Mutations blocking the transport of the influenza virus hemagglutinin between the rough endoplasmic reticulum and the Golgi apparatus

Serial Passaging of Seasonal H3N2 Influenza A/Singapore/G2-31.1/2014 Virus in MDCK-SIAT1 Cells and Primary Chick Embryo Cells Generates HA D457G Mutation and Other Variants in HA, NA, PB1, PB1-F2, and NS1

Influenza remains one of the most prevalent viruses circulating amongst humans and has resulted in several pandemics. The prevention and control of H3N2 influenza is complicated by its propensity for evolution, which leads to vaccine mismatch and reduced vaccine efficacies. This study employed the strategy of serial passaging to compare the evolution of the human seasonal influenza strain A/Singapore/G2-31.1/2014(H3N2) in MDCK-SIAT1 versus primary chick embryo fibroblast (CEF) cells. Genetic analysis of the HA, NS1, NA, and PB1 gene segments by Sanger sequencing revealed the presence of specific mutations and a repertoire of viral quasispecies following serial passaging. Most quasispecies were also found in PB1, which exhibited consistently high transversion-to-transition ratios in all five MDCK-SIAT1 passages. Most notably, passage 5 virus harbored the D457G substitution in the HA2 subunit, while passage 3 virus acquired K53Q and Q69H mutations in PB1-F2. An A971 variant leading to a non-synonymous R316Q substitution in PB1 was also identified in MDCK-SIAT1 passages 2 and 4. With an increasing number of passages, the proportion of D457G mutations progressively increased and was associated with larger virus plaque sizes. However, microneutralization assays revealed no significant differences in the neutralizing antibody profiles of human-influenza-immune serum samples against pre-passaged virus and passage 5 virus. In contrast, viable virus was only detected in passage 1 of CEF cells, which gave rise to multiple viral RNA quasispecies. Our findings highlight that serial passaging is able to drive differential adaptation of H3N2 influenza in different host species and may alter viral virulence. More studies are warranted to elucidate the complex relationships between H3N2 virus evolution, viral virulence changes, and low vaccine efficacy.

Substitution Arg140Gly in Hemagglutinin Reduced the Virulence of Highly Pathogenic Avian Influenza Virus H7N1

The H7 subtype of avian influenza viruses (AIV) stands out among other AIV. The H7 viruses circulate in ducks, poultry and equines and have repeatedly caused outbreaks of disease in humans. The laboratory strain A/chicken/Rostock/R0p/1934 (H7N1) (R0p), which was previously derived from the highly pathogenic strain A/FPV/Rostock/1934 (H7N1), was studied in this work to ascertain its biological property, genome stability and virulent changing mechanism. Several virus variants were obtained by serial passages in the chicken lungs. After 10 passages of this virus through the chicken lungs we obtained a much more pathogenic variant than the starting R0p. The study of intermediate passages showed a sharp increase in pathogenicity between the fifth and sixth passage. By cloning these variants, a pair of strains (R5p and R6p) was obtained, and the complete genomes of these strains were sequenced. Single amino acid substitution was revealed, namely reversion Gly140Arg in HA1. This amino acid is located at the head part of the hemagglutinin, adjacent to the receptor-binding site. In addition to the increased pathogenicity in chicken and mice, R6p differs from R5p in the shape of foci in cell culture and an increased affinity for a negatively charged receptor analogue, while maintaining a pattern of receptor-binding specificity and the pH of conformational change of HA.

Hemagglutinin Cleavability, Acid Stability, and Temperature Dependence Optimize Influenza B Virus for Replication in Human Airways

