Organization of viral RNA synthesis in Respiratory Syncytial Virus infected cells: Viral RNA granules?
Cells infected by respiratory syncytial virus (RVS) exhibit cytoplasmic inclusion (IBs) which concentrate the components of the viral polymerase (L, P, N and M2-1) and the viral genomic RNA. Using metabolic labelling we established that viral RNA synthesis occur in IBs. We identified and characterized a novel compartment within IBs designated as IB associated granules (IBAGs). Metabolic labelling and FISH experiments revealed that newly synthesized viral mRNA concentrate in IBAGs when genomic RNA are located in the remnant part of IBs. Interestingly, confocal microscopy and PALM/STORM super-resolution microscopy revealed that L, P and N while present in IB are excluded from IBAGs. In contrast, M2-1, a transcription processivity factor of RSV accumulated in IBAGs. Using a dicistronic minireplicon system we found that IBAGs formation is strictly dependent of viral RNA synthesis, but may occur in the absence of M2-1. Indeed, when omitting M2-1, FISH experiments revealed some IBAGs when using probes against the first ORF (which can be transcribed without M2-1), but not using probes against polyA or the second ORF. IBAGS are highly dynamic structures, which seem to release their content periodically into the cytosol as revealed by live cell microscopy and pulse chase experiments. Noteworthy IBAGs exhibit liquid behavior resembling this of liquid organelles. Like recently shown for Negri bodies, we found that RSV IBs exhibit properties of liquid organelles: spherical shape, fusion to form larger spherical structure, disappearance upon osmotic shock. Thus, IBAGs formation could be regarded as a liquid-liquid phase separation resulting from accumulation of viral mRNA in IBs. The strict dependence of IBAGs formation on viral RNA synthesis but not on M2-1 suggests that viral mRNA are the driving force of IBAGs formation.
INSERM U1173 Infection - Inflammation, Université Versailles Saint-Quentin en Yvelines
Domain 1 - UMR 9187 / U1196 - Chemistry, Modelling and Imaging for Biology