comprehension of the biological roles of glycan moieties has created a clear need for expression systems that can produce mammalian-type glycoproteins. In turn, this has intensified interest in understanding the protein glycosylation pathways of the heterologous hosts that are commonly used for recombinant glycoprotein expression. Among these, insect cells are the most widely used and, particularly in their role as hosts for baculovirus expression vectors, provide a powerful tool for biotechnology. Various studies of the glycosylation patterns of endogenous and recombinant glycoproteins produced by insect cells have revealed a large variety of O- and N-linked glycan structures and have established that the major processed O- and N-glycan species found on these glycoproteins are (Gal beta1,3)GalNAc-O-Ser/Thr and Man3(Fuc)GlcNAc2-N-Asn, respectively. However, the ability or inability of insect cells to synthesize and compartmentalize sialic acids and to produce sialylated glycans remains controversial. This is an important issue because terminal sialic acid residues play diverse biological roles in many glycoconjugates. While most work indicates that insect cell-derived glycoproteins are not sialylated, some well-controlled studies suggest that sialylation can occur. In evaluating this work, it is important to recognize that oligosaccharide structural determination is tedious work, due to the infinite diversity of this class of compounds. Furthermore, there is no universal method of glycan analysis; rather, various strategies and techniques can be used, which provide glycobiologists with relatively more or less precise and reliable results. Therefore, it is important to consider the methodology used to assess glycan structures when evaluating these studies. The purpose of this review is to survey the studies that have contributed to our current view of glycoprotein sialylation in insect cell systems, according to the methods used. Possible reasons for the disagreement on this topic in the literature, which include the diverse origins of biological material and experimental artifacts, will be discussed. In the final analysis, it appears that if insect cells have the genetic potential to perform sialylation of glycoproteins, this is a highly specialized function that probably occurs rarely. Thus, the production of sialylated recombinant glycoproteins in the baculovirus-insect cell system will require metabolic engineering efforts to extend the native protein glycosylation pathways of insect cells.

Engineering lepidopteran insect cells for sialoglycoprotein production by genetic transformation with mammalian beta 1,4-galactosyltransferase and alpha 2,6-sialyltransferase genes

Recombinant mammalian glycoproteins produced by the baculovirus-insect cell expression system usually do not have structurally authentic glycans. One reason for this limitation is the virtual absence in insect cells of certain glycosyltransferases, which are required for the biosynthesis of complex, terminally sialylated glycoproteins by mammalian cells. In this study, we genetically transformed insect cells with mammalian beta 1,4-galactosyltransferase and alpha 2,6-sialyltransferase genes. This produced a new insect cell line that can express both genes, serve as hosts for baculovirus infection, and produce foreign glycoproteins with terminally sialylated N-glycans.

N-glycan patterns of human transferrin produced in Trichoplusia ni insect cells: effects of mammalian galactosyltransferase

The N-glycans of human serum transferrin produced in Trichopulsia ni cells were analyzed to examine N-linked oligosaccharide processing in insect cells. Metabolic radiolabeling of the intra- and extracellular protein fractions revealed the presence of multiple transferrin glycoforms with molecular weights lower than that observed for native human transferrin. Consequently, the N-glycan structures of transferrin in the culture medium were determined using three-dimensional high performance liquid chromatography. The attached oligosaccharides included high mannose, paucimannosidic, and hybrid structures with over 50% of these structures containing one fucose, alpha(1,6)-, or two fucoses, alpha(1,6)- and alpha(1,3)-, linked to the Asn-linked N-acetylglucosamine. Neither sialic acid nor galactose was detected on any of the N-glycans. However, when transferrin was coexpressed with beta(1,4)-galactosyltransferase three additional galactose-containing hybrid oligosaccharides were obtained. The galactose attachments were exclusive to the alpha(1, 3)-mannose branch and the structures varied by the presence of zero, one, or two attached fucose residues. Furthermore, the presence of the galactosyltransferase appeared to reduce the number of paucimannosidic structures, which suggests that galactose attachment inhibits the ability of hexosaminidase activity to remove the terminal N-acetylglucosamine. The ability to promote galactosylation and reduce paucimannosidic N-glycans suggests that the oligosaccharide processing pathway in insect cells may be manipulated to mimic more closely that of mammalian cells.

N-Glycan processing by a lepidopteran insect alpha1,2-mannosidase

Protein glycosylation pathways are relatively poorly characterized in insect cells. As part of an overall effort to address this problem, we previously isolated a cDNA from Sf9 cells that encodes an insect alpha1,2-mannosidase (SfManI) which requires calcium and is inhibited by 1-deoxymannojirimycin. In the present study, we have characterized the substrate specificity of SfManI. A recombinant baculovirus was used to express a GST-tagged secreted form of SfManI which was purified from the medium using an immobilized glutathione column. The purified SfManI was then incubated with oligosaccharide substrates and the resulting products were analyzed by HPLC. These analyses showed that SfManI rapidly converts Man(9)GlcNAc(2)to Man(6)Glc-NAc(2)isomer C, then more slowly converts Man(6)GlcNAc(2)isomer C to Man(5)GlcNAc(2). The slow step in the processing of Man(9)GlcNAc(2)to Man(5)GlcNAc(2)by SfManI is removal of the alpha1,2-linked mannose on the middle arm of Man(9)GlcNAc(2). In this respect, SfManI is similar to mammalian alpha1,2-mannosidases IA and IB. However, additional HPLC and(1)H-NMR analyses demonstrated that SfManI converts Man(9)GlcNAc(2)to Man(5)GlcNAc(2)primarily through Man(7)GlcNAc(2)isomer C, the archetypal Man(9)GlcNAc(2)missing the lower arm alpha1,2-linked mannose residues. In this respect, SfManI differs from mammalian alpha1,2-mannosidases IA and IB, and is the first alpha1,2-mannosidase directly shown to produce Man(7)GlcNAc(2)isomer C as a major processing intermediate.

Electrophoretic analysis of glycoprotein glycans produced by lepidopteran insect cells infected with an immediate early recombinant baculovirus encoding mammalian beta1,4-galactosyltransferase

Glycosylation, the most extensive co- and post-translational modification of eukaryotic cells, can significantly affect biological activity and is particularly important for recombinant glycoproteins in human therapeutic applications. The baculovirus-insect cell expression system is a popular tool for the expression of heterologous proteins and has an excellent record of producing high levels of biologically active eukaryotic proteins. Insect cells are capable of glycosylation, but their N-glycosylation pathway is truncated in comparison with the pathway of mammalian cells. A previous study demonstrated that an immediate early recombinant baculovirus could be used to extend the insect cell N-glycosylation pathway by contributing bovine beta-1,4 galactosyltransferase (GalT) immediately after infection. Lectin blotting assays indicated that this ectopically expressed enzyme could transfer galactose to an N-linked glycan on a foreign glycoprotein expressed later in infection. In the current study, glycans were isolated from total Sf-9 cell glycoproteins after infection with the immediate early recombinant baculovirus encoding GalT, fluorescently conjugated and analyzed by electrophoresis in combination with exoglycosidase digestion. These direct analyses clearly demonstrated that Sf-9 cells infected with this recombinant baculovirus can synthesize galactosylated N-linked glycans.

