Co-opting templated aggregation to degrade pathogenic tau assemblies and improve motor function.

Protein aggregation causes a wide range of neurodegenerative diseases. Targeting and removing aggregates, but not the functional protein, is a considerable therapeutic challenge. Here, we describe a therapeutic strategy called "RING-Bait," which employs an aggregating protein sequence combined with an E3 ubiquitin ligase. RING-Bait is recruited into aggregates, whereupon clustering dimerizes the RING domain and activates its E3 function, resulting in the degradation of the aggregate complex. We exemplify this concept by demonstrating the specific degradation of tau aggregates while sparing soluble tau. Unlike immunotherapy, RING-Bait is effective against both seeded and cell-autonomous aggregation. RING-Bait removed tau aggregates seeded from Alzheimer's disease (AD) and progressive supranuclear palsy (PSP) brain extracts and was also effective in primary neurons. We used a brain-penetrant adeno-associated virus (AAV) to treat P301S tau transgenic mice, reducing tau pathology and improving motor function. A RING-Bait strategy could be applied to other neurodegenerative proteinopathies by replacing the Bait sequence to match the target aggregate.

Altered TMPRSS2 usage by SARS-CoV-2 Omicron impacts infectivity and fusogenicity.

The SARS-CoV-2 Omicron BA.1 variant emerged in 20211 and has multiple mutations in its spike protein2. Here we show that the spike protein of Omicron has a higher affinity for ACE2 compared with Delta, and a marked change in its antigenicity increases Omicron's evasion of therapeutic monoclonal and vaccine-elicited polyclonal neutralizing antibodies after two doses. mRNA vaccination as a third vaccine dose rescues and broadens neutralization. Importantly, the antiviral drugs remdesivir and molnupiravir retain efficacy against Omicron BA.1. Replication was similar for Omicron and Delta virus isolates in human nasal epithelial cultures. However, in lung cells and gut cells, Omicron demonstrated lower replication. Omicron spike protein was less efficiently cleaved compared with Delta. The differences in replication were mapped to the entry efficiency of the virus on the basis of spike-pseudotyped virus assays. The defect in entry of Omicron pseudotyped virus to specific cell types effectively correlated with higher cellular RNA expression of TMPRSS2, and deletion of TMPRSS2 affected Delta entry to a greater extent than Omicron. Furthermore, drug inhibitors targeting specific entry pathways3 demonstrated that the Omicron spike inefficiently uses the cellular protease TMPRSS2, which promotes cell entry through plasma membrane fusion, with greater dependency on cell entry through the endocytic pathway. Consistent with suboptimal S1/S2 cleavage and inability to use TMPRSS2, syncytium formation by the Omicron spike was substantially impaired compared with the Delta spike. The less efficient spike cleavage of Omicron at S1/S2 is associated with a shift in cellular tropism away from TMPRSS2-expressing cells, with implications for altered pathogenesis.

SARS-CoV-2 B.1.617.2 Delta variant replication and immune evasion.

The B.1.617.2 (Delta) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified in the state of Maharashtra in late 2020 and spread throughout India, outcompeting pre-existing lineages including B.1.617.1 (Kappa) and B.1.1.7 (Alpha)1. In vitro, B.1.617.2 is sixfold less sensitive to serum neutralizing antibodies from recovered individuals, and eightfold less sensitive to vaccine-elicited antibodies, compared with wild-type Wuhan-1 bearing D614G. Serum neutralizing titres against B.1.617.2 were lower in ChAdOx1 vaccinees than in BNT162b2 vaccinees. B.1.617.2 spike pseudotyped viruses exhibited compromised sensitivity to monoclonal antibodies to the receptor-binding domain and the amino-terminal domain. B.1.617.2 demonstrated higher replication efficiency than B.1.1.7 in both airway organoid and human airway epithelial systems, associated with B.1.617.2 spike being in a predominantly cleaved state compared with B.1.1.7 spike. The B.1.617.2 spike protein was able to mediate highly efficient syncytium formation that was less sensitive to inhibition by neutralizing antibody, compared with that of wild-type spike. We also observed that B.1.617.2 had higher replication and spike-mediated entry than B.1.617.1, potentially explaining the B.1.617.2 dominance. In an analysis of more than 130 SARS-CoV-2-infected health care workers across three centres in India during a period of mixed lineage circulation, we observed reduced ChAdOx1 vaccine effectiveness against B.1.617.2 relative to non-B.1.617.2, with the caveat of possible residual confounding. Compromised vaccine efficacy against the highly fit and immune-evasive B.1.617.2 Delta variant warrants continued infection control measures in the post-vaccination era.

