The HN protein of Newcastle disease virus induces cell apoptosis through the induction of lysosomal membrane permeabilization

Lysosomes are acidic organelles that mediate the degradation and recycling of cellular waste materials. Damage to lysosomes can cause lysosomal membrane permeabilization (LMP) and trigger different types of cell death, including apoptosis. Newcastle disease virus (NDV) can naturally infect most birds. Additionally, it serves as a promising oncolytic virus known for its effective infection of tumor cells and induction of intensive apoptotic responses. However, the involvement of lysosomes in NDV-induced apoptosis remains poorly understood. Here, we demonstrate that NDV infection profoundly triggers LMP, leading to the translocation of cathepsin B and D and subsequent mitochondria-dependent apoptosis in various tumor and avian cells. Notably, the released cathepsin B and D exacerbate NDV-induced LMP by inducing the generation of reactive oxygen species. Additionally, we uncover that the viral Hemagglutinin neuraminidase (HN) protein induces the deglycosylation and degradation of lysosome-associated membrane protein 1 (LAMP1) and LAMP2 dependent on its sialidase activity, which finally contributes to NDV-induced LMP and cellular apoptosis. Overall, our findings elucidate the role of LMP in NDV-induced cell apoptosis and provide novel insights into the function of HN during NDV-induced LMP, which provide innovative approaches for the development of NDV-based oncolytic agents.

The artificial amino acid change in the sialic acid-binding domain of the hemagglutinin neuraminidase of newcastle disease virus increases its specificity to HCT 116 colorectal cancer cells and tumor suppression effect

BACKGROUND

Oncolytic viruses are being studied and developed as novel cancer treatments. Using directed evolution technology, structural modification of the viral surface protein increases the specificity of the oncolytic virus for a particular cancer cell. Newcastle disease virus (NDV) does not show specificity for certain types of cancer cells during infection; therefore, it has low cancer cell specificity. Hemagglutinin is an NDV receptor-binding protein on the cell surface that determines host cell tropism. NDV selectivity for specific cancer cells can be increased by artificial amino acid changes in hemagglutinin neuraminidase HN proteins via directed evolution, leading to improved therapeutic effects.

METHODS

Sialic acid-binding sites (H domains) of the HN protein mutant library were generated using error-prone PCR. Variants of the H domain protein were screened by enzyme-linked immunosorbent assay using HCT 116 cancer cell surface molecules. The mutant S519G H domain protein showed the highest affinity for the surface protein of HCT 116 cells compared to that of different types of cancer cells. This showed that the S519G mutant H domain protein gene replaced the same part of the original HN protein gene, and S519G mutant recombinant NDV (rNDV) was constructed and recovered. S519G rNDV cancer cell killing effects were tested using the MTT assay with various cancer cell types, and the tumor suppression effect of the S519G mutant rNDV was tested in a xenograft mouse model implanted with cancer cells, including HCT 116 cells.

RESULTS

S519G rNDV showed increased specificity and enhanced killing ability of HCT 116 cells among various cancer cells and a stronger suppressive effect on tumor growth than the original recombinant NDV. Directed evolution using an artificial amino acid change in the NDV HN (S519G mutant) protein increased its specificity and oncolytic effect in colorectal cancer without changing its virulence.

CONCLUSION

These results provide a new methodology for the use of directed evolution technology for more effective oncolytic virus development.

Direct interaction of the molecular chaperone GRP78/BiP with the Newcastle disease virus hemagglutinin-neuraminidase protein plays a vital role in viral attachment to and infection of culture cells

Introduction

Glucose Regulated Proteins/Binding protein (GRP78/Bip), a representative molecular chaperone, effectively influences and actively participates in the replication processes of many viruses. Little is known, however, about the functional involvement of GRP78 in the replication of Newcastle disease virus (NDV) and the underlying mechanisms.

Methods

The method of this study are to establish protein interactomes between host cell proteins and the NDV Hemagglutinin-neuraminidase (HN) protein, and to systematically investigate the regulatory role of the GRP78-HN protein interaction during the NDV replication cycle.

Results

Our study revealed that GRP78 is upregulated during NDV infection, and its direct interaction with HN is mediated by the N-terminal 326 amino acid region. Knockdown of GRP78 by small interfering RNAs (siRNAs) significantly suppressed NDV infection and replication. Conversely, overexpression of GRP78 resulted in a significant increase in NDV replication, demonstrating its role as a positive regulator in the NDV replication cycle. We further showed that the direct interaction between GRP78 and HN protein enhanced the attachment of NDV to cells, and masking of GRP78 expressed on the cell surface with specific polyclonal antibodies (pAbs) inhibited NDV attachment and replication.

Discussion

These findings highlight the essential role of GRP78 in the adsorption stage during the NDV infection cycle, and, importantly, identify the critical domain required for GRP78-HN interaction, providing novel insights into the molecular mechanisms involved in NDV replication and infection.

C-terminal truncation of the hemagglutinin-neuraminidase (HN) protein enhances the virulence and immunogenicity of Newcastle disease virus (NDV) vaccine strain V4

The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) is a multifunctional protein with receptor recognition ability that plays an important role in the infection of cells by NDV. An alignment of NDV HN protein sequences of different genotypes showed that vaccine strains of NDV, such as the LaSota strain, generally have an HN protein of 577 amino acids. In comparison, the HN protein of the V4 strain has 616 amino acids, with 39 more amino acids at the C-terminus. In this study, we generated a recombinant NDV (rNDV) with a 39-amino-acid truncation at the HN C-terminus based on the full-length cDNA clone of the V4 strain. This rNDV, named rV4-HN-tr, displayed thermostability similar to that of the parental V4 strain. However, growth kinetics and pathogenicity analysis suggested that rV4-HN-tr is more virulent than the V4 strain. Notably, the C-terminus of HN affected the ability of the virus to adsorb onto cells. Structural predictions further suggested that the C-terminus of HN may obstruct the sialic acid binding site. Immunization of chickens with rV4-HN-tr induced a 3.5-fold higher level of NDV-specific antibodies than that obtained with the V4 strain and provided 100% immune protection against NDV challenge. Our study suggests that rV4-HN-tr is a thermostable, safe, and highly efficient vaccine candidate against Newcastle disease.

Molecular Evolution of Human Parainfluenza Virus Type 2 Based on Hemagglutinin-Neuraminidase Gene

To understand the molecular evolution of human parainfluenza virus type 2 (HPIV2), 21 Hemagglutinin-Neuraminidase (HN) gene sequences covering seven Chinese provinces in 2011 and 2017 to 2021 were combined with 90 published HN sequences worldwide for phylogenetic analysis. The result showed that global HPIV2 could be classified into two distinct clusters (I and II), five lineages (IA to IIE), and four sublineages (IB1 and 2, and IIE1 and 2). The minimum genetic distances between different clusters and lineages were 0.049 and 0.014, respectively. In the last decade, one lineage (IID) and three sublineages (IB1, IB2, and IIE1) have been cocirculating in China, with the sublineages IB2 and IIE1 dominating, while sublineages IB1 and IIE1 are dominant globally. In addition, the spread of HPIV2 had relative spatial clustering, and sublineage IB2 has only been detected in China thus far. The overall evolution rate of HPIV2 was relatively low, on the order of 10-4 substitutions/site/year, except for sublineage IB2 at 10-3 substitutions/site/year. Furthermore, human-animal transmission was observed, suggesting that the HPIV2 might have jumped out of animal reservoirs in approximately 1922, the predicted time of a common ancestor. The entire HN protein was under purifying/negative selection, and the specific amino acid changes and two novel N-glycosylation sites (N316 and N517) in sublineages IB1, IB2, and IIE1 were mostly located in the globular head region of the HN protein. In this study, preliminary evolutionary characteristics of HPIV2 based on the HN gene were obtained, increasing the recognition of the evolution and adaptation of HPIV2. IMPORTANCE The phylogenetic analysis showed that global HPIV2 could be classified into two distinct clusters (I and II) and five lineages (IA to IIE) with at least 0.049 and 0.014 genetic distances between clusters and lineages, respectively. Furthermore, lineages IB and IIE could be further divided into two sublineages (IB1-2 and IIE1-2). All China sequences belong to one lineage and three sublineages (IB1, IB2, IID, and IIE1), among which sublineages IB2 and IIE1 are predominant and cocirculating in China, while sublineages IB1 and IIE1 are dominant globally. The overall evolution rate of HPIV2 is on the order of 10-4 substitutions/site/year, with the highest rate of 2.18 × 10-3 for sublineage IB2. The entire HN protein is under purifying/negative selection, and the specific amino acid substitutions and two novel N-glycosylation sites (N316 and N517) in sublineages IB1, IB2, and IIE1 are mostly located in the globular head region of the HN protein.

