Yuko Sakai

Avian flu: isolation of drug-resistant H5N1 virus.

The persistence of H5N1 avian influenza viruses in many Asian countries and their ability to cause fatal infections in humans have raised serious concerns about a global flu pandemic. Here we report the isolation of an H5N1 virus from a Vietnamese girl that is resistant to the drug oseltamivir, which is an inhibitor of the viral enzyme neuraminidase and is currently used for protection against and treatment of influenza. Further investigation is necessary to determine the prevalence of oseltamivir-resistant H5N1 viruses among patients treated with this drug.

The paramyxovirus, Sendai virus, V protein encodes a luxury function required for viral pathogenesis

The Sendai virus (SeV) V protein is characterized by the unique cysteine‐rich domain in its carboxy‐terminal half which is fused to the amino‐terminal half of the P protein, but its function has remained enigmatic. The V protein‐directing mRNA is generated by a remarkable process known as mRNA editing involving the pseudotemplated addition of a single G residue at a specific septinucleotide locus in the P gene, whereas the unedited exact copy encodes the P protein. Here, we introduced two nucleotide changes in the septinucleotide motif (UUUUCCC to UUCUUCC) in a full‐length SeV cDNA and were able to recover a virus from the cDNA, which was devoid of mRNA editing and hence unable to synthesize the V protein. Compared with the parental wild‐type virus with regard to gene expression, replication and cytopathogenicity in various cell lines in vitro, the V(−) virus was found to be either potentiated or comparable but never attenuated. The V(−) virus, however, showed markedly attenuated in vivo replication capacity in and pathogenicity for mice. Thus, though categorized as a non‐essential gene product, SeV V protein encodes a luxury function required for in vivo pathogenicity.

Sendai virus C proteins are categorically nonessential gene products but silencing their expression severely impairs viral replication and pathogenesis

The P/C mRNA of Sendai virus (SeV), a prototypic member of the family Paramyxoviridae in the Mononegavirales superfamily comprising a large number of nonsegmented negative strand RNA viruses, encodes a nested set of accessory proteins, C′, C, Y1 and Y2, referred to collectively as C proteins, initiating, respectively, at ACG/81 and AUGs/114, 183, 201 in the +1 frame relative to the ORF of phospho (P) protein, the smaller subunit of RNA polymerase. Among them, C is the major species expressed in infected cells at a molar ratio which is several‐fold higher than the other three. However, their function has remained an enigma. It has not even been established whether or not the C proteins are essential for viral replication. Many other viruses in Mononegavirales encode C‐like proteins, but their roles also remain to be defined.

Accommodation of foreign genes into the Sendai virus genome: sizes of inserted genes and viral replication

Sendai virus (SeV) is an enveloped virus with a negative sense genome RNA of about 15.3 kb. We previously established a system to recover an infectious virus entirely from SeV cDNA and illustrated the feasibility of using SeV as a novel expression vector. Here, we have attempted to insert a series of foreign genes into SeV of different lengths to learn how far SeV can accommodate extra genes and how the length of inserted genes affects viral replication in cells cultured in vitro and in the natural host, mice. We show that a gene up to 3.2 kb can be inserted and efficiently expressed and that the replication speed as well as the final virus titers in cell culture are proportionally reduced as the inserted gene length increases. In vivo, such a size‐dependent effect was not very clear but a remarkably attenuated replication and pathogenicity were generally seen. Our data further confirmed reinforcement of foreign gene expression in vitro from the V(−) version of SeV in which the accessory V gene had been knocked out. Based on these results, we discuss the utility of SeV vector in terms of both efficiency and safety.

Versatility of the Accessory C Proteins of Sendai Virus: Contribution to Virus Assembly as an Additional Role

ABSTRACT The P/C mRNA of Sendai virus (SeV) encodes a nested set of accessory proteins, C′, C, Y1, and Y2, referred to collectively as C proteins, using the +1 frame relative to the open reading frame of phospho (P) protein and initiation codons at different positions. The C proteins appear to be basically nonstructural proteins as they are found abundantly in infected cells but greatly underrepresented in the virions. We previously created a 4C(−) SeV, which expresses none of the four C proteins, and concluded that the C proteins are categorically nonessential gene products but greatly contribute to viral full replication and infectivity (A. Kurotani et al., Genes Cells 3:111–124, 1998). Here, we further characterized the 4C(−) virus multiplication in cultured cells. The viral protein and mRNA synthesis was enhanced with the mutant virus relative to the parental wild-type (WT) SeV. However, the viral yields were greatly reduced. In addition, the 4C(−) virions appeared to be highly anomalous in size, shape, and sedimentation profile in a sucrose gradient and exhibited the ratios of infectivity to hemagglutination units significantly lower than those of the WT. In the WT infected cells, C proteins appeared to colocalize almost perfectly with the matrix (M) proteins, pretty well with an external envelope glycoprotein (hemagglutinin-neuraminidase [HN]), and very poorly with the internal P protein. In the absence of C proteins, there was a significant delay of the incorporation of M protein and both of the envelope proteins, HN and fusion (F) proteins, into progeny virions. These results strongly suggest that the accessory and basically nonstructural C proteins are critically required in the SeV assembly process. This role of C proteins was further found to be independent of their recently discovered function to counteract the antiviral action of interferon-α/β. SeV C proteins thus appear to be quite versatile.

