Acidic pH enhancement of the fusion of Newcastle disease virus with cultured cells

Newcastle Disease Virus Entry into Chicken Macrophages via a pH-Dependent, Dynamin and Caveola-Mediated Endocytic Pathway That Requires Rab5

The cellular entry pathways and the mechanisms of Newcastle disease virus (NDV) entry into cells are poorly characterized. In this study, we demonstrated that chicken interferon-induced transmembrane protein 1 (chIFITM1), which is located in the early endosomes, could limit the replication of NDV in chicken macrophage cell line HD11, suggesting the endocytic entry of NDV into chicken macrophages. Then, we presented a systematic study about the entry mechanism of NDV into chicken macrophages. First, we demonstrated that a low-pH condition and dynamin were required during NDV entry. However, NDV entry into chicken macrophages was independent of clathrin-mediated endocytosis. We also found that NDV entry was dependent on membrane cholesterol. The NDV entry and replication were significantly reduced by nystatin and phorbol 12-myristate 13-acetate treatment, overexpression of dominant-negative (DN) caveolin-1, or knockdown of caveolin-1, suggesting that NDV entry depends on caveola-mediated endocytosis. However, macropinocytosis did not play a role in NDV entry into chicken macrophages. In addition, we found that Rab5, rather than Rab7, was involved in the entry and traffic of NDV. The colocalization of NDV with Rab5 and early endosome suggested that NDV virion was transported to early endosomes in a Rab5-dependent manner after internalization. Of particular note, the caveola-mediated endocytosis was also utilized by NDV to enter primary chicken macrophages. Moreover, NDV entered different cell types using different pathways. Collectively, our findings demonstrate for the first time that NDV virion enters chicken macrophages via a pH-dependent, dynamin and caveola-mediated endocytosis pathway and that Rab5 is involved in the traffic and location of NDV. IMPORTANCE Although the pathogenesis of Newcastle disease virus (NDV) has been extensively studied, the detailed mechanism of NDV entry into host cells is largely unknown. Macrophages are the first-line defenders of host defense against infection of pathogens. Chicken macrophages are considered one of the main types of target cells during NDV infection. Here, we comprehensively investigated the entry mechanism of NDV in chicken macrophages. This is the first report to demonstrate that NDV enters chicken macrophages via a pH-dependent, dynamin and caveola-mediated endocytosis pathway that requires Rab5. The result is important for our understanding of the entry of NDV in chicken macrophages, which will further advance the knowledge of NDV pathogenesis and provide useful clues for the development of novel preventive or therapeutic strategies against NDV infection. In addition, this information will contribute to our further understanding of pathogenesis with regard to other members of the Avulavirus genus in the Paramyxoviridae family.

The nsp2 Hypervariable Region of Porcine Reproductive and Respiratory Syndrome Virus Strain JXwn06 Is Associated with Viral Cellular Tropism to Primary Porcine Alveolar Macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) poses a major threat to global pork production and has been notorious for its rapid genetic evolution in the field. The nonstructural protein 2 (nsp2) replicase protein represents the fastest evolving region of PRRSV, but the underlying biological significance has remained poorly understood. By deletion mutagenesis, we discovered that the nsp2 hypervariable region plays an important role in controlling the balance of genomic mRNA and a subset of subgenomic mRNAs. More significantly, we revealed an unexpected link of the nsp2 hypervariable region to viral tropism. Specifically, a mutant of the Chinese highly pathogenic PRRSV strain JXwn06 carrying a deletion spanning nsp2 amino acids 323 to 521 (nsp2Δ323-521) in its hypervariable region was found to lose infectivity in primary porcine alveolar macrophages (PAMs), although it could replicate relatively efficiently in the supporting cell line MARC-145. Consequently, this mutant failed to establish an infection in piglets. Further dissection of the viral life cycle revealed that the mutant had a defect (or defects) lying in the steps between virus penetration and negative-stranded RNA synthesis. Taken together, our results reveal novel functions of nsp2 in the PRRSV life cycle and provide important insights into the mechanisms of PRRSV RNA synthesis and cellular tropism.IMPORTANCE The PRRSV nsp2 replicase protein undergoes rapid and broad genetic variations in its middle region in the field, but the underlying significance has remained enigmatic. Here, we demonstrate that the nsp2 hypervariable region not only plays an important regulatory role in maintaining the balance of different viral mRNA species but also regulates PRRSV tropism to primary PAMs. Our results reveal novel functions for PRRSV nsp2 and have important implications for understanding the mechanisms of PRRSV RNA synthesis and cellular tropism.

Solution Structure, Self-Assembly, and Membrane Interactions of the Matrix Protein from Newcastle Disease Virus at Neutral and Acidic pH

