Structure of the haemagglutinin-neuraminidase from human parainfluenza virus type III

Functional and structural basis of human parainfluenza virus type 3 neutralization with human monoclonal antibodies

Human parainfluenza virus type 3 (hPIV3) is a respiratory pathogen that can cause severe disease in older people and infants. Currently, vaccines against hPIV3 are in clinical trials but none have been approved yet. The haemagglutinin-neuraminidase (HN) and fusion (F) surface glycoproteins of hPIV3 are major antigenic determinants. Here we describe naturally occurring potently neutralizing human antibodies directed against both surface glycoproteins of hPIV3. We isolated seven neutralizing HN-reactive antibodies and a pre-fusion conformation F-reactive antibody from human memory B cells. One HN-binding monoclonal antibody (mAb), designated PIV3-23, exhibited functional attributes including haemagglutination and neuraminidase inhibition. We also delineated the structural basis of neutralization for two HN and one F mAbs. MAbs that neutralized hPIV3 in vitro protected against infection and disease in vivo in a cotton rat model of hPIV3 infection, suggesting correlates of protection for hPIV3 and the potential clinical utility of these mAbs.

Protective human monoclonal antibodies target conserved sites of vulnerability on the underside of influenza virus neuraminidase

Continuously evolving influenza viruses cause seasonal epidemics and pose global pandemic threats. Although viral neuraminidase (NA) is an effective drug and vaccine target, our understanding of the NA antigenic landscape still remains incomplete. Here, we describe NA-specific human antibodies that target the underside of the NA globular head domain, inhibit viral propagation of a wide range of human H3N2, swine-origin variant H3N2, and H2N2 viruses, and confer both pre- and post-exposure protection against lethal H3N2 infection in mice. Cryo-EM structures of two such antibodies in complex with NA reveal non-overlapping epitopes covering the underside of the NA head. These sites are highly conserved among N2 NAs yet inaccessible unless the NA head tilts or dissociates. Our findings help guide the development of effective countermeasures against ever-changing influenza viruses by identifying hidden conserved sites of vulnerability on the NA underside.

Structural Studies of Henipavirus Glycoproteins

Henipaviruses are a genus of emerging pathogens that includes the highly virulent Nipah and Hendra viruses that cause reoccurring outbreaks of disease. Henipaviruses rely on two surface glycoproteins, known as the attachment and fusion proteins, to facilitate entry into host cells. As new and divergent members of the genus have been discovered and structurally characterized, key differences and similarities have been noted. This review surveys the available structural information on Henipavirus glycoproteins, complementing this with information from related biophysical and structural studies of the broader Paramyxoviridae family of which Henipaviruses are members. The process of viral entry is a primary focus for vaccine and drug development, and this review aims to identify critical knowledge gaps in our understanding of the mechanisms that drive Henipavirus fusion.

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.

Interplay of Modified Sialic Acid Inhibitors and the Human Parainfluenza Virus 1 Hemagglutinin-Neuraminidase Active Site

In the search for effective antivirals against Paramyxoviridae, the dynamics of human parainfluenza virus type 1 hemagglutinin-neuraminidase (hPIV1-HN) inhibition offers a promising perspective. This study focuses on the potential of C5- and C4-modified 2,3-unsaturated sialic acid (DANA) inhibitors and highlights their interaction with the hPIV1-HN enzyme. We show that a strategic substitution, replacing the C5 isopropyl group in BCX 2798 with a trifluoroacetyl function, increases inhibitory potency 3- to 4-fold. At the same time, we explore the special properties of the catalytic site of hPIV1-HN, which harbors only small substituents and favors a C4 sulfonylamido function over a carbonyl function, in contrast to the C4 pocket of Newcastle disease virus hemagglutinin-neuraminidase (NDV-HN). Based on these findings, we present a newly identified potent inhibitor that has the preferred C5 trifluoroacetamido and C4 trifluorosulfonylamide groups. The results of this study pave the way for a deeper understanding of the C4 and C5 binding pockets of hPIV1-HN and promote the development of new, more selective inhibitors.

Kinetic analysis of paramyxovirus-sialoglycan receptor interactions reveals virion motility

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

Design, Synthesis, and Antiviral Evaluation of Sialic Acid Derivatives as Inhibitors of Newcastle Disease Virus Hemagglutinin-Neuraminidase: A Translational Study on Human Parainfluenza Viruses

Global infections with viruses belonging to the Paramyxoviridae, such as Newcastle disease virus (NDV) or human parainfluenza viruses (hPIVs), pose a serious threat to animal and human health. NDV-HN and hPIVs-HN (HN hemagglutinin-neuraminidase) share a high degree of similarity in catalytic site structures; therefore, the development of an efficient experimental NDV host model (chicken) may be informative for evaluating the efficacy of hPIVs-HN inhibitors. As part of the broad research in pursuit of this goal and as an extension of our published work on antiviral drug development, we report here the biological results obtained with some newly synthesized C4- and C5-substituted 2,3-unsaturated sialic acid derivatives against NDV. All developed compounds showed high neuraminidase inhibitory activity (IC₅₀ 0.03-13 μM). Four molecules (9, 10, 23, 24) confirmed their high in vitro inhibitory activity, which caused a significant reduction of NDV infection in Vero cells, accompanied by very low toxicity.

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

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

Sialidase Inhibitors with Different Mechanisms

Sialidases, or neuraminidases, are enzymes that catalyze the hydrolysis of sialic acid (Sia)-containing molecules, mostly removal of the terminal Sia (desialylation). By desialylation, sialidase can modulate the functionality of the target compound and is thus often involved in biological pathways. Inhibition of sialidases with inhibitors is an important approach for understanding sialidase function and the underlying mechanisms and could serve as a therapeutic approach as well. Transition-state analogues, such as anti-influenza drugs oseltamivir and zanamivir, are major sialidase inhibitors. In addition, difluoro-sialic acids were developed as mechanism-based sialidase inhibitors. Further, fluorinated quinone methide-based suicide substrates were reported. Sialidase product analogue inhibitors were also explored. Finally, natural products have shown competitive inhibiton against viral, bacterial, and human sialidases. This Perspective describes sialidase inhibitors with different mechanisms and their activities and future potential, which include transition-state analogue inhibitors, mechanism-based inhibitors, suicide substrate inhibitors, product analogue inhibitors, and natural product inhibitors.

