Intracellular processing, glycosylation, and cell-surface expression of the measles virus fusion protein (F) encoded by a recombinant adenovirus

Strategies for enhancing intratumoral spread of oncolytic adenoviruses

Oncolytic viruses, effectively replicate viruses within malignant cells to lyse them without affecting normal ones, have recently shown great promise in developing therapeutic options for cancer. Adenoviruses (Ads) are one of the candidates in oncolytic virotheraoy due to its easily manipulated genomic DNA and expression of wide rane of its receptors on the various cancers. Although systematic delivery of oncolytic adenoviruses can target both primary and metastatic tumors, there are some drawbacks in the effective systematic delivery of oncolytic adenoviruses, including pre-existing antibodies and liver tropism. To overcome these limitations, intratumural (IT) administration of oncolytic viruses have been proposed. However, IT injection of Ads leaves much of the tumor mass unaffected and Ads are not able to disperse more in the tumor microenvironment (TME). To this end, various strategies have been developed to enhance the IT spread of oncolytic adenoviruses, such as using extracellular matrix degradation enzymes, junction opening peptides, and fusogenic proteins. In the present paper, we reviewed different oncolytic adenoviruses, their application in the clinical trials, and strategies for enhancing their IT spread.

Adenoviral Vectors Armed with Cell Fusion-Inducing Proteins as Anti-Cancer Agents

Cancer is a devastating disease that affects millions of patients every year, and causes an enormous economic burden on the health care system and emotional burden on affected families. The first line of defense against solid tumors is usually extraction of the tumor, when possible, by surgical methods. In cases where solid tumors can not be safely removed, chemotherapy is often the first line of treatment. As metastatic cancers often become vigorously resistant to treatments, the development of novel, more potent and selective anti-cancer strategies is of great importance. Adenovirus (Ad) is the most commonly used virus in cancer clinical trials, however, regardless of the nature of the Ad-based therapeutic, complete responses to treatment remain rare. A number of pre-clinical studies have shown that, for all vector systems, viral spread throughout the tumor mass can be a major limiting factor for complete tumor elimination. By expressing exogenous cell-fusion proteins, many groups have shown improved spread of Ad-based vectors. This review summarizes the research done to examine the potency of Ad vectors expressing fusogenic proteins as anti-cancer therapeutics.

RT-PCR and sequence analysis of the full-length fusion protein of Canine Distemper Virus from domestic dogs

During 2007-2014, 84 out of 236 (35.6%) samples from domestic dogs submitted to our laboratory for diagnostic purposes were positive for Canine Distemper Virus (CDV), as analyzed by RT-PCR amplification of a fragment of the nucleoprotein gene. Fifty-nine of them (70.2%) were from dogs that had been vaccinated against CDV. The full-length gene encoding the Fusion (F) protein of fifteen isolates was sequenced and compared with that of those of other CDVs, including wild-type and vaccine strains. Phylogenetic analysis using the F gene full-length sequences grouped all the Argentinean CDV strains in the SA2 clade. Sequence identity with the Onderstepoort vaccine strain was 89.0-90.6%, and the highest divergence was found in the 135 amino acids corresponding to the F protein signal-peptide, Fsp (64.4-66.7% identity). In contrast, this region was highly conserved among the local strains (94.1-100% identity). One extra putative N-glycosylation site was identified in the F gene of CDV Argentinean strains with respect to the vaccine strain. The present report is the first to analyze full-length F protein sequences of CDV strains circulating in Argentina, and contributes to the knowledge of molecular epidemiology of CDV, which may help in understanding future disease outbreaks.

Extended protection against phlebovirus infection conferred by recombinant adenovirus expressing consensus interferon (DEF201)

Punta Toro virus (PTV; Bunyaviridae, Phlebovirus) is related to Rift Valley fever virus (RVFV), a pathogenic agent which causes severe disease in humans and livestock primarily in the sub-Saharan region of Africa. The recent range expansion of RVFV and the potential for its intentional release into naïve populations pose a significant threat to public health and agriculture. Studies modeling disease in rodents and nonhuman primates have shown that PTV and RVFV are highly sensitive to the antiviral effects of alpha interferon (IFN-α), an important component of the innate antiviral host response. While recombinant IFN-α has high therapeutic value, its utility for the treatment of neglected tropical diseases is hindered by its short in vivo half-life and costly production of longer-lasting pegylated IFNs. Here, we demonstrate extended preexposure protection against lethal PTV challenge following a single intranasal administration of DEF201, which is a replication-deficient human adenovirus type 5 vector engineered to constitutively express consensus IFN-α (cIFN-α) from transduced host cells. DEF201 was also efficacious when administered within 24 h as a postexposure countermeasure. Serum concentrations of cIFN-α could be detected as early as 8 h following treatment and persisted for more than 1 week. The prolonged antiphlebovirus prophylactic effect, low production costs, and ease of administration make DEF201 a promising agent for intervention during natural disease outbreaks and for countering possible bioterrorist acts.

The Asia 2 specific signal peptide region and other domains in fusion protein genes characterized Asia 1 and Asia 2 canine distemper viruses

Background

Although the presence of Asia 2 group of canine distemper virus (CDV) was known by the sequencing and phylogenetic analysis of hemagglutinin (H) gene, the fusion (F) protein gene sequence of Asia 2 group had not been identified. So, the sequence analysis of F gene was carried out to elucidate the genotypic varaitons among Asian isolates.

Results

The phylogenetic analysis of F and H gene sequences from fourteen CDV isolates obtained from diseased dogs in Japan and Thailand indicated that the F genes had a new initiation codon and extra 27 nucleotides upstream of the usual open reading frame (ORF) and the F proteins had extra 9 amino acids at the N-terminal position only in Asia 2 isolates. On the contrary, the Asia 1 isolates had three extra putative N-glycosylation sites (two sites in the signal peptide region and one site in the F1 region) except for two strains of Th12 and Ac96I (two sites in signal peptide region) adding to four putative N-glycosylation sites that were conserved among all Asian isolates and Onderstepoort strain. In addition to this difference in N-glycosylation sites, the signal peptide region had a great diversity between Asia 1 and Asia 2 isolates. Also, characteristic amino acids were detected for some strains.