Viral nucleoprotein antibodies activate TRIM21 and induce T cell immunity.

Nucleoprotein (N) is an immunodominant antigen in many enveloped virus infections. While the diagnostic value of anti-N antibodies is clear, their role in immunity is not. This is because while they are non-neutralising, they somehow clear infection by coronavirus, influenza and LCMV in vivo. Here, we show that anti-N immune protection is mediated by the cytosolic Fc receptor and E3 ubiquitin ligase TRIM21. Exploiting LCMV as a model system, we demonstrate that TRIM21 uses anti-N antibodies to target N for cytosolic degradation and generate cytotoxic T cells (CTLs) against N peptide. These CTLs rapidly eliminate N-peptide-displaying cells and drive efficient viral clearance. These results reveal a new mechanism of immune synergy between antibodies and T cells and highlights N as an important vaccine target.

Liquid-liquid phase separation underpins the formation of replication factories in rotaviruses.

RNA viruses induce the formation of subcellular organelles that provide microenvironments conducive to their replication. Here we show that replication factories of rotaviruses represent protein-RNA condensates that are formed via liquid-liquid phase separation of the viroplasm-forming proteins NSP5 and rotavirus RNA chaperone NSP2. Upon mixing, these proteins readily form condensates at physiologically relevant low micromolar concentrations achieved in the cytoplasm of virus-infected cells. Early infection stage condensates could be reversibly dissolved by 1,6-hexanediol, as well as propylene glycol that released rotavirus transcripts from these condensates. During the early stages of infection, propylene glycol treatments reduced viral replication and phosphorylation of the condensate-forming protein NSP5. During late infection, these condensates exhibited altered material properties and became resistant to propylene glycol, coinciding with hyperphosphorylation of NSP5. Some aspects of the assembly of cytoplasmic rotavirus replication factories mirror the formation of other ribonucleoprotein granules. Such viral RNA-rich condensates that support replication of multi-segmented genomes represent an attractive target for developing novel therapeutic approaches.

IP6-stabilised HIV capsids evade cGAS/STING-mediated host immune sensing.

HIV-1 uses inositol hexakisphosphate (IP6) to build a metastable capsid capable of delivering its genome into the host nucleus. Here, we show that viruses that are unable to package IP6 lack capsid protection and are detected by innate immunity, resulting in the activation of an antiviral state that inhibits infection. Disrupting IP6 enrichment results in defective capsids that trigger cytokine and chemokine responses during infection of both primary macrophages and T-cell lines. Restoring IP6 enrichment with a single mutation rescues the ability of HIV-1 to infect cells without being detected. Using a combination of capsid mutants and CRISPR-derived knockout cell lines for RNA and DNA sensors, we show that immune sensing is dependent upon the cGAS-STING axis and independent of capsid detection. Sensing requires the synthesis of viral DNA and is prevented by reverse transcriptase inhibitors or reverse transcriptase active-site mutation. These results demonstrate that IP6 is required to build capsids that can successfully transit the cell and avoid host innate immune sensing.

Human coronaviruses activate and hijack the host transcription factor HSF1 to enhance viral replication.

Organisms respond to proteotoxic-stress by activating the heat-shock response, a cellular defense mechanism regulated by a family of heat-shock factors (HSFs); among six human HSFs, HSF1 acts as a proteostasis guardian regulating severe stress-driven transcriptional responses. Herein we show that human coronaviruses (HCoV), both low-pathogenic seasonal-HCoVs and highly-pathogenic SARS-CoV-2 variants, are potent inducers of HSF1, promoting HSF1 serine-326 phosphorylation and triggering a powerful and distinct HSF1-driven transcriptional-translational response in infected cells. Despite the coronavirus-mediated shut-down of the host translational machinery, selected HSF1-target gene products, including HSP70, HSPA6 and AIRAP, are highly expressed in HCoV-infected cells. Using silencing experiments and a direct HSF1 small-molecule inhibitor we show that, intriguingly, HCoV-mediated activation of the HSF1-pathway, rather than representing a host defense response to infection, is hijacked by the pathogen and is essential for efficient progeny particles production. The results open new scenarios for the search of innovative antiviral strategies against coronavirus infections.

