Silencing the morphogenesis of rotavirus

Viroporin activity is necessary for intercellular calcium signals that contribute to viral pathogenesis

Viruses engage in a variety of processes to subvert host defenses and create an environment amenable to replication. Here, using rotavirus as a prototype, we show that calcium conductance out of the endoplasmic reticulum by the virus encoded ion channel, NSP4, induces intercellular calcium waves that extend beyond the infected cell and contribute to pathogenesis. Viruses that lack the ability to induce this signaling show diminished viral shedding and attenuated disease in a mouse model of rotavirus diarrhea. This implicates nonstructural protein 4 (NSP4) as a virulence factor and provides mechanistic insight into its mode of action. Critically, this signaling induces a transcriptional signature characteristic of interferon-independent innate immune activation, which is not observed in response to a mutant NSP4 that does not conduct calcium. This implicates calcium dysregulation as a means of pathogen recognition, a theme broadly applicable to calcium-altering pathogens beyond rotavirus.

Rapid production of recombinant rotaviruses by overexpression of NSP2 and NSP5 genes with modified nucleotide sequences

Reverse genetics systems for rotaviruses (RV) facilitate the generation of genetically engineered RVs by transfection of 11 plasmids encoding 11 genomic viral RNA segments. In addition to viral genome expression, overexpression of NSP2 and NSP5 has been used to increase the rescue efficiency of recombinant RVs. Here, we showed that the overexpression of nucleotide sequence-modified NSP2 and NSP5 enabled the rapid and efficient production of recombinant RVs. Using improved reverse genetics, we established a reverse genetics system for human and bovine RV clinical isolates, as well as laboratory strains of bovine RV (NCDV and UK) and porcine RV (Gottfried). In addition, we rescued low-replicating recombinant RVs carrying a mutant NSP4 lacking the double-layered particle-binding domain, which was deficient in the efficient production of mature virions. These advancements in reverse genetics enabled the generation of molecular clones of RV clinical isolates and recombinant RVs harboring critical amino acid mutations, offering a versatile platform for investigating RV biology and pathogenesis.IMPORTANCERecombinant rotavirus (RV) synthesis via reverse genetics relies on both the viral propagation capacity and the efficiency of the experimental system. Since the establishment of our reverse genetics system, several enhancements have been implemented to augment the rescue efficiency. Nevertheless, challenges persist in generating RV clinical strains and recombinant viruses with low replication capacities. Notably, this improved reverse genetics system successfully facilitated the establishment of molecular clones of human and bovine RV clinical isolates. Fecal samples from patients with RV typically harbor quasi-species or, occasionally, multiple genotypes of RV. In the present study, we performed the genetic sequencing of clinical viral strains during the early propagation stages in cultured cells. Subsequently, infectious viruses were synthesized, allowing the characterization of circulating viruses in nature. This approach provides valuable insights into the genetic diversity and dynamics of RV populations and contributes to a more comprehensive understanding of viral pathogenesis and evolution.

Host and structure-specific codon usage of G genotype (VP7) among group A rotaviruses

Rotavirus A (RVA) infects a relatively wide host range. Studying the evolutionary dynamics of viral genomes and the evolution of host adaptations can inform the development of epidemiological models of disease transmission. Moreover, comprehending the adaptive evolution of viruses in the host could provide insights into how viruses promote evolutionary advantages on a larger scale at host level. This study aims to determine whether host specificity in codon usage existed. We used the Clustal W function within MEGA X software to perform sequence alignment, followed by construction of a phylogenetic tree based on the maximum-likelihood method. Additionally, Codon W software and EMBOSS were utilized for analysis of codon usage bias index. We analyzed codon usage bias (CUB) of host-specific G genotype VP7 to elucidate the molecular-dynamic evolutionary pattern and reveal the adaptive evolution of VP7 at the host level. The CUB of RV VP7 exhibits significant difference between human and other species. This bias can be primarily attributed to natural selection. In addition, the β-barrel structural domain, which plays a crucial role in viral transmembrane entry into cells, demonstrates a stronger CUB. Our results provide novel insights into the evolutionary dynamics of RVs, cross-species transmission, and virus-host adaptation.

Viroporin activity from rotavirus nonstructural protein 4 induces intercellular calcium waves that contribute to pathogenesis

Acute gastroenteritis remains the second leading cause of death among children under the age of 5 worldwide. While enteric viruses are the most common etiology, the drivers of their virulence remain incompletely understood. We recently found that cells infected with rotavirus, the most prevalent enteric virus in infants and young children, initiate hundreds of intercellular calcium waves that enhance both fluid secretion and viral spread. Understanding how rotavirus triggers intercellular calcium waves may allow us to design safer, more effective vaccines and therapeutics, but we still lack a mechanistic understanding of this process. In this study, we used existing virulent and attenuated rotavirus strains, as well as reverse engineered recombinants, to investigate the role of rotavirus nonstructural protein 4 (NSP4) in intercellular calcium wave induction using in vitro , organoid, and in vivo model systems. We found that the capacity to induce purinergic intercellular calcium waves (ICWs) segregated with NSP4 in both simian and murine-like rotavirus backgrounds, and NSP4 expression alone was sufficient to induce ICWs. NSP4's ability to function as a viroporin, which conducts calcium out of the endoplasmic reticulum, was necessary for ICW induction. Furthermore, viroporin activity and the resulting ICWs drove transcriptional changes indicative of innate immune activation, which were lost upon attenuation of viroporin function. Multiple aspects of RV disease severity in vivo correlated with the generation of ICWs, identifying a critical link between viroporin function, intercellular calcium waves, and enteric viral virulence.

The Role of the Host Cytoskeleton in the Formation and Dynamics of Rotavirus Viroplasms

Rotavirus (RV) replicates within viroplasms, membraneless electron-dense globular cytosolic inclusions with liquid-liquid phase properties. In these structures occur the virus transcription, replication, and packaging of the virus genome in newly assembled double-layered particles. The viroplasms are composed of virus proteins (NSP2, NSP5, NSP4, VP1, VP2, VP3, and VP6), single- and double-stranded virus RNAs, and host components such as microtubules, perilipin-1, and chaperonins. The formation, coalescence, maintenance, and perinuclear localization of viroplasms rely on their association with the cytoskeleton. A stabilized microtubule network involving microtubules and kinesin Eg5 and dynein molecular motors is associated with NSP5, NSP2, and VP2, facilitating dynamic processes such as viroplasm coalescence and perinuclear localization. Key post-translation modifications, particularly phosphorylation events of RV proteins NSP5 and NSP2, play pivotal roles in orchestrating these interactions. Actin filaments also contribute, triggering the formation of the viroplasms through the association of soluble cytosolic VP4 with actin and the molecular motor myosin. This review explores the evolving understanding of RV replication, emphasizing the host requirements essential for viroplasm formation and highlighting their dynamic interplay within the host cell.

