Porcine OSU rotavirus segment II sequence shows common features with the viral gene of human origin

Re-Examining Rotavirus Innate Immune Evasion: Potential Applications of the Reverse Genetics System

Rotaviruses represent one of the most successful pathogens in the world, with high infectivity and efficient transmission between the young of many animal species, including humans. To overcome host defenses, rotaviruses have evolved a plethora of strategies to effectively evade the innate immune response, establish initial infection in the small intestine, produce progeny, and shed into the environment. Previously, studying the roles and relative contributions of specific rotaviral factors in innate immune evasion had been challenging without a plasmid-only reverse genetics system. Although still in its infancy, current reverse genetics technology will help address important research questions regarding rotavirus innate immune evasion, host range restriction, and viral pathogenesis. In this review, we summarize the current knowledge about the antiviral host innate immune defense mechanisms, countermeasures of rotavirus-encoded factors, and strategies to better understand these interactions using the rotavirus reverse genetics system.

Rotavirus incapable of NSP6 expression can cause diarrhea in suckling mice

The group A rotavirus (RVA) genome comprising 11 double-stranded RNAs encodes six structural proteins (VP1-VP4, VP6, and VP7) and six non-structural proteins (NSP1-NSP6). Among these 12 rotaviral proteins, NSP6 has been less studied as to its function. We previously prepared a recombinant NSP6-deficient RVA derived from simian strain SA11-L2 by reverse genetics, and found that the NSP6-deficient virus grew well in cell culture, although its growth was less abundant than that of the parental SA11-L2 strain. In this study, we examined the potency of a recombinant RVA incapable of NSP6 expression to cause diarrhoea in suckling mice. The suckling mice infected with the NSP6-deficient virus apparently experienced diarrhoea, although the symptom was milder and the duration of diarrhoea was shorter than in the mice infected with the authentic SA11-L2 strain. Thus, together with the results obtained for cultured cells in the previous study, it can be concluded that NSP6 is not necessarily required for replication and pathogenicity in vitro and in vivo.

Recent advances in rotavirus reverse genetics and its utilization in basic research and vaccine development

Rotaviruses are segmented double-stranded RNA viruses with a high frequency of gene reassortment, and they are a leading cause of global diarrheal deaths in children less than 5 years old. Two-thirds of rotavirus-associated deaths occur in low-income countries. Currently, the available vaccines in developing countries have lower efficacy in children than those in developed countries. Due to added safety concerns and the high cost of current vaccines, there is a need to develop cost-effective next-generation vaccines with improved safety and efficacy. The reverse genetics system (RGS) is a powerful tool for investigating viral protein functions and developing novel vaccines. Recently, an entirely plasmid-based RGS has been developed for several rotaviruses, and this technological advancement has significantly facilitated novel rotavirus research. Here, we review the recently developed RGS platform and discuss its application in studying infection biology, gene reassortment, and development of vaccines against rotavirus disease.

Reverse Genetics System Demonstrates that Rotavirus Nonstructural Protein NSP6 Is Not Essential for Viral Replication in Cell Culture

The use of overlapping open reading frames (ORFs) to synthesize more than one unique protein from a single mRNA has been described for several viruses. Segment 11 of the rotavirus genome encodes two nonstructural proteins, NSP5 and NSP6. The NSP6 ORF is present in the vast majority of rotavirus strains, and therefore the NSP6 protein would be expected to have a function in viral replication. However, there is no direct evidence of its function or requirement in the viral replication cycle yet. Here, taking advantage of a recently established plasmid-only-based reverse genetics system that allows rescue of recombinant rotaviruses entirely from cloned cDNAs, we generated NSP6-deficient viruses to directly address its significance in the viral replication cycle. Viable recombinant NSP6-deficient viruses could be engineered. Single-step growth curves and plaque formation of the NSP6-deficient viruses confirmed that NSP6 expression is of limited significance for RVA replication in cell culture, although the NSP6 protein seemed to promote efficient virus growth.IMPORTANCE Rotavirus is one of the most important pathogens of severe diarrhea in young children worldwide. The rotavirus genome, consisting of 11 segments of double-stranded RNA, encodes six structural proteins (VP1 to VP4, VP6, and VP7) and six nonstructural proteins (NSP1 to NSP6). Although specific functions have been ascribed to each of the 12 viral proteins, the role of NSP6 in the viral replication cycle remains unknown. In this study, we demonstrated that the NSP6 protein is not essential for viral replication in cell culture by using a recently developed plasmid-only-based reverse genetics system. This reverse genetics approach will be successfully applied to answer questions of great interest regarding the roles of rotaviral proteins in replication and pathogenicity, which can hardly be addressed by conventional approaches.

