Establishment of different plasmid only-based reverse genetics systems for the recovery of African horse sickness virus

Highs and Lows in Calicivirus Reverse Genetics

In virology, the term reverse genetics refers to a set of methodologies in which changes are introduced into the viral genome and their effects on the generation of infectious viral progeny and their phenotypic features are assessed. Reverse genetics emerged thanks to advances in recombinant DNA technology, which made the isolation, cloning, and modification of genes through mutagenesis possible. Most virus reverse genetics studies depend on our capacity to rescue an infectious wild-type virus progeny from cell cultures transfected with an "infectious clone". This infectious clone generally consists of a circular DNA plasmid containing a functional copy of the full-length viral genome, under the control of an appropriate polymerase promoter. For most DNA viruses, reverse genetics systems are very straightforward since DNA virus genomes are relatively easy to handle and modify and are also (with few notable exceptions) infectious per se. This is not true for RNA viruses, whose genomes need to be reverse-transcribed into cDNA before any modification can be performed. Establishing reverse genetics systems for members of the Caliciviridae has proven exceptionally challenging due to the low number of members of this family that propagate in cell culture. Despite the early successful rescue of calicivirus from a genome-length cDNA more than two decades ago, reverse genetics methods are not routine procedures that can be easily extrapolated to other members of the family. Reports of calicivirus reverse genetics systems have been few and far between. In this review, we discuss the main pitfalls, failures, and delays behind the generation of several successful calicivirus infectious clones.

Recent insights into reverse genetics of norovirus

Norovirus is the leading cause of viral gastroenteritis globally, and poses substantial threats to public health. Despite substantial progress made in preventing norovirus diseases, the lack of a robust virus culture system has hampered biological research and effective strategies to combat this pathogen. Reverse genetic system is the technique to generate infectious viruses from cloned genetic constructs, which is a powerful tool for the investigation of viral pathogenesis and for the development of novel drugs and vaccines. The strategies of reverse genetics include bacterial artificial chromosomes, vaccinia virus vectors, and entirely plasmid-based systems. Since each strategy has its pros and cons, choosing appropriate approaches will greatly improve the efficiency of virus rescue. Reverse genetic systems that have been employed for norovirus greatly extend its life cycle and facilitate the development of medical countermeasures. In this review, we summarize the current knowledge on the structure, transmission, genetic evolution and clinical manifestations of norovirus, and describe recent advances in the studies of norovirus reverse genetics as well as its future prospects for therapeutics and vaccine development.

Recent advances in Reovirales viruses reverse genetics research

Reovirales are segmented double-strand RNA viruses with a broad host range that pose a serious threat to human and animal health. However, there are numerous viral species within the Reovirales, some of which have lagged behind other RNA viruses in the study of their biology due to the lack of an effective reverse genetics (RG) system. The RG systems are the most powerful tools for studying viral protein function, viral gene expression regulation, viral pathogenesis, and the generation of engineered vaccines. Recently, several entirely plasmid-based RG systems have been developed for several members of the Reovirales. This review outlines the development and future direction of the RG system for the best studied Reovirales viruses.

Investigating the Role of African Horse Sickness Virus VP7 Protein Crystalline Particles on Virus Replication and Release

African horse sickness is a deadly and highly infectious disease of equids, caused by African horse sickness virus (AHSV). AHSV is one of the most economically important members of the Orbivirus genus. AHSV is transmitted by the biting midge, Culicoides, and therefore replicates in both insect and mammalian cell types. Structural protein VP7 is a highly conserved major core protein of orbiviruses. Unlike any other orbivirus VP7, AHSV VP7 is highly insoluble and forms flat hexagonal crystalline particles of unknown function in AHSV-infected cells and when expressed in mammalian or insect cells. To examine the role of AHSV VP7 in virus replication, a plasmid-based reverse genetics system was used to generate a recombinant AHSV that does not form crystalline particles. We characterised the role of VP7 crystalline particle formation in AHSV replication in vitro and found that soluble VP7 interacted with viral proteins VP2 and NS2 similarly to wild-type VP7 during infection. Interestingly, soluble VP7 was found to form uncharacteristic tubule-like structures in infected cells which were confirmed to be as a result of unique VP7-NS1 colocalisation. Furthermore, it was found that VP7 crystalline particles play a role in AHSV release and yield. This work provides insight into the role of VP7 aggregation in AHSV cellular pathogenesis and contributes toward the understanding of the possible effects of viral protein aggregation in other human virus-borne diseases.