N-Glycosylation of Rotavirus NSP4 Protein Affects Viral Replication and Pathogenesis

Rotavirus (RV), the most common cause of gastroenteritis in children, carries a high economic and health burden worldwide. RV encodes six structural proteins and six nonstructural proteins (NSPs) that play different roles in viral replication. NSP4, a multifunctional protein involved in various viral replication processes, has two conserved N-glycosylation sites; however, the role of glycans remains elusive. Here, we used recombinant viruses generated by a reverse genetics system to determine the role of NSP4 N-glycosylation during viral replication and pathogenesis. The growth rate of recombinant viruses that lost one glycosylation site was as high as that of the wild-type virus. However, a recombinant virus that lost both glycosylation sites (glycosylation-defective virus) showed attenuated replication in cultured cell lines. Specifically, replications of glycosylation-defective virus in MA104 and HT29 cells were 10- and 100,000-fold lower, respectively, than that of the wild-type, suggesting that N-glycosylation of NSP4 plays a critical role in RV replication. The glycosylation-defective virus showed NSP4 mislocalization, delay of cytosolic Ca²⁺ elevation, and less viroplasm formation in MA104 cells; however, these impairments were not observed in HT29 cells. Further analysis revealed that assembly of glycosylation-defective virus was severely impaired in HT29 cells but not in MA104 cells, suggesting that RV replication mechanism is highly cell type dependent. In vivo mouse experiments also showed that the glycosylation-defective virus was less pathogenic than the wild-type virus. Taken together, the data suggest that N-glycosylation of NSP4 plays a vital role in viral replication and pathogenicity. IMPORTANCE Rotavirus is the main cause of gastroenteritis in young children and infants worldwide, contributing to 128,500 deaths each year. Here, we used a reverse genetics approach to examine the role of NSP4 N-glycosylation. An N-glycosylation-defective virus showed attenuated and cell-type-dependent replication in vitro. In addition, mice infected with the N-glycosylation-defective virus had less severe diarrhea than mice infected with the wild type. These results suggest that N-glycosylation affects viral replication and pathogenesis. Considering the reduced pathogenicity in vivo and the high propagation rate in MA104 cells, this glycosylation-defective virus could be an ideal live attenuated vaccine candidate.

Role of NS2 specific RNA binding and phosphorylation in liquid-liquid phase separation and virus assembly

Liquid-liquid phase separation (LLPS) has assumed a prominent role in biological cell systems, where it underpins the formation of subcellular compartments necessary for cell function. We investigated the underlying mechanism of LLPS in virus infected cells, where virus inclusion bodies are formed by an RNA-binding phosphoprotein (NS2) of Bluetongue virus to serve as sites for subviral particle assembly and virus maturation. We show that NS2 undergoes LLPS that is dependent on protein phosphorylation and RNA-binding and that LLPS occurrence is accompanied by a change in protein secondary structure. Site-directed mutagenesis identified two critical arginine residues in NS2 responsible for specific RNA binding and thus for NS2-RNA complex driven LLPS. Reverse genetics identified the same residues as essential for VIB assembly in infected cells and virus viability. Our findings suggest that a specific arginine-RNA interaction in the context of a phosphorylated state drives LLPS in this, and possibly other, virus infections.

Mature Rotavirus Particles Contain Equivalent Amounts of 7meGpppG-Capped and Noncapped Viral Positive-Sense RNAs

Viruses have evolved different strategies to overcome their recognition by the host innate immune system. The addition of caps at their 5' RNA ends is an efficient mechanism not only to ensure escape from detection by the innate immune system but also to ensure the efficient synthesis of viral proteins. Rotavirus mRNAs contain a type 1 cap structure at their 5' end that is added by the viral capping enzyme VP3, which is a multifunctional protein with all the enzymatic activities necessary to add the cap and also functions as an antagonist of the 2'-5'-oligoadenylate synthetase (OAS)/RNase L pathway. Here, the relative abundances of capped and noncapped viral RNAs during the replication cycle of rotavirus were determined. We found that both classes of rotaviral plus-sense RNAs (+RNAs) were encapsidated and that they were present in a 1:1 ratio in the mature infectious particles. The capping of viral +RNAs was dynamic, since different ratios of capped and noncapped RNAs were detected at different times postinfection. Similarly, when the relative amounts of capped and uncapped viral +RNAs produced in an in vitro transcription system were determined, we found that the proportions were very similar to those in the mature viral particles and in infected cells, suggesting that the capping efficiency of VP3, both in vivo and in vitro, might be close to 50%. Unexpectedly, when the effect of simultaneously knocking down the expression of VP3 and RNase L on the cap status of viral +RNAs was evaluated, we found that, even though at late times postinfection there was an increased proportion of capped viral RNAs in infected cells, the viral particles isolated from this condition contained equal ratios of capped and noncapped viral RNA, suggesting that there might be selective packaging of capped and noncapped RNAs. IMPORTANCE Rotaviruses have a genome composed of 11 segments of double-stranded RNA. Whether all 5' ends of the positive-sense genomic RNAs contained in the mature viral particles are modified by a cap structure is unknown. In this work, we characterized the relative proportions of capped and noncapped viral RNAs in rotavirus-infected cells and in viral particles by using a direct quantitative assay. We found that, independent of the relative proportions of capped/noncapped RNAs present in rotavirus-infected cells, there were similar proportions of these two kinds of 5'-modified positive-sense RNAs in the viral particles.

Functions of Viroporins in the Viral Life Cycle and Their Regulation of Host Cell Responses

Viroporins are virally encoded transmembrane proteins that are essential for viral pathogenicity and can participate in various stages of the viral life cycle, thereby promoting viral proliferation. Viroporins have multifaceted effects on host cell biological functions, including altering cell membrane permeability, triggering inflammasome formation, inducing apoptosis and autophagy, and evading immune responses, thereby ensuring that the virus completes its life cycle. Viroporins are also virulence factors, and their complete or partial deletion often reduces virion release and reduces viral pathogenicity, highlighting the important role of these proteins in the viral life cycle. Thus, viroporins represent a common drug-protein target for inhibiting drugs and the development of antiviral therapies. This article reviews current studies on the functions of viroporins in the viral life cycle and their regulation of host cell responses, with the aim of improving the understanding of this growing family of viral proteins.

Evaluation of a bivalent recombinant vaccine candidate targeting norovirus and rotavirus: Antibodies to rotavirus NSP4 exert antidiarrheal effects without virus neutralization

We previously found that when tandemly expressed with S_R69A -VP8*, nonstructural protein 4 (NSP4) of the rotavirus Wa strain exerts a minor effect on elevating the antibody responses targeting the rotavirus antigen VP8* of the 60-valent nanoparticle S_R69A -VP8* but could fully protect mice from diarrhea induced by the rotavirus strain Wa. In this study, we chose comparably less immunogenic norovirus 24-valent P particles with homogenous (i.e., VP8* from rotavirus) and heterogeneous (i.e., protruding domain of norovirus) antigens and in more challenging rotavirus SA11 strain-induced diarrhea mouse models to evaluate its main role in recombinant gastroenteritis virus-specific vaccines. The results showed that although as an adjuvant NSP4 exerted limited effects on the elevation of norovirus-specific or VP8*-specific neutralizing antibody production, as an antigen it could confer potent protection, particularly when synergized with VP8*, in rotavirus SA11 strain-induced diarrhea mouse models, possibly blocking the invasion of the intestinal wall by enterotoxin. NSP4 may be unnecessary for other recombinant vaccines as adjuvants, and its display mode should be evaluated specifically to avoid blocking coexpressed antigens in the norovirus P particles. This article is protected by copyright. All rights reserved.

Treading a HOSTile path: Mapping the dynamic landscape of host cell-rotavirus interactions to explore novel host-directed curative dimensions

Viruses are intracellular pathogens and are dependent on host cellular resources to carry out their cycles of perpetuation. Obtaining an integrative view of host-virus interaction is of utmost importance to understand the complex and dynamic interplay between viral components and host machineries. Besides its obvious scholarly significance, a comprehensive host-virus interaction profile also provides a platform where from host determinants of pro-viral and antiviral importance can be identified and further be subjected to therapeutic intervention. Therefore, adjunct to conventional methods of prophylactic vaccination and virus-directed antivirals, this host-targeted antiviral approach holds promising therapeutic potential. In this review, we present a comprehensive landscape of host cellular reprogramming in response to infection with rotavirus (RV) which causes profuse watery diarrhea in neonates and infants. In addition, an emphasis is given on how host determinants are either usurped or subverted by RV in course of infection and how therapeutic manipulation of specific host factors can effectively modulate the RV life cycle.

