Investigating the importance of co-expressed rotavirus proteins in the development of a selection-free rotavirus reverse genetics system
Wentzel, Johannes Frederik
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Reverse genetics is an innovative molecular biology tool that enables the manipulation of viral genomes at the cDNA level in order to generate particular mutants or artificial viruses. The reverse genetics system for the influenza virus is arguably one of the best illustrations of the potential power of this technology. This reverse genetics system is the basis for the ability to regularly adapt influenza vaccines strains. Today, reverse genetic systems have been developed for many animal RNA viruses. Selection-free reverse genetics systems have been developed for the members of the Re-oviridae family including, African horse-sickness virus, bluetongue virus and orthoreovirus. This ground-breaking technology has led to the generation of valuable evidence regarding the replication and pathogenesis of these viruses. Unfortunately, extrapolating either the plasmid-based or transcript-based reverse genetics systems to rotavirus has not yet been successful. The development of a selection-free rotavirus reverse genetics system will enable the systematic investigation of poorly understood aspects of the rotavirus replication cycle and aid the development of more effective vaccines, amongst other research avenues. This study investigated the importance of co-expressed rotavirus proteins in the development of a selection-free rotavirus reverse genetics system. The consensus sequences of the rotavirus strains Wa (RVA/Human-tc/USA/WaCS/1974/G1P) and SA11 (RVA/Simian-tc/ZAF/SA11/1958/G3P) where used to design rotavirus expression plasmids. The consensus nucleotide sequence of a human rotavirus Wa strain was determined by sequence-independent cDNA synthesis and amplification combined with next-generation 454® pyrosequencing. A total of 4 novel nucleotide changes, which also resulted in amino acid changes, were detected in genome segment 7 (NSP3), genome segment 9 (VP7) and genome segment 10 (NSP4). In silico analysis indicated that none of the detected nucleotide changes, and consequent amino acid variations, had any significant effect on viral structure. Evolutionary analysis indicated that the sequenced rotavirus WaCS was closely related to the ParWa and VirWa variants, which were derived from the original 1974 Wa isolate. Despite serial passaging in animals, as well as cell cultures, the Wa genome seems to be stable. Considering that the current reference sequence for the Wa strain is a composite sequence of various Wa variants, the rotavirus WaCS may be a more appropriate reference sequence. The rotavirus Wa and SA11 strains were selected for plasmid-based expression of rotavirus proteins, under control of a T7 promoter sequence, due to the fact that they propagate well in MA104 cells and the availability of their consensus sequences. The T7 RNA polymerase was provided by a recombinant fowlpox virus. After extensive transfection optimisation on a variety of mammalian cell lines, MA104 cells proved to be the best suited for the expression rotavirus proteins from plasmids. The expression of rotavirus Wa and SA11 VP1, VP6, NSP2 and NSP5 could be confirmed with immunostaining in MA104 and HEK 293H cells. Another approach involved the codon-optimised expression of the rotavirus replication complex scaffold in MA104 cells under the control of a CMV promoter sequence. This system was independent from the recombinant fowlpox virus. All three plasmid expression sets were designed to be used in combination with the transcript-based reverse genetics system in order to improve the odds of developing a successful rotavirus reverse genetics system. Rotavirus transcripts were generated using transcriptively active rotavirus SA11 double layered particles (DLPs). MA104 and HEK293H cells proved to be the best suited for the expression of rotavirus transcripts although expression of rotavirus VP6 could be demonstrated in all cell cultures examined (MA104, HEK 293H, BSR and COS-7) using immunostaining. In addition, the expression of transcript derived rotavirus VP1, NSP2 and NSP5 could be confirmed with immunofluorescence in MA104 and HEK 293H cells. This is the first report of rotavirus transcripts being translated in cultured cells. A peculiar cell death pattern was observed within 24 hours in response to transfection of rotavirus transcripts. This observed cell death, however does not seem to be related to normal viral cytopathic effect as no viable rotavirus could be recovered. In an effort to combine the transcript- and plasmid systems, a dual transfection strategy was followed where plasmids encoding rotavirus proteins were transfected first followed, 12 hours later, by the transfection of rotavirus SA11 transcripts. The codon- optimised plasmid system was designed as it was postulated that expression of the DLP-complex (VP1, VP2, VP3 and VP6), the rotavirus replication complex would form and assist with replication and/or packaging. Transfecting codon- optimized plasmids first noticeably delayed the mass cell death observed when transfecting rotavirus transcripts on their own. None of the examined coexpression systems were able to produce a viable rotavirus. Finally, the innate immune responses elicited by rotavirus transcripts and plasmid-derived rotavirus Wa and SA11 proteins were investigated. Quantitative RT-PCR (qRT-PCR) experiments indicated that rotavirus transcripts induced high levels of the expression of the cytokines IFN- α1, IFN-1β, IFN-λ1 and CXCL10. The expression of certain viral proteins from plasmids (VP3, VP7 and NSP5/6) was more likely to stimulate specific interferon responses, while other viral proteins (VP1, VP2, VP4 and NSP1) seem to be able to actively suppress the expression of certain cytokines. In the light of these suppression results, specific rotavirus proteins were expressed from transfected plasmids to investigate their potential in supressing the interferon responses provoked by rotavirus transcripts. qRT-PCR results indicated that cells transfected with the plasmids encoding SP1, NSP2 or a combination of NSP2 and NSP5 significantly reduced the expression of specific cytokines induced by rotavirus transcripts. These findings point to other possible viral innate suppression mechanisms in addition to the degradation of interferon regulatory factors by NSP1. The suppression of the strong innate immune response elicited by rotavirus transcripts might well prove to be vital in the quest to better understand the replication cycle of this virus and eventually lead to the development of a selection-free reverse genetics system for rotavirus.
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