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.

Furin cleavage of SARS-CoV-2 Spike promotes but is not essential for infection and cell-cell fusion.

Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) infects cells by binding to the host cell receptor ACE2 and undergoing virus-host membrane fusion. Fusion is triggered by the protease TMPRSS2, which processes the viral Spike (S) protein to reveal the fusion peptide. SARS-CoV-2 has evolved a multibasic site at the S1-S2 boundary, which is thought to be cleaved by furin in order to prime S protein for TMPRSS2 processing. Here we show that CRISPR-Cas9 knockout of furin reduces, but does not prevent, the production of infectious SARS-CoV-2 virus. Comparing S processing in furin knockout cells to multibasic site mutants reveals that while loss of furin substantially reduces S1-S2 cleavage it does not prevent it. SARS-CoV-2 S protein also mediates cell-cell fusion, potentially allowing virus to spread virion-independently. We show that loss of furin in either donor or acceptor cells reduces, but does not prevent, TMPRSS2-dependent cell-cell fusion, unlike mutation of the multibasic site that completely prevents syncytia formation. Our results show that while furin promotes both SARS-CoV-2 infectivity and cell-cell spread it is not essential, suggesting furin inhibitors may reduce but not abolish viral spread.

Recombinant Rotaviruses Rescued by Reverse Genetics Reveal the Role of NSP5 Hyperphosphorylation in the Assembly of Viral Factories.

Rotavirus (RV) replicates in round-shaped cytoplasmic viral factories, although how they assemble remains unknown. During RV infection, NSP5 undergoes hyperphosphorylation, which is primed by the phosphorylation of a single serine residue. The role of this posttranslational modification in the formation of viroplasms and its impact on virus replication remain obscure. Here, we investigated the role of NSP5 during RV infection by taking advantage of a modified fully tractable reverse-genetics system. A trans-complementing cell line stably producing NSP5 was used to generate and characterize several recombinant rotaviruses (rRVs) with mutations in NSP5. We demonstrate that an rRV lacking NSP5 was completely unable to assemble viroplasms and to replicate, confirming its pivotal role in rotavirus replication. A number of mutants with impaired NSP5 phosphorylation were generated to further interrogate the function of this posttranslational modification in the assembly of replication-competent viroplasms. We showed that the rRV mutant strains exhibited impaired viral replication and the ability to assemble round-shaped viroplasms in MA104 cells. Furthermore, we investigated the mechanism of NSP5 hyperphosphorylation during RV infection using NSP5 phosphorylation-negative rRV strains, as well as MA104-derived stable transfectant cell lines expressing either wild-type NSP5 or selected NSP5 deletion mutants. Our results indicate that NSP5 hyperphosphorylation is a crucial step for the assembly of round-shaped viroplasms, highlighting the key role of the C-terminal tail of NSP5 in the formation of replication-competent viral factories. Such a complex NSP5 phosphorylation cascade may serve as a paradigm for the assembly of functional viral factories in other RNA viruses.IMPORTANCE The rotavirus (RV) double-stranded RNA genome is replicated and packaged into virus progeny in cytoplasmic structures termed viroplasms. The nonstructural protein NSP5, which undergoes a complex hyperphosphorylation process during RV infection, is required for the formation of these virus-induced organelles. However, its roles in viroplasm formation and RV replication have never been directly assessed due to the lack of a fully tractable reverse-genetics (RG) system for rotaviruses. Here, we show a novel application of a recently developed RG system by establishing a stable trans-complementing NSP5-producing cell line required to rescue rotaviruses with mutations in NSP5. This approach allowed us to provide the first direct evidence of the pivotal role of this protein during RV replication. Furthermore, using recombinant RV mutants, we shed light on the molecular mechanism of NSP5 hyperphosphorylation during infection and its involvement in the assembly and maturation of replication-competent viroplasms.

Identification of a Small Molecule That Compromises the Structural Integrity of Viroplasms and Rotavirus Double-Layered Particles.

