Commensal bacteria : bacteria living on another host without causing harm or benefit Virome : the collection of viruses — or their genes — living in a given environment

Noroviruses (NoVs) are highly prevalent, positive-sense RNA viruses that infect a range of mammals, including humans and mice. Murine noroviruses (MuNoVs) are the most prevalent pathogens in biomedical research colonies, and they have been used extensively as a model system for human noroviruses (HuNoVs). Despite recent successes in culturing HuNoVs in the laboratory and a small animal host, studies of human viruses have inherent limitations. Thus, owing to its versatility, the MuNoV system—with its native host, reverse genetics, and cell culture systems—will continue to provide important insights into NoV and enteric virus biology. In the current review, we summarize recent findings from MuNoVs that increase our understanding of enteric virus pathogenesis and highlight similarities between human and murine NoVs that underscore the value of MuNoVs to inform studies of HuNoV biology. We also discuss the potential of endemic MuNoV infections to impact other disease models. 5.1 Review in Advance first posted online on July 2, 2015. (Changes may still occur before final publication online and in print.) Changes may still occur before final publication online and in print A nn ua l R ev ie w o f V ir ol og y 20 15 . 2 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y N ew Y or k U ni ve rs ity B ob st L ib ra ry o n 07 /0 4/ 15 . F or p er so na l u se o nl y. VI02CH05-Wobus ARI 19 June 2015 12:30 Commensal bacteria: bacteria living on another host without causing harm or benefit Virome: the collection of viruses—or their genes—living in a given environment, such as the gastrointestinal tract Genogroup: related viruses within a genus; can be further subdivided into genotype INTRODUCTION The breeding of specific-pathogen-free mice by exclusion of common mouse pathogens has enabled biomedical researchers to increase the consistency of disease models across laboratories and institutions by removing the potentially confounding effects of natural infections. However, the definition of specific pathogen free is unavoidably restricted to previously identified pathogens, most often discovered by their association with specific disease patterns. Murine noroviruses (MuNoVs) cause asymptomatic infections in many mouse strains, including immunocompetent mice and mice lacking adaptive immune responses (1, 2). Thus, their existence went unrecognized until 2003, when we serendipitously discovered the first MuNoV, called MNV-1, in our research facility (2). Since then, diagnostic MuNoV testing has been developed (3), and routine surveillance in mouse facilities has revealed that MuNoVs are the most prevalent type of pathogen in biomedical research facilities worldwide (4–7). Moreover, many MuNoV strains circulate in mouse research colonies and in the wild; over 100 different MuNoV sequences are currently deposited in GenBank. With the recent advances in high-throughput sequencing technology, the identification of infectious agents no longer depends on their ability to cause overt symptoms. In analogy with commensal bacteria, the virome (viruses quiescently inhabiting a mammalian host), and its likely influences on the health of its host, is receiving increasing attention (8). Thus, the academic research community needs to tackle the difficult questions of which virome constituents are tolerable in mouse colonies and how to control for their inevitable variability. Our analysis of MuNoVs 12 years after their initial discovery—in terms of their utility as a norovirus (NoV) model system and their potential to impact other research models as members of the mouse virome—is meant to inform and stimulate this conversation. Thus, this review will focus on (a) how MuNoV research has advanced our understanding of general NoV biology, including cell tropism, promotion of viral pathogenesis by commensal bacteria, persistence, and immunity, and (b) the implications of endemic MuNoV infections for other disease models, considering their presence in many barrier facilities. NOROVIRUSES: THE BASICS Norovirus Genomic Organization NoVs are positive-sense, single-stranded RNA viruses that constitute a genus in the Caliciviridae family (9). The genome is a ∼7.5-kb polyadenylated RNA that has a viral protein called VPg (viral protein, genome-linked) covalently attached to the 5′ end and is composed of three or four open reading frames (ORFs) (Figure 1a). ORF1 encodes the nonstructural (NS) proteins, which are expressed as a polyprotein that is cleaved by the virus-encoded protease into six mature proteins: NS1/2 (N-terminal protein), NS3 (p22), NS4 (3A-like protein), NS5 (VPg), NS6 (protease), and NS7 [RNA-dependent RNA polymerase (RdRp)] (Figure 1a). ORF2 encodes the major capsid protein VP1, which is divided into shell (S) and protruding (P) domains and forms nonenveloped, icosahedral particles that are 27–40 nm in size (Figure 1b). ORF3 encodes the minor capsid protein VP2, of which a few copies in the interior of the virion are thought to play a role in genome packaging (10, 11). MuNoVs also contain a fourth ORF that is expressed in an alternate reading frame from ORF2 and encodes VF1 (virulence factor 1), an antagonist of the innate immune response (12, 13).