Mutational analysis of the N-linked glycans on Autographa californica nucleopolyhedrovirus gp64

gp64 is the major envelope glycoprotein in the budded form of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). gp64 is essential for AcMNPV infection, as it mediates penetration of budded virus into host cells via the endocytic pathway. In this study, we used site-directed mutagenesis to map the positions of the N-linked glycans on AcMNPV gp64, characterize their structures, and evaluate their influence on gp64 function. We found that four of the five consensus N-glycosylation sites in gp64 are used, and we mapped the positions of those sites to amino acids 198, 355, 385, and 426 in the polypeptide chain. Endoglycosidase H sensitivity assays showed that N-linked glycans located at different positions are processed to various degrees. Lectin blotting analyses showed that each N-linked glycan on gp64 contains alpha-linked mannose, all but one contains alpha-linked fucose, and none contains detectable beta-linked galactose or alpha2,6-linked sialic acid. The amounts of infectious progeny produced by AcMNPV mutants lacking one, two, or three N-linked glycans on gp64 were about 10- to 100-fold lower than wild-type levels. This reduction did not correlate with reductions in the expression, transport, or inherent fusogenic activity of the mutant gp64s or in the gp64 content of mutant budded virus particles. However, all of the mutant viruses bound more slowly than the wild type. Therefore, elimination of one or more N-glycosylation sites in AcMNPV gp64 impairs binding of budded virus to the cell, which explains why viruses containing these mutant forms of gp64 produce less infectious progeny.

Engineering N-glycosylation pathways in the baculovirus-insect cell system

The inability to produce eukaryotic glycoproteins with complex N-linked glycans is a major limitation of the baculovirus-insect cell expression system. Recent studies have demonstrated that metabolic engineering can be used to extend the glycoprotein processing capabilities of lepidopteran insect cells. This approach is being used to develop new baculovirus-insect cell expression systems that can produce more authentic recombinant glycoproteins and obtain new information on insect N-glycosylation pathways.

Stable expression of mammalian beta 1,4-galactosyltransferase extends the N-glycosylation pathway in insect cells

An established lepidopteran insect cell line (Sf9) was cotransfected with expression plasmids encoding neomycin phosphotransferase and bovine beta 1,4-galactosyltransferase. Neomycin-resistant transformants were selected, assayed for beta 1,4-galactosyltransferase activity, and the transformant with the highest level of enzymatic activity was characterized. Southern blots indicated that this transformed Sf9 cell derivative contained multiple copies of the galactosyltransferase-encoding expression plasmid integrated at a single site in its genome. One-step growth curves showed that these cells supported normal levels of baculovirus replication. Baculovirus infection of the transformed cells stimulated beta 1,4-galactosyltransferase activity almost 5-fold by 12 h postinfection. This was followed by a gradual decline in activity, but the infected cells still had about as much activity as uninfected controls as late as 48 h after infection and they were able to produce a beta 1,4-galactosylated virion glycoprotein during infection. Infection of the transformed cells with a conventional recombinant baculovirus expression vector encoding human tissue plasminogen activator also resulted in the production of a galactosylated end-product. These results demonstrate that stable transformation can be used to add a functional mammalian glycosyltransferase to lepidopteran insect cells and extend their N-glycosylation pathway. Furthermore, stably-transformed insect cells can be used as modified hosts for conventional baculovirus expression vectors to produce foreign glycoproteins with "mammalianized" glycans which more closely resemble those produced by higher eucaryotes.

Isolation and characterization of an alpha 1,2-mannosidase cDNA from the lepidopteran insect cell line Sf9

As part of our ongoing efforts to characterize the N-glycosylation pathway of lepidopteran insect cells, we have isolated an alpha 1,2-mannosidase homolog from an Sf9 cDNA library. This cDNA contains an open reading frame which encodes a 670 amino acid protein with a calculated molecular weight of 75,225 Da. This protein has two potential N-glycosylation sites, two consensus calcium binding sequences, and is predicted to be a type II integral membrane protein with a 22 amino acid transmembrane domain (residues 31-52). The amino acid sequence of this protein is 35-57% identical to Drosophila, human, murine, and yeast alpha 1,2-mannosidases. A transcript of approximately 6 kilobases was detected by Northern blot analysis of Sf9 mRNA. Genomic Southern blots probed with an intron-free fragment of the alpha 1,2-mannosidase gene indicated that there are at least two copies or cross-hybridizing variants of this gene in the Sf9 genome. In vivo expression of the cDNA using a recombinant baculovirus produced a protein that released [3H]mannose from [3H]Man9GlcNAc. This activity required calcium, but not magnesium, and was inhibited by 1-deoxymannojirimycin. These results indicate that Sf9 cells encode and express an alpha 1,2-mannosidase with properties similar to those of other eukaryotic processing alpha 1,2-mannosidases.

Isolation and characterization of a class II alpha-mannosidase cDNA from lepidopteran insect cells

