Modes of paramyxovirus fusion: a Henipavirus perspective

Crystal structure and solution state of the C-terminal head region of the narmovirus receptor binding protein

IMPORTANCE

Genetically diverse paramyxoviruses are united in their presentation of a receptor-binding protein (RBP), which works in concert with the fusion protein to facilitate host-cell entry. The C-terminal head region of the paramyxoviral RBP, a primary determinant of host-cell tropism and inter-species transmission potential, forms structurally distinct classes dependent upon protein and glycan receptor specificity. Here, we reveal the architecture of the C-terminal head region of the RBPs from Nariva virus (NarV) and Mossman virus (MosV), two archetypal rodent-borne paramyxoviruses within the recently established genus Narmovirus, family Paramyxoviridae. Our analysis reveals that while narmoviruses retain the general architectural features associated with paramyxoviral RBPs, namely, a six-bladed β-propeller fold, they lack the structural motifs associated with known receptor-mediated host-cell entry pathways. This investigation indicates that the RBPs of narmoviruses exhibit pathobiological features that are distinct from those of other paramyxoviruses.

Structure and antigenicity of divergent Henipavirus fusion glycoproteins

In August 2022, a novel henipavirus (HNV) named Langya virus (LayV) was isolated from patients with severe pneumonic disease in China. This virus is closely related to Mòjiāng virus (MojV), and both are divergent from the bat-borne HNV members, Nipah (NiV) and Hendra (HeV) viruses. The spillover of LayV is the first instance of a HNV zoonosis to humans outside of NiV and HeV, highlighting the continuing threat this genus poses to human health. In this work, we determine the prefusion structures of MojV and LayV F proteins via cryogenic electron microscopy to 2.66 and 3.37 Å, respectively. We show that despite sequence divergence from NiV, the F proteins adopt an overall similar structure but are antigenically distinct as they do not react to known antibodies or sera. Glycoproteomic analysis revealed that while LayV F is less glycosylated than NiV F, it contains a glycan that shields a site of vulnerability previously identified for NiV. These findings explain the distinct antigenic profile of LayV and MojV F, despite the extent to which they are otherwise structurally similar to NiV. Our results carry implications for broad-spectrum HNV vaccines and therapeutics, and indicate an antigenic, yet not structural, divergence from prototypical HNVs.

Nipah Virus Impairs Autocrine IFN Signaling by Sequestering STAT1 and STAT2 into Inclusion Bodies

Nipah virus (NiV) is an emerging zoonotic paramyxovirus that causes fatal infections in humans. As with most disease-causing viruses, the pathogenic potential of NiV is linked to its ability to block antiviral responses, e.g., by antagonizing IFN signaling through blocking STAT proteins. One of the STAT1/2-binding proteins of NiV is the phosphoprotein (P), but its functional role in IFN antagonism in a full viral context is not well defined. As NiV P is required for genome replication and specifically accumulates in cytosolic inclusion bodies (IBs) of infected cells, we hypothesized that this compartmentalization might play a role in P-mediated IFN antagonism. Supporting this notion, we show here that NiV can inhibit IFN-dependent antiviral signaling via a NiV P-dependent sequestration of STAT1 and STAT2 into viral IBs. Consequently, the phosphorylation/activation and nuclear translocation of STAT proteins in response to IFN is limited, as indicated by the lack of nuclear pSTAT in NiV-infected cells. Blocking autocrine IFN signaling by sequestering STAT proteins in IBs is a not yet described mechanism by which NiV could block antiviral gene expression and provides the first evidence that cytosolic NiV IBs may play a functional role in IFN antagonism.

Cell-Cell Fusion Assays to Study Henipavirus Entry and Evaluate Therapeutics

Henipaviruses include the deadly zoonotic Nipah (NiV) and Hendra (HeV) paramyxoviruses, which have caused recurring outbreaks in human populations. A hallmark of henipavirus infection is the induction of cell-cell fusion (syncytia), caused by the expression of the attachment (G) and fusion (F) glycoproteins on the surface of infected cells. The interactions of G and F with each other and with receptors on cellular plasma membranes drive both viral entry and syncytia formation and are thus of great interest. While F shares structural and functional homologies with class I fusion proteins of other viruses such as influenza and human immunodeficiency viruses, the intricate interactions between the G and F glycoproteins allow for unique approaches to studying the class I membrane fusion process. This allows us to study cell-cell fusion and viral entry kinetics for BSL-4 pathogens such as NiV and HeV under BSL-2 conditions using recombinant DNA techniques. Here, we present approaches to studying henipavirus-induced membrane fusion for currently identified and emerging henipaviruses, including more traditional syncytia counting-based cell-cell fusion assay and a new heterologous fluorescent dye exchange cell-cell fusion assay.

Henipavirus Immune Evasion and Pathogenesis Mechanisms: Lessons Learnt from Natural Infection and Animal Models

Nipah henipavirus (NiV) and Hendra henipavirus (HeV) are zoonotic emerging paramyxoviruses causing severe disease outbreaks in humans and livestock, mostly in Australia, India, Malaysia, Singapore and Bangladesh. Both are bat-borne viruses and in humans, their mortality rates can reach 60% in the case of HeV and 92% for NiV, thus being two of the deadliest viruses known for humans. Several factors, including a large cellular tropism and a wide zoonotic potential, con-tribute to their high pathogenicity. This review provides an overview of HeV and NiV pathogenicity mechanisms and provides a summary of their interactions with the immune systems of their different host species, including their natural hosts bats, spillover-hosts pigs, horses, and humans, as well as in experimental animal models. A better understanding of the interactions between henipaviruses and their hosts could facilitate the development of new therapeutic strategies and vaccine measures against these re-emerging viruses.

Investigating host-virus interaction mechanism and phylogenetic analysis of viral proteins involved in the pathogenesis

Since the emergence of yellow fever in the Americas and the devastating 1918 influenza pandemic, biologists and clinicians have been drawn to human infecting viruses to understand their mechanisms of infection better and develop effective therapeutics against them. However, the complex molecular and cellular processes that these viruses use to infect and multiply in human cells have been a source of great concern for the scientific community since the discovery of the first human infecting virus. Viral disease outbreaks, such as the recent COVID-19 pandemic caused by a novel coronavirus, have claimed millions of lives and caused significant economic damage worldwide. In this study, we investigated the mechanisms of host-virus interaction and the molecular machinery involved in the pathogenesis of some common human viruses. We also performed a phylogenetic analysis of viral proteins involved in host-virus interaction to understand the changes in the sequence organization of these proteins during evolution for various strains of viruses to gain insights into the viral origin's evolutionary perspectives.

Cooperativity mediated by rationally selected combinations of human monoclonal antibodies targeting the henipavirus receptor binding protein

Hendra virus and Nipah virus (NiV), members of the Henipavirus (HNV) genus, are zoonotic paramyxoviruses known to cause severe disease across six mammalian orders, including humans. We isolated a panel of human monoclonal antibodies (mAbs) from the B cells of an individual with prior exposure to equine Hendra virus (HeV) vaccine, targeting distinct antigenic sites. The most potent class of cross-reactive antibodies achieves neutralization by blocking viral attachment to the host cell receptors ephrin-B2 and ephrin-B3, with a second class being enhanced by receptor binding. mAbs from both classes display synergistic activity in vitro. In a stringent hamster model of NiV Bangladesh (NiV_B) infection, antibodies from both classes reduce morbidity and mortality and achieve synergistic protection in combination. These candidate mAbs might be suitable for use in a cocktail therapeutic approach to achieve synergistic potency and reduce the risk of virus escape.

Doyle et al. describe two human monoclonal antibodies that target the henipavirus receptor-binding protein, HENV-103 and HENV-117, that display highly potent activity in vitro and enhanced therapeutic efficacy in vivo when delivered as a cocktail.

