The Norovirus NS3 Protein Is a Dynamic Lipid- and Microtubule-Associated Protein Involved in Viral RNA Replication
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VIRUS-CELL INTERACTIONS crossm The Norovirus NS3 Protein Is a Dynamic Lipid- and Microtubule-Associated Protein Involved in Viral RNA Replication Ben T. Cotton,a,b Jennifer L. Hyde,a,c* Soroush T. Sarvestani,b Stanislav V. Sosnovtsev,d Kim Y. Green,d Peter A. White,e Jason M. Mackenzieb Downloaded from Department of Physiology, Anatomy and Microbiology, La Trobe University, Melbourne, Victoria, Australiaa; Department of Microbiology and Immunology, University of Melbourne, Peter Doherty Institute for Infection and Immunity, Melbourne, Victoria, Australiab; School of Chemical and Biological Sciences, University of Queensland, Brisbane, Queensland, Australiac; Norovirus Gastroenteritis Unit, Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, USAd; School of Biotechnology and Biomolecular Sciences, Faculty of Science, University of New South Wales, Sydney, New South Wales, Australiae http://jvi.asm.org/ ABSTRACT Norovirus (NoV) infections are a significant health burden to society, yet the lack of reliable tissue culture systems has hampered the development of appro- Received 31 October 2016 Accepted 18 priate antiviral therapies. Here we show that the NoV NS3 protein, derived from mu- November 2016 Accepted manuscript posted online 23 rine NoV (MNV), is intimately associated with the MNV replication complex and the November 2016 viral replication intermediate double-stranded RNA (dsRNA). We observed that when Citation Cotton BT, Hyde JL, Sarvestani ST, expressed individually, MNV NS3 and NS3 encoded by human Norwalk virus (NV) in- Sosnovtsev SV, Green KY, White PA, Mackenzie duced the formation of distinct vesicle-like structures that did not colocalize with JM. 2017. The norovirus NS3 protein is a dynamic lipid- and microtubule-associated on March 19, 2017 by UQ Library any particular protein markers to cellular organelles but localized to cellular mem- protein involved in viral RNA replication. J Virol branes, in particular those with a high cholesterol content. Both proteins also 91:e02138-16. https://doi.org/10.1128/ showed some degree of colocalization with the cytoskeleton marker -tubulin. Al- JVI.02138-16. though the distribution of MNV and NV NS3s were similar, NV NS3 displayed a Editor Terence S. Dermody, University of Pittsburgh School of Medicine higher level of colocalization with the Golgi apparatus and the endoplasmic reticu- Copyright © 2017 American Society for lum (ER). However, we observed that although both proteins colocalized in mem- Microbiology. All Rights Reserved. branes counterstained with filipin, an indicator of cholesterol content, MNV NS3 dis- Address correspondence to Jason M. played a greater association with flotillin and stomatin, proteins known to associate Mackenzie, [email protected]. with sphingolipid- and cholesterol-rich microdomains. Utilizing time-lapse epifluores- * Present address: Jennifer L. Hyde, Department of Pathology and Immunology, cence microscopy, we observed that the membrane-derived vesicular structures in- Washington University School of Medicine, St. duced by MNV NS3 were highly motile and dynamic in nature, and their movement Louis, Missouri, USA. was dependent on intact microtubules. These results begin to interrogate the func- tions of NoV proteins during virus replication and highlight the conserved properties of the NoV NS3 proteins among the seven Norovirus genogroups. IMPORTANCE Many mechanisms involved in the replication of norovirus still remain unclear, including the role for the NS3 protein, one of seven nonstructural viral pro- teins, which remains to be elucidated. This study reveals that murine norovirus (MNV) NS3 is intimately associated with the viral replication complex and dsRNA. We observed that the NS3 proteins of both MNV and Norwalk virus (NV) induce promi- nent vesicular structures and that this formation is dependent on microtubules and cellular cholesterol. Thus, this study contributes to our understanding of protein function within different Norovirus genogroups and expands a growing knowledge base on the interaction between positive-strand RNA [(ϩ)RNA] viruses and cellu- lar membranes that contribute to the biogenesis of virus-induced membrane or- ganelles. This study contributes to our understanding of viral protein function and the ability of a viral protein to recruit specific cellular organelles and lipids that enable replication. KEYWORDS norovirus, cytoskeleton, virus replication, cellular lipids February 2017 Volume 91 Issue 3 e02138-16 Journal of Virology jvi.asm.org 1 Cotton et al. Journal of Virology oroviruses (NoVs) belong to the Caliciviridae family and are small, nonenveloped, Npositive-sense