Kinetic analysis of paramyxovirus-sialoglycan receptor interactions reveals virion motility

Many viruses initiate infection by binding to sialoglycan receptors at the cell surface. Binding to such receptors comes at a cost, however, as the sheer abundance of sialoglycans e.g. in mucus, may immobilize virions to non-functional decoy receptors. As a solution, sialoglycan-binding as well as sialoglycan-cleavage activities are often present in these viruses, which for paramyxoviruses are combined in the hemagglutinin-neuraminidase (HN) protein. The dynamic interactions of sialoglycan-binding paramyxoviruses with their receptors are thought to be key determinants of species tropism, replication and pathogenesis. Here we used biolayer interferometry to perform kinetic analyses of receptor interactions of animal and human paramyxoviruses (Newcastle disease virus, Sendai virus, and human parainfluenza virus 3). We show that these viruses display strikingly different receptor interaction dynamics, which correlated with their receptor-binding and -cleavage activities and the presence of a second sialic acid binding site. Virion binding was followed by sialidase-driven release, during which virions cleaved sialoglycans until a virus-specific density was reached, which was largely independent of virion concentration. Sialidase-driven virion release was furthermore shown to be a cooperative process and to be affected by pH. We propose that paramyxoviruses display sialidase-driven virion motility on a receptor-coated surface, until a threshold receptor density is reached at which virions start to dissociate. Similar motility has previously been observed for influenza viruses and is likely to also apply to sialoglycan-interacting embecoviruses. Analysis of the balance between receptor-binding and -cleavage increases our understanding of host species tropism determinants and zoonotic potential of viruses.

Subnanometer structure of an enveloped virus fusion complex on viral surface reveals new entry mechanisms

Paramyxoviruses-including important pathogens like parainfluenza, measles, and Nipah viruses-use a receptor binding protein [hemagglutinin-neuraminidase (HN) for parainfluenza] and a fusion protein (F), acting in a complex, to enter cells. We use cryo-electron tomography to visualize the fusion complex of human parainfluenza virus 3 (HN/F) on the surface of authentic clinical viruses at a subnanometer resolution sufficient to answer mechanistic questions. An HN loop inserts in a pocket on F, showing how the fusion complex remains in a ready but quiescent state until activation. The globular HN heads are rotated with respect to each other: one downward to contact F, and the other upward to grapple cellular receptors, demonstrating how HN/F performs distinct steps before F activation. This depiction of viral fusion illuminates potentially druggable targets for paramyxoviruses and sheds light on fusion processes that underpin wide-ranging biological processes but have not been visualized in situ or at the present resolution.

α2,3-Linked Sialic Acids Are the Potential Attachment Receptor for Shaan Virus Infection in MARC-145 Cells

Shaan virus (ShaV), a novel species of the genus Jeilongvirus, family Paramyxoviridae, was isolated from an insectivore bat (Miniopterus schreibersii) in Korea in 2016. ShaV particles contain a hemagglutinin-neuraminidase (HN) glycoprotein in their envelope that allows the virus to target cells. Typically, diverse paramyxoviruses with HN glycoprotein are reported to interact predominantly with sialic acids, but there are no studies of receptors for ShaV. In this study, the identification of potential receptors for ShaV was demonstrated using sialidase treatments, glycan microarray, magnetic bead-based virus binding assay, and neuraminidase inhibitor treatments. Pretreatment of MARC-145 cells with sialidase, which cleaves α2,3-linked sialic acids, showed higher inhibition of viral infection than α2,6-linked-specific sialidase. These data were supported by the binding of ShaV to predominantly α2,3-linked sialylated glycans in the screening of sialyl linkage patterns through glycan microarray. To further confirm the direct interaction between ShaV and α2,3-linked sialic acids, ShaV was incubated with α2,3- or α2,6-linked sialylated glycans conjugated to magnetic beads, and binding signals were detected only for α2,3-linked sialylated glycans. In addition, the potential of sialic acids as a receptor was demonstrated by the viral replication inhibitory effect of the neuraminidase inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminicacid (DANA) in the mature virion release steps. Collectively, these results support that α2,3-linked sialic acids are the potential receptor for ShaV infection in MARC-145 cells. IMPORTANCE Bats host major mammalian paramyxoviruses, and novel paramyxoviruses are increasingly being reported around the world. Shaan virus (ShaV), from the genus Jeilongvirus, family Paramyxoviridae, has a potential attachment glycoprotein, HN. Here, we identify that ShaV binds to sialic acid and demonstrate that α2,3-linked sialic acids are the potential receptor for ShaV infection. The presented data regarding ShaV receptor specificity will enable studies into the viral tropism for the host and contribute to the development of new antiviral strategies targeting viral receptors.

Quantitative regulation of the thermal stability of enveloped virus vaccines by surface charge engineering to prevent the self-aggregation of attachment glycoproteins

The development of thermostable vaccines can relieve the bottleneck of existing vaccines caused by thermal instability and subsequent poor efficacy, which is one of the predominant reasons for the millions of deaths caused by vaccine-preventable diseases. Research into the mechanism of viral thermostability may provide strategies for developing thermostable vaccines. Using Newcastle disease virus (NDV) as model, we identified the negative surface charge of attachment glycoprotein as a novel determinant of viral thermostability. It prevented the temperature-induced aggregation of glycoprotein and subsequent detachment from virion surface. Then structural stability of virion surface was improved and virus could bind to and infect cells efficiently after heat-treatment. Employing the approach of surface charge engineering, thermal stability of NDV and influenza A virus (IAV) vaccines was successfully improved. The increase in the level of vaccine thermal stability was determined by the value-added in the negative surface charge of the attachment glycoprotein. The engineered live and inactivated vaccines could be used efficiently after storage at 37°C for at least 10 and 60 days, respectively. Thus, our results revealed a novel surface-charge-mediated link between HN protein and NDV thermostability, which could be used to design thermal stable NDV and IAV vaccines rationally.

[Expression of NDV HN protein in rice and development of a semi-quantitative rapid method for detection of antibodies]

The aim of this study was to develop a semi-quantitative immunochromatographic method for rapid detection of Newcastle disease virus (NDV) antibodies by expressing HN protein in rice endosperm bioreactor. The recombinant plasmid pUC57-HN was digested by MlyⅠ and XhoⅠ to retrieve the HN gene, while the intermediate vector pMP3 containing promoter, signal peptide and terminator was digested by NaeⅠ and XhoⅠ. The HN gene and the linearized pMP3 were purified and ligated to form a recombinant plasmid pMP3-HN1. Subsequently, pMP3-HN1 and plant vector pCAMBIA1300 were digested by EcoRⅠ and Hind Ⅲ, and the HN1 gene was cloned into pCAMBIA1300. The recombinant plasmid pCAMBIA1300-HN1 was introduced into Agrobacterium tumefaciens EHA105 by electrotransformation, and the pCAMBIA1300-HN1 was transferred into rice callus by agrobacterium-mediated method. After dark culture, callus screening, differentiation, rooting and transplanting, transgenic rice seeds were obtained 4 months later. PCR identified that the HN gene has been inserted into the rice genome. SDS-PAGE and Western blotting indicated that the HN protein was successfully expressed in the positive rice endosperm. The purity of the HN protein was more than 90% by SP cation exchange chromatography and gel filtration chromatography. According to the national standards for the diagnostic techniques of Newcastle disease HI test (HI≥4log₂, positive antibody reaction), a colloidal gold labeled purified HN protein was used to prepare a semi-quantitative test strip by double-antibody sandwich method for rapid detection of NDV antibody. The results showed that the test strip did not cross-react with positive sera against other viruses, and the sensitivity of the test strip reached 1:102 400 for standard positive sera of Newcastle disease. Testing of a total of 308 clinical sera showed that the compliance rate of the test strip with HI test was 97.08%, and the Kappa value was 0.942. In conclusion, high purity recombinant HN protein was obtained from rice endosperm, and a simple, rapid, highly sensitive and highly specific semi-quantitative immunochromatographic strip was developed. The test strip could be used for immune evaluation of the Newcastle disease vaccine.