Lepidopteran insect cells are used routinely as hosts for foreign glycoprotein expression by recombinant baculoviruses, but the precise nature of their N-glycosylation pathway remains poorly defined. These cells clearly have processing glucosidases and mannosidases that can convert precursors to Man3GlcNAc2 structures and fucosyltransferases that can add fucose to the oligosaccharide core. However, their ability to extend these structures to produce complex side chains like those found in mammalian cells remains to be determined. To begin to examine this pathway at the molecular genetic level, we isolated and characterized a class II alpha-mannosidase (alpha-mannosidase II) cDNA from Sf9, a lepidopteran insect cell line. In mammalian cells, this enzyme catalyzes the committed step in the pathway converting N-linked carbohydrates to complex forms. Degenerate primers against conserved regions in known class II alpha-mannosidase protein sequences were used to generate an alpha-mannosidase II-specific PCR product from Sf9 cell DNA. Sequence information from this product was used to isolate a partial cDNA clone, the 5' end was isolated by ligation-anchored PCR, and the full length alpha-mannosidase II cDNA was assembled. This cDNA contained a long open reading frame predicted to encode an 1130 amino acid protein with 37% identity to human Golgi alpha-mannosidase II and with a type II membrane topology, a feature of all known Golgi processing enzymes. Southern blotting indicated that alpha-mannosidase II is a single copy gene in Sf9 cells. Other Lepidoptera had related alpha-mannosidase II genes, but there was variation among different genera, and the Sf9 alpha-mannosidase II cDNA did not cross-hybridize with DNA from animals outside Lepidoptera. Steady-state levels of alpha-mannosidase II RNA were low in uninfected Sf9 cells and even lower after baculovirus infection. The in vitro-translated Sf9 alpha-mannosidase II protein had the expected size and was translocated and N-glycosylated by microsomal membranes. Expression of the Sf9 alpha-mannosidase II cDNA in the baculovirus system produced large amounts of a protein with the expected size and swainsonine-sensitive alpha-mannosidase II activity towards an aryl-alpha-mannoside substrate. These results demonstrate that Sf9 cells encode and express an alpha-mannosidase II with properties similar to those of the mammalian enzyme.

Variation in nebulizer aerosol output and weight output from the Mefar dosimeter: implications for multicentre studies

The active aerosol component of nebulizers is less than 100% of output by weight, and may vary between nebulizers in different batches from the same manufacturer. A measure of bronchial responsiveness to methacholine, which can overcome this problem, is required. One hundred and sixty nebulizers from 21 centres in the European Community Respiratory Health Survey (ECRHS) were calibrated for aerosol and weight output. Methacholine challenge data were obtained for 1,021 subjects in three English centres of the ECRHS. The dose producing a 20% fall in forced expiratory volume in one second (PD20), and log-slope, the regression slope of percentage decline in FEV1 with log (dose), were calculated, with and without calibration of nebulizers by weight. Within-centre variation in nebulizer percentage aerosol output had a coefficient of variation of less than 10%. Unlike PD20, log-slope is unaffected by constant percentage overestimation of nebulizer output. Variation in output by weight of nebulizers of 10% had little affect on log-slope. It is, however, affected by the scheduled range of doses. Log-slope shows advantages in analysis, and is less affected by variation in nebulizer output. It can be used for multicentre comparisons, with restriction to a common dose protocol.

The role of the AcMNPV 25K gene, "FP25," in baculovirus polh and p10 expression

A previous study showed that an Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) 25K mutant produced less polyhedrin protein than wild-type (Jarvis et al., J. Virol. 66, 6903-6911, 1992). In this study, the role of the 25K gene product (AcMNPV ORF 61) in baculovirus gene expression was further investigated. Five different viral 25K mutants expressed lower levels of polyhedrin protein and less CAT activity under the control of the polh promoter compared to wild-type. Polh RNA was equally stable in wild-type and mutant virus-infected cells while the rate of polh transcription was significantly reduced in mutant-infected cells. In comparison, steady-state levels of p10 RNA were not reduced in 25K mutant-infected cells, indicating that the reduction in polh RNA did not reflect a general effect on very late gene transcription. Expression of ie-1, which also appears to influence polh expression (Choi and Guarino, Virology 209, 90-98, 1995), was not influenced by 25K mutation. These results show that the 25K protein is important for maintaining optimal levels of polh transcription by a mechanism that does not involve maintaining ie-1 expression.

Dynamic phosphorylation of Autographa californica nuclear polyhedrosis virus pp31

Autographa californica nuclear polyhedrosis virus (AcMNPV) pp31 is a nuclear phosphoprotein that accumulates in the virogenic stroma, which is the viral replication center in the infected-cell nucleus, binds to DNA, and serves as a late expression factor. Considering that reversible phosphorylation could influence its functional properties, we examined phosphorylation and dephosphorylation of pp31 in detail. Our results showed that pp31 is posttranslationally phosphorylated by both cellular and virus-encoded or -induced kinases. Threonine phosphorylation of pp31 by the virus-specific kinase activity was sensitive to aphidicolin, indicating that it requires late viral gene expression. We also found that pp31 is dephosphorylated by a virus-encoded or -induced phosphatase(s), indicating that phosphorylation of pp31 is a dynamic process. Analysis of pp31 fusion proteins showed that pp31 contains at least three phosphorylation sites. The amino-terminal 100 amino acids of pp31 include at least one serine residue that is phosphorylated by a cellular kinase(s). The C-terminal 67 amino acids of pp31 include at least one threonine residue that is phosphorylated by the virus-specific kinase(s). Finally, this C-terminal domain of pp31 includes at least one serine that is phosphorylated by either a host or viral kinase(s). Interestingly, site-directed mutagenesis of the consensus threonine phosphorylation sites in the C-terminal domain of pp31 failed to prevent threonine phosphorylation, suggesting that the virus-specific kinase is unique and has an undetermined recognition site.

Modifying the insect cell N-glycosylation pathway with immediate early baculovirus expression vectors

The baculovirus-insect cell expression system is well-suited for recombinant glycoprotein production because baculovirus vectors can provide high levels of expression and insect cells can modify newly synthesized proteins in eucaryotic fashion. However, the N-glycosylation pathway of baculovirus-infected insect cells differs from the pathway found in higher eucaryotes, as indicated by the fact that glycoproteins produced in the baculovirus system typically lack complex biantennary N-linked oligosaccharide side chains containing penultimate galactose and terminal sialic acid residues. We recently developed a new type of baculovirus vector that can express foreign genes immediately after infection under the control of the viral ie1 promoter. These immediate early baculovirus expression vectors can be used to modify the insect cell N-glycosylation pathway and produce a foreign glycoprotein with more extensively processed N-linked oligosaccharides. These vectors can also be used to study the influence of the late steps in N-linked oligosaccharide processing on glycoprotein function. Further development could lead to baculovirus-insect cell expression systems that can produce recombinant glycoproteins with complex biantennary N-linked oligosaccharides structurally identical to those produced by higher eucaryotes.