Development of an entirely plasmid-based reverse genetics system for 12-segmented double-stranded RNA viruses

The family Reoviridae is a nonenveloped virus group with a double-stranded (ds) RNA genome comprising 9 to 12 segments. In the family Reoviridae, the genera Cardoreovirus, Phytoreovirus, Seadornavirus, Mycoreovirus, and Coltivirus contain virus species having 12-segmented dsRNA genomes. Reverse genetics systems used to generate recombinant infectious viruses are powerful tools for investigating viral gene function and for developing vaccines and therapeutic interventions. Generally, this methodology has been utilized for Reoviridae viruses such as Orthoreovirus, Orbivirus, Cypovirus, and Rotavirus, which have genomes with 10 or 11 segments, respectively. However, no reverse genetics system has been developed for Reoviridae viruses with a genome harboring 12 segments. Herein, we describe development of an entire plasmid-based reverse genetics system for Tarumizu tick virus (TarTV) (genus Coltivirus, family Reoviridae), which has a genome of 12 segments. Recombinant TarTVs were generated by transfection of 12 cloned complementary DNAs encoding the TarTV genome into baby hamster kidney cells expressing T7 RNA polymerase. Using this technology, we generated VP12 mutant viruses and demonstrated that VP12 is an N-glycosylated protein. We also generated a reporter virus expressing the HiBiT-tagged VP8 protein. This reverse genetics system will increase our understanding of not only the biology of the genus Coltivirus but also the replication machinery of the family Reoviridae.

Rotavirus reverse genetics systems: Development and application

Rotaviruses (RVs) cause acute gastroenteritis in infants and young children. Since 2006, live-attenuated vaccines have reduced the number of RV-associated deaths; however, RV is still responsible for an estimated 228,047 annual deaths worldwide. RV, a member of the family Reoviridae, has an 11-segmented double-stranded RNA genome contained within a non-enveloped, triple layered virus particle. In 2017, a long-awaited helper virus-free reverse genetics system for RV was established. Since then, numerous studies have reported the generation of recombinant RVs; these studies verify the robustness of reverse genetics systems. This review provides technical insight into current reverse genetics systems for RVs, as well as discussing basic and applied studies that have used these systems.

Reverse genetics approaches: a novel strategy for African horse sickness virus vaccine design

African horse sickness (AHS) is a devastating disease caused by African horse sickness virus (AHSV) and transmitted by arthropods between its equine hosts. AHSV is endemic in sub-Saharan Africa, where polyvalent live attenuated vaccine is in use even though it is associated with safety risks. This review article summarizes and compares new strategies to generate safe and effective AHSV vaccines based on protein, virus like particles, viral vectors and reverse genetics technology. Manipulating the AHSV genome to generate synthetic viruses by means of reverse genetic systems has led to the generation of potential safe vaccine candidates that are under investigation.

Development and optimization of a DNA-based reverse genetics systems for epizootic hemorrhagic disease virus

Epizootic hemorrhagic disease virus (EHDV) is a member of the genus Orbivirus, family Reoviridae, and has a genome consisting of 10 linear double-stranded (ds) RNA segments. The current reverse genetics system (RGS) for engineering the EHDV genome relies on the use of in vitro-synthesized capped viral RNA transcripts. To obtain more-efficient and simpler RGSs for EHDV, we developed an entirely DNA (plasmid or PCR amplicon)-based RGS for viral rescue. This RGS enabled the rescue of infectious EHDV from BSR-T7 cells following co-transfection with seven helper viral protein expression plasmids and 10 cDNA rescue plasmids or PCR amplicons representing the EHDV genome. Furthermore, we optimized the DNA-based systems and confirmed that some of the helper expression plasmids were not essential for the recovery of infectious EHDV. Thus, DNA-based RGSs may offer a more efficient method of recombinant virus recovery and accelerate the study of the biological characteristics of EHDV and the development of novel vaccines.

African Horse Sickness: A Review of Current Understanding and Vaccine Development

African horse sickness is a devastating disease that causes great suffering and many fatalities amongst horses in sub-Saharan Africa. It is caused by nine different serotypes of the orbivirus African horse sickness virus (AHSV) and it is spread by Culicoid midges. The disease has significant economic consequences for the equine industry both in southern Africa and increasingly further afield as the geographic distribution of the midge vector broadens with global warming and climate change. Live attenuated vaccines (LAV) have been used with relative success for many decades but carry the risk of reversion to virulence and/or genetic re-assortment between outbreak and vaccine strains. Furthermore, the vaccines lack DIVA capacity, the ability to distinguish between vaccine-induced immunity and that induced by natural infection. These concerns have motivated interest in the development of new, more favourable recombinant vaccines that utilize viral vectors or are based on reverse genetics or virus-like particle technologies. This review summarizes the current understanding of AHSV structure and the viral replication cycle and also evaluates existing and potential vaccine strategies that may be applied to prevent or control the disease.