Aspects of Biological Replication and Evolution Independent of the Central Dogma: Insights from Protein-Free Vesicular Transformations and Protein-Mediated Membrane Remodeling

Biological membrane remodeling is central to living systems. In spite of serving as “containers” of whole-living systems and functioning as dynamic compartments within living systems, biological membranes still find a “blue collar” treatment compared to the “white collar” nucleic acids and proteins in biology. This may be attributable to the fact that scientific literature on biological membrane remodeling is only 50 years old compared to ~ 150 years of literature on proteins and a little less than 100 years on nucleic acids. However, recently, evidence for symbiotic origins of eukaryotic cells from data only on biological membranes was reported. This, coupled with appreciation of reproducible amphiphilic self-assemblies in aqueous environments (mimicking replication), has already initiated discussions on origins of life beyond nucleic acids and proteins. This work presents a comprehensive compilation and meta-analyses of data on self-assembly and vesicular transformations in biological membranes—starting from model membranes to establishment of Influenza Hemagglutinin-mediated membrane fusion as a prototypical remodeling system to a thorough comparison between enveloped mammalian viruses and cellular vesicles. We show that viral membrane fusion proteins, in addition to obeying “stoichiometry-driven protein folding”, have tighter compositional constraints on their amino acid occurrences than general-structured proteins, regardless of type/class. From the perspective of vesicular assemblies and biological membrane remodeling (with and without proteins) we find that cellular vesicles are quite different from viruses. Finally, we propose that in addition to pre-existing thermodynamic frameworks, kinetic considerations in de novo formation of metastable membrane structures with available “third-party” constituents (including proteins) were not only crucial for origins of life but also continue to offer morphological replication and/or functional mechanisms in modern life forms, independent of the central dogma.

Parainfluenza virus entry at the onset of infection

Parainfluenza viruses, members of the enveloped, negative-sense, single stranded RNA Paramyxoviridae family, impact global child health as the cause of significant lower respiratory tract infections. Parainfluenza viruses enter cells by fusing directly at the cell surface membrane. How this fusion occurs via the coordinated efforts of the two molecules that comprise the viral surface fusion complex, and how these efforts may be blocked, are the subjects of this chapter. The receptor binding protein of parainfluenza forms a complex with the fusion protein of the virus, remaining stably associated until a receptor is reached. At that point, the receptor binding protein actively triggers the fusion protein to undergo a series of transitions that ultimately lead to membrane fusion and viral entry. In recent years it has become possible to examine this remarkable process on the surface of viral particles and to begin to understand the steps in the transition of this molecular machine, using a structural biology approach. Understanding the steps in entry leads to several possible strategies to prevent fusion and inhibit infection.

Evolution and diversity of bat and rodent Paramyxoviruses from North America

Paramyxoviruses are a diverse group of negative-sense, single-stranded RNA viruses of which several species cause significant mortality and morbidity. In recent years the collection of paramyxoviruses sequences detected in wild mammals has substantially grown, however little is known about paramyxovirus diversity in North American mammals. To better understand natural paramyxovirus diversity, host range, and host specificity, we sought to comprehensively characterize paramyxoviruses across a range of diverse co-occurring wild small mammals in Southern Arizona. We used highly degenerate primers to screen fecal and urine samples and obtained a total of 55 paramyxovirus sequences from 12 rodent species and 6 bat species. We also performed illumina RNA-seq and de novo assembly on 14 of the positive samples to recover a total of five near full-length viral genomes. We show there are at least two clades of rodent-borne paramyxoviruses in Arizona, while bat-associated paramyxoviruses formed a putative single clade. Using structural homology modeling of the viral attachment protein, we infer that three of the five novel viruses likely bind sialic acid in a manner similar to other Respiroviruses, while the other two viruses from Heteromyid rodents likely bind a novel host receptor. We find no evidence for cross-species transmission, even among closely related sympatric host species. Taken together, these data suggest paramyxoviruses are a common viral infection in some bat and rodent species present in North America, and illuminate the evolution of these viruses. Importance There are a number of viral lineages that are potential zoonotic threats to humans. One of these, paramyxoviruses, have jumped into humans multiple times from wild and domestic animals. We conducted one of the largest viral surveys of wild mammals in the United States to better understand paramyxovirus diversity and evolution.

Roles of conserved residues in the receptor binding sites of human parainfluenza virus type 3 HN protein

Human parainfluenza virus type 3 (hPIV-3) entry and intrahost spread through membrane fusion are initiated by two envelope glycoproteins, hemagglutinin-neuraminidase (HN) and fusion (F) protein. Binding of HN protein to the cellular receptor via its receptor-binding sites triggers conformational changes in the F protein leading to virus-cell fusion. However, little is known about the roles of individual amino acids that comprise the receptor-binding sites in the fusion process. Here, residues R192, D216, E409, R424, R502, Y530 and E549 located within the receptor-binding site Ⅰ, and residues N551 and H552 at the putative site Ⅱ were replaced by alanine with site-directed mutagenesis. All mutants except N551A displayed statistically lower hemadsorption activities ranging from 16.4% to 80.2% of the wild-type (wt) level. With standardization of the number of bound erythrocytes, similarly, other than N551A, all mutants showed reduced fusogenic activity at three successive stages: lipid mixing (hemifusion), content mixing (full fusion) and syncytium development. Kinetic measurements of the hemifusion process showed that the initial hemifusion extent for R192A, D216A, E409A, R424A, R502A, Y530A, E549A and H552A was decreased to 69.9%, 80.6%, 71.3%, 67.3%, 50.6%, 87.4%, 84.9% and 25.1%, respectively, relative to the wt, while the initial rate of hemifusion for the E409A, R424A, R502A and H552A mutants was reduced to 69.0%, 35.4%, 62.3%, 37.0%, respectively. In addition, four mutants with reduced initial hemifusion rates also showed decreased percentages of F protein cleavage from 43.4% to 56.3% of the wt. Taken together, Mutants R192A, D216A, E409A, R424A, R502A, Y530A, E549A and H552A may lead to damage on the fusion activity at initial stage of hemifusion, of which decreased extent and rate may be associated with impaired receptor binding activity resulting in the increased activation barrier of F protein and the cleavage of it, respectively.