Newcastle disease virus (NDV) is an enveloped paramyxovirus. The matrix protein of the virus (M-NDV) has an innate propensity to produce virus-like particles budding from the plasma membrane of the expressing cell without recruiting other viral proteins. The virus predominantly infects the host cell via fusion with the host plasma membrane or, alternatively, can use receptor-mediated endocytic pathways. The question arises as to what are the mechanisms supporting such diversity, especially concerning the assembling and membrane binding properties of the virus protein scaffold under both neutral and acidic pH conditions. Here, we suggest a novel method of M-NDV isolation in physiological ionic strength and employ a combination of small-angle X-ray scattering, atomic force microscopy with complementary structural techniques, and membrane interaction measurements to characterize the solution behavior/structure of the protein as well as its binding to lipid membranes at pH 4.0 and pH 7.0. We demonstrate that the minimal structural unit of the protein in solution is a dimer that spontaneously assembles in a neutral milieu into hollow helical oligomers by repeating the protein tetramers. Acidic pH conditions decrease the protein oligomerization state to the individual dimers, tetramers, and octamers without changing the density of the protein layer and lipid membrane affinity, thus indicating that the endocytic pathway is a possible facilitator of NDV entry into a host cell through enhanced scaffold disintegration.IMPORTANCE The matrix protein of the Newcastle disease virus (NDV) is one of the most abundant viral proteins that regulates the formation of progeny virions. NDV is an avian pathogen that impacts the economics of bird husbandry due to its resulting morbidity and high mortality rates. Moreover, it belongs to the Avulavirus subfamily of the Paramyxoviridae family of Mononegavirales that include dangerous representatives such as respiratory syncytial virus, human parainfluenza virus, and measles virus. Here, we investigate the solution structure and membrane binding properties of this protein at both acidic and neutral pH to distinguish between possible virus entry pathways and propose a mechanism of assembly of the viral matrix scaffold. This work is fundamental for understanding the mechanisms of viral entry as well as to inform subsequent proposals for the possible use of the virus as an adequate template for future drug or vaccine delivery.

Newcastle disease virus strain AF2240 as an oncolytic virus: A review

The discovery of tumour selective virus-mediated apoptosis marked the birth of an alternative cancer treatment in the form of oncolytic viruses. Even though, its oncolytic efficiency was demonstrated more than 50 years ago, safety concerns which resulted from mild to lethal side effects hampered the progress of oncolytic virus research. Since the classical oncolytic virus studies rely heavily on its natural oncolytic ability, virus manipulation was limited, thereby, restricted efforts to improve its safety. In order to circumvent such restriction, experiments involving non-human viruses such as the avian Newcastle disease virus (NDV) was conducted using cultured cells, animal models and human subjects. The corresponding reports on its significant tumour cytotoxicity along with impressive safety profile initiated immense research interest in the field of oncolytic NDV. The varying degree of oncolytic efficiency and virulency among NDV strains encouraged researchers from all around the world to experiment with their respective local NDV isolates in order to develop an oncolytic virus with desirable characteristics. Such desirable features include high tumour-killing ability, selectivity and low systemic cytotoxicity. The Malaysian field outbreak isolate, NDV strain AF2240, also currently, receives significant research attention. Apart from its high cytotoxicity against tumour cells, this strain also provided fundamental insight into NDV-mediated apoptosis mechanism which involves Bax protein recruitment as well as death receptor engagement. Studies on its ability to selectively induce apoptosis in tumour cells also resulted in a proposed p38 MAPK/NF-κB/IκBα pathway. The immunogenicity of AF2240 was also investigated through PBMC stimulation and macrophage infection. In addition, the enhanced oncolytic ability of this strain under hypoxic condition signifies its dynamic tumour tropism. This review is aimed to introduce and discuss the aforementioned details of the oncolytic AF2240 strain along with its current challenges which outlines the future research direction of this virus.

Peste des Petits Ruminants Virus Enters Caprine Endometrial Epithelial Cells via the Caveolae-Mediated Endocytosis Pathway

Peste des petits ruminants virus (PPRV) causes an acute and highly contagious disease of sheep and goats and has spread with alarming speed around the world. The pathology of Peste des petits ruminants is linked to retrogressive changes and necrotic lesions in lymphoid tissues and epithelial cells. However, the process of PPRV entry into host epithelial cells remains largely unknown. Here, we performed a comprehensive study of the entry mechanism of PPRV into caprine endometrial epithelial cells (EECs). We clearly demonstrated that PPRV internalization was inhibited by chloroquine and ammonium chloride, which elevate the pH of various organelles. However, PPRV entry was not affected by chlorpromazine and knockdown of the clathrin heavy chain in EECs. In addition, we found that the internalization of PPRV was dependent on dynamin and membrane cholesterol and was suppressed by silencing of caveolin-1. Macropinocytosis did not play a role, but phosphatidylinositol 3-kinase (PI3K) was required for PPRV internalization. Cell type and receptor-dependent differences indicated that PPRV entry into caprine fetal fibroblast cells (FFCs) occurred via a different route. Taken together, our findings demonstrate that PPRV enters EECs through a cholesterol-dependent caveolae-mediated uptake mechanism that is pH-dependent and requires dynamin and PI3K but is independent of clathrin. This potentially provides insight into the entry mechanisms of other morbilliviruses.

Transient activation of the PI3K/Akt pathway promotes Newcastle disease virus replication and enhances anti-apoptotic signaling responses

Viral infection activates a host's cellular phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, which is involved in cell differentiation, growth, survival, and apoptosis. To elucidate molecular mechanisms in the pathogenesis of Newcastle disease virus (NDV), we demonstrated that NDV transiently activates the PI3K/Akt pathway in chicken cells at an early phase of infection. Its activation was observed as early as 15 min post-infection and gradually weakened after 24 h. Incubating cells with a PI3K inhibitor, LY294002 or wortmannin, prior to NDV infection decreased NDV progeny yields and suppressed Akt phosphorylation at early times post-infection. Akt activation is triggered by NDV-GM or NDV-F48E9 and is abolished by methyl β-cyclodextrin and chlorpromazine. Treatment following NDV-La Sota infection had no obvious effect. However, inhibiting PI3K activation promoted apoptotic responses during an early stage of NDV infection. The pan caspase inhibitor ZVAD-FMK mitigated the reduction in Akt phosphorylation by inhibiting PI3K activation, which indicates the signaling pathway promotes cell survival and, in turn, facilitates viral replication. By suppressing premature apoptosis upon NDV infection, the PI3K/Akt pathway enhances the anti-apoptotic response.