Transfection of exogenous rotavirus rearranged RNA segments in cells infected with a WT rotavirus results in subsequent gene rearrangements

Group A rotaviruses, members of the family Reoviridae, are a major cause of infantile acute gastroenteritis. The rotavirus genome consists of 11 dsRNA segments. In some cases, an RNA segment is replaced by a rearranged RNA segment, which is derived from its standard counterpart by partial sequence duplication. It has been shown that some rearranged segments are preferentially encapsidated into viral progenies after serial passages in cell culture. Based on this characteristic, a reverse genetics system was used previously to introduce exogenous segment 7 rearrangements into an infectious rotavirus. This study extends this reverse genetics system to RNA segments 5 and 11. Transfection of exogenous rotavirus rearranged RNA segment 5 or 11 into cells infected with a WT helper rotavirus (bovine strain RF) resulted in subsequent gene rearrangements in the viral progeny. Whilst recombinant viruses were rescued with an exogenous rearranged segment 11, the exogenous segment was modified by a secondary rearrangement. The occurrence of spontaneous rearrangements of WT or exogenous segments is a major hindrance to the use of this reverse genetics approach.

Impaired hyperphosphorylation of rotavirus NSP5 in cells depleted of casein kinase 1alpha is associated with the formation of viroplasms with altered morphology and a moderate decrease in virus replication

The rotavirus (RV) non-structural protein 5, NSP5, is encoded by the smallest of the 11 genomic segments and localizes in 'viroplasms', cytoplasmic inclusion bodies in which viral RNA replication and packaging take place. NSP5 is essential for the replicative cycle of the virus because, in its absence, viroplasms are not formed and viral RNA replication and transcription do not occur. NSP5 is produced early in infection and undergoes a complex hyperphosphorylation process, leading to the formation of proteins differing in electrophoretic mobility. The role of hyperphosphorylation of NSP5 in the replicative cycle of rotavirus is unknown. Previous in vitro studies have suggested that the cellular kinase CK1alpha is responsible for the NSP5 hyperphosphorylation process. Here it is shown, by means of specific RNA interference, that in vivo, CK1alpha is the enzyme that initiates phosphorylation of NSP5. Lack of NSP5 hyperphosphorylation affected neither its interaction with the virus VP1 and NSP2 proteins normally found in viroplasms, nor the production of viral proteins. In contrast, the morphology of viroplasms was altered markedly in cells in which CK1alpha was depleted and a moderate decrease in the production of double-stranded RNA and infectious virus was observed. These data show that CK1alpha is the kinase that phosphorylates NSP5 in virus-infected cells and contribute to further understanding of the role of NSP5 in RV infection.

Characterization of the NSP6 protein product of rotavirus gene 11

The 12kDa non-structural protein 6 (NSP6) is the least studied of the rotavirus proteins. In an attempt to further characterize this protein mono-specific antisera was generated using purified protein expressed in E. coli. Pulse/chase radio-labeling of virus infected cells was used to show that it is expressed at a steady but low rate throughout the virus replication cycle. In contrast to the other rotavirus non-structural proteins, NSP6 was found to have a high rate of turnover, being completely degraded within 2h of synthesis. NSP6 tagged with GFP was used to probe the intracellular distribution of the protein, perinuclear aggregates were observed in the cytoplasm of transfected cells. Following virus infection of these transfected cells the aggregates were seen to redistribute to the viroplasms. Consistent with its localization to the site of viral genome replication and packaging, NSP6 was found to be a sequence independent nucleic acid binding protein, with similar affinities for ssRNA and dsRNA.

