Rotavirus reverse genetics systems: Development and application

Chimeric Viruses Enable Study of Antibody Responses to Human Rotaviruses in Mice

The leading cause of gastroenteritis in children under the age of five is rotavirus infection, accounting for 37% of diarrhoeal deaths in infants and young children globally. Oral rotavirus vaccines have been widely incorporated into national immunisation programs, but whilst these vaccines have excellent efficacy in high-income countries, they protect less than 50% of vaccinated individuals in low- and middle-income countries. In order to facilitate the development of improved vaccine strategies, a greater understanding of the immune response to existing vaccines is urgently needed. However, the use of mouse models to study immune responses to human rotavirus strains is currently limited as rotaviruses are highly species-specific and replication of human rotaviruses is minimal in mice. To enable characterisation of immune responses to human rotavirus in mice, we have generated chimeric viruses that combat the issue of rotavirus host range restriction. Using reverse genetics, the rotavirus outer capsid proteins (VP4 and VP7) from either human or murine rotavirus strains were encoded in a murine rotavirus backbone. Neonatal mice were infected with chimeric viruses and monitored daily for development of diarrhoea. Stool samples were collected to quantify viral shedding, and antibody responses were comprehensively evaluated. We demonstrated that chimeric rotaviruses were able to efficiently replicate in mice. Moreover, the chimeric rotavirus containing human rotavirus outer capsid proteins elicited a robust antibody response to human rotavirus antigens, whilst the control chimeric murine rotavirus did not. This chimeric human rotavirus therefore provides a new strategy for studying human-rotavirus-specific immunity to the outer capsid, and could be used to investigate factors causing variability in rotavirus vaccine efficacy. This small animal platform therefore has the potential to test the efficacy of new vaccines and antibody-based therapeutics.

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.

Recovery of Recombinant Rotaviruses by Reverse Genetics

Rotaviruses are the primary cause of severe gastroenteritis in infants and young children throughout the world. To combat rotavirus illness, several live oral vaccines have been developed, or are under development, that are formulated from attenuated human or human-animal reassortant strains of rotavirus. While the effectiveness of these vaccines is generally high in developed countries, the same vaccines are significantly less effective in many developing countries, where the need for rotavirus vaccines is greatest. Recently, reverse genetics systems have been developed that allow modification of the segmented double-stranded (ds)RNA genome of rotavirus, including reprogramming the genome to allow expression of additional proteins that may stimulate expanded neutralizing antibody responses in vaccinated children. The use of reverse genetics systems may not only lead to the development of more potent classes of vaccines but can be used to better explore the intricacies of rotavirus molecular biology and pathogenesis. In this article, we share protocols that can be used to generate recombinant rotaviruses, including modified strains that express foreign proteins.

Genetic engineering strategy for generating a stable dsRNA virus vector using a virus-like codon-modified transgene

IMPORTANCE

The stabilities of transgenes in RNA virus vectors differ between the genes of interest, but the molecular mechanisms determining genetic stability remain unknown. This study demonstrated that the stability of a transgene was affected by the nucleotide composition, and altering the codon usage of transgenes to resemble that of the viral genome significantly increased transgene stability in double-stranded RNA virus vectors. The virus-like codon modification strategy enabled generation of stable rotavirus and mammalian orthoreovirus vectors, which could be developed as machinery for gene delivery to the intestines and/or respiratory organs. This technology has further potential to be expanded to other RNA viruses.

Engineering Non-Human RNA Viruses for Cancer Therapy

Alongside the development and progress in cancer immunotherapy, research in oncolytic viruses (OVs) continues advancing novel treatment strategies to the clinic. With almost 50 clinical trials carried out over the last decade, the opportunities for intervention using OVs are expanding beyond the old-fashioned concept of "lytic killers", with promising breakthrough therapeutic strategies focused on leveraging the immunostimulatory potential of different viral platforms. This review presents an overview of non-human-adapted RNA viruses engineered for cancer therapy. Moreover, we describe the diverse strategies employed to manipulate the genomes of these viruses to optimize their therapeutic capabilities. By focusing on different aspects of this particular group of viruses, we describe the insights into the promising advancements in the field of virotherapy and its potential to revolutionize cancer treatment.

Rotavirus Particle Disassembly and Assembly In Vivo and In Vitro

Rotaviruses (RVs) are non-enveloped multilayered dsRNA viruses that are major etiologic agents of diarrheal disease in humans and in the young in a large number of animal species. The viral particle is composed of three different protein layers that enclose the segmented dsRNA genome and the transcriptional complexes. Each layer defines a unique subparticle that is associated with a different phase of the replication cycle. Thus, while single- and double-layered particles are associated with the intracellular processes of selective packaging, genome replication, and transcription, the viral machinery necessary for entry is located in the third layer. This modular nature of its particle allows rotaviruses to control its replication cycle by the disassembly and assembly of its structural proteins. In this review, we examine the significant advances in structural, molecular, and cellular RV biology that have contributed during the last few years to illuminating the intricate details of the RV particle disassembly and assembly processes.