Sendai Virus-Vectored Vaccines That Express Envelope Glycoproteins of Respiratory Viruses

Human respiratory syncytial virus (HRSV), human metapneumovirus (HMPV), and human parainfluenza viruses (HPIVs) are leading causes of respiratory disease in young children, the elderly, and individuals of all ages with immunosuppression. Vaccination strategies against these pneumoviruses and paramyxoviruses are vast in number, yet no licensed vaccines are available. Here, we review development of Sendai virus (SeV), a versatile pediatric vaccine that can (a) serve as a Jennerian vaccine against HPIV1, (b) serve as a recombinant vaccine against HRSV, HPIV2, HPIV3, and HMPV, (c) accommodate foreign genes for viral glycoproteins in multiple intergenic positions, (d) induce durable, mucosal, B-cell, and T-cell immune responses without enhanced immunopathology, (e) protect cotton rats, African green monkeys, and chimpanzees from infection, and (f) be formulated into a vaccine cocktail. Clinical phase I safety trials of SeV have been completed in adults and 3–6-year-old children. Clinical testing of SeVRSV, an HRSV fusion (F) glycoprotein gene recombinant, has also been completed in adults. Positive results from these studies, and collaborative efforts with the National Institutes of Health and the Serum Institute of India assist advanced development of SeV-based vaccines. Prospects are now good for vaccine successes in infants and consequent protection against serious viral disease.

Potent Henipavirus Neutralization by Antibodies Recognizing Diverse Sites on Hendra and Nipah Virus Receptor Binding Protein

Hendra (HeV) and Nipah (NiV) viruses are emerging zoonotic pathogens in the Henipavirus genus causing outbreaks of disease with very high case fatality rates. Here, we report the first naturally occurring human monoclonal antibodies (mAbs) against HeV receptor binding protein (RBP). All isolated mAbs neutralized HeV, and some also neutralized NiV. Epitope binning experiments identified five major antigenic sites on HeV-RBP. Animal studies demonstrated that the most potent cross-reactive neutralizing mAbs, HENV-26 and HENV-32, protected ferrets in lethal models of infection with NiV Bangladesh 3 days after exposure. We solved the crystal structures of mAb HENV-26 in complex with both HeV-RBP and NiV-RBP and of mAb HENV-32 in complex with HeV-RBP. The studies reveal diverse sites of vulnerability on RBP recognized by potent human mAbs that inhibit virus by multiple mechanisms. These studies identify promising prophylactic antibodies and define protective epitopes that can be used in rational vaccine design.

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

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

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.

Receptor-mediated cell entry of paramyxoviruses: Mechanisms, and consequences for tropism and pathogenesis

Research in the last decade has uncovered many new paramyxoviruses, airborne agents that cause epidemic diseases in animals including humans. Most paramyxoviruses enter epithelial cells of the airway using sialic acid as a receptor and cause only mild disease. However, others cross the epithelial barrier and cause more severe disease. For some of these viruses, the host receptors have been identified, and the mechanisms of cell entry have been elucidated. The tetrameric attachment proteins of paramyxoviruses have vastly different binding affinities for their cognate receptors, which they contact through different binding surfaces. Nevertheless, all input signals are converted to the same output: conformational changes that trigger refolding of trimeric fusion proteins and membrane fusion. Experiments with selectively receptor-blinded viruses inoculated into their natural hosts have provided insights into tropism, identifying the cells and tissues that support growth and revealing the mechanisms of pathogenesis. These analyses also shed light on diabolically elegant mechanisms used by morbilliviruses, including the measles virus, to promote massive amplification within the host, followed by efficient aerosolization and rapid spread through host populations. In another paradigm of receptor-facilitated severe disease, henipaviruses, including Nipah and Hendra viruses, use different members of one protein family to cause zoonoses. Specific properties of different paramyxoviruses, like neurotoxicity and immunosuppression, are now understood in the light of receptor specificity. We propose that research on the specific receptors for several newly identified members of the Paramyxoviridae family that may not bind sialic acid is needed to anticipate their zoonotic potential and to generate effective vaccines and antiviral compounds.

Diversity of sialidases found in the human body - A review

Sialidases are enzymes essential for numerous organisms including humans. Hydrolytic sialidases (EC 3.2.1.18), trans-sialidases and anhydrosialidases (intramolecular trans-sialidases, EC 4.2.2.15) are glycoside hydrolase enzymes that cleave the glycosidic linkage and release sialic acid residues from sialyl substrates. The paper summarizes diverse sialidases present in the human body and their potential impact on development of antiviral compounds - inhibitors of viral neuraminidases. It includes a brief overview of catalytic mechanisms of action of sialidases and describes the origin of sialidases in the human body. This is followed by description of the structure and function of sialidase families with a special focus on the GH33 and GH34 families. Various effects of sialidases on human body are also briefly described. Modulation of sialidase activity may be considered a useful tool for effective treatment of various diseases. In some cases, it is desired to completely suppress the activity of sialidases by suitable inhibitors. Specific sialidase inhibitors are useful for the treatment of influenza, epilepsy, Alzheimer's disease, diabetes, different types of cancer, or heart defects. Challenges and future directions are shortly depicted in the final part of the paper.

Disruption of the Dimer-Dimer Interaction of the Mumps Virus Attachment Protein Head Domain, Aided by an Anion Located at the Interface, Compromises Membrane Fusion Triggering

Mumps virus (MuV), an enveloped negative-strand RNA virus belonging to the family Paramyxoviridae, enters the host cell through membrane fusion mediated by two viral envelope proteins, an attachment protein hemagglutinin-neuraminidase (MuV-HN) and a fusion (F) protein. However, how the binding of MuV-HN to glycan receptors triggers membrane fusion is not well understood. The crystal structure of the MuV-HN head domain forms a tetramer (dimer of dimers) like other paramyxovirus attachment proteins. In the structure, a sulfate ion (SO₄ ^2-) was found at the interface between two dimers, which may be replaced by a hydrogen phosphate ion (HPO₄ ^2-) under physiological conditions. The anion is captured by the side chain of a positively charged arginine residue at position 139 of one monomer each from both dimers. Substitution of alanine or lysine for arginine at this position compromised the fusion support activity of MuV-HN without affecting its cell surface expression, glycan-receptor binding, and interaction with the F protein. Furthermore, the substitution appeared to affect the tetramer formation of the head domain as revealed by blue native-PAGE analysis. These results, together with our previous similar findings with the measles virus attachment protein head domain, suggest that the dimer-dimer interaction within the tetramer may play an important role in triggering membrane fusion during paramyxovirus entry.IMPORTANCE Despite the use of effective live vaccines, mumps outbreaks still occur worldwide. Mumps virus (MuV) infection typically causes flu-like symptoms and parotid gland swelling but sometimes leads to orchitis, oophoritis, and neurological complications, such as meningitis, encephalitis, and deafness. MuV enters the host cell through membrane fusion mediated by two viral proteins, a receptor-binding attachment protein, and a fusion protein, but its detailed mechanism is not fully understood. In this study, we show that the tetramer (dimer of dimers) formation of the MuV attachment protein head domain is supported by an anion located at the interface between two dimers and that the dimer-dimer interaction plays an important role in triggering the activation of the fusion protein and causing membrane fusion. These results not only further our understanding of MuV entry but provide useful information about a possible target for antiviral drugs.