Immediate-early baculovirus vectors for foreign gene expression in transformed or infected insect cells

Baculovirus expression vectors are used routinely for foreign gene expression and are under intense development as improved biological pesticides. Conventional baculovirus expression vectors are recombinant viruses that can express a foreign gene in insect cells under the control of the polyhedrin promoter, which provides high-level transcription during the very late phase of infection. For some applications, including foreign glycoprotein production and insect pest control, it might be advantageous to have baculovirus vectors that could express foreign gene products in uninfected cells or earlier after infection. To fulfill this need, we have constructed a new set of plasmids that can be used to clone and express foreign genes under the control of a baculovirus ie1 promoter, which is active in uninfected insect cells and throughout infection. We used a subset of these new plasmids to isolate recombinant baculoviruses containing various foreign genes and compared expression of these genes by the resulting immediate-early baculovirus vectors and by conventional baculovirus vectors. As expected, the immediate-early vectors began to express each foreign gene earlier in infection but, by 36-48 h postinfection, the conventional vectors had produced more of each foreign protein. Conventional baculovirus vectors also produced more enzymatic activity from two different procaryotic genes than the immediate-early baculovirus vectors. However, immediate-early vectors produced as much or more enzymatic activity from two different eucaryotic genes encoding secretory pathway proteins than the conventional vectors, even at 48 h postinfection. Hence, this report describes a new set of plasmids that can be used to clone and express foreign genes under the control of the baculovirus ie1 promoter and suggests that immediate-early baculovirus vectors might be as useful as conventional baculovirus expression vectors for producing biologically active eucaryotic secretory pathway proteins.

Mapping functional domains in AcMNPV pp31

Autographa californica nuclear polyhedrosis virus (AcMNPV) replicates in the nucleus and produces a viral-modified form of the nuclear matrix called the virogenic stroma. The virogenic stroma is the site of viral DNA packaging and nucleocapsid assembly and is thought to be the site of viral DNA replication and RNA transcription. AcMNPV encodes a phosphoprotein, pp31, which localizes to the nucleus of uninfected insect cells and to the virogenic stroma of infected insect cells. pp31 has DNA binding activity and has been identified as a late expression factor. Thus, the intracellular location of pp31, its DNA binding activity, and its identification as a late transcription factor suggest that it participates in replicative events that occur in the virogenic stroma during AcMNPV infection. The purpose of this study was to map the pp31 domains needed for nuclear localization, virogenic stroma localization, and DNA binding. We focused on four basic amino acid regions (BRs 1-4) and used site-directed mutagenesis and gene fusion techniques to probe their functions. The amino-terminal basic region (BR1) was most important for nuclear localization of pp31 in uninfected cells. Three of the four BRs were needed to efficiently localize pp31 to the nucleus and virogenic stroma in infected cells. BR3 was identified as the DNA binding domain of pp31. These data indicated that BR1, BR3, and BR4 are important functional or multifunctional domains within the AcMNPV pp31 protein.

Biochemical analysis of the N-glycosylation pathway in baculovirus-infected lepidopteran insect cells

The baculovirus-insect cell system is used routinely for foreign glycoprotein production, but the precise nature of the N-glycosylation pathway in this system remains unclear. Some studies indicate that these cells cannot process N-linked oligosaccharides to complex forms containing outer-chain galactose and sialic acid, while others indicate that they can. In this study, we used the major virion envelope glycoprotein of the baculovirus Autographa california multicapsid nuclear polyhedrosis virus (AcMNPV) to probe the N-glycosylation pathway in baculovirus-infected lepidopteran insect cells. The results showed that gp64 contained mannose, fucose, and probably N-acetylglucosamine, but no detectable galactose or sialic acid. These same results were observed with gp64 produced in any one of three different lepidopteran insect cell lines derived from Spodoptera frugiperda, Trichoplusia ni, or Estigmene acrea, whether it was produced at relatively earlier or later times after infection. These results indicated that the gp64 produced in AcMNPV-infected lepidopteran insect cells lacks complex N-linked oligosaccharides containing outer-chain galactose and sialic acid. By contrast, gp64 produced in mammalian cells contained both galactose and sialic acid, and endoglycosidase digestions revealed that these sugars were constituents of N-linked, not O-linked, oligosaccharides. This showed that at least one N-linked side chain on gp64 has the potential to be processed to a complex form. Together, these results suggest either that AcMNPV-infected lepidopteran insect cells are unable to convert any of the N-linked side chains on gp64 to complex structures or that outer-chain galactose and sialic acid residues are added to gp64 and then removed by cellular or viral exoglycosidases.

Biosynthesis and processing of the Autographa californica nuclear polyhedrosis virus gp64 protein

gp64 is a major virion envelope glycoprotein of the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV). gp64 plays an important role in AcMNPV infection, probably mediating penetration of one form of the virus into host cells through the endocytic pathway. gp64 also represents an excellent probe for studying the membrane glycoprotein processing capabilities of baculovirus-infected insect cells, which are used widely as a eucaryotic expression system. The goals of this study were to characterize gp64 biosynthesis and processing and determine how N-glycosylation and N-linked oligosaccharide processing influence the fate and function of gp64 in AcMNPV-infected insect cells. We found that gp64 was synthesized in a biphasic fashion, with peaks at 8 and 24 hr postinfection in both the intracellular and extracellular fractions. Interestingly, the first peak preceded detectable budded virus (BV) production, suggesting that gp64 is shed from infected cells early in infection. Transcriptional regulation accounted for the biphasic mode of gp64 protein synthesis, as transcription initiated at a consensus early motif during early times of infection, at a late motif during late times of infection, and there was a lag between the peak of early and the onset of late transcription. In vitro transcription-translation assays showed that the second ATG in the AcMNPV gp64 long open reading frame is used as the translational initiation codon and that downstream sequences encode a functional signal peptide. Pulse-chase analyses, endoglycosidases, and various inhibitors were used to show that some N-linked oligosaccharides on gp64 are processed by glucosidases and alpha-mannosidases in AcMNPV-infected insect cells. These experiments also revealed that at least two differentially processed gp64 glycoforms are produced in these cells and that both can reach the cell surface and assemble into progeny BV. However, N-linked oligosaccharide processing was not required for gp64 cell surface expression, its assembly into infectious BV, or its fusogenic activity. This suggested that any gp64 glycoform produced during infection, regardless of its N-linked carbohydrate structure, can have essentially normal biological properties. By contrast, transport of gp64 to the cell surface, production of infectious BV, and fusogenic activity were reduced in the absence of N-glycosylation, indicating that this modification is necessary for optimal gp64 function.

Long-term stability of baculoviruses stored under various conditions

The long-term stability of baculoviruses under various storage conditions was investigated. The results showed that the titers of baculovirus working stocks were significantly reduced after the viruses had been stored in polypropylene tubes for several months or a year at 4 degrees C. The virus was not lost by sticking to the plastic tubes, because the same results were obtained when the virus was stored in glass tubes. The major factor contributing to the loss of viral infectivity was exposure to light. Virus stocks stored at temperatures ranging from -85 degrees to 37 degrees C were quite stable, as long as they were protected from light. Finally, viral infectivity was not significantly reduced by multiple freeze-thaw cycles. These results demonstrated that baculovirus working stocks may be stored frozen or refrigerated with little long-term loss of infectivity, as long as they are protected from light.