Influenza A virus (IAV) and influenza B virus (IBV) cause yearly epidemics with significant morbidity and mortality. When zoonotic IAVs enter the human population, the viral hemagglutinin (HA) requires adaptation to achieve sustained virus transmission. In contrast, IBV has been circulating in humans, its only host, for a long period of time. Whether this entailed adaptation of IBV HA to the human airways is unknown. To address this question, we compared two seasonal IAVs (A/H1N1 and A/H3N2) and two IBVs (B/Victoria and B/Yamagata lineages) with regard to host-dependent activity of HA as the mediator of membrane fusion during viral entry. We first investigated proteolytic activation of HA by covering all type II transmembrane serine protease (TTSP) and kallikrein enzymes, many of which proved to be present in human respiratory epithelium. The IBV HA0 precursor is cleaved by a broader panel of TTSPs and activated with much higher efficiency than IAV HA0. Accordingly, knockdown of a single protease, TMPRSS2, abrogated spread of IAV but not IBV in human respiratory epithelial cells. Second, the HA fusion pH values proved similar for IBV and human-adapted IAVs (with one exception being the HA of 1918 IAV). Third, IBV HA exhibited higher expression at 33°C, a temperature required for membrane fusion by B/Victoria HA. This indicates pronounced adaptation of IBV HA to the mildly acidic pH and cooler temperature of human upper airways. These distinct and intrinsic features of IBV HA are compatible with extensive host adaptation during prolonged circulation of this respiratory virus in the human population.IMPORTANCE Influenza epidemics are caused by influenza A and influenza B viruses (IAV and IBV, respectively). IBV causes substantial disease; however, it is far less studied than IAV. While IAV originates from animal reservoirs, IBV circulates in humans only. Virus spread requires that the viral hemagglutinin (HA) is active and sufficiently stable in human airways. We resolve here how these mechanisms differ between IBV and IAV. Whereas human IAVs rely on one particular protease for HA activation, this is not the case for IBV. Superior activation of IBV by several proteases should enhance shedding of infectious particles. IBV HA exhibits acid stability and a preference for 33°C, indicating pronounced adaptation to the human upper airways, where the pH is mildly acidic and a cooler temperature exists. These adaptive features are rationalized by the long existence of IBV in humans and may have broader relevance for understanding the biology and evolution of respiratory viruses.

Progressive glycosylation of the haemagglutinin of avian influenza H5N1 modulates virus replication, virulence and chicken-to-chicken transmission without significant impact on antigenic drift

Highly pathogenic H5N1 avian influenza virus (A/H5N1) devastated the poultry industry and continues to pose a pandemic threat. Studying the progressive genetic changes in A/H5N1 after long-term circulation in poultry may help us to better understand A/H5N1 biology in birds. A/H5N1 clade 2.2.1.1 antigenic drift viruses have been isolated from vaccinated commercial poultry in Egypt. They exhibit a peculiar stepwise accumulation of glycosylation sites (GS) in the haemagglutinin (HA) with viruses carrying, beyond the conserved 5 GS, additional GS at amino acid residues 72, 154, 236 and 273 resulting in 6, 7, 8 or 9 GS in the HA. Available information about the impact of glycosylation on virus fitness and pathobiology is mostly derived from mammalian models. Here, we generated recombinant viruses imitating the progressive acquisition of GS in HA and investigated their biological relevance in vitro and in vivo. Our in vitro results indicated that the accumulation of GS correlated with increased glycosylation, increased virus replication, neuraminidase activity, cell-to-cell spread and thermostability, however, strikingly, without significant impact on virus escape from neutralizing antibodies. In vivo, glycosylation modulated virus virulence, tissue tropism, replication and chicken-to-chicken transmission. Predominance in the field was towards viruses with hyperglycosylated HA. Together, progressive glycosylation of the HA may foster persistence of A/H5N1 by increasing replication, stability and bird-to-bird transmission without significant impact on antigenic drift.

Membrane accumulation of influenza A virus hemagglutinin triggers nuclear export of the viral genome via protein kinase Calpha-mediated activation of ERK signaling

Replication and transcription of the influenza virus genome takes place exclusively within the nucleus of the infected cells. The viral RNA genome, polymerase subunits, and nucleoprotein form ribonucleoprotein (RNP) complexes. Late in the infectious cycle RNPs have to be exported from the nucleus to be enwrapped into budding progeny virions at the cell membrane. This process requires viral activation of the cellular Raf/MEK/ERK (mitogen-activated protein kinase (MAPK)) signaling cascade that is activated late in the infection cycle. Accordingly, block of the cascade results in retardation of RNP export and reduced titers of progeny virus. In the present study we have analyzed the importance of cell-membrane association of the viral hemagglutinin glycoprotein for viral MAPK activation. We show that hemagglutinin membrane accumulation and its tight association with lipid-raft domains trigger activation of the MAPK cascade via protein kinase Calpha activation and induces RNP export. This may represent an auto-regulative mechanism that coordinates timing of RNP export to a point when all viral components are ready for virus budding.