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.

Molecular evolution of the hemagglutinin-neuraminidase (HN) gene in human respirovirus 3

Human respirovirus 3 (HRV3) is a major causative agent of acute respiratory infections in humans. HRV3 can manifest as a recurrent infection, although exactly how is not known. In the present study, we conducted detailed molecular evolutionary analyses of the major antigen-coding hemagglutinin-neuraminidase (HN) gene of this virus detected/isolated in various countries. We performed analyses of time-scaled evolution, similarity, selective pressure, phylodynamics, and conformational epitope prediction by mapping to HN protein models. In this way, we estimated that a common ancestor of the HN gene of HRV3 and bovine respirovirus 3 diverged around 1815 and formed many lineages in the phylogenetic tree. The evolutionary rates of the HN gene were 1.1 × 10^-3 substitutions/site/year, although the majority of these substitutions were synonymous. Some positive and many negative selection sites were predicted in the HN protein. Phylodynamic fluctuations of the gene were observed, and these were different in each lineage. Furthermore, most of the predicted B cell epitopes did not correspond to the neutralization-related mouse monoclonal antibody binding sites. The lack of a link between the conformational epitopes and neutralization sites may explain the naturally occurring HRV3 reinfection.

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.

Inhibiting Human Parainfluenza Virus Infection by Preactivating the Cell Entry Mechanism

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), and the fusion protein (F) facilitate viral fusion and entry into cells through a process involving HN activation by receptor binding, which triggers conformational changes in F to activate it to reach its fusion-competent state. Interfering with this process through premature activation of the F protein may be an effective antiviral strategy in vitro. We identified and optimized small compounds that implement this antiviral strategy through an interaction with HN, causing HN to activate F in an untimely fashion. To address that mechanism, we produced novel anti-HPIV3 F conformation-specific antibodies that can be used to assess the functionality of compounds designed to induce F activation. Both the novel antiviral compounds that we present and these newly characterized postfusion antibodies are novel tools for the exploration and development of antiviral approaches.

Paramyxoviruses, specifically, the childhood pathogen human parainfluenza virus type 3, are internalized into host cells following fusion between the viral and target cell membranes. The receptor binding protein, hemagglutinin (HA)-neuraminidase (HN), and the fusion protein (F) facilitate viral fusion and entry into the cell through a coordinated process involving HN activation by receptor binding, which triggers conformational changes in the F protein to activate it to reach its fusion-competent state. Interfering with this process through premature activation of the F protein has been shown to be an effective antiviral strategy in vitro. Conformational changes in the F protein leading to adoption of the postfusion form of the protein—prior to receptor engagement of HN at the host cell membrane—render the virus noninfectious. We previously identified a small compound (CSC11) that implements this antiviral strategy through an interaction with HN, causing HN to activate F in an untimely process. To assess the functionality of such compounds, it is necessary to verify that the postfusion state of F has been achieved. As demonstrated by Melero and colleagues, soluble forms of the recombinant postfusion pneumovirus F proteins and of their six helix bundle (6HB) motifs can be used to generate postfusion-specific antibodies. We produced novel anti-HPIV3 F conformation-specific antibodies that can be used to assess the functionality of compounds designed to induce F activation. In this study, using systematic chemical modifications of CSC11, we synthesized a more potent derivative of this compound, CM9. Much like CSC11, CM9 causes premature triggering of the F protein through an interaction with HN prior to receptor engagement, thereby preventing fusion and subsequent infection. In addition to validating the potency of CM9 using plaque reduction, fusion inhibition, and binding avidity assays, we confirmed the transition to a postfusion conformation of F in the presence of CM9 using our novel anti-HPIV3 conformation-specific antibodies. We present both CM9 and these newly characterized postfusion antibodies as novel tools to explore and develop antiviral approaches. In turn, these advances in both our molecular toolset and our understanding of HN-F interaction will support development of more-effective antivirals. Combining the findings described here with our recently described physiologically relevant ex vivo system, we have the potential to inform the development of therapeutics to block viral infection.

Human parainfluenza virus infection in severe acute respiratory infection cases in Beijing, 2014-2016: A molecular epidemiological study

Background

Severe acute respiratory infection (SARI) threatens human health and even survival, causing a huge number of hospitalized patients every year. However, as one of the most common respiratory viruses circulated worldwide, the epidemiological and phylogenetic characteristics of human parainfluenza virus (HPIV) in these cases were not well known.

Objectives

To reveal the epidemiological features of HPIV infection in SARIs in Beijing area from September 2014 to August 2016.

Methods

A total of 1229 SARI cases in Beijing area were enrolled, investigated, sampled, and tested by multiplex real‐time PCR to identify HPIVs and other common respiratory viruses. Eighteen HPIV‐3 viruses isolated from all HPIV‐positive samples in these SARI cases were sequenced and analyzed.

Results

Among all enrolled cases, 0.81%, 0.73%, 4.48%, and 0.57% were positive for HPIV‐1 to HPIV‐4, respectively. The highest yield rate of HPIV infection occurred in children under 5 years old (9.07%), followed by the patients over 60 years old (6.02%). The phylogenetic information of HPIV‐3 showed that all viruses belonged to Cluster C3a.

Conclusions

Besides the young children, the elders older than 60 years also showed a relatively high infection rate of HPIVs, which should be given comparable attentions. Moreover, the HPIV‐3 circulating in China undergoes continued evolution, suggesting the potential risk of evolved HPIV infection should not be overlooked.