Heterologous protein expression affects the death kinetics of baculovirus-infected insect cell cultures: a quantitative study by use of n-target theory

The death of cultured insect cells after baculovirus infection is a time-dependent event. Without a quantitative model, it is difficult to characterize its kinetics. Our group has shown that the cell survival rate can be characterized by use of the n-target theory, which involves only two parameters: the number of hypothetical inactivation targets (n) and the first-order death rate (k). In this study, we used different recombinant viruses to examine the effect of heterologous protein expression on the cell survival rate. The proteins expressed were beta-galactosidase, human T-cell leukemia virus type I p40x, human interleukin-2, and human tissue plasminogen activator (tPA). The survival rate was affected by protein expression, but the n value remained constant if the protein expression level was high (above 30 mg/L). Low-level expression of secreted, glycosylated tPA resulted in a reduced n value, which was restored to the normal value when the tPA signal peptide and prosequence were deleted. In addition, if the n value was normal (10-11), the level of protein expression correlated negatively with the death rate. However, if the n value was reduced by unfavorable culture conditions or foreign protein expression, the expression level correlated positively with the death rate. A dimensionless plot with kt as the dimensionless time shows that alteration of the k value while retaining constant n is equivalent to a rescaling of time. Therefore, the survival curves with constant n reduce to a single curve on the dimensionless plot.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of different signal peptides and prosequences on expression and secretion of human tissue plasminogen activator in the baculovirus system

Foreign secretory pathway proteins are often produced in surprisingly low amounts in the baculovirus/insect cell expression system. One possible reason for this is that heterologous signal peptides might be inefficiently recognized by the insect cell protein translocation machinery. This idea was supported by a recent study showing that secretion of a plant protein in the baculovirus system was enhanced when its signal peptide was replaced with an insect-derived signal peptide (Tessier, D. C., Thomas, D. Y., Khouri, H. E., Laliberte, F., and Vernet, T. (1991) Gene (Amst.) 98, 177-183). We have extended these observations by measuring the effects of different signal peptide and signal peptide-prosequence combinations on baculovirus-mediated expression and secretion of human tissue plasminogen activator (t-PA). Replacement of the native prepropeptide with signal peptides from a lepidopteran insect secretory protein (cecropin B), a major baculovirus structural glycoprotein (64K), or an abundant, highly conserved lumenal protein of the rough endoplasmic reticulum (GRP78/BiP, a 78-kDa glucose-regulated protein/immunoglobulin heavy chain-binding protein), had no significant effect on t-PA expression or secretion. The same results were obtained with the signal peptide from honeybee prepromellitin, which was able to enhance secretion of plant propapain (Tessier et al., 1991 (above)). Similar results were obtained when heterologous signal peptides were combined with the native prosequence or when the intact cecropin B preprosequence was used. Translational initiation at an upstream, in-frame ATT, which could functionally inactivate any signal peptide, did not explain the low efficiency of t-PA secretion. Finally, deletion of the native signal peptide, prosequence, or both, failed to increase t-PA production. These results showed that insect-derived signal peptides and/or prosequences cannot always enhance the expression and/or secretion of foreign secretory pathway proteins in the baculovirus system. They also suggested that the inability of insect cells to recognize the processing signals in human t-PA efficiently is probably not the major factor preventing its high level production in this system.

Effects of baculovirus infection on IE1-mediated foreign gene expression in stably transformed insect cells

Previously, we produced transformed insect cell lines that can express a selected foreign protein constitutively, in the absence of baculovirus infection (D. L. Jarvis, J. G. W. Fleming, G. R. Kovacs, M. D. Summers, and L. A. Guarino, Bio/Technology 8:950-955, 1990). These cells contain stably integrated copies of chimeric genes consisting of the promoter from an immediate-early baculovirus gene, IE1, and the sequences encoding either human tissue plasminogen activator or Escherichia coli beta-galactosidase. Transcription of the integrated genes in these cells is specifically controlled by the IE1 promoter. The purpose of this study was to determine how baculovirus infection influences IE1-mediated foreign protein production by these stably transformed insect cell lines. The results showed that viral infection transiently stimulated and then strongly inhibited the production of both tissue plasminogen activator, a secreted protein, and beta-galactosidase, an intracellular protein. These effects reflected virus-induced changes in the steady-state levels of RNA produced by the integrated genes. Transient assays showed that expression of the viral IEN gene alone could account for the increased levels of RNA observed early in infection. The precise mechanism accounting for the decreased levels of RNA observed later in infection was not determined. However, we obtained evidence that the native IE1 promoter remains active throughout infection, which suggested indirectly that the integrated IE1 promoter is transcriptionally inactivated at late times of baculovirus infection. Thus, the same promoter behaved quite differently late in infection, depending on its local environment. Neither methylation nor degradation appeared to be responsible for inactivating IE1-mediated expression of the integrated genes. The significance of these results with respect to the baculovirus-host interaction and the practical applications of stably transformed insect cell lines are discussed.

Insect cell hosts for baculovirus expression vectors contain endogenous exoglycosidase activity

Four different insect cell lines that can be used as hosts for baculovirus infection were assayed for the presence of endogenous exoglycosidases. All four cell lines, derived from Spodoptera frugiperda, Trichoplusia ni, Bombyx mori, or Malacosoma disstria, contained N-acetyl-beta-glucosaminidase, N-acetyl-beta-galactosaminidase, beta-galactosidase, and sialidase activities. Exoglycosidase activities were found in cell lysates as well as cell-free supernatants from uninfected and wild-type baculovirus infected cells. Oligosaccharide analysis of cellular glycoproteins using lectins recognizing Gal beta 1, 3GalNAc, Gal beta 1, 4GlcNAc, and NeuAc alpha 2,6Gal demonstrated that only Gal beta 1,3GalNAc was present. The demonstration that these cells contain exoglycosidases raises the possibility that the oligosaccharides of baculovirus-expressed glycoproteins are subject to enzymatic degradation.

