Transient expression and sequence of the matrix (M1) gene of WSN influenza A virus in a vaccinia vector

Filamentous versus Spherical Morphology: A Case Study of the Recombinant A/WSN/33 (H1N1) Virus

Influenza A virus is a serious human pathogen that assembles enveloped virions on the plasma membrane of the host cell. The pleiomorphic morphology of influenza A virus, represented by spherical, elongated, or filamentous particles, is important for the spread of the virus in nature. Using fixative protocols for sample preparation and negative staining electron microscopy, we found that the recombinant A/WSN/33 (H1N1) (rWSN) virus, a strain considered to be strictly spherical, may produce filamentous particles when amplified in the allantoic cavity of chicken embryos. In contrast, the laboratory WSN strain and the rWSN virus amplified in Madin-Darby canine kidney cells exhibited a spherical morphology. Next-generation sequencing (NGS) suggested a rare Ser126Cys substitution in the M1 protein of rWSN, which was confirmed by the mass spectrometric analysis. No structurally relevant substitutions were found by NGS in other proteins of rWSN. Bioinformatics algorithms predicted a neutral structural effect of the Ser126Cys mutation. The mrWSN_M1_126S virus generated after the introduction of the reverse Cys126Ser substitution exhibited a similar host-dependent partially filamentous phenotype. We hypothesize that a shortage of some as-yet-undefined cellular components involved in virion budding and membrane scission may result in the appearance of filamentous particles in the case of usually "nonfilamentous" virus strains.

Association of influenza virus matrix protein with ribonucleoproteins

To characterize the sites and nature of binding of influenza A virus matrix protein (M1) to ribonucleoprotein (RNP), M1 of A/WSN/33 was altered by deletion or site-directed mutagenesis, expressed in vitro, and allowed to attach to RNP under a variety of conditions. Approximately 70% of the wild-type (Wt) M1 bound to RNP at pH 7.0, but less than 5% of M1 associated with RNP at pH 5.0. Increasing the concentration of NaCl reduced M1 binding, but even at a high salt concentration (0.6 M NaCl), approximately 20% of the input M1 was capable of binding to RNP. Mutations altering potential M1 RNA-binding regions (basic amino acids 101RKLKR105 and the zinc finger motif at amino acids 148 to 162) had varied effect: mutations of amino acids 101 to 105 reduced RNP binding compared to the Wt M1, but mutations of zinc finger motif did not. Treatment of RNP with RNase reduced M1 binding by approximately half, but even M1 mutants lacking RNA-binding regions had residual binding to RNase-treated RNP provided that the N-terminal 76 amino acids of M1 (containing two hydrophobic domains) were intact. Addition of detergent to the reaction mixture further reduced binding related to the N-terminal 76 amino acids and showed the greatest effect for mutations affecting the RNA-binding regions of basic amino acids. The data suggest that M1 interacts with both the RNA and protein components of RNP in assembly and disassembly of influenza A viruses.

Nucleus-targeting domain of the matrix protein (M1) of influenza virus

The matrix protein M1 of influenza virus A/WSN/33 was shown by immunofluorescent staining to be transported into the nuclei of transfected cells without requiring other viral proteins. We postulated the existence of a potential signal sequence at amino acids 101 to 105 (RKLKR) that is required for nuclear localization of the M1 protein. When CV1 cells were transfected with recombinant vectors expressing the entire M1 protein (amino acids 1 to 252) or just the first 112 N-terminal amino acids, both the complete M1 protein and the truncated M1 protein were transported to the nucleus. In contrast, expression in CV1 cells of vectors coding for M1 proteins with deletions from amino acids 77 to 202 or amino acids 1 to 134 resulted only in cytoplasmic immunofluorescent staining of these truncated M1 proteins without protein being transported to the nucleus. Moreover, no nuclear membrane translocation occurred when CV1 cells were transfected with recombinant vectors expressing M1 proteins with deletions of amino acids 101 to 105 or with substitution at amino acids 101 to 105 of SNLNS for RKLKR. Furthermore, a synthetic oligopeptide corresponding to M1 protein amino acids 90 to 108 was also transported into isolated nuclei derived from CV1 cells, whereas oligopeptides corresponding to amino acid sequences 25 to 40, 67 to 81, and 135 to 164 were not transported into the isolated cell nuclei. These data suggest that the amino acid sequence 101RKLKR105 is the nuclear localization signal of the M1 protein.

Specific changes in the M1 protein during adaptation of influenza virus to mouse

The matrix (M) gene of influenza virus has been implicated as a determinant of virulence for mouse brain and lung. Comparison of the M gene sequences of the mouse brain adapted variants A/NWS/33 and A/WSN/33 to their parent, A/WS/33, identified two specific amino acid substitutions in the M1 protein which correlated with virulence for mouse: Ala41-->Val and Thr139-->Ala.