Structural Insights into Human Parainfluenza Virus 3 Hemagglutinin-Neuraminidase Using Unsaturated 3- N-Substituted Sialic Acids as Probes

A novel approach to human parainfluenza virus 3 (hPIV-3) inhibitor design has been evaluated by targeting an unexplored pocket within the active site region of the hemagglutinin-neuraminidase (HN) of the virus that is normally occluded upon ligand engagement. To explore this opportunity, we developed a highly efficient route to introduce nitrogen-based functionalities at the naturally unsubstituted C-3 position on the neuraminidase inhibitor template N-acyl-2,3-dehydro-2-deoxy-neuraminic acid ( N-acyl-Neu2en), via a regioselective 2,3-bromoazidation. Introduction of triazole substituents at C-3 on this template provided compounds with low micromolar inhibition of hPIV-3 HN neuraminidase activity, with the most potent having 48-fold improved potency over the corresponding C-3 unsubstituted analogue. However, the C-3-triazole N-acyl-Neu2en derivatives were significantly less active against the hemagglutinin function of the virus, with high micromolar IC₅₀ values determined, and showed insignificant in vitro antiviral activity. Given the different pH optima of the HN protein's neuraminidase (acidic pH) and hemagglutinin (neutral pH) functions, the influence of pH on inhibitor binding was examined using X-ray crystallography and STD NMR spectroscopy, providing novel insights into the multifunctionality of hPIV-3 HN. While the 3-phenyltriazole- N-isobutyryl-Neu2en derivative could bind HN at pH 4.6, suitable for neuraminidase inhibition, at neutral pH binding of the inhibitor was substantially reduced. Importantly, this study clearly demonstrates for the first time that potent inhibition of HN neuraminidase activity is not necessarily directly correlated with a strong antiviral activity, and suggests that strong inhibition of the hemagglutinin function of hPIV HN is crucial for potent antiviral activity. This highlights the importance of designing hPIV inhibitors that primarily target the receptor-binding function of hPIV HN.

Functional analysis of amino acids at stalk/head interface of human parainfluenza virus type 3 hemagglutinin-neuraminidase protein in the membrane fusion process

Human parainfluenza virus type 3 (hPIV3) is an important respiratory pathogen that causes the majority of viral pneumonia of infants and young children. hPIV3 can infect host cells through the synergistic action of hemagglutinin-neuraminidase (HN) protein and the homotypic fusion (F) protein on the viral surface. HN protein plays a variety of roles during the virus invasion process, such as promoting viral particles to bind to receptors, cleaving sialic acid, and activating the F protein. Crystal structure research shows that HN tetramer adopted a "heads-down" conformation, at least two heads dimmer on flank of the four-helix bundle stalk, which forms a symmetrical interaction interface. The stalk region determines interactions and activation of F protein in specificity, and the heads in down position statically shield these residues. In order to make further research on the function of these amino acids at the hPIV3 HN stalk/head interface, fifteen mutations (8 sites from stalk and 7 sites from head) were engineered into this interface by site-directed mutagenesis in this study. Alanine substitution in this region of hPIV3 HN had various effects on cell fusion promotion, receptor binding, and neuraminidase activity. Besides, L151A also affected surface protein expression efficiency. Moreover, I112A, D120A, and R122A mutations of the stalk region that were masked by global head in down position had influence on the interaction between F and HN proteins.

In vitro sensitivity of human parainfluenza 3 clinical isolates to ribavirin, favipiravir and zanamivir

BACKGROUND

Human parainfluenza type 3 (HPIV3) is an important respiratory pathogen. Although a number of potential therapeutic candidates exist, there is currently no licensed therapy or vaccine. Ribavirin (RBV), favipiravir (FVP) and zanamivir (ZNV) are inhibitors with proven activity against influenza and with potential inhibitory activity against HPIV3 laboratory adapted strains in vitro.

OBJECTIVES

To evaluate RBV, FVP and ZNV as inhibitors of minimally passaged UK clinical strains of HPIV3 as well as a laboratory adapted strain MK9 in vitro.

STUDY DESIGN

The inhibitory action of RBV, FVP and ZNV was evaluated against nine minimally passaged clinical strains and a laboratory adapted strain MK9 using plaque reduction and growth curve inhibition in a cell culture model.

RESULTS

Clinical isolates were found to be at least as susceptible as the laboratory adapted strains to RBV and FVP and significantly more susceptible to ZNV. However the inhibitory concentrations achieved by ZNV against clinical strains remain prohibitively high in vivo.

CONCLUSIONS

RBV, FVP and ZNV were found to be effective inhibitors of HPIV3 in vitro. The lack of efficacy of RBV in vivo may be due to inability to reach required therapeutic levels. FVP, on the other hand, is a good potential therapeutic agent against HPIV3. Further studies using wild type clinical strains, as well as better formulation and delivery mechanisms may improve the utility of these three inhibitors.

Detailed genetic analyses of the HN gene in human respirovirus 3 detected in children with acute respiratory illness in the Iwate Prefecture, Japan

We performed detailed genetic analyses of the partial hemagglutinin-neuraminidase (HN) gene in 34 human respirovirus 3 (HRV3) strains from children with acute respiratory illness during 2013-2015 in Iwate Prefecture, Japan. In addition, we performed analyses of the evolutionary timescale of the gene using the Bayesian Markov chain Monte Carlo (MCMC) method. Furthermore, we analyzed pairwise distances and performed selective pressure analyses followed by linear B-cell epitope mapping and N-glycosylation and phylodynamic analyses. A phylogenetic tree showed that the strains diversified at around 1939, and the rate of molecular evolution was 7.6 × 10^-4 substitutions/site/year. Although the pairwise distances were relatively short (0.03 ± 0.018 [mean ± standard deviation, SD]), two positive selection sites (Cys544Trp and Leu555Ser) and no amino acid substitutions were found in the active/catalytic sites. Six epitopes were estimated in this study, and three mouse monoclonal antibody binding sites (amino acid positions 278, 281, and 461) overlapped with two epitopes belonging to subcluster C3 strains. Bayesian skyline plot analyses indicated that subcluster C3 strains have been increasing from 2004, whereas subcluster C1 strains have declined from 2004. Based on these results, Iwate strains were divided into two subclusters and each subcluster evolved independently. Moreover, our results suggested that some predicted linear epitopes (epitopes 3 and 5) are candidates for an HRV3 vaccine motif. To better understand the details of the molecular evolution of HRV, further studies are needed.

Vaccines for the Paramyxoviruses and Pneumoviruses: Successes, Candidates, and Hurdles

Human parainfluenza viruses (family Paramyxoviridae), human metapneumovirus, and respiratory syncytial virus (family Pneumoviridae) infect most infants and children within the first few years of life and are the etiologic agents for many serious acute respiratory illnesses. These virus infections are also associated with long-term diseases that impact quality of life, including asthma. Despite over a half-century of vaccine research, development, and clinical trials, no vaccine has been licensed to date for the paramyxoviruses or pneumoviruses for the youngest infants. In this study, we describe the recent reclassification of paramyxoviruses and pneumoviruses into distinct families by the International Committee on the Taxonomy of Viruses. We also discuss some past unsuccessful vaccine trials and some currently preferred vaccine strategies. Finally, we discuss hurdles that must be overcome to support successful respiratory virus vaccine development for the youngest children.