From Protein to Pandemic: The Transdisciplinary Approach Needed to Prevent Spillover and the Next Pandemic

Pandemics are a consequence of a series of processes that span scales from viral biology at 10-9 m to global transmission at 106 m. The pathogen passes from one host species to another through a sequence of events that starts with an infected reservoir host and entails interspecific contact, innate immune responses, receptor protein structure within the potential host, and the global spread of the novel pathogen through the naive host population. Each event presents a potential barrier to the onward passage of the virus and should be characterized with an integrated transdisciplinary approach. Epidemic control is based on the prevention of exposure, infection, and disease. However, the ultimate pandemic prevention is prevention of the spillover event itself. Here, we focus on the potential for preventing the spillover of henipaviruses, a group of viruses derived from bats that frequently cross species barriers, incur high human mortality, and are transmitted among humans via stuttering chains. We outline the transdisciplinary approach needed to prevent the spillover process and, therefore, future pandemics.

Viral Interactions with Adaptor-Protein Complexes: A Ubiquitous Trait among Viral Species

Numerous viruses hijack cellular protein trafficking pathways to mediate cell entry or to rearrange membrane structures thereby promoting viral replication and antagonizing the immune response. Adaptor protein complexes (AP), which mediate protein sorting in endocytic and secretory transport pathways, are one of the conserved viral targets with many viruses possessing AP-interacting motifs. We present here different mechanisms of viral interference with AP complexes and the functional consequences that allow for efficient viral propagation and evasion of host immune defense. The ubiquity of this phenomenon is evidenced by the fact that there are representatives for AP interference in all major viral families, covered in this review. The best described examples are interactions of human immunodeficiency virus and human herpesviruses with AP complexes. Several other viruses, like Ebola, Nipah, and SARS-CoV-2, are pointed out as high priority disease-causative agents supporting the need for deeper understanding of virus-AP interplay which can be exploited in the design of novel antiviral therapies.

Engineering Protease-Resistant Peptides to Inhibit Human Parainfluenza Viral Respiratory Infection

The lower respiratory tract infections affecting children worldwide are in large part caused by the parainfluenza viruses (HPIVs), particularly HPIV3, along with human metapneumovirus and respiratory syncytial virus, enveloped negative-strand RNA viruses. There are no vaccines for these important human pathogens, and existing treatments have limited or no efficacy. Infection by HPIV is initiated by viral glycoprotein-mediated fusion between viral and host cell membranes. A viral fusion protein (F), once activated in proximity to a target cell, undergoes a series of conformational changes that first extend the trimer subunits to allow insertion of the hydrophobic domains into the target cell membrane and then refold the trimer into a stable postfusion state, driving the merger of the viral and host cell membranes. Lipopeptides derived from the C-terminal heptad repeat (HRC) domain of HPIV3 F inhibit infection by interfering with the structural transitions of the trimeric F assembly. Clinical application of this strategy, however, requires improving the in vivo stability of antiviral peptides. We show that the HRC peptide backbone can be modified via partial replacement of α-amino acid residues with β-amino acid residues to generate α/β-peptides that retain antiviral activity but are poor protease substrates. Relative to a conventional α-lipopeptide, our best α/β-lipopeptide exhibits improved persistence in vivo and improved anti-HPIV3 antiviral activity in animals.

Functional Analysis of the Fusion and Attachment Glycoproteins of Mojiang Henipavirus

Mojiang virus (MojV) is the first henipavirus identified in a rodent and known only by sequence data, whereas all other henipaviruses have been isolated from bats (Hendra virus, Nipah virus, Cedar virus) or discovered by sequence data from material of bat origin (Ghana virus). Ephrin-B2 and -B3 are entry receptors for Hendra and Nipah viruses, but Cedar virus can utilize human ephrin-B1, -B2, -A2 and -A5 and mouse ephrin-A1. However, the entry receptor for MojV remains unknown, and its species tropism is not well characterized. Here, we utilized recombinant full-length and soluble forms of the MojV fusion (F) and attachment (G) glycoproteins in membrane fusion and receptor tropism studies. MojV F and G were functionally competent and mediated cell–cell fusion in primate and rattine cells, albeit with low levels and slow fusion kinetics. Although a relative instability of the pre-fusion conformation of a soluble form of MojV F was observed, MojV F displayed significantly greater fusion activity when heterotypically paired with Ghana virus G. An exhaustive investigation of A- and B-class ephrins indicated that none serve as a primary receptor for MojV. The MojV cell fusion phenotype is therefore likely the result of receptor restriction rather than functional defects in recombinant MojV F and G glycoproteins.

Hijacking the Fusion Complex of Human Parainfluenza Virus as an Antiviral Strategy

Paramyxoviruses, including human parainfluenza virus type 3, are internalized into host cells by fusion between viral and target cell membranes. The receptor binding protein, hemagglutinin-neuraminidase (HN), upon binding to its cell receptor, triggers conformational changes in the fusion protein (F). This action of HN activates F to reach its fusion-competent state. Using small molecules that interact with HN, we can induce the premature activation of F and inactivate the virus. To obtain highly active pretriggering compounds, we carried out a virtual modeling screen for molecules that interact with a sialic acid binding site on HN that we propose to be the site involved in activating F. We use cryo-electron tomography of authentic intact viral particles for the first time to directly assess the mechanism of action of this treatment on the conformation of the viral F protein and present the first direct observation of the induced conformational rearrangement in the viral F protein.

The receptor binding protein of parainfluenza virus, hemagglutinin-neuraminidase (HN), is responsible for actively triggering the viral fusion protein (F) to undergo a conformational change leading to insertion into the target cell and fusion of the virus with the target cell membrane. For proper viral entry to occur, this process must occur when HN is engaged with host cell receptors at the cell surface. It is possible to interfere with this process through premature activation of the F protein, distant from the target cell receptor. Conformational changes in the F protein and adoption of the postfusion form of the protein prior to receptor engagement of HN at the host cell membrane inactivate the virus. We previously identified small molecules that interact with HN and induce it to activate F in an untimely fashion, validating a new antiviral strategy. To obtain highly active pretriggering candidate molecules we carried out a virtual modeling screen for molecules that interact with sialic acid binding site II on HN, which we propose to be the site responsible for activating F. To directly assess the mechanism of action of one such highly effective new premature activating compound, PAC-3066, we use cryo-electron tomography on authentic intact viral particles for the first time to examine the effects of PAC-3066 treatment on the conformation of the viral F protein. We present the first direct observation of the conformational rearrangement induced in the viral F protein.

Differential Features of Fusion Activation within the Paramyxoviridae

Paramyxovirus (PMV) entry requires the coordinated action of two envelope glycoproteins, the receptor binding protein (RBP) and fusion protein (F). The sequence of events that occurs during the PMV entry process is tightly regulated. This regulation ensures entry will only initiate when the virion is in the vicinity of a target cell membrane. Here, we review recent structural and mechanistic studies to delineate the entry features that are shared and distinct amongst the Paramyxoviridae. In general, we observe overarching distinctions between the protein-using RBPs and the sialic acid- (SA-) using RBPs, including how their stalk domains differentially trigger F. Moreover, through sequence comparisons, we identify greater structural and functional conservation amongst the PMV fusion proteins, as compared to the RBPs. When examining the relative contributions to sequence conservation of the globular head versus stalk domains of the RBP, we observe that, for the protein-using PMVs, the stalk domains exhibit higher conservation and find the opposite trend is true for SA-using PMVs. A better understanding of conserved and distinct features that govern the entry of protein-using versus SA-using PMVs will inform the rational design of broader spectrum therapeutics that impede this process.

