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

Open AccessPublished:January 16, 2020DOI:https://doi.org/10.1074/jbc.REV119.009961
      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.

      Introduction

      In the last decade, many new paramyxoviruses have been identified. These viruses, transmitted mainly through airborne routes, are similar to pathogens that have caused the loss of hundreds of millions of human lives (
      ,
      • Rota P.A.
      • Moss W.J.
      • Takeda M.
      • de Swart R.L.
      • Thompson K.M.
      • Goodson J.L.
      Measles.
      ). Animal paramyxoviruses continue to cause crippling economic losses when they spread in domesticated mammals and in poultry, they are threatening the extinction of certain species, and they often cause debilitating diseases and significant mortality in companion animals (
      • Rota P.A.
      • Moss W.J.
      • Takeda M.
      • de Swart R.L.
      • Thompson K.M.
      • Goodson J.L.
      Measles.
      ,
      • Zeltina A.
      • Bowden T.A.
      • Lee B.
      Emerging paramyxoviruses: receptor tropism and zoonotic potential.
      ).
      This review focuses on insights recently gained on the processes by which two of the paramyxoviruses most relevant for human health, measles virus (MeV)
      The abbreviations used are: MeV
      measles virus
      RPV
      rinderpest virus
      HeV
      Hendra virus
      NiV
      Nipah virus
      MuV
      mumps virus
      NDV
      Newcastle disease virus
      PIV5
      parainfluenza virus 5
      SeV
      Sendai virus
      CDV
      canine distemper virus
      CedV
      Cedar virus
      F
      fusion protein
      nt
      nucleotide(s)
      L
      large protein, or polymerase
      N
      nucleocapsid
      M
      matrix protein
      HN
      hemagglutinin/neuraminidases
      H
      hemagglutinin
      G
      glycoprotein
      HPIV
      human parainfluenza virus
      CNS
      central nervous system
      SLAM
      signaling lymphocytic activation molecule
      CD46
      cluster of differentiation 46, or membrane cofactor protein
      CD4
      cluster of differentiation 4
      PDB
      Protein Data Bank
      aa
      amino acids.
      and Nipah virus (NiV), infect their hosts. Whereas most paramyxoviruses enter epithelial cells of the airway using sialic acid as a receptor, causing mild disease, MeV and NiV cross the epithelial barrier and cause severe disease. The receptors that allow entry of these viruses and the cells and tissues that support their growth were only recently unambiguously identified and characterized, providing insights into viral tropism and mechanisms of pathogenesis.

      The Paramyxoviridae

      A few months ago, growth of the Paramyxoviridae family to 72 members prompted the International Committee on Virus Taxonomy to restructure it into four subfamilies and 16 genera (
      • Rima B.
      • Balkema-Buschmann A.
      • Dundon W.G.
      • Duprex P.
      • Easton A.
      • Fouchier R.
      • Kurath G.
      • Lamb R.
      • Lee B.
      • Rota P.
      • Wang L.
      • Consortium I.R.
      ICTV Virus Taxonomy Profile: Paramyxoviridae.
      ). Rather than illustrating the new classification, Fig. 1 focuses on the genetic relationships of the currently most relevant paramyxoviruses. These include MeV that still causes about 140,000 deaths annually (WHO Key Facts, https://www.who.int/news-room/fact-sheets/detail/measles)
      Please note that the JBC is not responsible for the long-term archiving and maintenance of this site or any other third party hosted site.
      and is targeted for eradication by the World Health Organization (
      • Rota P.A.
      • Moss W.J.
      • Takeda M.
      • de Swart R.L.
      • Thompson K.M.
      • Goodson J.L.
      Measles.
      ). Eradication has been successful for the animal morbillivirus rinderpest (RPV), which had major economic impact on cattle rearing and was lethal for wild species of even-toed ungulates (
      • Morens D.M.
      • Holmes E.C.
      • Davis A.S.
      • Taubenberger J.K.
      Global rinderpest eradication: lessons learned and why humans should celebrate too.
      ). The emerging henipaviruses, Hendra virus (HeV) and NiV, have a broad mammalian host range, including humans and domestic animals, causing severe and often fatal respiratory and neurological diseases. High case fatality rates and a lack of approved therapeutics or vaccines have earned these viruses the highest biosafety classification (level 4). Medically relevant paramyxoviruses also include mumps virus (MuV) and the human parainfluenza viruses (HPIV1–4), which are among the most prevalent human viruses known.
      Figure thumbnail gr1
      Figure 1Phylogenetic analysis of attachment proteins of selected paramyxoviruses. Attachment protein sequences of the reference species of each virus were aligned to form an unrooted tree. Viruses for which attachment protein structures have been solved are indicated in boldface type. The five genera of the family Paramyxoviridae are indicated by colored ellipses, according to the nomenclature used until 2019. Purple, genus Henipavirus; green, genus Avulavirus; pink, genus Rubulavirus; lilac, genus Respirovirus, tan, genus Morbillivirus. Sequences were aligned with Clustal Omega (
      • Sievers F.
      • Wilm A.
      • Dineen D.
      • Gibson T.J.
      • Karplus K.
      • Li W.
      • Lopez R.
      • McWilliam H.
      • Remmert M.
      • Söding J.
      • Thompson J.D.
      • Higgins D.G.
      Fast, scalable generation of high-quality protein multiple sequence alignments using Clustal Omega.
      ), and the cladogram was generated using FigTree version 1.4.4 (http://tree.bio.ed.ac.uk/software/figtree/). (Please note that the JBC is not responsible for the long-term archiving and maintenance of this site or any other third party hosted site.) Sequence information was from the Uniprot database for proteins without solved structures or from the Protein Data Bank (PDB) for proteins with solved structures. Accession codes used were as follows (clockwise from the genus Henipavirus): CedV, PDB code 6P72 (
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ); NiV, PDB code 2VSM (
      • Bowden T.A.
      • Aricescu A.R.
      • Gilbert R.J.
      • Grimes J.M.
      • Jones E.Y.
      • Stuart D.I.
      Structural basis of Nipah and Hendra virus attachment to their cell-surface receptor ephrin-B2.
      ); HeV, PDB code 6CMG (
      • Xu K.
      • Rockx B.
      • Xie Y.
      • DeBuysscher B.L.
      • Fusco D.L.
      • Zhu Z.
      • Chan Y.P.
      • Xu Y.
      • Luu T.
      • Cer R.Z.
      • Feldmann H.
      • Mokashi V.
      • Dimitrov D.S.
      • Bishop-Lilly K.A.
      • Broder C.C.
      • Nikolov D.B.
      Crystal structure of the Hendra virus attachment G glycoprotein bound to a potent cross-reactive neutralizing human monoclonal antibody.
      ); Tupaia paramyxovirus (TPMV), Q9JFN4; NDV, PDB code 1E8T (
      • Takimoto T.
      • Taylor G.L.
      • Crennell S.J.
      • Scroggs R.A.
      • Portner A.
      Crystallization of Newcastle disease virus hemagglutinin-neuraminidase glycoprotein.
      ); HPIV4a, P21526; MuV, PDB code 5B2C (
      • Kubota M.
      • Takeuchi K.
      • Watanabe S.
      • Ohno S.
      • Matsuoka R.
      • Kohda D.
      • Nakakita S.I.
      • Hiramatsu H.
      • Suzuki Y.
      • Nakayama T.
      • Terada T.
      • Shimizu K.
      • Shimizu N.
      • Shiroishi M.
      • Yanagi Y.
      • Hashiguchi T.
      Trisaccharide containing α2,3-linked sialic acid is a receptor for mumps virus.
      ); PIV5, PDB code 1Z4X (
      • Yuan P.
      • Thompson T.B.
      • Wurzburg B.A.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structural studies of the parainfluenza virus 5 hemagglutinin-neuraminidase tetramer in complex with its receptor, sialyllactose.
      ); HPIV2, P25465; HPIV3, PDB code 4MZA, 4XJR (
      • Xu R.
      • Palmer S.G.
      • Porotto M.
      • Palermo L.M.
      • Niewiesk S.
      • Wilson I.A.
      • Moscona A.
      Interaction between the hemagglutinin-neuraminidase and fusion glycoproteins of human parainfluenza virus type III regulates viral growth in vivo.
      ); bovine parainfluenza 3 virus (BPIV-3), P06167; SeV, P04853; HPIV1, P16071; phocine distemper virus (PDV), P28882; CDV (strain A92-6), Q66000; dolphin morbillivirus (DMV), Q66411; Peste-des-petits-ruminants virus (PPRV), Q2TT33; MeV, PDB code 4GJT (
      • Zhang X.
      • Lu G.
      • Qi J.
      • Li Y.
      • He Y.
      • Xu X.
      • Shi J.
      • Zhang C.W.
      • Yan J.
      • Gao G.F.
      Structure of measles virus hemagglutinin bound to its epithelial receptor nectin-4.
      ); RPV, P41355.
      Several animal viruses are also shown in Fig. 1. Newcastle disease virus (NDV), an avulavirus, is an avian pathogen that can be destructive for the poultry industry. Four other animal viruses are especially relevant for fundamental research: parainfluenza virus 5 (PIV5) is a rubulavirus that has been instrumental to understanding the molecular biology of the family and the interactions with the cellular innate immune response. Sendai virus (SeV) is a respirovirus that infects mice, providing a convenient model for pathogenesis. Canine distemper virus (CDV) is a morbillivirus that infects many carnivores, including ferrets, and is used to characterize the pathogenesis of morbilliviruses. Cedar virus (CedV), a henipavirus isolated from bats, is important for the study of receptor interactions and virus emergence.
      The Paramyxoviridae are enveloped negative-strand RNA viruses that share different characteristics with two other families of negative-strand RNA viruses (
      ). Their envelope glycoproteins have similar structure and function as those of the Orthomyxoviridae, including the influenza viruses. However, the Paramyxoviridae genome is nonsegmented, sharing a similar organization and gene expression strategy with the Rhabdoviridae like rabies virus and vesicular stomatitis virus. Before focusing on the mechanisms of paramyxovirus cell entry and on the consequences of receptor-specific cell entry for tropism and pathogenesis, we briefly review their genome structure and replication mechanisms. This is important because, whereas receptor recognition is the main determinant of paramyxovirus tropism, post-entry mechanisms are crucial for efficient virus spread.

      Genomes and replication

      Fig. 2 illustrates the RNA genome (top) and a particle (bottom) of MeV, a typical paramyxovirus (
      • Rota P.A.
      • Moss W.J.
      • Takeda M.
      • de Swart R.L.
      • Thompson K.M.
      • Goodson J.L.
      Measles.
      ,
      • Rima B.
      • Balkema-Buschmann A.
      • Dundon W.G.
      • Duprex P.
      • Easton A.
      • Fouchier R.
      • Kurath G.
      • Lamb R.
      • Lee B.
      • Rota P.
      • Wang L.
      • Consortium I.R.
      ICTV Virus Taxonomy Profile: Paramyxoviridae.
      ). The MeV genomic RNA, as that of all negative-strand RNA viruses, serves two functions. First, it is the template for synthesis of mRNAs. And second, it is the template for the synthesis of the antigenome positive-strand RNA (shown in Fig. 2A). Negative-strand RNA viruses encode and package their own RNA polymerase, which transcribes mRNAs once the virus envelope has fused with the plasma membrane of the infected cell. Viral replication, which occurs after transcription, requires the continuous synthesis of viral proteins. Newly replicated antigenomes serve as template for amplification of negative-strand genomic RNAs, which are the templates for secondary transcription (
      • Pfaller C.K.
      • Cattaneo R.
      • Schnell M.J.
      Reverse genetics of Mononegavirales: how they work, new vaccines, and new cancer therapeutics.
      ).
      Figure thumbnail gr2
      Figure 2Antigenome, particle, and membrane fusion apparatus of a typical paramyxovirus, MeV. A, genome shown as a positive strand. The protein-coding regions are color-coded, noncoding regions are in black, and the M-F boundary is shown with a gray dot. B, MeV particle with its six major components: N, P, M, F, H, and L. Particles can contain multiple genomes, as represented by the three genomes in this particle. C, enlarged representations of the H tetramer and F trimer. The stalk of the H tetramer is modeled on the solved NDV HN structure (
      • Navaratnarajah C.K.
      • Negi S.
      • Braun W.
      • Cattaneo R.
      Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
      ,
      • Yuan P.
      • Swanson K.A.
      • Leser G.P.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the Newcastle disease virus hemagglutinin-neuraminidase (HN) ectodomain reveals a four-helix bundle stalk.
      ). A green cylinder represents the membrane distal region of the stalk that was not in the solved structure. A blue star denotes a kink in the parallel four-helix bundle structure of the stalk. Four blue hexameric heads represent the six-bladed β-propellers of the receptor-binding domains. The heads are connected to the stalk by flexible dimeric linkers (green/blue) and four monomeric connectors (purple) (
      • Navaratnarajah C.K.
      • Rosemarie Q.
      • Cattaneo R.
      A structurally unresolved head segment of defined length favors proper measles virus hemagglutinin tetramerization and efficient membrane fusion triggering.
      ,
      • Herren M.
      • Shrestha N.
      • Wyss M.
      • Zurbriggen A.
      • Plattet P.
      Regulatory role of the morbillivirus attachment protein head-to-stalk linker module in membrane fusion triggering.
      ). The MeV F trimer ectodomain structure (PDB code 5YXW) (
      • Hashiguchi T.
      • Fukuda Y.
      • Matsuoka R.
      • Kuroda D.
      • Kubota M.
      • Shirogane Y.
      • Watanabe S.
      • Tsumoto K.
      • Kohda D.
      • Plemper R.K.
      • Yanagi Y.
      Structures of the prefusion form of measles virus fusion protein in complex with inhibitors.
      ) is shown on the right with monomers represented as blue, purple, and orange. F-protein residues critical for receiving the fusion triggering signal from H are shaded red. Interrupted lines represent the unstructured segments of the ectodomains.
      Genomes of Paramyxoviridae are about 15,000–19,000 bases in length and contain six or more genes in a conserved order. The 15,894-nucleotide (nt) MeV genome begins with a 52-nt 3′ region, the leader, and ends with a 40-nt region, the trailer. These control regions flank the six contiguous transcription units (genes), which are separated by three untranscribed nucleotides. For MeV there are six genes coding for eight proteins, in the order (positive strand): 5′-N-P/V/C-M-F-H-L-3′ (Fig. 2A). The P gene of paramyxoviruses uses overlapping reading frames to code for up to four proteins, P, C, V, and W. The polymerase (known as large protein, L) transcribes the viral genome with a sequential “stop-start” mechanism. It accesses the genome through an entry site located near its 3′ end. It then transcribes the first gene (N) with high processivity; caps, methylates, and polyadenylates the N mRNA; and reinitiates P mRNA synthesis. The frequency of reinitiation is less than 100%, resulting in a gradient of transcript levels: N is transcribed at the highest levels and L at the lowest (
      • Cattaneo R.
      • Rebmann G.
      • Schmid A.
      • Baczko K.
      • ter Meulen V.
      • Billeter M.A.
      Altered transcription of a defective measles virus genome derived from a diseased human brain.
      ).
      RNA genomes, which are protected by helically arranged N proteins, have characteristic size and shape: 190-Å diameter, 49.5-Å pitch, and about 1 μm in length for MeV (
      • Gutsche I.
      • Desfosses A.
      • Effantin G.
      • Ling W.L.
      • Haupt M.
      • Ruigrok R.W.
      • Sachse C.
      • Schoehn G.
      Structural virology. Near-atomic cryo-EM structure of the helical measles virus nucleocapsid.
      ). Recent structural studies have revealed how the 3′ end of the genomic RNA becomes accessible to the viral polymerase complex (
      • Desfosses A.
      • Milles S.
      • Jensen M.R.
      • Guseva S.
      • Colletier J.P.
      • Maurin D.
      • Schoehn G.
      • Gutsche I.
      • Ruigrok R.W.H.
      • Blackledge M.
      Assembly and cryo-EM structures of RNA-specific measles virus nucleocapsids provide mechanistic insight into paramyxoviral replication.
      ), constituted of one copy of the L polymerase and four copies of the P co-factor (
      • Du Pont V.
      • Jiang Y.
      • Plemper R.K.
      Bipartite interface of the measles virus phosphoprotein X domain with the large polymerase protein regulates viral polymerase dynamics.
      ). Another replication complex component, the C protein, interacts with P and improves polymerase processivity and accuracy (
      • Pfaller C.K.
      • Mastorakos G.M.
      • Matchett W.E.
      • Ma X.
      • Samuel C.E.
      • Cattaneo R.
      Measles virus defective interfering RNAs are generated frequently and early in the absence of C protein and can be destabilized by adenosine deaminase acting on RNA-1-like hypermutations.
      ,
      • Pfaller C.K.
      • Bloyet L.M.
      • Donohue R.C.
      • Huff A.L.
      • Bartemes W.P.
      • Yousaf I.
      • Urzua E.
      • Claviere M.
      • Zachary M.
      • de Masson d'Autume V.
      • Carson S.
      • Schieferecke A.J.
      • Meyer A.J.
      • Gerlier D.
      • Cattaneo R.
      The C protein is recruited to measles virus ribonucleocapsids by the phosphoprotein.
      ). The third and fourth proteins expressed by the P gene, V and W, are nonstructural proteins that control innate immunity by antagonizing both type I interferon signaling and interferon production (
      • Donohue R.C.
      • Pfaller C.K.
      • Cattaneo R.
      Cyclical adaptation of measles virus quasispecies to epithelial and lymphocytic cells: to V, or not to V.
      ,
      • Shaw M.L.
      • Cardenas W.B.
      • Zamarin D.
      • Palese P.
      • Basler C.F.
      Nuclear localization of the Nipah virus W protein allows for inhibition of both virus- and Toll-like receptor 3-triggered signaling pathways.
      ,
      • Devaux P.
      • Hudacek A.W.
      • Hodge G.
      • Reyes-Del Valle J.
      • McChesney M.B.
      • Cattaneo R.
      A recombinant measles virus unable to antagonize STAT1 function cannot control inflammation and is attenuated in rhesus monkeys.
      ,
      • Svitek N.
      • Gerhauser I.
      • Goncalves C.
      • Grabski E.
      • Döring M.
      • Kalinke U.
      • Anderson D.E.
      • Cattaneo R.
      • von Messling V.
      Morbillivirus control of the interferon response: relevance of STAT2 and mda5 but not STAT1 for canine distemper virus virulence in ferrets.
      ,
      • Satterfield B.A.
      • Cross R.W.
      • Fenton K.A.
      • Agans K.N.
      • Basler C.F.
      • Geisbert T.W.
      • Mire C.E.
      The immunomodulating V and W proteins of Nipah virus determine disease course.
      ). In addition, these proteins, which are post-entry determinants of virus tropism, may modulate tissue-specific virus gene expression (
      • Donohue R.C.
      • Pfaller C.K.
      • Cattaneo R.
      Cyclical adaptation of measles virus quasispecies to epithelial and lymphocytic cells: to V, or not to V.
      ).

