Nipah and Hendra Viruses Use an Adjustable Latch in Receptor Engagement.

Cellular entry of paramyxoviruses requires the coordinated action of both the attachment (G/H/HN) and fusion (F) glycoproteins, but how receptor binding activates G to trigger F-mediated fusion during viral entry is not known. Here, we identify a receptor (ephrinB2)-induced allosteric activation site in Nipah virus (NiV) G involved in triggering F-mediated fusion. We first generated a conformational monoclonal antibody (monoclonal antibody 45 (Mab45)) whose binding to NiV-G was enhanced upon NiV-G-ephrinB2 binding. However, Mab45 also inhibited viral entry, and its receptor binding-enhanced (RBE) epitope was temperature-dependent, suggesting that the Mab45 RBE epitope on G may be involved in triggering F. The Mab45 RBE epitope was mapped to the base of the globular domain (beta6S4/beta1H1). Alanine scan mutants within this region that did not exhibit this RBE epitope were also non-fusogenic despite their ability to bind ephrinB2, oligomerize, and associate with F at wild-type (WT) levels. Although circular dichroism revealed conformational changes in the soluble ectodomain of WT NiV-G upon ephrinB2 addition, no such changes were detected with soluble RBE epitope mutants or short-stalk G mutants. Additionally, WT G, but not a RBE epitope mutant, could dissociate from F upon ephrinB2 engagement. Finally, using a biotinylated HR2 peptide to detect pre-hairpin intermediate formation, a cardinal feature of F-triggering, we showed that ephrinB2 binding to WT G, but not the RBE-epitope mutants, could trigger F. In sum, we implicate the coordinated interaction between the base of NiV-G globular head domain and the stalk domain in mediating receptor-induced F triggering during viral entry.

Henipaviruses and Fruit Bats, Papua New Guinea

Hendra and Nipah viruses are classified as biosafety level 4 (BSL-4) agents in the United States and Australia, as there is a risk of infection of laboratory personnel and appropriate precautions must be taken. Widespread presence of Nipah virus antigens could be seen by immunohistochemistry (IHC) in endothelial and smooth muscle cells of blood vessels and in various parenchymal cells, particularly in neurons. Primer sequences can be designed to detect both Hendra and Nipah viruses or to detect each virus separately. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) assays can facilitate research into the pathogenesis of Hendra and Nipah viruses. African green monkey kidney (Vero) cells inoculated with high-titer samples contain antigen within a few days of inoculation. Cytopathic effect (CPE) is usually seen within 3 to 5 days. The CPE is noticeable by the formation of syncytia containing several nuclei. To confirm the presence and identity of the virus, fixed cells are tested by immunofluorescence assay, or supernatant fluids can be tested for evidence of viral replication by RT-PCR techniques. Hendra and Nipah viruses can be reliably distinguished from each other only by neutralization tests. Other cells, such as rabbit kidney cells (RK-13), are susceptible to Hendra and Nipah viruses. Hyperimmune mouse ascites fluid, which recognizes all of the structural proteins of Nipah or Hendra viruses, is used as the positive control. The enzyme-linked immunosorbent assay (ELISA) immunoglobulin M (IgM) test has been the most important technique for serological confirmation in acute patients.

Medically important paramyxoviruses, such as measles, mumps, parainfluenza, Nipah, and Hendra viruses, infect host cells by directing fusion of the viral and cellular plasma membranes. Upon infection, paramyxoviruses cause a second type of membrane fusion, cell-cell fusion (syncytium formation), which is linked to pathogenicity. Host cell receptor binding causes conformational changes in the attachment glycoprotein (HN, H, or G) that trigger a conformational cascade in the fusion (F) glycoprotein that mediates membrane fusion. F, a class I fusion protein, contains the archetypal heptad repeat regions 1 (HR1) and 2 (HR2). It is well established that binding of HR1 and HR2 is key to fusing viral and cellular membranes. In this study, we uncovered a novel fusion-modulatory role of a third structurally conserved helical region (HR3) in F. Based on its location within the F structure, and structural differences between its prefusion and postfusion conformations, we hypothesized that the HR3 modulates triggering of the F conformational cascade (still requiring G). We used the deadly Nipah virus (NiV) as an important paramyxoviral model to perform alanine scan mutagenesis and a series of multidisciplinary structural/functional analyses that dissect the various states of the membrane fusion cascade. Remarkably, we found that specific residues within the HR3 modulate not only early F-triggering but also late extensive fusion pore expansion steps in the membrane fusion cascade. Our results characterize these novel fusion-modulatory roles of the F HR3, improving our understanding of the membrane fusion process for NiV and likely for the related Henipavirus genus and possibly Paramyxoviridae family members.IMPORTANCE The Paramyxoviridae family includes important human and animal pathogens, such as measles, mumps, and parainfluenza viruses and the deadly henipaviruses Nipah (NiV) and Hendra (HeV) viruses. Paramyxoviruses infect the respiratory tract and the central nervous system (CNS) and can be highly infectious. Most paramyxoviruses have a limited host range. However, the biosafety level 4 NiV and HeV are highly pathogenic and have a wide mammalian host range. Nipah viral infections result in acute respiratory syndrome and severe encephalitis in humans, leading to 40 to 100% mortality rates. The lack of licensed vaccines or therapeutic approaches against NiV and other important paramyxoviruses underscores the need to understand viral entry mechanisms. In this study, we uncovered a novel role of a third helical region (HR3) of the NiV fusion glycoprotein in the membrane fusion process that leads to viral entry. This discovery sets HR3 as a new candidate target for antiviral strategies for NiV and likely for related viruses.