A key region of molecular specificity orchestrates unique ephrin-B1 utilization by Cedar virus

The emergent zoonotic henipaviruses, Hendra, and Nipah are responsible for frequent and fatal disease outbreaks in domestic animals and humans. Specificity of henipavirus attachment glycoproteins (G) for highly species-conserved ephrin ligands underpins their broad host range and is associated with systemic and neurological disease pathologies. Here, we demonstrate that Cedar virus (CedV)-a related henipavirus that is ostensibly nonpathogenic-possesses an idiosyncratic entry receptor repertoire that includes the common henipaviral receptor, ephrin-B2, but, distinct from pathogenic henipaviruses, does not include ephrin-B3. Uniquely among known henipaviruses, CedV can use ephrin-B1 for cellular entry. Structural analyses of CedV-G reveal a key region of molecular specificity that directs ephrin-B1 utilization, while preserving a universal mode of ephrin-B2 recognition. The structural and functional insights presented uncover diversity within the known henipavirus receptor repertoire and suggest that only modest structural changes may be required to modulate receptor specificities within this group of lethal human pathogens.

Predicting and designing therapeutics against the Nipah virus

Despite Nipah virus outbreaks having high mortality rates (>70% in Southeast Asia), there are no licensed drugs against it. In this study, we have considered all 9 Nipah proteins as potential therapeutic targets and computationally identified 4 putative peptide inhibitors (against G, F and M proteins) and 146 small molecule inhibitors (against F, G, M, N, and P proteins). The computations include extensive homology/ab initio modeling, peptide design and small molecule docking. An important contribution of this study is the increased structural characterization of Nipah proteins by approximately 90% of what is deposited in the PDB. In addition, we have carried out molecular dynamics simulations on all the designed protein-peptide complexes and on 13 of the top shortlisted small molecule ligands to check for stability and to estimate binding strengths. Details, including atomic coordinates of all the proteins and their ligand bound complexes, can be accessed at http://cospi.iiserpune.ac.in/Nipah. Our strategy was to tackle the development of therapeutics on a proteome wide scale and the lead compounds identified could be attractive starting points for drug development. To counter the threat of drug resistance, we have analysed the sequences of the viral strains from different outbreaks, to check whether they would be sensitive to the binding of the proposed inhibitors.

A structural basis for antibody-mediated neutralization of Nipah virus reveals a site of vulnerability at the fusion glycoprotein apex

Nipah virus (NiV) is a highly pathogenic paramyxovirus that causes frequent outbreaks of severe neurologic and respiratory disease in humans with high case fatality rates. The 2 glycoproteins displayed on the surface of the virus, NiV-G and NiV-F, mediate host-cell attachment and membrane fusion, respectively, and are targets of the host antibody response. Here, we provide a molecular basis for neutralization of NiV through antibody-mediated targeting of NiV-F. Structural characterization of a neutralizing antibody (nAb) in complex with trimeric prefusion NiV-F reveals an epitope at the membrane-distal domain III (DIII) of the molecule, a region that undergoes substantial refolding during host-cell entry. The epitope of this monoclonal antibody (mAb66) is primarily protein-specific and we observe that glycosylation at the periphery of the interface likely does not inhibit mAb66 binding to NiV-F. Further characterization reveals that a Hendra virus-F-specific nAb (mAb36) and many antibodies in an antihenipavirus-F polyclonal antibody mixture (pAb835) also target this region of the molecule. Integrated with previously reported paramyxovirus F-nAb structures, these data support a model whereby the membrane-distal region of the F protein is targeted by the antibody-mediated immune response across henipaviruses. Notably, our domain-specific sequence analysis reveals no evidence of selective pressure at this region of the molecule, suggestive that functional constraints prevent immune-driven sequence variation. Combined, our data reveal the membrane-distal region of NiV-F as a site of vulnerability on the NiV surface.

Detailed Molecular Biochemistry for Novel Therapeutic Design Against Nipah and Hendra Virus: A Systematic Review

BACKGROUND

Nipah virus (NiV) and Hendra virus (HeV) of genus Henipavirus are the deadliest zoonotic viruses, which cause severe respiratory ailments and fatal encephalitis in humans and other susceptible animals. The fatality rate for these infections had been alarmingly high with no approved treatment available to date. Viral attachment and fusion with host cell membrane is essential for viral entry and is the most essential event of viral infection. Viral attachment is mediated by interaction of Henipavirus attachment glycoprotein (G) with the host cell receptor: Ephrin B2/B3, while viral fusion and endocytosis are mediated by the combined action of both viral glycoprotein (G) and fusion protein (F).

CONCLUSION

This review highlights the mechanism of viral attachment, fusion and also explains the basic mechanism and pathobiology of this infection in humans. The drugs and therapeutics used either experimentally or clinically against NiV and HeV infection have been documented and classified in detail. Some amino acid residues essential for the functionality of G and F proteins were also emphasized. Therapeutic designing to target and block these residues can serve as a promising approach in future drug development against NiV and HeV.

Transmembrane Domain Dissociation Is Required for Hendra Virus F Protein Fusogenic Activity

Hendra virus (HeV) is a zoonotic paramyxovirus that utilizes a trimeric fusion (F) protein within its lipid bilayer to mediate membrane merger with a cell membrane for entry. Previous HeV F studies showed that transmembrane domain (TMD) interactions are important for stabilizing the prefusion conformation of the protein prior to triggering. Thus, the current model for HeV F fusion suggests that modulation of TMD interactions is critical for initiation and completion of conformational changes that drive membrane fusion. HeV F constructs (T483C/V484C, V484C/N485C, and N485C/P486C) were generated with double cysteine substitutions near the N-terminal region of the TMD to study the effect of altered flexibility in this region. Oligomeric analysis showed that the double cysteine substitutions successfully promoted intersubunit disulfide bond formation in HeV F. Subsequent fusion assays indicated that the introduction of disulfide bonds in the mutants prohibited fusion events. Further testing confirmed that T483C/V484C and V484C/N485C were expressed at the cell surface at levels that would allow for fusion. Attempts to restore fusion with a reducing agent were unsuccessful, suggesting that the introduced disulfide bonds were likely buried in the membrane. Conformational analysis showed that T483C/V484C and V484C/N485C were able to bind a prefusion conformation-specific antibody prior to cell disruption, indicating that the introduced disulfide bonds did not significantly affect protein folding. This study is the first to report that TMD dissociation is required for HeV F fusogenic activity and strengthens our model for HeV fusion.IMPORTANCE The paramyxovirus Hendra virus (HeV) causes severe respiratory illness and encephalitis in humans. To develop therapeutics for HeV and related viral infections, further studies are needed to understand the mechanisms underlying paramyxovirus fusion events. Knowledge gained in studies of the HeV fusion (F) protein may be applicable to a broad span of enveloped viruses. In this study, we demonstrate that disulfide bonds introduced between the HeV F transmembrane domains (TMDs) block fusion. Depending on the location of these disulfide bonds, HeV F can still fold properly and bind a prefusion conformation-specific antibody prior to cell disruption. These findings support our current model for HeV membrane fusion and expand our knowledge of the TMD and its role in HeV F stability and fusion promotion.

Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus

Cedar virus (CedV) is a bat-borne henipavirus related to Nipah virus (NiV) and Hendra virus (HeV), zoonotic agents of fatal human disease. CedV receptor-binding protein (G) shares only ∼30% sequence identity with those of NiV and HeV, although they can all use ephrin-B2 as an entry receptor. We demonstrate that CedV also enters cells through additional B- and A-class ephrins (ephrin-B1, ephrin-A2, and ephrin-A5) and report the crystal structure of the CedV G ectodomain alone and in complex with ephrin-B1 or ephrin-B2. The CedV G receptor-binding site is structurally distinct from other henipaviruses, underlying its capability to accommodate additional ephrin receptors. We also show that CedV can enter cells through mouse ephrin-A1 but not human ephrin-A1, which differ by 1 residue in the key contact region. This is evidence of species specific ephrin receptor usage by a henipavirus, and implicates additional ephrin receptors in potential zoonotic transmission.