Detection and identification of human influenza viruses by the polymerase chain reaction

A series of oligonucleotide primers are described which hybridize to conserved regions of influenza virus cDNA and prime DNA synthesis in Taq polymerase catalyzed amplification reactions (PCR). Primers were designed to hybridize as nested pairs and, following a two-step amplification, produce uniquely sized DNA fragments diagnostic for viral type and subtype. Influenza A and B matrix-protein genes and the influenza C haemagglutinin gene were targets for the type-specific primers. Subtype-specific primers targeted conserved sequences within the three haemagglutinin or two neuraminidase subtypes of different human influenza isolates. The utility of this method was demonstrated using computer search methods and by accurately amplifying DNA from a variety of influenza A, B, and C strains. Type-specific primer sets showed a broad type specificity and amplified DNA from viral strains of unknown sequence. Restriction mapping and DNA sequencing showed that fragments amplified in this manner derived from the input template, confirming the accuracy of the method and demonstrating how PCR can be used to quickly derive sufficient sequence information for analysis of viral relatedness. Subtyping primers were able to distinguish accurately between the three haemagglutinin (H1, H2, H3) and two neuraminidase (N1, N2) alleles of human influenza A isolates. Again DNA was amplified from viruses of unknown sequence confirming that most of these primer sets may prove useful as broad range subtyping reagents. In order to simplify the work associated with analysis of many samples, we have also devised a rapid method for the isolation of viral RNA and synthesis of cDNA. Using this 'mini-prep' technique, it is possible to detect, amplify, and identify picogram quantities of influenza virus in a single day, confirming that PCR provides a useful alternative to existing methods of influenza detection.

Vaccinia virus vectors: new strategies for producing recombinant vaccines

The development and continued refinement of techniques for the efficient insertion and expression of heterologous DNA sequences from within the genomic context of infectious vaccinia virus recombinants are among the most promising current approaches towards effective immunoprophylaxis against a variety of protozoan, viral, and bacterial human pathogens. Because of its medical relevance, this area is the subject of intense research interest and has evolved rapidly during the past several years. This review (i) provides an updated overview of the technology that exists for assembling recombinant vaccinia virus strains, (ii) discusses the advantages and disadvantages of these approaches, (iii) outlines the areas of outgoing research directed towards overcoming the limitations of current techniques, and (iv) provides some insight (i.e., speculation) about probable future refinements in the use of vaccinia virus as a vector.

The phosphorylation of the integral membrane (M1) protein of influenza virus

The phosphorylation of the internal and integral membrane (M1) protein of influenza virus was studied. Four points can be made based on the data: (1) The M1 contains at least two moles of phosphate per mole of M1. (2) Phosphorylation of M1 is conserved between influenza A, B and C viruses. Other characteristics of the M1 are also conserved, such as solubility in organic solvent, heterogeneity and ability to partition into lipid vesicles. (3) M1 is phosphorylated in cells infected with a vaccinia recombinant (vP273) containing only the gene of M1, either as a result of a vaccinia virus associated kinase or a cellular one. (4) The phosphate is located within or in close proximity to the major stretch of neutral and hydrophobic amino acids found in M1, as determined by analyzing cyanogen bromide fragments.

M protein (M1) of influenza virus: antigenic analysis and intracellular localization with monoclonal antibodies

A panel of 16 monoclonal antibodies recognizing M protein (M1) of influenza virus was generated. Competition analyses resulted in localization of 14 monoclonal antibodies to three antigenic sites. Three monoclonal antibodies localized to site 1B recognized a peptide synthesized to M1 (residues 220 to 236) with enzyme-linked immunosorbent assay titers equivalent to or greater than that seen with purified M1; therefore, site 1B is located near the C terminus of M1. Sites 2 and 3 localize to the N-terminal half of M1. Antigenic variation of M proteins was seen when the monoclonal antibodies were tested against 14 strains of type A influenza viruses. Several monoclonal antibodies showed specific recognition of A/PR/8/34 and A/USSR/90/77 M proteins and little or no reactivity for all other strains tested. Immunofluorescence analysis with the monoclonal antibodies showed migration of M protein to the nucleus during the replicative cycle and demonstrated association of M protein with actin filaments in the cytoplasm. Use of a vaccinia virus recombinant containing the M-protein gene demonstrated migration of M protein to the nucleus in the absence of synthesis of gene products from other influenza virus RNA segments.

Epitope mapping by deletion mutants and chimeras of two vesicular stomatitis virus glycoprotein genes expressed by a vaccinia virus vector