Employing RNA viruses to fight cancer: novel insights into oncolytic virotherapy

Within recent decades, viruses that specifically target tumor cells have emerged as novel therapeutic agents against cancer. These viruses do not only act via their cell-lytic properties, but also harbor immunostimulatory features to re-direct the tumor microenvironment and stimulate tumor-directed immune responses. Furthermore, oncolytic viruses are considered to be superior to classical cancer therapies due to higher selectivity towards tumor cell destruction and, consequently, less collateral damage of non-transformed healthy tissue. In particular, the field of oncolytic RNA viruses is rapidly developing since these agents possess alternative tumor-targeting strategies compared to established oncolytic DNA viruses. Thus, oncolytic RNA viruses have broadened the field of virotherapy facilitating new strategies to fight cancer. In addition to several naturally occurring oncolytic viruses, genetically modified RNA viruses that are armed to express foreign factors such as immunostimulatory molecules have been successfully tested in early clinical trials showing promising efficacy. This review aims to provide an overview of the most promising RNA viruses in clinical development, to summarize the current knowledge of clinical trials using these viral agents, and to discuss the main issues as well as future perspectives of clinical approaches using oncolytic RNA viruses.

Synthesis and chemical characterization of several perfluorinated sialic acid glycals and evaluation of their in vitro antiviral activity against Newcastle disease virus

Newcastle Disease Virus (NDV), belonging to the Paramyxoviridae family, causes a serious infectious disease in birds, resulting in severe losses in the poultry industry every year. Haemagglutinin neuraminidase glycoprotein (HN) has been recognized as a key protein in the viral infection mechanism, and its inhibition represents an attractive target for the development of new drugs based on sialic acid glycals, with the 2-deoxy-2,3-didehydro-d-N-acetylneuraminic acid (Neu5Ac2en) as their backbone. Herein we report the synthesis of several Neu5Ac2en glycals and of their perfluorinated C-5 modified derivatives, including their respective stereoisomers at C-4, together with evaluation of their in vitro antiviral activity. While all synthesized compounds were found to be active HN inhibitors in the micromolar range, we found that their potency was influenced by the chain-length of the C-5 perfluorinated acetamido functionality. Thus, the binding modes of the inhibitors were also investigated by performing a docking study. Moreover, the perfluorinated glycals were found to be more active than the corresponding normal C-5 acylic derivatives. Finally, cell-cell fusion assays on NDV infected cells revealed that the addition of a newly synthesized C-4α heptafluorobutyryl derivative almost completely inhibited NDV-induced syncytium formation.

Paramyxovirus Glycoproteins and the Membrane Fusion Process

The family Paramyxoviridae includes many viruses that significantly affect human and animal health. An essential step in the paramyxovirus life cycle is viral entry into host cells, mediated by virus-cell membrane fusion. Upon viral entry, infection results in expression of the paramyxoviral glycoproteins on the infected cell surface. This can lead to cell-cell fusion (syncytia formation), often linked to pathogenesis. Thus membrane fusion is essential for both viral entry and cell-cell fusion and an attractive target for therapeutic development. While there are important differences between viral-cell and cell-cell membrane fusion, many aspects are conserved. The paramyxoviruses generally utilize two envelope glycoproteins to orchestrate membrane fusion. Here, we discuss the roles of these glycoproteins in distinct steps of the membrane fusion process. These findings can offer insights into evolutionary relationships among Paramyxoviridae genera and offer future targets for prophylactic and therapeutic development.

Stability, biophysical properties and effect of ultracentrifugation and diafiltration on measles virus and mumps virus

Measles virus and mumps virus (MeV and MuV) are enveloped RNA viruses used for production of live attenuated vaccines for prophylaxis of measles and mumps disease, respectively. For biotechnological production of and basic research on these viruses, the preparation of highly purified and infectious viruses is a prerequisite, and to meet that aim, knowledge of their stability and biophysical properties is crucial. Our goal was to carry out a detailed investigation of the stability of MeV and MuV under various pH, temperature, shear stress, filtration and storage conditions, as well as to evaluate two commonly used purification techniques, ultracentrifugation and diafiltration, with regard to their efficiency and effect on virus properties. Virus titers were estimated by CCID50 assay, particle size and concentration were measured by Nanoparticle tracking analysis (NTA) measurements, and the host cell protein content was determined by ELISA. The results demonstrated the stability of MuV and MeV at pH <9 and above pH 4 and 5, respectively, and aggregation was observed at pH >9. Storage without stabilizer did not result in structural changes, but the reduction in infectivity after 24 hours was significant at +37 °C. Vortexing of the viruses resulted in significant particle degradation, leading to lower virus titers, whereas pipetting had much less impact on virus viability. Diafiltration resulted in higher recovery of both total and infectious virus particles than ultracentrifugation. These results provide important data for research on all upstream and downstream processes on these two viruses regarding biotechnological production and basic research.