A structure-based rationale for sialic acid independent host-cell entry of Sosuga virus

The bat-borne paramyxovirus, Sosuga virus (SosV), is one of many paramyxoviruses recently identified and classified within the newly established genus Pararubulavirus, family Paramyxoviridae The envelope surface of SosV presents a receptor-binding protein (RBP), SosV-RBP, which facilitates host-cell attachment and entry. Unlike closely related hemagglutinin neuraminidase RBPs from other genera of the Paramyxoviridae, SosV-RBP and other pararubulavirus RBPs lack many of the stringently conserved residues required for sialic acid recognition and hydrolysis. We determined the crystal structure of the globular head region of SosV-RBP, revealing that while the glycoprotein presents a classical paramyxoviral six-bladed β-propeller fold and structurally classifies in close proximity to paramyxoviral RBPs with hemagglutinin-neuraminidase (HN) functionality, it presents a receptor-binding face incongruent with sialic acid recognition. Hemadsorption and neuraminidase activity analysis confirms the limited capacity of SosV-RBP to interact with sialic acid in vitro and indicates that SosV-RBP undergoes a nonclassical route of host-cell entry. The close overall structural conservation of SosV-RBP with other classical HN RBPs supports a model by which pararubulaviruses only recently diverged from sialic acid binding functionality.

Authentic Modeling of Human Respiratory Virus Infection in Human Pluripotent Stem Cell-Derived Lung Organoids

Respiratory viruses are among the first pathogens encountered by young children, and the significant impact of these viral infections on the developing lung is poorly understood. Circulating viruses are suited to the environment of the human lung and are different from those of viruses grown in cultured cells. We modeled respiratory virus infections that occur in children or infect the distal lung using lung organoids that represent the entire developing infant lung. These 3D lung organoids, derived from human pluripotent stem cells, develop into branching airway and alveolar structures and provide a tissue environment that maintains the authentic viral genome. The lung organoids can be genetically engineered prior to differentiation, thereby generating tissues bearing or lacking specific features that may be relevant to viral infection, a feature that may have utility for the study of host-pathogen interaction for a range of lung pathogens.

Infectious viruses so precisely fit their hosts that the study of natural viral infection depends on host-specific mechanisms that affect viral infection. For human parainfluenza virus 3, a prevalent cause of lower respiratory tract disease in infants, circulating human viruses are genetically different from viruses grown in standard laboratory conditions; the surface glycoproteins that mediate host cell entry on circulating viruses are suited to the environment of the human lung and differ from those of viruses grown in cultured cells. Polarized human airway epithelium cultures have been used to represent the large, proximal airways of mature adult airways. Here we modeled respiratory virus infections that occur in children or infect the distal lung using lung organoids that represent the entire developing infant lung. These 3D lung organoids derived from human pluripotent stem cells contain mesoderm and pulmonary endoderm and develop into branching airway and alveolar structures. Whole-genome sequencing analysis of parainfluenza viruses replicating in the organoids showed maintenance of nucleotide identity, suggesting that no selective pressure is exerted on the virus in this tissue. Infection with parainfluenza virus led to viral shedding without morphological changes, while respiratory syncytial virus infection induced detachment and shedding of infected cells into the lung organoid lumens, reminiscent of parainfluenza and respiratory syncytial virus in human infant lungs. Measles virus infection, in contrast, induced syncytium formation. These human stem cell-derived lung organoids may serve as an authentic model for respiratory viral pathogenesis in the developing or infant lung, recapitulating respiratory viral infection in the host.

Quantitative investigation of the direct interaction between Hemagglutinin and fusion proteins of Peste des petits ruminant virus using surface Plasmon resonance

Background

The specific and dynamic interaction between the hemagglutinin (H) and fusion (F) proteins of morbilliviruses is a prerequisite for the conformational rearrangements and membrane fusion during infection process. The two heptad repeat regions (HRA and HRB) of F protein are both important for the triggering of F protein.

Methods

In this study, the direct interactions of Peste des petits ruminants virus (PPRV) H with F, HRA and HRB were quantitatively evaluated using biosensor surface plasmon resonance (SPR).

Results

The binding affinities of immobilized pCMV-HA-H (HA-H) interacted with proteins pCMV-HA-F (HA-F) and pCMV-HA-HRB (HA-HRB) (K_D = 1.91 × 10^− 8 M and 2.60 × 10^− 7 M, respectively) reacted an order of magnitude more strongly than that of pCMV-HA-HRA (HA-HRA) and pCMV-HA-Tp IGFR-LD (HA) (K_D = 1.08 × 10^− 4 M and 1.43 × 10^− 4 M, respectively).

Conclusions

The differences of the binding affinities suggested that HRB is involved in functionally important intermolecular interaction in the fusion process.

Monomeric ephrinB2 binding induces allosteric changes in Nipah virus G that precede its full activation

Nipah virus is an emergent paramyxovirus that causes deadly encephalitis and respiratory infections in humans. Two glycoproteins coordinate the infection of host cells, an attachment protein (G), which binds to cell surface receptors, and a fusion (F) protein, which carries out the process of virus-cell membrane fusion. The G protein binds to ephrin B2/3 receptors, inducing G conformational changes that trigger F protein refolding. Using an optical approach based on second harmonic generation, we show that monomeric and dimeric receptors activate distinct conformational changes in G. The monomeric receptor-induced changes are not detected by conformation-sensitive monoclonal antibodies or through electron microscopy analysis of G:ephrinB2 complexes. However, hydrogen/deuterium exchange experiments confirm the second harmonic generation observations and reveal allosteric changes in the G receptor binding and F-activating stalk domains, providing insights into the pathway of receptor-activated virus entry.Nipah virus causes encephalitis in humans. Here the authors use a multidisciplinary approach to study the binding of the viral attachment protein G to its host receptor ephrinB2 and show that monomeric and dimeric receptors activate distinct conformational changes in G and discuss implications for receptor-activated virus entry.

Stimulation of Nipah Fusion: Small Intradomain Changes Trigger Extensive Interdomain Rearrangements

Nipah is an emerging paramyxovirus that is of serious concern to human health. It invades host cells using two of its membrane proteins-G and F. G binds to host ephrins and this stimulates G to activate F. Upon activation, F mediates virus-host membrane fusion. Here we focus on mechanisms that underlie the stimulation of G by ephrins. Experiments show that G interacts with ephrin and F through separate sites located on two different domains, the receptor binding domain (RBD) and the F activation domain (FAD). No models explain this allosteric coupling. In fact, the analogous mechanisms in other paramyxoviruses also remain undetermined. The structural organization of G is such that allosteric coupling must involve at least one of the two interfaces-the RBD-FAD interface and/or the RBD-RBD interface. Here we examine using molecular dynamics the effect of ephrin binding on the RBD-RBD interface. We find that despite inducing small changes in individual RBDs, ephrin reorients the RBD-RBD interface extensively, and in a manner that will enhance solvent exposure of the FAD. While this finding supports a proposed model of G stimulation, we also find from additional simulations that ephrin induces a similar RBD-RBD reorientation in a stimulation-deficient G mutant, V₂₀₉ VG → AAA. Together, our simulations suggest that while inter-RBD reorientation may be important, it is not, by itself, a sufficient condition for G stimulation. Additionally, we find that the mutation affects the conformational ensemble of RBD globally, including the RBD-FAD interface, suggesting the latter's role in G stimulation. Because ephrin induces small changes in individual RBDs, a proper analysis of conformational ensembles required that they are compared directly-we employ a method we developed recently, which we now release at SimTK, and show that it also performs excellently for non-Gaussian distributions.

Zoonotic Potential of Emerging Paramyxoviruses: Knowns and Unknowns

The risk of spillover of enzootic paramyxoviruses and the susceptibility of recipient human and domestic animal populations are defined by a broad collection of ecological and molecular factors that interact in ways that are not yet fully understood. Nipah and Hendra viruses were the first highly lethal zoonotic paramyxoviruses discovered in modern times, but other paramyxoviruses from multiple genera are present in bats and other reservoirs that have unknown potential to spillover into humans. We outline our current understanding of paramyxovirus reservoir hosts and the ecological factors that may drive spillover, and we explore the molecular barriers to spillover that emergent paramyxoviruses may encounter. By outlining what is known about enzootic paramyxovirus receptor usage, mechanisms of innate immune evasion, and other host-specific interactions, we highlight the breadth of unexplored avenues that may be important in understanding paramyxovirus emergence.