CRISPR-Csy4-Mediated Editing of Rotavirus Double-Stranded RNA Genome

CRISPR-nucleases have been widely applied for editing cellular and viral genomes, but nuclease-mediated genome editing of double-stranded RNA (dsRNA) viruses has not yet been reported. Here, by engineering CRISPR-Csy4 nuclease to localize to rotavirus viral factories, we achieve the nuclease-mediated genome editing of rotavirus, an important human and livestock pathogen with a multisegmented dsRNA genome. Rotavirus replication intermediates cleaved by Csy4 is edited through the formation of precise deletions in the targeted genome segments in a single replication cycle. Using CRISPR-Csy4-mediated editing of rotavirus genome, we label the products of rotavirus secondary transcription made by newly assembled viral particles during rotavirus replication, demonstrating that this step largely contributes to the overall production of viral proteins. We anticipate that the nuclease-mediated cleavage of dsRNA virus genomes will promote an advanced level of understanding of viral replication and host-pathogen interactions, also offering opportunities to develop therapeutics.

Rotavirus viroplasm biogenesis involves microtubule-based dynein transport mediated by an interaction between NSP2 and dynein intermediate chain

Rotaviruses are the causative agents of severe and dehydrating gastroenteritis in children, piglets, and many other young animals. They replicate their genomes and assemble double-layered particles in cytoplasmic electron-dense inclusion bodies called 'viroplasms'. The formation of viroplasms is reportedly associated with the stability of microtubules. Although material transport is an important function of microtubules, whether and how microtubule-based transport influences the formation of viroplasms is still unclear. Here, we demonstrate that the small viroplasms move and fuse in living cells. We show that microtubule-based dynein transport affects rotavirus infection, viroplasm formation, and the assembly of transient enveloped particles (TEPs) and triple-layered particles (TLPs). The dynein intermediate chain (DIC) is shown to localize in the viroplasm and to interact directly with non-structural protein 2 (NSP2), indicating that DIC is responsible for connecting the viroplasm to dynein. The WD40 repeat domain of DIC regulates the interaction between DIC and NSP2, and the knockdown of DIC inhibited rotaviral infection, viroplasm formation, and the assembly of TEPs and TLPs. Our findings show that rotavirus viroplasms hijack dynein transport for fusion events, required for maximal assembly of infectious viral progeny. This study provides novel insights into the intracellular transport of viroplasms, which is involved in their biogenesis. Importance Because the viroplasm is the viral factory for rotavirus replication, viroplasm formation undoubtedly determines the effective production of progeny rotavirus. Therefore, understanding the virus-host interactions involved in the biogenesis of the viroplasm is critical for the future development of prophylactic and therapeutic strategies. Previous studies have reported that the formation of viroplasms is associated with the stability of microtubules, whereas little is known about its specific mechanism. Here, we demonstrate that rotavirus viroplasm formation takes advantage of microtubule-based dynein transport mediated by an interaction between NSP2 and DIC. These findings provide new insight into the intracellular transport of viroplasms.

Development and characterization of a plant-derived rotavirus-like particle vaccine

BACKGROUND

Virus-like particles (VLPs) are unable to replicate in the recipient but stimulate the immune system through recognition of repetitive subunits. Parenterally delivered rotavirus-VLP (Ro-VLP) vaccine could have the potential to overcome the weaknesses of licensed oral live-attenuated rotavirus vaccines, namely, low efficacy in low-income and high mortality settings and a potential risk of intussusception.

METHODS

A monovalent Ro-VLP composed of viral protein (VP) 7, VP6 and VP2 of G1 genotype specificity was produced in Nicotiana benthamiana using Agrobacterium tumefaciens infiltration-based transient recombinant expression system. Plants expressing recombinant G1 Ro-VLP were harvested, then the resultant biomass was processed through a series of clarification and purification steps including standard extraction, filtration, ultrafiltration and chromatography. The purified G1 Ro-VLP was subsequently examined for its immunogenicity and toxicological profile using animal models.

RESULTS

G1 Ro-VLP had a purity of ≥90% and was structurally similar to triple-layered rotavirus particles as determined by cryogenic transmission electron microscopy. Two doses of aluminum hydroxide-adjuvanted G1 Ro-VLP (1 μg, 5 μg or 30 μg), administered intramuscularly, elicited a robust homotypic neutralizing antibody response in rats. Also, rabbits administered G1 Ro-VLP (10 μg or 30 μg) four times intramuscularly with aluminum hydroxide adjuvant did not show any significant toxicity.

CONCLUSIONS

Plant-derived Ro-VLP composed of VP7, VP6 and VP2 structural proteins would be a plausible alternative to live-attenuated oral rotavirus vaccines currently distributed worldwide.

Advances in the Development of Antiviral Compounds for Rotavirus Infections

Group A rotaviruses (RVAs) are the major cause of severe acute gastroenteritis (AGE) in children under 5 years of age, annually resulting in nearly 130,000 deaths worldwide. Social conditions in developing countries that contribute to decreased oral rehydration and vaccine efficacy and the lack of approved antiviral drugs position RVA as a global health concern. In this minireview, we present an update in the field of antiviral compounds, mainly in relation to the latest findings in RVA virion structure and the viral replication cycle. In turn, we attempt to provide a perspective on the possible treatments for RVA-associated AGE, with special focus on novel approaches, such as those representing broad-spectrum therapeutic options. In this context, the modulation of host factors, lipid droplets, and the viral polymerase, which is highly conserved among AGE-causing viruses, are analyzed as possible drug targets.

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.

Rotaviruses Associate with Distinct Types of Extracellular Vesicles

Rotaviruses are the leading cause of viral gastroenteritis among children under five years of age. Rotavirus cell entry has been extensively studied; however, rotavirus cell release is still poorly understood. Specifically, the mechanism by which rotaviruses leave the cell before cell lysis is not known. Previous works have found rotavirus proteins and viral particles associated with extracellular vesicles secreted by cells. These vesicles have been shown to contain markers of exosomes; however, in a recent work they presented characteristics more typical of microparticles, and they were associated with an increase in the infectivity of the virus. In this work, we purified different types of vesicles from rotavirus-infected cells. We analyzed the association of virus with these vesicles and their possible role in promotion of rotavirus infection. We confirmed a non-lytic rotavirus release from the two cell lines tested, and observed a notable stimulation of vesicle secretion following rotavirus infection. A fraction of the secreted viral particles present in the cell supernatant was protected from protease treatment, possibly through its association with membranous vesicles; the more pronounced association of the virus was with fractions corresponding to cell membrane generated microvesicles. Using electron microscopy, we found different size vesicles with particles resembling rotaviruses associated from both- the outside and the inside. The viral particles inside the vesicles were refractory to neutralization with a potent rotavirus neutralizing monoclonal antibody, and were able to infect cells even without trypsin activation. The association of rotavirus particles with extracellular vesicles suggests these might have a role in virus spread.