Spectrum of Newcastle disease virus stability in gradients of temperature and pH

Newcastle disease (ND) is one of the highly pathogenic viral diseases of avian species. The disease is endemic in many developing countries where agriculture serves as the primary source of national income. Newcastle disease virus (NDV) belongs to the family Paramyxoviridae and is well characterized member among the avian paramyxovirus serotypes. The failure of vaccination is one of the major causes of NDV outbreaks in field condition. The present study gives a brief picture about the biology of NDV genome and its proteins under different conditions of temperature and pH. Our results indicate that the NDV is non-infective above 42 °C and unstable above 72 °C. The study will be useful in defining an optimum storage condition for NDV without causing any deterioration in its viability.

Entry of Newcastle Disease Virus into the host cell: role of acidic pH and endocytosis

Most paramyxoviruses enter the cell by direct fusion of the viral envelope with the plasma membrane. Our previous studies have shown the colocalization of Newcastle Disease Virus (NDV) with the early endosome marker EEA1 and the inhibition of NDV fusion by the caveolin-phosphorylating drug phorbol 12-myristate 13-acetate (PMA) prompted us to propose that NDV enters the cells via endocytosis. Here we show that the virus-cell fusion and cell-cell fusion promoted by NDV-F are increased by about 30% after brief exposure to low pH in HeLa and ELL-0 cells but not in NDV receptor- deficient cell lines such as GM95 or Lec1. After a brief low-pH exposure, the percentage of NDV fusion at 29 °C was similar to that at 37 °C without acid-pH stimulation, meaning that acid pH would decrease the energetic barrier to enhance fusion. Furthermore, preincubation of cells with the protein kinase C inhibitor bisindolylmaleimide led to the inhibition of about 30% of NDV infectivity, suggesting that a population of virus enters cells through receptor-mediated endocytosis. Moreover, the involvement of the GTPase dynamin in NDV entry is shown as its specific inhibitor, dynasore, also impaired NDV fusion and infectivity. Optimal infection of the host cells was significantly affected by drugs that inhibit endosomal acidification such as concanamycin A, monensin and chloroquine. These results support our hypothesis that entry of NDV into ELL-0 and HeLa cells occurs through the plasma membrane as well as by dynamin- low pH- and receptor- dependent endocytosis.

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A pulse of low-pH enhanced NDV fusion and infectivity in a cell-dependent manner.

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NDV infectivity was impaired by a protein kinase C inhibitor.

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A specific inhibitor of the GTPase dynamin impaired NDV fusion and infectivity.

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Inhibition of endosomal acidification inhibited NDV fusion and infectivity.

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NDV may enter by dynamin-acid- and receptor-dependent endocytosis.

Fluorosomes: fluorescent virus-like nanoparticles that represent a convenient tool to visualize receptor-ligand interactions

Viruses are the smallest life forms and parasitize on many eukaryotic organisms, including humans. Consequently, the study of viruses and viral diseases has had an enormous impact on diverse fields of biology and medicine. Due to their often pathogenic properties, viruses have not only had a strong impact on the development of immune cells but also on shaping entire immune mechanisms in their hosts. In order to better characterize virus-specific surface receptors, pathways of virus entry and the mechanisms of virus assembly, diverse methods to visualize virus particles themselves have been developed in the past decades. Apart from characterization of virus-specific mechanisms, fluorescent virus particles also serve as valuable platforms to study receptor-ligand interactions. Along those lines the authors have developed non-infectious virus-like nanoparticles (VNP), which can be decorated with immune receptors of choice and used for probing receptor-ligand interactions, an especially interesting application in the field of basic but also applied immunology research. To be able to better trace receptor-decorated VNP the authors have developed technology to introduce fluorescent proteins into such particles and henceforth termed them fluorosomes (FS). Since VNP are assembled in a simple expression system relying on HEK-293 cells, gene-products of interest can be assembled in a simple and straightforward fashion—one of the reasons why the authors like to call fluorosomes ‘the poor-man's staining tool’. Within this review article an overview on virus particle assembly, chemical and recombinant methods of virus particle labeling and examples on how FS can be applied as sensors to monitor receptor-ligand interactions on leukocytes are given.

Macropinocytosis-like HIV-1 internalization in macrophages is CCR5 dependent and leads to efficient but delayed degradation in endosomal compartments

HIV-1 endocytosis by a macropinocytosis-like mechanism has been shown to lead to productive infection in macrophages. However, little is known of this pathway. In this study, we examined HIV-1 endocytosis using biochemical approaches and imaging techniques in order to better understand the mechanisms that allow for productive infection of these cells via the endosomal pathway. We show here that this macropinocytosis-like mechanism is not the sole pathway involved in HIV-1 endocytosis in macrophages. However, this pathway specifically requires CCR5 engagement at the cell surface, which in turn suggests that the virus and its coreceptor are present in the endosomal environment simultaneously. Furthermore, although we observed efficient viral degradation following endocytosis, analyses of HIV-1 transport through the endolysosomal pathway revealed that viral degradation is delayed following endosomal internalization, possibly allowing the virus to complete its fusion.

