The Morphology and Assembly of Respiratory Syncytial Virus Revealed by Cryo-Electron Tomography.

Human respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract disease in young children. With repeat infections throughout life, it can also cause substantial disease in the elderly and in adults with compromised cardiac, pulmonary and immune systems. RSV is a pleomorphic enveloped RNA virus in the Pneumoviridae family. Recently, the three-dimensional (3D) structure of purified RSV particles has been elucidated, revealing three distinct morphological categories: spherical, asymmetric, and filamentous. However, the native 3D structure of RSV particles associated with or released from infected cells has yet to be investigated. In this study, we have established an optimized system for studying RSV structure by imaging RSV-infected cells on transmission electron microscopy (TEM) grids by cryo-electron tomography (cryo-ET). Our results demonstrate that RSV is filamentous across several virus strains and cell lines by cryo-ET, cryo-immuno EM, and thin section TEM techniques. The viral filament length varies from 0.5 to 12 µm and the average filament diameter is approximately 130 nm. Taking advantage of the whole cell tomography technique, we have resolved various stages of RSV assembly. Collectively, our results can facilitate the understanding of viral morphogenesis in RSV and other pleomorphic enveloped viruses.

Architecture of respiratory syncytial virus revealed by electron cryotomography.

Human respiratory syncytial virus is a human pathogen that causes severe infection of the respiratory tract. Current information about the structure of the virus and its interaction with host cells is limited. We carried out an electron cryotomographic characterization of cell culture-grown human respiratory syncytial virus to determine the architecture of the virion. The particles ranged from 100 nm to 1,000 nm in diameter and were spherical, filamentous, or a combination of the two. The filamentous morphology correlated with the presence of a cylindrical matrix protein layer linked to the inner leaflet of the viral envelope and with local ordering of the glycoprotein spikes. Recombinant viruses with only the fusion protein in their envelope showed that these glycoproteins were predominantly in the postfusion conformation, but some were also in the prefusion form. The ribonucleocapsids were left-handed, randomly oriented, and curved inside the virions. In filamentous particles, they were often adjacent to an intermediate layer of protein assigned to M2-1 (an envelope-associated protein known to mediate association of ribonucleocapsids with the matrix protein). Our results indicate important differences in structure between the Paramyxovirinae and Pneumovirinae subfamilies within the Paramyxoviridae, and provide fresh insights into host cell exit of a serious pathogen.

A critical phenylalanine residue in the respiratory syncytial virus fusion protein cytoplasmic tail mediates assembly of internal viral proteins into viral filaments and particles.

UNLABELLED: Respiratory syncytial virus (RSV) is a single-stranded RNA virus in the Paramyxoviridae family that assembles into filamentous structures at the apical surface of polarized epithelial cells. These filaments contain viral genomic RNA and structural proteins, including the fusion (F) protein, matrix (M) protein, nucleoprotein (N), and phosphoprotein (P), while excluding F-actin. It is known that the F protein cytoplasmic tail (FCT) is necessary for filament formation, but the mechanism by which the FCT mediates assembly into filaments is not clear. We hypothesized that the FCT is necessary for interactions with other viral proteins in order to form filaments. In order to test this idea, we expressed the F protein with cytoplasmic tail (CT) truncations or specific point mutations and determined the abilities of these variant F proteins to form filaments independent of viral infection when coexpressed with M, N, and P. Deletion of the terminal three FCT residues (amino acids Phe-Ser-Asn) or mutation of the Phe residue resulted in a loss of filament formation but did not affect F-protein expression or trafficking to the cell surface. Filament formation could be restored by addition of residues Phe-Ser-Asn to an FCT deletion mutant and was unaffected by mutations to Ser or Asn residues. Second, deletion of residues Phe-Ser-Asn or mutation of the Phe residue resulted in a loss of M, N, and P incorporation into virus-like particles. These data suggest that a C-terminal Phe residue in the FCT mediates assembly through incorporation of internal virion proteins into virus filaments at the cell surface. IMPORTANCE: Respiratory syncytial virus (RSV) is a leading cause of bronchiolitis and pneumonia in infants and the elderly worldwide. There is no licensed RSV vaccine and only limited therapeutics for use in infected patients. Many aspects of the RSV life cycle have been studied, but the mechanisms that drive RSV assembly at the cell surface are not well understood. This study provides evidence that a specific residue in the RSV fusion protein cytoplasmic tail coordinates assembly into viral filaments by mediating the incorporation of internal virion proteins. Understanding the mechanisms that drive RSV assembly could lead to targeted development of novel antiviral drugs. Moreover, since RSV exits infected cells in an ESCRT (endosomal sorting complexes required for transport)-independent manner, these studies may contribute new knowledge about a general strategy by which ESCRT-independent viruses mediate outward bud formation using viral protein-mediated mechanisms during assembly and budding.