      Envelope

      Enveloped particles of Paramyxoviridae have been observed to be pleomorphic or spherical (Fig. 2B), depending on the methods used for purification. MeV particles are particularly heterogeneous in size. Their diameter is in the 120–300-nm range, but they can vary from 100 to 1000 nm in length, implying that their cargo volume differs. Indeed, large particles can contain multiple genomes, as deduced initially from sedimentation and UV inactivation studies and then confirmed by genetic complementation experiments (
      • Rager M.
      • Vongpunsawad S.
      • Duprex W.P.
      • Cattaneo R.
      Polyploid measles virus with hexameric genome length.
      ). The envelope includes two glycoproteins, the tetrameric attachment protein and the trimeric fusion (F) protein (Fig. 2C). The F-protein of Paramyxoviridae causes membrane fusion at neutral pH, although exceptions to this rule have been described (
      • Chang A.
      • Dutch R.E.
      Paramyxovirus fusion and entry: multiple paths to a common end.
      ). The F and H oligomers form “spikes” that extend ∼8–12 nm from the surface of the particle membrane (
      • Gui L.
      • Jurgens E.M.
      • Ebner J.L.
      • Porotto M.
      • Moscona A.
      • Lee K.K.
      Electron tomography imaging of surface glycoproteins on human parainfluenza virus 3: association of receptor binding and fusion proteins before receptor engagement.
      ). The matrix (M) protein bridges the envelope with the nucleocapsid. In MeV, M is observed as a two-dimensional paracrystalline array associated with the inner leaflet of the plasma membrane (
      • Ke Z.
      • Strauss J.D.
      • Hampton C.M.
      • Brindley M.A.
      • Dillard R.S.
      • Leon F.
      • Lamb K.M.
      • Plemper R.K.
      • Wright E.R.
      Promotion of virus assembly and organization by the measles virus matrix protein.
      ).
      The paramyxovirus attachment proteins that use neuraminic acid (α2,3-linked sialic acid) as a receptor are known as hemagglutinin/neuraminidases (HN), because they also have receptor-cleaving neuraminidase activity. The attachment proteins of MeV and of the other morbilliviruses are named hemagglutinins (H), whereas those of the henipaviruses are called glycoproteins (G). In truth, H is a misnomer. Originally, the attachment protein of an attenuated MeV strain was named H because cells infected with this virus absorb the erythrocytes of New World primate species. However, only the attachment protein of that attenuated strain, and not those of WT strains of MeV or other morbilliviruses, has this function. Eventually, it was shown that hemabsorption depends on binding a specific protein, CD46, rather than sialic acid (
      • Naniche D.
      • Varior-Krishnan G.
      • Cervoni F.
      • Wild T.F.
      • Rossi B.
      • Rabourdin-Combe C.
      • Gerlier D.
      Human membrane cofactor protein (CD46) acts as a cellular receptor for measles virus.
      ,
      • Dörig R.E.
      • Marcil A.
      • Chopra A.
      • Richardson C.D.
      The human CD46 molecule is a receptor for measles virus (Edmonston strain).
      ). Binding to CD46, which is expressed on the erythrocytes of New World monkeys but not on those of humans (
      • Hsu E.C.
      • Dörig R.E.
      • Sarangi F.
      • Marcil A.
      • Iorio C.
      • Richardson C.D.
      Artificial mutations and natural variations in the CD46 molecules from human and monkey cells define regions important for measles virus binding.
      ), is thus implied in MeV attenuation (
      • Navaratnarajah C.K.
      • Vongpunsawad S.
      • Oezguen N.
      • Stehle T.
      • Braun W.
      • Hashiguchi T.
      • Maenaka K.
      • Yanagi Y.
      • Cattaneo R.
      Dynamic interaction of the measles virus hemagglutinin with its receptor signaling lymphocytic activation molecule (SLAM, CD150).
      ,
      • Lecouturier V.
      • Fayolle J.
      • Caballero M.
      • Carabaña J.
      • Celma M.L.
      • Fernandez-Muñoz R.
      • Wild T.F.
      • Buckland R.
      Identification of two amino acids in the hemagglutinin glycoprotein of measles virus (MV) that govern hemadsorption, HeLa cell fusion, and CD46 downregulation: phenotypic markers that differentiate vaccine and wild-type MV strains.
      ).

      Receptors, tropism, and pathogenesis

      Most paramyxoviruses that transmit through airborne routes (Fig. 3A) enter epithelial cells of the airways using protein-linked sialic acid as receptor and cause relatively mild disease. However, some paramyxoviruses cross the epithelial barrier and cause systemic, severe disease. We discuss the tropism and pathogenesis of three representative viruses: human parainfluenza virus (HPIV3), which causes acute respiratory disease but does not spread systemically; NiV, which spreads systemically and can cause lethal disease in humans (
      • Spiropoulou C.F.
      Nipah virus outbreaks: still small but extremely lethal.
      ); and MeV, which replicates in receptor-defined ecological niches, causing both acute respiratory disease and delayed immunosuppression (
      • Mina M.J.
      • Kula T.
      • Leng Y.
      • Li M.
      • de Vries R.D.
      • Knip M.
      • Siljander H.
      • Rewers M.
      • Choy D.F.
      • Wilson M.S.
      • Larman H.B.
      • Nelson A.N.
      • Griffin D.E.
      • de Swart R.L.
      • Elledge S.J.
      Measles virus infection diminishes preexisting antibodies that offer protection from other pathogens.
      ) (Fig. 3B).
      Figure thumbnail gr3
      Figure 3Routes of entry and organs affected by infections with different paramyxoviruses. A, host entry. MeV and HPIV3 enter the human body through the respiratory route, whereas NiV can enter via both respiratory and oral routes. B, infection kinetics and cells and organs most affected by infections. Infected cells and organs are shown. x axis, time of infection in days, unless otherwise noted; y axis, viral loads in blood (red lines) or specific organs (blue lines). Top, HPIV3 infects epithelial cells of the upper respiratory tract during early stages of infection and cells of the lower respiratory tract during late stages. Middle, NiV infects lung epithelial cells, spreads to the vascular system, and, during late stages, infects different organs, causing multiple-organ failure. Relapse-encephalitis may occur in the brain up to 13 months post-infection. Bottom, MeV infects alveolar macrophages and dendritic cells, which transfer the virus to draining lymph nodes. After extensive replication in lymphatic tissues, MeV spreads to the upper-airway epithelia. Rarely, MeV infects the brain, and its persistence can cause lethal disorders years after acute infection.

      HPIV3 binds sialic acid and causes acute respiratory disease

      HPIV3 represents the respiroviruses (Fig. 1). The HPIV3 attachment protein, HN, uses α2,3-linked sialic acid as a receptor. We do not know to what type of moieties this receptor is attached, but we think that it could be on many different proteins. This oligosaccharide is predominantly found on nasopharyngeal, bronchiolar, bronchial, and tracheal epithelial cells and alveolar pneumocytes. There is some indication of ocular distribution of α2,3-linked sialic acid as well, but very little is known about the exact levels (
      • Kumlin U.
      • Olofsson S.
      • Dimock K.
      • Arnberg N.
      Sialic acid tissue distribution and influenza virus tropism.
      ). Tissue distribution of the receptor accounts for the symptoms caused by HPIV3 infections, which include not only mild respiratory disease but also bronchiolitis, bronchitis, and pneumonia in infants and children.
      HPIV3 infection starts with virus entering through the nasal route (Fig. 3A), where it infects nasopharyngeal cells. As virus replicates and amplifies, it moves on to infect bronchiolar, bronchial, and tracheal epithelial cells and eventually spreads to alveolar pneumocytes in the lungs (
      • Schomacker H.
      • Schaap-Nutt A.
      • Collins P.L.
      • Schmidt A.C.
      Pathogenesis of acute respiratory illness caused by human parainfluenza viruses.
      ). HPIV3 replication peaks within the first week of infection (Fig. 3B, top). Different animal models have suggested that HPIV3 replication can start as early as 1 day after exposure and peaks between 2 and 5 days (
      • Durbin A.P.
      • Elkins W.R.
      • Murphy B.R.
      African green monkeys provide a useful nonhuman primate model for the study of human parainfluenza virus types-1, -2, and -3 infection.
      ,
      • Ottolini M.G.
      • Porter D.D.
      • Blanco J.C.
      • Prince G.A.
      A cotton rat model of human parainfluenza 3 laryngotracheitis: virus growth, pathology, and therapy.
      ,
      • Porter D.D.
      • Prince G.A.
      • Hemming V.G.
      • Porter H.G.
      Pathogenesis of human parainfluenza virus 3 infection in two species of cotton rats: Sigmodon hispidus develops bronchiolitis, while Sigmodon fulviventer develops interstitial pneumonia.
      ). HPIV3 is usually cleared by the immune system, and infection is resolved within 7–10 days; however, in immunocompromised individuals, it can lead to death (
      • Aguayo-Hiraldo P.I.
      • Arasaratnam R.J.
      • Tzannou I.
      • Kuvalekar M.
      • Lulla P.
      • Naik S.
      • Martinez C.A.
      • Piedra P.A.
      • Vera J.F.
      • Leen A.M.
      Characterizing the cellular immune response to parainfluenza virus 3.
      ,
      • Cortez K.J.
      • Erdman D.D.
      • Peret T.C.
      • Gill V.J.
      • Childs R.
      • Barrett A.J.
      • Bennett J.E.
      Outbreak of human parainfluenza virus 3 infections in a hematopoietic stem cell transplant population.
      ,
      • Maziarz R.T.
      • Sridharan P.
      • Slater S.
      • Meyers G.
      • Post M.
      • Erdman D.D.
      • Peret T.C.
      • Taplitz R.A.
      Control of an outbreak of human parainfluenza virus 3 in hematopoietic stem cell transplant recipients.
      ).