Conclusion

Asia 2 isolates were distinguished from other CDV lineages by the extra 27 nucleotide sequence. The signal peptide region of F gene gives a remarkable differentiation between Asia 1 and Asia 2 isolates. Strains Th12 and Ac96I were differentiated from other Asia 1 strains by the F protein glycosylation sites.

Domain architecture and oligomerization properties of the paramyxovirus PIV 5 hemagglutinin-neuraminidase (HN) protein

The mechanism by which the paramyxovirus hemagglutinin-neuraminidase (HN) protein couples receptor binding to activation of virus entry remains to be fully understood, but the HN stalk is thought to play an important role in the process. We have characterized ectodomain constructs of the parainfluenza virus 5 HN to understand better the underlying architecture and oligomerization properties that may influence HN functions. The PIV 5 neuraminidase (NA) domain is monomeric whereas the ectodomain forms a well-defined tetramer. The HN stalk also forms tetramers and higher order oligomers with high alpha-helical content. Together, the data indicate that the globular NA domains form weak intersubunit interactions at the end of the HN stalk tetramer, while stabilizing the stalk and overall oligomeric state of the ectodomain. Electron microscopy of the HN ectodomain reveals flexible arrangements of the NA and stalk domains, which may be important for understanding how these two HN domains impact virus entry.

Effect of the alterations in the fusion protein of measles virus isolated from brains of patients with subacute sclerosing panencephalitis on syncytium formation

Measles virus (MV) is the causative agent of subacute sclerosing panencephalitis (SSPE) and viruses isolated from brains of the patients contain numerous mutations. We have previously demonstrated that the hemagglutinin (H) protein of MV SSPE strains can interact with the signaling lymphocyte activation molecule (SLAM) and an unidentified molecule on Vero cells, but not with CD46, as a receptor. The mechanism by which MV SSPE strains can induce cell-cell fusion in SLAM-negative Vero cells is not understood. We report here on the effect of mutations in the fusion (F) proteins of three MV SSPE strains on syncytium formation. The F proteins of the three SSPE strains were functional and co-expression with H protein from the MV wild-type or SSPE strains in this study induced formation of large syncytia in Vero cells as well as in cell lines expressing SLAM or CD46. Expression of chimeric F proteins of SSPE strains showed that amino acid substitutions in the F protein extracellular as well as cytoplasmic domain contributed to enhanced cell-cell fusion in Vero cells. These findings suggest a common molecular mechanism and a key role of the F protein for syncytium formation in cells expressing an unidentified third receptor for MV.

Interactions among measles virus hemagglutinin, fusion protein and cell receptor signaling lymphocyte activation molecule (SLAM) indicating a new fusion-trimer model

For measles viruses, fusion on the cell membrane is an important initial step in the entry into the infected cells. The recent research indicated that hemagglutinin firstly leads the conformational changes in the fusion protein then co-mediates the membrane fusion. In the work, we use the co-immunoprecipitation and pull-down techniques to identify the interactions among fusion protein, hemagglutinin and signaling lymphocyte activation molecule (SLAM), which reveal that the three proteins can form a functional complex to mediate the SLAM-dependent fusion. Moreover, under the confocal microscope, fusion protein and hemagglutinin protein can show the cocapping mediated by the SLAM. So fusion protein not only is involved in the fusion but also might directly interact with the SLAM to be a new fusion-trimer model, which might account for the infection mechanism of measles virus.

Mechanism of CD150 (SLAM) down regulation from the host cell surface by measles virus hemagglutinin protein

Measles virus has been reported to enter host cells via either of two cellular receptors, CD46 and CD150 (SLAM). CD46 is found on most cells of higher primates, while SLAM is expressed on activated B, T, and dendritic cells and is an important regulatory molecule of the immune system. Previous reports have shown that measles virus can down regulate expression of its two cellular receptors on the host cell surface during infection. In this study, the process of down regulation of SLAM by measles virus was investigated. We demonstrated that expression of the hemagglutinin (H) protein of measles virus was sufficient for down regulation. Our studies provided evidence that interactions between H and SLAM in the endoplasmic reticulum (ER) can promote the down regulation of SLAM but not CD46. In addition, we demonstrated that interactions between H and SLAM at the host cell surface can also contribute to SLAM down regulation. These results indicate that two mechanisms involving either intracellular interactions between H and SLAM in the ER or receptor-mediated binding to H at the surfaces of host cells can lead to the down regulation of SLAM during measles virus infection.

Role for CCR5Delta32 protein in resistance to R5, R5X4, and X4 human immunodeficiency virus type 1 in primary CD4+ cells

CCR5Delta32 is a loss-of-function mutation that abolishes cell surface expression of the human immunodeficiency virus (HIV) coreceptor CCR5 and provides genetic resistance to HIV infection and disease progression. Since CXCR4 and other HIV coreceptors also exist, we hypothesized that CCR5Delta32-mediated resistance may be due not only to the loss of CCR5 function but also to a gain-of-function mechanism, specifically the active inhibition of alternative coreceptors by the mutant CCR5Delta32 protein. Here we demonstrate that efficient expression of the CCR5Delta32 protein in primary CD4(+) cells by use of a recombinant adenovirus (Ad5/Delta32) was able to down-regulate surface expression of both wild-type CCR5 and CXCR4 and to confer broad resistance to R5, R5X4, and X4 HIV type 1 (HIV-1). This may be important clinically, since we found that CD4(+) cells purified from peripheral blood mononuclear cells of individuals who were homozygous for CCR5Delta32, which expressed the mutant protein endogenously, consistently expressed lower levels of CXCR4 and showed less susceptibility to X4 HIV-1 isolates than cells from individuals lacking the mutation. Moreover, CD4(+) cells from individuals who were homozygous for CCR5Delta32 expressed the mutant protein in five of five HIV-exposed, uninfected donors tested but not in either of two HIV-infected donors tested. The mechanism of inhibition may involve direct scavenging, since we were able to observe a direct interaction of CCR5 and CXCR4 with CCR5Delta32, both by genetic criteria using the yeast two-hybrid system and by biochemical criteria using the coimmunoprecipitation of heterodimers. Thus, these results suggest that at least two distinct mechanisms may account for genetic resistance to HIV conferred by CCR5Delta32: the loss of wild-type CCR5 surface expression and the generation of CCR5Delta32 protein, which functions as a scavenger of both CCR5 and CXCR4.

