Noroviruses are small, positive-sense RNA viruses within the family Caliciviridae, and are now accepted widely as a major cause of acute gastroenteritis in both developed and developing countries. Despite their impact, our understanding of the life cycle of noroviruses has lagged behind that of other RNA viruses due to the inability to culture human noroviruses (HuNVs). Our knowledge of norovirus biology has improved significantly over the past decade as a result of numerous technological advances. The use of a HuNV replicon, improved biochemical and cellbased assays, combined with the discovery of a murine norovirus capable of replication in cell culture, has improved greatly our understanding of the molecular mechanisms of norovirus genome translation and replication, as well as the interaction with host cell processes. In this review, the current state of knowledge of the intracellular life of noroviruses is discussed with particular emphasis on the mechanisms of viral gene expression and viral genome replication.
IntroductionThe prototype norovirus, Norwalk virus, was first described in 1972 as the aetiological agent responsible for an outbreak of acute gastroenteritis in an elementary school in Norwalk, OH, USA (Kapikian, 2000). Noroviruses are now accepted as a leading cause of gastroenteritis in developed and developing countries (Glass et al., 2009;Hall et al., 2013). Spread primarily via the faecal-oral route, norovirus infections are typically an acute self-limiting gastrointestinal infection. Norovirus gastroenteritis has recently been identified as a significant cause of morbidity and mortality in the immunocompromised, and can result in long-term persistent disease (reviewed by Bok & Green, 2012). Norovirus infection has also been associated with a number of more significant clinical outcomes: necrotizing enterocolitis (Turcios-Ruiz et al., 2008), seizures in infants (Medici et al., 2010), encephalopathy (Ito et al., 2006), pneumatosis intestinalis (Chan et al., 2010;Kim et al., 2011) and disseminated intravascular coagulation (CDC, 2002), to name but a few. In developing countries, an estimated 200 000 deaths in children ,5 years of age are thought to be due to norovirus infections (Patel et al., 2008) and they have recently been reported as the second leading cause of gastroenteritis-related deaths in the USA, typically resulting in 797 deaths per annum (Hall et al., 2013). Despite their significant impact, noroviruses remain one of the most poorly characterized groups of RNA viruses, due largely to the fact that, despite numerous attempts (Duizer et al., 2004;Papafragkou et al., 2013; Takanashi et al., 2013), human noroviruses (HuNVs) have yet to be cultured efficiently in cell culture.Noroviruses are members of the family Caliciviridae of small, positive-sense RNA viruses, which is divided currently into five genera: Vesivirus, Lagovirus, Nebovirus, Sapovirus and Norovirus. Members of the genera Norovirus and Sapovirus are able to infect humans and cause gastroenteritis. The genus Norovirus is s...