Identification of the sites for suppressor mutations on the hemagglutinin molecule to temperature-sensitive phenotype of the influenza virus

A temperature-sensitive (ts) mutant of the influenza virus A/WSN/33 strain, ts-134, possessed a defect in intracellular transport at the nonpermissive temperature and marked thermolability of hemagglutinin (HA) activity at 51 C. These were caused by a change at amino acid residue 157 from tyrosine to histidine in the HA protein. We isolated 37 spontaneous revertant clones from ts-134 at the nonpermissive temperature and determined their HA sequences. The deduced amino acid sequences demonstrated that one was a true revertant and the others were revertants with suppressor mutations, each of which had an additional amino acid change besides those of ts-134. The changed amino acids were located at 14 positions on the HA molecule, and eight of them were found in multiple revertants. These were located in five to six distinct regions on the three-dimensional structure of the HA molecule. However, the heat stability of HAs in the revertants was recovered differently depending on the sites of the changed amino acids. The kinetics of transport of the HA protein in the revertants were slightly delayed compared to the wild-type both at permissive and nonpermissive temperatures.

Marburg virus gene 4 encodes the virion membrane protein, a type I transmembrane glycoprotein

Gene 4 of Marburg virus, strain Musoke, was subjected to nucleotide sequence analysis. It is 2,844 nucleotides long and extends from genome position 5821 to position 8665 (EMBL Data Library, emnew: MVREPCYC [accession no. Z12132]). The gene is flanked by transcriptional signal sequences (start signal, 3'-UACUUCUUGUAAUU-5'; termination signal, 3'-UAAUUCUUUUU-5') which are conserved in all Marburg virus genes. The major open reading frame encodes a polypeptide of 681 amino acids (M(r), 74,797). After in vitro transcription and translation, as well as expression in Escherichia coli, this protein was identified by its immunoreactivity with specific antisera as the unglycosylated form of the viral membrane glycoprotein (GP). The GP is characterized by the following four different domains: (i) a hydrophobic signal peptide at the amino terminus (1 to 18), (ii) a predominantly hydrophilic external domain (19 to 643), (iii) a hydrophobic transmembrane anchor (644 to 673), and (iv) a small hydrophilic cytoplasmic tail at the carboxy terminus (674 to 681). Amino acid analysis indicated that the signal peptide is removed from the mature GP. The GP therefore has the structural features of a type I transmembrane glycoprotein. The external domain of the protein has 19 N-glycosylation sites and several clusters of hydroxyamino acids and proline residues that are likely to be the attachment sites for about 30 O-glycosidic carbohydrate chains. The region extending from positions 585 to 610 shows significant homology to a domain observed in the envelope proteins of several retroviruses and Ebola virus that has been suspected to be responsible for immunosuppressive properties of these viruses. A second open reading frame of gene 4 has the coding capacity for an unidentified polypeptide 112 amino acids long.

Marburg virus, a filovirus: messenger RNAs, gene order, and regulatory elements of the replication cycle

The genome of Marburg virus (MBG), a filovirus, is 19.1 kb in length and thus the largest one found with negative-strand RNA viruses. The gene order - 3' untranslated region-NP-VP35-VP40-GP-VP30-VP24-L-5' untranslated region-resembles that of other non-segmented negative-strand (NNS) RNA viruses. Six species of polyadenylated subgenomic RNAs, isolated from MBG-infected cells, are complementary to the negative-strand RNA genome. They can be translated in vitro into the known structural proteins NP, GP (non-glycosylated form), VP40, VP35, VP30 and VP24. At the gene boundaries conserved transcriptional start (3'-NNCUNCNUNUAAUU-5') and stop signals (3'-UAAUUCUUUUU-5') are located containing the highly conserved pentamer 3'-UAAUU-5'. Comparison with other NNS RNA viruses shows conservation primarily in the termination signals, whereas the start signals are more variable. The intergenic regions vary in length and nucleotide composition. All genes have relatively long 3' and 5' end non-coding regions. The putative 3' and 5' leader RNA sequences of the MBG genome resemble those of other NNS RNA viruses in length, conservation at the 3' and 5' ends, and in being complementary at their extremities. The data support the concept of a common taxonomic order Mononegavirales comprising the Filoviridae, Paramyxoviridae, and Rhabdoviridae families.