Inhibitory potential of Crotalus durissus terrificus venom on measles virus growth

This paper presents the antiviral activity found in a snake with Crotalus durissus terrificus venom (Cdt), studied by use of microplate inhibition assay, using measles virus (MV). Cdt at concentrations below 100 microg/ml showed no cytotoxicity for Vero cells. This study shows the optimal conditions for cell treatment and infection. Two factors that affect virus binding and infection efficiency were studied: the use of an adsorption step, where infection volume was varied; and the concentration of fetal bovine serum (FBS). The adsorption step, with or without FBS, increased the bound virus percentage, whereas it increased bound virus at equilibrium only in FBS-free until 2.5% FBS. In contrast, the addition of 10% FBS decreased the bound virus percentage. The inhibition of MV replication in Vero cells was observed when Cdt was added either before or during cell infection with virus. Its inhibitory concentration against MV replication was 0.1 until 100 microg/ml, respectively. The anti-MV effect of the Cdt was gradually decreased when it was added before or during infection, and little inhibition was observed when Cdt was added 1 h after infection, suggesting that the MV infection was inhibited at the time of the initial events such as at the moment of adsorption and penetration of the viral cycle. In conclusion, Cdt contains anti-MV effects that may be of potential clinical interest.

Experimental vaccines against measles in a world of changing epidemiology

Vaccination with the current live attenuated measles vaccine is one of the most successful and cost-effective medical interventions. However, as a result of persisting maternal antibodies and immaturity of the infant immune system, this vaccine is poorly immunogenic in children <9 months old. Immunity against the live vaccine is less robust than natural immunity and protection less durable. There may also be some concern about (vaccine) virus spread during the final stage of an eventual measles eradication program. Opinions may differ with respect to the potential threat that some of these concerns may be to the World Health Organisation goal of measles elimination, but there is a consensus that the development of new measles vaccines cannot wait. Candidate vaccines are based on viral or bacterial vectors expressing recombinant viral proteins, naked DNA, immune stimulating complexes or synthetic peptides mimicking neutralising epitopes. While some of these candidate vaccines have proven their efficacy in monkey studies, aerosol formulated live attenuated measles vaccine are evaluated in clinical trials.

Regulation of fusion activity by the cytoplasmic domain of a paramyxovirus F protein

SER virus is a member of the family Paramyxoviridae, genus Rubulavirus, which has been isolated from pigs. It is very closely related to SV5 virus serologically, in protein profile, and in nucleotide sequence. However, unlike SV5, SER induces minimal syncytium formation in infected CV-1 or BHK cells. Fluorescence transfer experiments between labeled erythrocytes and infected MDBK cells revealed that SER also induces hemifusion and pore formation with reduced efficiency. The virion polypeptide profiles of SER and SV5 are very similar, except that the SER F1 subunit shows an apparent molecular weight that is about 2 kDa higher than that of SV5. Comparison of the deduced amino acid sequences revealed the SER F (551 aa) to be longer than SV5 F (529 aa) by 22 residues in the cytoplasmic tail (CT) domain. The HN and M gene sequences of the viruses were found to be very similar. The SER F showed minimal fusion activity when coexpressed with either SV5 or SER HN. In contrast, SV5 F was highly fusogenic when coexpressed with either HN protein, indicating that the restricted fusion capacity of SER virus is a property of its F protein. Truncation in the CT of SER F by 22 residues completely rescued its ability to cause syncytium formation, whereas other truncations rescued syncytium formation partially. These results demonstrate that an elongated CT of a paramyxovirus F protein suppresses its membrane fusion activity.

Multiple glycosylated forms of the respiratory syncytial virus fusion protein are expressed in virus-infected cells

Analysis of the respiratory syncytial virus (RSV) fusion (F) protein in RSV-infected Vero cells showed the presence of a single F1 subunit and at least two different forms of the F2 subunit, designated F2a (21 kDa) and F2b (16 kDa), which were collectively referred to as [F2](a/b). Enzymatic deglycosylation of [F2](a/b) produced a single 10 kDa product suggesting that [F2](a/b) arises from differences in the glycosylation pattern of F2a and F2b. The detection of [F2](a/b) was dependent upon the post-translational cleavage of the F protein by furin, since its appearance was prevented in RSV-infected Vero cells treated with the furin inhibitor dec-RVKR-cmk. Analysis by protein cross-linking revealed that the F1 subunit interacted with [F2](a/b), via disulphide bonding, to produce equivalent F protein trimers, which were expressed on the surface of infected cells. Collectively, these data show that multiple F protein species are expressed in RSV-infected cells.

Leucine at position 278 of the AIK-C measles virus vaccine strain fusion protein is responsible for reduced syncytium formation

The live measles virus (MV) vaccine strain AIK-C was attenuated from the wild-type strain Edmonston by plaque purification at 33 degrees C. Strain AIK-C grew well at 33 degrees C with a mixture of small-and medium-sized plaques in Vero cells, but did not grow well at 40 degrees C. To investigate fusion inducibility, expression plasmids for the fusion (F) and haemagglutinin (H) protein regions of MV strains AIK-C (pAIK-F01 and pAIK-H) and Edmonston (pEdm-F and pEdm-H) were constructed. pEdm-F induced extensive cell fusion in B95a and Vero cells under the control of T7 RNA polymerase, whereas a sharp reduction in syncytium formation was observed when pAIK-F01 was used. Six amino acid differences were determined between pAIK-F01 and pEdm-F. Direct sequencing showed that the seed strain AIK-C contained either Leu or Phe at position 278 of the F protein. Experiments using recombinant F protein plasmids demonstrated that those with Leu at position 278 induced poor syncytium formation, while those with Phe at position 278 (Edmonston type) induced extensive cell fusion. Replacement of Phe with Leu at position 278 of pEdm-F reduced fusion-inducing capability. A full-length infectious clone of AIK-C with Leu at position 278 of the F protein was constructed. The rescued virus produced small plaques in Vero cells. However, the same rescued virus with Phe at position 278 produced large plaques. It was concluded that Leu at position 278 of the F protein of the MV vaccine strain AIK-C is responsible for the formation of small plaques.