Paramyxovirus fusion and entry: multiple paths to a common end

The paramyxovirus family contains many common human pathogenic viruses, including measles, mumps, the parainfluenza viruses, respiratory syncytial virus, human metapneumovirus, and the zoonotic henipaviruses, Hendra and Nipah. While the expression of a type 1 fusion protein and a type 2 attachment protein is common to all paramyxoviruses, there is considerable variation in viral attachment, the activation and triggering of the fusion protein, and the process of viral entry. In this review, we discuss recent advances in the understanding of paramyxovirus F protein-mediated membrane fusion, an essential process in viral infectivity. We also review the role of the other surface glycoproteins in receptor binding and viral entry, and the implications for viral infection. Throughout, we concentrate on the commonalities and differences in fusion triggering and viral entry among the members of the family. Finally, we highlight key unanswered questions and how further studies can identify novel targets for the development of therapeutic treatments against these human pathogens.

Measles virus glycoprotein-pseudotyped lentiviral vector-mediated gene transfer into quiescent lymphocytes requires binding to both SLAM and CD46 entry receptors

Gene transfer into quiescent T and B cells is of importance for gene therapy and immunotherapy approaches to correct hematopoietic disorders. Previously, we generated lentiviral vectors (LVs) pseudotyped with the Edmonston measles virus (MV) hemagglutinin and fusion glycoproteins (Hgps and Fgps) (H/F-LVs), which, for the first time, allowed efficient transduction of quiescent human B and T cells. These target cells express both MV entry receptors used by the vaccinal Edmonston strain, CD46 and signaling lymphocyte activation molecule (SLAM). Interestingly, LVs pseudotyped with an MV Hgp, blind for the CD46 binding site, were completely inefficient for resting-lymphocyte transduction. Similarly, SLAM-blind H mutants that recognize only CD46 as the entry receptor did not allow stable LV transduction of resting T cells. The CD46-tropic LVs accomplished vector-cell binding, fusion, entry, and reverse transcription at levels similar to those achieved by the H/F-LVs, but efficient proviral integration did not occur. Our results indicate that both CD46 and SLAM binding sites need to be present in cis in the Hgp to allow successful stable transduction of quiescent lymphocytes. Moreover, the entry mechanism utilized appears to be crucial: efficient transduction was observed only when CD46 and SLAM were correctly engaged and an entry mechanism that strongly resembles macropinocytosis was triggered. Taken together, our results suggest that although vector entry can occur through the CD46 receptor, SLAM binding and subsequent signaling are also required for efficient LV transduction of quiescent lymphocytes to occur.

Antigenic and immunogenic investigation of the virulence motif of the Newcastle disease virus fusion protein

Newcastle disease (ND) caused by virulent Newcastle disease virus (NDV) is a highly contagious viral disease of poultry. Virulent NDVs characteristically have a multibasic amino acid sequence (virulence motif) such as (112)RRQKRF(117) at the cleavage site of the precusor fusion (F0) protein. The antigenic and immunogenic characteristics of the virulence motif (112)RRQKRF(117) in the F0 protein of virulent NDVs were investigated. Epitope mapping analysis revealed that a RRQKRF-specific monoclonal antibody 4G2 recognized the KRF section of the motif. A synthetic peptide bearing the RRQKRF motif reacted strongly with sera from virulent NDV (with RRQKRF motif)-infected chickens. These sera also showed reactivity to peptides bearing other virulence motifs ((112)KRQKRF(117), (112)RRQRRF(117) and (112)RRRKRF(117)) but not an avirulence motif ((112)GRQGRL(117)) by ELISA. The synthetic bearing RRQKRF motif reacted with 60% to 91% of sera taken from surviving chickens on ND outbreak farms but not with sera from vaccinated birds, even though most of the sera had antibody to NDV due to vaccination. This indicates that the virulence motif has the potential to differentiate virulent NDV infected birds from vaccinated birds.

Mutations in the ectodomain of newcastle disease virus fusion protein confer a hemagglutinin-neuraminidase-independent phenotype

The entry of enveloped viruses into host cells is preceded by membrane fusion, which in paramyxoviruses is triggered by the fusion (F) protein. Refolding of the F protein from a metastable conformation to a highly stable postfusion form is critical for the promotion of fusion, although the mechanism is still not well understood. Here we examined the effects of mutations of individual residues of the F protein of Newcastle disease virus, located at critical regions of the protein, such as the C terminus of the N-terminal heptad repeat (HRA) and the N terminus of the C-terminal heptad repeat (HRB). Seven of the mutants were expressed at the cell surface, showing differences in antibody reactivity in comparison with the F wild type. The N211A, L461A, I463A, and I463F mutants showed a hyperfusogenic phenotype both in syncytium and in dye transfer assays. The four mutants promoted fusion more efficiently at lower temperatures than the wild type did, meaning they probably had lower energy requirements for activation. Moreover, the N211A, I463A, and I463F mutants exhibited hemagglutinin-neuraminidase (HN)-independent activity when influenza virus hemagglutinin (HA) was coexpressed as an attachment protein. The data are discussed in terms of alterations of the refolding pathway and/or the stability of the prefusion and fusion conformations.