Nipah and Hendra Viruses

Henipaviruses are the deadliest human pathogens of the Paramyxoviridae family. The genus Henipavirus includes two closely related, zoonotic, and highly pathogenic paramyxoviruses, Nipah virus (NiV) and Hendra virus (HeV), which cause high morbidity and mortality in animals and humans. There are no approved therapeutics to treat or prevent henipavirus infections in humans. The bat species Pteropus are the natural reservoir of henipaviruses, and zoonotic transmission can occur directly or through an intermediate host; horses for HeV and pigs for NiV. The unavailability of a treatment for these infections, the recent detection of HeV antibodies in family pets and the emergence of NiV infections in new geographic areas (Philippines) are an illustration of the risk that these viruses possess and their momentum of becoming a global threat. Unless an effective therapeutic against these viruses is developed, they will continue to emerge and cause significant challenges and threats in the coming years. Key Concepts The newly established genus Henipavirus of Paramyxoviridae family has three members Hendra, Nipah and Cedar viruses. Fruit bats of the genus Pteropus have been identified as the natural reservoir for henipaviruses. Both Nipah and Hendra viruses are highly pathogenic agents that cause acute encephalitis and respiratory illness with high mortality rate in humans, pigs and horses. There is no approved therapeutics for the treatment of henipaviruses, and they continue to pose a threat in Australia (Hendra virus) and East and Southeast Asia (Nipah virus) with a great potential to spread beyond these regions. M 102.4 is an experimental monoclonal antibody that targets the viral entry stage of Henipavirus and was proven to treat henipavirus infections in animals.

Hendra virus is a negative-sense single-stranded RNA virus within the Paramyxoviridae family which, together with Nipah virus, forms the Henipavirus genus. Infection with bat-borne Hendra virus leads to a disease with high mortality rates in humans. We determined the crystal structure of the unliganded six-bladed beta-propeller domain and compared it to the previously reported structure of Hendra virus attachment glycoprotein (HeV-G) in complex with its cellular receptor, ephrin-B2. As observed for the related unliganded Nipah virus structure, there is plasticity in the Glu579-Pro590 and Lys236-Ala245 ephrin-binding loops prior to receptor engagement. These data reveal that henipaviral attachment glycoproteins undergo common structural transitions upon receptor binding and further define the structural template for antihenipaviral drug design. Our analysis also provides experimental evidence for a dimeric arrangement of HeV-G that exhibits striking similarity to those observed in crystal structures of related paramyxovirus receptor-binding glycoproteins. The biological relevance of this dimer is further supported by the positional analysis of glycosylation sites from across the paramyxoviruses. In HeV-G, the sites lie away from the putative dimer interface and remain accessible to alpha-mannosidase processing on oligomerization. We therefore propose that the overall mode of dimer assembly is conserved for all paramyxoviruses; however, while the geometry of dimerization is rather closely similar for those viruses that bind flexible glycan receptors, significant (up to 60 degrees ) and different reconfigurations of the subunit packing (associated with a significant decrease in the size of the dimer interface) have accompanied the independent switching to high-affinity protein receptor binding in Hendra and measles viruses.

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.