Three membrane-proximal amino acids in the human parainfluenza type 2 (HPIV 2) F protein are critical for fusogenic activity

In this study, we investigated the role of the membrane-proximal region of the human parainfluenza virus type 2 (HPIV2) F protein by mutational analysis, including deletion, insertion, and substitution. Deletion or replacement of the entire 12 amino acid region (aa 474-485) of the HPIV2 F protein completely abolished its fusion activity when coexpressed with the HPIV2 HN protein. Deletion of groups of four of aa 478-485, single alanine, or other amino acid substitutions among aa 478-485 had minimal or limited effects on HPIV2 F/HN-induced cell fusion. However, a significant reduction in, or complete inhibition of, fusion activity was observed when aa 474-477 were deleted, or the N475, F476, or F477 residues were singly substituted with alanine. In addition, insertions of four amino acids at this region or deletion of eight or more amino acids significantly reduced F protein fusion activity. The oligomerization patterns and levels of cell surface expression of the mutant F proteins were compared to those of the wild-type HPIV2 F protein. The mutant HPIV2 F proteins defective in fusion were also found to be unable to initiate hemifusion, indicating that there is a specific requirement for three specific amino acids as well as the spacing in this region for initiating lipid mixing.

Recovery and characterization of a chimeric rinderpest virus with the glycoproteins of peste-des-petits-ruminants virus: homologous F and H proteins are required for virus viability

Rinderpest (RP) and peste-des-petits-ruminants (PPR) are two important diseases of domestic ruminants. To improve on currently available vaccines against PPR, we have created cDNA copies of the RP virus genome in which either the fusion (F) or hemagglutinin (H) gene, or both, was replaced with the corresponding gene from PPR virus. It was necessary to develop a modified rescue system in which the T7 RNA polymerase was provided by a recombinant fowlpox virus and the entire rescue procedure took place in Vero cells before we could obtain live virus from these chimeric constructs. No virus was recovered when only one of the glycoprotein genes was changed, but a chimeric virus containing both F and H genes from PPR virus was reproducibly rescued from cDNA, indicating that a virus-specific functional interaction takes place between the F and H proteins. The rescued virus expressing the PPR glycoproteins grew more slowly in tissue culture than either parental virus and formed abnormally large syncytia. Goats infected with the chimera showed no adverse reaction, as assessed by clinical signs, temperature, leukocyte count, virus isolation, and serology, and were protected from subsequent challenge with wild-type PPR virus.

A recombinant measles vaccine virus expressing wild-type glycoproteins: consequences for viral spread and cell tropism

Wild-type, lymphotropic strains of measles virus (MV) and tissue culture-adapted MV vaccine strains possess different cell tropisms. This observation has led to attempts to identify the viral receptors and to characterize the functions of the MV glycoproteins. We have functionally analyzed the interactions of MV hemagglutinin (H) and fusion (F) proteins of vaccine (Edmonston) and wild-type (WTF) strains in different combinations in transfected cells. Cell-cell fusion occurs when both Edmonston F and H proteins are expressed in HeLa or Vero cells. The expression of WTF glycoproteins in HeLa cells did not result in syncytia, yet they fused efficiently with cells of lymphocytic origin. To further investigate the role of the MV glycoproteins in virus cell entry and also the role of other viral proteins in cell tropism, we generated recombinant vaccine MVs containing one or both glycoproteins from WTF. These viruses were viable and grew similarly in lymphocytic cells. Recombinant viruses expressing the WTFH protein showed a restricted spread in HeLa cells but spread efficiently in Vero cells. Parental WTF remained restricted in both cell types. Therefore, not only differential receptor usage but also other cell-specific factors are important in determining MV cell tropism.

The nucleotide and predicted amino acid sequence of the fusion protein of recent isolates of canine distemper virus in Japan

Analysis of the molecular properties of fusion (F) proteins of field isolates of canine distemper virus (CDV) by immunoprecipitation analysis revealed an identical molecular mass of F protein of 3 field isolates as well as the Onderstepoort laboratory strain. Sequencing showed that the F gene of a field isolate (the Yanaka strain) shared 90.1% and 95.7% identities with the Onderstepoort strain at nucleotide and amino acid levels, respectively. All of the 13 cysteine residues and 4 potential asparagine-linked glycosylation sites were completely conserved amongst these strains. These results indicate that the F proteins is much less heterogeneous than that observed in the hemagglutinin proteins of CDV.

A 45,000-M(r) glycoprotein in the Sendai virus envelope triggers virus-cell fusion

Sendai virus envelopes devoid of hemagglutinin-neuraminidase but containing the fusion protein (F-virosomes) were prepared. F-virosomes exhibited discernible serine protease activity at neutral pH. Electrophoretic analysis of the protein profile of the F-virosomes under nonreducing conditions, by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing, led to the identification of a previously unknown glycoprotein with a relative molecular weight of 45,000 (45K protein) associated with the F protein. The identity of the 45K protein, as distinct from F protein, was established by Western blot analysis with F- and 45K-specific antibodies. This 45K protein forms a nexus with the F protein through noncovalent hydrophobic interactions, as proved by its sensitivity to urea treatment, and it is essential for the proteolytic activity of the F-virosomes as well as for the fusion of the viral envelope with host cell membrane. N-terminal sequence analysis (first 11 amino acids) of this protein showed strong homology (> 90%) to flavivirus NS3 serine proteases but no similarity to any of the Sendai viral proteins. On the basis of the N-terminal sequence, oligonucleotides were designed corresponding to the sense and antisense DNA sequences. Dot blot hybridization and primer extension with these oligonucleotides with the viral and the host genome confirmed the host origin of this protein. Further, the limited proteolytic digestion of the target membrane resulted in significant inhibition of viral fusion with it. On the basis of these results, we postulate a model for the molecular mechanism of F protein-induced membrane fusion, which may provide a rationale for other paramyxoviruses.