Idiosyncratic Mòjiāng virus attachment glycoprotein directs a host-cell entry pathway distinct from genetically related henipaviruses

In 2012, cases of lethal pneumonia among Chinese miners prompted the isolation of a rat-borne henipavirus (HNV), Mòjiāng virus (MojV). Although MojV is genetically related to highly pathogenic bat-borne henipaviruses, the absence of a conserved ephrin receptor-binding motif in the MojV attachment glycoprotein (MojV-G) indicates a differing host-cell recognition mechanism. Here we find that MojV-G displays a six-bladed β-propeller fold bearing limited similarity to known paramyxoviral attachment glycoproteins, in particular at host receptor-binding surfaces. We confirm the inability of MojV-G to interact with known paramyxoviral receptors in vitro, indicating an independence from well-characterized ephrinB2/B3, sialic acid and CD150-mediated entry pathways. Furthermore, we find that MojV-G is antigenically distinct, indicating that MojV would less likely be detected in existing large-scale serological screening studies focused on well-established HNVs. Altogether, these data indicate a unique host-cell entry pathway for this emerging and potentially pathogenic HNV.

The attachment glycoprotein (HNV-G) of henipaviruses interacts with host receptors at the cell surface and is a major determinant of species tropism. Here the authors provide structural and functional evidence that the emergent henipavirus, Mòjiang virus, uses an entry mechanism that is independent of known paramyoxviral cellular receptors.

The impact of the butterfly effect on human parainfluenza virus haemagglutinin-neuraminidase inhibitor design

Human parainfluenza viruses represent a leading cause of lower respiratory tract disease in children, with currently no available approved drug or vaccine. The viral surface glycoprotein haemagglutinin-neuraminidase (HN) represents an ideal antiviral target. Herein, we describe the first structure-based study on the rearrangement of key active site amino acid residues by an induced opening of the 216-loop, through the accommodation of appropriately functionalised neuraminic acid-based inhibitors. We discovered that the rearrangement is influenced by the degree of loop opening and is controlled by the neuraminic acid’s C-4 substituent’s size (large or small). In this study, we found that these rearrangements induce a butterfly effect of paramount importance in HN inhibitor design and define criteria for the ideal substituent size in two different categories of HN inhibitors and provide novel structural insight into the druggable viral HN protein.

Genetic analysis of human parainfluenza virus type 3 obtained in Croatia, 2011-2015

PURPOSE

This study investigated the HPIV3 circulating strains in Croatia and whether the other parts of HPIV3 genome (F gene and HN 582 nucleotides fragment) could be equally suitable for genetic and phylogenetic analysis.

METHODOLOGY

Clinical materials were collected in period 2011-2015 from children suffering from respiratory illnesses. In positive HPIV3 samples viral genome was partially amplified and sequenced for HN and F genes. Obtained sequences were analysed by phylogenetic analysis and genetic characterization was performed.

RESULTS

All samples from this study belonged to subcluster C and over a short period of time, genetic lineage C3a gained prevalence over the other C genetic lineages, from 39 % in 2011 to more than 90 % in 2013 and 2014. Phylogenetic classifications of HPIV3 based on the entire HN gene, HN 582 nt fragment and entire fusion (F) gene showed identical classification results for Croatian strains and the reference strains. Molecular analysis of the F and HN glycoproteins, showed their similar nucleotide diversity (Fcds P=0.0244 and HNcds P=0.0231) and similar Ka/Ks ratios (F Ka/Ks=0.0553 and HN Ka/Ks=0.0428). Potential N-glycosylation sites, cysteine residues and antigenic sites are generally strongly conserved in HPIV3 glycoproteins from both our and the reference samples.

CONCLUSION

The HPIV3 subclaster C3 (genetic lineage C3a) became the most detected circulating HPIV3 strain in Croatia. The results indicated that the HN 582 nt and the entire F gene sequences were as good for phylogenetic analysis as the entire HN gene sequence.

Neuraminidase activity of blue eye disease porcine rubulavirus: Specificity, affinity and inhibition studies

Porcine rubulavirus (PorPV), also known as La Piedad Michoacan Virus (LPMV) causes encephalitis and reproductive failure in newborn and adult pigs, respectively. The hemagglutinin-neuraminidase (HN) glycoprotein is the most exposed and antigenic of the virus proteins. HN plays central roles in PorPV infection; i.e., it recognizes sialic acid-containing cell receptors that mediate virus attachment and penetration; in addition, its neuraminidase (sialic acid releasing) activity has been proposed as a virulence factor. This work describes the purification and characterization of PorPV HN protein (isolate PAC1). The specificity of neuraminidase is restricted to sialyl(α2,3)lactose (3SL). HN showed typical Michaelis-Menten kinetics with fetuin as substrate (km=0.029μM, Vmax=522.8nmolmin^-1mg^-1). When 3SL was used as substrate, typical cooperative kinetics were found (S₅₀=0.15μM, Vmax=154.3nmolmin^-1mg^-1). The influenza inhibitor zanamivir inhibited the PorPV neuraminidase with IC₅₀ of 0.24μM. PorPV neuraminidase was activated by Ca²⁺ and inhibited by nucleoside triphosphates with the level of inhibition depending on phosphorylation level. The present results open possibilities to study the role of neuraminidase in the pathogenicity of PorPV infection and its potential inhibitors.

Flexibility of the Head-Stalk Linker Domain of Paramyxovirus HN Glycoprotein Is Essential for Triggering Virus Fusion

The Paramyxoviridae comprise a large family of enveloped, negative-sense, single-stranded RNA viruses with significant economic and public health implications. For nearly all paramyxoviruses, infection is initiated by fusion of the viral and host cell plasma membranes in a pH-independent fashion. Fusion is orchestrated by the receptor binding protein hemagglutinin-neuraminidase (HN; also called H or G depending on the virus type) protein and a fusion (F) protein, the latter undergoing a major refolding process to merge the two membranes. Mechanistic details regarding the coupling of receptor binding to F activation are not fully understood. Here, we have identified the flexible loop region connecting the bulky enzymatically active head and the four-helix bundle stalk to be essential for fusion promotion. Proline substitution in this region of HN of parainfluenza virus 5 (PIV5) and Newcastle disease virus HN abolishes cell-cell fusion, whereas HN retains receptor binding and neuraminidase activity. By using reverse genetics, we engineered recombinant PIV5-EGFP viruses with mutations in the head-stalk linker region of HN. Mutations in this region abolished virus recovery and infectivity. In sum, our data suggest that the loop region acts as a "hinge" around which the bulky head of HN swings to-and-fro to facilitate timely HN-mediate F-triggering, a notion consistent with the stalk-mediated activation model of paramyxovirus fusion.