Despite the availability of two attenuated vaccines, rotavirus (RV) gastroenteritis remains an important cause of mortality among children in developing countries, causing about 215,000 infant deaths annually. Currently, there are no specific antiviral therapies available. RV is a nonenveloped virus with a segmented double-stranded RNA genome. Viral genome replication and assembly of transcriptionally active double-layered particles (DLPs) take place in cytoplasmic viral structures called viroplasms. In this study, we describe strong impairment of the early stages of RV replication induced by a small molecule known as an RNA polymerase III inhibitor, ML-60218 (ML). This compound was found to disrupt already assembled viroplasms and to hamper the formation of new ones without the need for de novo transcription of cellular RNAs. This phenotype was correlated with a reduction in accumulated viral proteins and newly made viral genome segments, disappearance of the hyperphosphorylated isoforms of the viroplasm-resident protein NSP5, and inhibition of infectious progeny virus production. In in vitro transcription assays with purified DLPs, ML showed dose-dependent inhibitory activity, indicating the viral nature of its target. ML was found to interfere with the formation of higher-order structures of VP6, the protein forming the DLP outer layer, without compromising its ability to trimerize. Electron microscopy of ML-treated DLPs showed dose-dependent structural damage. Our data suggest that interactions between VP6 trimers are essential, not only for DLP stability, but also for the structural integrity of viroplasms in infected cells.IMPORTANCE Rotavirus gastroenteritis is responsible for a large number of infant deaths in developing countries. Unfortunately, in the countries where effective vaccines are urgently needed, the efficacy of the available vaccines is particularly low. Therefore, the development of antivirals is an important goal, as they might complement the available vaccines or represent an alternative option. Moreover, they may be decisive in fighting the acute phase of infection. This work describes the inhibitory effect on rotavirus replication of a small molecule initially reported as an RNA polymerase III inhibitor. The molecule is the first chemical compound identified that is able to disrupt viroplasms, the viral replication machinery, and to compromise the stability of DLPs by targeting the viral protein VP6. This molecule thus represents a starting point in the development of more potent and less cytotoxic compounds against rotavirus infection.

A Single Nucleoside Viral Polymerase Inhibitor Against Norovirus, Rotavirus, and Sapovirus-Induced Diarrhea.

A safe and highly efficient antiviral is needed for the prophylaxis and/or treatment of viral diarrhea. We here demonstrate the in vitro antiviral activity of four 2'-C-methyl nucleoside analogues against noro-, rota-, and sapoviruses. The most potent nucleoside analogue, 7-deaza-2'-C-methyladenosine, inhibits replication of these viruses with a 50% effective concentration < 5 µM. Mechanistically, we demonstrate that the 2'-C-methyl nucleoside analogues act by inhibiting transcription of the rotavirus genome. This provides the first evidence that a single viral-diarrhea-targeted treatment can be developed through a viral-polymerase-targeting small molecule.

A Respiratory Syncytial Virus Vaccine Vectored by a Stable Chimeric and Replication-Deficient Sendai Virus Protects Mice without Inducing Enhanced Disease.

Respiratory syncytial virus (RSV) is a major cause of severe respiratory infections in children and elderly people, and no marketed vaccine exists. In this study, we generated and analyzed a subunit vaccine against RSV based on a novel genome replication-deficient Sendai virus (SeV) vector. We inserted the RSV F protein, known to be a genetically stable antigen, into our vector in a specific way to optimize the vaccine features. By exchanging the ectodomain of the SeV F protein for its counterpart from RSV, we created a chimeric vectored vaccine that contains the RSV F protein as an essential structural component. In this way, the antigen is actively expressed on the surfaces of vaccine particles in its prefusion conformation, and as recently reported for other vectored vaccines, the occurrence of silencing mutations of the transgene in the vaccine genome can be prevented. In addition, its active gene expression contributes to further stimulation of the immune response. In order to understand the best route of immunization, we compared vaccine efficacies after intranasal (i.n.) or intramuscular (i.m.) immunization of BALB/c mice. Via both routes, substantial RSV-specific immune responses were induced, consisting of serum IgG and neutralizing antibodies, as well as cytotoxic T cells. Moreover, i.n. immunization was also able to stimulate specific mucosal IgA in the upper and lower respiratory tract. In virus challenge experiments, animals were protected against RSV infection after both i.n. and i.m. immunization without inducing vaccine-enhanced disease. Above all, the replication-deficient SeV appeared to be safe and well tolerated.IMPORTANCE Respiratory syncytial virus (RSV) is a major cause of respiratory diseases in young children and elderly people worldwide. There is a great demand for a licensed vaccine. Promising existing vaccine approaches based on live-attenuated vaccines or viral vectors have suffered from unforeseen drawbacks related to immunogenicity and attenuation. We provide a novel RSV vaccine concept based on a genome replication-deficient Sendai vector that has many favorable vaccine characteristics. The specific vaccine design guarantees genetic stability of the transgene; furthermore, it supports a favorable presentation of the antigen, activating the adaptive response, features that other vectored vaccine approaches have often had difficulties with. Wide immunological and pathological analyses in mice confirmed the validity and efficacy of this approach after both parenteral and mucosal administration. Above all, this concept is suitable for initiating clinical studies, and it could also be applied to other infectious diseases.