Quantitative measurement of paramyxovirus fusion: differences in requirements of glycoproteins between simian virus 5 and human parainfluenza virus 3 or Newcastle disease virus

To compare the requirements for paramyxovirus-mediated cell fusion, the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of simian virus 5 (SV5), human parainfluenza virus 3 (HPIV-3), and Newcastle disease virus (NDV) were expressed individually or coexpressed in either homologous or heterologous combinations in CV-1 or HeLa-T4 cells, using the vaccinia virus-T7 polymerase transient expression system. The contribution of individual glycoproteins in virus-induced membrane fusion was examined by using a quantitative assay for lipid mixing based on the relief of self-quenching (dequenching) of fluorescence of the lipid probe octadecyl rhodamine (R18) and a quantitative assay for content mixing based on the cytoplasmic activation of a reporter gene, beta-galactosidase. In these assays, expression of the individual F glycoproteins did not induce significant levels of cell fusion and no cell fusion was observed in experiments when cells individually expressing homologous F or HN proteins were mixed. However, coexpression of homologous F and HN glycoproteins resulted in extensive cell fusion. The kinetics of fusion were found to be very similar for all three paramyxoviruses studied. With NDV and HPIV-3, no cell fusion was detected when F proteins were coexpressed with heterologous HN proteins or influenza virus hemagglutinin (HA). In contrast, SV5 F protein exhibited a considerable degree of fusion activity when coexpressed with either NDV or HPIV-3 HN or with influenza virus HA, although the kinetics of fusion were two- to threefold higher when the homologous SV5 F and HN proteins were coexpressed. Thus, these data indicate that among the paramyxoviruses tested, SV5 has different requirements for cell fusion.

Improved adenovirus vector provides herpes simplex virus ribonucleotide reductase R1 and R2 subunits very efficiently

We have constructed a new adenovirus (Ad) expression vector, pAdBM5, that allows for the production of unprecedented levels of recombinant protein in the human 293 cell line using the Ad expression system. The main feature of this vector is a combination of enhancer sequences that increases the activity of the ectopic major late promoter (MLP) in recombinant Ad. In 293 cells infected with helper-free Ad recombinants generated with the pAdBM5 transfer vector, both herpes simplex virus (HSV) ribonucleotide reductase R1 and R2 subunits represent the most abundant polypeptides, accounting for as much as 15-20% of total cellular proteins. Our data suggest that this level of expression is probably very close to the upper limit of the system. Furthermore, when compared to the widely utilized baculovirus (Bac)/Sf9 expression system, the improved Ad vector showed a better performance for the production and purification of active HSV-2 ribonucleotide reductase R1 and R2 subunits. The R2 subunit was about 5-fold more abundant in recombinant Ad-infected 293 cells than in Bac-infected Sf9 cells while the R1 subunit was produced at roughly similar levels with either system. However, the amount of active soluble R1 obtained from recombinant Ad-infected 293 cells was at least 5 times higher because most of the R1 produced in Sf9 cells was insoluble.

Functional and structural interactions between measles virus hemagglutinin and CD46

We analyzed the roles of the individual measles virus (MV) surface glycoproteins in mediating functional and structural interactions with human CD46, the primary MV receptor. On one cell population, recombinant vaccinia virus vectors were used to produce the MV hemagglutinin (H) and fusion (F) glycoproteins. As fusion partner cells, various cell types were examined, without or with human CD46 (endogenous or recombinant vaccinia virus encoded). Fusion between the two cell populations was monitored by a quantitative reporter gene activation assay and by syncytium formation. MV glycoproteins promoted fusion with primate cells but not with nonprimate cells; recombinant CD46 rendered nonprimate cells competent for MV glycoprotein-mediated fusion. Markedly different fusion specificity was observed for another morbillivirus, canine distemper virus (CDV): recombinant CDV glycoproteins promoted fusion with primate and nonprimate cells independently of CD46. Fusion by the recombinant MV and CDV glycoproteins required coexpression of H plus F in either homologous or heterologous combinations. To assess the role of H versus F in determining the CD46 dependence of MV fusion, we examined the fusion specificities of cells producing heterologous glycoprotein combinations. The specificity of HMV plus FCDV paralleled that observed for the homologous MV glycoproteins: fusion occurred with primate cells but not with nonprimate cells unless they produced recombinant CD46. By contrast, the specificity of HCDV plus FMV paralleled that for the homologous CDV glycoproteins: fusion occurred with either primate or nonprimate cells with no dependence on CD46. Thus, for both MV and CDV, fusion specificity was determined by H. In particular, the results demonstrate a functional interaction between HMV and CD46. Flow cytometry and antibody coprecipitation studies provided a structural correlate to this functional interaction: CD46 formed a molecular complex with HMV but not with FMV or with either CDV glycoprotein. These results highlight the critical role of the H glycoprotein in determining MV specificity for CD46-positive cells.

Human parainfluenza virus type 3: analysis of the cytoplasmic tail and transmembrane anchor of the hemagglutinin-neuraminidase protein in promoting cell fusion

The role of the cytoplasmic tail and transmembrane anchor of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN) glycoprotein in promoting cell fusion was investigated. A series of amino terminal deletion mutants (d10, d20, d27, d31, d40, d44, and d73) were compared for processing, cell surface expression, and maintenance of their biological attributes by recombinant expression of mutant genes using a plasmid vector (pcDL-SR alpha-296) in CV-1 and HeLa cells. To determine the fusion promoting activity (FPA) of the various mutant proteins, a simple assay was developed which quantified the fusion of two different HeLa cell types. One of the cell types, HeLa-tat, constitutively expressed the human immunodeficiency virus type I (HIV-1) tat protein from a Moloney murine leukemia virus long terminal repeat (LTR), while the second cell type, HeLa beta-gal, contained a reporter gene, beta-galactosidase, under the control of an HIV1-LTR. Fusion of mixed HeLa cell monolayers (50:50, HeLa-tat: HeLa beta-gal), following transfection with appropriate plasmids, resulted in transactivation of the reporter gene which was then measured by direct staining of cells or using cell lysates with appropriate substrates. Cell fusion was observed only when both the HPIV3 F and functional HN proteins were both co-transfected into cells. Of the seven deletion mutants examined, only d10, d20, d27 and d31 were expressed to significant levels on the cell surface and only these four mutant proteins maintained FPA. Compared with the wt HN at 48 h post transfection, d10 and d20 had enhanced FPA (119% and 158%, respectively), while d27 and d31 were diminished (74% and > 4%, respectively). Analysis of protein expression suggested that the reason for the increase in FPA of the mutant proteins was that the levels of protein expressed at the cell surface was twofold or threefold higher for d10 and d20, respectively, compared to the wt HN.

