Abstract
Many viruses replicate most efficiently in specific phases of the cell cycle, establishing or exploiting favorable conditions for viral replication, although little is known about the relationship between caliciviruses and the cell cycle. Microarray and Western blot analysis of murine norovirus 1 (MNV-1)-infected cells showed changes in cyclin transcript and protein levels indicative of a G(1) phase arrest. Cell cycle analysis confirmed that MNV-1 infection caused a prolonging of the G(1) phase and an accumulation of cells in the G(0)/G(1) phase. The accumulation in G(0)/G(1) phase was caused by a reduction in cell cycle progression through the G(1)/S restriction point, with MNV-1-infected cells released from a G(1) arrest showing reduced cell cycle progression compared to mock-infected cells. MNV-1 replication was compared in populations of cells synchronized into specific cell cycle phases and in asynchronously growing cells. Cells actively progressing through the G(1) phase had a 2-fold or higher increase in virus progeny and capsid protein expression over cells in other phases of the cell cycle or in unsynchronized populations. These findings suggest that MNV-1 infection leads to prolonging of the G(1) phase and a reduction in S phase entry in host cells, establishing favorable conditions for viral protein production and viral replication. There is limited information on the interactions between noroviruses and the cell cycle, and this observation of increased replication in the G(1) phase may be representative of other members of the Caliciviridae.
IMPORTANCE
Noroviruses have proven recalcitrant to growth in cell culture, limiting our understanding of the interaction between these viruses and the infected cell. In this study, we used the cell-culturable MNV-1 to show that infection of murine macrophages affects the G(1)/S cell cycle phase transition, leading to an arrest in cell cycle progression and an accumulation of cells in the G(0)/G(1) phase. Furthermore, we show that MNV replication is enhanced in the G(1) phase compared to other stages of the cell cycle. Manipulating the cell cycle or adapting to cell cycle responses of the host cell is a mechanism to enhance virus replication. To the best of our knowledge, this is the first report of a norovirus interacting with the host cell cycle and exploiting the favorable conditions of the G(0)/G(1) phase for RNA virus replication.