Deletion mutants and chimeras of the glycoprotein (G) genes of vesicular stomatitis virus serotypes Indiana (VSV-Ind) and New Jersey (VSV-NJ) were cloned in plasmids and vaccinia virus vectors under control of the bacteriophage T7 polymerase promoter for expression in CV-1 cells co-infected with a T7 polymerase-expressing vaccinia virus recombinant. Truncated and chimeric G proteins expressed by these vectors were tested for their capacity to react with VSV-Ind and VSV-NJ epitope-specific monoclonal antibodies (MAbs) by Western blot analysis for those antigenic determinants not affected by disulfide-bond reducing conditions or by immuno dotblot analysis for those that are. These experiments allowed us to create putative epitope maps for glycoproteins of both serotypes based on binding affinity and cross-reactivity of VSV-Ind and VSV-NJ MAbs for truncated or chimeric G proteins of known amino acid sequences. Seven of the 9 VSV-NJ G epitopes, including all 4 epitopes involved in virus neutralization by MAbs, mapped to the center (amino acid sequence 193-289) of the 517 amino acid VSV-NJ G protein. Four of the 11 VSV-Ind G epitopes, including 2 neutralizable epitopes, mapped to the cysteine-rich amino-terminal domain (amino acid sequence 80-183) of the 511 amino acid VSV-Ind G protein; the remaining 7 VSV-Ind G epitopes, including 2 involved in virus neutralization, were clustered in the cysteine-poor carboxy-terminal domain (amino acid sequence 286-428). In site-specific mutants of the VSV-Ind G gene defective in one or both glycosylation sites, only the amino-terminal epitopes of the VSV-Ind G protein were affected by deletion of the carbohydrate chain at residue 179; deletion of the carbohydrate chain at residue 336 did not alter reactivity of the G protein with any of the relevant monoclonal antibodies. These results are discussed in relation to earlier attempts to map the antigenic determinants of VSV-NJ and VSV-Ind G proteins by proteolysis of the G protein and by sequencing the G genes of mutant viruses selected for their resistance to neutralization by epitope-specific monoclonal antibodies.

Effects of cell differentiation on replication of A/WS/33, WSN, and A/PR/8/34 influenza viruses in mouse brain cell cultures: biological and immunological characterization of products

The responses of mouse embryo brain (MEB) cell cultures and of Madin-Darby canine kidney cells and chicken embryo fibroblasts to infection with A/PR/8/34 (PR8), A/WS/33 (WS), or the neurovirulent WSN variant were compared in terms of (i) single-cycle yields of hemagglutinating and associated neuraminidase (NA) activities and plaque-forming particles, the latter with or without trypsin activation [PFU(TR++) or PFU(TR--), respectively], and (ii) expression of nucleoprotein (NP), M1, and NS1 protein, determined for specific cell types by immunostaining, for whole culture lysates by Western blot analysis of NP and M1. Primary MEB cultures grown in serum-enriched medium were infected after 6 days (young), when none of the cells reacted specifically and exclusively with any of the nerve cell marker antibodies used, or after greater than or equal to 21 days (aged), when astrocytes (the predominant cell type), neurons, and oligodendrocytes were morphologically and immunologically mature. Secondary astrocyte-enriched cultures were used when they contained 90 to 99% of their cells as astrocytes at an early stage of differentiation. By all criteria, young MEB cultures were only marginally less permissive for each of the three viruses than were chicken embryo fibroblasts or Madin-Darby canine kidney cells. Aged MEB cultures, by comparison, produced undiminished NP, hemagglutinin, and neuraminidase, but yields of PFU(TR++) and expression of M1 protein (relative to NP) were reduced for all three viruses, most for PR8 and least for WSN; relative reduction of NS1 protein was demonstrable only in PR8-infected aged cultures. Immunostaining revealed low levels of M1 and NS1 expression only in astrocytes, not in oligodendrocytes and neurons. In PR8-infected mature astrocytes, NP accumulated in the nucleus; it persisted in some cells for at least 8 weeks after infection. The presence of NP did not seem to interfere with cell division. Secondary MEB cultures containing 90 to 99% immature astrocytes were less restricted than were aged primary cultures. Thus, it appears that reduced permissivity of nerve cell cultures, as measured in this study, is most closely correlated with advancing differentiation and maturity of astroglial cells. Assembled virions, including those that score as PFU(TR++) in restricted cultures (e.g., PR8-infected aged MEB), may be mainly products of mature oligodendrocytes and neurons.

Growth restriction of influenza A virus by M2 protein antibody is genetically linked to the M1 protein

The M2 protein of influenza A virus is a 97-amino acid integral membrane protein expressed at the surface of infected cells. Recent studies have shown that a monoclonal antibody (14C2) recognizes the N terminus of M2 and restricts the replication of certain influenza A viruses. To investigate the mechanism of M2 antibody growth restriction, 14C2 antibody-resistant variants of strain A/Udorn/72 have been isolated. Most of the variant viruses are not conventional antigenic variants as their M2 protein is still recognized by the 14C2 antibody. A genetic analysis of reassortant influenza viruses prepared from the 14C2 antibody-resistant variants and an antibody-sensitive parent virus indicates that M2 antibody growth restriction is linked to RNA segment 7, which encodes both the membrane protein (M1) and the M2 integral membrane protein. Nucleotide sequence analysis of RNA segment 7 from the variant viruses predicts single amino acid substitutions in the cytoplasmic domain of M2 at positions 71 and 78 or at the N terminus of the M1 protein at residues 31 and 41. To further examine the genetic basis for sensitivity and resistance to the 14C2 antibody, the nucleotide sequences of RNA segment 7 of several natural isolates of influenza virus have been obtained. Differences in the M1 and M2 amino acid sequences for some of the naturally resistant strains correlate with those found for the M2 antibody variant viruses. The possible interaction of M1 and M2 in virion assembly is discussed.