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Plus-strand RNA viruses are proficient at remodeling host cell membranes for optimal viral genome replication and the production of infectious progeny. These ultrastructural alterations result in the formation of viral membranous organelles and may be observed by different imaging techniques, providing nanometric resolution. Guided by confocal and electron microscopy, this study describes the generation of wide-field volumes using cryogenic soft-X-ray tomography (cryo-SXT) on SARS-CoV-2-infected human lung adenocarcinoma cells. Confocal microscopy showed accumulation of double-stranded RNA (dsRNA) and nucleocapsid (N) protein in compact perinuclear structures, preferentially found around centrosomes at late stages of the infection. Transmission electron microscopy (TEM) showed accumulation of membranous structures in the vicinity of the infected cell nucleus, forming a viral replication organelle containing characteristic double-membrane vesicles and virus-like particles within larger vesicular structures. Cryo-SXT revealed viral replication organelles very similar to those observed by TEM but indicated that the vesicular organelle observed in TEM sections is indeed a vesiculo-tubular network that is enlarged and elongated at late stages of the infection. Overall, our data provide additional insight into the molecular architecture of the SARS-CoV-2 replication organelle.

The main objective of the studies within COCID Work Package 6 is to understand basic mechanisms by which viral replication machineries are removed from cells after pharmacological interruption of viral replication using functional as well as imaging techniques, including soft X-ray tomography. In this document, we highlight the progress in establishing the HCV replication models (replicons), the antiviral treatment chosen to eliminate viral structures from the host cells and the correlation between elimination of the viral replication machinery and restoration of a normal host cell homeostasis, using markers of HCV-induced stress. These data are essential to frame the experimental setup chosen for the imaging process required in subsequent stps of the project.

Although their origin, nature and structure are not identical, a common feature of positive-strand RNA viruses is their ability to subvert host lipids and intracellular membranes to generate replication and assembly complexes. Recently, lipin1, a cellular enzyme that converts phosphatidic acid into diacylglycerol, has been implicated in the formation of the membranous web that hosts hepatitis C virus (HCV) replicase. In the liver, lipin1 cooperates with lipin2 to maintain glycerolipid homeostasis. We extended our previous study of the lipin family on HCV infection, by determining the impact of the lipin2 silencing on viral replication. Our data reveal that lipin2 silencing interferes with HCV virion secretion at late stages of the infection, without significantly affecting viral replication or assembly. Moreover, uninfected lipin2-, but not lipin1-deficient cells display alterations in mitochondrial and Golgi apparatus morphology, suggesting that lipin2 contributes to the maintenance of the overall organelle architecture. Finally, our data suggest a broader function of lipin2 for replication of HCV and other RNA viruses, in contrast with the specific impact of lipin1 silencing on HCV replication. Overall, this study reveals distinctive functions of lipin1 and lipin2 in cells of hepatic origin, a context in which they are often considered functionally redundant.