Root-preferential expression of Newcastle virus glycoproteins driven by NtREL1 promoter in tobacco hairy roots and evaluation of oral delivery in mice

Newcastle disease virus (NDV) is a lethal virus in avian species with a disastrous effect on the poultry industry. NDV is enveloped by a host-derived membrane with two glycosylated haemagglutinin-neuraminidase (HN) and Fusion (F) proteins. NDV infection usually leads to death within 2-6 days, so the preexisting antibodies provide the most critical protection for this infection. The HN and F glycoproteins are considered the main targets of the immune system. In the present study, two constructs harboring the HN or F epitopes are sub-cloned separately under the control of a root-specific promoter NtREL1 or CaMV35S (35S Cauliflower Mosaic Virus promoter) as a constitutive promoter. The recombinant vectors were transformed into the Agrobacterium tumefaciens strain LBA4404 and then introduced to tobacco (Nicotiana tabacum L.) leaf disk explants. PCR with specific primers was performed to confirm the presence of the hn and f genes in the genome of the regenerated plants. Then, the positive lines were transformed via non-recombinant A. rhizogenes (strain ATCC15834) to develop hairy roots.HN and F were expressed at 0.37% and 0.33% of TSP using the CaMV35S promoter and at 0.75% and 0.54% of TSP using the NtREL1 promoter, respectively. Furthermore, the mice fed transgenic hairy roots showed a high level of antibody responses (IgG and IgA) against rHN and rF proteins.

Single-virus assay reveals membrane determinants and mechanistic features of Sendai virus binding

Sendai virus (SeV, formally murine respirovirus) is a membrane-enveloped, negative-sense RNA virus in the Paramyxoviridae family and is closely related to human parainfluenza viruses. SeV has long been utilized as a model paramyxovirus and has recently gained attention as a viral vector candidate for both laboratory and clinical applications. To infect host cells, SeV must first bind to sialic acid glycolipid or glycoprotein receptors on the host cell surface via its hemagglutinin-neuraminidase (HN) protein. Receptor binding induces a conformational change in HN, which allosterically triggers the viral fusion (F) protein to catalyze membrane fusion. While it is known that SeV binds to α2,3-linked sialic acid receptors, and there has been some study into the chemical requirements of those receptors, key mechanistic features of SeV binding remain unknown, in part because traditional approaches often convolve binding and fusion. Here, we develop and employ a fluorescence microscopy-based assay to observe SeV binding to supported lipid bilayers (SLBs) at the single-particle level, which easily disentangles binding from fusion. Using this assay, we investigate mechanistic questions of SeV binding. We identify chemical structural features of ganglioside receptors that influence viral binding and demonstrate that binding is cooperative with respect to receptor density. We measure the characteristic decay time of unbinding and provide evidence supporting a "rolling" mechanism of viral mobility following receptor binding. We also study the dependence of binding on target cholesterol concentration. Interestingly, we find that although SeV binding shows striking parallels in cooperative binding with a prior report of Influenza A virus, it does not demonstrate a similar sensitivity to cholesterol concentration and receptor nanocluster formation.

Matrix metalloproteinase-14 regulates collagen degradation and migration of mononuclear cells during infection with genotype VII Newcastle disease virus

Upregulation of matrix metalloproteinase (MMP)-14, a major driven force of extracellular-matrix (ECM) remodelling and cell migration, correlates with ECM breakdown and pathologic manifestation of genotype VII Newcastle disease virus (NDV) in chickens. However, the functional relevance between MMP-14 and pathogenesis of genotype VII NDV remains to be investigated. In this study, expression, biofunction and regulation of MMP-14 induced by genotype VII NDV were analysed in chicken peripheral blood mononuclear cells (PBMCs). The results showed that JS5/05 significantly increased expression and membrane accumulation of MMP-14 in PBMCs, correlating to enhanced collagen degradation and cell migration. Specific MMP-14 inhibition significantly impaired collagen degradation and migration of JS5/05-infected cells, suggesting dependence of these features on MMP-14. In addition, MMP-14 upregulation correlated with activation of the extracellular signal-regulated kinase (ERK) pathway upon JS5/05 infection, and blockage of the ERK signalling significantly suppressed MMP-14-mediated collagen degradation and migration of JS5/05-infected cells. Using a panel of chimeric NDVs derived from gene exchange between genotype VII and IV NDV, the fusion and haemagglutinin-neuraminidase genes were identified as the major viral determinants for MMP-14 expression and activity. In conclusion, MMP-14 was defined as a critical regulator of collagen degradation and cell migration of chicken PBMCs infected with genotype VII NDV, which may contribute to pathology of the virus. Our findings add novel information to the body of knowledge regarding virus-host biology and NDV pathogenesis.

Human parainfluenza virus fusion complex glycoproteins imaged in action on authentic viral surfaces

Infection by human parainfluenza viruses (HPIVs) causes widespread lower respiratory diseases, including croup, bronchiolitis, and pneumonia, and there are no vaccines or effective treatments for these viruses. HPIV3 is a member of the Respirovirus species of the Paramyxoviridae family. These viruses are pleomorphic, enveloped viruses with genomes composed of single-stranded negative-sense RNA. During viral entry, the first step of infection, the viral fusion complex, comprised of the receptor-binding glycoprotein hemagglutinin-neuraminidase (HN) and the fusion glycoprotein (F), mediates fusion upon receptor binding. The HPIV3 transmembrane protein HN, like the receptor-binding proteins of other related viruses that enter host cells using membrane fusion, binds to a receptor molecule on the host cell plasma membrane, which triggers the F glycoprotein to undergo major conformational rearrangements, promoting viral entry. Subsequent fusion of the viral and host membranes allows delivery of the viral genetic material into the host cell. The intermediate states in viral entry are transient and thermodynamically unstable, making it impossible to understand these transitions using standard methods, yet understanding these transition states is important for expanding our knowledge of the viral entry process. In this study, we use cryo-electron tomography (cryo-ET) to dissect the stepwise process by which the receptor-binding protein triggers F-mediated fusion, when forming a complex with receptor-bearing membranes. Using an on-grid antibody capture method that facilitates examination of fresh, biologically active strains of virus directly from supernatant fluids and a series of biological tools that permit the capture of intermediate states in the fusion process, we visualize the series of events that occur when a pristine, authentic viral particle interacts with target receptors and proceeds from the viral entry steps of receptor engagement to membrane fusion.

Lysosome-Associated Membrane Proteins Support the Furin-Mediated Processing of the Mumps Virus Fusion Protein

Mumps virus (MuV), an enveloped RNA virus of the Paramyxoviridae family and the causative agent of mumps, affects the salivary glands and other glandular tissues as well as the central nervous system. The virus enters the cell by inducing the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by MuV envelope proteins: the hemagglutinin-neuraminidase and fusion (F) protein. Cleavage of the MuV F protein (MuV-F) into two subunits by the cellular protease furin is a prerequisite for fusion and virus infectivity. Here, we show that 293T (a derivative of HEK293) cells do not produce syncytia upon expression of MuV envelope proteins or MuV infection. This failure is caused by the inefficient MuV-F cleavage despite the presence of functional furin in 293T cells. An expression cloning strategy revealed that overexpression of lysosome-associated membrane proteins (LAMPs) confers on 293T cells the ability to produce syncytia upon expression of MuV envelope proteins. The LAMP family comprises the ubiquitously expressed LAMP1 and LAMP2, the interferon-stimulated gene product LAMP3, and the cell type-specific proteins. The expression level of the LAMP3 gene, but not of LAMP1 and LAMP2 genes, differed markedly between 293T and HEK293 cells. Overexpression of LAMP1, LAMP2, or LAMP3 allowed 293T cells to process MuV-F efficiently. Furthermore, these LAMPs were found to interact with both MuV-F and furin. Our results indicate that LAMPs support the furin-mediated cleavage of MuV-F and that, among them, LAMP3 may be critical for the process, at least in certain cells.IMPORTANCE The cellular protease furin mediates proteolytic cleavage of many host and pathogen proteins and plays an important role in viral envelope glycoprotein maturation. MuV, an enveloped RNA virus of the Paramyxoviridae family and an important human pathogen, enters the cell through the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by the viral attachment protein and the F protein. Cleavage of MuV-F into two subunits by furin is a prerequisite for fusion and virus infectivity. Here, we show that LAMPs support the furin-mediated cleavage of MuV-F. Expression levels of LAMPs affect the processing of MuV-F and MuV-mediated membrane fusion. Among LAMPs, the interferon-stimulated gene product LAMP3 is most critical in certain cells. Our study provides potential targets for anti-MuV therapeutics.