Receptor-Targeted Nipah Virus Glycoproteins Improve Cell-Type Selective Gene Delivery and Reveal a Preference for Membrane-Proximal Cell Attachment

Receptor-targeted lentiviral vectors (LVs) can be an effective tool for selective transfer of genes into distinct cell types of choice. Moreover, they can be used to determine the molecular properties that cell surface proteins must fulfill to act as receptors for viral glycoproteins. Here we show that LVs pseudotyped with receptor-targeted Nipah virus (NiV) glycoproteins effectively enter into cells when they use cell surface proteins as receptors that bring them closely enough to the cell membrane (less than 100 Å distance). Then, they were flexible in receptor usage as demonstrated by successful targeting of EpCAM, CD20, and CD8, and as selective as LVs pseudotyped with receptor-targeted measles virus (MV) glycoproteins, the current standard for cell-type specific gene delivery. Remarkably, NiV-LVs could be produced at up to two orders of magnitude higher titers compared to their MV-based counterparts and were at least 10,000-fold less effectively neutralized than MV glycoprotein pseudotyped LVs by pooled human intravenous immunoglobulin. An important finding for NiV-LVs targeted to Her2/neu was an about 100-fold higher gene transfer activity when particles were targeted to membrane-proximal regions as compared to particles binding to a more membrane-distal epitope. Likewise, the low gene transfer activity mediated by NiV-LV particles bound to the membrane distal domains of CD117 or the glutamate receptor subunit 4 (GluA4) was substantially enhanced by reducing receptor size to below 100 Å. Overall, the data suggest that the NiV glycoproteins are optimally suited for cell-type specific gene delivery with LVs and, in addition, for the first time define which parts of a cell surface protein should be targeted to achieve optimal gene transfer rates with receptor-targeted LVs.

Pseudotyping of lentiviral vectors (LVs) with glycoproteins from other enveloped viruses has not only often been revealing in mechanistic studies of particle assembly and entry, but is also of practical importance for gene delivery. LVs pseudotyped with engineered glycoproteins allowing free choice of receptor usage are expected to overcome current limitations in cell-type selectivity of gene transfer. Here we describe for the first time receptor-targeted Nipah virus glycoproteins as important step towards this goal. LV particles carrying the engineered Nipah virus glycoproteins were substantially more efficient in gene delivery than their state-of-the-art measles virus-based counterparts, now making the production of receptor-targeted LVs for clinical applications possible. Moreover, the data define for the first time the molecular requirements for membrane fusion with respect to the position of the receptor binding site relative to the cell membrane, a finding with implications for the molecular evolution of paramyxoviruses using proteinaceous receptors for cell entry.

Hendra virus and Nipah virus animal vaccines

Hendra virus (HeV) and Nipah virus (NiV) are zoonotic viruses that emerged in the mid to late 1990s causing disease outbreaks in livestock and people. HeV appeared in Queensland, Australia in 1994 causing a severe respiratory disease in horses along with a human case fatality. NiV emerged a few years later in Malaysia and Singapore in 1998-1999 causing a large outbreak of encephalitis with high mortality in people and also respiratory disease in pigs which served as amplifying hosts. The key pathological elements of HeV and NiV infection in several species of mammals, and also in people, are a severe systemic and often fatal neurologic and/or respiratory disease. In people, both HeV and NiV are also capable of causing relapsed encephalitis following recovery from an acute infection. The known reservoir hosts of HeV and NiV are several species of pteropid fruit bats. Spillovers of HeV into horses continue to occur in Australia and NiV has caused outbreaks in people in Bangladesh and India nearly annually since 2001, making HeV and NiV important transboundary biological threats. NiV in particular possesses several features that underscore its potential as a pandemic threat, including its ability to infect humans directly from natural reservoirs or indirectly from other susceptible animals, along with a capacity of limited human-to-human transmission. Several HeV and NiV animal challenge models have been developed which have facilitated an understanding of pathogenesis and allowed for the successful development of both active and passive immunization countermeasures.

Henipaviruses

The first henipaviruses, Hendra virus (HeV), and Nipah virus (NiV) were pathogenic zoonoses that emerged in the mid to late 1990s causing serious disease outbreaks in livestock and humans. HeV was recognized in Australia 1994 in horses exhibiting respiratory disease along with a human case fatality, and then NiV was identified during a large outbreak of human cases of encephalitis with high mortality in Malaysia and Singapore in 1998–1999 along with respiratory disease in pigs which served as amplifying hosts. The recently identified third henipavirus isolate, Cedar virus (CedPV), is not pathogenic in animals susceptible to HeV and NiV disease. Molecular detection of additional henipavirus species has been reported but no additional isolates of virus have been reported. Central pathological features of both HeV and NiV infection in humans and several susceptible animal species is a severe systemic and often fatal neurologic and/or respiratory disease. In people, both viruses can also manifest relapsed encephalitis following recovery from an acute infection, particularly NiV. The recognized natural reservoir hosts of HeV, NiV, and CedPV are pteropid bats, which do not show clinical illness when infected. With spillovers of HeV continuing to occur in Australia and NiV in Bangladesh and India, these henipaviruses continue to be important transboundary biological threats. NiV in particular possesses several features that highlight a pandemic potential, such as its ability to infect humans directly from natural reservoirs or indirectly from other susceptible animals along with a capacity of limited human-to-human transmission. Several henipavirus animal challenge models have been developed which has aided in understanding HeV and NiV pathogenesis as well as how they invade the central nervous system, and successful active and passive immunization strategies against HeV and NiV have been reported which target the viral envelope glycoproteins.

Structure and stabilization of the Hendra virus F glycoprotein in its prefusion form

Hendra virus (HeV) is one of the two prototypical members of the Henipavirus genus of paramyxoviruses, which are designated biosafety level 4 (BSL-4) organisms due to the high mortality rate of Nipah virus (NiV) and HeV in humans. Paramyxovirus cell entry is mediated by the fusion protein, F, in response to binding of a host receptor by the attachment protein. During posttranslational processing, the fusion peptide of F is released and, upon receptor-induced triggering, inserts into the host cell membrane. As F undergoes a dramatic refolding from its prefusion to postfusion conformation, the fusion peptide brings the host and viral membranes together, allowing entry of the viral RNA. Here, we present the crystal structure of the prefusion form of the HeV F ectodomain. The structure shows very high similarity to the structure of prefusion parainfluenza virus 5 (PIV5) F, with the main structural differences in the membrane distal apical loops and the fusion peptide cleavage loop. Functional assays of mutants show that the apical loop can tolerate perturbation in length and surface residues without loss of function, except for residues involved in the stability and conservation of the F protein fold. Structure-based disulfide mutants were designed to anchor the fusion peptide to conformationally invariant residues of the F head. Two mutants were identified that inhibit F-mediated fusion by stabilizing F in its prefusion conformation.

Crystal Structure of the Pre-fusion Nipah Virus Fusion Glycoprotein Reveals a Novel Hexamer-of-Trimers Assembly

Nipah virus (NiV) is a paramyxovirus that infects host cells through the coordinated efforts of two envelope glycoproteins. The G glycoprotein attaches to cell receptors, triggering the fusion (F) glycoprotein to execute membrane fusion. Here we report the first crystal structure of the pre-fusion form of the NiV-F glycoprotein ectodomain. Interestingly this structure also revealed a hexamer-of-trimers encircling a central axis. Electron tomography of Nipah virus-like particles supported the hexameric pre-fusion model, and biochemical analyses supported the hexamer-of-trimers F assembly in solution. Importantly, structure-assisted site-directed mutagenesis of the interfaces between F trimers highlighted the functional relevance of the hexameric assembly. Shown here, in both cell-cell fusion and virus-cell fusion systems, our results suggested that this hexamer-of-trimers assembly was important during fusion pore formation. We propose that this assembly would stabilize the pre-fusion F conformation prior to cell attachment and facilitate the coordinated transition to a post-fusion conformation of all six F trimers upon triggering of a single trimer. Together, our data reveal a novel and functional pre-fusion architecture of a paramyxoviral fusion glycoprotein.

