Abstract
Human norovirus (HuNV) is a member of the calicivirus family and is a major cause of viral gastroenteritis worldwide. Due to the absence of a suitable cell culture system, HuNoV replication mechanisms are poorly understood, but two animal caliciviruses, Feline calicivirus (FCV) and Murine Norovirus (MNV) provide models to increase our understanding of norovirus biology. Unlike cellular mRNAs, the calicivirus RNA genome does not possess a 5' cap structure but instead has a 13–15 kDa viral protein, genome linked (VPg) directing translation, hijacking the host protein synthesis machinery. The viral life cycle requires separated events occurring at different times since viral transcripts are used as the template both for translation (mRNA) and replication (genomic RNA). Therefore mechanisms are required to control the viral RNA fate. In eukaryotes, during stress conditions, mRNAs can be stored in subcellular compartments such as stress granules to stall their translation or in processing bodies to be degraded. Recent evidence indicates that these compartments also play an important role during the viral life cycle. Therefore, using immunofluorescence microscopy we set out to investigate how FCV and MNV infection regulate the formation of G3BP1- and PABP-1-containing stress granules and DCP-1-containing processing bodies to address whether these cytoplasmic granules could play a role during the viral life cycle. We have now shown that FCV has the ability to prevent stress granules formation during infection and that this is important for replication in CRFK and FEA cells. Using FCV-free supernatant from infected CRFK cells and immunofluorescence microscopy, we have also shown that during infection, the formation of stress granules is induced in a paracrine manner in uninfected cells via a messenger molecule released from infected cells. We hypothesize that this could reflect a new antiviral role for stress granules. Furthermore, MNV and FCV infection also led to the disruption of processing-bodies assembly. Overall, this study revealed that caliciviruses modulate the RNA granules during infection and that this could be part of viral mechanism to counteract the antiviral response.