A Genetically Engineered Rotavirus NSP2 Phosphorylation Mutant Impaired in Viroplasm Formation and Replication Shows an Early Interaction between vNSP2 and Cellular Lipid Droplets

Many RNA viruses replicate in cytoplasmic compartments (virus factories or viroplasms) composed of viral and cellular proteins, but the mechanisms required for their formation remain largely unknown. Rotavirus (RV) replication in viroplasms requires interactions between virus nonstructural proteins NSP2 and NSP5, which are associated with components of lipid droplets (LDs). We previously identified two forms of NSP2 in RV-infected cells, a cytoplasmically dispersed form (dNSP2) and a viroplasm-specific form (vNSP2), which interact with hypophosphorylated and hyperphosphorylated NSP5, respectively, indicating that a coordinated phosphorylation cascade controls viroplasm assembly. The cellular kinase CK1α phosphorylates NSP2 on serine 313, triggering the localization of vNSP2 to sites of viroplasm assembly and its association with hyperphosphorylated NSP5. Using reverse genetics, we generated a rotavirus with a phosphomimetic NSP2 (S313D) mutation to directly evaluate the role of CK1α NSP2 phosphorylation in viroplasm formation. Recombinant rotavirus NSP2 S313D (rRV NSP2 S313D) is significantly delayed in viroplasm formation and in virus replication and interferes with wild-type RV replication in coinfection. Taking advantage of the delay in viroplasm formation, the NSP2 phosphomimetic mutant was used as a tool to observe very early events in viroplasm assembly. We show that (i) viroplasm assembly correlates with NSP5 hyperphosphorylation and (ii) vNSP2 S313D colocalizes 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.IMPORTANCE Reverse genetics was used to generate a recombinant rotavirus with a single phosphomimetic mutation in nonstructural protein 2 (NSP2 S313D) that exhibits delayed viroplasm formation, delayed replication, and an interfering phenotype during coinfection with wild-type rotavirus, indicating the importance of this amino acid during virus replication. Exploiting the delay in viroplasm assembly, we found that viroplasm-associated NSP2 colocalizes with rotavirus-induced lipid droplets prior to the accumulation of other rotavirus proteins that are required for viroplasm formation and that NSP5 hyperphosphorylation is required for viroplasm assembly. These data suggest that NSP2 phospho-S313 is sufficient for interaction with lipid droplets and may be the virus factor that induces lipid droplet biogenesis in rotavirus-infected cells. Lipid droplets are cellular organelles critical for the replication of many viral and bacterial pathogens, and thus, understanding the mechanism of NSP2-mediated viroplasm/lipid droplet initiation and interaction will lead to new insights into this important host-pathogen interaction.

COPII Vesicle Transport Is Required for Rotavirus NSP4 Interaction with the Autophagy Protein LC3 II and Trafficking to Viroplasms

Many viruses that replicate in the cytoplasm dramatically remodel and stimulate the accumulation of host cell membranes for efficient replication by poorly understood mechanisms. For rotavirus, a critical step in virion assembly requires the accumulation of membranes adjacent to virus replication centers called viroplasms. Early electron microscopy studies describe viroplasm-associated membranes as "swollen" endoplasmic reticulum (ER). We previously demonstrated that rotavirus infection initiates cellular autophagy and that membranes containing the autophagy marker protein LC3 and the rotavirus ER-synthesized transmembrane glycoprotein NSP4 traffic to viroplasms, suggesting that NSP4 must exit the ER. This study aimed to address the mechanism of NSP4 exit from the ER and determine whether the viroplasm-associated membranes are ER derived. We report that (i) NSP4 exits the ER in COPII vesicles, resulting in disrupted COPII vesicle transport and ER exit sites; (ii) COPII vesicles are hijacked by LC3 II, which interacts with NSP4; and (iii) NSP4/LC3 II-containing membranes accumulate adjacent to viroplasms. In addition, the ER transmembrane proteins SERCA and calnexin were not detected in viroplasm-associated membranes, providing evidence that the rotavirus maturation process of "budding" occurs through autophagy-hijacked COPII vesicle membranes. These findings reveal a new mechanism for rotavirus maturation dependent on intracellular host protein transport and autophagy for the accumulation of membranes required for virus replication.IMPORTANCE In a morphogenic step that is exceedingly rare for nonenveloped viruses, immature rotavirus particles assemble in replication centers called viroplasms, and bud through cytoplasmic cellular membranes to acquire the outer capsid proteins for infectious particle assembly. Historically, the intracellular membranes used for particle budding were thought to be endoplasmic reticulum (ER) because the rotavirus nonstructural protein NSP4, which interacts with the immature particles to trigger budding, is synthesized as an ER transmembrane protein. This present study shows that NSP4 exits the ER in COPII vesicles and that the NSP4-containing COPII vesicles are hijacked by the cellular autophagy machinery, which mediates the trafficking of NSP4 to viroplasms. Changing the paradigm for rotavirus maturation, we propose that the cellular membranes required for immature rotavirus particle budding are not an extension of the ER but are COPII-derived autophagy isolation membranes.

The Guanine Nucleotide Exchange Factor GBF1 Participates in Rotavirus Replication

Cellular and viral factors participate in the replication cycle of rotavirus. We report that the guanine nucleotide exchange factor GBF1, which activates the small GTPase Arf1 to induce COPI transport processes, is required for rotavirus replication since knocking down GBF1 expression by RNA interference or inhibiting its activity by treatment with brefeldin A (BFA) or Golgicide A (GCA) significantly reduces the yield of infectious viral progeny. This reduction in virus yield was related to a block in virus assembly, since in the presence of either BFA or GCA, the assembly of infectious mature triple-layered virions was significantly prevented and only double-layered particles were detected. We report that the catalytic activity of GBF1, but not the activation of Arf1, is essential for the assembly of the outer capsid of rotavirus. We show that both BFA and GCA, as well as interfering with the synthesis of GBF1, alter the electrophoretic mobility of glycoproteins VP7 and NSP4 and block the trimerization of the virus surface protein VP7, a step required for its incorporation into virus particles. Although a posttranslational modification of VP7 (other than glycosylation) could be related to the lack of trimerization, we found that NSP4 might also be involved in this process, since knocking down its expression reduces VP7 trimerization. In support, recombinant VP7 protein overexpressed in transfected cells formed trimers only when cotransfected with NSP4.IMPORTANCE Rotavirus, a member of the family Reoviridae, is the major cause of severe diarrhea in children and young animals worldwide. Despite significant advances in the characterization of the biology of this virus, the mechanisms involved in morphogenesis of the virus particle are still poorly understood. In this work, we show that the guanine nucleotide exchange factor GBF1, relevant for COPI/Arf1-mediated cellular vesicular transport, participates in the replication cycle of the virus, influencing the correct processing of viral glycoproteins VP7 and NSP4 and the assembly of the virus surface proteins VP7 and VP4.

Nanoscale organization of rotavirus replication machineries

Rotavirus genome replication and assembly take place in cytoplasmic electron dense inclusions termed viroplasms (VPs). Previous conventional optical microscopy studies observing the intracellular distribution of rotavirus proteins and their organization in VPs have lacked molecular-scale spatial resolution, due to inherent spatial resolution constraints. In this work we employed super-resolution microscopy to reveal the nanometric-scale organization of VPs formed during rotavirus infection, and quantitatively describe the structural organization of seven viral proteins within and around the VPs. The observed viral components are spatially organized as five concentric layers, in which NSP5 localizes at the center of the VPs, surrounded by a layer of NSP2 and NSP4 proteins, followed by an intermediate zone comprised of the VP1, VP2, VP6. In the outermost zone, we observed a ring of VP4 and finally a layer of VP7. These findings show that rotavirus VPs are highly organized organelles.

Rotavirus RNAs sponge host cell RNA binding proteins and interfere with their subcellular localization

Cellular mRNAs cycle between translating and non-translating pools, polysomes compose the translating pool, while RNA granules contain translationally-silenced mRNAs, where the RNAs are either stored in stress granules, or accumulate in processing bodies (PBs) or GW-bodies, which have an important role in RNA degradation. Viruses have developed measures to prevent the deleterious effects of these structures during their replication. Rotavirus, the most common agent of viral gastroenteritis, is capable of establishing a successful infection by counteracting several of the antiviral responses of its host. Here, we describe that in rotavirus-infected cells the distribution of several RNA binding proteins is changed causing the disaggregation of PBs, the relocalization of GW-body proteins, and the cytoplasmic accumulation of HuR, a predominantly nuclear protein. We show that this redistribution of proteins is more likely caused by the accumulation of viral RNA in the cytoplasm of infected-cells, where it might be acting as an RBP sponge.