      NiV binds ephrin-B2 or ephrin-B3 and can cause lethal systemic disease

      NiV represents the henipaviruses (Fig. 1). NiV, like HeV, is endemic in certain pteropid fruit bat species, which serve as reservoir hosts (
      • Halpin K.
      • Hyatt A.D.
      • Fogarty R.
      • Middleton D.
      • Bingham J.
      • Epstein J.H.
      • Rahman S.A.
      • Hughes T.
      • Smith C.
      • Field H.E.
      • Daszak P.
      • Henipavirus Ecology Research Group
      Pteropid bats are confirmed as the reservoir hosts of henipaviruses: a comprehensive experimental study of virus transmission.
      ). NiV and HeV have an exceptionally broad species tropism, and natural or experimental infection has been documented to span six mammalian orders. Affected species include pigs, horses, cats, dogs, guinea pigs, mice, hamsters, ferrets, squirrel monkeys, African green monkeys, and humans (
      • Broder C.C.
      • Weir D.L.
      • Reid P.A.
      Hendra virus and Nipah virus animal vaccines.
      ). NiV causes severe and often fatal respiratory and neurological diseases. The NiV attachment protein, G, utilizes ephrin-B2 and ephrin-B3, as receptors for cell entry. Ephrin-B2 is expressed in arteries, capillaries, and bronchial and pulmonary alveolar type II epithelial cells (
      • Bennett K.M.
      • Afanador M.D.
      • Lal C.V.
      • Xu H.
      • Persad E.
      • Legan S.K.
      • Chenaux G.
      • Dellinger M.
      • Savani R.C.
      • Dravis C.
      • Henkemeyer M.
      • Schwarz M.A.
      Ephrin-B2 reverse signaling increases α5β1 integrin-mediated fibronectin deposition and reduces distal lung compliance.
      ,
      • Hafner C.
      • Schmitz G.
      • Meyer S.
      • Bataille F.
      • Hau P.
      • Langmann T.
      • Dietmaier W.
      • Landthaler M.
      • Vogt T.
      Differential gene expression of Eph receptors and ephrins in benign human tissues and cancers.
      ,
      • Gale N.W.
      • Baluk P.
      • Pan L.
      • Kwan M.
      • Holash J.
      • DeChiara T.M.
      • McDonald D.M.
      • Yancopoulos G.D.
      Ephrin-B2 selectively marks arterial vessels and neovascularization sites in the adult, with expression in both endothelial and smooth-muscle cells.
      ). Ephrin-B3 is predominantly found in the central nervous system (CNS) and at lower levels in the vasculature (
      • Hafner C.
      • Schmitz G.
      • Meyer S.
      • Bataille F.
      • Hau P.
      • Langmann T.
      • Dietmaier W.
      • Landthaler M.
      • Vogt T.
      Differential gene expression of Eph receptors and ephrins in benign human tissues and cancers.
      ,
      • Xu K.
      • Broder C.C.
      • Nikolov D.B.
      Ephrin-B2 and ephrin-B3 as functional henipavirus receptors.
      ). High CNS expression of NiV receptors may account for its neurological disease potential, which is often lethal.
      NiV enters the host through the nasal or oronasal route, from respiratory secretions or contaminated food (Fig. 3A) (
      • Clayton B.A.
      • Middleton D.
      • Bergfeld J.
      • Haining J.
      • Arkinstall R.
      • Wang L.
      • Marsh G.A.
      Transmission routes for Nipah virus from Malaysia and Bangladesh.
      ,
      • Munster V.J.
      • Prescott J.B.
      • Bushmaker T.
      • Long D.
      • Rosenke R.
      • Thomas T.
      • Scott D.
      • Fischer E.R.
      • Feldmann H.
      • de Wit E.
      Rapid Nipah virus entry into the central nervous system of hamsters via the olfactory route.
      ,
      • Sazzad H.M.
      • Hossain M.J.
      • Gurley E.S.
      • Ameen K.M.
      • Parveen S.
      • Islam M.S.
      • Faruque L.I.
      • Podder G.
      • Banu S.S.
      • Lo M.K.
      • Rollin P.E.
      • Rota P.A.
      • Daszak P.
      • Rahman M.
      • Luby S.P.
      Nipah virus infection outbreak with nosocomial and corpse-to-human transmission, Bangladesh.
      ,
      • Singh R.K.
      • Dhama K.
      • Chakraborty S.
      • Tiwari R.
      • Natesan S.
      • Khandia R.
      • Munjal A.
      • Vora K.S.
      • Latheef S.K.
      • Karthik K.
      • Singh Malik Y.
      • Singh R.
      • Chaicumpa W.
      • Mourya D.T.
      Nipah virus: epidemiology, pathology, immunobiology and advances in diagnosis, vaccine designing and control strategies: a comprehensive review.
      ). It initially replicates in ephrin-B2–positive bronchial and pulmonary alveolar type II epithelial cells (
      • Clayton B.A.
      • Middleton D.
      • Bergfeld J.
      • Haining J.
      • Arkinstall R.
      • Wang L.
      • Marsh G.A.
      Transmission routes for Nipah virus from Malaysia and Bangladesh.
      ) and lung endothelial cells, as early as 2 days post-infection (
      • Geisbert T.W.
      • Daddario-DiCaprio K.M.
      • Hickey A.C.
      • Smith M.A.
      • Chan Y.P.
      • Wang L.F.
      • Mattapallil J.J.
      • Geisbert J.B.
      • Bossart K.N.
      • Broder C.C.
      Development of an acute and highly pathogenic nonhuman primate model of Nipah virus infection.
      ). It then enters the bloodstream by infecting arterial endothelium, arterial smooth muscles, and pericytes that also express ephrin-B2; in a nonhuman primate model, replication in blood vessels is observed starting 4 days post-infection (
      • Geisbert T.W.
      • Daddario-DiCaprio K.M.
      • Hickey A.C.
      • Smith M.A.
      • Chan Y.P.
      • Wang L.F.
      • Mattapallil J.J.
      • Geisbert J.B.
      • Bossart K.N.
      • Broder C.C.
      Development of an acute and highly pathogenic nonhuman primate model of Nipah virus infection.
      ). Within the first week of infection, NiV disseminates to different organs, including spleen, kidneys, heart, and liver, as documented in different animal models (Fig. 3B, center) (
      • Geisbert T.W.
      • Daddario-DiCaprio K.M.
      • Hickey A.C.
      • Smith M.A.
      • Chan Y.P.
      • Wang L.F.
      • Mattapallil J.J.
      • Geisbert J.B.
      • Bossart K.N.
      • Broder C.C.
      Development of an acute and highly pathogenic nonhuman primate model of Nipah virus infection.
      ,
      • Guillaume V.
      • Wong K.T.
      • Looi R.Y.
      • Georges-Courbot M.C.
      • Barrot L.
      • Buckland R.
      • Wild T.F.
      • Horvat B.
      Acute Hendra virus infection: analysis of the pathogenesis and passive antibody protection in the hamster model.
      ,
      • Mire C.E.
      • Satterfield B.A.
      • Geisbert J.B.
      • Agans K.N.
      • Borisevich V.
      • Yan L.
      • Chan Y.P.
      • Cross R.W.
      • Fenton K.A.
      • Broder C.C.
      • Geisbert T.W.
      Pathogenic differences between Nipah virus Bangladesh and Malaysia strains in primates: implications for antibody therapy.
      ,
      • Wong K.T.
      • Grosjean I.
      • Brisson C.
      • Blanquier B.
      • Fevre-Montange M.
      • Bernard A.
      • Loth P.
      • Georges-Courbot M.C.
      • Chevallier M.
      • Akaoka H.
      • Marianneau P.
      • Lam S.K.
      • Wild T.F.
      • Deubel V.
      A golden hamster model for human acute Nipah virus infection.
      ). NiV can also directly enter the CNS via the olfactory nerve and possibly the hematogenous route (
      • Munster V.J.
      • Prescott J.B.
      • Bushmaker T.
      • Long D.
      • Rosenke R.
      • Thomas T.
      • Scott D.
      • Fischer E.R.
      • Feldmann H.
      • de Wit E.
      Rapid Nipah virus entry into the central nervous system of hamsters via the olfactory route.
      ,
      • Weingartl H.
      • Czub S.
      • Copps J.
      • Berhane Y.
      • Middleton D.
      • Marszal P.
      • Gren J.
      • Smith G.
      • Ganske S.
      • Manning L.
      • Czub M.
      Invasion of the central nervous system in a porcine host by Nipah virus.
      ). Once inside the brain, it infects ephrin-B3– and/or ephrin-B2–expressing neurons and other parenchymal cells within the CNS, causing encephalitis. This eventually leads to neurological disease (
      • Bossart K.N.
      • Zhu Z.
      • Middleton D.
      • Klippel J.
      • Crameri G.
      • Bingham J.
      • McEachern J.A.
      • Green D.
      • Hancock T.J.
      • Chan Y.P.
      • Hickey A.C.
      • Dimitrov D.S.
      • Wang L.F.
      • Broder C.C.
      A neutralizing human monoclonal antibody protects against lethal disease in a new ferret model of acute Nipah virus infection.
      ), multiple organ failure, and death of the host. Survivors of acute NiV infection may suffer a lethal relapse-encephalitis 8–13 months after initial disease resolution (Fig. 3B) (
      • Tan C.T.
      • Goh K.J.
      • Wong K.T.
      • Sarji S.A.
      • Chua K.B.
      • Chew N.K.
      • Murugasu P.
      • Loh Y.L.
      • Chong H.T.
      • Tan K.S.
      • Thayaparan T.
      • Kumar S.
      • Jusoh M.R.
      Relapsed and late-onset Nipah encephalitis.
      ,
      • Wong K.T.
      • Robertson T.
      • Ong B.B.
      • Chong J.W.
      • Yaiw K.C.
      • Wang L.F.
      • Ansford A.J.
      • Tannenberg A.
      Human Hendra virus infection causes acute and relapsing encephalitis.
      ,
      • Liu J.
      • Coffin K.M.
      • Johnston S.C.
      • Babka A.M.
      • Bell T.M.
      • Long S.Y.
      • Honko A.N.
      • Kuhn J.H.
      • Zeng X.
      Nipah virus persists in the brains of nonhuman primate survivors.
      ).

      MeV binds two proteins, causing both respiratory disease and immunosuppression

      MeV is a human morbillivirus, a genus that also includes several important animal pathogens (Fig. 1). Even though disease severity caused by different morbilliviruses varies, the underlying pathogenic mechanisms and clinical manifestations are very similar. The attachment protein of all morbilliviruses binds two receptors, the signaling lymphocytic activation molecule (SLAM) and nectin-4. SLAM is found on immune cells, namely immature thymocytes, activated and memory T cells, naive and activated B cells, macrophages, and dendritic cells (
      • De Salort J.
      • Sintes J.
      • Llinàs L.
      • Matesanz-Isabel J.
      • Engel P.
      Expression of SLAM (CD150) cell-surface receptors on human B-cell subsets: from pro-B to plasma cells.
      ,
      • Wang N.
      • Satoskar A.
      • Faubion W.
      • Howie D.
      • Okamoto S.
      • Feske S.
      • Gullo C.
      • Clarke K.
      • Sosa M.R.
      • Sharpe A.H.
      • Terhorst C.
      The cell surface receptor SLAM controls T cell and macrophage functions.
      ,
      • Laksono B.M.
      • de Vries R.D.
      • Verburgh R.J.
      • Visser E.G.
      • de Jong A.
      • Fraaij P.L.A.
      • Ruijs W.L.M.
      • Nieuwenhuijse D.F.
      • van den Ham H.J.
      • Koopmans M.P.G.
      • van Zelm M.C.
      • Osterhaus A.D.M.E.
      • de Swart R.L.
      Studies into the mechanism of measles-associated immune suppression during a measles outbreak in the Netherlands.
      ,
      • Laksono B.M.
      • Grosserichter-Wagener C.
      • de Vries R.D.
      • Langeveld S.A.G.
      • Brem M.D.
      • van Dongen J.J.M.
      • Katsikis P.D.
      • Koopmans M.P.G.
      • van Zelm M.C.
      • de Swart R.L.
      In vitro measles virus infection of human lymphocyte subsets demonstrates high susceptibility and permissiveness of both naive and memory B cells.
      ). Nectin-4 is expressed at high levels by epithelial cells of the nasopharynx, trachea, and epidermal keratinocytes (
      • Reymond N.
      • Fabre S.
      • Lecocq E.
      • Adelaïde J.
      • Dubreuil P.
      • Lopez M.
      Nectin4/PRR4, a new afadin-associated member of the nectin family that trans-interacts with nectin1/PRR1 through V domain interaction.
      ) and localizes to the adherens junctions (
      • Brancati F.
      • Fortugno P.
      • Bottillo I.
      • Lopez M.
      • Josselin E.
      • Boudghene-Stambouli O.
      • Agolini E.
      • Bernardini L.
      • Bellacchio E.
      • Iannicelli M.
      • Rossi A.
      • Dib-Lachachi A.
      • Stuppia L.
      • Palka G.
      • Mundlos S.
      • et al.
      Mutations in PVRL4, encoding cell adhesion molecule nectin-4, cause ectodermal dysplasia-syndactyly syndrome.
      ).
      MeV enters the body through the nasal route and infects SLAM-positive alveolar macrophages and dendritic cells (Fig. 3A) (
      • Tatsuo H.
      • Ono N.
      • Tanaka K.
      • Yanagi Y.
      SLAM (CDw150) is a cellular receptor for measles virus.
      ,
      • de Swart R.L.
      • Ludlow M.
      • de Witte L.
      • Yanagi Y.
      • van Amerongen G.
      • McQuaid S.
      • Yüksel S.
      • Geijtenbeek T.B.
      • Duprex W.P.
      • Osterhaus A.D.
      Predominant infection of CD150+ lymphocytes and dendritic cells during measles virus infection of macaques.
      ). These cells ferry the infection through the epithelial barrier and spread it to the local lymph nodes (
      • Lemon K.
      • de Vries R.D.
      • Mesman A.W.
      • McQuaid S.
      • van Amerongen G.
      • Yüksel S.
      • Ludlow M.
      • Rennick L.J.
      • Kuiken T.
      • Rima B.K.
      • Geijtenbeek T.B.
      • Osterhaus A.D.
      • Duprex W.P.
      • de Swart R.L.
      Early target cells of measles virus after aerosol infection of non-human primates.
      ,
      • Ferreira C.S.
      • Frenzke M.
      • Leonard V.H.
      • Welstead G.G.
      • Richardson C.D.
      • Cattaneo R.
      Measles virus infection of alveolar macrophages and dendritic cells precedes spread to lymphatic organs in transgenic mice expressing human signaling lymphocytic activation molecule (SLAM, CD150).
      ). The cellular distribution of SLAM overlaps with the susceptibility of different cell types to WT MeV infection (
      • Condack C.
      • Grivel J.C.
      • Devaux P.
      • Margolis L.
      • Cattaneo R.
      Measles virus vaccine attenuation: suboptimal infection of lymphatic tissue and tropism alteration.
      ). Another argument for the central role of SLAM in morbillivirus tropism is that CDV and RPV also enter immune cells through the cognate SLAM protein (
      • Tatsuo H.
      • Ono N.
      • Yanagi Y.
      Morbilliviruses use signaling lymphocyte activation molecules (CD150) as cellular receptors.
      ). Indeed, genetically modified MeV and CDV unable to enter cells through SLAM are attenuated in primate and ferret models, respectively (
      • von Messling V.
      • Svitek N.
      • Cattaneo R.
      Receptor (SLAM [CD150]) recognition and the V protein sustain swift lymphocyte-based invasion of mucosal tissue and lymphatic organs by a morbillivirus.
      ,
      • Leonard V.H.
      • Hodge G.
      • Reyes-Del Valle J.
      • McChesney M.B.
      • Cattaneo R.
      Measles virus selectively blind to signaling lymphocytic activation molecule (SLAM; CD150) is attenuated and induces strong adaptive immune responses in rhesus monkeys.
      ).
      Massive amplification of MeV and the other morbilliviruses in lymph nodes and primary immune tissues sets the stage for synchronous, massive invasion of tissues expressing the morbillivirus epithelial receptor, nectin-4 (
      • Mühlebach M.D.
      • Mateo M.
      • Sinn P.L.
      • Prüfer S.
      • Uhlig K.M.
      • Leonard V.H.
      • Navaratnarajah C.K.
      • Frenzke M.
      • Wong X.X.
      • Sawatsky B.
      • Ramachandran S.
      • McCray Jr., P.B.
      • Cichutek K.
      • von Messling V.
      • Lopez M.
      • Cattaneo R.
      Adherens junction protein nectin-4 is the epithelial receptor for measles virus.
      ,
      • Noyce R.S.
      • Bondre D.G.
      • Ha M.N.
      • Lin L.T.
      • Sisson G.
      • Tsao M.S.
      • Richardson C.D.
      Tumor cell marker PVRL4 (nectin 4) is an epithelial cell receptor for measles virus.
      ). Contrary to initial assumptions, morbilliviruses enter epithelia from the basolateral side, being delivered there by infected immune cells (
      • Frenzke M.
      • Sawatsky B.
      • Wong X.X.
      • Delpeut S.
      • Mateo M.
      • Cattaneo R.
      • von Messling V.
      Nectin-4-dependent measles virus spread to the cynomolgus monkey tracheal epithelium: role of infected immune cells infiltrating the lamina propria.
      ,
      • Ludlow M.
      • Lemon K.
      • de Vries R.D.
      • McQuaid S.
      • Millar E.L.
      • van Amerongen G.
      • Yüksel S.
      • Verburgh R.J.
      • Osterhaus A.D.
      • de Swart R.L.
      • Duprex W.P.
      Measles virus infection of epithelial cells in the macaque upper respiratory tract is mediated by subepithelial immune cells.
      ). Indeed, nectin-4 is located on the basolateral side of epithelial cells, and airway infection is restricted to epithelia that express nectin-4, including the trachea and the upper respiratory tract (Fig. 3B, bottom) (
      • Mühlebach M.D.
      • Mateo M.
      • Sinn P.L.
      • Prüfer S.
      • Uhlig K.M.
      • Leonard V.H.
      • Navaratnarajah C.K.
      • Frenzke M.
      • Wong X.X.
      • Sawatsky B.
      • Ramachandran S.
      • McCray Jr., P.B.
      • Cichutek K.
      • von Messling V.
      • Lopez M.
      • Cattaneo R.
      Adherens junction protein nectin-4 is the epithelial receptor for measles virus.
      ,
      • Noyce R.S.
      • Bondre D.G.
      • Ha M.N.
      • Lin L.T.
      • Sisson G.
      • Tsao M.S.
      • Richardson C.D.
      Tumor cell marker PVRL4 (nectin 4) is an epithelial cell receptor for measles virus.
      ). Because the trachea is the anatomical location most useful to support particle aerosolization, this two-phase process of amplification in immune cells followed by massive, synchronous invasion of the trachea accounts for the extremely contagious nature of MeV infections (
      • Leonard V.H.
      • Sinn P.L.
      • Hodge G.
      • Miest T.
      • Devaux P.
      • Oezguen N.
      • Braun W.
      • McCray Jr., P.B.
      • McChesney M.B.
      • Cattaneo R.
      Measles virus blind to its epithelial cell receptor remains virulent in rhesus monkeys but cannot cross the airway epithelium and is not shed.
      ,
      • Sawatsky B.
      • Wong X.X.
      • Hinkelmann S.
      • Cattaneo R.
      • von Messling V.
      Canine distemper virus epithelial cell infection is required for clinical disease but not for immunosuppression.
      ,
      • Mateo M.
      • Navaratnarajah C.K.
      • Cattaneo R.
      Structural basis of efficient contagion: measles variations on a theme by parainfluenza viruses.
      ,
      • Delpeut S.
      • Sawatsky B.
      • Wong X.X.
      • Frenzke M.
      • Cattaneo R.
      • von Messling V.
      Nectin-4 interactions govern measles virus virulence in a new model of pathogenesis, the squirrel monkey (Saimiri sciureus).
      ).
      Weeks after contagion, MeV can cause rare neurological complications, like primary measles encephalitis, in about 1 of 1000 cases. Moreover, 5–10 years after resolution of the primary infection, persistent MeV infections cause lethal subacute sclerosing panencephalitis in about 1 in 10,000 cases, or possibly at a higher incidence when very young children are infected (
      • Bellini W.J.
      • Rota J.S.
      • Lowe L.E.
      • Katz R.S.
      • Dyken P.R.
      • Zaki S.R.
      • Shieh W.J.
      • Rota P.A.
      Subacute sclerosing panencephalitis: more cases of this fatal disease are prevented by measles immunization than was previously recognized.
      ,
      • Wendorf K.A.
      • Winter K.
      • Zipprich J.
      • Schechter R.
      • Hacker J.K.
      • Preas C.
      • Cherry J.D.
      • Glaser C.
      • Harriman K.
      Subacute sclerosing panencephalitis: the devastating measles complication that might be more common than previously estimated.
      ). A neural receptor accounting for these pathologies has been sought but not yet identified (
      • Watanabe S.
      • Shirogane Y.
      • Sato Y.
      • Hashiguchi T.
      • Yanagi Y.
      New insights into measles virus brain infections.
      ). Alternative mechanisms that may account for neuro-invasion include MeV delivery to the brain by SLAM-expressing infected immune cells and a newly discovered form of cytoplasm transfer occurring between nectin-4–expressing epithelial cells and nectin-1–expressing neurons (
      • Generous A.R.
      • Harrison O.J.
      • Troyanovsky R.B.
      • Mateo M.
      • Navaratnarajah C.K.
      • Donohue R.C.
      • Pfaller C.K.
      • Alekhina O.
      • Sergeeva A.P.
      • Indra I.
      • Thornburg T.
      • Kochetkova I.
      • Billadeau D.D.
      • Taylor M.P.
      • Troyanovsky S.M.
      • et al.
      Trans-endocytosis elicited by nectins transfers cytoplasmic cargo including infectious material between cells.
      ).