Molecular Mechanism of the Flexible Glycan Receptor Recognition by Mumps Virus

Mumps virus (MuV) is an important aerosol-transmitted human pathogen causing epidemic parotitis, meningitis, encephalitis, and deafness. MuV preferentially uses a trisaccharide containing α2,3-linked sialic acid as a receptor. However, given the MuV tropism toward glandular tissues and the central nervous system, an additional glycan motif(s) may also serve as a receptor. Here, we performed a large-scale glycan array screen with MuV hemagglutinin-neuraminidase (MuV-HN) attachment proteins by using 600 types of glycans from The Consortium for Functional Glycomics Protein-Glycan Interaction Core in an effort to find new glycan receptor motif(s). According to the results of the glycan array, we successfully determined the crystal structures of MuV-HN proteins bound to newly identified glycan motifs, sialyl LewisX (SLeX) and the oligosaccharide portion of the GM2 ganglioside (GM2-glycan). Interestingly, the complex structures showed that SLeX and GM2-glycan share the same configuration with the reported trisaccharide motif, 3'-sialyllactose (3'-SL), at the binding site of MuV-HN, while SLeX and GM2-glycan have several unique interactions compared with those of 3'-SL. Thus, MuV-HN protein can allow an additional spatial modification in GM2-glycan and SLeX at the second and third carbohydrates from the nonreducing terminus of the core trisaccharide structure, respectively. Importantly, MuV entry was efficiently inhibited in the presence of 3'-SL, SLeX, or GM2-glycan derivatives, which indicates that these motifs can serve as MuV receptors. The α2,3-sialylated oligosaccharides, such as SLeX and 3'-sialyllactosamine, are broadly expressed in various tissues, and GM2 exists mainly in neural tissues and the adrenal gland. The distribution of these glycan motifs in human tissues/organs may have bearing on MuV tropism.IMPORTANCE Mumps virus (MuV) infection is characterized by parotid gland swelling and can cause pancreatitis, orchitis, meningitis, and encephalitis. MuV-related hearing loss is also a serious complication because it is usually irreversible. MuV outbreaks have been reported in many countries, even in high-vaccine-coverage areas. MuV has tropism toward glandular tissues and the central nervous system. To understand the unique MuV tropism, revealing the mechanism of receptor recognition by MuV is very important. Here, using a large-scale glycan array and X-ray crystallography, we show that MuV recognizes sialyl LewisX and GM2 ganglioside as receptors, in addition to a previously reported MuV receptor, a trisaccharide containing an α2,3-linked sialic acid. The flexible recognition of these glycan receptors by MuV may explain the unique tropism and pathogenesis of MuV. Structures will also provide a template for the development of effective entry inhibitors targeting the receptor-binding site of MuV.

Virion-Associated Cholesterol Regulates the Infection of Human Parainfluenza Virus Type 3

The matrix (M) proteins of paramyxoviruses bind to the nucleocapsids and cytoplasmic tails of glycoproteins, thus mediating the assembly and budding of virions. We first determined the budding characterization of the HPIV3 Fusion (F) protein to investigate the assembly mechanism of human parainfluenza virus type 3 (HPIV3). Our results show that expression of the HPIV3 F protein alone is sufficient to initiate the release of virus-like particles (VLPs), and the F protein can regulate the VLP-forming ability of the M protein. Furthermore, HPIV3_F-Flag, which is a recombinant HPIV3 with a Flag tag at the C-terminus of the F protein, was constructed and recovered. We found that the M, F, and hemagglutinin-neuraminidase (HN) proteins and the viral genome can accumulate in lipid rafts in HPIV3_F-Flag-infected cells, and the F protein mainly exists in the form of F₁ in VLPs, lipid rafts, and purified virions. Furthermore, the function of cholesterol in the viral envelope and cell membrane was assessed via the elimination of cholesterol by methyl-β-cyclodextrin (MβCD). Our results suggest that the infectivity of HPIV3 was markedly reduced, due to defective internalization ability in the absence of cholesterol. These results reveal that HPIV3 might assemble in the lipid rafts to acquire cholesterol for the envelope of HPIV3, which suggests the that disruption of the cholesterol composition of HPIV3 virions might be a useful method for the design of anti-HPIV3 therapy.

Enhanced growth of influenza A virus by coinfection with human parainfluenza virus type 2

It has been reported that dual or multiple viruses can coinfect epithelial cells of the respiratory tract. However, little has been reported on in vitro interactions of coinfected viruses. To explore how coinfection of different viruses affects their biological property, we examined growth of influenza A virus (IAV) and human parainfluenza virus type 2 (hPIV2) during coinfection of Vero cells. We found that IAV growth was enhanced by coinfection with hPIV2. The enhanced growth of IAV was not reproduced by coinfection with an hPIV2 mutant with reduced cell fusion activity, or by ectopic expression of the V protein of hPIV2. In contrast, induction of cell fusion by ectopic expression of the hPIV2 HN and F proteins augments IAV growth. hPIV2 coinfection supported IAV growth in cells originated from the respiratory epithelium. The enhancement correlated closely with cell fusion ability of hPIV2 in those cells. These results indicate that cell fusion induced by hPIV2 infection is beneficial to IAV replication and that enhanced viral replication by coinfection with different viruses can modify their pathological consequences.

Mutations in the DI-DII Linker of Human Parainfluenza Virus Type 3 Fusion Protein Result in Diminished Fusion Activity

Human parainfluenza virus type 3 (HPIV3) can cause severe respiratory tract diseases in infants and young children, but no licensed vaccines or antiviral agents are currently available for treatment. Fusing the viral and target cell membranes is a prerequisite for its entry into host cells and is directly mediated by the fusion (F) protein. Although several domains of F are known to have important effects on regulating the membrane fusion activity, the roles of the DI-DII linker (residues 369–374) of the HPIV3 F protein in the fusogenicity still remains ill-defined. To facilitate our understanding of the role of this domain might play in F-induced cell-cell fusion, nine single mutations were engineered into this domain by site-directed mutagenesis. A vaccinia virus-T7 RNA polymerase transient expression system was employed to express the wild-type or mutated F proteins. These mutants were analyzed for membrane fusion activity, cell surface expression, and interaction between F and HN protein. Each of the mutated F proteins in this domain has a cell surface expression level similar to that of wild-type F. All of them resulted in a significant reduction in fusogenic activity in all steps of membrane fusion. Furthermore, all these fusion-deficient mutants reduced the amount of the HN-F complexes at the cell surface. Together, the results of our work suggest that this region has an important effect on the fusogenic activity of F.

Cell-Specific Inhibition of Paramyxovirus Maturation by Proteasome Inhibitors

Effects of proteasome inhibitors on the replication of a paramyxovirus in comparison with the effects on replication of an orthomyxovirus and rhabdovirus were investigated. Treatment of Sendai virus (SeV)-infected LLC-MK2 cells with 50 microM MG132 reduced virus growth to ca. 1/10,000, and treatment with different concentrations of MG132 reduced virus growth in a dose-dependent manner. Released amounts of viral proteins were reduced in correspondence with decrease in infectivity. The inhibition of virus maturation was confirmed by an SeV-like particle formation system. Lactacystin also impaired SeV growth and zLL impaired the growth to a lesser extent, suggesting involvement of proteasomes in the restriction of virus growth. In the presence of MG132, localizations of the M protein and viral F and HN glycoproteins on the cell membrane appeared to be partly dissociated, although the viral glycoproteins were normally transported to the cell surface. These results suggest that an early step of SeV assembly was disturbed by proteasome inhibitors. The relationship of the results with ubiquitin is also discussed. SeV maturation was less susceptible and resistant to MG132 in CV1 cells and A549 cells, respectively, indicating cell specificity of the drug effect. Release of vesicular stomatitis virus also showed high susceptibility to MG132 and release of influenza virus A/WSN/33 was only mildly susceptible to the drug in LLC-MK2 cells. Effects of proteasome inhibitors on virus maturation are thus highly cell-specific and partly virus-specific.

[Study on functions of N-carbohydrate chains in human parainfluenza virus type 3 hemagglutinin-neuraminidase protein]

To determine the functions of N-carbohydrate chains in human parainfluenza virus type 3 hemagglutinin-neuraminidase(HN) protein, a PCR-based site-directed mutagenesis method was used to obtain N-glycan mutants. Protein electrophoresis rate, cell surface expression,receptor binding activity, neuraminidase activity and cell fusion promotion activity were determined. The HN proteins of single mutants (G1, G2, and G4) and multiple mutants (G12, G14, G24 and G124) migrated faster than the wild-type (wt) HN protein on polyacrylamide gels, while G3-mutated protein and wt HN protein migrated at the same position. There was no statistic difference in cell surface expression and neuraminidase activity between wt and each mutant HN protein (P>0.05), but receptor binding activity and cell fusion promotion activity of each mutant protein was reduced to significant extent (P<0.05). G1, G2 and G4 mutants exhibited re duced receptor binding activity, which was 83.94%, 76.45% and 55.32% of the wt level, respectively. G1, G2 and G4-mutated proteins also showed reductions in fusion promotion activity, which was 80.84%, 77.83% and 64.16%, respectively. Multiple mutants with G12-, G14-, G24- and G124- substitutions could further reduce receptor binding activities, 33.07%, 20.67%, 19.96% and 15.11% of the wt HN level, respectively. G12, G14, G24 and G124 mutants exhibited levels of fusion promotion activity that were only 46.360, 12.04%, 13.43% and 4.05% of the wt amount, respectively. As N-glycans of hPIV3 HN protein play an important role in receptor binding activity and cell fusion promotion activity of HN protein. We propose that the loss of N-glycans change the conformation or orientation of globular domain that is responsible for receptor binding and lower receptor binding activity and cell fusion promotion activi ty.