Phylogenetic analysis of human G9P[8] rotavirus strains circulating in Jiangsu, China between 2010 and 2016

Rotavirus A (RVA) is the leading cause of acute viral gastroenteritis in children under 5 years of age worldwide. G9P[8] is a common RVA genotype that has been persistently prevalent in Jiangsu, China. To determine the genetic diversity of G9P[8] RVAs, 7 representative G9P[8] strains collected from Suzhou Children's Hospital between 2010 and 2016 (named JS2010-JS2016) were analyzed through whole-genome sequencing. All evaluated strains showed the Wa-like constellation G9-P[8]-I1-R1-C1-M1-A1-N1-T1-E1-H1. Furthermore, phylogenetic analysis revealed that the VP7 genes of all strains clustered into lineage G9-III and G9-VI. With the exception of strain JS2012 (P[8]-4), the VP4 sequences of all strains belonged to the P[8]-3 lineage. Sequencing further revealed that amino acid substitutions were present in the antigenic regions of the VP7 and VP4 genes of all strains. Moreover, there were multiple substitutions in antigenic sites I and II of the nonstructural protein 4 (NSP4) genes, whereas the other NSP genes were relatively conserved. In conclusion, our phylogenetic analysis of these 7 G9P[8] strains suggests that RVA varied across regions and time. Therefore, our findings suggest that continued surveillance is necessary to explore the molecular evolutionary characteristics of RVA for better prevention and treatment of acute viral gastroenteritis.

Actin-Dependent Nonlytic Rotavirus Exit and Infectious Virus Morphogenetic Pathway in Nonpolarized Cells

During the late stages of rotavirus morphogenesis, the surface proteins VP4 and VP7 are assembled onto the previously structured double-layered virus particles to yield a triple-layered, mature infectious virus. The current model for the assembly of the outer capsid is that it occurs within the lumen of the endoplasmic reticulum. However, it has been shown that VP4 and infectious virus associate with lipid rafts, suggesting that the final assembly of the rotavirus spike protein VP4 involves a post-endoplasmic reticulum event. In this work, we found that the actin inhibitor jasplakinolide blocks the cell egress of rotavirus from nonpolarized MA104 cells at early times of infection, when there is still no evidence of cell lysis. These findings contrast with the traditional assumption that rotavirus is released from nonpolarized cells by a nonspecific mechanism when the cell integrity is lost. Inspection of the virus present in the extracellular medium by use of density flotation gradients revealed that a fraction of the released virus is associated with low-density membranous structures. Furthermore, the intracellular localization of VP4, its interaction with lipid rafts, and its targeting to the cell surface were shown to be prevented by jasplakinolide, implying a role for actin in these processes. Finally, the VP4 present at the plasma membrane was shown to be incorporated into the extracellular infectious virus, suggesting the existence of a novel pathway for the assembly of the rotavirus spike protein.IMPORTANCE Rotavirus is a major etiological agent of infantile acute severe diarrhea. It is a nonenveloped virus formed by three concentric layers of protein. The early stages of rotavirus replication, including cell attachment and entry, synthesis and translation of viral mRNAs, replication of the genomic double-stranded RNA (dsRNA), and the assembly of double-layered viral particles, have been studied widely. However, the mechanisms involved in the later stages of infection, i.e., viral particle maturation and cell exit, are less well characterized. It has been assumed historically that rotavirus exits nonpolarized cells following cell lysis. In this work, we show that the virus exits cells by a nonlytic, actin-dependent mechanism, and most importantly, we describe that VP4, the spike protein of the virus, is present on the cell surface and is incorporated into mature, infectious virus, indicating a novel pathway for the assembly of this protein.

Hepatitis C Virus Lipoviroparticles Assemble in the Endoplasmic Reticulum (ER) and Bud off from the ER to the Golgi Compartment in COPII Vesicles

Hepatitis C virus (HCV) exists as a lipoprotein-virus hybrid lipoviroparticle (LVP). In vitro studies have demonstrated the importance of apolipoproteins in HCV secretion and infectivity, leading to the notion that HCV coopts the secretion of very-low-density lipoprotein (VLDL) for its egress. However, the mechanisms involved in virus particle assembly and egress are still elusive. The biogenesis of VLDL particles occurs in the endoplasmic reticulum (ER), followed by subsequent lipidation in the ER and Golgi compartment. The secretion of mature VLDL particles occurs through the Golgi secretory pathway. HCV virions are believed to latch onto or fuse with the nascent VLDL particle in either the ER or the Golgi compartment, resulting in the generation of LVPs. In our attempt to unravel the collaboration between HCV and VLDL secretion, we studied HCV particles budding from the ER en route to the Golgi compartment in COPII vesicles. Biophysical characterization of COPII vesicles fractionated on an iodixanol gradient revealed that HCV RNA is enriched in the highly buoyant COPII vesicle fractions and cofractionates with apolipoprotein B (ApoB), ApoE, and the HCV core and envelope proteins. Electron microscopy of immunogold-labeled microsections revealed that the HCV envelope and core proteins colocalize with apolipoproteins and HCV RNA in Sec31-coated COPII vesicles. Ultrastructural analysis also revealed the presence of HCV structural proteins, RNA, and apolipoproteins in the Golgi stacks. These findings support the hypothesis that HCV LVPs assemble in the ER and are transported to the Golgi compartment in COPII vesicles to embark on the Golgi secretory route.IMPORTANCE HCV assembly and release accompany the formation of LVPs that circulate in the sera of HCV patients and are also produced in an in vitro culture system. The pathway of HCV morphogenesis and secretion has not been fully understood. This study investigates the exact site where the association of HCV virions with host lipoproteins occurs. Using immunoprecipitation of COPII vesicles and immunogold electron microscopy (EM), we characterize the existence of LVPs that cofractionate with lipoproteins, viral proteins, RNA, and vesicular components. Our results show that this assembly occurs in the ER, and LVPs thus formed are carried through the Golgi network by vesicular transport. This work provides a unique insight into the HCV LVP assembly process within infected cells and offers opportunities for designing antiviral therapeutic cellular targets.

Analysis of NSP4 Gene and Its Association with Genotyping of Rotavirus Group A in Stool Samples

Background

Non-structural protein 4 (NSP4) is a critical protein for rotavirus (RV) replication and assembly. This protein has multiple domains and motifs that predispose its function and activity. NSP4 has a sequence divergence in human and animal RVs. Recently, 14 genotypes (E1-E14) of NSP4 have been identified, and E1 and E2 have been shown to be the most common genotypes in human.

Methods

The gene and protein sequence of NSP4 in RV-positive samples were inspected with the aim of NSP4 genotyping and variation analysis in viroporin and other domains. P and G typings of RV samples were carried out by WHO primers using a semi-multiplex PCR method. Non-typeable RV samples were amplified by conserved primers and sequenced.

Results

In viroporin and enterotoxin, conserved sequence was detected, and amino acids substitution with the same biochemical properties was found.

Conclusion

Association of NSP4 genotype with P or G genotyping G1/G9 correlates with E1 genogroups. In electrophoretyping of RV, E2 genotype had a short pattern when compared to E1.

