Rotavirus is a major cause of acute diarrhea in children under 5 years old, leading to approximately 600,000 annual deaths in the world . Although two live vaccines, an attenuated human rotavirus strain (Rotarix™) and a pentavalent human-bovine reassortant (Rotateq™), have been demonstrated to protect recipients from rotavirus infection effectively and safely in clinical trials and have been licensed in several countries, the protective mechanisms of rotavirus vaccines and the pathogenic mechanisms of rotavirus are not fully understood [2, 3]. A better understanding of the pathogenic mechanisms of rotavirus infection, especially how rotaviruses subvert and evade host antiviral responses are essential for identifying novel strategies to develop antiviral reagents and new vaccines.
The type I interferon (IFN) mediated immune response constitutes the first line of host defense against virus infection . Host cells respond to viral infection by producing IFNs, which further trigger the expression of a variety of genes involved in antiviral responses through the Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) pathway . IFNs also stimulate downstream immune events, leading to the activation of specific immune cells involved in adaptive immune responses [6, 7]. To counteract antiviral responses induced by IFN-α/β, most viruses have evolved viral products to suppress the IFN-mediated signaling pathways . For example, NS1 of influenza virus, NS1/NS2 of respiratory syncytial virus (RSV), VP35 of Ebola virus, E6 protein of human papilloma virus (HPV), and 3C of enterovirus 71 suppress IFN induction by inhibiting IFN signaling pathways [9–14].
Rotaviruses, members of the Reoviridae family, are non-enveloped icosahedra viruses containing 11 segments of a double stranded RNA (dsRNA) genome within a triple-layered particle. The rotavirus genome encodes six structural proteins (VPs) and six nonstructural proteins (NSPs). The structural proteins (VP1-4, VP6-7) form the virion. The NSPs (NSP1-6) function in dsRNA replication, transcription and translation of viral mRNA, and maturation of viral particles . Rotavirus NSP1, a 55-kDa RNA binding protein, is the product of the rotavirus gene 5. It has been shown that the interaction between NSP1 and host signaling proteins is essential for rotaviruses to subvert innate immune responses. NSP1 inhibits innate immune signaling by the following mechanisms. First, NSP1 induces proteasome-dependent degradation of the interferon transcription factors (IRF3, IRF7, and IRF5) to inhibit the IFN response [15–17]. Second, NSP1 inhibits nuclear factor-κB (NF-κB) activation by inducing proteasome-dependent degradation of β-transducin repeat containing protein (β-TrCP) and subsequent IFN-β gene transcription . Third, rotavirus efficiently antagonizes cellular antivirus responses by preventing the nuclear accumulation of STAT1, STAT2, and NF-κB .
NSP1 is the least conserved protein among rotavirus strains . The effect of NSP1 on innate immunity appears rotavirus strain-specific . Investigations on the NSP1 proteins of different rotavirus strains have shown that some degrade IRFs, some degrade β-TrCP, and some target both . For instance, the porcine OSU strain NSP1 cannot induce IRF3 degradation, but it induces the degradation of β-TrCP . We hypothesize that, aside from IRFs and β-TrCP, NSP1 might target other cellular substrates involved in antiviral signaling pathways.
In this study, we investigated whether NSP1 targets other proteins involved in IFN response. We found that NSP1 can inhibit virus-induced activation of IFN-β promoter independent of IRF3 degradation. Furthermore, we show that retinoic acid inducible gene I (RIG-I)-mediated induction of IFN-β is inhibited by NSP1. Our study also revealed that NSP1 interacts with RIG-I and mediates RIG-I down-regulation in a proteasome-independent way. Thus, RIG-I may be an additional target that is antagonized by rotavirus NSP1.