Baculovirus phosphoprotein pp31 is associated with virogenic stroma

The PstI K fragment of Autographa californica nuclear polyhedrosis virus (AcMNPV) encodes a protein with a molecular weight of 31,000. To define the role of this protein (pp31) in virus infection further, it was overexpressed in bacteria and used to produce polyclonal antiserum. Radioimmunoprecipitation analysis indicated that pp31 was synthesized during both the early and late phases of virus infection, consistent with previous analyses indicating that the gene was regulated by tandem early and late promoters. Metabolic labeling of cells with carrier-free phosphate indicated that pp31 was phosphorylated. Biochemical fractionation experiments showed that pp31 was localized in the nucleus and that it was more stably associated with the nucleus at later times of infection. Immunoblot analysis of subnuclear fractions indicated that pp31 was associated predominantly with the chromatin and nuclear matrix fractions. Immunofluorescence experiments confirmed that the pp31 protein was localized in the nucleus. Nuclear staining was relatively uniform early but was more centrally nuclear later in infection. Immunoelectron microscopy indicated that the pp31 protein was a component of virogenic stroma. Southwestern (DNA-protein) blot analysis demonstrated that pp31 is a DNA-binding protein. These findings suggest a possible role for pp31 in the virus life cycle.

Enhancement of polyhedrin nuclear localization during baculovirus infection

Polyhedrin is the major component of the nuclear viral occlusions produced during replication of the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV). Since viral occlusions are responsible for the horizontal transmission of AcMNPV in nature, the biosynthesis, localization, and assembly of polyhedrin are important events in the viral replication cycle. We recently defined the sequence requirements for nuclear localization and assembly of polyhedrin. In this study, we examined the localization of polyhedrin at different times of infection. The results showed that nuclear localization of polyhedrin becomes more efficient as the occlusion phase of infection progresses. Several different factors were identified that might contribute to this overall effect, including a higher rate of polyhedrin nuclear localization and a higher rate of polyhedrin biosynthesis. We also examined the biosynthesis and processing of polyhedrin in cells infected with an AcMNPV few polyhedra (FP) mutant, which produces smaller numbers of viral occlusions that contain few or no virions. Compared with wild type, the FP mutant produced polyhedrin more slowly and localized it to the nucleus less efficiently at the beginning of the occlusion phase of infection (24 h postinfection). This supported the idea that the efficiency of polyhedrin nuclear localization is tightly coupled to its rate of biosynthesis. It also revealed that expression of the viral 25K gene, which is inactivated in the FP mutant, is directly or indirectly associated with an enhancement of polyhedrin biosynthesis and nuclear localization at the beginning of the occlusion phase of infection. This enhancement effect appears to be necessary to ensure the normal assembly of viral occlusions.

Requirements for nuclear localization and supramolecular assembly of a baculovirus polyhedrin protein

This study defines the requirements for the nuclear localization, stable nuclear association, and supramolecular assembly of a baculovirus polyhedrin protein in lepidopteran insect cells. Fragments of the polyhedrin protein were genetically fused to two different nonnuclear reporter proteins and the intracellular distribution of the fusion proteins was analyzed in infected insect cells. Analysis by indirect immunofluorescence showed that the domain between amino acids 30 and 57 could mediate nuclear localization of polyhedrin. However, biochemical fractionation experiments showed that this domain was not sufficient for a detergent-stable association of polyhedrin with the nucleus. This required a slightly larger domain, between amino acids 30 and 110. Differential interference-contrast microscopy showed that the supramolecular assembly of polyhedrin into nuclear occlusion-like particles required the domain between amino acids 19 and 110. The most likely candidate for a minimal nuclear localization signal was the sequence KRKK, located between amino acids 32 and 35. Therefore, oligonucleotide-directed mutagenesis was used to change this sequence to NGNN and the intracellular distribution of the mutant protein was analyzed. The results showed that the mutant protein was predominantly localized in the cytoplasm of infected cells, where it assembled into large, cubic, occlusion-like particles. Thus, the KRKK sequence is necessary for the nuclear localization of polyhedrin, but nuclear localization is not required for its supramolecular assembly into occlusion-like particles.

Translocation and cleavage of rubella virus envelope glycoproteins: identification and role of the E2 signal sequence

The structural proteins of rubella virus (RV) are translated as a large polyprotein precursor, p110, which is processed to produce the mature virion components, the 33K capsid protein (C) and the two envelope glycoproteins, E1 (58K) and E2 (42K to 47K). The precise processing mechanism has not been elucidated; however it must include at least two proteolytic cleavages to release the individual virion components from the polyprotein, and it must provide for their dichotomous intracellular distribution. The C protein remains in the cytoplasm where it participates in the formation of nucleocapsids, while the envelope glycoproteins enter the cellular secretory pathway and are N-glycosylated and cleaved. Sequence analysis of the 24S mRNA encoding the polyprotein precursor suggests that both E1 and E2 are preceded by signal peptides for translocation across the membrane of the rough endoplasmic reticulum. A recent study has provided direct evidence that the putative signal peptide preceding E1 can in fact mediate translocation of E1. In this study, we have used in vitro translation-translocation assays to examine further the processing of RV glycoproteins. We have shown that the putative signal sequence preceding E2 can mediate translocation of the E2 protein in the absence of an intact E1 signal peptide. The experiments also revealed that cleavage of the E2-E1 polyprotein requires (i) the E2 signal peptide, (ii) microsomal membranes and (iii) sequences beyond the proximal half of the E1 signal peptide. Together these results suggest that separation of the E2 signal sequence as well as the proteolytic cleavage of E1 from E2 is performed by the cellular enzyme, signal peptidase.

Use of Early Baculovirus Promoters for Continuous Expression and Efficient Processing of Foreign Gene Products in Stably Transformed Lepidopteran Cells

Baculoviruses are currently used as vectors for the transient high-level expression of foreign gene products in insect cells. In this study, we demonstrate that baculoviruses can also be made to continuously express a foreign gene product by using the promoter from IE1, an immediate early viral gene, to produce stably-transformed insect cells. This approach gave levels of foreign gene expression lower than those usually obtained with the lytic baculovirus expression vector system. Expression, however, was continuous and stable, and a complex human glycoprotein (tissue plasminogen activator) was processed more efficiently. We conclude that stable transformation is a feasible approach for baculovirus-mediated foreign gene expression in lepidopteran cells, particularly for products that are relatively poorly-expressed and/or processed in lytically infected cells.