Induction of apoptosis in Vero cells by Newcastle disease virus requires viral replication, de-novo protein synthesis and caspase activation

Newcastle disease virus causes (NDV) apoptotic death of infected cells. In the present study, the stimulus that provoked the induction of apoptosis in infected cells was examined. Vero cells infected with NDV developed apoptosis as characterized by DNA fragmentation and decreased DNA content. In presence of ammonium chloride, infected cells did not show reduced DNA content indicating the requirement of virus entry for the induction of apoptosis. UV-inactivated NDV did not induce apoptosis in cells suggesting the need of virus replication. Although cycloheximide blocked NDV-induced apoptosis, actinomycin-D did not, suggesting that de-novo viral protein synthesis was critical for the induction of apoptosis. In addition, activation of caspases was also detected by flowcytometry, indirect fluorescent and colorimetric assays. Based on the results, it was concluded that NDV-induced apoptosis in Vero cells required virus replication, de-novo protein synthesis and caspase activation.

Newcastle disease virus may enter cells by caveolae-mediated endocytosis

The entry into cells of Newcastle disease virus (NDV), a prototype member of the paramyxoviruses, is believed to occur by direct fusion at the plasma membrane through a pH-independent mechanism. In addition, NDV may enter host cells by an endocytic pathway. Treatment of cells with drugs that block caveolae-dependent endocytosis reduced NDV fusion and infectivity, the degree of inhibition being dependent on virus concentration. The inhibitory effect was reduced greatly when drugs were added after virus adsorption. Cells treated with methyl beta-cyclodextrin, a drug that sequesters cholesterol from membranes, reduced the extent of fusion, infectivity and virus-cell binding; this indicates that cholesterol plays a role in NDV entry. Double-labelling immunofluorescence assays performed with anti-NDV monoclonal antibodies and antibodies against the early endosome marker EEA1 revealed the localization of the virus in these intracellular structures. Using fluorescence microscopy, it was found that cell-cell fusion was enhanced at low pH. It is concluded that NDV may infect cells through a caveolae-dependent endocytic pathway, suggesting that this pathway could be an alternative route for virus entry into cells.

Paramyxovirus fusion: real-time measurement of parainfluenza virus 5 virus-cell fusion

Although cell-cell fusion assays are useful surrogate methods for studying virus fusion, differences between cell-cell and virus-cell fusion exist. To examine paramyxovirus fusion in real time, we labeled viruses with fluorescent lipid probes and monitored virus-cell fusion by fluorimetry. Two parainfluenza virus 5 (PIV5) isolates (W3A and SER) and PIV5 containing mutations within the fusion protein (F) were studied. Fusion was specific and temperature-dependent. Compared to many low pH-dependent viruses, the kinetics of PIV5 fusion was slow, approaching completion within several minutes. As predicted from cell-cell fusion assays, virus containing an F protein with an extended cytoplasmic tail (rSV5 F551) had reduced fusion compared to wild-type virus (W3A). In contrast, virus-cell fusion for SER occurred at near wild-type levels, despite the fact that this isolate exhibits a severely reduced cell-cell fusion phenotype. These results support the notion that virus-cell and cell-cell fusion have significant differences.

[Viral fusion mechanisms]

The majority of viral fusion proteins can be divided into two classes. The influenza hemagglutinin (HA) belongs to the class I fusion proteins and undergoes a series of conformational changes at acidic pH, leading to membrane fusion. The crystal structures of the prefusion and the postfusion forms of HA have been revealed in 1981 and 1994, respectively. On the basis of these structures, a model for the mechanism of membrane fusion mediated by the conformational changes of HA has been proposed. The flavivirus E and alphavirus E1 proteins belong to the class II fusion proteins and mediate membrane fusion at acidic pH. Their prefusion structures are distinct from that of HA. Last year, however, it has become evident that the postfusion structures of these class I and class II fusion proteins are similar. The paramyxovirus F protein belongs to the class I fusion proteins. In contrast to HA, an interaction between F and its homologous attachment protein is required for F to undergo the conformational changes. Since F mediates fusion at neutral pH, the infected cells can fuse with neighboring uninfected cells. The crystal structures of F and the attachment protein HN have recently been clarified, which will facilitate studies of the molecular mechanism of F-mediated membrane fusion.

Role of sialic acid-containing molecules in paramyxovirus entry into the host cell: A minireview

Sialic acid-containing compounds play a key role in the initial steps of the paramyxovirus life cycle. As enveloped viruses, their entry into the host cell consists of two main events: binding to the host cell and membrane fusion. Virus adsorption occurs at the surface of the host cell with the recognition of specific receptor molecules located at the cell membrane by specific viral attachment proteins. The viral attachment protein present in some paramyxoviruses (Respirovirus, Rubulavirus and Avulavirus) is the HN glycoprotein, which binds to cellular sialic acid-containing molecules and exhibits sialidase and fusion promotion activities. Gangliosides of the gangliotetraose series bearing the sialic acid N-acetylneuraminic (Neu5Ac) on the terminal galactose attached in alpha2-3 linkage, such as GD1a, GT1b, and GQ1b, and neolacto-series gangliosides are the major receptors for Sendai virus. Much less is known about the receptors for other paramyxoviruses than for Sendai virus. Human parainfluenza viruses 1 and 3 preferentially recognize oligosaccharides containing N-acetyllactosaminoglycan branches with terminal Neu5Acalpha2-3Gal. In the case of Newcastle disease virus, has been reported the absence of a specific pattern of the gangliosides that interact with the virus. Additionally, several works have described the use of sialylated glycoproteins as paramyxovirus receptors. Accordingly, the design of specific sialic acid analogs to inhibit the sialidase and/or receptor binding activity of viral attachment proteins is an important antiviral strategy. In spite of all these data, the exact nature of paramyxovirus receptors, apart from their sialylated nature, and the mechanism(s) of viral attachment to the cell surface are poorly understood.