A correlation between capsid protein VP2 and the plaque morphology of African horse sickness virus in cell culture

The attenuated live virus vaccine that is used in South Africa to protect against African horse sickness infection was developed more than 50 years ago. With the selection of the vaccine strains by cell culture passage, a correlation between the size of plaques formed in monolayer Vero cultures and attenuation of virus virulence in horses was found. The large plaque phenotype was used as an indication of cell culture adaptation and strongly correlated with attenuation of virulence in horses. There was never any investigation into the genetic causes of either the variation in plaque size, or the loss of virulence. An understanding of the underlying mechanisms of attenuation would benefit the production of a safer AHSV vaccine. To this end, the genomes of different strains of two African horse sickness isolates, producing varying plaque sizes, were compared and the differences between them identified. This comparison suggested that proteins VP2, VP3, VP5 and NS3 were most likely involved in the determination of the plaque phenotype. Comparison between genome sequences (obtained from GenBank) of low and high passage strains from two additional serotypes indicated that VP2 was the only protein with amino acid substitutions in all four serotypes. The amino acid substitutions all occurred within the same hydrophilic area, resulting in increased hydrophilicity of VP2 in the large plaque strains.

African Swine Fever Virus NP868R Capping Enzyme Promotes Reovirus Rescue during Reverse Genetics by Promoting Reovirus Protein Expression, Virion Assembly, and RNA Incorporation into Infectious Virions

Reoviruses, like many eukaryotic viruses, contain an inverted 7-methylguanosine (m7G) cap linked to the 5' nucleotide of mRNA. The traditional functions of capping are to promote mRNA stability, protein translation, and concealment from cellular proteins that recognize foreign RNA. To address the role of mRNA capping during reovirus replication, we assessed the benefits of adding the African swine fever virus NP868R capping enzyme during reovirus rescue. C3P3, a fusion protein containing T7 RNA polymerase and NP868R, was found to increase protein expression 5- to 10-fold compared to T7 RNA polymerase alone while enhancing reovirus rescue from the current reverse genetics system by 100-fold. Surprisingly, RNA stability was not increased by C3P3, suggesting a direct effect on protein translation. A time course analysis revealed that C3P3 increased protein synthesis within the first 2 days of a reverse genetics transfection. This analysis also revealed that C3P3 enhanced processing of outer capsid μ1 protein to μ1C, a previously described hallmark of reovirus assembly. Finally, to determine the rate of infectious-RNA incorporation into new virions, we developed a new recombinant reovirus S1 gene that expressed the fluorescent protein UnaG. Following transfection of cells with UnaG and infection with wild-type virus, passage of UnaG through progeny was significantly enhanced by C3P3. These data suggest that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation.IMPORTANCE Our findings expand our understanding of how viruses utilize capping, suggesting that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation, in addition to enhancing protein translation. Beyond providing mechanistic insight into reovirus replication, our findings also show that reovirus reverse genetics rescue is enhanced 100-fold by the NP868R capping enzyme. Since reovirus shows promise as a cancer therapy, efficient reovirus reverse genetics rescue will accelerate production of recombinant reoviruses as candidates to enhance therapeutic potency. NP868R-assisted reovirus rescue will also expedite production of recombinant reovirus for mechanistic insights into reovirus protein function and structure.

[A plasmid-based reverse genetics system for rotaviruses]

Rotavirus (RV), a non-enveloped icosahedral virus containing eleven gene segments of double-stranded RNA, is the leading cause of severe, acute diarrhea among infants and young children worldwide. Safe and effective rotavirus vaccines have been available since 2006, and have markedly reduced the number of deaths by severe gastroenteritis. However, rotaviruses are still responsible for approximately 200,000 deaths annually worldwide. Reverse genetics systems for the manipulation of viral genomes are a powerful approach for studying viral replication and pathogenesis, and for developing vaccines and viral vectors. The understanding of the molecular mechanisms underlying RV pathogenesis, or development of next generation vaccines, has been hampered by the lack of a complete reverse genetics system. Recently, we developed a novel reverse genetics system which enabled recovery of recombinant RVs entirely from cloned cDNAs. This new strategy requires co-expression of a small transmembrane protein that accelerates cell-to-cell fusion and vaccinia virus capping enzyme. In this review, the strategies and history of the development of reverse genetics systems for the family Reoviridae are described.