IMPORTANCE

Paramyxovirus fusion with the host cell plasma membrane is essential for virus infection. Membrane fusion is orchestrated via interaction of the receptor binding protein (HN, H, or G) with the viral fusion glycoprotein (F). Two distinct models have been suggested to describe the mechanism of fusion: these include "the clamp" and the "provocateur" model of activation. By using biochemical and reverse genetics tools, we have obtained strong evidence in favor of the HN stalk-mediated activation of paramyxovirus fusion. Specifically, our data strongly support the notion that the short linker between the head and stalk plays a role in "conformational switching" of the head group to facilitate F-HN interaction and triggering.

Trisaccharide containing α2,3-linked sialic acid is a receptor for mumps virus

Mumps virus (MuV) remains an important pathogen worldwide, causing epidemic parotitis, orchitis, meningitis, and encephalitis. Here we show that MuV preferentially uses a trisaccharide containing α2,3-linked sialic acid in unbranched sugar chains as a receptor. Crystal structures of the MuV attachment protein hemagglutinin-neuraminidase (MuV-HN) alone and in complex with the α2,3-sialylated trisaccharide revealed that in addition to the interaction between the MuV-HN active site residues and sialic acid, other residues, including an aromatic residue, stabilize the third sugar of the trisaccharide. The importance of the aromatic residue and the third sugar in the MuV-HN-receptor interaction was confirmed by computational energy calculations, isothermal titration calorimetry studies, and glycan-binding assays. Furthermore, MuV-HN was found to bind more efficiently to unbranched α2,3-sialylated sugar chains compared with branched ones. Importantly, the strategically located aromatic residue is conserved among the HN proteins of sialic acid-using paramyxoviruses, and alanine substitution compromised their ability to support cell-cell fusion. These results suggest that not only the terminal sialic acid but also the adjacent sugar moiety contribute to receptor function for mumps and these paramyxoviruses. The distribution of structurally different sialylated glycans in tissues and organs may explain in part MuV's distinct tropism to glandular tissues and the central nervous system. In the crystal structure, the epitopes for neutralizing antibodies are located around the α-helices of MuV-HN that are not well conserved in amino acid sequences among different genotypes of MuV. This may explain the fact that MuV reinfection sometimes occurs.

A molecular epidemiological study of human parainfluenza virus type 3 at a tertiary university hospital during 2013-2015 in Catalonia, Spain

Human parainfluenza virus type 3 (HPIV-3) is one of the most common respiratory viruses particularly among young children and immunocompromised patients. The seasonality, prevalence and genetic diversity of HPIV-3 at a Spanish tertiary-hospital from 2013 to 2015 are reported. HPIV-3 infection was laboratory-confirmed in 102 patients (76%, under 5 years of age). Among <5 years-old patients, 9 (11.5%) were under any degree of immunosuppression, whereas this percentage was significantly higher (19; 79.2%) among patients older than 5 years. HPIV-3 was detected at varying levels, but mainly during spring and summer. All characterized HN/F sequences fell within C1b, C5 and in other two closely C3a-related groups. Furthermore, a new genetic lineage (C1c) was described. Genetic similarity and epidemiological data confirmed some nosocomial infections, highlighting the importance of the HPIV-3 surveillance, particularly in high-risk patients. This study provides valuable information on HPIV-3 diversity due to the scarce information in Europe.

•

Children and immunosuppressed adults showed a great susceptibility to infection.

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Valuable information about the current genetic diversity in Europe is provided.

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Different lineages, including a first described, were locally circulating.

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Genetic similarity and epidemiological data confirmed some nosocomial infections.

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The present study highlights the importance of the HPIV-3 surveillance.

Paramyxovirus Glycoproteins and the Membrane Fusion Process

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

Immobilization of the N-terminal helix stabilizes prefusion paramyxovirus fusion proteins

Parainfluenza virus 5 (PIV5) is an enveloped, single-stranded, negative-sense RNA virus of the Paramyxoviridae family. PIV5 fusion and entry are mediated by the coordinated action of the receptor-binding protein, hemagglutinin-neuraminidase (HN), and the fusion protein (F). Upon triggering by HN, F undergoes an irreversible ATP- and pH-independent conformational change, going down an energy gradient from a metastable prefusion state to a highly stable postfusion state. Previous studies have highlighted key conformational changes in the F-protein refolding pathway, but a detailed understanding of prefusion F-protein metastability remains elusive. Here, using two previously described F-protein mutations (S443D or P22L), we examine the capacity to modulate PIV5 F stability and the mechanisms by which these point mutants act. The S443D mutation destabilizes prefusion F proteins by disrupting a hydrogen bond network at the base of the F-protein globular head. The introduction of a P22L mutation robustly rescues destabilized F proteins through a local hydrophobic interaction between the N-terminal helix and a hydrophobic pocket. Prefusion stabilization conferred by a P22L-homologous mutation is demonstrated in the F protein of Newcastle disease virus, a paramyxovirus of a different genus, suggesting a conserved stabilizing structural element within the paramyxovirus family. Taken together, the available data suggest that movement of the N-terminal helix is a necessary early step for paramyxovirus F-protein refolding and presents a novel target for structure-based drug design.

Measles Virus Fusion Protein: Structure, Function and Inhibition

Measles virus (MeV), a highly contagious member of the Paramyxoviridae family, causes measles in humans. The Paramyxoviridae family of negative single-stranded enveloped viruses includes several important human and animal pathogens, with MeV causing approximately 120,000 deaths annually. MeV and canine distemper virus (CDV)-mediated diseases can be prevented by vaccination. However, sub-optimal vaccine delivery continues to foster MeV outbreaks. Post-exposure prophylaxis with antivirals has been proposed as a novel strategy to complement vaccination programs by filling herd immunity gaps. Recent research has shown that membrane fusion induced by the morbillivirus glycoproteins is the first critical step for viral entry and infection, and determines cell pathology and disease outcome. Our molecular understanding of morbillivirus-associated membrane fusion has greatly progressed towards the feasibility to control this process by treating the fusion glycoprotein with inhibitory molecules. Current approaches to develop anti-membrane fusion drugs and our knowledge on drug resistance mechanisms strongly suggest that combined therapies will be a prerequisite. Thus, discovery of additional anti-fusion and/or anti-attachment protein small-molecule compounds may eventually translate into realistic therapeutic options.