The hemagglutinin envelope protein of canine distemper virus (CDV) confers cell tropism as illustrated by CDV and measles virus complementation analysis

Measles virus (MV) and canine distemper virus (CDV) are morbilliviruses that cause acute illnesses and several persistent central nervous system infections in humans and in dogs, respectively. Characteristically, the cytopathic effect of these viruses is the formation of syncytia in permissive cells. In this study, a vaccinia virus expression system was used to express MV and CDV hemagglutinin (HA) and fusion (F) envelope proteins. We found that cotransfecting F and HA genes of MV or F and HA genes of CDV resulted in extensive syncytium formation in permissive cells while transfecting either F or HA alone did not. Similar experiments with heterologous pairs of proteins, CDV-F with MV-HA or MV-F with CDV-HA, caused significant cell fusion in both cases. These results indicate that in this expression system, cell fusion requires both F and HA; however, the functions of these proteins are interchangeable between the two types of morbilliviruses. Human-mouse somatic hybrids were used to determine the human chromosome conferring susceptibility to either MV and CDV. Of the 12 hybrids screened, none were sensitive to MV. Two of the hybrids containing human chromosome 19 formed syncytia following CDV infection. In addition, these two hybrids underwent cell fusion when cotransfected with CDV-F and CDV-HA (but not MV-F and MV-HA) glycoproteins by using the vaccinia virus expression system. To discover the viral component responsible for cell specificity, complementation experiments coexpressing CDV-HA with MV-F or CDV-F with MV-HA in the CDV-sensitive hybrids were performed. We found that syncytia were formed only in the presence of CDV-HA. These results support the idea that the HA protein is responsible for cell tropism. Furthermore, while the F protein is necessary for the fusion process, it is interchangeable with the F protein from other morbilliviruses.

Transfection of Sendai virus F gene cDNA with mutations at its cleavage site and HN gene cDNA into COS cells induces cell fusion

In contrast to the wild type Sendai virus fusion protein (F), a mutated F to possess a cleavage site similar to that of virulent Newcastle disease virus F, could be cleaved by proteases present in COS cells. When mutated F and hemagglutinin-neuraminidase (HN) were coexpressed at the cell surface, syncytium formation was observed.

Characterization of a cleavage mutant of the measles virus fusion protein defective in syncytium formation

Membrane fusion caused by measles virus (MV) is a function of the fusion (F) protein. This process is essential for penetration into the host cell and subsequent initiation of the virus replicative cycle. The biological activity of the MV F protein is generated by endoproteolytic cleavage of a precursor protein (F0) into a large F1 subunit and a smaller F2 subunit held together by disulfide bonds. The cleavage site consists of a cluster of five basic amino acids (amino acids 108 to 112) within the predicted primary structure of the F protein. To investigate the role of the arginine residue at the carboxy terminus of the F2 subunit (arginine 112), site-directed mutagenesis was used to construct a cleavage mutant of the MV F protein in which this arginine residue was changed to a leucine residue. The mutated F gene, encoding four out of the five basic amino acids at the cleavage site, was inserted into the genome of vaccinia virus. The resulting recombinant virus was used to study expression of the mutant F protein in infected cells. Analysis of the Leu-112 mutant protein made in infected cells demonstrated that this single-amino-acid substitution resulted in a reduced rate of transport of the mutant protein to the cell surface, despite its efficient cleavage to yield F1 and F2 subunits. However, the electrophoretic mobilities of the Leu-112 polypeptides suggested that the protein was cleaved incorrectly. This aberrant cleavage appears to have abolished the ability of the F protein to cause syncytium formation. The data indicate that the arginine 112 residue is critical for the correct proteolytic cleavage that is required for the membrane fusion activity of the MV F protein.

Functional analysis of N-linked glycosylation mutants of the measles virus fusion protein synthesized by recombinant vaccinia virus vectors

The role of N-linked glycosylation in the biological activity of the measles virus (MV) fusion (F) protein was analyzed by expressing glycosylation mutants with recombinant vaccinia virus vectors. There are three potential N-linked glycosylation sites located on the F2 subunit polypeptide of MV F, at asparagine residues 29, 61, and 67. Each of the three potential glycosylation sites was mutated separately as well as in combination with the other sites. Expression of mutant proteins in mammalian cells showed that all three sites are used for the addition of N-linked oligosaccharides. Cell surface expression of mutant proteins was reduced by 50% relative to the wild-type level when glycosylation at either Asn-29 or Asn-61 was abolished. Despite the similar levels of cell surface expression, the Asn-29 and Asn-61 mutant proteins had different biological activities. While the Asn-61 mutant was capable of inducing syncytium formation, the Asn-29 mutant protein did not exhibit any significant cell fusion activity. Inactivation of the Asn-67 glycosylation site also reduced cell surface transport of mutant protein but had little effect on its ability to cause cell fusion. However, when the Asn-67 mutation was combined with mutations at either of the other two sites, cleavage-dependent activation, cell surface expression, and cell fusion activity were completely abolished. Our data show that the loss of N-linked oligosaccharides markedly impaired the proteolytic cleavage, stability, and biological activity of the MV F protein. The oligosaccharide side chains in MV F are thus essential for optimum conformation of the extracellular F2 subunit that is presumed to bind cellular membranes.

gp120-independent fusion mediated by the human immunodeficiency virus type 1 gp41 envelope glycoprotein: a reassessment

In a natural context, membrane fusion mediated by the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins involves both the exterior envelope glycoprotein (gp120) and the transmembrane glycoprotein (gp41). Perez et al. (J. Virol. 66:4134-4143, 1992) reported that a mutant HIV-1 envelope glycoprotein containing only the signal peptide and carboxyl terminus of the gp120 exterior glycoprotein fused to the complete gp41 glycoprotein was properly cleaved and that the resultant gp41 glycoprotein was able to induce the fusion of even CD4-negative cells. In the studies reported herein, mutant proteins identical or similar to those studied by Perez et al. lacked detectable cell fusion activity. The proteolytic processing of these proteins was very inefficient, and one processed product identified by Perez et al. as the authentic gp41 glycoprotein was shown to contain carboxyl-terminal gp120 sequences. Furthermore, no fusion activity was observed for gp41 glycoproteins exposed after shedding of the gp120 glycoprotein by soluble CD4. Thus, evidence supporting a gp120-independent cell fusion activity for the HIV-1 gp41 glycoprotein is currently lacking.