Nipah virus fusion protein: Importance of the cytoplasmic tail for endosomal trafficking and bioactivity

Nipah virus (NiV) is a highly pathogenic paramyxovirus which encodes two surface glycoproteins: the receptor-binding protein G and the fusion protein F. As for all paramyxoviruses, proteolytic activation of the NiV-F protein is an indispensable prerequisite for viral infectivity. Interestingly, proteolytic activation of NiV-F differs principally from other paramyxoviruses with respect to protease usage (cathepsins instead of trypsin- or furin-like proteases), and the subcellular localization where cleavage takes place (endosomes instead of Golgi or plasma membrane). To allow efficient F protein activation needed for productive virus replication and cell-to-cell fusion, the NiV-F cytoplasmic tail contains a classical tyrosine-based endocytosis signal (Y₅₂₅RSL) that we have shown earlier to be needed for F uptake and proteolytic activation. In this report, we furthermore revealed that an intact endocytosis signal alone is not sufficient for full bioactivity. The very C-terminus of the cytoplasmic tail is needed in addition. Deletions of more than four residues did not affect F uptake or endosomal cleavage but downregulated the surface expression, likely by delaying the intracellular trafficking through endosomal-recycling compartments. Given that the NiV-F cytoplasmic tail is needed for timely and correct intracellular trafficking, endosomal cleavage and fusion activity, the influence of tail truncations on NiV-mediated cell-to-cell fusion and on pseudotyping lentiviral vectors is discussed.

Molecular recognition of human ephrinB2 cell surface receptor by an emergent African henipavirus

The discovery of African henipaviruses (HNVs) related to pathogenic Hendra virus (HeV) and Nipah virus (NiV) from Southeast Asia and Australia presents an open-ended health risk. Cell receptor use by emerging African HNVs at the stage of host-cell entry is a key parameter when considering the potential for spillover and infection of human populations. The attachment glycoprotein from a Ghanaian bat isolate (GhV-G) exhibits <30% sequence identity with Asiatic NiV-G/HeV-G. Here, through functional and structural analysis of GhV-G, we show how this African HNV targets the same human cell-surface receptor (ephrinB2) as the Asiatic HNVs. We first characterized this virus-receptor interaction crystallographically. Compared with extant HNV-G-ephrinB2 structures, there was significant structural variation in the six-bladed β-propeller scaffold of the GhV-G receptor-binding domain, but not the Greek key fold of the bound ephrinB2. Analysis revealed a surprisingly conserved mode of ephrinB2 interaction that reflects an ongoing evolutionary constraint among geographically distal and phylogenetically divergent HNVs to maintain the functionality of ephrinB2 recognition during virus-host entry. Interestingly, unlike NiV-G/HeV-G, we could not detect binding of GhV-G to ephrinB3. Comparative structure-function analysis further revealed several distinguishing features of HNV-G function: a secondary ephrinB2 interaction site that contributes to more efficient ephrinB2-mediated entry in NiV-G relative to GhV-G and cognate residues at the very C terminus of GhV-G (absent in Asiatic HNV-Gs) that are vital for efficient receptor-induced fusion, but not receptor binding per se. These data provide molecular-level details for evaluating the likelihood of African HNVs to spill over into human populations.

Evidence for ubiquitin-regulated nuclear and subnuclear trafficking among Paramyxovirinae matrix proteins

The paramyxovirus matrix (M) protein is a molecular scaffold required for viral morphogenesis and budding at the plasma membrane. Transient nuclear residence of some M proteins hints at non-structural roles. However, little is known regarding the mechanisms that regulate the nuclear sojourn. Previously, we found that the nuclear-cytoplasmic trafficking of Nipah virus M (NiV-M) is a prerequisite for budding, and is regulated by a bipartite nuclear localization signal (NLS_bp), a leucine-rich nuclear export signal (NES), and monoubiquitination of the K258 residue within the NLSbp itself (NLS_bp-lysine). To define whether the sequence determinants of nuclear trafficking identified in NiV-M are common among other Paramyxovirinae M proteins, we generated the homologous NES and NLS_bp-lysine mutations in M proteins from the five major Paramyxovirinae genera. Using quantitative 3D confocal microscopy, we determined that the NES and NLS_bp-lysine are required for the efficient nuclear export of the M proteins of Nipah virus, Hendra virus, Sendai virus, and Mumps virus. Pharmacological depletion of free ubiquitin or mutation of the conserved NLS_bp-lysine to an arginine, which inhibits M ubiquitination, also results in nuclear and nucleolar retention of these M proteins. Recombinant Sendai virus (rSeV-eGFP) bearing the NES or NLS_bp-lysine M mutants rescued at similar efficiencies to wild type. However, foci of cells expressing the M mutants displayed marked fusogenicity in contrast to wild type, and infection did not spread. Recombinant Mumps virus (rMuV-eGFP) bearing the homologous mutations showed similar defects in viral morphogenesis. Finally, shotgun proteomics experiments indicated that the interactomes of Paramyxovirinae M proteins are significantly enriched for components of the nuclear pore complex, nuclear transport receptors, and nucleolar proteins. We then synthesize our functional and proteomics data to propose a working model for the ubiquitin-regulated nuclear-cytoplasmic trafficking of cognate paramyxovirus M proteins that show a consistent nuclear trafficking phenotype.

Elucidating virus-cell interactions is fundamental to understanding viral replication and identifying targets for therapeutic control of viral infection. Paramyxoviruses include human and animal pathogens of medical and agricultural significance. Their matrix (M) structural protein organizes virion assembly at the plasma membrane and mediates viral budding. While nuclear localization of M proteins has been described for some paramyxoviruses, the underlying mechanisms of nuclear trafficking and the biological relevance of this observation have remained largely unexamined. Through comparative analyses of M proteins across five Paramyxovirinae genera, we identify M proteins from at least three genera that exhibit similar nuclear trafficking phenotypes regulated by an NLS_bp as well as an NES sequence within M that may mediate the interaction of M with host nuclear transport receptors. Additionally, a conserved lysine within the NLS_bp of some M proteins is required for nuclear export by regulating M ubiquitination. Sendai virus engineered to express a ubiquitination-defective M does not produce infectious virus but instead displays extensive cell-cell fusion while M is retained in the nucleolus. Thus, some Paramyxovirinae M proteins undergo regulated and active nuclear and subnuclear transport, a prerequisite for viral morphogenesis, which also suggests yet to be discovered roles for M in the nucleus.

Unity in diversity: shared mechanism of entry among paramyxoviruses

The Paramyxoviridae family includes many viruses that are pathogenic in humans, including parainfluenza viruses, measles virus, respiratory syncytial virus, and the emerging zoonotic Henipaviruses. No effective treatments are currently available for these viruses, and there is a need for efficient antiviral therapies. Paramyxoviruses enter the target cell by binding to a cell surface receptor and then fusing the viral envelope with the target cell membrane, allowing the release of the viral genome into the cytoplasm. Blockage of these crucial steps prevents infection and disease. Binding and fusion are driven by two virus-encoded glycoproteins, the receptor-binding protein and the fusion protein, that together form the viral "fusion machinery." The development of efficient antiviral drugs requires a deeper understanding of the mechanism of action of the Paramyxoviridae fusion machinery, which is still controversial. Here, we review recent structural and functional data on these proteins and the current understanding of the mechanism of the paramyxovirus cell entry process.

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.