Structure of the ulster strain newcastle disease virus hemagglutinin-neuraminidase reveals auto-inhibitory interactions associated with low virulence

Paramyxovirus hemagglutinin-neuraminidase (HN) plays roles in viral entry and maturation, including binding to sialic acid receptors, activation of the F protein to drive membrane fusion, and enabling virion release during virus budding. HN can thereby directly influence virulence and in a subset of avirulent Newcastle disease virus (NDV) strains, such as NDV Ulster, HN must be proteolytically activated to remove a C-terminal extension not found in other NDV HN proteins. Ulster HN is 616 amino acids long and the 45 amino acid C-terminal extension present in its precursor (HN₀) form has to be cleaved to render HN biologically active. Here we show that Ulster HN contains an inter-subunit disulfide bond within the C-terminal extension at residue 596, which regulates HN activities and neuraminidase (NA) domain dimerization. We determined the crystal structure of the dimerized NA domain containing the C-terminal extension, which extends along the outside of the sialidase β-propeller domain and inserts C-terminal residues into the NA domain active site. The C-terminal extension also engages a secondary sialic acid binding site present in NDV HN proteins, which is located at the NA domain dimer interface, that most likely blocks its attachment function. These results clarify how the Ulster HN C-terminal residues lead to an auto-inhibited state of HN, the requirement for proteolytic activation of HN₀ and associated reduced virulence.

Antiviral activity of methylelaiophylin, an alpha-glucosidase inhibitor

Methylelaiophylin isolated from Streptomyces melanosporofaciens was selected as an alpha-glucosidase inhibitor with an IC50 value of 10 micrometer. It showed mixedtype inhibition of alpha-glucosidase with a Ki value of 5.94 micrometer. In addition, methylelaiophylin inhibited the intracellular trafficking of hemagglutinin-neuramidase (HN), a glycoprotein of Newcastle disease virus (NDV), in baby hamster kidney (BHK) cells. Methylelaiophylin inhibited the cell surface expression of NDV-HN glycoprotein without significantly affecting HN glycoprotein synthesis in NDV-infected BHK cells.

Sendai virus particle production: basic requirements and role of the SYWST motif present in HN cytoplasmic tail

Sendai virus (SeV) HN protein is dispensable for virus particle production. HN incorporation into virions strictly depends on a cytoplasmic domain SYWST motif. HNAFYKD, with SYWST replaced with the analogous sequence of measles virus (MeV) H (AFYKD), is not incorporated in virus particles produced by LLCMK2 cells, although it is normally expressed at the plasma membrane. Unlike HNSYWST, HNAFYKD is not internalized to late endosomes, raising the possibility that HN internalization is required for uptake into virus particles. Various mosaic MeV-H containing increasing amounts of the SeV-HN all failed to be taken up in SeV virions. However, when co-expressed with HNAFYKD these MeV-H chimera induced HNAFYKD uptake into virions showing that internalization is not a prerequisite for HN uptake into particles. We propose that HN incorporation in virus particles requires first neutralization by HN of a putative inhibitor of infectious particle formation.

Expression of hemagglutinin-neuraminidase glycoprotein of newcastle disease Virus in agroinfiltrated Nicotiana benthamiana plants

The worldwide need for producing safer and less expensive vaccines with minor manufacture and processing requirements, together with the advances made through biotechnology, has promoted the development of efficient alternative tools to conventional vaccines. One of these is the use of plants or plant cell culture as production platforms of vaccine antigens with potential use as immunogens. We have already described the use of transgenic potato plants as immunogens against Newcastle Disease Virus (NDV), although the amount of the recombinant antigen recovered was low. The main objective of the work presented here was to enhance the expression of the HN glycoprotein of NDV through a protein targeting strategy and a promoter change. We have cloned the HN coding region under the regulation of the rubisco small subunit promoter in 5 different versions in a subcellular localization strategy, and we have established that the construct harboring the complete HN gene with its own signal peptide, fused to KDEL retention peptide, rendered the best expressed/accumulated HN protein level whether a transient or a stable transformation assay was performed. We conclude that agroinfiltration results in a simple and useful tool for selecting suitable genetic constructions to be used in stable plant transformation and, moreover, it could be used as a method to produce immunogens for vaccine developments.

[Cloning and expression of the six coding genes of sendai virus BB1 strain]

Six genes for nucleoprotein, phosphoprotein, matrix protein, hemagglutinin neuramindase protein, fusion protein and large protein were obtained by reverse transcription and PCR methods based on our previous work of sequencing full length genome of sendai virus BB1 strain (DQ219803 in GenBank). Sequencing showed the six genes were completely identical to that we reported. In order to supply the function necessary for rescuing and packaging of sendai virus vector in trans, the N, P, M, F, HN and L genes were separately cloned into an adenoviral shuttle expression vector pDC316 resulting in six recombinant adenoviral plasimds. Six replicating defective recombinant adenoviruses Ad5-N, Ad5-P, Ad5-M, Ad5-F, Ad5-HN and Ad5-L were obtained by separately cotransfection of pDC316 carrying N, P, M, F, HN and L genes with the adenoviral genomic plasmid pBHGloxdeltaE1, 3Cre into HEK293cells. Restrictive enzymatic results indicated that the six recombinant plasmids were correctly constructed. PCR results showed the recombinant adenoviruses contained the respective SeV genes . Western blotting as well as immunofluorescence assay indicated the expression of the corresponding proteins of sendai virus. These work laid the basis for the construction of the full length genome plasmid of sendai virus BB1 strain and the setup of SeV virus vector system based on SeV BB1 strain.

[Biological characterization of three natural isolates of the porcine rubulavirus (Mexico)]

Biological characterization of three natural isolates of the porcine rubulavirus (Mexico). Porcine rubulavirus (PoRV) produces a neurological and reproductive syndrome in pigs called the blue-eye disease, known only from Mexico. Several isolates were grouped by the main symptoms presented during outbreaks: a) neurotropic in piglets, (b) broadly neurotropic in piglets and gonadotropic in adults, and (c) gonadotropic in adults. We studied some biological properties of three strains, which fall in one of each virus group: La Piedad Michoacán (LPM) and Producci6n Animal Cerdos 1 (PAC1) and 3 (PAC3), respectively. The analyzed viral properties are mainly related with the trans-membrane hemagglutinin-neuraminidase (HN) and fusion (F) proteins, such as cytopathic effect, hemolysis, hemagglutinating (HA) and neuraminidase (NA) activities. In the infection assays PAC1 strain presented the highest fusogenicity level; however, the most cytolytic strain was PAC3. In addition, HA and NA activities and viral genome of PAC3 strain was detected in supernatants during cell infection earlier than in the other two strains, which shows that PAC3 virions release from the host cell earlier than LPM and PAC1. Experimental determination in purified viruses shows that PAC3 presented a higher HA and NA activities; however, PAC1 shows other interesting properties, such as a high thermostability of HN and differences about substrate profile respect to LPM and PAC3. Our data suggest that NA activity is associated with the virulence of RVP.

Paramyxoviruses: different receptors - different mechanisms of fusion

Paramyxovirus-mediated membrane fusion usually requires an interaction between the viral-attachment and -fusion proteins. The mechanism by which this interaction regulates fusion differs between paramyxoviruses that bind to sialic acid-containing receptors and those that recognize specific proteins. The recently solved structure of the globular head of the measles virus hemagglutinin suggests that this difference might be related to the location of the receptor-binding sites on the attachment proteins of the two classes of paramyxoviruses.