Paramyxovirus mediated cell fusion requires co-expression of both the fusion and hemagglutinin-neuraminidase glycoproteins

Syncytia formation in either CV-1 or HeLa T4+ cells required recombinant expression of both fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins from the human parainfluenza virus type 3 (HPIV3), human parainfluenza virus type 2 (HPIV2), and simian virus 5 (SV5). In this system, recombinant T7 transcription vectors (pT7-5 or pGEM) containing F or HN, were transfected individually or in combination into cells previously infected with a recombinant vaccinia virus expressing T7 RNA polymerase (vTF7-3). While both proteins were processed and expressed at the cell surface, syncytia formation occurred only when both glycoproteins were co-expressed. The function of HN in the fusion process could not be replaced using lectins or by co-expression of heterologous F and HN proteins. Further, cell fusion was not observed when experiments were performed using individually expressed F and HN proteins in adjacent cells. The data presented in this report support the notion that a specific interaction between both paramyxoviral glycoproteins is required for the formation of syncytia in tissue culture monolayers.

Establishment and characterisation of murine cells constitutively expressing the fusion, nucleoprotein and matrix proteins of measles virus

To advance our understanding of the immunobiology of measles virus (MV) infections, we have investigated the possibility of establishing cell lines constitutively expressing the individual MV antigens. In contrast to previously published studies, we show that it is possible to establish cell lines expressing high levels of fusion (F), nucleoprotein (NP) and matrix (M) MV proteins. Once cloned, the cell lines were stable with high levels of expression for more than six months. The size and cell distribution of the NP and F proteins were similar to those observed in MV- or vaccinia-MV recombinant-infected cells. In contrast, the distribution of the M protein, although being similar to that of MV-infected cells, differed from that of Vaccinia-M recombinant virus-infected cells. Preliminary results suggest that these cell lines will be useful tools for studying the contribution of individual MV antigens to the cell-mediated immune response to this virus.

Hemagglutinin-neuraminidase enhances F protein-mediated membrane fusion of reconstituted Sendai virus envelopes with cells

Reconstituted Sendai virus envelopes containing both the fusion (F) protein and the hemagglutinin-neuraminidase (HN) (F,HN-virosomes) or only the F protein (F-virosomes) were prepared by solubilization of the intact virus with Triton X-100 followed by its removal by using SM2 Bio-Beads. Viral envelopes containing HN whose disulfide bonds were irreversibly reduced (HNred) were also prepared by treating the envelopes with dithiothreitol followed by dialysis (F,HNred-virosomes). Both F-virosomes and F,HNred-virosomes induced hemolysis of erythrocytes in the presence of wheat germ agglutinin, but the rates and extents were markedly lower than those for hemolysis induced by F,HN-virosomes. Using an assay based on the relief of self-quenching of a lipid probe incorporated in the Sendai virus envelopes, we demonstrate the fusion of both F,HN-virosomes and F-virosomes with cultured HepG2 cells containing the asialoglycoprotein receptor, which binds to a terminal galactose moiety of F. By desialylating the HepG2 cells, the entry mediated by HN-terminal sialic acid receptor interactions was bypassed. We show that both F-virosomes and F,HN-virosomes fuse with desialylated HepG2 cells, although the rate was two- to threefold higher if HN was included in the viral envelope. We also observed enhancement of fusion rates when both F and HN envelope proteins were attached to their specific receptors.

Fusion mutants of Newcastle disease virus selected with monoclonal antibodies to the hemagglutinin-neuraminidase

The Australia-Victoria (AV) isolate of Newcastle disease virus (NDV) induces fusion from within but not fusion from without. L1, a neuraminidase (NA)-deficient virus derived from AV, has the opposite fusion phenotype from the wild-type virus. It fails to induce the former mode of fusion, but has gained a limited ability to promote the latter. Monoclonal antibodies to antigenic site 23 on the hemagglutinin-neuraminidase (HN) glycoprotein have previously been shown to select variants of the AV isolate that have altered NA activity or receptor-binding affinity. By using an antibody to this site, variants of L1 have been selected. Three of the variants have gained an increased affinity for sialic acid-containing receptors, as evidenced by the resistance of their hemagglutinating activity to the presence of reduced amounts of sialic acid on the surface of chicken erythrocytes. All four variants still have very low levels of NA activity, comparable to that of the parent virus, L1. The alteration in receptor-binding affinity results in a decreased potential for elution from cellular receptors and correlates with an increased ability to promote both modes of fusion. A single amino acid substitution in the HN protein of each variant, responsible for its escape from neutralization, has been identified. These studies identify two HN residues, 193 and 203, at which monoclonal antibody-selected substitution influences the receptor recognition properties of NDV and may influence its ability to promote syncytium formation.

Biological activity of paramyxovirus fusion proteins: factors influencing formation of syncytia

The fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of the paramyxovirus simian virus 5 (SV5) were expressed individually or coexpressed in CV-1 cells by using SV40-based vectors and recombinant vaccinia viruses. The extent of detectable fusion in a syncytium formation assay was found to be affected by the expression system used. In addition, when HN was coexpressed with F, it was found that the expression vector system influenced the contribution of HN in forming syncytia. The abilities of the SV5, human parainfluenza virus type 3, and Newcastle disease virus F glycoproteins to cause fusion, when expressed alone or coexpressed with HN, were directly compared by using the SV40-based vector system in CV-1 cells. The F proteins exhibited various degrees of fusion activity independent of HN expression, but the formation of syncytia could be enhanced to different extents by the coexpression of the homotypic HN protein.