Rotavirus replication is correlated with S/G2 interphase arrest of the host cell cycle

In infected cells rotavirus (RV) replicates in viroplasms, cytosolic structures that require a stabilized microtubule (MT) network for their assembly, maintenance of the structure and perinuclear localization. Therefore, we hypothesized that RV could interfere with the MT-breakdown that takes place in mitosis during cell division. Using synchronized RV-permissive cells, we show that RV infection arrests the cell cycle in S/G2 phase, thus favoring replication by improving viroplasms formation, viral protein translation, and viral assembly. The arrest in S/G2 phase is independent of the host or viral strain and relies on active RV replication. RV infection causes cyclin B1 down-regulation, consistent with blocking entry into mitosis. With the aid of chemical inhibitors, the cytoskeleton network was linked to specific signaling pathways of the RV-induced cell cycle arrest. We found that upon RV infection Eg5 kinesin was delocalized from the pericentriolar region to the viroplasms. We used a MA104-Fucci system to identify three RV proteins (NSP3, NSP5, and VP2) involved in cell cycle arrest in the S-phase. Our data indicate that there is a strong correlation between the cell cycle arrest and RV replication.

Transfer of Anti-Rotavirus Antibodies during Pregnancy and in Milk Following Maternal Vaccination with a Herpes Simplex Virus Type-1 Amplicon Vector

Rotaviruses (RVs) are important enteric pathogens of newborn humans and animals, causing diarrhea and in rare cases death, especially in very young individuals. Rotavirus vaccines presently used are modified live vaccines that lack complete biological safety. Previous work from our laboratory suggested that vaccines based on in situ produced, non-infectious rotavirus-like particles (RVLPs) are efficient while being entirely safe. However, using either vaccine, active mucosal immunization cannot induce protective immunity in newborns due to their immature immune system. We therefore hypothesized that offspring from vaccinated dams are passively immunized either by transfer of maternal antibodies during pregnancy or by taking up antibodies from milk. Using a codon optimized polycistronic gene expression cassette packaged into herpesvirus particles, the simultaneous expression of the RV capsid genes led to the intracellular formation of RVLPs in various cell lines. Vaccinated dams developed a strong RV specific IgG antibody response determined in sera and milk of both mother and pups. Moreover, sera of naïve pups nursed by vaccinated dams also had RV specific antibodies suggesting a lactogenic transfer of antibodies. Although full protection of pups was not achieved in this mouse model, our observations are important for the development of improved vaccines against RV in humans as well as in various animal species.

In vitro antiviral activity of Lactobacillus casei and Bifidobacterium adolescentis against rotavirus infection monitored by NSP4 protein production

AIMS

The aim of this study was to determine the antiviral activity of four probiotic metabolites (Lactobacillus and Bifidobacetrium species) against rotavirus in vitro infection monitored by the NSP4 protein production and Ca(2+) release.

METHODS AND RESULTS

The antiviral effect of the metabolites was performed due a comparison between a blocking model and an intracelullar model on MA104 cells, with the response of NSP4 production and Ca(2+) liberation measured by flow cytometry. Significant results were obtained with the metabolites of Lactobacillus casei, and Bifidobacterium adolescentis in the reduction of the protein production (P = 0·04 and P = 0·014) and Ca(2+) liberation (P = 0·094 and P = 0·020) in the intracellular model, which suggests a successful antiviral activity against RV infection.

CONCLUSIONS

This study demonstrates that probiotic metabolites were able to interfere with the final amount of intracellular NSP4 protein and a successful Ca(2+) regulation, which suggests a new approach to the mechanism exerted by probiotics against the rotavirus infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

A novel anti-rotaviral effect exerted by probiotic metabolites monitored by the NSP4 protein during the RV in vitro infection and the effect on the Ca(2+) release is reported; suggesting a reduction on the impact of the infection by decreasing the damage of the cells preventing the electrolyte loss.

Rotavirus and Serotonin Cross-Talk in Diarrhoea

Rotavirus (RV) has been shown to infect and stimulate secretion of serotonin from human enterochromaffin (EC) cells and to infect EC cells in the small intestine of mice. It remains to identify which intracellularly expressed viral protein(s) is responsible for this novel property and to further establish the clinical role of serotonin in RV infection. First, we found that siRNA specifically silencing NSP4 (siRNA^NSP4) significantly attenuated secretion of serotonin from Rhesus rotavirus (RRV) infected EC tumor cells compared to siRNA^VP4, siRNA^VP6 and siRNA^VP7. Second, intracellular calcium mobilization and diarrhoeal capacity from virulent and avirulent porcine viruses correlated with the capacity to release serotonin from EC tumor cells. Third, following administration of serotonin, all (10/10) infants, but no (0/8) adult mice, responded with diarrhoea. Finally, blocking of serotonin receptors using Ondansetron significantly attenuated murine RV (strain EDIM) diarrhoea in infant mice (2.9 vs 4.5 days). Ondansetron-treated mice (n = 11) had significantly (p < 0.05) less diarrhoea, lower diarrhoea severity score and lower total diarrhoea output as compared to mock-treated mice (n = 9). Similarly, Ondansetron-treated mice had better weight gain than mock-treated animals (p < 0.05). A most surprising finding was that the serotonin receptor antagonist significantly (p < 0.05) also attenuated total viral shedding. In summary, we show that intracellularly expressed NSP4 stimulates release of serotonin from human EC tumor cells and that serotonin participates in RV diarrhoea, which can be attenuated by Ondansetron.

Inflammatory and oxidative stress in rotavirus infection

Rotaviruses are the single leading cause of life-threatening diarrhea affecting children under 5 years of age. Rotavirus entry into the host cell seems to occur by sequential interactions between virion proteins and various cell surface molecules. The entry mechanisms seem to involve the contribution of cellular molecules having binding, chaperoning and oxido-reducing activities. It appears to be that the receptor usage and tropism of rotaviruses is determined by the species, cell line and rotavirus strain. Rotaviruses have evolved functions which can antagonize the host innate immune response, whereas are able to induce endoplasmic reticulum (ER) stress, oxidative stress and inflammatory signaling. A networking between ER stress, inflammation and oxidative stress is suggested, in which release of calcium from the ER increases the generation of mitochondrial reactive oxygen species (ROS) leading to toxic accumulation of ROS within ER and mitochondria. Sustained ER stress potentially stimulates inflammatory response through unfolded protein response pathways. However, the detailed characterization of the molecular mechanisms underpinning these rotavirus-induced stressful conditions is still lacking. The signaling events triggered by host recognition of virus-associated molecular patterns offers an opportunity for the development of novel therapeutic strategies aimed at interfering with rotavirus infection. The use of N-acetylcysteine, non-steroidal anti-inflammatory drugs and PPARγ agonists to inhibit rotavirus infection opens a new way for treating the rotavirus-induced diarrhea and complementing vaccines.

The tyrosine kinase inhibitor genistein induces the detachment of rotavirus particles from the cell surface

Group A rotaviruses are a major cause of severe gastroenteritis in young infants. In this work we evaluated the potential role of protein tyrosine kinases on rotavirus infectivity and viral progeny production. From the broad-spectrum inhibitors tested, only genistein, a flavonoid, inhibited rotavirus infectivity. The inhibition observed was dose and strain dependent, with more than 10-fold IC50 differences for some rotavirus strains, and the effect of the drug was shown to be dependent of their activity as a protein tyrosine kinase inhibitor, since the inactive analogue of genistein, daidzein, had no effect on virus infection. Investigation of the stage of virus replication blocked by the drug showed that it interferes with the early interactions of the virus with receptors and/or co-receptors, since treatment of the cells with genistein promoted the detachment of the virus from the cell surface.

Viral Membrane Channels: Role and Function in the Virus Life Cycle

Viroporins are small, hydrophobic trans-membrane viral proteins that oligomerize to form hydrophilic pores in the host cell membranes. These proteins are crucial for the pathogenicity and replication of viruses as they aid in various stages of the viral life cycle, from genome uncoating to viral release. In addition, the ion channel activity of viroporin causes disruption in the cellular ion homeostasis, in particular the calcium ion. Fluctuation in the calcium level triggers the activation of the host defensive programmed cell death pathways as well as the inflammasome, which in turn are being subverted for the viruses’ replication benefits. This review article summarizes recent developments in the functional investigation of viroporins from various viruses and their contributions to viral replication and virulence.