Incorporation of functional HN-F glycoprotein-containing complexes into newcastle disease virus is dependent on cholesterol and membrane lipid raft integrity

Newcastle disease virus assembles in plasma membrane domains with properties of membrane lipid rafts, and disruption of these domains by cholesterol extraction with methyl-beta-cyclodextrin resulted in the release of virions with irregular protein composition, abnormal particle density, and reduced infectivity (J. P. Laliberte, L. W. McGinnes, M. E. Peeples, and T. G. Morrison, J. Virol. 80:10652-10662, 2006). In the present study, these results were confirmed using Niemann-Pick syndrome type C cells, which are deficient in normal membrane rafts due to mutations affecting cholesterol transport. Furthermore, cholesterol extraction of infected cells resulted in the release of virions that attached to target cells at normal levels but were defective in virus-cell membrane fusion. The reduced fusion capacity of particles released from cholesterol-extracted cells correlated with significant loss of HN-F glycoprotein-containing complexes detected in the virion envelopes of these particles and with detection of cell-associated HN-F protein-containing complexes in extracts of cholesterol-extracted cells. Extraction of cholesterol from purified virions had no effect on virus-cell attachment, virus-cell fusion, particle infectivity, or the levels of glycoprotein-containing complexes. Taken together, these results suggest that cholesterol and membrane rafts are required for the formation or maintenance of HN-F glycoprotein-containing complexes in cells but not the stability of preformed glycoprotein complexes once assembled into virions.

Evaluation of Newcastle disease virus chimeras expressing the Hemagglutinin-Neuraminidase protein of velogenic strains in the context of a mesogenic recombinant virus backbone

A major factor in the pathogenicity of Newcastle disease virus (NDV) is the amino acid sequence of the fusion protein cleavage site, but the role of other viral genes that contribute to virulence and different clinical forms of the disease remain undefined. To assess the role of other NDV genes in virus pathogenicity, a reverse genetics system was developed using the mesogenic NDV Anhinga strain to provide a backbone for generating gene mutations or gene exchanges in attempts to enhance or attenuate the virulence of that virus. Chimeras created by exchange of the Anhinga Hemagglutinin-Neuraminidase (HN) gene with HN genes of neurotropic and viscerotropic velogenic viruses produced no significant change in virus pathogenicity as assessed by conducting the mean death time and intracerebral pathogenicity index assays and by inoculation of susceptible day-old SPF chickens. Inclusion in the recombinant construct of homotypic F genes, obtained from the parental viruses, also failed to enhance the pathotype of the recombinant viruses to a velogenic pathotype. A HN gene exchange alone within the context of the NDV Anhinga backbone failed to increase virus virulence from mesogenic to velogenic pathotype and suggests a multigenic role for NDV pathogenicity.

Fusion promotion by a paramyxovirus hemagglutinin-neuraminidase protein: pH modulation of receptor avidity of binding sites I and II

Paramyxoviruses, including the childhood respiratory pathogen human parainfluenza virus type 3 (HPIV3), possess an envelope protein hemagglutinin-neuraminidase (HN) that has receptor-cleaving (neuraminidase), as well as receptor-binding, activity. HN is a type II transmembrane glycoprotein, present on the surface of the virus as a tetramer composed of two dimers. HN is also essential for activating the fusion protein (F) to mediate merger of the viral envelope with the host cell membrane. This initial step of viral entry occurs at the host cell surface at neutral pH. The HN molecule carries out these three different critical activities at specific points in the process of viral entry, and understanding the regulation of these activities is key for the design of strategies that block infection. One bifunctional site (site I) on the HN of HPIV3 possesses both receptor binding and neuraminidase activities, and we recently obtained experimental evidence for a second receptor binding site (site II) on HPIV3 HN. Mutation of HN at specific residues at this site, which is next to the HN dimer interface, confers enhanced fusion properties, without affecting neuraminidase activity or receptor binding at neutral pH. We now demonstrate that mutations at this site II, as well as at site I, confer pH dependence on HN's receptor avidity. These mutations permit pH to modulate the binding and fusion processes of the virus, potentially providing regulation at specific stages of the viral life cycle.

Synergistic Effects of Adjuvants Interferon-γ and Levamisole on DNA Vaccination against Infection with Newcastle Disease Virus

Both humoral and cell-mediated immune responses are important to protect animals from initial acute viral infection and establishment of chronic infection. Adjuvants for DNA vaccines can influence the balance between humoral and cell-mediated immunities. In this study, a DNA vaccine encoding the hemagglutinin-neuraminidase and fusion genes of Newcastle disease virus (NDV) incorporated with chicken interferon(provax-chIFN-gamma) cDNA as a molecular adjuvant and levamisole (LMS) as a chemical adjuvant was tested for its efficacy in protection against NDV lethal challenge. Compared with DNA vaccine alone, the DNA vaccine with provax-chIFN-gamma plus LMS induced significantly higher humoral and cell-mediated responses, as shown by higher levels of hemagglutination inhibition (HI) titers and T cell proliferation. In addition, the DNA vaccine with provax-chIFN-gamma plus LMS formulation increased the expression of IFN-gamma, interleukin (IL)-2, IL-4, IL-12, and IL-13, suggesting that the effectiveness of the IFN-gamma and LMS formulation is partly due to the enhancement of balanced cytokine production. Furthermore, the two adjuvants yielded 80% protection in chickens against challenge with a lethal dose of the virulent NDV strain. This study demonstrates that the synergistic effects of provax-chIFN-gamma plus LMS as the adjuvants in NDV DNA vaccination could be used to improve protective efficacy in chickens.

A second receptor binding site on human parainfluenza virus type 3 hemagglutinin-neuraminidase contributes to activation of the fusion mechanism

The hemagglutinin-neuraminidase (HN) protein of paramyxoviruses carries out three discrete activities that each affect the ability of HN to promote viral fusion and entry: receptor binding, receptor cleaving (neuraminidase), and triggering of the fusion protein. The interrelationship between the receptor binding and fusion-triggering functions of HN has not been clear. For human parainfluenza type 3 (HPIV3), one bifunctional site on HN can carry out both receptor binding and neuraminidase activities, and this site's receptor binding can be inhibited by the small receptor analog zanamivir. We now report experimental evidence, complemented by computational data, for a second receptor binding site near the HPIV3 HN dimer interface. This second binding site can mediate receptor binding even in the presence of zanamivir, and it differs from the second receptor binding site of the paramyxovirus Newcastle disease virus in its function and its relationship to the primary binding site. This second binding site of HPIV3 HN is involved in triggering F. We suggest that the two receptor binding sites on HPIV3 HN each contribute in distinct ways to virus-cell interaction; one is the multifunctional site that contains both binding and neuraminidase activities, and the other contains binding activity and also is involved in fusion promotion.

Deduced amino acid sequences surrounding the fusion glycoprotein cleavage site and of the carboxyl-terminus of haemagglutinin-neuraminidase protein of the avirulent thermostable vaccine strain I-2 of Newcastle disease virus

A single-tube RT-PCR technique generated a 387 bp or 300 bp cDNA amplicon covering the F(0) cleavage site or the carboxyl (C)-terminus of the HN gene, respectively, of Newcastle disease virus (NDV) strain I-2. Sequence analysis was used to deduce the amino acid sequences of the cleavage site of F protein and the C-terminus of HN protein, which were then compared with sequences for other NDV strains. The cleavage site of NDV strain I-2 had a sequence motif of (112) RKQGRLIG(119), consistent with an avirulent phenotype. Nucleotide sequencing and deduction of amino acids at the C-terminus of HN revealed that strain I-2 had a 7-amino-acid extension (VEILKDGVREARSSR. This differs from the virulent viruses that caused outbreaks of Newcastle disease in Australia in the 1930s and 1990s, which have HN extensions of 0 and 9 amino acids, respectively. Amino acid sequence analyses of the F and HN genes of strain I-2 confirmed its avirulent nature and its Australian origin.

Structural analysis of a designed inhibitor complexed with the hemagglutinin-neuraminidase of Newcastle disease virus

Viruses of the Paramyxoviridae family are the leading cause of respiratory disease in children. The human parainfluenza viruses (hPIV) are members of the Paramyxovirinae subfamily, which also includes mumps virus, Newcastle disease virus (NDV), Sendai virus (SV) and simian type 5 virus (SV5). On the surface of these viruses is the glycoprotein hemagglutinin-neuraminidase (HN), which is responsible for cell attachment, promotion of fusion and release of progeny virions. This multifunctional nature of HN makes it an attractive target for the development of inhibitors as a treatment for childhood respiratory diseases. Here we report the crystal structure of NDV HN in complex with a derivative of 2-deoxy-2,3-dehydro-N-acetylneuraminic acid, Neu5Ac2en, that has a functional group designed to occupy a large conserved binding pocket around the active site. The purpose of this study was to examine the effect of a bulky hydrophobic group at the O4 position of Neu5Ac2en, given the hydrophobic nature of the binding pocket. This derivative, with a benzyl group added to the O4 position of Neu5Ac2en, has an IC(50) of approximately 10 microM in a neuraminidase assay against hPIV3 HN. The IC(50) value of the parent compound, Neu5Ac2en, in the same assay is approximately 25 microM. These results highlight the striking difference between the influenza neuraminidase and paramyxovirus HN active sites, and provide a platform for the development of improved HN inhibitors.