The recruitment of TRiC chaperonin in rotavirus viroplasms correlates with virus replication.

Rotavirus (RV) replication takes place in the viroplasms, cytosolic inclusions that allow the synthesis of virus genome segments and their encapsidation in the core shell, followed by the addition of the second layer of the virion. The viroplasms are composed of several viral proteins, including NSP5, which serves as the main building block. Microtubules, lipid droplets, and miRNA-7 are among the host components recruited in viroplasms. We investigated the interaction between RV proteins and host components of the viroplasms by performing a pull-down assay of lysates from RV-infected cells expressing NSP5-BiolD2. Subsequent tandem mass spectrometry identified all eight subunits of the tailless complex polypeptide I ring complex (TRiC), a cellular chaperonin responsible for folding at least 10% of the cytosolic proteins. Our confirmed findings reveal that TRiC is brought into viroplasms and wraps around newly formed double-layered particles. Chemical inhibition of TRiC and silencing of its subunits drastically reduced virus progeny production. Through direct RNA sequencing, we show that TRiC is critical for RV replication by controlling dsRNA genome segment synthesis, particularly negative-sense single-stranded RNA. Importantly, cryo-electron microscopy analysis shows that TRiC inhibition results in defective virus particles lacking genome segments and polymerase complex (VP1/VP3). Moreover, TRiC associates with VP2 and NSP5 but not with VP1. Also, VP2 is shown to be essential for recruiting TRiC in viroplasms and preserving their globular morphology. This study highlights the essential role of TRiC in viroplasm formation and in facilitating virion assembly during the RV life cycle. IMPORTANCE: The replication of rotavirus takes place in cytosolic inclusions termed viroplasms. In these inclusions, the distinct 11 double-stranded RNA genome segments are co-packaged to complete a genome in newly generated virus particles. In this study, we show for the first time that the tailless complex polypeptide I ring complex (TRiC), a cellular chaperonin responsible for the folding of at least 10% of the cytosolic proteins, is a component of viroplasms and is required for the synthesis of the viral negative-sense single-stranded RNA. Specifically, TRiC associates with NSP5 and VP2, the cofactor involved in RNA replication. Our study adds a new component to the current model of rotavirus replication, where TRiC is recruited to viroplasms to assist replication.

Principles of RNA recruitment to viral ribonucleoprotein condensates in a segmented dsRNA virus.

Rotaviruses transcribe 11 distinct RNAs that must be co-packaged prior to their replication to make an infectious virion. During infection, nontranslating rotavirus transcripts accumulate in cytoplasmic protein-RNA granules known as viroplasms that support segmented genome assembly and replication via a poorly understood mechanism. Here, we analysed the RV transcriptome by combining DNA-barcoded smFISH of rotavirus-infected cells. Rotavirus RNA stoichiometry in viroplasms appears to be distinct from the cytoplasmic transcript distribution, with the largest transcript being the most enriched in viroplasms, suggesting a selective RNA enrichment mechanism. While all 11 types of transcripts accumulate in viroplasms, their stoichiometry significantly varied between individual viroplasms. Accumulation of transcripts requires the presence of 3' untranslated terminal regions and viroplasmic localisation of the viral polymerase VP1, consistent with the observed lack of polyadenylated transcripts in viroplasms. Our observations reveal similarities between viroplasms and other cytoplasmic RNP granules and identify viroplasmic proteins as drivers of viral RNA assembly during viroplasm formation.

Multivalent bicyclic peptides are an effective antiviral modality that can potently inhibit SARS-CoV-2.