      Cell entry

      Receptor binding

      Cell entry of all paramyxoviruses is mediated by the concerted action of attachment protein (HN, H, or G) tetramers with F-protein trimers (Fig. 2C). After receptors bind one or more attachment protein heads, the heads move and change their conformation, leading either to a structural change in the stalk or the exposure of parts of the stalk, which elicits refolding of F-trimers (
      • Bishop K.A.
      • Hickey A.C.
      • Khetawat D.
      • Patch J.R.
      • Bossart K.N.
      • Zhu Z.
      • Wang L.F.
      • Dimitrov D.S.
      • Broder C.C.
      Residues in the stalk domain of the Hendra virus G glycoprotein modulate conformational changes associated with receptor binding.
      ,
      • Bose S.
      • Zokarkar A.
      • Welch B.D.
      • Leser G.P.
      • Jardetzky T.S.
      • Lamb R.A.
      Fusion activation by a headless parainfluenza virus 5 hemagglutinin-neuraminidase stalk suggests a modular mechanism for triggering.
      ,
      • Liu Q.
      • Stone J.A.
      • Bradel-Tretheway B.
      • Dabundo J.
      • Benavides Montano J.A.
      • Santos-Montanez J.
      • Biering S.B.
      • Nicola A.V.
      • Iorio R.M.
      • Lu X.
      • Aguilar H.C.
      Unraveling a three-step spatiotemporal mechanism of triggering of receptor-induced Nipah virus fusion and cell entry.
      ,
      • Liu Q.
      • Bradel-Tretheway B.
      • Monreal A.I.
      • Saludes J.P.
      • Lu X.
      • Nicola A.V.
      • Aguilar H.C.
      Nipah virus attachment glycoprotein stalk C-terminal region links receptor binding to fusion triggering.
      ,
      • Navaratnarajah C.K.
      • Oezguen N.
      • Rupp L.
      • Kay L.
      • Leonard V.H.
      • Braun W.
      • Cattaneo R.
      The heads of the measles virus attachment protein move to transmit the fusion-triggering signal.
      ,
      • Navaratnarajah C.K.
      • Negi S.
      • Braun W.
      • Cattaneo R.
      Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
      ,
      • Navaratnarajah C.K.
      • Kumar S.
      • Generous A.
      • Apte-Sengupta S.
      • Mateo M.
      • Cattaneo R.
      The measles virus hemagglutinin stalk: structures and functions of the central fusion activation and membrane-proximal segments.
      ). This causes fusion of the particle envelope with the plasma membrane, a process that occurs at neutral pH (
      • Mateo M.
      • Navaratnarajah C.K.
      • Cattaneo R.
      Structural basis of efficient contagion: measles variations on a theme by parainfluenza viruses.
      ,
      • Jardetzky T.S.
      • Lamb R.A.
      Activation of paramyxovirus membrane fusion and virus entry.
      ,
      • Plattet P.
      • Alves L.
      • Herren M.
      • Aguilar H.C.
      Measles virus fusion protein: structure, function and inhibition.
      ).
      The attachment proteins of paramyxoviruses are type II membrane glycoproteins comprised of a short cytoplasmic tail, a membrane-spanning segment, and an ectodomain with a stalk and a six-blade β-propeller headgroup that contacts the receptors (Fig. 2C, left). The F-proteins are type I membrane glycoproteins that form trimers, initially folding to a metastable, prefusion conformation (Fig. 2C, right), and upon activation refolding to catalyze membrane fusion. Proteolytic cleavage of F into two subunits is required for fusion activity. The MeV F-protein is cleaved by furin-like proteases in a trans-Golgi compartment as it is trafficked to the cell surface (
      • Richardson C.
      • Hull D.
      • Greer P.
      • Hasel K.
      • Berkovich A.
      • Englund G.
      • Bellini W.
      • Rima B.
      • Lazzarini R.
      The nucleotide sequence of the mRNA encoding the fusion protein of measles virus (Edmonston strain): a comparison of fusion proteins from several different paramyxoviruses.
      ). Whereas most paramyxovirus F-proteins are similarly processed prior to arrival at the plasma membrane, the henipavirus F-proteins reach the cell surface as inactive precursors. Henipavirus F-activation requires endocytosis, which allows F-cleavage by an endosome-resident protease, cathepsin L (
      • Pager C.T.
      • Dutch R.E.
      Cathepsin L is involved in proteolytic processing of the Hendra virus fusion protein.
      ,
      • Pager C.T.
      • Craft Jr, W.W.
      • Patch J.
      • Dutch R.E.
      A mature and fusogenic form of the Nipah virus fusion protein requires proteolytic processing by cathepsin L.
      ,
      • Diederich S.
      • Thiel L.
      • Maisner A.
      Role of endocytosis and cathepsin-mediated activation in Nipah virus entry.
      ).

      Receptor-binding affinities

      Table 1 compares the receptor-binding constants of HN, H, and G attachment proteins with those of other enveloped RNA viruses, such as influenza A virus and HIV. The influenza attachment protein (HA) interacts with sialic acid, whereas the HIV attachment protein (gp120) binds the CD4 protein. Binding affinities of the paramyxovirus attachment proteins to their receptors are widely different, ranging from very weak (Kd > 50 μm, similar to influenza HA) for HN-proteins, to intermediate (Kd = 20–200 nm, similar to HIV gp120) for most H-proteins, to very strong (Kd = 0.1–3 nm) for most G-proteins (
      • Navaratnarajah C.K.
      • Vongpunsawad S.
      • Oezguen N.
      • Stehle T.
      • Braun W.
      • Hashiguchi T.
      • Maenaka K.
      • Yanagi Y.
      • Cattaneo R.
      Dynamic interaction of the measles virus hemagglutinin with its receptor signaling lymphocytic activation molecule (SLAM, CD150).
      ,
      • Santiago C.
      • Björling E.
      • Stehle T.
      • Casasnovas J.M.
      Distinct kinetics for binding of the CD46 and SLAM receptors to overlapping sites in the measles virus hemagglutinin protein.
      ,
      • Bonaparte M.I.
      • Dimitrov A.S.
      • Bossart K.N.
      • Crameri G.
      • Mungall B.A.
      • Bishop K.A.
      • Choudhry V.
      • Dimitrov D.S.
      • Wang L.F.
      • Eaton B.T.
      • Broder C.C.
      Ephrin-B2 ligand is a functional receptor for Hendra virus and Nipah virus.
      ,
      • Negrete O.A.
      • Wolf M.C.
      • Aguilar H.C.
      • Enterlein S.
      • Wang W.
      • Mühlberger E.
      • Su S.V.
      • Bertolotti-Ciarlet A.
      • Flick R.
      • Lee B.
      Two key residues in ephrinB3 are critical for its use as an alternative receptor for Nipah virus.
      ,
      • Mateo M.
      • Navaratnarajah C.K.
      • Syed S.
      • Cattaneo R.
      The measles virus hemagglutinin beta-propeller head β4-β5 hydrophobic groove governs functional interactions with nectin-4 and CD46 but not those with the signaling lymphocytic activation molecule.
      ,
      • Fukuhara H.
      • Ito Y.
      • Sako M.
      • Kajikawa M.
      • Yoshida K.
      • Seki F.
      • Mwaba M.H.
      • Hashiguchi T.
      • Higashibata M.A.
      • Ose T.
      • Kuroki K.
      • Takeda M.
      • Maenaka K.
      Specificity of morbillivirus hemagglutinins to recognize SLAM of different species.
      ,
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ).
      Table 1Binding constants of attachment proteins to receptors
      VirusAttachment proteinReceptor(s)KonKoffKdReferences
      m−1 s−1s−1nm
      MuVHNSialic acidNA
      NA, not available.
      NA5.6 × 104
      • Kubota M.
      • Takeuchi K.
      • Watanabe S.
      • Ohno S.
      • Matsuoka R.
      • Kohda D.
      • Nakakita S.I.
      • Hiramatsu H.
      • Suzuki Y.
      • Nakayama T.
      • Terada T.
      • Shimizu K.
      • Shimizu N.
      • Shiroishi M.
      • Yanagi Y.
      • Hashiguchi T.
      Trisaccharide containing α2,3-linked sialic acid is a receptor for mumps virus.
      MeVHhSLAM2.5 × 1042.0 × 10−380
      • Navaratnarajah C.K.
      • Vongpunsawad S.
      • Oezguen N.
      • Stehle T.
      • Braun W.
      • Hashiguchi T.
      • Maenaka K.
      • Yanagi Y.
      • Cattaneo R.
      Dynamic interaction of the measles virus hemagglutinin with its receptor signaling lymphocytic activation molecule (SLAM, CD150).
      Nectin-41.8 × 1053.5 × 10−320
      • Mateo M.
      • Navaratnarajah C.K.
      • Syed S.
      • Cattaneo R.
      The measles virus hemagglutinin beta-propeller head β4-β5 hydrophobic groove governs functional interactions with nectin-4 and CD46 but not those with the signaling lymphocytic activation molecule.
      CD467.2 × 1047.3 × 10−3100
      • Santiago C.
      • Björling E.
      • Stehle T.
      • Casasnovas J.M.
      Distinct kinetics for binding of the CD46 and SLAM receptors to overlapping sites in the measles virus hemagglutinin protein.
      CDVHdSLAM6.1 × 1042.4 × 10−3347
      • Fukuhara H.
      • Ito Y.
      • Sako M.
      • Kajikawa M.
      • Yoshida K.
      • Seki F.
      • Mwaba M.H.
      • Hashiguchi T.
      • Higashibata M.A.
      • Ose T.
      • Kuroki K.
      • Takeda M.
      • Maenaka K.
      Specificity of morbillivirus hemagglutinins to recognize SLAM of different species.
      NiVGephrin-B29.7 × 1051.1 × 10−40.11
      • Negrete O.A.
      • Wolf M.C.
      • Aguilar H.C.
      • Enterlein S.
      • Wang W.
      • Mühlberger E.
      • Su S.V.
      • Bertolotti-Ciarlet A.
      • Flick R.
      • Lee B.
      Two key residues in ephrinB3 are critical for its use as an alternative receptor for Nipah virus.
      ephrin-B36.9 × 1051.9 × 10−32.83
      • Negrete O.A.
      • Wolf M.C.
      • Aguilar H.C.
      • Enterlein S.
      • Wang W.
      • Mühlberger E.
      • Su S.V.
      • Bertolotti-Ciarlet A.
      • Flick R.
      • Lee B.
      Two key residues in ephrinB3 are critical for its use as an alternative receptor for Nipah virus.
      HeVGephrin-B21.3 × 1051.4 × 10−41
      • Bonaparte M.I.
      • Dimitrov A.S.
      • Bossart K.N.
      • Crameri G.
      • Mungall B.A.
      • Bishop K.A.
      • Choudhry V.
      • Dimitrov D.S.
      • Wang L.F.
      • Eaton B.T.
      • Broder C.C.
      Ephrin-B2 ligand is a functional receptor for Hendra virus and Nipah virus.
      ephrin-B3NANA24
      • Negrete O.A.
      • Chu D.
      • Aguilar H.C.
      • Lee B.
      Single amino acid changes in the Nipah and Hendra virus attachment glycoproteins distinguish ephrinB2 from ephrinB3 usage.
      CedVGephrin-B11.2 × 1062.9 × 10−40.24
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ephrin-B21.8 × 1061.0 × 10−50.56
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ephrin-A23.2 × 1046.4 × 10−3196
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ephrin-A57.0 × 1037.9 × 10−4113
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      HIVgp120CD48.3 × 1041.6 × 10−319.3
      • Wu H.
      • Myszka D.G.
      • Tendian S.W.
      • Brouillette C.G.
      • Sweet R.W.
      • Chaiken I.M.
      • Hendrickson W.A.
      Kinetic and structural analysis of mutant CD4 receptors that are defective in HIV gp120 binding.
      InfluenzaHASialic acidNANA7.6 × 103 to 7.7 × 104
      • Wu W.
      • Air G.M.
      Binding of influenza viruses to sialic acids: reassortant viruses with A/NWS/33 hemagglutinin bind to α2,8-linked sialic acid.
      a NA, not available.
      These striking differences in binding affinity are explained in part by how many receptors the different attachment proteins need to engage. Due to the abundance of sialic acid expressed on the cell surface as a terminal component of sugar chains, the HN low affinity translates into high virus-binding avidities (
      • Xiong X.
      • Coombs P.J.
      • Martin S.R.
      • Liu J.
      • Xiao H.
      • McCauley J.W.
      • Locher K.
      • Walker P.A.
      • Collins P.J.
      • Kawaoka Y.
      • Skehel J.J.
      • Gamblin S.J.
      Receptor binding by a ferret-transmissible H5 avian influenza virus.
      ). In contrast, only several H-protein interactions with the cognate receptors may suffice to promote strong binding of morbillivirus particles to cells. The extremely strong binding affinity of the henipavirus G-proteins to certain ephrin receptors (Kd = 0.1 nm for NiV G with ephrin-B2 and Kd = 0.24 nm for CedV with ephrin-B1) suggests that perhaps only a single receptor interaction may be sufficient to stabilize henipavirus particle binding to cells. The range of binding affinities exhibited by paramyxoviruses may reflect, in part, the relative abundance of their respective receptors.