Role of glycosylation in the transport of recombinant glycoproteins through the secretory pathway of lepidopteran insect cells

Cell lines established from the Lepidopteran insect Spodoptera frugiperda (e.g., Sf9) are used routinely as hosts for the expression of foreign proteins by baculovirus vectors. Previously, we showed that human tissue plasminogen activator (t-PA) was expressed, N-glycosylated, and secreted by Sf9 cells infected with a recombinant baculovirus (Jarvis DL, Summers MD: Mol Cell Biol 9:214-223, 1989). We also showed that t-PA secretion was blocked by tunicamycin (TM), an inhibitor of N-glycosylation, but not by castanospermine (CS) or N-methyldeoxynojirimycin, inhibitors of the initial steps in N-linked oligosaccharide processing. This suggested that the addition, but not the processing, of N-linked oligosaccharides is required for the secretion of recombinant t-PA from baculovirus-infected Sf9 cells. In this study, we present a more generalized evaluation of the role of N-glycosylation in the transport of recombinant glycoproteins through the Sf9 cell secretory pathway. Several different secretory or membrane-bound glycoproteins were expressed in control, TM-treated, or CS-treated Sf9 cells, and their appearance in the medium or on the cell surface was measured. The results showed that TM blocked the transport of some, but not all, of these proteins, whereas CS did not block the transport of any. This suggests that N-glycosylation is sometimes required for the transport of recombinant glycoproteins through the Sf9 secretory pathway, while processing of the oligosaccharides is not. At least two other proteins, p80 and p31, consistently coimmunoprecipitated with the nonglycosylated precursors of recombinant glycoproteins expressed in TM-treated Sf9 cells. Neither was antigenically related to any of the recombinant proteins. Relatively larger amounts of p80 and p31 were coprecipitated when transport was completely blocked by TM compared to when transport was only reduced or was unaffected. These results suggest that p80 and p31 block the transport of some nonglycosylated glycoprotein precursors in TM-treated Sf9 cells by binding to them and producing transport-incompetent heterooligomeric complexes. If this speculation is correct, then p80 and p31 are functionally analogous to the mammalian immunoglobulin heavy chain binding/glucose-regulated 78 kilodalton protein (BiP/GRP78).

SV40 T-antigen as a dual oncogene: structure and function of the plasma membrane-associated population

SV40 T-antigen (T-ag) is localized in both the nucleus (nT-ag) and plasma membrane (pmT-ag) of cells and provides multiple functions necessary for cell transformation. The pmT-ag population is structurally very similar to the nT-ag. Transport to the cell surface is by an unknown mechanism that does not involve the secretory pathway. The disposition of T-ag in the membrane exposes both the amino and the carboxyl terminus on the exterior of the cell. Nuclear-transport-defective mutants of T-ag can transform established cells in culture, but not primary cells, suggesting that non-nuclear forms of T-ag may mediate some transformation-related process(es). A non-cytolytic protein extraction technique utilizing 1-butanol solubilized from SV40-transformed cells a multimeric complex composed of pmT-ag and at least five cellular proteins ranging in size from 35,000 (35K) to 60K M. Both amino- and carboxylterminal T-ag-specific monoclonal antibodies co-precipitated T-ag and the 35-60K Mr proteins, but antibodies against the internal portion of T-ag precipitated only uncomplexed T-ag. The growth state of the cells markedly influenced the expression of the T-ag-containing surface complexes; more complexes were recovered from actively dividing cells than from confluent cell cultures, and suspension cells yielded more complexes than cells on a substratum. The complex exhibited a highly dynamic association with the cell membrane, as demonstrated by pulse-chase analysis. The characteristics of growth-dependent expression and rapid turnover rate suggest a functional role for the membrane complex. The identities of the cellular proteins in the complex with pmT-ag are unknown, although one member (56K) is recognized by p53-specific monoclonal antibodies.

Glycosylation and secretion of human tissue plasminogen activator in recombinant baculovirus-infected insect cells

Cell lines established from the lepidopteran insect Spodoptera frugiperda (fall armyworm; Sf9) are used routinely as hosts for the expression of foreign proteins by recombinant baculovirus vectors. We have examined the pathway of protein glycosylation and secretion in these cells, using human tissue plasminogen activator (t-PA) as a model. t-PA expressed in Sf9 cells was both N glycosylated and secreted. At least a subset of the N-linked oligosaccharides in extracellular t-PA was resistant to endo-beta-N-acetyl-D-glucosaminidase H, which removes immature, high-mannose-type oligosaccharides. This refutes the general conclusion from previous studies that Sf9 cells cannot process immature N-linked oligosaccharides to an endo-beta-N-acetyl-D-glucosaminidase H-resistant form. A nonglycosylated t-PA precursor was not detected in Sf9 cells, even with very short pulse-labeling times. This suggests that the mammalian signal sequence of t-PA is efficiently recognized in Sf9 cells and that it can mediate rapid translocation across the membrane of the rough endoplasmic reticulum, where cotranslational N glycosylation takes place. However, t-PA was secreted rather slowly, with a half-time of about 1.6 h. Thus, a rate-limiting step(s) in secretion occurs subsequent to translocation and N glycosylation of the t-PA polypeptide. Treatment of Sf9 cells with tunicamycin, but not with inhibitors of oligosaccharide processing, prevented the appearance of t-PA in the extracellular medium. This suggests that N glycosylation per se, but not processing of the N-linked oligosaccharides, is required directly or indirectly in baculovirus-infected Sf9 cells for the secretion of t-PA. Finally, the relative efficiency of secretion decreased dramatically with time of infection, suggesting that the Sf9 host cell secretory pathway is compromised during the later stages of baculovirus infection.

Biochemical properties of SV40 large tumor antigen as a glycosylated protein

The large tumor antigen (T-ag) of SV40 is a virus-encoded polypeptide that provides multiple biological activities required for virus replication and cellular transformation. T-ag is an exceptional model for the study of protein processing, because it displays a variety of chemical modifications and an unusual dual subcellular distribution. The cellular mechanisms responsible for the synthesis and processing of T-ag are unknown. With respect to glycosylation, this has been related to a lack of knowledge of the biochemical properties of T-ag as a glycoprotein. Several such properties are characterized here. We found that T-ag is glycosylated at multiple sites on the polypeptide chain. The oligosaccharides appear to belong to a single size class, molecular weight approximately 400, and the linkage between the polypeptide and the carbohydrate side chain is sensitive to beta-elimination under mild alkaline conditions. At least one glycosylation site was localized to the region between amino acids 1 and 272 (probably between residues 83 and 272), and at least one additional site was localized to a separate region, between amino acids 523 and 708. The results of cycloheximide experiments suggested that glycosylation of T-ag is a cotranslational event, and both the nuclear and the membrane-associated forms of T-ag appeared to be glycosylated. The results of these studies verify previous conclusions that the cellular secretory pathway is not involved in the glycosylation of T-ag; instead, a cytoplasmic mechanism might be involved.