Hendra virus (HeV) and Nipah virus (NiV) are closely related emerging viruses comprising the Henipavirus genus of the Paramyxovirinae. Each has a broad species tropism and can cause disease with high mortality in both animal and human hosts. These viruses infect cells by a pH-independent membrane fusion event mediated by their attachment (G) and fusion (F) envelope glycoproteins (Envs). Seven Fabs, m101 to -7, were selected for their significant binding to a soluble form of Hendra G (sG) which was used as the antigen for panning of a large naïve human antibody library. The selected Fabs inhibited, to various degrees, cell fusion mediated by the HeV or NiV Envs and virus infection. The conversion of the most potent neutralizer of infectious HeV, Fab m101, to immunoglobulin G1 (IgG1) significantly increased its cell fusion inhibitory activity: the 50% inhibitory concentration was decreased more than 10-fold to approximately 1 microg/ml. The IgG1 m101 was also exceptionally potent in neutralizing infectious HeV; complete (100%) neutralization was achieved with 12.5 microg/ml, and 98% neutralization required only 1.6 microg/ml. The inhibition of fusion and infection correlated with binding of the Fabs to full-length G as measured by immunoprecipitation and less with binding to sG as measured by enzyme-linked immunosorbent assay and Biacore. m101 and m102 competed with the ephrin-B2, which we recently identified as a functional receptor for both HeV and NiV, indicating a possible mechanism of neutralization by these antibodies. The m101, m102, and m103 antibodies competed with each other, suggesting that they bind to overlapping epitopes which are distinct from the epitopes of m106 and m107. In an initial attempt to localize the epitopes of m101 and m102, we measured their binding to a panel of 11 G alanine-scanning mutants and identified two mutants, P185A and Q191 K192A, which significantly decreased binding to m101 and one, G183, which decreased binding of m102 to G. These results suggest that m101 to -7 are specific for HeV or NiV or both and exhibit various neutralizing activities; they are the first human monoclonal antibodies identified against these viruses and could be used for treatment, prophylaxis, and diagnosis and as research reagents and could aid in the development of vaccines.

The article is devoted to the development of a method for detection of viral RNA of two highly pathogenic zoonotic viruses from the genus Henipavirus - Hendra and Nipah using real-time reverse transcription polymerase chain reaction. In the natural environment, these viruses are carried by flying foxes in the family Pteropodidae. Horses and pigs, respectively, are susceptible to infection. The diseases are also transmitted to humans through contact with sick animals, their biological excreta and from person to person. In infected humans and animals, clinical signs of infection may be asymptomatic, or may present with flu-like symptoms at the onset of the disease and progress to neurologic disease and acute respiratory infection, followed by death. In Australia, the subunit vaccine HeV-sG is used against Hendra virus in horses. There is no treatment or vaccine for Hendra or Nipah viruses for humans. The need to develop new detection methods and search for new viral targets remains an urgent task due to the large area of distribution of the described viruses, high contagiousness and mortality of animals and humans. The study describes the original designed primers and probes for conserved regions of the genomes of two viruses: the gene encoding the nucleocapsid protein of Hendra virus and the gene encoding the glycoprotein of Nipah virus. Synthetic controls for the extraction and reverse transcription PCR stages have been created, confirming the quality of the developed method. Biological samples from healthy people (blood plasma, swabs from oral and nasopharyngeal mucous membranes, cerebrospinal fluid) with the addition of artificial controls passed the stages of sample extraction and real-time reverse transcription PCR, thus confirming the quality of control samples. The detection limit of the described viral RNA identification methods was determined as 100 copies/mL for Hendra virus and 1000 copies/mL for Nipah virus. The amplification transit time is less than 90 minutes. The developed method will help in epidemiologic control of the spread of these infections, can be used in the diagnosis of Hendra and Nipah viruses and for solving research tasks to study the properties of these pathogens.

The henipaviruses, including Nipah virus (NiV) and Hendra virus (HeV), are biosafety level 4 (BSL-4) zoonotic pathogens that cause severe neurological and respiratory disease in humans. To study the replication machinery of these viruses, we developed robust minigenome systems that can be safely used in BSL-2 conditions. The nucleocapsid (N), phosphoprotein (P), and large protein (L) of henipaviruses are critical elements of their replication machinery and thus essential support components of the minigenome systems. Here, we tested the effects of diverse combinations of the replication support proteins on the replication capacity of the NiV and HeV minigenomes by exchanging the helper plasmids coding for these proteins among the two viruses. We demonstrate that all combinations including one or more heterologous proteins were capable of replicating both the NiV and HeV minigenomes. Sequence alignment showed identities of 92% for the N protein, 67% for P, and 87% for L. Notably, variations in amino acid residues were not concentrated in the N-P and P-L interacting regions implying that dissimilarities in amino acid composition among NiV and HeV polymerase complex proteins may not impact their interactions. The observed indiscriminate activity of NiV and HeV polymerase complex proteins is different from related viruses, which can support the replication of heterologous genomes only when the whole polymerase complex belongs to the same virus. This newly observed promiscuous property of the henipavirus polymerase complex proteins likely attributed to their conserved interaction regions could potentially be harnessed to develop universal anti-henipavirus antivirals.IMPORTANCEGiven the severity of disease induced by Hendra and Nipah viruses in humans and the continuous emergence of new henipaviruses as well as henipa-like viruses, it is necessary to conduct a more comprehensive investigation of the biology of henipaviruses and their interaction with the host. The replication of henipaviruses and the development of antiviral agents can be studied in systems that allow experiments to be performed under biosafety level 2 conditions. Here, we developed robust minigenome systems for the Nipah virus (NiV) and Hendra virus (HeV) that provide a convenient alternative for studying NiV and HeV replication. Using these systems, we demonstrate that any combination of the three polymerase complex proteins of NiV and HeV could effectively initiate the replication of both viral minigenomes, which suggests that the interaction regions of the polymerase complex proteins could be effective targets for universal and effective anti-henipavirus interventions.