Rotavirus Spike Protein VP4 Mediates Viroplasm Assembly by Association to Actin Filaments

Rotavirus (RV) viroplasms are cytosolic inclusions where both virus genome replication and primary steps of virus progeny assembly take place. A stabilized microtubule cytoskeleton and lipid droplets are required for the viroplasm formation, which involves several virus proteins. The viral spike protein VP4 has not previously been shown to have a direct role in viroplasm formation. However, it is involved with virus-cell attachment, endocytic internalization, and virion morphogenesis. Moreover, VP4 interacts with actin cytoskeleton components, mainly in processes involving virus entrance and egress, and thereby may have an indirect role in viroplasm formation. In this study, we used reverse genetics to construct a recombinant RV, rRV/VP4-BAP, that contains a biotin acceptor peptide (BAP) in the K145-G150 loop of the VP4 lectin domain, permitting live monitoring. The recombinant virus was replication competent but showed a reduced fitness. We demonstrate that rRV/VP4-BAP infection, as opposed to rRV/wt infection, did not lead to a reorganized actin cytoskeleton as viroplasms formed were insensitive to drugs that depolymerize actin and inhibit myosin. Moreover, wild-type (wt) VP4, but not VP4-BAP, appeared to associate with actin filaments. Similarly, VP4 in coexpression with NSP5 and NSP2 induced a significant increase in the number of viroplasm-like structures. Interestingly, a small peptide mimicking loop K145-G150 rescued the phenotype of rRV/VP4-BAP by increasing its ability to form viroplasms and hence improve virus progeny formation. Collectively, these results provide a direct link between VP4 and the actin cytoskeleton to catalyze viroplasm assembly.

IMPORTANCE The spike protein VP4 participates in diverse steps of the rotavirus (RV) life cycle, including virus-cell attachment, internalization, modulation of endocytosis, virion morphogenesis, and virus egress. Using reverse genetics, we constructed for the first time a recombinant RV, rRV/VP4-BAP, harboring a heterologous peptide in the lectin domain (loop K145-G150) of VP4. The rRV/VP4-BAP was replication competent but with reduced fitness due to a defect in the ability to reorganize the actin cytoskeleton, which affected the efficiency of viroplasm assembly. This defect was rescued by adding a permeable small-peptide mimicking the wild-type VP4 loop K145-G150. In addition to revealing a new role of VP4, our findings suggest that rRV harboring an engineered VP4 could be used as a new dual vaccination platform providing immunity against RV and additional heterologous antigens.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

Rotavirus research: 2014-2020

Rotaviruses are major causes of acute gastroenteritis in infants and young children worldwide and also cause disease in the young of many other mammalian and of avian species. During the recent 5-6 years rotavirus research has benefitted in a major way from the establishment of plasmid only-based reverse genetics systems, the creation of human and other mammalian intestinal enteroids, and from the wide application of structural biology (cryo-electron microscopy, cryo-EM tomography) and complementary biophysical approaches. All of these have permitted to gain new insights into structure-function relationships of rotaviruses and their interactions with the host. This review follows different stages of the viral replication cycle and summarizes highlights of structure-function studies of rotavirus-encoded proteins (both structural and non-structural), molecular mechanisms of viral replication including involvement of cellular proteins and lipids, the spectrum of viral genomic and antigenic diversity, progress in understanding of innate and acquired immune responses, and further developments of prevention of rotavirus-associated disease.

Rotavirus as an Expression Platform of Domains of the SARS-CoV-2 Spike Protein

Among vaccines administered to children are those targeting rotavirus, a segmented double-stranded RNA virus that represents a major cause of severe gastroenteritis. To explore the feasibility of establishing a combined rotavirus-SARS-CoV-2 vaccine, we generated recombinant (r)SA11 rotaviruses with modified segment 7 RNAs that contained coding cassettes for NSP3, a translational 2A stop-restart signal, and a FLAG-tagged portion of the SARS-CoV-2 spike (S) protein: S1 fragment, N-terminal domain (NTD), receptor-binding domain (RBD), extended RBD (ExRBD), or S2 core (CR) domain. Generation of rSA11 containing the S1 coding sequence required a sequence insertion of 2.2 kbp, the largest such insertion yet introduced into the rotavirus genome. Immunoblotting showed that rSA11 viruses containing the smaller NTD, RBD, ExRBD, and CR coding sequences expressed S-protein products of expected size, with ExRBD expressed at highest levels. These rSA11 viruses were genetically stable during serial passage. In contrast, the rSA11 virus containing the full-length S coding sequence (rSA11/NSP3-fS1) failed to express its expected 80 kDa fS1 product, for unexplained reasons. Moreover, rSA11/NSP3-fS1 was genetically unstable, with variants lacking the S1 insertion appearing during serial passage. Nonetheless, these results emphasize the potential usefulness of rotavirus vaccines as expression vectors of immunogenic portions of the SARS-CoV-2 S protein, including NTD, RBD, ExRBD, and CR, that have sizes smaller than the S1 fragment.