The nucleotide sequence of the L gene of Marburg virus, a filovirus: homologies with paramyxoviruses and rhabdoviruses

The nucleotide sequence of the L gene of Marburg virus, strain Musoke, has been determined. The L gene has a single long open reading frame encoding a polypeptide of 2330 amino acids (MW 267,175) that represents the viral RNA-dependent RNA polymerase. The putative transcription start signal (3'CUACCUAUAAUU 5') and the termination signal (3' UAAUUCUUUUU 5') of the gene could be identified. Computer-assisted comparison of the L protein with L proteins of other nonsegmented negative-stranded RNA viruses (Paramyxoviridae: Sendai virus, Newcastle disease virus, human parainfluenza 3 virus, measles virus, human respiratory syncytial virus; Rhabdoviridae: vesicular stomatitis virus, rabies virus) revealed significant homologies primarily in the N-terminal half of the proteins. We have identified three common conserved boxes (A, B, and C) among filo-, paramyxo-, and rhabdovirus L proteins, which are probably involved in the polymerase function. The L proteins can be divided into an N-terminal half, which seems to accommodate the common enzymatic sites, and a C-terminal half carrying virus specific peculiarities. The data presented here suggest a common evolutionary history for all nonsegmented negative-stranded RNA viruses and show that filoviruses are more closely related to paramyxo- than to rhabdoviruses.

Direct detection of influenza virus antigen in nasopharyngeal specimens by direct enzyme immunoassay in comparison with quantitating virus shedding

We developed a direct enzyme immunoassay [EIA; Enzygnost Influenza A(Ag) and Enzygnost Influenza B(Ag)] for the direct detection of influenza A and B virus antigens in nasopharyngeal secretion specimens (NPS). The test is performed without sonification of specimens, and results are obtained within 4 h. A direct comparison between direct EIA and quantitation of virus shedding for influenza A and B virus antigen detection was carried out. A total of 210 NPS and 98 nasopharyngeal wash specimens (NPW) were investigated. We isolated influenza A viruses from 79 (37.6%) of 210 NPS; of these 79 cell-culture-positive NPS, 70 (88.6%) were also positive by direct EIA. Of 29 (13.8%) NPS from which influenza B virus was isolated, 24 (82.8%) NPS were positive by direct EIA. Virus shedding was determined quantitatively in 48 NPS from patients with influenza A and in 24 NPS from patients with influenza B. Only a crude correlation between optical density values and virus concentrations was observed. Detection of influenza virus antigens in NPS by direct EIA showed sensitivities of 89.7% for influenza A virus and 87.9% for influenza B virus and specificities of 99.3% for influenza A virus and 100% for influenza B virus. With direct EIA, all NPW were negative for influenza A virus, although virus was isolated from 21 (21.4%) NPW. Of 15 NPW from which influenza B virus was isolated, 7 showed positive results in direct EIA. In addition, direct EIA is suitable for detecting influenza A and B viruses in cell cultures before the appearance of any cytopathic effects and can be used as a cell culture confirmation test.

Structure and assembly of hemagglutinin mutants of fowl plague virus with impaired surface transport

Five temperature-sensitive mutants of influenza virus A/FPV/Rostock/34 (H7N1), ts206, ts293, ts478, ts482, and ts651, displaying correct hemagglutinin (HA) insertion into the apical plasma membrane of MDCK cells at the permissive temperature but defective transport to the cell surface at the restrictive temperature, have been investigated. Nucleotide sequence analysis of the HA gene of the mutants and their revertants demonstrated that with each mutant a single amino acid change is responsible for the transport block. The amino acid substitutions were compared with those of mutants ts1 and ts227, which have been analyzed previously (W. Schuy, C. Will, K. Kuroda, C. Scholtissek, W. Garten, and H.-D. Klenk, EMBO J. 5:2831-2836, 1986). With the exception of ts206, the changed amino acids of all mutants and revertants accumulate in three distinct areas of the three-dimensional HA model: (i) at the tip of the 80-A (8-nm)-long alpha helix, (ii) at the connection between the globular region and stem, and (iii) in the basal domain of the stem. The concept that these areas are critical for HA assembly and hence for transport is supported by the finding that the mutants that are unable to leave the endoplasmic reticulum at the nonpermissive temperature do not correctly trimerize. Upon analysis by density gradient centrifugation, cross-linking, and digestion with trypsin and endoglucosaminidase H, two groups can be discriminated among these mutants: with ts1, ts227, and ts478, the HA forms large irreversible aggregates, whereas with ts206 and ts293, it is retained in the monomeric form in the endoplasmic reticulum. With a third group, comprising mutants ts482 and ts651 that enter the Golgi apparatus, trimerization was not impaired.