RNA viruses known to cause acute and chronic diseases in humans (1, 2) and other mammals (3). The clinical symptoms of humans infected with NoV include vomiting, abdominal cramps, and nonbloody loose or watery diarrhea that can lead to severe dehydration (4, 5). NoV infections result in an estimated 230,000 deaths per year, particularly in developing nations (6). This pathogen is highly infectious (7) and transmitted primarily from person to person within enclosed settings such as schools, cruise ships, hospitals, and aged-care and child care facilities (8, 9). NoV is the only virus associated with global pandemics of acute gastroenteritis that emerge every 2 to 3 years, affecting hundreds of millions of people (10, 11). NoV pandemics have occurred globally since the mid-1990s with increasing frequency (10–12). Conse- quently, NoV-associated gastroenteritis has become a major public health concern for which there is no available antiviral agent or preventative vaccine. Downloaded from NoV host cell tropism appears to be exceptionally restricted (13); therefore, human NoVs are difficult to study due to the lack of a reliable cell culture system or any small-animal models. Recently, it was observed that B cells could be infected with human NoV but only in the presence of histo-blood group antigen-expressing enteric bacteria, and infection was observed to be inefficient (14). Subsequently, multiple human NoV strains have been more robustly cultivated in stem cell-derived enterocytes forming human intestinal enteroid monolayer cultures (15). Consequently, the molec- http://jvi.asm.org/ ular mechanisms of human NoV pathogenesis and immunity remain poorly defined (5). In 2003, murine norovirus (MNV) was discovered and has been used to study the immunity and pathogenesis of NoV infection in a mouse model (16). Subsequently, the tropism of MNV for mononuclear cells was identified and led to the establishment of the first in vivo culture system to study NoV replication, a significant advancement in NoV biology (17). NoV harbors an RNA genome of ϳ7,500 nucleotides that is protein linked at the 5= on March 19, 2017 by UQ Library end by VPg (encoded by the viral gene NS5), polyadenylated at the 3= end, and divided into 3 open reading frames (ORFs) (4 for MNV). ORF1 encodes a large polyprotein that undergoes proteolytic cleavage by the virus-encoded protease (NS6) to produce the mature viral nonstructural proteins (18). The two structural proteins, VP1 (major capsid protein) and VP2 (minor capsid protein), are encoded by ORF2 and ORF3, respectively, and can be additionally translated from a viral subgenomic RNA (18). Importantly, the ORF1/ORF2 overlap has been shown to be the breakpoint for genetic recombination (19–22). Our group has been instrumental in identifying and interrogating the intracellular site of MNV replication (20, 21). We have shown that the MNV replication complex (RC) is a collection of irregular-sized vesicles tightly associated with the endoplasmic reticulum (ER), the trans-Golgi network, endosomes, and the centrosome (20, 22). In addition, we have identified the discrete intracellular localization patterns of the MNV ORF1 proteins and their relationship with cellular organelles and membranes utilized during MNV replication (23). We showed that MNV NS1-2 localizes to the ER, NS4 localizes to endosomes, and NS6 localizes to mitochondria (23). Interestingly, NS3 induced discrete vesicular structures that were unidentifiable by typical cell organelle markers but showed some association with microtubules (23, 24). In this study, we aimed to further investigate the function of MNV NS3 during the replication cycle and the origin of the vesicular structures induced by MNV NS3 expression, comparing it to the expression pattern of human NoV (Norwalk virus [NV]) NS3. We show that MNV NS3 is intimately associated with double-stranded RNA (dsRNA) within the MNV RC and show that the localization pattern of transiently expressed human NV NS3 is similar to that of MNV NS3, although there are some subtle differences. Additionally, we show that the vesicular structures induced during NS3 expression contain lipids, and using time-lapse epifluorescence microscopy, we dem- onstrate that these structures are highly dynamic and undergo microtubule-dependent transport. February 2017 Volume 91 Issue 3 e02138-16 jvi.asm.org 2 Norovirus NS3 Protein—Lipids and Microtubules Journal of Virology Downloaded from http://jvi.asm.org/ on March 19, 2017 by UQ Library FIG 1 The MNV NS3 protein associates with the viral replication complex. Thawed cryosections of MNV-infected RAW264.7 cells were fixed at 12 h p.i. (A and B), 18 h p.i. (C and D), or 24 h p.i. (E and F) and were immunolabeled with anti-NS3 antibodies and 10-nm protein A gold. Bars, 200 nm in all cases. RESULTS