RNA interference of rotavirus segment 11 mRNA reveals the essential role of NSP5 in the virus replicative cycle

Rotavirus genomes contain 11 double-stranded (ds) RNA segments. Genome segment 11 encodes the non-structural protein NSP5 and, in some strains, also NSP6. NSP5 is produced soon after viral infection and localizes in cytoplasmic viroplasms, where virus replication takes place. RNA interference by small interfering (si) RNAs targeted to genome segment 11 mRNA of two different strains blocked production of NSP5 in a strain-specific manner, with a strong effect on the overall replicative cycle: inhibition of viroplasm formation, decreased production of other structural and non-structural proteins, synthesis of viral genomic dsRNA and production of infectious particles. These effects were shown not to be due to inhibition of NSP6. The results obtained strengthen the importance of secondary transcription/translation in rotavirus replication and demonstrate that NSP5 is essential for the assembly of viroplasms and virus replication.

Rotavirus NS26 is modified by addition of single O-linked residues of N-acetylglucosamine

We studied the post-translational modification of NS26, the protein product of rotavirus gene 11 segment. Based on the presence of a putative N-glycosylation site and the high content of serine and threonine residues in gene 11 amino acid sequence we investigated whether NS26 is modified by carbohydrate addition. Specific antibodies raised against the gene 11 product expressed in Escherichia coli recognized in infected cells two polypeptides with apparent molecular weight of 26,000 (26-kDa polypeptide) and 28,000 (28-kDa polypeptide). Pulse-chase experiments demonstrated that the 26-kDa product was processed to the 28-kDa polypeptide. Both polypeptides were metabolically labeled with [3H]glucosamine, indicating the presence of a carbohydrate moiety on the protein. NS26 was found to be resistant to endo-beta-N-acetylglucosaminidase H and endo-beta-N-acetylglucosaminidase F/peptide:N-glycosidase F treatment, but sensitive to removal by alkali-induced beta-elimination, suggesting that the saccharide chain was attached to the protein via an O-glycosidic linkage. Chromatographic analysis of total acid hydrolysates of [3H]glucosamine-labeled NS26-bound carbohydrate indicated the presence of N-acetylglucosamine. In addition, mild alkaline treatment of NS26 in the presence of NaB3H4 identified the O-linked carbohydrate moiety as N-acetylglucosamine. Taken together, these data demonstrate that NS26 is processed to a 28-kDa polypeptide by addition of O-linked monosaccharide residues of N-acetylglucosamine.

Expression of rotavirus proteins encoded by alternative open reading frames of genome segment 11

The nucleotide sequence of rotavirus genome segment 11 shows that this gene contains three potential open reading frames. We used several approaches to determine whether any polypeptides other than NS26, the primary protein product, are expressed. In particular, we sought to determine whether the strong out-of-phase start codon present at nucleotides 80-82, which would encode a protein of 92 amino acids, is used in vivo or in cell-free systems. Several modifications of gene 11 were made and found to produce proteins from the different initiation codons in cell-free transcription-translation systems. The protein from the out-of-phase open reading frame was shown to be expressed in rotavirus-infected MA104 cells; this was demonstrated using monospecific sera prepared to this protein expressed in Spodoptera frugiperda insect cells infected with a baculovirus recombinant containing only the out-of-phase open reading frame. The origin of some of the lower-molecular-weight bands serologically related to the primary product of gene 11, NS26, was also studied by selective immunoprecipitation using two different sera made from recombinant baculovirus lysates. All of these polypeptides are present in infected cells in a complex which is still incompletely defined.

Conservation of a potential metal binding motif despite extensive sequence diversity in the rotavirus nonstructural protein NS53

The nucleotide sequence for the simian rotavirus SA11 gene segment 5 has been determined. The gene is 1611 nucleotides in length and contains a single open reading frame of 1485 nucleotides. The segment codes for the nonstructural protein NS53 which is predicted to be a polypeptide of 495 amino acids with a molecular weight of 58,484. When compared to the sequence of bovine RF gene segment 5 there are homologies of only 49 and 36% at the nucleotide and amino acid levels, respectively. This is in marked contrast to the situation with other rotavirus nonstructural proteins which are highly conserved between isolates. Nevertheless, there is a conserved region between amino acids 37-81 which contains a generalized motif for a metal binding domain. All eight cysteine and two histidine residues in this short sequence are conserved between the simian and bovine NS53 proteins. The conservation of this domain despite extensive sequence diversity in the remainder of the protein suggests that this region is functionally important.