Influenza virus ts61S hemagglutinin is significantly defective in polypeptide folding and intracellular transport at the permissive temperature

The influenza virus hemagglutinin (HA) temperature-sensitive (ts) mutant, ts61S, contains a nucleotide change in RNA segment 4 which leads to an amino acid change at HA1 residue 110 of serine to proline. When ts61S HA is synthesized and maintained at the nonpermissive temperature (39.5 degrees), the HA is defective in transport in the exocytic pathway and is retained in the endoplasmic reticulum (S. Nakajima, D. J. Brown, M., Ueda, K., Nakajima, A. Suguira, A. K. Pattnaik, and D. P. Nayak, 1986, Virology 154, 279-285). In a comparison of the biochemical properties of ts61S HA and A/WSN/33 HA (wt) expressed at the permissive temperature (33 degrees), we have found that ts61S HA is extensively debilitated. A large proportion of ts61S HA fails to gain reactivity with conformation-specific monoclonal antibodies and does not become resistant to protease digestion. In turn, a large population of the molecules are not transported from the ER to the Golgi apparatus or cell surface with the same kinetics or efficiency as wt HA. These data suggest that the serine to proline change at HA1 residue 110 leads to partial impairment of folding at the permissive temperature with complete impairment at the nonpermissive temperature.

Retarded processing of influenza virus hemagglutinin in insect cells

When expressed in Spodoptera frugiperda cells by a baculovirus vector, the hemagglutinin of fowl plague virus has been found to contain palmitic acid in covalent hydroxylamine-sensitive linkage, indicating that these cells have the capacity to acylate foreign proteins at cysteine residues. Centrifugation on sucrose density gradients and immune precipitation with conformation-specific antibodies were used to compare trimerization of the hemagglutinin in insect cells and in fowl plague virus-infected MDCK cells. Trimerization of the hemagglutinin was incomplete in insect cells, and the kinetics of this reaction were about three times slower than in vertebrate cells. Similarly, post-translational proteolytic cleavage occurred in insect cells with a half-time of 90 min, and a substantial fraction of the hemagglutinin persisted in uncleaved form. In contrast, hemagglutinin was almost completely cleaved in MDCK cells, and the half-time of cleavage was only 30 min. The data indicate that in insect cells trimerization and, as a result, the subsequent processing steps of the hemagglutinin, are retarded and less efficient. The possible roles of aberrant glycosylation, acidic milieu, and lack of other influenza virus proteins in hemagglutinin trimerization are discussed.

The oligosaccharides of influenza virus hemagglutinin expressed in insect cells by a baculovirus vector

The hemagglutinin of fowl plague virus has been expressed in Spodoptera frugiperda (SF) cell cultures using a baculovirus vector. To elucidate the structure of the carbohydrate side chains, radioactively labeled oligosaccharides were liberated by treatment with endoglucosaminidase H and glycopeptidase F. Sequential degradation with exoglycosidases and chromatographic analyses revealed the presence of oligomannosidic side chains, predominantly of the structures Man5-9GlcNAc2, and the truncated oligosaccharide cores Man3GlcNAc2 and Man3[Fuc]GlcNAc2. Polyacrylamide gel electrophoresis of endoglycosidase-treated hemagglutinin showed that most side chains of the HA1 subunit are truncated, whereas the HA2 subunit has one oligomannosidic and one truncated oligosaccharide. Comparison of these results with the glycosylation pattern of hemagglutinin obtained from vertebrate cells allowed a tentative allocation of the oligosaccharides to individual glycosylation sites. The results indicate that SF cells have the capacity to trim N-glycans to trimannosyl cores and to further process these by the addition of fucose. Thus, the complex oligosaccharides found on hemagglutinin from vertebrate hosts are replaced on hemagglutinin derived from insect cells by small truncated side chains.