Analysis of the thermal and pH stability of human respiratory syncytial virus.

Respiratory syncytial virus (RSV) was studied as a function of pH (3-8) and temperature (10-85 degrees C) by fluorescence, circular dichroism, and high-resolution second-derivative absorbance spectroscopies, as well as dynamic light scattering and optical density as a measurement of viral aggregation. The results indicate that the secondary, tertiary, and quaternary structures of RSV are both pH and temperature labile. Derivative ultraviolet absorbance and fluorescence spectroscopy (intrinsic and extrinsic) analyses suggest that the stability of tertiary structure of RSV proteins is maximized near neutral pH. In agreement with these results, the secondary structure of RSV polypeptides seems to be more stable at pH 7-8, as evaluated by circular dichroism spectroscopy. The integrity of the viral particles studied by turbidity and dynamic light scattering also revealed that RSV is more thermally stable near neutral pH and particularly prone to aggregation below pH 6. By combination of the spectroscopic data employing a multidimensional eigenvector phase space approach, an empirical phase diagram for RSV was constructed. The pharmaceutical utility of this approach and the optimal formulation conditions are discussed.

The pathogenesis of respiratory syncytial virus disease in childhood.

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory infection in infants and children. RSV is an RNA virus whose genome encodes 10 proteins. The G protein is responsible for viral attachment to cells whilst the F protein promotes syncytia formation. These proteins are also important in the immune response to RSV. Both the innate and adaptive arms of the cellular immune system are involved in the immunological response to RSV. The cytopathic effects of the virus explain many of the pathological findings in RSV disease. However, there is compelling evidence to suggest that the host cell immune response also has a prominent role in disease pathogenesis. Non-immunological factors may also be important.

In vitro growth profiles of respiratory syncytial virus in the presence of influenza virus.

To elucidate epidemiological interference between respiratory syncytial (RSV) and influenza viruses, the influence of influenza A (HlN1) virus on the growth of RSV was examined. Although RSV grew in MDCK cells, coinfection with influenza A virus led to a reduction of progeny RSV. The degree of growth interference depended on the time of infection with influenza A virus post infection (p.i.) with RSV. In fact, infection with influenza A virus 12 hrs p.i. with RSV did not influence growth of the latter virus. On the contrary, growth suppression of influenza A virus by RSV was observed when the coinfection began at the later stages of RSV infection. Suppression of the growth of RSV by influenza A infection was further demonstrated at the level of viral protein synthesis. An indirect immunofluorescence (IF) test revealed that a large proportion of infected cells synthesized both RSV and influenza A virus antigens. Scanning electron microscopic (SEM) examination demonstrated that influenza A and RSV virions possessing surface antigens specific for each virus were selectively released from dually infected cells. In the present study, we proved for the first time that the growth of RSV is blocked by competitive infection with influenza A virus in a susceptible cell population, competitive protein synthesis and selective budding of RSV and influenza viruses from the same infected cells.