Amino acid change 335 E to K affects the sialic-acid-binding and neuraminidase activities of Urabe AM9 mumps virus hemagglutinin-neuraminidase glycoprotein

A mutation coding for the amino acid change E335 to K is frequently found in the hemagglutinin-neuraminidase (HN) gene of Urabe AM9 mumps viruses isolated during post-vaccination meningitis cases. To identify if this mutation modifies the biological activities of the HN glycoprotein, two variants of Urabe AM9 vaccine differing at amino acid 335 (HN-E335 and HN-K335) were isolated and their receptor-binding specificity was determined by means of competence assays. Pre-incubation of the viruses with sialic acids inhibited both syncytia formation in Vero cells and replication in SH-SY5Y cells. Thus, HN-K335 showed higher affinity towards sialylalpha2,6lactose, whereas HN-G335 preferred sialylalpha2,3lactose. These results are relevant because a high expression of sialylalpha2,6lactose in nerve cells was confirmed by means of Sambucus nigra lectin-cytochemistry. In addition, kinetics assays showed that HN-K335 and HN-E335 also differ in their hydrolysis rate (Vmax values of 37.5 vs. 3.5 nmol min-1mg-1, respectively). Therefore, HN-K335 variant presented a neuraminidase activity level 11-fold higher than that of HN-E335 variant. In conclusion, the mutation affects the receptor-binding and neuraminidase activities of Urabe AM9 mumps virus variants.

Antiviral effects of glycosylation and glucose trimming inhibitors on human parainfluenza virus type 3

Endoplasmic reticulum (ER) alpha-glucosidase inhibitors block the trimming of N-linked glycosylation and thus prevent the production of several viruses. The present study investigates the antiviral effects of the alpha-glucosidase and alpha-mannosidase inhibitors (castanospermine, 1-deoxynojirimycin, bromoconduritol, deoxymannojirimycin and swainsonine) on human parainfluenza virus type 3 (HPIV3). The alpha-glucosidase inhibitors (castanospermine, 1-deoxynojirimycin) in recombinant expression systems reduced the surface and intracellular expression of both HPIV3 F and HN proteins. On the other hand, alpha-mannosidase inhibitors prevented processing of the oligosaccharides on HPIV3 glycoproteins into the complex form. Consequently, alpha-glycosidase inhibitors (castanospermine and 1-deoxynojirimycin) significantly inhibited viral fusion activity. We demonstrated that the alpha-glucosidase inhibitors (castanospermine and 1-deoxynojirimycin) reduced the infectivity of newly released viral particles. We postulate that alpha-glucosidase inhibitors can prevent the first steps of HPIV3 envelope glycoprotein processing and that the inhibition of glucose trimming has antiviral effects.

Effects of Heptad Repeat Regions of F Protein on the Specific Membrane Fusion in Paramyxoviruses

OBJECTIVES

To identify the effects of heptad repeat regions (HR1 and HR2) of F on the specific membrane fusion in paramyxoviruses.

METHODS

Site-directed mutagenesis was used to create same enzyme sites on the F genes of Newcastle disease virus (NDV) and human parainfluenza virus (hPIV). Gene recombination was used to get chimeric F proteins NDV C-HR1 and hPIV C-HR1 by exchanging HR1 fragments each other; NDV C-HR2 and hPIV C-HR2 were also obtained by the same way. All the chimeric F proteins were co-expressed with their homologous or heterogeneous HN in eukaryocytes. Cell fusion functions were assayed by Giemsa staining and reporter gene method. The expression efficiencies of F proteins were assayed with fluorescence-activated cell sorter (FACS).

RESULTS

NDV C-HR1 and hPIV C-HR1 had 53.91 and 83.15% of fusion activities, and NDV C-HR2 and hPIV C-HR2 had 107.23 and 12.01% of fusion activities, respectively, as compared with their relevant wild types. The analysis of FACS indicated that the expression efficiencies of all the chimeric F proteins except NDV C-HR2 were lower than those of their relevant wild types.

CONCLUSIONS

HR1 of NDV F might be important for its specific membrane fusion, but HR2 of NDV F may not; both HR1 and HR2 of hPIV F may be important for its specific membrane fusion.

Specific binding of heat shock protein 70 with HN-protein inhibits the HN-protein assembly in Sendai virus-infected Vero cells

The production of hemagglutinating virus of Japan (HVJ; Sendai virus) was inhibited at 41 degrees C, whereas it was normal at 37 degrees C. In the infected Vero cells, viral specific proteins were synthesized even at 41 degrees C, but the synthesized HN protein was not integrated into the cell membrane, resulting in the inhibition of viral production. To investigate the relationship of HSP70 to the inhibition of HN-protein integration, the expression of HSP70 was induced by prostaglandin A1 (PGA1) at 37 degrees C, and the influence on viral infection was examined. The induction of HSP70 at 37 degrees C inhibited the viral production. Viral proteins were also synthesized, even in the presence of PGA1. However, HN protein was not as present on the cell membrane following PGA1-treatment as it was at 41 degrees C, whereas F protein was detected. An immunoprecipitation assay showed that HSP70 was coprecipitated with HN protein, but not with F protein. The results suggested that the specific interaction of HSP70 with HN protein prevented the protein from integrating into the cell membrane. In addition, the abnormal virus-like particles, of which HN protein and nucleocapsid were ablated, were released in the culture medium at 41 degrees C, although the size was smaller than the normal viral virions. The results suggest that HN protein is necessary for viral morphogenesis.

Inhibition of receptor binding stabilizes Newcastle disease virus HN and F protein-containing complexes

Receptor binding of paramyxovirus attachment proteins and the interactions between attachment and fusion (F) proteins are thought to be central to activation of the F protein activity; however, mechanisms involved are unclear. To explore the relationships between Newcastle disease virus (NDV) HN and F protein interactions and HN protein attachment to sialic acid receptors, HN and F protein-containing complexes were detected and quantified by reciprocal coimmunoprecipitation from extracts of transfected avian cells. To inhibit HN protein receptor binding, cells transfected with HN and F protein cDNAs were incubated with neuraminidase from the start of transfection. Under these conditions, no fusion was observed, but amounts of HN and F protein complexes increased twofold over amounts detected in extracts of untreated cells. Stimulation of attachment by incubation of untransfected target cells with neuraminidase-treated HN and F protein-expressing cells resulted in a twofold decrease in amounts of HN and F protein complexes. In contrast, high levels of complexes containing HN protein and an uncleaved F protein (F-K115Q) were detected, and those levels were unaffected by neuraminidase treatment of cell monolayers or by incubation with target cells. These results suggest that HN and F proteins reside in a complex in the absence of receptor binding. Furthermore, the results show that not only receptor binding but also F protein cleavage are necessary for disassociation of the HN and F protein-containing complexes.

Paramyxovirus membrane fusion: lessons from the F and HN atomic structures

Paramyxoviruses enter cells by fusion of their lipid envelope with the target cell plasma membrane. Fusion of the viral membrane with the plasma membrane allows entry of the viral genome into the cytoplasm. For paramyxoviruses, membrane fusion occurs at neutral pH, but the trigger mechanism that controls the viral entry machinery such that it occurs at the right time and in the right place remains to be elucidated. Two viral glycoproteins are key to the infection process—an attachment protein that varies among different paramyxoviruses and the fusion (F) protein, which is found in all paramyxoviruses. For many of the paramyxoviruses (parainfluenza viruses 1–5, mumps virus, Newcastle disease virus and others), the attachment protein is the hemagglutinin/neuraminidase (HN) protein. In the last 5 years, atomic structures of paramyxovirus F and HN proteins have been reported. The knowledge gained from these structures towards understanding the mechanism of viral membrane fusion is described.

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.

Addition of N-glycans in the stalk of the Newcastle disease virus HN protein blocks its interaction with the F protein and prevents fusion

Most paramyxovirus fusion (F) proteins require the coexpression of the homologous attachment (HN) protein to promote membrane fusion, consistent with the existence of a virus-specific interaction between the two proteins. Analysis of the fusion activities of chimeric HN proteins indicates that the stalk region of the HN spike determines its F protein specificity, and analysis of a panel of site-directed mutants indicates that the F-interactive site resides in this region. Here, we use the addition of oligosaccharides to further explore the role of the HN stalk in the interaction with F. N-glycans were individually added at several positions in the stalk to determine their effects on the activities of HN, as well as its structure. N-glycan addition at positions 69 and 77 in the stalk specifically blocks fusion and the HN-F interaction without affecting either HN structure or its other activities. N-glycans added at other positions in the stalk modulate activities that reside in the globular head of HN. This correlates with an alteration of the tetrameric structure of the protein, as indicated by sucrose gradient sedimentation analyses. Finally, N-glycan addition in another region of HN (residues 124 to 152), predicted by a peptide-based analysis to mediate the interaction with F, does not significantly reduce the level of fusion, arguing strongly against this site being part of the F-interactive domain in HN. Our data support the idea that the F-interactive site on HN is defined by the stalk region of the protein.