      Receptor-binding modes

      Crystal structures of receptor-bound attachment proteins, further validated by biochemical and functional studies, have revealed much about the receptor-binding modes of paramyxoviruses (
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ,
      • Yuan P.
      • Thompson T.B.
      • Wurzburg B.A.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structural studies of the parainfluenza virus 5 hemagglutinin-neuraminidase tetramer in complex with its receptor, sialyllactose.
      ,
      • Bowden T.A.
      • Aricescu A.R.
      • Gilbert R.J.
      • Grimes J.M.
      • Jones E.Y.
      • Stuart D.I.
      Structural basis of Nipah and Hendra virus attachment to their cell-surface receptor ephrin-B2.
      ,
      • Xu K.
      • Rajashankar K.R.
      • Chan Y.P.
      • Himanen J.P.
      • Broder C.C.
      • Nikolov D.B.
      Host cell recognition by the henipaviruses: crystal structures of the Nipah G attachment glycoprotein and its complex with ephrin-B3.
      ,
      • Santiago C.
      • Celma M.L.
      • Stehle T.
      • Casasnovas J.M.
      Structure of the measles virus hemagglutinin bound to the CD46 receptor.
      ,
      • Hashiguchi T.
      • Ose T.
      • Kubota M.
      • Maita N.
      • Kamishikiryo J.
      • Maenaka K.
      • Yanagi Y.
      Structure of the measles virus hemagglutinin bound to its cellular receptor SLAM.
      ,
      • Zhang X.
      • Lu G.
      • Qi J.
      • Li Y.
      • He Y.
      • Xu X.
      • Shi J.
      • Zhang C.W.
      • Yan J.
      • Gao G.F.
      Structure of measles virus hemagglutinin bound to its epithelial receptor nectin-4.
      ). Fig. 4A presents a top view of the attachment protein heads of HPIV3, HeV, and MeV and illustrates their modes of receptor binding. The overall structure of the three attachment proteins is conserved, but important differences in receptor-binding geometries exist.
      Figure thumbnail gr4
      Figure 4Structure and receptor-binding modes of the attachment proteins of three paramyxoviruses. A, top view of one head of each protein (HN, G, or H). Left, the HPIV3 HN monomeric head is shown looking down the barrel of the six-bladed β-propeller, indicated by a hexagon, analogously to the hexagonal schematic heads used in C for MeV H. In this HN-head atomic structure (PDB codes 4MZA and 4XJR) (
      • Xu R.
      • Palmer S.G.
      • Porotto M.
      • Palermo L.M.
      • Niewiesk S.
      • Wilson I.A.
      • Moscona A.
      Interaction between the hemagglutinin-neuraminidase and fusion glycoproteins of human parainfluenza virus type III regulates viral growth in vivo.
      ), residues interacting with sialic acid are indicated in orange. Center, HeV G (PDB code 2X9M) (
      • Bowden T.A.
      • Crispin M.
      • Harvey D.J.
      • Jones E.Y.
      • Stuart D.I.
      Dimeric architecture of the Hendra virus attachment glycoprotein: evidence for a conserved mode of assembly.
      ) residues contacting ephrin-B2 are indicated in purple. Right, H-protein contact sites for SLAM (blue residues) and nectin-4 (yellow residues) are indicated on the MeV H-head structure (PDB code 2ZB5) (
      • Hashiguchi T.
      • Kajikawa M.
      • Maita N.
      • Takeda M.
      • Kuroki K.
      • Sasaki K.
      • Kohda D.
      • Yanagi Y.
      • Maenaka K.
      Crystal structure of measles virus hemagglutinin provides insight into effective vaccines.
      ). Green residues bind both SLAM and nectin-4. B, side views of the tetrameric stalks represented by four cylinders. The stalks are comprised of a dimer of dimers (green and blue cylinders, with length proportional to number of aa). Arrows represent flexible linkers that connect to the globular head domains. The white stars indicate a kink in the parallel four-helix bundle organization of the stalks. Blue wavy line, cytoplasmic tail, shown for only one subunit. Gray box, plasma membrane. Center, thick red lines connecting yellow dots (Cys residues) indicate disulfide bonds that stabilize the HeV G-dimers or the tetramer. Right, thick red lines indicate disulfide bonds that stabilize the MeV H-dimer.
      HPIV3 HN interactions with sialic acid occur toward the central funnel of the β-barrel (Fig. 4A, left) (
      • Yuan P.
      • Thompson T.B.
      • Wurzburg B.A.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structural studies of the parainfluenza virus 5 hemagglutinin-neuraminidase tetramer in complex with its receptor, sialyllactose.
      ), within an active site that conserves essential catalytic residues found in other neuraminidases (
      • Yen H.L.
      • Hoffmann E.
      • Taylor G.
      • Scholtissek C.
      • Monto A.S.
      • Webster R.G.
      • Govorkova E.A.
      Importance of neuraminidase active-site residues to the neuraminidase inhibitor resistance of influenza viruses.
      ). Neuraminidase function is reduced at neutral pH, enabling attachment and entry, but is activated at lower pH, enabling sialic acid removal during particle budding.
      The HeV G-protein residues interacting with ephrin-B2/-B3 are also located in the central pocket of the β-barrel (Fig. 4A, center) (
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ,
      • Bowden T.A.
      • Aricescu A.R.
      • Gilbert R.J.
      • Grimes J.M.
      • Jones E.Y.
      • Stuart D.I.
      Structural basis of Nipah and Hendra virus attachment to their cell-surface receptor ephrin-B2.
      ,
      • Xu K.
      • Rajashankar K.R.
      • Chan Y.P.
      • Himanen J.P.
      • Broder C.C.
      • Nikolov D.B.
      Host cell recognition by the henipaviruses: crystal structures of the Nipah G attachment glycoprotein and its complex with ephrin-B3.
      ). Interestingly, this binding mode is reminiscent of that of HN with sialic acid, and could have evolved from it, and completely different from the protein receptor using morbilliviruses. However, a closer analysis of the binding interface reveals a flexible, exposed loop of the ephrin receptors (G-H loop, Fig. 5) (
      • Bonaparte M.I.
      • Dimitrov A.S.
      • Bossart K.N.
      • Crameri G.
      • Mungall B.A.
      • Bishop K.A.
      • Choudhry V.
      • Dimitrov D.S.
      • Wang L.F.
      • Eaton B.T.
      • Broder C.C.
      Ephrin-B2 ligand is a functional receptor for Hendra virus and Nipah virus.
      ,
      • Bowden T.A.
      • Aricescu A.R.
      • Gilbert R.J.
      • Grimes J.M.
      • Jones E.Y.
      • Stuart D.I.
      Structural basis of Nipah and Hendra virus attachment to their cell-surface receptor ephrin-B2.
      ,
      • Xu K.
      • Rajashankar K.R.
      • Chan Y.P.
      • Himanen J.P.
      • Broder C.C.
      • Nikolov D.B.
      Host cell recognition by the henipaviruses: crystal structures of the Nipah G attachment glycoprotein and its complex with ephrin-B3.
      ) that fits snugly into a cavity of the henipavirus G–binding pocket in an induced-fit lock-and-key mechanism (Fig. 5A) (
      • Xu K.
      • Broder C.C.
      • Nikolov D.B.
      Ephrin-B2 and ephrin-B3 as functional henipavirus receptors.
      ,
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ). This intimate interaction between the residues of the G-H loop and the G-protein receptor-binding cavity may account for the strong receptor-binding affinities.
      Figure thumbnail gr5
      Figure 5Receptor interactions of the henipavirus G-proteins. A, G-H loop insertion of ephrin-B1/-B2 into the CedV G receptor-binding site. Pink, ephrin-B1 + CedV G; blue, ephrin-B2 + CedV G; green, ephrin-B2 + HeV G; yellow, ephrin-B2 + GhV G. B and C, top (top panels) and side views (bottom panels) of the ephrin-B2 G-H loop (four residues shaded blue) interacting with the receptor-binding pockets of CedV G and HeV G, respectively. Top panels, a view of the receptor-binding pockets indicating the interactions with four critical residues at the tip of the ephrin-B2 G-H loop (blue residues). G-protein residues critical for receptor interaction and/or the formation of the binding cavity are indicated. The bottom panels depict a side, cut-away view of the receptor-binding pocket at a 90° rotation of the view depicted in the top panels. P1–P3, hydrophobic pockets 1–3 (B, bottom). P1–P4, hydrophobic pockets 1–4 (C, bottom). The critical P4 pocket-forming residue HeV Trp-504 (W504HeV) (C, top) was substituted by Tyr-525 in CedV G (B, top). This substitution allows CedV Tyr-525 (Y525CedV side-chain stabilization by π stacking with CedV residue Phe-459 (F459CedV) and swings out of the pocket region (B, top). In the vertical direction, another pocket P4 boundary-forming residue, HeV Leu-305 (L305HeV (C, top), is replaced by CedV Asp-328 (not visible), which points away from the pocket due to the lack of hydrophobic interaction. These amino acid changes result in the loss of pocket P4 and the enlargement of pocket P3 in CedV G (compare B and C, bottom panels). Adapted from Ref.
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      .This research was originally published in Proceedings of the National Academy of Sciences of the United States of America. Laing, E. D., Navaratnarajah, C. K., Cheliout Da Silva, S., Petzing, S. R., Xu, Y., Sterling, S. L., Marsh, G. A., Wang, L.-F., Amaya, M., Nikolov, D. B., Cattaneo, R., Broder, C. C., and Xu, K. Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus. Proc. Natl. Acad. Sci. U.S.A. 2019; 116:20707–20715. © United States National Academy of Sciences.
      Ephrins differ in their ability to serve as henipavirus receptors, primarily due to differences in their G-H loop sequences (
      • Negrete O.A.
      • Wolf M.C.
      • Aguilar H.C.
      • Enterlein S.
      • Wang W.
      • Mühlberger E.
      • Su S.V.
      • Bertolotti-Ciarlet A.
      • Flick R.
      • Lee B.
      Two key residues in ephrinB3 are critical for its use as an alternative receptor for Nipah virus.
      ). CedV, the most recent henipavirus isolate, binds ephrin-B1, in addition to ephrin-B2, but not ephrin-B3 (
      • Marsh G.A.
      • de Jong C.
      • Barr J.A.
      • Tachedjian M.
      • Smith C.
      • Middleton D.
      • Yu M.
      • Todd S.
      • Foord A.J.
      • Haring V.
      • Payne J.
      • Robinson R.
      • Broz I.
      • Crameri G.
      • Field H.E.
      • Wang L.F.
      Cedar virus: a novel Henipavirus isolated from Australian bats.
      ,
      • Laing E.D.
      • Amaya M.
      • Navaratnarajah C.K.
      • Feng Y.R.
      • Cattaneo R.
      • Wang L.F.
      • Broder C.C.
      Rescue and characterization of recombinant cedar virus, a non-pathogenic Henipavirus species.
      ). The crystal structure of the CedV G-head domain in complex with ephrin-B2 revealed that the binding cavity is comprised of three pockets (Fig. 5B, P1–P3), whereas the HeV G–binding cavity has four pockets (Fig. 5C, P1–P4) (
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ). This structural alteration allows accommodation of residues with larger side chains in the pocket P3, accounting for the altered receptor specificity of CedV (
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ,
      • Pryce R.
      • Azarm K.
      • Rissanen I.
      • Harlos K.
      • Bowden T.A.
      • Lee B.
      A key region of molecular specificity orchestrates unique ephrin-B1 utilization by Cedar virus.
      ).
      Whereas the mode of receptor binding to henipavirus G-proteins retains vestiges of sialic acid binding to HN, receptor binding to MeV H must have evolved independently from the original interaction localized to the central cavity: all three MeV receptors interact with one side of the β-barrel (Fig. 4A, right) (
      • Santiago C.
      • Celma M.L.
      • Stehle T.
      • Casasnovas J.M.
      Structure of the measles virus hemagglutinin bound to the CD46 receptor.
      ,
      • Hashiguchi T.
      • Ose T.
      • Kubota M.
      • Maita N.
      • Kamishikiryo J.
      • Maenaka K.
      • Yanagi Y.
      Structure of the measles virus hemagglutinin bound to its cellular receptor SLAM.
      ,
      • Zhang X.
      • Lu G.
      • Qi J.
      • Li Y.
      • He Y.
      • Xu X.
      • Shi J.
      • Zhang C.W.
      • Yan J.
      • Gao G.F.
      Structure of measles virus hemagglutinin bound to its epithelial receptor nectin-4.
      ). All three binding sites overlap, but nectin-4 and CD46 functionally interact with a groove between two β-sheets (β4-β5) (
      • Zhang X.
      • Lu G.
      • Qi J.
      • Li Y.
      • He Y.
      • Xu X.
      • Shi J.
      • Zhang C.W.
      • Yan J.
      • Gao G.F.
      Structure of measles virus hemagglutinin bound to its epithelial receptor nectin-4.
      ), whereas SLAM does not penetrate the groove, but rather lies over it and interacts with residues in blades β5 and β6 (
      • Mateo M.
      • Navaratnarajah C.K.
      • Syed S.
      • Cattaneo R.
      The measles virus hemagglutinin beta-propeller head β4-β5 hydrophobic groove governs functional interactions with nectin-4 and CD46 but not those with the signaling lymphocytic activation molecule.
      ).
      Because all three receptors bind distinct but overlapping sites on the MeV H-protein, this precise location may be critical for function. This hypothesis was tested by positioning 6-histidine tags in exposed loops of all blades of the MeV H-protein head and testing their function by pulling on the tags utilizing specific membrane-bound antibodies. Indeed, only pulling on those tags located near the binding sites of the natural receptors triggered membrane fusion (
      • Navaratnarajah C.K.
      • Oezguen N.
      • Rupp L.
      • Kay L.
      • Leonard V.H.
      • Braun W.
      • Cattaneo R.
      The heads of the measles virus attachment protein move to transmit the fusion-triggering signal.
      ).
      Altogether, these studies indicate that different types of receptors (proteins or carbohydrates) can trigger the paramyxovirus fusion apparatus by binding its attachment protein with vastly different affinities. Receptors contact the heads of the HN and G attachment proteins from the top, through the central funnel of the β-barrel, but contact the head of the H-protein through a lateral groove. Nevertheless, all input signals are converted to the same output, F-trimer triggering leading to membrane fusion.

