Generation of recombinant rotaviruses from just 11 cDNAs encoding a viral genome

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 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.

Strategy for generation of replication-competent recombinant rotaviruses expressing multiple foreign genes

With the recent establishment of robust reverse genetics systems for rotavirus, rotavirus is being developed as a vector to express foreign genes. However, insertion of larger sequences such as those encoding multiple foreign genes into the rotavirus genome has been challenging because the virus segments are small. In this paper, we attempted to insert multiple foreign genes into a single gene segment of rotavirus to determine whether it can efficiently express multiple exogenous genes from its genome. At first, we engineered a truncated NSP1 segment platform lacking most of the NSP1 open reading frame and including a self-cleaving 2A sequence (2A), which made it possible to generate a recombinant rotavirus stably expressing NanoLuc (Nluc) luciferase as a model foreign gene. Based on this approach, we then demonstrated the generation of a replication-competent recombinant rotavirus expressing three reporter genes (Nluc, EGFP, and mCherry) by separating them with self-cleaving 2As, indicating the capacity of rotaviruses as to the insertion of multiple foreign genes. Importantly, the inserted multiple foreign genes remained genetically stable during serial passages in cell culture, indicating the potential of rotaviruses as attractive expression vectors. The strategy described here will serve as a model for the generation of rotavirus-based vectors designed for the expression and/or delivery of multiple foreign genes.

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.

Plasmid-based reverse genetics for probing phosphorylation-dependent viroplasm formation in rotaviruses

Rotavirus (RV) replication occurs in cytoplasmic compartments, known as viroplasms, that are composed of viral and cellular proteins. Viroplasm formation requires RV nonstructural proteins NSP2 and NSP5 and cellular lipid droplets (LDs); however, the mechanisms required for viroplasm assembly remain largely unknown. We previously identified two conformationally-distinct forms of NSP2 (dNSP2, vNSP2) found in RV-infected cells that interact differentially with hypo- and hyperphosphorylated NSP5, respectively, and indicate a coordinated phosphorylation-dependent mechanism regulating viroplasm assembly. We also reported that phosphorylation of dNSP2 on serine 313 by the cellular kinase CK1α triggers the localization of vNSP2 to sites of viroplasm assembly and its association with hyperphosphorylated NSP5. To directly evaluate the role of CK1α-mediated NSP2 phosphorylation on viroplasm formation, we used a recently published plasmid-based reverse genetics method to generate a recombinant rotavirus (rRV) with a phosphomimetic NSP2 mutation (rRV NSP2 S313D). The rRV NSP2 S313D virus is significantly delayed in viroplasm formation, virus replication, and interferes with wild type RV replication during co-infection. The rRV NSP2 S313A virus was not rescued. Taking advantage of the delay in viroplasm formation, the NSP2 S313D phosphomimetic mutant was used as a tool to observe very early events in viroplasm assembly. We show that (1) viroplasm assembly correlates with NSP5 hyperphosphorylation, and (2) that vNSP2 S313D co-localizes with RV-induced LDs without NSP5, suggesting that vNSP2 phospho-S313 is sufficient for interacting with LDs and may be the virus factor required for RV-induced LD formation. Further studies with the rRV NSP2 S313D virus are expected to reveal new aspects of viroplasm and LD initiation and assembly.