Function and immunogenicity of human parainfluenza virus 3 glycoproteins expressed by recombinant adenoviruses

Human parainfluenza virus type 3 fusion (F) and hemagglutinin-neuraminidase (HN) cDNA sequences were inserted into the E3 region of the adenovirus type 5 genome. Cells infected with recombinant adenoviruses containing HPIV3 F (AdF) and HN (AdHN) sequences were shown to express HPIV3 F and HN proteins that were functional and immunogenic. The HN protein produced following AdHN infection was glycosylated, expressed on the surface of infected cells and exhibited both hemagglutinin and neuraminidase activities. AdF infection led to the synthesis of both the HPIV3 F0 precursor and its proteolytic cleavage product, F1. F proteins produced by AdF were glycosylated and expressed on the infected cell surface. Syncytium formation was observed in HeLa T4 cell monolayers upon coinfection with AdF and AdHN. The F and HN proteins expressed by recombinant adenoviruses were recognized by HPIV3 F- and HN-specific monoclonal antibodies. Mice injected intraperitoneally with AdF or AdHN produced antibodies that immunoprecipitated the appropriate HPIV3 glycoproteins and sera from immunized mice effectively neutralized HPIV3 virions. These results support future work using recombinant adenoviruses to study the immune response to individual HPIV3 glycoproteins as well as in protection studies using animal models.

Expression of mumps virus glycoproteins in mammalian cells from cloned cDNAs: both F and HN proteins are required for cell fusion

Two recombinant plasmids were constructed by inserting the cDNAs of either the fusion (F) or the hemagglutinin-neuraminidase (HN) protein genes of mumps virus into the pcDL-SR alpha expression vector. Both the F and the HN proteins expressed in COS7 cells transfected with their respective recombinant plasmids were indistinguishable in terms of electrophoretic mobility from their counterparts synthesized in mumps virus-infected cells. The F protein was cleaved and expressed on the cell surface, but uncleaved forms were also detected. The expressed HN protein was transported to the cell surface and adsorbed guinea pig erythrocytes. Syncytium formation was induced when COS7 cells were transfected with both recombinant plasmid DNAs together, but not with the recombinant plasmid only carrying the F gene. This observation indicates that cell fusion mediated by mumps virus requires both the F and the HN glycoproteins.

Nonreplicating viral vectors as potential vaccines: recombinant canarypox virus expressing measles virus fusion (F) and hemagglutinin (HA) glycoproteins

The development of canarypox virus (CPV) recombinants expressing the hemagglutinin (HA) and fusion (F) glycoproteins of measles virus (MV) is described. Inoculation of the CPV-MV recombinants into avian or nonavian tissue culture substrates led to the expression of authentic MVF and MVHA as determined by radioimmunoprecipitation and surface immunofluorescence. In contrast to avian-derived tissue culture, no productive replication of the CPV recombinant was evident in tissue culture cells derived from nonavian origin. On inoculation of dogs, a species restricted for avipoxvirus replication, the recombinants elicited a protective immune response against a lethal canine distemper virus (CDV) challenge. The level of MV neutralizing antibodies and the level of protection induced against CDV challenge achieved by the host-restricted CPV vector were equivalent to that obtained by vaccinia virus vectors expressing the same MV antigens.

Vaccinia virus recombinants expressing either the measles virus fusion or hemagglutinin glycoprotein protect dogs against canine distemper virus challenge

cDNA clones of the genes encoding either the hemagglutinin (HA) or fusion (F) proteins of the Edmonston strain of measles virus (MV) were expressed in vaccinia virus recombinants. Immunofluorescence analysis detected both proteins on the plasma membranes of unfixed cells as well as internally in fixed cells. Immunoprecipitation of metabolically radiolabeled infected-cell extracts by using specific sera demonstrated a 76-kDa HA polypeptide and gene products of 60, 44, and 23 kDa which correspond to a MV F precursor and cleavage products F0, F1, and F2, respectively. Neither recombinant induced cell fusion of Vero cells when inoculated individually, but efficient cell fusion was readily observed upon coinfection of cells with both recombinants. Inoculation of dogs with the vaccinia virus-MV F recombinant (VV-MVF) did not give rise to detectable MV-neutralizing antibody. Inoculation of dogs with the vaccinia virus-MV HA recombinant (VV-MVHA) or coinoculation with both recombinants (VV-MVF and VV-MVHA) induced significant MV-neutralizing titers that were increased following a booster inoculation. Inoculation of dogs with the vaccinia virus recombinants or with MV failed to induce canine distemper virus (CDV)-neutralizing antibodies. Upon challenge with a lethal dose of virulent CDV, signs of infection were observed in dogs inoculated with (VV-MVF). No symptoms of disease were observed in dogs that had been vaccinated with VV-MVHA or with VV-MVHA and VV-MVF and then challenged with CDV. All dogs vaccinated with the recombinant viruses as well as those inoculated with MV or a vaccine strain of CDV survived CDV challenge.

The fusion and hemagglutinin-neuraminidase glycoproteins of human parainfluenza virus 3 are both required for fusion

Recombinant vaccinia viruses, VF and VHN, expressing the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of human parainfluenza virus 3 (HPIV3) were constructed. Infection of HeLa T4 cells with VF and VHN led to the synthesis of glycoproteins, with the correct apparent molecular weights, that were recognized by monoclonal antibodies specific for HPIV3F and HN. The HN glycoprotein was present on the surface of cells infected with VHN and these cells demonstrated both hemadsorbing and neuraminidase activities. The F glycoprotein was present in cleaved and uncleaved forms and was also expressed on the surface of VF-infected cells. Fusion activity, however, as evidenced by syncytium formation and lysis of human erythrocytes, could only be demonstrated when HeLa T4 cells were coinfected with VF and VHN. Fusion events that are mediated by HPIV3, therefore, require both the F and HN glycoproteins.

Generation of mammalian cells expressing stably measles virus proteins via bicistronic RNA

The proteins of measles virus are believed to be cytotoxic, and have never been expressed stably from the cloned genes in cultured cells. We found that measles viral proteins can be expressed via a bicistronic RNA. The dominantly selectable DHFR* protein-coding region encoding a mutant dihydrofolate reductase was inserted into the 3'-untranslated regions of the measles viral genes encoding nucleoprotein (N), matrix (M) protein, and hemagglutinin (H). The tandemly arranged cistrons were placed under control by the inducible promoter of human metallothionein IIA gene, or the noninducible early promoter of simian virus 40. Upon transfecting into mammalian cells, these gene constructs synthesized bicistronic RNAs. The downstream DHFR* gene conferred resistance to methotrexate (MTX). Cells that survived MTX selection expressed stably the N, M, or H protein of measles virus. Expression of N protein was further inducible by cadmium chloride treatment. This system will be useful for studying the protein functions of measles virus, and could be applied to express other potentially toxic gene products.