      Membrane fusion mechanisms

      The cell entry processes of all paramyxoviruses evolved from the same core mechanism. These processes rely on modular, exchangeable components: the heads of the G and H attachment proteins can be exchanged, provided that the contact of their stalk with the cognate F-trimer is maintained (
      • Mirza A.M.
      • Aguilar H.C.
      • Zhu Q.
      • Mahon P.J.
      • Rota P.A.
      • Lee B.
      • Iorio R.M.
      Triggering of the Newcastle disease virus fusion protein by a chimeric attachment protein that binds to Nipah virus receptors.
      ,
      • Talekar A.
      • Moscona A.
      • Porotto M.
      Measles virus fusion machinery activated by sialic acid binding globular domain.
      ). It is also possible to trigger membrane fusion by appending a foreign binding domain to the attachment proteins and pulling on it with its cognate receptor, as shown for MeV H (
      • Schneider U.
      • Bullough F.
      • Vongpunsawad S.
      • Russell S.J.
      • Cattaneo R.
      Recombinant measles viruses efficiently entering cells through targeted receptors.
      ,
      • Hammond A.L.
      • Plemper R.K.
      • Zhang J.
      • Schneider U.
      • Russell S.J.
      • Cattaneo R.
      Single-chain antibody displayed on a recombinant measles virus confers entry through the tumor-associated carcinoembryonic antigen.
      ) and NiV G (
      • Bender R.R.
      • Muth A.
      • Schneider I.C.
      • Friedel T.
      • Hartmann J.
      • Plückthun A.
      • Maisner A.
      • Buchholz C.J.
      Receptor-targeted Nipah virus glycoproteins improve cell-type selective gene delivery and reveal a preference for membrane-proximal cell attachment.
      ). However, triggering occurs only when the contact of the attachment protein stalk with F-trimers remains intact. Thus, the attachment protein stalk has a central role in membrane fusion triggering.

      Structure and function of the attachment protein stalks

      The stalks of the HN-, H-, and G-proteins have many similarities and some interesting differences. Crystal structure analyses of HN-stalks indicate that they are organized into parallel four-helix bundles (
      • Yuan P.
      • Swanson K.A.
      • Leser G.P.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the Newcastle disease virus hemagglutinin-neuraminidase (HN) ectodomain reveals a four-helix bundle stalk.
      ,
      • Bose S.
      • Welch B.D.
      • Kors C.A.
      • Yuan P.
      • Jardetzky T.S.
      • Lamb R.A.
      Structure and mutagenesis of the parainfluenza virus 5 hemagglutinin-neuraminidase stalk domain reveals a four-helix bundle and the role of the stalk in fusion promotion.
      ,
      • Welch B.D.
      • Yuan P.
      • Bose S.
      • Kors C.A.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the parainfluenza virus 5 (PIV5) hemagglutinin-neuraminidase (HN) ectodomain.
      ), and biochemical evidence suggests that the henipavirus G- and morbillivirus H-protein stalks organize into similar structures (
      • Bishop K.A.
      • Hickey A.C.
      • Khetawat D.
      • Patch J.R.
      • Bossart K.N.
      • Zhu Z.
      • Wang L.F.
      • Dimitrov D.S.
      • Broder C.C.
      Residues in the stalk domain of the Hendra virus G glycoprotein modulate conformational changes associated with receptor binding.
      ,
      • Liu Q.
      • Bradel-Tretheway B.
      • Monreal A.I.
      • Saludes J.P.
      • Lu X.
      • Nicola A.V.
      • Aguilar H.C.
      Nipah virus attachment glycoprotein stalk C-terminal region links receptor binding to fusion triggering.
      ,
      • Navaratnarajah C.K.
      • Negi S.
      • Braun W.
      • Cattaneo R.
      Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
      ,
      • Navaratnarajah C.K.
      • Kumar S.
      • Generous A.
      • Apte-Sengupta S.
      • Mateo M.
      • Cattaneo R.
      The measles virus hemagglutinin stalk: structures and functions of the central fusion activation and membrane-proximal segments.
      ,
      • Ader N.
      • Brindley M.A.
      • Avila M.
      • Origgi F.C.
      • Langedijk J.P.M.
      • Örvell C.
      • Vandevelde M.
      • Zurbriggen A.
      • Plemper R.K.
      • Plattet P.
      Structural rearrangements of the central region of the morbillivirus attachment protein stalk domain trigger F protein refolding for membrane fusion.
      ,
      • Ader N.
      • Brindley M.
      • Avila M.
      • Örvell C.
      • Horvat B.
      • Hiltensperger G.
      • Schneider-Schaulies J.
      • Vandevelde M.
      • Zurbriggen A.
      • Plemper R.K.
      • Plattet P.
      Mechanism for active membrane fusion triggering by morbillivirus attachment protein.
      ,
      • Ader-Ebert N.
      • Khosravi M.
      • Herren M.
      • Avila M.
      • Alves L.
      • Bringolf F.
      • Orvell C.
      • Langedijk J.P.
      • Zurbriggen A.
      • Plemper R.K.
      • Plattet P.
      Sequential conformational changes in the morbillivirus attachment protein initiate the membrane fusion process.
      ,
      • Apte-Sengupta S.
      • Navaratnarajah C.K.
      • Cattaneo R.
      Hydrophobic and charged residues in the central segment of the measles virus hemagglutinin stalk mediate transmission of the fusion-triggering signal.
      ). The PIV5 four-helix bundle stalk structure shows adjacent segments of 11-mer and 7-mer hydrophobic repeat regions, with a kink at their junction (Fig. 2C, left, star) (
      • Bose S.
      • Welch B.D.
      • Kors C.A.
      • Yuan P.
      • Jardetzky T.S.
      • Lamb R.A.
      Structure and mutagenesis of the parainfluenza virus 5 hemagglutinin-neuraminidase stalk domain reveals a four-helix bundle and the role of the stalk in fusion promotion.
      ). This structure has been used to model the MeV H-stalk, and mutagenesis experiments in both systems have reached similar conclusions, localizing the F activation site to a few amino acids surrounding the kink (
      • Navaratnarajah C.K.
      • Negi S.
      • Braun W.
      • Cattaneo R.
      Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
      ,
      • Bose S.
      • Welch B.D.
      • Kors C.A.
      • Yuan P.
      • Jardetzky T.S.
      • Lamb R.A.
      Structure and mutagenesis of the parainfluenza virus 5 hemagglutinin-neuraminidase stalk domain reveals a four-helix bundle and the role of the stalk in fusion promotion.
      ). Functional evidence for the requirement of a conformational change of the HN- and H-stalks for F-trimer activation was obtained by cross-linking specific positions by engineered disulfide bonds, which inhibits function; removal of certain bonds by reduction restores function (
      • Navaratnarajah C.K.
      • Negi S.
      • Braun W.
      • Cattaneo R.
      Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
      ,
      • Bose S.
      • Welch B.D.
      • Kors C.A.
      • Yuan P.
      • Jardetzky T.S.
      • Lamb R.A.
      Structure and mutagenesis of the parainfluenza virus 5 hemagglutinin-neuraminidase stalk domain reveals a four-helix bundle and the role of the stalk in fusion promotion.
      ). Similarly, there is evidence that the henipavirus G-stalks modulate conformational changes (
      • Bishop K.A.
      • Hickey A.C.
      • Khetawat D.
      • Patch J.R.
      • Bossart K.N.
      • Zhu Z.
      • Wang L.F.
      • Dimitrov D.S.
      • Broder C.C.
      Residues in the stalk domain of the Hendra virus G glycoprotein modulate conformational changes associated with receptor binding.
      ).
      On the membrane-distal side of the stalk, the interactions among the four subunits of the HN-, G-, and H-proteins have interesting differences (Fig. 4B). Certain HN-protein dimers are stabilized by one disulfide bond at the top of the stalk (
      • Yuan P.
      • Thompson T.B.
      • Wurzburg B.A.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structural studies of the parainfluenza virus 5 hemagglutinin-neuraminidase tetramer in complex with its receptor, sialyllactose.
      ,
      • Welch B.D.
      • Yuan P.
      • Bose S.
      • Kors C.A.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the parainfluenza virus 5 (PIV5) hemagglutinin-neuraminidase (HN) ectodomain.
      ,
      • Crennell S.
      • Takimoto T.
      • Portner A.
      • Taylor G.
      Crystal structure of the multifunctional paramyxovirus hemagglutinin-neuraminidase.
      ,
      • Takimoto T.
      • Taylor G.L.
      • Crennell S.J.
      • Scroggs R.A.
      • Portner A.
      Crystallization of Newcastle disease virus hemagglutinin-neuraminidase glycoprotein.
      ), whereas others, such as HPIV3 HN, lack this stabilization (Fig. 4B, left) (
      • Collins P.L.
      • Mottet G.
      Homooligomerization of the hemagglutinin-neuraminidase glycoprotein of human parainfluenza virus type 3 occurs before the acquisition of correct intramolecular disulfide bonds and mature immunoreactivity.
      ,
      • Lawrence M.C.
      • Borg N.A.
      • Streltsov V.A.
      • Pilling P.A.
      • Epa V.C.
      • Varghese J.N.
      • McKimm-Breschkin J.L.
      • Colman P.M.
      Structure of the haemagglutinin-neuraminidase from human parainfluenza virus type III.
      ). On the other hand, both henipavirus G-proteins and the morbillivirus H-proteins contain two disulfide bonds at the top of the stalk for dimer stabilization (Fig. 4B, center and right). The G-protein has a third disulfide bond in the stalk that serves to stabilize the tetramer (Fig. 4B, center, C146) (
      • Maar D.
      • Harmon B.
      • Chu D.
      • Schulz B.
      • Aguilar H.C.
      • Lee B.
      • Negrete O.A.
      Cysteines in the stalk of the Nipah virus G glycoprotein are located in a distinct subdomain critical for fusion activation.
      ).
      The mechanisms of signal transmission through the attachment protein also have interesting variations, as discussed below.

      Membrane fusion by HN-proteins

      Structural analyses of the NDV HN-stalk revealed extensive interactions with HN-heads: HN-heads are positioned along the stalk in a “four heads down” state likely to interfere with F-trimer interactions (
      • Yuan P.
      • Swanson K.A.
      • Leser G.P.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the Newcastle disease virus hemagglutinin-neuraminidase (HN) ectodomain reveals a four-helix bundle stalk.
      ). Based on this structure, Jardetzky and Lamb (
      • Jardetzky T.S.
      • Lamb R.A.
      Activation of paramyxovirus membrane fusion and virus entry.
      ) proposed a stalk-exposure/induced-fit model of fusion triggering by HN-proteins. In this model, HN-proteins would be initially folded in the “four heads down” conformation, which would prevent premature triggering of the F-trimer during intracellular transport.
      Structural analysis of another HN tetramer, namely the PIV5 ectodomain complexed with a glycan receptor, revealed another unanticipated conformation, named “two heads up/two heads down” (
      • Welch B.D.
      • Yuan P.
      • Bose S.
      • Kors C.A.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the parainfluenza virus 5 (PIV5) hemagglutinin-neuraminidase (HN) ectodomain.
      ). Evidence was presented suggesting that this conformation, which allows interaction of the F-protein from one side of the stalk, represents an intermediate before HN reaches a final “four heads up” conformation after receptor binding to both dimers.
      This would expose the F-interacting segment at the top of the HN-stalk, allowing HN to contact F and trigger fusion via a putative induced fit mechanism (
      • Bose S.
      • Welch B.D.
      • Kors C.A.
      • Yuan P.
      • Jardetzky T.S.
      • Lamb R.A.
      Structure and mutagenesis of the parainfluenza virus 5 hemagglutinin-neuraminidase stalk domain reveals a four-helix bundle and the role of the stalk in fusion promotion.
      ). In this model, the stalk is a “provocateur” of fusion; consistent with this model, headless HN-stalks can trigger fusion in the absence of receptors, albeit at low efficiency (
      • Bose S.
      • Zokarkar A.
      • Welch B.D.
      • Leser G.P.
      • Jardetzky T.S.
      • Lamb R.A.
      Fusion activation by a headless parainfluenza virus 5 hemagglutinin-neuraminidase stalk suggests a modular mechanism for triggering.
      ). Notably, the rearrangement of the HN-heads is facilitated by the flexibility of the head-stalk linker segment (Fig. 4B) (
      • Bose S.
      • Heath C.M.
      • Shah P.A.
      • Alayyoubi M.
      • Jardetzky T.S.
      • Lamb R.A.
      Mutations in the parainfluenza virus 5 fusion protein reveal domains important for fusion triggering and metastability.
      ). Porotto and co-workers (
      • Gui L.
      • Jurgens E.M.
      • Ebner J.L.
      • Porotto M.
      • Moscona A.
      • Lee K.K.
      Electron tomography imaging of surface glycoproteins on human parainfluenza virus 3: association of receptor binding and fusion proteins before receptor engagement.
      ) suggest that, in addition to inducing rearrangement of the HN-heads about the stalks, receptors also promote clustering of HN/F complexes and that this additional step is required for HN to transmit the F-triggering signal.

      Membrane fusion by G-proteins

      Analyses of the NiV fusion process indicate that interactions of the F-trimer with both the G-head and the G-stalk occur prior to receptor binding (
      • Bishop K.A.
      • Hickey A.C.
      • Khetawat D.
      • Patch J.R.
      • Bossart K.N.
      • Zhu Z.
      • Wang L.F.
      • Dimitrov D.S.
      • Broder C.C.
      Residues in the stalk domain of the Hendra virus G glycoprotein modulate conformational changes associated with receptor binding.
      ,
      • Bossart K.N.
      • Fusco D.L.
      • Broder C.C.
      Paramyxovirus entry.
      ,
      • Aguilar H.C.
      • Henderson B.A.
      • Zamora J.L.
      • Johnston G.P.
      Paramyxovirus glycoproteins and the membrane fusion process.
      ). Therefore, Aguilar and colleagues (
      • Liu Q.
      • Stone J.A.
      • Bradel-Tretheway B.
      • Dabundo J.
      • Benavides Montano J.A.
      • Santos-Montanez J.
      • Biering S.B.
      • Nicola A.V.
      • Iorio R.M.
      • Lu X.
      • Aguilar H.C.
      Unraveling a three-step spatiotemporal mechanism of triggering of receptor-induced Nipah virus fusion and cell entry.
      ) have proposed a “bidentate” model for G-protein fusion triggering. According to this model, the initial conformation of the globular heads atop the stalks prevents F-activation by the G-stalk (
      • Maar D.
      • Harmon B.
      • Chu D.
      • Schulz B.
      • Aguilar H.C.
      • Lee B.
      • Negrete O.A.
      Cysteines in the stalk of the Nipah virus G glycoprotein are located in a distinct subdomain critical for fusion activation.
      ). Similar to HN, expression of just the G-protein stalk is sufficient to trigger fusion (
      • Liu Q.
      • Bradel-Tretheway B.
      • Monreal A.I.
      • Saludes J.P.
      • Lu X.
      • Nicola A.V.
      • Aguilar H.C.
      Nipah virus attachment glycoprotein stalk C-terminal region links receptor binding to fusion triggering.
      ).
      In the bidentate model, receptor binding causes the G-protein head domain to undergo two sequential conformational changes, leading to the exposure of the membrane-distal end of the G-protein stalk that acts to trigger the F-protein conformational change (
      • Liu Q.
      • Stone J.A.
      • Bradel-Tretheway B.
      • Dabundo J.
      • Benavides Montano J.A.
      • Santos-Montanez J.
      • Biering S.B.
      • Nicola A.V.
      • Iorio R.M.
      • Lu X.
      • Aguilar H.C.
      Unraveling a three-step spatiotemporal mechanism of triggering of receptor-induced Nipah virus fusion and cell entry.
      ). In both the HN and the G activation models, the top of the attachment protein stalk is a functional and structural linker between the head domain and the rest of the stalk, serving to propagate the F-triggering signal by undergoing conformational changes.