Bitter-sweet symphony: glycan-lectin interactions in virus biology

Glycans are carbohydrate modifications typically found on proteins or lipids, and can act as ligands for glycan-binding proteins called lectins. Glycans and lectins play crucial roles in the function of cells and organs, and in the immune system of animals and humans. Viral pathogens use glycans and lectins that are encoded by their own or the host genome for their replication and spread. Recent advances in glycobiological research indicate that glycans and lectins mediate key interactions at the virus-host interface, controlling viral spread and/or activation of the immune system. This review reflects on glycan-lectin interactions in the context of viral infection and antiviral immunity. A short introduction illustrates the nature of glycans and lectins, and conveys the basic principles of their interactions. Subsequently, examples are discussed highlighting specific glycan-lectin interactions and how they affect the progress of viral infections, either benefiting the host or the virus. Moreover, glycan and lectin variability and their potential biological consequences are discussed. Finally, the review outlines how recent advances in the glycan-lectin field might be transformed into promising new approaches to antiviral therapy.

Co-assembly of viral envelope glycoproteins regulates their polarized sorting in neurons

Newly synthesized envelope glycoproteins of neuroinvasive viruses can be sorted in a polarized manner to the somatodendritic and/or axonal domains of neurons. Although critical for transneuronal spread of viruses, the molecular determinants and interregulation of this process are largely unknown. We studied the polarized sorting of the attachment (NiV-G) and fusion (NiV-F) glycoproteins of Nipah virus (NiV), a paramyxovirus that causes fatal human encephalitis, in rat hippocampal neurons. When expressed individually, NiV-G exhibited a non-polarized distribution, whereas NiV-F was specifically sorted to the somatodendritic domain. Polarized sorting of NiV-F was dependent on interaction of tyrosine-based signals in its cytosolic tail with the clathrin adaptor complex AP-1. Co-expression of NiV-G with NiV-F abolished somatodendritic sorting of NiV-F due to incorporation of NiV-G•NiV-F complexes into axonal transport carriers. We propose that faster biosynthetic transport of unassembled NiV-F allows for its proteolytic activation in the somatodendritic domain prior to association with NiV-G and axonal delivery of NiV-G•NiV-F complexes. Our study reveals how interactions of viral glycoproteins with the host's transport machinery and between themselves regulate their polarized sorting in neurons.

Neurons are highly polarized cells exhibiting somatodendritic and axonal domains with distinct protein and lipid compositions. Some enveloped viruses target neurons by binding of the viral envelope glycoproteins to neuronal surface receptors. The ensuing fusion of the viral and neuronal membranes delivers the genetic material of the virus into the neurons. During viral replication in neurons, newly synthesized envelope glycoproteins are sorted to the somatodendritic and/or axonal domains. Although critical for viral propagation, the mechanisms responsible for this sorting are largely unknown. We studied the neuronal sorting of the attachment (NiV-G) and fusion (NiV-F) glycoproteins of Nipah virus, a pathogen that causes fatal human encephalitis. When analyzed individually, NiV-G was delivered to both the axonal and somatodendritic domains. In contrast, NiV-F was exclusively targeted to the somatodendritic domain by virtue of interaction of specific signals in this protein with AP-1, a component of the neuronal protein transport machinery. Assembly with NiV-G, however, abolished somatodendritic sorting of NiV-F due to incorporation of complexes into axon-bound vesicles. Thus, coordinated interactions of viral glycoproteins with the host's sorting machinery and between themselves allow temporal and spatial regulation of their distribution in neurons. We propose that this coordination facilitates viral spread among neurons.

Structural basis of efficient contagion: measles variations on a theme by parainfluenza viruses

A quartet of attachment proteins and a trio of fusion protein subunits play the cell entry concert of parainfluenza viruses. While many of these viruses bind sialic acid to enter cells, wild type measles binds exclusively two tissue-specific proteins, the lymphatic receptor signaling lymphocytic activation molecule (SLAM), and the epithelial receptor nectin-4. SLAM binds near the stalk-head junction of the hemagglutinin. Nectin-4 binds a hydrophobic groove located between blades 4 and 5 of the hemagglutinin β-propeller head. The mutated vaccine strain hemagglutinin binds in addition the ubiquitous protein CD46, which explains attenuation. The measles virus entry concert has four movements. Andante misterioso: the virus takes over the immune system. Allegro con brio: it rapidly spreads in the upper airway's epithelia. 'Targeting' fugue: the versatile orchestra takes off. Presto furioso: the virus exits the host with thunder. Be careful: music is contagious.

The measles virus hemagglutinin stalk: structures and functions of the central fusion activation and membrane-proximal segments

The measles virus (MeV) membrane fusion apparatus consists of a fusion protein trimer and an attachment protein tetramer. To trigger membrane fusion, the heads of the MeV attachment protein, hemagglutinin (H), bind cellular receptors while the 96-residue-long H stalk transmits the triggering signal. Structural and functional studies of the triggering mechanism of other paramyxoviruses suggest that receptor binding to their hemagglutinin-neuraminidase (HN) results in signal transmission through the central segments of their stalks. To gain insight into H-stalk structure and function, we individually replaced its residues with cysteine. We then assessed how stable the mutant proteins are, how efficiently they can be cross-linked by disulfide bonds, whether cross-linking results in loss of function, and, in this case, whether disulfide bond reduction restores function. While many residues in the central segment of the stalk and in the spacer segment above it can be efficiently cross-linked by engineered disulfide bonds, we report here that residues 59 to 79 cannot, suggesting that the 20 membrane-proximal residues are not engaged in a tetrameric structure. Rescue-of-function studies by disulfide bond reduction resulted in the redefinition and extension of the central fusion-activation segment as covering residues 84 to 117. In particular, we identified four residues located between positions 92 and 99, the function of which cannot be restored by disulfide bond reduction after cysteine mutagenesis. These mutant H proteins reached the cell surface as complex oligomers but could not trigger membrane fusion. We discuss these observations in the context of the stalk exposure model of membrane fusion triggering by paramyxoviruses.

IMPORTANCE

Measles virus, while being targeted for eradication, still causes significant morbidity and mortality. Here, we seek to understand how it enters cells by membrane fusion. Two viral integral membrane glycoproteins (hemagglutinin tetramers and fusion protein trimers) mediate the concerted receptor recognition and membrane fusion processes. Since previous studies have suggested that the hemagglutinin stalk transmits the triggering signal to the fusion protein trimer, we completed an analysis of its structure and function by systematic Cys mutagenesis. We report that while certain residues of the central stalk segment confer specificity to the interaction with the fusion protein trimer, others are necessary to allow folding of the H-oligomer in a standard conformation conducive to fusion triggering, and still other residues sustain the conformational change that transmits the fusion-triggering signal.

Interaction between the hemagglutinin-neuraminidase and fusion glycoproteins of human parainfluenza virus type III regulates viral growth in vivo

Paramyxoviruses, enveloped RNA viruses that include human parainfluenza virus type 3 (HPIV3), cause the majority of childhood viral pneumonia. HPIV3 infection starts when the viral receptor-binding protein engages sialic acid receptors in the lung and the viral envelope fuses with the target cell membrane. Fusion/entry requires interaction between two viral surface glycoproteins: tetrameric hemagglutinin-neuraminidase (HN) and fusion protein (F). In this report, we define structural correlates of the HN features that permit infection in vivo. We have shown that viruses with an HN-F that promotes growth in cultured immortalized cells are impaired in differentiated human airway epithelial cell cultures (HAE) and in vivo and evolve in HAE into viable viruses with less fusogenic HN-F. In this report, we identify specific structural features of the HN dimer interface that modulate HN-F interaction and fusion triggering and directly impact infection. Crystal structures of HN, which promotes viral growth in vivo, show a diminished interface in the HN dimer compared to the reference strain's HN, consistent with biochemical and biological data indicating decreased dimerization and decreased interaction with F protein. The crystallographic data suggest a structural explanation for the HN's altered ability to activate F and reveal properties that are critical for infection in vivo.