A dual drug regimen synergistically blocks human parainfluenza virus infection

Human parainfluenza type-3 virus (hPIV-3) is one of the principal aetiological agents of acute respiratory illness in infants worldwide and also shows high disease severity in the elderly and immunocompromised, but neither therapies nor vaccines are available to treat or prevent infection, respectively. Using a multidisciplinary approach we report herein that the approved drug suramin acts as a non-competitive in vitro inhibitor of the hPIV-3 haemagglutinin-neuraminidase (HN). Furthermore, the drug inhibits viral replication in mammalian epithelial cells with an IC₅₀ of 30 μM, when applied post-adsorption. Significantly, we show in cell-based drug-combination studies using virus infection blockade assays, that suramin acts synergistically with the anti-influenza virus drug zanamivir. Our data suggests that lower concentrations of both drugs can be used to yield high levels of inhibition. Finally, using NMR spectroscopy and in silico docking simulations we confirmed that suramin binds HN simultaneously with zanamivir. This binding event occurs most likely in the vicinity of the protein primary binding site, resulting in an enhancement of the inhibitory potential of the N-acetylneuraminic acid-based inhibitor. This study offers a potentially exciting avenue for the treatment of parainfluenza infection by a combinatorial repurposing approach of well-established approved drugs.

Morbillivirus and henipavirus attachment protein cytoplasmic domains differently affect protein expression, fusion support and particle assembly

The amino-terminal cytoplasmic domains of paramyxovirus attachment glycoproteins include trafficking signals that influence protein processing and cell surface expression. To characterize the role of the cytoplasmic domain in protein expression, fusion support and particle assembly in more detail, we constructed chimeric Nipah virus (NiV) glycoprotein (G) and canine distemper virus (CDV) haemagglutinin (H) proteins carrying the respective heterologous cytoplasmic domain, as well as a series of mutants with progressive deletions in this domain. CDV H retained fusion function and was normally expressed on the cell surface with a heterologous cytoplasmic domain, while the expression and fusion support of NiV G was dramatically decreased when its cytoplasmic domain was replaced with that of CDV H. The cell surface expression and fusion support functions of CDV H were relatively insensitive to cytoplasmic domain deletions, while short deletions in the corresponding region of NiV G dramatically decreased both. In addition, the first 10 residues of the CDV H cytoplasmic domain strongly influence its incorporation into virus-like particles formed by the CDV matrix (M) protein, while the co-expression of NiV M with NiV G had no significant effect on incorporation of G into particles. The cytoplasmic domains of both the CDV H and NiV G proteins thus contribute differently to the virus life cycle.

Mutations in the Leucine Zipper-Like Motif of the Human Parainfluenza Virus 3 Fusion Protein Impair Fusion Activity

Objective

To investigate the effect of the leucine zipper-like motif between HRA and HRB of the human parainfluenza virus 3 fusion protein on fusion activity.

Methods

Site-directed mutagenesis was utilized to substitute the heptadic residues at 257, 264, 271, 278, 285, 292, and 299 in this motif with alanine. Additionally, 3 middle heptadic leucine residues at 271, 278, and 285 were replaced with alanine singly or in combination. A vaccinia virus-T7 RNA polymerase transient expression system was employed to express the wild-type or mutated fusion (F) proteins. Three different types of membrane fusion assays were performed to analyze the fusogenic activity, fluorescence-activated cell sorting (FACS) analysis was executed to examine the cell surface expression level, and a coimmunoprecipitation assay was conducted to probe the hemagglutinin-neuraminidase (HN)-F interaction at the cell surface.

Results

All of the substitutions in this motif exhibited diminished or even lost fusion activity in all stages of fusion, although they all had no effect on cell surface expression. In the coimmunoprecipitation assay, all mutants resulted in decreased detection of the HN-F complexes compared with that of the wild-type F protein.

Conclusions

This motif has an important influence on fusion activity, and its integrality is indispensable for membrane fusion.

Canine Distemper Virus Fusion Activation: Critical Role of Residue E123 of CD150/SLAM

Measles virus (MeV) and canine distemper virus (CDV) possess tetrameric attachment proteins (H) and trimeric fusion proteins, which cooperate with either SLAM or nectin 4 receptors to trigger membrane fusion for cell entry. While the MeV H-SLAM cocrystal structure revealed the binding interface, two distinct oligomeric H assemblies were also determined. In one of the conformations, two SLAM units were sandwiched between two discrete H head domains, thus spotlighting two binding interfaces ("front" and "back"). Here, we investigated the functional relevance of both interfaces in activating the CDV membrane fusion machinery. While alanine-scanning mutagenesis identified five critical regulatory residues in the front H-binding site of SLAM, the replacement of a conserved glutamate residue (E at position 123, replaced with A [E123A]) led to the most pronounced impact on fusion promotion. Intriguingly, while determination of the interaction of H with the receptor using soluble constructs revealed reduced binding for the identified SLAM mutants, no effect was recorded when physical interaction was investigated with the full-length counterparts of both molecules. Conversely, although mutagenesis of three strategically selected residues within the back H-binding site of SLAM did not substantially affect fusion triggering, nevertheless, the mutants weakened the H-SLAM interaction recorded with the membrane-anchored protein constructs. Collectively, our findings support a mode of binding between the attachment protein and the V domain of SLAM that is common to all morbilliviruses and suggest a major role of the SLAM residue E123, located at the front H-binding site, in triggering the fusion machinery. However, our data additionally support the hypothesis that other microdomain(s) of both glycoproteins (including the back H-binding site) might be required to achieve fully productive H-SLAM interactions.

IMPORTANCE

A complete understanding of the measles virus and canine distemper virus (CDV) cell entry molecular framework is still lacking, thus impeding the rational design of antivirals. Both viruses share many biological features that partially rely on the use of analogous Ig-like host cell receptors, namely, SLAM and nectin 4, for entering immune and epithelial cells, respectively. Here, we provide evidence that the mode of binding between the membrane-distal V domain of SLAM and the attachment protein (H) of morbilliviruses is very likely conserved. Moreover, although structural information revealed two discrete conformational states of H, one of the structures displayed two H-SLAM binding interfaces ("front" and "back"). Our data not only spotlight the front H-binding site of SLAM as the main determinant of membrane fusion promotion but suggest that the triggering efficiency of the viral entry machinery may rely on a local conformational change within the front H-SLAM interactive site rather than the binding affinity.