Topology of membrane insertion in vitro and plasma membrane assembly in vivo of the yeast arginine permease

We have examined the topology of the yeast arginine permease, a plasma-membrane protein with multiple membrane-spanning domains. Using fusions of the permease with the glycosylatable secreted yeast protein, acid phosphatase, we identified membrane-spanning sequences that can translocate adjacent acid phosphatase across the membrane of the endoplasmic reticulum (ER), as measured by in vitro glycosylation. Examination for the presence or absence of glycosylation in a systematic series of such fusions gave an internally consistent model for the lumenal or cytoplasmic disposition of the acid phosphatase reporter, defining the topology of the permease. The phenotypes of a further set of arginine permease gene fusions with portions of the gene for the secreted protein, killer toxin, suggest that the pathways of export of membrane and secreted proteins need not be functionally distinct.

The structure of serotype H10 hemagglutinin of influenza A virus: comparison of an apathogenic avian and a mammalian strain pathogenic for mink

The primary structure of the hemagglutinin of the apathogenic avian influenza virus A/chick/Germany/N/49 (H10N7) and of the serologically related strain A/mink/Sweden/84 (H10N4) pathogenic for mink has been elucidated by nucleotide sequence analysis, and the carbohydrates attached to the polypeptide have been determined. The H10 hemagglutinin has 65, 52, 46, 45, and 44% amino acid sequence homology with serotypes H7, H3, H1, H2, and H5, respectively. H10 and H7 hemagglutinins are also most closely related in their glycosylation patterns. There is a high sequence homology between both H10 strains supporting the concept that the mink virus has obtained its hemagglutinin from an avian strain. The sequence homology includes the cleavage site which consists of a single arginine as is the case with most other hemagglutinins exhibiting low susceptibility to proteolytic activation. The similarity in hemagglutinin structure between both H10 strains is discussed in light of the distinct differences in the pathogenicity of both viruses.

Lectins as probes for the assay of rhabdovirus infections in plants

Thirteen different, biotinylated plant lectins were tested for their ability to recognize specifically the glycoproteins of the two different plant rhabdoviruses potato yellow dwarf virus and eggplant mottled dwarf virus. All viruses were propagated on the same plant host species, Nicotiana rustica L. The lectin-binding to the viral proteins was tested after electrophoretic separation and transfer to nitrocellulose membranes. Besides purified virus also partially pure virus preparations were used for the tests, in order to determine the specificity. The lectins had been selected for specificities to either one of the following monosaccharides: mannose, glucose, galactose, N-acetyl-D-galactosamine, N-acetyl-D-glucosamine and fucose. In the test panel of thirteen lectins, seven were found to react with the viral glycoproteins. Among these, four (LCA, VFA, PSA, Con A) belonged to the mannosyl- or glycosyl-specific group. However, these four lectins reacted also with other host proteins when partially pure virus preparations were used as samples. The other three lectins (GSA2b, STA, WGA) were specific for N-acetyl-D-glucosamine and detected almost exclusively the viral glycoproteins. Two of these lectins, STA and WGA, were extremely suitable for virus-specific assays, since they did not react with glycoproteins in healthy controls that were identical or comparable in their electrophoretic mobility with the rhabdovirus glycoproteins. No binding to viral glycoproteins was observed with galactose-, N-acetyl-galactosamine- and fucose-specific lectins. The assay for rhabdovirus glycoproteins in plants with the lectins was approximately 8-16 times less sensitive than with virus-specific antibodies.

Variations in glycosylation of the influenza virus hemagglutinin of subtype H7. Brief report

The numbers of the carbohydrate side chains have been analyzed on a series of subtype 7 hemagglutinins of influenza A virus. The procedure employed involves growth of the virus in the presence of the trimming inhibitor N-methyl-1-desoxynojirimycin followed by controlled removal of the resulting mannose-rich oligosaccharides with endo-N-acetylglucosaminidase H. By comparative analysis with virus grown in the absence of the inhibitor oligomannosidic side chains can be discriminated from complex ones. The results show that all H7 hemagglutinins have a complex and an oligomannosidic side chain on HA2, whereas the oligosaccharides of HA1 vary in number.