The cellular secretory pathway is not utilized for biosynthesis, modification, or intracellular transport of the simian virus 40 large tumor antigen

Unlike most proteins, which are localized within a single subcellular compartment in the eucaryotic cell, the simian virus 40 (SV40) large tumor antigen (T-ag) is associated with both the nucleus and the plasma membrane. Current knowledge of protein processing would predict a role for the secretory pathway in the biosynthesis and transport of at least a subpopulation of T-ag to account for certain of its chemical modifications and for its ability to reach the cell surface. We have examined this prediction by using in vitro translation and translocation experiments. Preliminary experiments established that translation of T-ag was detectable with as little as 0.1 microgram of the total cytoplasmic RNA from SV40-infected cells. Therefore, by using a 100-fold excess of this RNA, the sensitivity of the assays was above the limits necessary to detect the theoretical fraction of RNA equivalent to the subpopulation of plasma-membrane-associated T-ag (2 to 5% of total T-ag). In contrast to a control rotavirus glycoprotein, the electrophoretic mobility of T-ag was not changed by the addition of microsomal vesicles to the in vitro translation mixture. Furthermore, T-ag did not undergo translocation in the presence of microsomal vesicles, as evidenced by its sensitivity to trypsin treatment and its absence in the purified vesicles. Identical results were obtained with either cytoplasmic RNA from SV40-infected cells or SV40 early RNA transcribed in vitro from a recombinant plasmid containing the SP6 promoter. SV40 early mRNA in infected cells was detected in association with free, but not with membrane-bound, polyribosomes. Finally, monensin, an inhibitor of Golgi function, failed to specifically prevent either glycosylation or cell surface expression of T-ag, although it did depress overall protein synthesis in TC-7 cells. We conclude from these observations that the constituent organelles of the secretory pathway are not involved in the biosynthesis, modification, or intracellular transport of T-ag. The initial step in the pathway of T-ag biosynthesis appears to be translation on free cytoplasmic polyribosomes. With the exclusion of the secretory pathway, we suggest that T-ag glycosylation, palmitylation, and transport to the plasma membrane are accomplished by previously unrecognized cellular mechanisms.

Absence of a structural basis for intracellular recognition and differential localization of nuclear and plasma membrane-associated forms of simian virus 40 large tumor antigen

The simian virus 40 large tumor antigen (T-ag) is found in both the nuclei (nT-ag) and plasma membranes (mT-ag) of simian virus 40-infected or -transformed cells. It is not known how newly synthesized T-ag molecules are recognized, sorted, and transported to their ultimate subcellular destinations. One possibility is that these events depend upon structural differences between nT-ag and mT-ag. To test this possibility, we compared the structures of nT-ag and mT-ag from simian virus 40-infected cells. No differences between the two forms of T-ag were detected by migration in polyacrylamide gels, by Staphylococcus aureus V8 partial proteolytic mapping of methionine- or proline-containing peptides, or by two-dimensional tryptic peptide mapping of methionine-containing peptides. The carboxy-terminal, methionine-containing tryptic peptide was identified in the two-dimensional maps and was shown to be identical in nT-ag and mT-ag. Thus, a structural basis for the recognition and differential localization of T-ags could not be demonstrated. The carboxy terminus of the T-ag encoded by mutant dlA2413 is derived from the alternate open reading frame of the simian virus 40 early region, in analogy with the theoretical early gene product, T*-ag. We used this mutant to identify peptides unique to T*-ag. None of these peptides were detected in maps of mT-ag; only wild-type T-ag-specific peptides were found. These findings suggest that T*-ag does not represent the membrane-associated form of T-ag, but that mT-ag is encoded within the same reading frame used for nT-ag.

Modification of simian virus 40 large tumor antigen by glycosylation

The SV40-encoded transforming protein, large tumor antigen (T-ag), is multifunctional. Chemical modifications of the T-ag polypeptide may be important for its multifunctional capacity. T-ag is additionally modified by glycosylation. T-ag was metabolically labeled in SV40-infected cells with tritiated galactose or glucosamine, but not with mannose or fucose. The identity of glycosylated T-ag was established by immunoprecipitation with a variety of T-ag-specific antisera, including monoclonal antibodies. Incorporation of labeled sugar into T-ag was inhibited in the presence of excess unlabeled sugars, but not in the presence of excess unlabeled amino acids. Labeled monosaccharides could be preferentially removed from T-ag with a mixture of glycosidic enzymes. In addition, galactose was removed from purified T-ag by acid hydrolysis and identified as such by thin-layer chromatography. T-ag oligosaccharides were resistant to treatment with EndoH, and glycosylation was not inhibited by tunicamycin. Together, these data strongly suggest that T-ag is glycosylated. Several characteristics, including lack of mannose labeling, EndoH resistance, and tunicamycin resistance, suggest that T-ag is not an N-linked glycoprotein. Rather, these properties are more consistent with the identification of T-ag as an O-linked glycoprotein.

Structural comparisons of wild-type and nuclear transport-defective simian virus 40 large tumor antigens

PARA(nT) is a defective SV40-adenovirus 7 hybrid virus which contains the entire early region of the SV40 genome and codes for the synthesis of SV40 large tumor antigen (T-ag). A transport-defective variant of this hybrid, PARA(cT), encodes T-ag that is not transported to the nucleus, but accumulates in the cytoplasm. The structures of T-ags extracted from wild-type (WT) SV40-, PARA(nT)-, and PARA(cT)-infected cells were compared by peptide mapping. All three types of T-ag underwent considerable degradation when extracted using Tris-buffered Nonidet P-40 at pH 8.0. The addition of 200 microM leupeptin to the extraction buffer significantly inhibited this degradation. Comparison of methionine-containing tryptic peptides revealed no differences among the T-ags, suggesting that their primary structures are similar or identical. Phosphopeptide mapping revealed no differences between SV40- and PARA(nT)-encoded T-ags. In contrast, PARA(cT)-encoded T-ag lacked a prominent phosphopeptide that was present in both of the others. The possible relevance of this difference in phosphorylation to the transport defect is discussed.

Modification of the poliovirus capsid by ultraviolet light

Ultraviolet (UV) irradiation of type I poliovirus resulted in a modified (M) particle that had lost infectivity, lacked ability to adsorb to HeLa cells, lacked VP4, and reduced in S value. Additional irradiation resulted in the loss of VP2, further reduction in S value, and permeability of the capsid to RNAse, This particle (C) as well as M contain the genome. Acid pH (5.5-65) and sulfhydryl-reducing substances (dithiothreitol. reduced glutathione, and L-cysteine) inhibited UV-induced modification of the capsid. UV irradiation at alkaline pH (7.5-8.5) resulted in more extensive modification of the capsid than irradiation at neutral pH. Ionic compounds were found to inhibit the modifying reaction.