Rotaviruses: Extraction and Isolation of RNA, Reassortant Strains, and NSP4 Protein

Rotavirus (RV) contains 11 double-stranded RNA segments that encode for twelve structural and nonstructural proteins. The separation and isolation of viral RNA is a necessary precursor for many experimental techniques and can be useful for rapid RV RNA typing and sequencing of different rotavirus strains. The segmented genome enables RV to recombine easily. These recombinant viruses are essential for many purposes, including generation of potential vaccine strains. Rotavirus gene 10 expresses the viral enterotoxin, NSP4, which has been the focus of several studies due to the influence of NSP4 on rotavirus replication, morphogenesis, and pathogenesis. This unit will describe the isolation and separation of viral RNAs, the production characterization of recombinant RV in culture, and the expression and isolation of NSP4 in mammalian and insect cells.

New tags for recombinant protein detection and O-glycosylation reporters

Monoclonal antibodies (mAbs), because of their unique specificity, are irreplaceable tools for scientific research. Precise mapping of the antigenic determinants allows the development of epitope tagging approaches to be used with recombinant proteins for several purposes. Here we describe a new family of tags derived from the epitope recognized by a single highly specific mAb (anti-roTag mAb), which was obtained from a pool of mAbs reacting with the rotavirus nonstructural protein 5 (NSP5). The variable regions of the anti-roTag mAb were identified and their binding capacity verified upon expression as a single-chain/miniAb. The minimal epitope, termed roTag, was identified as a 10 amino acid sequence (SISSSIFKNE). The affinity of the anti-roTag/roTag interaction was found to be comparable to that of the anti-SV5/SV5 tag interaction. roTag was successfully used for detection of several recombinant cytosolic, secretory and membrane proteins. Two additional variants of roTag of 10 and 13 amino acids containing O-glycosylation susceptible sites (termed OG-tag and roTagO) were constructed and characterised. These tags were useful to detect proteins passing through the Golgi apparatus, the site of O-glycosylation.

Sequence and structural analyses of NSP4 proteins from human group A rotavirus strains detected in Tunisia

BACKGROUND

The NSP4 protein of group A rotavirus (RVA) has been recognized as a viral enterotoxin and plays important roles in viral pathogenesis and morphogenesis. Domains involved in structural and functional interactions have been proposed mainly based on the simian SA11 strain.

METHODS

NSP4 has been classified into 15 different genotypes (E1-E15), and the aim of this study was to analyze the sequences of 46 RVA strains in order to determine the aminoacid (aa) differences between E1 and E2 genotypes. Another aspect was to characterize the structural and physicochemical properties of these strains.

RESULTS

Comparison of deduced aa sequences of the NSP4 protein showed that divergences between NSP4 genotypes E1 and E2 were mostly observed in the VP4-binding, the interspecies variable domain (ISVD) and the double-layered particle (DLP) binding domains. Interestingly, uncommon variations in residues 131 and 138, which are known to be important aa in pathogenesis, were found in one unusual animal derived strain belonging to the E2 genotype. Concerning the structural aspect, no significant differences were noted.

CONCLUSION

The presence of punctual aa variations in the NSP4 genotypes may indicate that NSP4 mutates mainly via accumulation of point mutations.

Mutational analysis of the rotavirus NSP4 enterotoxic domain that binds to caveolin-1

Background

Rotavirus (RV) nonstructural protein 4 (NSP4) is the first described viral enterotoxin, which induces early secretory diarrhea in neonatal rodents. Our previous data show a direct interaction between RV NSP4 and the structural protein of caveolae, caveolin-1 (cav-1), in yeast and mammalian cells. The binding site of cav-1 mapped to the NSP4 amphipathic helix, and led us to examine which helical face was responsible for the interaction.

Methods

A panel of NSP4 mutants were prepared and tested for binding to cav-1 by yeast two hybrid and direct binding assays. The charged residues of the NSP4 amphipathic helix were changed to alanine (NSP4_46-175-ala6); and three residues in the hydrophobic face were altered to charged amino acids (NSP4_46-175-HydroMut). In total, twelve mutants of NSP4 were generated to define the cav-1 binding site. Synthetic peptides corresponding to the hydrophobic and charged faces of NSP4 were examined for structural changes by circular dichroism (CD) and diarrhea induction by a neonatal mouse study.

Results

Mutations of the hydrophilic face (NSP4_46-175-Ala6) bound cav-1 akin to wild type NSP4. In contrast, disruption of the hydrophobic face (NSP4_46-175-HydroMut) failed to bind cav-1. These data suggest NSP4 and cav-1 associate via a hydrophobic interaction. Analyses of mutant synthetic peptides in which the hydrophobic residues in the enterotoxic domain of NSP4 were altered suggested a critical hydrophobic residue. Both NSP4_{HydroMut112-140,} that contains three charged amino acids (aa113, 124, 131) changed from the original hydrophobic residues and NSP4_{AlaAcidic112-140} that contained three alanine residues substituted for negatively charged (aa114, 125, 132) amino acids failed to induce diarrhea. Whereas peptides NSP4wild type _112−140 and NSP4_{AlaBasic112-140} that contained three alanine substituted for positively charged (aa115, 119, 133) amino acids, induced diarrhea.

Conclusions

These data show that the cav-1 binding domain is within the hydrophobic face of the NSP4 amphipathic helix. The integrity of the helical structure is important for both cav-1 binding and diarrhea induction implying a connection between NSP4 functional and binding activities.

Vesicular transport of progeny parvovirus particles through ER and Golgi regulates maturation and cytolysis

Progeny particles of non-enveloped lytic parvoviruses were previously shown to be actively transported to the cell periphery through vesicles in a gelsolin-dependent manner. This process involves rearrangement and destruction of actin filaments, while microtubules become protected throughout the infection. Here the focus is on the intracellular egress pathway, as well as its impact on the properties and release of progeny virions. By colocalization with cellular marker proteins and specific modulation of the pathways through over-expression of variant effector genes transduced by recombinant adeno-associated virus vectors, we show that progeny PV particles become engulfed into COPII-vesicles in the endoplasmic reticulum (ER) and are transported through the Golgi to the plasma membrane. Besides known factors like sar1, sec24, rab1, the ERM family proteins, radixin and moesin play (an) essential role(s) in the formation/loading and targeting of virus-containing COPII-vesicles. These proteins also contribute to the transport through ER and Golgi of the well described analogue of cellular proteins, the secreted Gaussia luciferase in absence of virus infection. It is therefore likely that radixin and moesin also serve for a more general function in cellular exocytosis. Finally, parvovirus egress via ER and Golgi appears to be necessary for virions to gain full infectivity through post-assembly modifications (e.g. phosphorylation). While not being absolutely required for cytolysis and progeny virus release, vesicular transport of parvoviruses through ER and Golgi significantly accelerates these processes pointing to a regulatory role of this transport pathway.

Previously, it was thought that non-enveloped lytic parvoviruses were released through a lytic burst of cells at the end of infection. However, recent work demonstrated that these small non-enveloped single-stranded DNA viruses are actively transported through vesicles from the nucleus, the site of replication and assembly, to the cell periphery. The current investigation demonstrates that progeny particles become engulfed into COPII-vesicles in the endoplasmic reticulum (ER) and are transported through the Golgi to the plasma membrane (PM). ERM family proteins radixin and moesin appear to play an essential role in this cellular secretion pathway. While passing through ER and Golgi cisternae, PVs maturate through post-assembly modifications, which significantly increase the infectivity of progeny virions. Finally, the vesicular transport of parvoviral particles was shown to regulate virus-induced cytolysis, thereby accelerating the further release and spread of progeny virions. As rodent PVs are currently viewed as oncolytic agents for cancer virotherapy, it is important to further investigate the mechanism of PV egress — not only to improve the spreading of these agents through the tumor mass, but also to optimize the induction of an anti-tumor immune response upon virus — induced cytolysis.

