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

Abstract

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.

Keywords: Nipah virus; animal virus; cell invasion; host-pathogen interaction; infection; measles; microbiology; morbillivirus; negative-strand RNA virus; paramyxovirus; pathogenesis; receptor; sialic acid; virology; virus entry.

Figure 1.

Phylogenetic 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 (150), 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 (100); NiV, PDB code 2VSM (103); HeV, PDB code 6CMG (152); Tupaia paramyxovirus (TPMV), Q9JFN4; NDV, PDB code 1E8T (125); HPIV4a, P21526; MuV, PDB code 5B2C (146); PIV5, PDB code 1Z4X (102); HPIV2, P25465; HPIV3, PDB code 4MZA, 4XJR (153); 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 (107); RPV, P41355.

Figure 2.

Antigenome, 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 (87, 117). 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) (133, 134). The MeV F trimer ectodomain structure (PDB code 5YXW) (154) 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.

Figure 3.

Routes 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.

Figure 4.

Structure 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 Fig. 2C for MeV H. In this HN-head atomic structure (PDB codes 4MZA and 4XJR) (153), residues interacting with sialic acid are indicated in orange. Center, HeV G (PDB code 2X9M) (155) 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) (156). 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.

Figure 5.

Receptor 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 (W504_HeV) (C, top) was substituted by Tyr-525 in CedV G (B, top). This substitution allows CedV Tyr-525 (Y525_CedV side-chain stabilization by π stacking with CedV residue Phe-459 (F459_CedV) and swings out of the pocket region (B, top). In the vertical direction, another pocket P4 boundary-forming residue, HeV Leu-305 (L305_HeV (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. .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.

Figure 6.

Morbillivirus 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) (117). 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) (154). 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 (132). 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) (151).