Influence of the human parainfluenza virus 3 attachment protein's neuraminidase activity on its capacity to activate the fusion protein

In order to examine functions of the hemagglutinin-neuraminidase (HN) protein that quantitatively influence fusion promotion, human parainfluenza virus 3 (HPIV3) variants with alterations in HN were studied. The variant HNs have mutations that affect either receptor binding avidity, neuraminidase activity, or fusion protein (F) activation. Neuraminidase activity was regulated by manipulation of temperature and pH. F activation was assessed by quantitating the irreversible binding of target erythrocytes (RBC) to HN/F-coexpressing cells in the presence of 4-GU-DANA (zanamivir) to release target cells bound only by HN-receptor interactions; the remaining, irreversibly bound target cells are retained via the fusion protein. In cells coexpressing wild-type (wt) or variant HNs with wt F, the fusion promotion capacity of HN was distinguished from target cell binding by measuring changes with time in the amounts of target RBC that were (i) reversibly bound by HN-receptor interaction (released only upon the addition of 4-GU-DANA), (ii) released by HN's neuraminidase, and (iii) irreversibly bound by F-insertion or fusion (F triggered). For wt HN, lowering the pH (to approach the optimum for HPIV3 neuraminidase) decreased F triggering via release of HN from its receptor. An HN variant with increased receptor binding avidity had F-triggering efficiency like that of wt HN at pH 8.0, but this efficiency was not decreased by lowering the pH to 5.7, which suggested that the variant HN's higher receptor binding activity counterbalanced the receptor dissociation promoted by increased neuraminidase activity. To dissect the specific contribution of neuraminidase to triggering, two variant HNs that are triggering-defective due to a mutation in the HN stalk were evaluated. One of these variants has, in addition, a mutation in the globular head that renders it neuraminidase dead, while the HN with the stalk mutation alone has 30% of wt neuraminidase. While the variant without neuraminidase activity triggered F effectively at 37 degrees C irrespective of pH, the variant possessing effective neuraminidase activity completely failed to activate F at pH 5.7 and was capable of only minimal triggering activity even at pH 8.0. These results demonstrate that neuraminidase activity impacts the extent of HPIV3-mediated fusion by releasing HN from contact with receptor. Any particular HN's competence to promote F-mediated fusion depends on the balance between its inherent F-triggering efficacy and its receptor-attachment regulatory functions (binding and receptor cleavage).

Virosome and ISCOM vaccines against Newcastle disease: preparation, characterization and immunogenicity

The purpose of this study was to prepare and characterize virosomes and ISCOMs containing envelope proteins of Newcastle disease virus (NDV) and to evaluate their immunogenicity in target animals (chickens). Virosomes were prepared by solubilization of virus with either Triton X-100 or octyl glucoside (OG) followed by detergent removal. Biochemical analysis revealed that these virosomes contained both the haemagglutinin-neuraminidase protein (HN) and the fusion protein (F), with preserved biological activity. Acidic environment triggered the fusion between virosomes and chicken erythrocyte ghosts. Formation of ISCOMs was achieved by solubilizing phospholipids, cholesterol, envelope protein antigen and Quil A in Triton X-100. The ISCOM particles were formed by removal of the detergent. In each formulation the relative HN content correlated with the capability to agglutinate red blood cells. The immunogenicity of these lipid-based subunit vaccines was determined in chickens after subcutaneous immunization. The relative HN content of the subunit vaccines correlated with the haemagglutination-inhibition (HI) antibody titres. Virosomes prepared with Triton X-100 and ISCOMs offered high clinical protection (> 80%) upon challenge with virulent NDV. Virosomes prepared with OG yielded lower clinical protection despite high HI antibody titres. Virosomes with reduced antigen density showed poor immunogenicity and protection. In conclusion, ND virosomes and ISCOMs were found to be immunogenic and provided good protection.

Gangliosides and N-glycoproteins function as Newcastle disease virus receptors

The interaction of enveloped viruses with cell surface receptors is the first step in the viral cycle and an important determinant of viral host range. Although it is established that the paramyxovirus Newcastle Disease Virus binds to sialic acid-containing glycoconjugates the exact nature of the receptors has not yet been determined. Accordingly, here we attempted to characterize the cellular receptors for Newcastle disease virus. Treatment of cells with tunicamycin, an inhibitor of protein N-glycosylation, blocked fusion and infectivity, while the inhibitor of O-glycosylation benzyl-N-acetyl-alpha-D-galactosamide had no effect. Additionally, the inhibitor of glycolipid biosynthesis 1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol blocked viral fusion and infectivity. These results suggest that N-linked glycoproteins and glycolipids would be involved in viral entry but not O-linked glycoproteins. The ganglioside content of COS-7 cells was analyzed showing that GD1a was the major ganglioside component; the presence of GM1, GM2 and GM3 was also established. In a thin-layer chromatographic binding assay, we analyzed the binding of the virus to different gangliosides, detecting the interaction with monosialogangliosides such as GM3, GM2 and GM1; disialogangliosides such as GD1a and GD1b, and trisialogangliosides such as GT1b. Unlike with other viruses, our results seem to point to the absence of a specific pattern of gangliosides that interact with Newcastle disease virus. In conclusion, our results suggest that Newcastle disease virus requires different sialic acid-containing compounds, gangliosides and glycoproteins for entry into the target cell. We propose that gangliosides would act as primary receptors while N-linked glycoproteins would function as the second receptor critical for viral entry.