COVID-19 has stimulated the rapid development of new antibody and small molecule therapeutics to inhibit SARS-CoV-2 infection. Here we describe a third antiviral modality that combines the drug-like advantages of both. Bicycles are entropically constrained peptides stabilized by a central chemical scaffold into a bi-cyclic structure. Rapid screening of diverse bacteriophage libraries against SARS-CoV-2 Spike yielded unique Bicycle binders across the entire protein. Exploiting Bicycles' inherent chemical combinability, we converted early micromolar hits into nanomolar viral inhibitors through simple multimerization. We also show how combining Bicycles against different epitopes into a single biparatopic agent allows Spike from diverse variants of concern (VoC) to be targeted (Alpha, Beta, Delta and Omicron). Finally, we demonstrate in both male hACE2-transgenic mice and Syrian golden hamsters that both multimerized and biparatopic Bicycles reduce viraemia and prevent host inflammation. These results introduce Bicycles as a potential antiviral modality to tackle new and rapidly evolving viruses.

Rotavirus reverse genetics: A tool for understanding virus biology.

Rotaviruses (RVs) are considered to be one of the most common causes of viral gastroenteritis in young children and infants worldwide. Before recent developments, studies on rotavirus biology have suffered from the lack of an effective reverse genetics (RG) system to generate recombinant rotaviruses and study the precise roles of the viral proteins in the context of RV infection. Lately a fully-tractable plasmid-only based RG system for rescuing recombinant rotaviruses has been developed leading to a breakthrough in the RV field. Since then, the reproducibility and improvements of this technology have led to the generation of several recombinant rotaviruses with modifications on different gene segments, which has allowed the manipulation of viral genes to characterise the precise roles of viral proteins during RV replication cycle or to encode exogenous proteins for different purposes. This review will recapitulate the different RG approaches developed so far, highlighting any similarities, differences and limitations of the systems as well as the gene segments involved. The review will further summarise the latest recombinant rotaviruses generated using the plasmid-only based RG system showing the enormous potentials of this technique to shed light on the still unanswered questions in rotavirus biology.

Viroplasms: Assembly and Functions of Rotavirus Replication Factories.

Viroplasms are cytoplasmic, membraneless structures assembled in rotavirus (RV)-infected cells, which are intricately involved in viral replication. Two virus-encoded, non-structural proteins, NSP2 and NSP5, are the main drivers of viroplasm formation. The structures (as far as is known) and functions of these proteins are described. Recent studies using plasmid-only-based reverse genetics have significantly contributed to elucidation of the crucial roles of these proteins in RV replication. Thus, it has been recognized that viroplasms resemble liquid-like protein-RNA condensates that may be formed via liquid-liquid phase separation (LLPS) of NSP2 and NSP5 at the early stages of infection. Interactions between the RNA chaperone NSP2 and the multivalent, intrinsically disordered protein NSP5 result in their condensation (protein droplet formation), which plays a central role in viroplasm assembly. These droplets may provide a unique molecular environment for the establishment of inter-molecular contacts between the RV (+)ssRNA transcripts, followed by their assortment and equimolar packaging. Future efforts to improve our understanding of RV replication and genome assortment in viroplasms should focus on their complex molecular composition, which changes dynamically throughout the RV replication cycle, to support distinct stages of virion assembly.

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.

Sequences in the cytoplasmic tail of SARS-CoV-2 Spike facilitate expression at the cell surface and syncytia formation.

The Spike (S) protein of SARS-CoV-2 binds ACE2 to direct fusion with host cells. S comprises a large external domain, a transmembrane domain, and a short cytoplasmic tail. Understanding the intracellular trafficking of S is relevant to SARS-CoV-2 infection, and to vaccines expressing full-length S from mRNA or adenovirus vectors. Here we report a proteomic screen for cellular factors that interact with the cytoplasmic tail of S. We confirm interactions with the COPI and COPII vesicle coats, ERM family actin regulators, and the WIPI3 autophagy component. The COPII binding site promotes exit from the endoplasmic reticulum, and although binding to COPI should retain S in the early Golgi where viral budding occurs, there is a suboptimal histidine residue in the recognition motif. As a result, S leaks to the surface where it accumulates and can direct the formation of multinucleate syncytia. Thus, the trafficking signals in the tail of S indicate that syncytia play a role in the SARS-CoV-2 lifecycle.