Two clones obtained from Urabe AM9 mumps virus vaccine differ in their replicative efficiency in neuroblastoma cells

A high rate of post-vaccinal aseptic meningitis for Urabe AM9 mumps virus strain is well documented. This strain is composed of two virus variants differing at the nt 1081 (A/G) region in the hemagglutinin-neuraminidase (HN) gene. An association of HN-A(1081) variant with neurovirulence has been proposed. In order to test for neurotropism we isolated the HN-A(1081) and HN-G(1081) virus variants from Urabe AM9 mumps virus vaccine. Sequential passages were performed in monkey kidney Vero cells and human neuroblastoma SH-SY5Y cells. Viral replication was determined by conventional and real-time RT-PCR. The results show that clone HN-A(1081) can replicate efficiently in both cell types. However, a defective replication of clone HN-G(1081), lacking its genetic marker, was observed after the third passage in neuroblastoma cells. Kinetics assays showed that clone HN-A(1081) replicates faster than clone HN-G(1081). Viral clones were also inoculated into the brains of newborn rats. Clone HN-A(1081) replicated 14 times, while clone HN-G(1081) merely duplicated its level over the initial inoculum. These results suggest that there is a selective replication of HN-A(1081) mumps virus variants in cells of nervous origin.

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).

The novel parainfluenza virus hemagglutinin-neuraminidase inhibitor BCX 2798 prevents lethal synergism between a paramyxovirus and Streptococcus pneumoniae

An association exists between respiratory viruses and bacterial infections. Prevention or treatment of the preceding viral infection is a logical goal for reducing this important cause of morbidity and mortality. The ability of the novel, selective parainfluenza virus hemagglutinin-neuraminidase inhibitor BCX 2798 to prevent the synergism between a paramyxovirus and Streptococcus pneumoniae was examined in this study. A model of secondary bacterial pneumonia after infection with a recombinant Sendai virus whose hemagglutinin-neuraminidase gene was replaced with that of human parainfluenza virus type 1 [rSV(hHN)] was established in mice. Challenge of mice with a sublethal dose of S. pneumoniae 7 days after a sublethal infection with rSV(hHN) (synergistic group) caused 100% mortality. Bacterial infection preceding viral infection had no effect on survival. The mean bacterial titers in the synergistic group were significantly higher than in mice infected with bacteria only. The virus titers were similar in mice infected with rSV(hHN) alone and in dually infected mice. Intranasal administration of BCX 2798 at 10 mg/kg per day to the synergistic group of mice starting 4 h before virus infection protected 80% of animals from death. This effect was accompanied by a significant reduction in lung viral and bacterial titers. Treatment of mice 24 h after the rSV(hHN) infection showed no protection against synergistic lethality. Together, our results indicate that parainfluenza viruses can prime for secondary bacterial infections. Prophylaxis of parainfluenza virus infections with antivirals might be an effective strategy for prevention of secondary bacterial complications in humans.

Inhibition of parainfluenza virus type 3 and Newcastle disease virus hemagglutinin-neuraminidase receptor binding: effect of receptor avidity and steric hindrance at the inhibitor binding sites

Zanamivir (4-guanidino-Neu5Ac2en [4-GU-DANA]) inhibits not only the neuraminidase activity but also the receptor interaction of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN), blocking receptor binding and subsequent fusion promotion. All activities of the HPIV3 variant ZM1 HN (T193I/I567V) are less sensitive to 4-GU-DANA's effects. The T193I mutation in HN confers both increased receptor binding and increased neuraminidase activity, as well as reduced sensitivities of both activities to 4-GU-DANA inhibition, consistent with a single site on the HN molecule carrying out both catalysis and binding. We now provide evidence that the HPIV3 variant's resistance to receptor-binding inhibition by 4-GU-DANA is related to a reduced affinity of the HN receptor-binding site for this compound as well as to an increase in the avidity of HN for the receptor. Newcastle disease virus (NDV) HN and HPIV3 HN respond differently to inhibition in ways that suggest a fundamental distinction between them. NDV HN-receptor binding is less sensitive than HPIV3 HN-receptor binding to 4-GU-DANA, while its neuraminidase activity is highly sensitive. Both HPIV3 and NDV HNs are sensitive to receptor-binding inhibition by the smaller molecule DANA. However, for NDV HN, some receptor binding cannot be inhibited. These data are consistent with the presence in NDV HN of a second receptor-binding site that is devoid of enzyme activity and has a negligible, if any, affinity for 4-GU-DANA. Avidity for the receptor contributes to resistance by allowing the receptor to compete effectively with inhibitors for interaction with HN, while the further determinant of resistance is the reduced binding of the inhibitor molecule to the binding pocket on HN. Based upon our data and recent three-dimensional structural information on the HPIV3 and NDV HNs, we propose mechanisms for the observed sensitivity and resistance of HN to receptor-binding inhibition and discuss the implications of these mechanisms for the distribution of HN functions.

Lentiviral Vectors Pseudotyped with Envelope Glycoproteins Derived from Human Parainfluenza Virus Type 3

We describe the generation of lentiviruses pseudotyped with human parainfluenza type 3 envelope (HPIV3) glycoproteins. Lentivirus particles, expressed in 293T/17 cells, incorporate HPIV3 hemagglutinin-neuraminidase (HN) and fusion (F) proteins into their lipid bilayers and are able to transduce human kidney epithelial cells and polarized MDCK cells. Neuraminidase, AZT, and anti-HPIV3 antisera block transduction, which is consistent with lentiviral-mediated transduction via sialated receptors for HPIV3. Our findings show that HPIV3 pseudotyped lentiviruses can be formed and may have a number of useful properties for human gene transfer.

High cell surface expression of Newcastle disease virus proteins via replicon vectors demonstrates syncytia forming activity of F and fusion promotion activity of HN molecules

For functional studies, the hemagglutinin-neuraminidase (HN) and the fusion protein (F) of Newcastle disease virus (NDV) were expressed in BHK cells using two vectors which are based on the Semliki Forest virus (SFV) replicon. The first system of high protein expression works by transfection of RNA which before has been in vitro transcribed from a vector containing the gene for the SFV self-amplifying replicase (REP) and a foreign gene using the SP6 promoter. A high level of protein (HN or F) expression was detected 18-20 h after transfection. To study the host range of this expression system, a panel of different cell lines were compared for transfections with SFV RNA. A wide range of expression efficiency was observed, the highest being BHK cells. The second system is based on a DNA plasmid in which the SFV-REP and a foreign gene are expressed in cells under the transcriptional and translational control of the cytomegalovirus immediate-early enhancer T7 promoter. DNA-electroporated BHK cells expressed also high levels of the recombinant proteins but at a delayed time point (40-48 h) as compared with the corresponding RNA. Co-expression of the two NDV proteins, HN and F, via this DNA vector in the same cells led to syncytium formation in the cell monolayer, showing that both proteins expressed in this way, were functionally active. F alone, expressed via this vector, displayed residual fusion activity suggesting its proteolytic cleavage and its functional independence from HN.

Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells

Human parainfluenza virus type 3 (HPIV-3) is an airborne pathogen that infects human lung epithelial cells from the apical (luminal) plasma membrane domain. In the present study, we have identified cell surface-expressed nucleolin as a cellular cofactor required for the efficient cellular entry of HPIV-3 into human lung epithelial A549 cells. Nucleolin was enriched on the apical cell surface domain of A549 cells, and HPIV-3 interacted with nucleolin during entry. The importance of nucleolin during HPIV-3 replication was borne out by the observation that HPIV-3 replication was significantly inhibited following (i). pretreatment of cells with antinucleolin antibodies and (ii). preincubation of HPIV-3 with purified nucleolin prior to its addition to the cells. Moreover, HPIV-3 cellular internalization and attachment assays performed in the presence of antinucleolin antibodies and purified nucleolin revealed the requirement of nucleolin during HPIV-3 internalization but not during attachment. Thus, these results suggest that nucleolin expressed on the surfaces of human lung epithelial A549 cells plays an important role during HPIV-3 cellular entry.