Conformational changes of Newcastle disease virus envelope glycoproteins triggered by gangliosides

We have investigated the conformational changes of Newcastle disease virus (NDV) glycoproteins in response to receptor binding, using 1,1-bis(4-anilino)naphthalene-5,5-disulfonic acid (bis-ANS) as a hydrophobicity-sensitive probe. Temperature- and pH-dependent conformational changes were detected in the presence of free bovine gangliosides. The fluorescence of bis-ANS was maximal at pH 5. The binding of bis-ANS to NDV was not affected by chemicals that denature the fusion glycoprotein, such as reducing agents, nor by the presence of neuraminidase inhibitors such as N-acetyl neuramicic acid. Gangliosides partially inhibited fusion and hemadsorption, but not neuraminidase hemagglutinin-neuraminidase glycoprotein (HN) activity. A conformational intermediate of HN, triggered by the presence of gangliosides acting as receptor mimics, was detected. Our results indicate that, upon binding to free gangliosides, HN undergoes a certain conformational change that does not affect the fusion glycoprotein.

Activation of fusion by the SER virus F protein: a low-pH-dependent paramyxovirus entry process

SER virus, a paramyxovirus closely related to simian virus 5, induces no syncytium formation. The SER virus F protein has a long cytoplasmic tail (CT), and truncation or mutations of the CT result in enhanced syncytium formation (S. Seth, A. Vincent, and R. W. Compans, J. Virol. 77:167-178, 2003; S. Tong, M. Li, A. Vincent, R. W. Compans, E. Fritsch, R. Beier, C. Klenk, M. Ohuchi, and H.-D. Klenk, Virology 301:322-333, 2002). We hypothesized that the presence of the long CT serves to stabilize the metastable conformation of the F protein. We observed that the hemifusion, cytoplasmic content mixing, and syncytium formation ability of the wild-type SER virus F coexpressed with the SER virus hemagglutinin-neuraminidase (HN) protein was enhanced, both qualitatively and quantitatively, at elevated temperatures. We also observed enhanced hemifusion, content mixing, and syncytium formation in SER virus F- and HN-expressing cells at reduced pH conditions ranging between 4.8 and 6.2. We have obtained evidence that in contrast to other paramyxoviruses, entry of SER virus into cells occurs by a low-pH-dependent process, indicating that the conversion to the fusion-active state for SER virus F is triggered by exposure to reduced pH.

Mode of action of two inhibitory peptides from heptad repeat domains of the fusion protein of Newcastle disease virus

Peptides derived from heptad repeat (HR) sequences of viral fusion proteins from several enveloped viruses have been shown to inhibit virus-mediated membrane fusion but the mechanism remains unknown. To further investigate this, the inhibition mechanism of two HR-derived peptides from the fusion protein of the paramyxovirus Newcastle disease virus (NDV) was investigated. Peptide N24 (residues 145-168) derived from HR1 was found to be 145-fold more inhibitory in a syncytium assay than peptide C24 (residues 474-496), derived from HR2. Both peptides failed to block lipid-mixing between R18-labeled virus and cells. None of the peptides interfered with the binding of hemagglutinin-neuraminidase (HN) protein to the target cells, as demonstrated by hemagglutining assays. When both peptides were mixed at equimolar concentrations, their inhibitory effect was abolished. In addition, both peptides induced the aggregation of negatively charged and zwitterionic phospholipid membranes. The ability of the peptides to interact with each other in solution suggests that these peptides may bind to the opposite HR region on the protein whereas their ability to interact with membranes as well as their failure to block lipid transfer suggest a second binding site. Taken together these results, suggest a mode of action for C24 and N24 in which both peptides have two different targets on the F protein: the opposite HR sequence and their corresponding domains.

Fusogenic activity of reconstituted newcastle disease virus envelopes: a role for the hemagglutinin-neuraminidase protein in the fusion process

Enveloped viruses, such as newcastle disease virus (NDV), make their entry into the host cell by membrane fusion. In the case of NDV, the fusion step requires both transmembrane hemagglutinin-neuraminidase (HN) and fusion (F) viral envelope glycoproteins. The HN protein should show fusion promotion activity. To date, the nature of HN-F interactions is a controversial issue. In this work, we aim to clarify the role of the HN glycoprotein in the membrane fusion step. Four types of reconstituted detergent-free NDV envelopes were used, on differing in their envelope protein contents. Fusion of the different virosomes and erythrocyte ghosts was monitored using the octadecyl rhodamine B chloride assay. Only the reconstituted envelopes having the F protein, even in the absence of HN protein, displayed residual fusion activity. Treatment of such virosomes with denaturing agents affecting the F protein abolished fusion, indicating that the fusion detected was viral protein-dependent. Interestingly, the rate of fusion in the reconstituted systems was similar to that of intact viruses in the presence of the inhibitor of HN sialidase activity 2,3-dehydro-2-deoxy-N-acetylneuraminic acid. The results show that the residual fusion activity detected in the reconstituted systems was exclusively due to F protein activity, with no contribution from the fusion promotion activity of HN protein.