Cedar virus (CedV), closely related to the Hendra and Nipah viruses, is a novel Henipavirus that was originally isolated from flying foxes in Australia in 2012. Although its glycoprotein G exhibits relatively low sequence similarity with its counterparts of the Hendra and Nipah viruses, CedV also uses ephrin receptors, i.e., ephrins B1, B2, A2 and A5, to enters human cells. Nevertheless, the entry mechanism of CedV into bat cells remains unexplored. Considering that Rousettus aegyptiacus (Egyptian Rousette bat, ERB) is postulated to be a reservoir host for henipaviruses, we aim to reveal the receptors utilized by CedV to enable its entry into ERB cells. To this end, we cloned the class A and B ephrins of ERB and generated CHO-K1 cells stably expressing individual ephrins. We also developed a lentivirus-based pseudovirus system containing the firefly luciferase reporter. Assessment of the luciferase activity in cells expressing single ephrins demonstrated that the ERB ephrin B1 and B2 mediated CedV pseudovirus entry. Further, we generated a recombinant CedV expressing the fluorescent protein TurboFP635 (rCedV-nTurbo635). By performing high-content microscopy and flow cytometry, we unveiled that, in addition to ephrin B1 and B2, ephrin A5 was also able to mediate rCedV-nTurbo635 entry, although to a much lesser extent. In contrast to human ephrin A2, ERB ephrin A2 failed to mediate rCedV-nTurbo635 entry. Finally, we generated ERB epithelial cells with ephrin B1 and/or ephrin B2 knockdown (KD). The entry of rCedV-nTurbo635 into ERB epithelial cells was drastically impaired by ephrin B1/B2 KD, validating the importance of ephrin B1 and B2 in its entry. Altogether, we conclude that CedV primarily employs ERB ephrin B1, B2 and, possibly, A5 for its entry into ERB cells.

Nipah virus (NiV) is a zoonotic paramyxovirus that causes high mortality rates in humans, with no approved treatment or vaccine available for human use. Viral entry into host cells relies on two viral envelope glycoproteins: the attachment (G) and fusion (F) glycoproteins. Binding of G to the ephrinB2 or ephrinB3 cell receptors triggers conformational changes in G that in turn cause F to undergo conformational changes that result in virus-host cell membrane fusion and viral entry. It is currently unknown, however, which specific regions of G and F interact during membrane fusion. Past efforts to determine the interacting regions have relied mainly on coimmunoprecipitation, a technique with some pitfalls. We developed a flow-cytometric assay to study membrane protein-protein interactions, and using this assay we report a bidentate interaction whereby both the head and stalk regions of NiV G interact with NiV F, a new finding for the paramyxovirus family.

Regulated nucleocytoplasmic transport of proteins is central to cellular function and dysfunction during processes such as viral infection. Active protein trafficking into and out of the nucleus is dependent on the presence within cargo proteins of intrinsic specific modular signals for nuclear import (nuclear localization signals, NLSs) and export (nuclear export signals, NESs). Rabies virus (RabV) phospho (P) protein, which is largely responsible for antagonising the host anti-viral response, is expressed as five isoforms (P1-P5). The subcellular trafficking of these isoforms is thought to depend on a balance between the activities of a dominant N-terminal NES (N-NES) and a distinct C-terminal NLS (C-NLS). Specifically, the N-NES-containing isoforms P1 and P2 are cytoplasmic, whereas the shorter P3-P5 isoforms, which lack the N-NES, are believed to be nuclear through the activity of the C-NLS. Here, we show for the first time that RabV P contains an additional strong NLS in the N-terminal region (N-NLS), which, intriguingly, overlaps with the N-NES. This arrangement represents a novel nuclear trafficking module where the N-NLS is inactive in P1 but becomes activated in P3, concomitant with truncation of the N-NES, to become the principal targeting signal conferring nuclear accumulation. Understanding this unique switch arrangement of overlapping, co-regulated NES/NLS sequences is vital to delineating the critical role of RabV P protein in viral infection.

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