Rotavirus prevents the expression of host responses by blocking the nucleocytoplasmic transport of polyadenylated mRNAs

Rotaviruses are the most important agent of severe gastroenteritis in young children. Early in infection, these viruses take over the host translation machinery, causing a severe shutoff of cell protein synthesis while viral proteins are efficiently synthesized. In infected cells, there is an accumulation of the cytoplasmic poly(A)-binding protein in the nucleus, induced by the viral protein NSP3. Here we found that poly(A)-containing mRNAs also accumulate and become hyperadenylated in the nuclei of infected cells. Using reporter genes bearing the untranslated regions (UTRs) of cellular or viral genes, we found that the viral UTRs do not determine the efficiency of translation of mRNAs in rotavirus-infected cells. Furthermore, we showed that while a polyadenylated reporter mRNA directly delivered into the cytoplasm of infected cells was efficiently translated, the same reporter introduced as a plasmid that needs to be transcribed and exported to the cytoplasm was poorly translated. Altogether, these results suggest that nuclear retention of poly(A)-containing mRNAs is one of the main strategies of rotavirus to control cell translation and therefore the host antiviral and stress responses.

How viruses use the endoplasmic reticulum for entry, replication, and assembly

To cause infection, a virus enters a host cell, replicates, and assembles, with the resulting new viral progeny typically released into the extracellular environment to initiate a new infection round. Virus entry, replication, and assembly are dynamic and coordinated processes that require precise interactions with host components, often within and surrounding a defined subcellular compartment. Accumulating evidence pinpoints the endoplasmic reticulum (ER) as a crucial organelle supporting viral entry, replication, and assembly. This review focuses on the molecular mechanism by which different viruses co-opt the ER to accomplish these crucial infection steps. Certain bacterial toxins also hijack the ER for entry. An interdisciplinary approach, using rigorous biochemical and cell biological assays coupled with advanced microscopy strategies, will push to the next level our understanding of the virus-ER interaction during infection.

Rotavirus viroplasm fusion and perinuclear localization are dynamic processes requiring stabilized microtubules

Rotavirus viroplasms are cytosolic, electron-dense inclusions corresponding to the viral machinery of replication responsible for viral template transcription, dsRNA genome segments replication and assembly of new viral cores. We have previously observed that, over time, those viroplasms increase in size and decrease in number. Therefore, we hypothesized that this process was dependent on the cellular microtubular network and its associated dynamic components. Here, we present evidence demonstrating that viroplasms are dynamic structures, which, in the course of an ongoing infection, move towards the perinuclear region of the cell, where they fuse among each other, thereby gaining considerably in size and, simultaneouly, explaining the decrease in numbers. On the viral side, this process seems to depend on VP2 for movement and on NSP2 for fusion. On the cellular side, both the temporal transition and the maintenance of the viroplasms are dependent on the microtubular network, its stabilization by acetylation, and, surprisingly, on a kinesin motor of the kinesin-5 family, Eg5. Thus, we provide for the first time deeper insights into the dynamics of rotavirus replication, which can explain the behavior of viroplasms in the infected cell.

Rotavirus infection of cells in culture induces activation of RhoA and changes in the actin and tubulin cytoskeleton

Rotavirus infection induces an increase in [Ca²⁺]_cyto, which in turn may affect the distribution of the cytoskeleton proteins in the infected cell. Changes in microfilaments, including the formation of stress fibers, were observed starting at 0.5 h.p.i. using fluorescent phalloidin. Western blot analysis indicated that RhoA is activated between 0.5 and 1 h.p.i. Neither the phosphorylation of RhoA nor the formation of stress fibers were observed in cells infected with virions pre-treated with an anti-VP5* non-neutralizing mAb, suggesting that RhoA activation is stimulated by the interaction of the virus with integrins forming the cell receptor complex. In addition, the structure of the tubulin cytoskeleton was also studied. Alterations of the microtubules were evident starting at 3 h.p.i. and by 7 h.p.i. when microtubules were markedly displaced toward the periphery of the cell cytoplasm. Loading of rotavirus-infected cells with either a Ca²⁺ chelator (BAPTA) or transfection with siRNAs to silence NSP4, reversed the changes observed in both the microfilaments and microtubules distribution, but not the appearance of stress fibers. These results indicate that alterations in the distribution of actin microfilaments are initiated early during infection by the activation of RhoA, and that latter changes in the Ca²⁺ homeostasis promoted by NSP4 during infection may be responsible for other alterations in the actin and tubulin cytoskeleton.

Rotavirus non-structural proteins: structure and function

The replication of rotavirus is a complex process that is orchestrated by an exquisite interplay between the rotavirus non-structural and structural proteins. Subsequent to particle entry and genome transcription, the non-structural proteins coordinate and regulate viral mRNA translation and the formation of electron-dense viroplasms that serve as exclusive compartments for genome replication, genome encapsidation and capsid assembly. In addition, non-structural proteins are involved in antagonizing the antiviral host response and in subverting important cellular processes to enable successful virus replication. Although far from complete, new structural studies, together with functional studies, provide substantial insight into how the non-structural proteins coordinate rotavirus replication. This brief review highlights our current knowledge of the structure-function relationships of the rotavirus non-structural proteins, as well as fascinating questions that remain to be understood.

Genetics and reverse genetics of rotavirus

Rotavirus is a member of the family Reoviridae, which have genomes consisting of 10-12 double-stranded RNA segments. The functions of proteins encoded by each segment of the rotavirus genome have been studied extensively by several methods including reassortants, temperature-sensitive mutants, isolates with rearranged RNA segments, RNAi analysis, and other procedures. However, as found for most RNA viruses, the technique of reverse genetics is required for precise genotype/phenotype correlation, for the analysis of the role of specific mutation in replication process and pathogenesis, and for the development of vectors and vaccines. In 2006, we presented the first description of a reverse genetics system for rotavirus, although a helper virus and a selection system are required. Since then, two other approaches have been reported for rotavirus reverse genetics, both requiring the presence of a helper virus. A tractable, helper virus-free reverse genetics system for rotavirus has not been developed so far, in contrast to the recent developments of plasmid only-based reverse genetics systems for other members of the Reoviridae.

HSV-1 amplicon vectors launch the production of heterologous rotavirus-like particles and induce rotavirus-specific immune responses in mice

Virus-like particles (VLPs) are promising vaccine candidates because they represent viral antigens in the authentic conformation of the virion and are therefore readily recognized by the immune system. As VLPs do not contain genetic material they are safer than attenuated virus vaccines. In this study, herpes simplex virus type 1 (HSV-1) amplicon vectors were constructed to coexpress the rotavirus (RV) structural genes VP2, VP6, and VP7 and were used as platforms to launch the production of RV-like particles (RVLPs) in vector-infected mammalian cells. Despite the observed splicing of VP6 RNA, full-length VP6 protein and RVLPs were efficiently produced. Intramuscular injection of mice with the amplicon vectors as a two-dose regimen without adjuvants resulted in RV-specific humoral immune responses and, most importantly, immunized mice were partially protected at the mucosal level from challenge with live wild-type (wt) RV. This work provides proof of principle for the application of HSV-1 amplicon vectors that mediate the efficient production of heterologous VLPs as genetic vaccines.