      Membrane fusion by H-proteins

      Another variation on the triggering theme is the “safety catch” model (
      • Plattet P.
      • Alves L.
      • Herren M.
      • Aguilar H.C.
      Measles virus fusion protein: structure, function and inhibition.
      ). This model was proposed by Plattet et al. (
      • Plattet P.
      • Alves L.
      • Herren M.
      • Aguilar H.C.
      Measles virus fusion protein: structure, function and inhibition.
      ) to account for the fusion-triggering mechanism of morbilliviruses (Fig. 6). Biochemical analyses of H-stalk mutants characterized different functional modules: the tetrameric central segment (aa 85–119), a tetrameric spacer (aa 120–139), two dimeric linkers with hinges (aa 140–154), and four monomeric connectors to the heads (aa 155–188) (Fig. 6A) (
      • Navaratnarajah C.K.
      • Negi S.
      • Braun W.
      • Cattaneo R.
      Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
      ).
      Figure thumbnail gr6
      Figure 6Morbillivirus fusion triggering mechanism. One H-tetramer and one F-trimer are shown. Only one monomer of each H-head dimer is shown for clarity. The MeV H-stalk was modeled based on the NDV stalk structure (PDB code 3T1E) (
      • Yuan P.
      • Swanson K.A.
      • Leser G.P.
      • Paterson R.G.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the Newcastle disease virus hemagglutinin-neuraminidase (HN) ectodomain reveals a four-helix bundle stalk.
      ). A green cylinder is used to represent the head-proximal region of the stalk, which acts as a spacer. Blue lines, the flexible linkers, which, together with the connectors (purple lines), link the stalk to the head domains. +, location of the hydrophobic hinge; yellow circles, Cys residues. Blue star, kink in the stalk centered on a module (red helices) that is critical for F-triggering. The MeV F-trimer head domain crystal structure is presented with the three monomers indicated by different shades of gray (PDB code 5YXW) (
      • Hashiguchi T.
      • Fukuda Y.
      • Matsuoka R.
      • Kuroda D.
      • Kubota M.
      • Shirogane Y.
      • Watanabe S.
      • Tsumoto K.
      • Kohda D.
      • Plemper R.K.
      • Yanagi Y.
      Structures of the prefusion form of measles virus fusion protein in complex with inhibitors.
      ). Residues critical for receiving the triggering signal from H are shaded yellow and define a groove made by the interface of two adjacent F-monomers (
      • Apte-Sengupta S.
      • Negi S.
      • Leonard V.H.
      • Oezguen N.
      • Navaratnarajah C.K.
      • Braun W.
      • Cattaneo R.
      Base of the measles virus fusion trimer head receives the signal that triggers membrane fusion.
      ). A, the glycoprotein complex prior to receptor interaction. B, receptors bind and pull on the H-heads, leading to a conformational change centered on the hydrophobic hinge of the linker (circled black plus sign). This signal is transmitted down the stalk to induce a conformational change (blue star with black border) that triggers F-protein refolding. C, post-fusion, the F-trimer refolds, fusing the viral membrane with the plasma membrane. The post-fusion form of F is represented by the HPIV3 F post-fusion structure (PDB code 1ZTM) (
      • Yin H.S.
      • Paterson R.G.
      • Wen X.
      • Lamb R.A.
      • Jardetzky T.S.
      Structure of the uncleaved ectodomain of the paramyxovirus (hPIV3) fusion protein.
      ).
      The central segment, which includes the kink at the junction of the 11-mer and 7-mer hydrophobic repeat regions, triggers F-trimer refolding (
      • Navaratnarajah C.K.
      • Kumar S.
      • Generous A.
      • Apte-Sengupta S.
      • Mateo M.
      • Cattaneo R.
      The measles virus hemagglutinin stalk: structures and functions of the central fusion activation and membrane-proximal segments.
      ,
      • Ader N.
      • Brindley M.A.
      • Avila M.
      • Origgi F.C.
      • Langedijk J.P.M.
      • Örvell C.
      • Vandevelde M.
      • Zurbriggen A.
      • Plemper R.K.
      • Plattet P.
      Structural rearrangements of the central region of the morbillivirus attachment protein stalk domain trigger F protein refolding for membrane fusion.
      ,
      • Apte-Sengupta S.
      • Negi S.
      • Leonard V.H.
      • Oezguen N.
      • Navaratnarajah C.K.
      • Braun W.
      • Cattaneo R.
      Base of the measles virus fusion trimer head receives the signal that triggers membrane fusion.
      ). The hydrophobic hinge maintains the H-protein in an autorepressed state prior to receptor binding (
      • Herren M.
      • Shrestha N.
      • Wyss M.
      • Zurbriggen A.
      • Plattet P.
      Regulatory role of the morbillivirus attachment protein head-to-stalk linker module in membrane fusion triggering.
      ). The head connectors govern proper H-protein tetramerization (
      • Navaratnarajah C.K.
      • Rosemarie Q.
      • Cattaneo R.
      A structurally unresolved head segment of defined length favors proper measles virus hemagglutinin tetramerization and efficient membrane fusion triggering.
      ). According to the model, membrane-bound receptors pull on the heads (Fig. 6B) (
      • Navaratnarajah C.K.
      • Oezguen N.
      • Rupp L.
      • Kay L.
      • Leonard V.H.
      • Braun W.
      • Cattaneo R.
      The heads of the measles virus attachment protein move to transmit the fusion-triggering signal.
      ). This causes refolding of the hinge region, followed by transduction of the signal down to the central segment of the stalk, which conformational change triggers F-trimer refolding and membrane fusion (Fig. 6C).
      In this model, H initially folds into a conformation that precludes F-activation (
      • Ader-Ebert N.
      • Khosravi M.
      • Herren M.
      • Avila M.
      • Alves L.
      • Bringolf F.
      • Orvell C.
      • Langedijk J.P.
      • Zurbriggen A.
      • Plemper R.K.
      • Plattet P.
      Sequential conformational changes in the morbillivirus attachment protein initiate the membrane fusion process.
      ). This is analogous to the “four heads down” conformation observed for HN-proteins, although the initial H-conformation does not prohibit H-F interactions (
      • Plemper R.K.
      • Hammond A.L.
      • Cattaneo R.
      Measles virus envelope glycoproteins hetero-oligomerize in the endoplasmic reticulum.
      ), allowing formation of H-F complexes without F-triggering (
      • Brindley M.A.
      • Chaudhury S.
      • Plemper R.K.
      Measles virus glycoprotein complexes preassemble intracellularly and relax during transport to the cell surface in preparation for fusion.
      ). Analogously to HN- and G-proteins, expression of just the H-protein stalk is sufficient to trigger fusion (
      • Brindley M.A.
      • Suter R.
      • Schestak I.
      • Kiss G.
      • Wright E.R.
      • Plemper R.K.
      A stabilized headless measles virus attachment protein stalk efficiently triggers membrane fusion.
      ).

      Three variations on one cell entry mechanism

      In summary, the cell entry mechanisms of all paramyxoviruses share one core process: upon receptor binding, attachment protein tetramers trigger F-trimer refolding and membrane fusion. The provocateur, bidentate, and safety catch models have many similarities, including movement of the heads upon receptor binding and involvement of the stalk in signal transmission.
      The models also have interesting differences. The provocateur model accounts for structural evidence suggesting that the HN-protein heads physically hinder the interactions of their stalk with F-trimers; no such evidence was presented for the H-proteins. The safety catch model accounts for formation of H-tetramer and F-trimer complexes during transport to the cell surface; in contrast, HN-F complexes form only at the cell surface. Finally, the bidentate model combines elements of the other two models and proposes a sequence of conformational changes by which the G-heads elicit F-trimer refolding.
      The dissociation constants from the cognate receptors also differ, ranging from subnanomolar for G-protein, to 20–200 nm for H-protein, to micromolar for HN-proteins. Thus, viruses with HN-proteins must contact many receptors before their attachment is stabilized, whereas only a few receptor contacts are required to stabilize the attachment of viruses with G-proteins. The geometry of receptor binding also differs, with receptors pulling the G- and HN-heads from their central funnel but the H-heads from one side. Despite these differences, all input signals are converted to the same output, F-trimer refolding and membrane fusion.

      Conclusions and future directions

      The paramyxoviruses of deepest human disease concern include those that have evolved away from binding cells through sialic acid and have gained the ability to cross the epithelial barrier by binding specific proteins. Morbilliviruses have adapted to a complex ecological niche defined by two receptors. First, these viruses use SLAM to infect immune cells. They rapidly and massively reproduce therein and can reach all of the organs of the host within these cells. Morbilliviruses then target certain epithelia through a protein expressed on their basolateral side, nectin-4, which is expressed most abundantly in the tracheal epithelium. Additional virus amplification at a site facilitating aerosolization contributes to extremely efficient contagion.
      In another paradigm for the study of viral disease, henipaviruses, including NiV and HeV, use different members of the ephrin receptor protein family to spread within their hosts. Ephrins are ubiquitous membrane proteins, and their high conservation among mammals facilitates henipavirus zoonosis and allows for broad host tropism. Ephrin-B2, the receptor shared by all henipaviruses isolated to date, is expressed in the respiratory epithelium (
      • Bennett K.M.
      • Afanador M.D.
      • Lal C.V.
      • Xu H.
      • Persad E.
      • Legan S.K.
      • Chenaux G.
      • Dellinger M.
      • Savani R.C.
      • Dravis C.
      • Henkemeyer M.
      • Schwarz M.A.
      Ephrin-B2 reverse signaling increases α5β1 integrin-mediated fibronectin deposition and reduces distal lung compliance.
      ), vascular endothelium (
      • Gale N.W.
      • Baluk P.
      • Pan L.
      • Kwan M.
      • Holash J.
      • DeChiara T.M.
      • McDonald D.M.
      • Yancopoulos G.D.
      Ephrin-B2 selectively marks arterial vessels and neovascularization sites in the adult, with expression in both endothelial and smooth-muscle cells.
      ), and neurons (
      • Dines M.
      • Grinberg S.
      • Vassiliev M.
      • Ram A.
      • Tamir T.
      • Lamprecht R.
      The roles of Eph receptors in contextual fear conditioning memory formation.
      ). This broad receptor distribution accounts for the multisystemic vasculitis and encephalitis observed during both HeV and NiV infections (
      • Wong K.T.
      • Tan C.T.
      Clinical and pathological manifestations of human henipavirus infection.
      ).
      Fueled both by the identification of proven infectious agents and by computational analyses of sequences present in cells of different species, the Paramyxoviridae family more than doubled its members within a few years (
      • Zeltina A.
      • Bowden T.A.
      • Lee B.
      Emerging paramyxoviruses: receptor tropism and zoonotic potential.
      ,
      • Rima B.
      • Balkema-Buschmann A.
      • Dundon W.G.
      • Duprex P.
      • Easton A.
      • Fouchier R.
      • Kurath G.
      • Lamb R.
      • Lee B.
      • Rota P.
      • Wang L.
      • Consortium I.R.
      ICTV Virus Taxonomy Profile: Paramyxoviridae.
      ,
      • Marsh G.A.
      • de Jong C.
      • Barr J.A.
      • Tachedjian M.
      • Smith C.
      • Middleton D.
      • Yu M.
      • Todd S.
      • Foord A.J.
      • Haring V.
      • Payne J.
      • Robinson R.
      • Broz I.
      • Crameri G.
      • Field H.E.
      • Wang L.F.
      Cedar virus: a novel Henipavirus isolated from Australian bats.
      ,
      • Drexler J.F.
      • Corman V.M.
      • Müller M.A.
      • Maganga G.D.
      • Vallo P.
      • Binger T.
      • Gloza-Rausch F.
      • Cottontail V.M.
      • Rasche A.
      • Yordanov S.
      • Seebens A.
      • Knörnschild M.
      • Oppong S.
      • Adu Sarkodie Y.
      • Pongombo C.
      • et al.
      Bats host major mammalian paramyxoviruses.
      ). New members include viruses that encode HN-proteins that interact with sialic acid and others that encode G-proteins that may bind protein receptors. Two of the newly classified taxa (narmoviruses and pararubuloviruses) are predicted to have protein receptors due to the absence of the neuraminic acid binding motif (
      • Rima B.
      • Balkema-Buschmann A.
      • Dundon W.G.
      • Duprex P.
      • Easton A.
      • Fouchier R.
      • Kurath G.
      • Lamb R.
      • Lee B.
      • Rota P.
      • Wang L.
      • Consortium I.R.
      ICTV Virus Taxonomy Profile: Paramyxoviridae.
      ). Study of the receptor interactions and of the critical post-entry processes, such as innate immune control, will be crucial to understand the tissue and species tropism of these emerging paramyxoviruses.
      For example, CedV is a henipavirus that is apathogenic in animals known to be susceptible to Nipah and Hendra disease. This can be attributed mainly to its inability to control the innate immune response (
      • Marsh G.A.
      • de Jong C.
      • Barr J.A.
      • Tachedjian M.
      • Smith C.
      • Middleton D.
      • Yu M.
      • Todd S.
      • Foord A.J.
      • Haring V.
      • Payne J.
      • Robinson R.
      • Broz I.
      • Crameri G.
      • Field H.E.
      • Wang L.F.
      Cedar virus: a novel Henipavirus isolated from Australian bats.
      ,
      • Laing E.D.
      • Amaya M.
      • Navaratnarajah C.K.
      • Feng Y.R.
      • Cattaneo R.
      • Wang L.F.
      • Broder C.C.
      Rescue and characterization of recombinant cedar virus, a non-pathogenic Henipavirus species.
      ,
      • Lieu K.G.
      • Marsh G.A.
      • Wang L.F.
      • Netter H.J.
      The non-pathogenic Henipavirus Cedar paramyxovirus phosphoprotein has a compromised ability to target STAT1 and STAT2.
      ,
      • Schountz T.
      • Campbell C.
      • Wagner K.
      • Rovnak J.
      • Martellaro C.
      • DeBuysscher B.L.
      • Feldmann H.
      • Prescott J.
      Differential innate immune responses elicited by Nipah virus and Cedar virus correlate with disparate in vivo pathogenesis in hamsters.
      ), but probably also to its different receptor interactions (
      • Laing E.D.
      • Navaratnarajah C.K.
      • Cheliout Da Silva S.
      • Petzing S.R.
      • Xu Y.
      • Sterling S.L.
      • Marsh G.A.
      • Wang L.-F.
      • Amaya M.
      • Nikolov D.B.
      • Cattaneo R.
      • Broder C.C.
      • Xu K.
      Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.
      ,
      • Pryce R.
      • Azarm K.
      • Rissanen I.
      • Harlos K.
      • Bowden T.A.
      • Lee B.
      A key region of molecular specificity orchestrates unique ephrin-B1 utilization by Cedar virus.
      ). Thus, CedV, the only nonbiosafety level 4 henipavirus currently available, affords us with a model system to study the interplay between receptor interactions and innate immunity control and determine how these processes contribute to tropism and virulence of henipaviruses.
      Finally, research on the newly identified paramyxoviruses is required to anticipate their zoonotic potential and to generate effective vaccines and antiviral compounds. Because fusion requires refolding of the F-protein, paramyxovirus antivirals can be developed to block this conformational shift. Indeed, in small-animal models, inhibitors developed to capture F in its prefusion state have been effective in preventing MeV entry and spread (
      • Mathieu C.
      • Huey D.
      • Jurgens E.
      • Welsch J.C.
      • DeVito I.
      • Talekar A.
      • Horvat B.
      • Niewiesk S.
      • Moscona A.
      • Porotto M.
      Prevention of measles virus infection by intranasal delivery of fusion inhibitor peptides.
      ,
      • Figueira T.N.
      • Palermo L.M.
      • Veiga A.S.
      • Huey D.
      • Alabi C.A.
      • Santos N.C.
      • Welsch J.C.
      • Mathieu C.
      • Horvat B.
      • Niewiesk S.
      • Moscona A.
      • Castanho M.A.R.B.
      • Porotto M.
      In vivo efficacy of measles virus fusion protein-derived peptides is modulated by the properties of self-assembly and membrane residence.
      ). In an alternative approach, small-molecule inhibitors of the viral polymerase can protect from lethal morbillivirus infection (
      • Krumm S.A.
      • Yan D.
      • Hovingh E.S.
      • Evers T.J.
      • Enkirch T.
      • Reddy G.P.
      • Sun A.
      • Saindane M.T.
      • Arrendale R.F.
      • Painter G.
      • Liotta D.C.
      • Natchus M.G.
      • von Messling V.
      • Plemper R.K.
      An orally available, small-molecule polymerase inhibitor shows efficacy against a lethal morbillivirus infection in a large animal model.
      ). Importantly, recent analyses of paramyxovirus tropism in natural hosts have identified both entry and post-entry steps that result in virus attenuation. This knowledge will facilitate engineering of effective vaccines for currently emerging zoonotic viruses, as well as for pathogens with receptor specificities predestining them to cause zoonotic disease.