Naturally Occurring Deletional Mutation in the C-Terminal Cytoplasmic Tail of CCR5 Affects Surface Trafficking of CCR5

ABSTRACT CCR5 is an essential coreceptor for the cellular entry of R5 strains of human immunodeficiency virus type 1 (HIV-1). CCR5-893(−) is a single-nucleotide deletion mutation which is observed exclusively in Asians (M. A. Ansari-Lari, et al., Nat. Genet. 16:221–222, 1997). This mutant gene produces a CCR5 which lacks the entire C-terminal cytoplasmic tail. To assess the effect of CCR5-893(−) on HIV-1 infection, we generated a recombinant Sendai virus expressing the mutant CCR5 and compared its HIV-1 coreceptor activity with that of wild-type CCR5. Although the mutant CCR5 has intact extracellular domains, its coreceptor activity was much less than that of wild-type CCR5. Flow cytometric analyses and confocal microscopic observation of cells expressing the mutant CCR5 revealed that surface CCR5 levels were greatly reduced in these cells, while cytoplasmic CCR5 levels of the mutant CCR5 were comparable to that of the wild type. Peripheral blood CD4+ T cells obtained from individuals heterozygous for this allele expressed very low levels of CCR5. These data suggest that the CCR5-893(−) mutation affects intracellular transport of CCR5 and raise the possibility that this mutation also affects HIV-1 transmission and disease progression.

Interaction between secretogranin III and carboxypeptidase E facilitates prohormone sorting within secretory granules

Secretogranin III (SgIII) and carboxypeptidase E (CPE) bind specifically to cholesterol-rich secretory granule (SG) membranes. We previously showed that SgIII binds chromogranin A (CgA) and targets CgA to the SGs in endocrine cells. We investigated the binding of SgIII and CPE because they frequently localize close to the periphery of SGs, and they bind each other in mouse corticotrope-derived AtT-20 cells. In Cpefat mouse corticotropes, which have defective CPE, proopiomelanocortin (POMC)-derived adrenocorticotrophin hormone (ACTH)-containing peptides were distributed over the entire surface of the SGs, and displayed a regulated secretion by secretagogues. The Cpefat pituitary exhibited elevated levels of SgIII and CgA, which suggests that they compensate for a sorting function of CPE for POMC and its intermediates to ACTH. Indeed, both SgIII and CgA were able to bind POMC-derived intermediates. In a competitive pull-down assay, excessive SgIII led to a decrease in CPE-bound POMC-derived intermediate molecules, and SgIII pulled-down by anti-ACTH antibody increased proportionately. We suggest that SgIII and CPE form the separate functional sorting complex by anchoring to cholesterol-rich SG membranes, and POMC-derived peptides are transferred from CPE to SgIII, and subsequently to CgA.

Sendai virus‐based expression of HIV‐1 gp120: reinforcement by the V(−) version

We have established a system for recovering Sendai virus (SeV), a nonsegmented negative strand RNA virus, entirely from cDNA at an extremely high rate, and have succeeded in creating a V(−) SeV whose gene expression was greatly enhanced by the deletion of the nonessential V gene. Because of its extreme medical importance, there has been a strong need for the establishment of a better system to express the gp120 envelope glycoprotein of the human immunodeficiency virus type 1 (HIV‐1) in sufficient quantity and purity. It also remains to be established to produce gp120 in in vitro natural host cells for HIV‐1 such as human primary blood mononuclear cells, macrophages or established T cell lines.

Importance of the N-glycan in the V3 loop of HIV-1 envelope protein for CXCR-4- but not CCR-5-dependent fusion

The V3 region of HIV‐1 envelope protein possesses a single N‐linked sugar chain, which is conserved in most HIV‐1 strains. We studied its role in the life cycle of HIV‐1 strains with different co‐receptor usage. Removal of the glycan appeared to cause a marked reduction of CXCR‐4‐ but not CCR‐5‐dependent virus entry. A basic amino acid substitution at the 11th position of V3 markedly compensated for the removal of the N‐glycan. These results indicate that the N‐glycan plays an important role for CXCR‐4‐dependent virus entry and that this role is exerted in a particular context of the peptide backbone.

A Subset of p23 Localized on Secretory Granules in Pancreatic β-cells

Proteins on the membrane of secretory granules (SGs) involved in their biogenesis and exocytosis are poorly characterized compared with those of synaptic vesicle in neurons. Thus the secretory granule membrane was prepared from a mouse pancreatic β-cell line MIN6 by subcellular fractionation, and protein constituents were analyzed by microscale two-dimensional liquid chromatography coupled with electrospray ionization tandem mass spectrometry. Using this proteomics approach, one of the p24 family proteins, p23, was unexpectedly found in the granule fraction, although p24 proteins are generally regarded as functioning in the early secretory pathways between the endoplasmic reticulum and the Golgi apparatus. We further showed that p23 is expressed at high levels in endocrine cells. Furthermore, immunocytochemical analyses of pancreatic β-cells at the light and electron microscopic levels demonstrated that a significant amount of p23 is localized on the insulin granule membrane, although it is most intensely concentrated at the cis-Golgi compartment as previously shown in non-endocrine cells. These findings suggest that a fraction of p23 enters post-Golgi compartments and may function in the biogenesis and/or quality control of SGs.