IMPORTANCE

Human parainfluenza viruses cause the majority of childhood cases of croup, bronchiolitis, and pneumonia worldwide. Enveloped viruses must fuse their membranes with the target cell membranes in order to initiate infection. Parainfluenza fusion proceeds via a multistep reaction orchestrated by the two glycoproteins that make up its fusion machine. In vivo, viruses adapt for survival by evolving to acquire a set of fusion machinery features that provide key clues about requirements for infection in human beings. Infection of the lung by parainfluenzavirus is determined by specific interactions between the receptor binding molecule (hemagglutinin-neuraminidase [HN]) and the fusion protein (F). Here we identify specific structural features of the HN dimer interface that modulate HN-F interaction and fusion and directly impact infection. The crystallographic and biochemical data point to a structural explanation for the HN's altered ability to activate F for fusion and reveal properties that are critical for infection by this important lung virus in vivo.

Crystal structure of the Hendra virus attachment G glycoprotein bound to a potent cross-reactive neutralizing human monoclonal antibody

The henipaviruses, represented by Hendra (HeV) and Nipah (NiV) viruses are highly pathogenic zoonotic paramyxoviruses with uniquely broad host tropisms responsible for repeated outbreaks in Australia, Southeast Asia, India and Bangladesh. The high morbidity and mortality rates associated with infection and lack of licensed antiviral therapies make the henipaviruses a potential biological threat to humans and livestock. Henipavirus entry is initiated by the attachment of the G envelope glycoprotein to host cell membrane receptors. Previously, henipavirus-neutralizing human monoclonal antibodies (hmAb) have been isolated using the HeV-G glycoprotein and a human naïve antibody library. One cross-reactive and receptor-blocking hmAb (m102.4) was recently demonstrated to be an effective post-exposure therapy in two animal models of NiV and HeV infection, has been used in several people on a compassionate use basis, and is currently in development for use in humans. Here, we report the crystal structure of the complex of HeV-G with m102.3, an m102.4 derivative, and describe NiV and HeV escape mutants. This structure provides detailed insight into the mechanism of HeV and NiV neutralization by m102.4, and serves as a blueprint for further optimization of m102.4 as a therapeutic agent and for the development of entry inhibitors and vaccines.

Since their initial emergence, henipaviruses have continued to cause spillover events in both human and livestock populations, posing significant biothreats. Currently there are no licensed or approved therapies for treatment of henipavirus infection and the human case mortality rates average >70%. We used X-ray crystallography to determine the high-resolution structures of the Hendra virus G glycoprotein in complex with a cross-reactive neutralizing human monoclonal antibody. The structures provide detailed insight into the mechanism of HeV and NiV neutralization by this potent and clinically-relevant human monoclonal antibody that is currently in development for use in humans. This monoclonal antibody was recently shown to be an effective post-exposure therapy in non-human models of lethal Hendra virus infection. Indeed, it has already been used in four people on a compassionate use request, three in Australia and one in the United States, as a therapeutic agent. Furthermore, we identified and characterized two escape mutants generated in vitro and evaluated their mechanism of escape. Our results serve as a blueprint for further optimization of this antibody and for the development of novel entry inhibitors and vaccines. This report also supports the additional pre-clinical data required for eventual licensure by detailing the antibody's mechanism of henipavirus neutralization.

Measles virus fusion machinery activated by sialic acid binding globular domain

Paramyxoviruses, including the human pathogen measles virus (MV) and the avian Newcastle disease virus (NDV), enter host cells through fusion of the viral envelope with the target cell membrane. This fusion is driven by the concerted action of two viral envelope glycoproteins: the receptor binding protein and the fusion protein (F). The MV receptor binding protein (hemagglutinin [H]) attaches to proteinaceous receptors on host cells, while the receptor binding protein of NDV (hemagglutinin-neuraminidase [HN]) interacts with sialic acid-containing receptors. The receptor-bound HN/H triggers F to undergo conformational changes that render it competent to mediate fusion of the viral and cellular membranes. The mechanism of fusion activation has been proposed to be different for sialic acid-binding viruses and proteinaceous receptor-binding viruses. We report that a chimeric protein containing the NDV HN receptor binding region and the MV H stalk domain can activate MV F to fuse, suggesting that the signal to the stalk of a protein-binding receptor binding molecule can be transmitted from a sialic acid binding domain. By engineering the NDV HN globular domain to interact with a proteinaceous receptor, the fusion activation signal was preserved. Our findings are consistent with a unified mechanism of fusion activation, at least for the Paramyxovirinae subfamily, in which the receptor binding domains of the receptor binding proteins are interchangeable and the stalk determines the specificity of F activation.

Identification of a region in the stalk domain of the nipah virus receptor binding protein that is critical for fusion activation

Paramyxoviruses, including the emerging lethal human Nipah virus (NiV) and the avian Newcastle disease virus (NDV), enter host cells through fusion of the viral and target cell membranes. For paramyxoviruses, membrane fusion is the result of the concerted action of two viral envelope glycoproteins: a receptor binding protein and a fusion protein (F). The NiV receptor binding protein (G) attaches to ephrin B2 or B3 on host cells, whereas the corresponding hemagglutinin-neuraminidase (HN) attachment protein of NDV interacts with sialic acid moieties on target cells through two regions of its globular domain. Receptor-bound G or HN via its stalk domain triggers F to undergo the conformational changes that render it competent to mediate fusion of the viral and cellular membranes. We show that chimeric proteins containing the NDV HN receptor binding regions and the NiV G stalk domain require a specific sequence at the connection between the head and the stalk to activate NiV F for fusion. Our findings are consistent with a general mechanism of paramyxovirus fusion activation in which the stalk domain of the receptor binding protein is responsible for F activation and a specific connecting region between the receptor binding globular head and the fusion-activating stalk domain is required for transmitting the fusion signal.

A treatment for and vaccine against the deadly Hendra and Nipah viruses

Hendra virus and Nipah virus are bat-borne paramyxoviruses that are the prototypic members of the genus Henipavirus. The henipaviruses emerged in the 1990s, spilling over from their natural bat hosts and causing serious disease outbreaks in humans and livestock. Hendra virus emerged in Australia and since 1994 there have been 7 human infections with 4 case fatalities. Nipah virus first appeared in Malaysia and subsequent outbreaks have occurred in Bangladesh and India. In total, there have been an estimated 582 human cases of Nipah virus and of these, 54% were fatal. Their broad species tropism and ability to cause fatal respiratory and/or neurologic disease in humans and animals make them important transboundary biological threats. Recent experimental findings in animals have demonstrated that a human monoclonal antibody targeting the viral G glycoprotein is an effective post-exposure treatment against Hendra and Nipah virus infection. In addition, a subunit vaccine based on the G glycoprotein of Hendra virus affords protection against Hendra and Nipah virus challenge. The vaccine has been developed for use in horses in Australia and is the first vaccine against a Biosafety Level-4 (BSL-4) agent to be licensed and commercially deployed. Together, these advances offer viable approaches to address Hendra and Nipah virus infection of livestock and people.

Envelope protein dynamics in paramyxovirus entry

Paramyxoviruses include major pathogens with significant global health and economic impact. This large family of enveloped RNA viruses infects cells by employing two surface glycoproteins that tightly cooperate to fuse their lipid envelopes with the target cell plasma membrane, an attachment and a fusion (F) protein. Membrane fusion is believed to depend on receptor-induced conformational changes within the attachment protein that lead to the activation and subsequent refolding of F. While structural and mechanistic studies have considerably advanced our insight into paramyxovirus cell adhesion and the structural basis of F refolding, how precisely the attachment protein links receptor engagement to F triggering remained poorly understood. Recent reports based on work with several paramyxovirus family members have transformed our understanding of the triggering mechanism of the membrane fusion machinery. Here, we review these recent findings, which (i) offer a broader mechanistic understanding of the paramyxovirus cell entry system, (ii) illuminate key similarities and differences between entry strategies of different paramyxovirus family members, and (iii) suggest new strategies for the development of novel therapeutics.