      Acknowledgments

      We thank Bert Rima, Christopher C. Broder, Kai Xu, and Christian Pfaller for helpful suggestions on the manuscript.

      References

      1. Lamb R.A. Parks G. Paramyxoviridae. 1. Lippincott Williams & Wilkins, Philadelphia2013: 957-995
        • Rota P.A.
        • Moss W.J.
        • Takeda M.
        • de Swart R.L.
        • Thompson K.M.
        • Goodson J.L.
        Measles.
        Nat. Rev. Dis. Primers. 2016; 2 (27411684): 16049
        • Zeltina A.
        • Bowden T.A.
        • Lee B.
        Emerging paramyxoviruses: receptor tropism and zoonotic potential.
        PLoS Pathog. 2016; 12 (26915013): e1005390
        • Rima B.
        • Balkema-Buschmann A.
        • Dundon W.G.
        • Duprex P.
        • Easton A.
        • Fouchier R.
        • Kurath G.
        • Lamb R.
        • Lee B.
        • Rota P.
        • Wang L.
        • Consortium I.R.
        ICTV Virus Taxonomy Profile: Paramyxoviridae.
        J. Gen. Virol. 2019; 100 (31609197): 1593-1594
        • Morens D.M.
        • Holmes E.C.
        • Davis A.S.
        • Taubenberger J.K.
        Global rinderpest eradication: lessons learned and why humans should celebrate too.
        J. Infect. Dis. 2011; 204 (21653230): 502-505
        • Chang A.
        • Dutch R.E.
        Paramyxovirus fusion and entry: multiple paths to a common end.
        Viruses. 2012; 4 (22590688): 613-636
        • Pfaller C.K.
        • Cattaneo R.
        • Schnell M.J.
        Reverse genetics of Mononegavirales: how they work, new vaccines, and new cancer therapeutics.
        Virology. 2015; 479 (25702088): 331-344
        • Cattaneo R.
        • Rebmann G.
        • Schmid A.
        • Baczko K.
        • ter Meulen V.
        • Billeter M.A.
        Altered transcription of a defective measles virus genome derived from a diseased human brain.
        EMBO J. 1987; 6 (3582370): 681-688
        • Gutsche I.
        • Desfosses A.
        • Effantin G.
        • Ling W.L.
        • Haupt M.
        • Ruigrok R.W.
        • Sachse C.
        • Schoehn G.
        Structural virology. Near-atomic cryo-EM structure of the helical measles virus nucleocapsid.
        Science. 2015; 348 (25883315): 704-707
        • Desfosses A.
        • Milles S.
        • Jensen M.R.
        • Guseva S.
        • Colletier J.P.
        • Maurin D.
        • Schoehn G.
        • Gutsche I.
        • Ruigrok R.W.H.
        • Blackledge M.
        Assembly and cryo-EM structures of RNA-specific measles virus nucleocapsids provide mechanistic insight into paramyxoviral replication.
        Proc. Natl. Acad. Sci. U.S.A. 2019; 116 (30787192): 4256-4264
        • Du Pont V.
        • Jiang Y.
        • Plemper R.K.
        Bipartite interface of the measles virus phosphoprotein X domain with the large polymerase protein regulates viral polymerase dynamics.
        PLoS Pathog. 2019; 15 (31381607): e1007995
        • Pfaller C.K.
        • Mastorakos G.M.
        • Matchett W.E.
        • Ma X.
        • Samuel C.E.
        • Cattaneo R.
        Measles virus defective interfering RNAs are generated frequently and early in the absence of C protein and can be destabilized by adenosine deaminase acting on RNA-1-like hypermutations.
        J. Virol. 2015; 89 (25972541): 7735-7747
        • Pfaller C.K.
        • Bloyet L.M.
        • Donohue R.C.
        • Huff A.L.
        • Bartemes W.P.
        • Yousaf I.
        • Urzua E.
        • Claviere M.
        • Zachary M.
        • de Masson d'Autume V.
        • Carson S.
        • Schieferecke A.J.
        • Meyer A.J.
        • Gerlier D.
        • Cattaneo R.
        The C protein is recruited to measles virus ribonucleocapsids by the phosphoprotein.
        J. Virol. 2019; 94 (31748390): e01733-19
        • Donohue R.C.
        • Pfaller C.K.
        • Cattaneo R.
        Cyclical adaptation of measles virus quasispecies to epithelial and lymphocytic cells: to V, or not to V.
        PLoS Pathog. 2019; 15 (30768648): e1007605
        • Shaw M.L.
        • Cardenas W.B.
        • Zamarin D.
        • Palese P.
        • Basler C.F.
        Nuclear localization of the Nipah virus W protein allows for inhibition of both virus- and Toll-like receptor 3-triggered signaling pathways.
        J. Virol. 2005; 79 (15857993): 6078-6088
        • Devaux P.
        • Hudacek A.W.
        • Hodge G.
        • Reyes-Del Valle J.
        • McChesney M.B.
        • Cattaneo R.
        A recombinant measles virus unable to antagonize STAT1 function cannot control inflammation and is attenuated in rhesus monkeys.
        J. Virol. 2011; 85 (20980517): 348-356
        • Svitek N.
        • Gerhauser I.
        • Goncalves C.
        • Grabski E.
        • Döring M.
        • Kalinke U.
        • Anderson D.E.
        • Cattaneo R.
        • von Messling V.
        Morbillivirus control of the interferon response: relevance of STAT2 and mda5 but not STAT1 for canine distemper virus virulence in ferrets.
        J. Virol. 2014; 88 (24371065): 2941-2950
        • Satterfield B.A.
        • Cross R.W.
        • Fenton K.A.
        • Agans K.N.
        • Basler C.F.
        • Geisbert T.W.
        • Mire C.E.
        The immunomodulating V and W proteins of Nipah virus determine disease course.
        Nat. Commun. 2015; 6 (26105519): 7483
        • Rager M.
        • Vongpunsawad S.
        • Duprex W.P.
        • Cattaneo R.
        Polyploid measles virus with hexameric genome length.
        EMBO J. 2002; 21 (12006489): 2364-2372
        • Gui L.
        • Jurgens E.M.
        • Ebner J.L.
        • Porotto M.
        • Moscona A.
        • Lee K.K.
        Electron tomography imaging of surface glycoproteins on human parainfluenza virus 3: association of receptor binding and fusion proteins before receptor engagement.
        MBio. 2015; 6 (25691596): e02393-14
        • Ke Z.
        • Strauss J.D.
        • Hampton C.M.
        • Brindley M.A.
        • Dillard R.S.
        • Leon F.
        • Lamb K.M.
        • Plemper R.K.
        • Wright E.R.
        Promotion of virus assembly and organization by the measles virus matrix protein.
        Nat. Commun. 2018; 9 (29712906): 1736
        • Naniche D.
        • Varior-Krishnan G.
        • Cervoni F.
        • Wild T.F.
        • Rossi B.
        • Rabourdin-Combe C.
        • Gerlier D.
        Human membrane cofactor protein (CD46) acts as a cellular receptor for measles virus.
        J. Virol. 1993; 67 (8371352): 6025-6032
        • Dörig R.E.
        • Marcil A.
        • Chopra A.
        • Richardson C.D.
        The human CD46 molecule is a receptor for measles virus (Edmonston strain).
        Cell. 1993; 75 (8402913): 295-305
        • Hsu E.C.
        • Dörig R.E.
        • Sarangi F.
        • Marcil A.
        • Iorio C.
        • Richardson C.D.
        Artificial mutations and natural variations in the CD46 molecules from human and monkey cells define regions important for measles virus binding.
        J. Virol. 1997; 71 (9223509): 6144-6154
        • Navaratnarajah C.K.
        • Vongpunsawad S.
        • Oezguen N.
        • Stehle T.
        • Braun W.
        • Hashiguchi T.
        • Maenaka K.
        • Yanagi Y.
        • Cattaneo R.
        Dynamic interaction of the measles virus hemagglutinin with its receptor signaling lymphocytic activation molecule (SLAM, CD150).
        J. Biol. Chem. 2008; 283 (18292085): 11763-11771
        • Lecouturier V.
        • Fayolle J.
        • Caballero M.
        • Carabaña J.
        • Celma M.L.
        • Fernandez-Muñoz R.
        • Wild T.F.
        • Buckland R.
        Identification of two amino acids in the hemagglutinin glycoprotein of measles virus (MV) that govern hemadsorption, HeLa cell fusion, and CD46 downregulation: phenotypic markers that differentiate vaccine and wild-type MV strains.
        J. Virol. 1996; 70 (8676439): 4200-4204
        • Spiropoulou C.F.
        Nipah virus outbreaks: still small but extremely lethal.
        J. Infect. Dis. 2019; 219 (30365002): 1855-1857
        • Mina M.J.
        • Kula T.
        • Leng Y.
        • Li M.
        • de Vries R.D.
        • Knip M.
        • Siljander H.
        • Rewers M.
        • Choy D.F.
        • Wilson M.S.
        • Larman H.B.
        • Nelson A.N.
        • Griffin D.E.
        • de Swart R.L.
        • Elledge S.J.
        Measles virus infection diminishes preexisting antibodies that offer protection from other pathogens.
        Science. 2019; 366 (31672891): 599-606
        • Kumlin U.
        • Olofsson S.
        • Dimock K.
        • Arnberg N.
        Sialic acid tissue distribution and influenza virus tropism.
        Influenza Other Respir. Viruses. 2008; 2 (19453419): 147-154
        • Schomacker H.
        • Schaap-Nutt A.
        • Collins P.L.
        • Schmidt A.C.
        Pathogenesis of acute respiratory illness caused by human parainfluenza viruses.
        Curr. Opin. Virol. 2012; 2 (22709516): 294-299
        • Durbin A.P.
        • Elkins W.R.
        • Murphy B.R.
        African green monkeys provide a useful nonhuman primate model for the study of human parainfluenza virus types-1, -2, and -3 infection.
        Vaccine. 2000; 18 (10738104): 2462-2469
        • Ottolini M.G.
        • Porter D.D.
        • Blanco J.C.
        • Prince G.A.
        A cotton rat model of human parainfluenza 3 laryngotracheitis: virus growth, pathology, and therapy.
        J. Infect. Dis. 2002; 186 (12447755): 1713-1717
        • Porter D.D.
        • Prince G.A.
        • Hemming V.G.
        • Porter H.G.
        Pathogenesis of human parainfluenza virus 3 infection in two species of cotton rats: Sigmodon hispidus develops bronchiolitis, while Sigmodon fulviventer develops interstitial pneumonia.
        J. Virol. 1991; 65 (1845878): 103-111
        • Aguayo-Hiraldo P.I.
        • Arasaratnam R.J.
        • Tzannou I.
        • Kuvalekar M.
        • Lulla P.
        • Naik S.
        • Martinez C.A.
        • Piedra P.A.
        • Vera J.F.
        • Leen A.M.
        Characterizing the cellular immune response to parainfluenza virus 3.
        J. Infect. Dis. 2017; 216 (28472480): 153-161
        • Cortez K.J.
        • Erdman D.D.
        • Peret T.C.
        • Gill V.J.
        • Childs R.
        • Barrett A.J.
        • Bennett J.E.
        Outbreak of human parainfluenza virus 3 infections in a hematopoietic stem cell transplant population.
        J. Infect. Dis. 2001; 184 (11598830): 1093-1097
        • Maziarz R.T.
        • Sridharan P.
        • Slater S.
        • Meyers G.
        • Post M.
        • Erdman D.D.
        • Peret T.C.
        • Taplitz R.A.
        Control of an outbreak of human parainfluenza virus 3 in hematopoietic stem cell transplant recipients.
        Biol. Blood Marrow Transplant. 2010; 16 (19781656): 192-198
        • Halpin K.
        • Hyatt A.D.
        • Fogarty R.
        • Middleton D.
        • Bingham J.
        • Epstein J.H.
        • Rahman S.A.
        • Hughes T.
        • Smith C.
        • Field H.E.
        • Daszak P.
        • Henipavirus Ecology Research Group
        Pteropid bats are confirmed as the reservoir hosts of henipaviruses: a comprehensive experimental study of virus transmission.
        Am. J. Trop. Med. Hyg. 2011; 85 (22049055): 946-951
        • Broder C.C.
        • Weir D.L.
        • Reid P.A.
        Hendra virus and Nipah virus animal vaccines.
        Vaccine. 2016; 34 (27154393): 3525-3534
        • Bennett K.M.
        • Afanador M.D.
        • Lal C.V.
        • Xu H.
        • Persad E.
        • Legan S.K.
        • Chenaux G.
        • Dellinger M.
        • Savani R.C.
        • Dravis C.
        • Henkemeyer M.
        • Schwarz M.A.
        Ephrin-B2 reverse signaling increases α5β1 integrin-mediated fibronectin deposition and reduces distal lung compliance.
        Am. J. Respir. Cell Mol. Biol. 2013; 49 (23742148): 680-687
        • Hafner C.
        • Schmitz G.
        • Meyer S.
        • Bataille F.
        • Hau P.
        • Langmann T.
        • Dietmaier W.
        • Landthaler M.
        • Vogt T.
        Differential gene expression of Eph receptors and ephrins in benign human tissues and cancers.
        Clin. Chem. 2004; 50 (14726470): 490-499
        • Gale N.W.
        • Baluk P.
        • Pan L.
        • Kwan M.
        • Holash J.
        • DeChiara T.M.
        • McDonald D.M.
        • Yancopoulos G.D.
        Ephrin-B2 selectively marks arterial vessels and neovascularization sites in the adult, with expression in both endothelial and smooth-muscle cells.
        Dev. Biol. 2001; 230 (11161569): 151-160
        • Xu K.
        • Broder C.C.
        • Nikolov D.B.
        Ephrin-B2 and ephrin-B3 as functional henipavirus receptors.
        Semin. Cell Dev. Biol. 2012; 23 (22227101): 116-123
        • Clayton B.A.
        • Middleton D.
        • Bergfeld J.
        • Haining J.
        • Arkinstall R.
        • Wang L.
        • Marsh G.A.
        Transmission routes for Nipah virus from Malaysia and Bangladesh.
        Emerg. Infect. Dis. 2012; 18 (23171621): 1983-1993