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

Identification of a cooperative effect between amino acids 169 and 174 in the rotavirus NSP4 double-layered particle-binding domain

Segmented RNA viruses are capable of exchanging genome segments via reassortment as a means of immune evasion and to maintain viral fitness. Reassortments of single-genome segments are common among group A rotaviruses. Multiple instances of co-reassortment of two genome segments, GS6(VP6) and GS10(NSP4), have been documented in surveillance. Specifically, a division between NSP4 genotypes has been observed in the NSP4 double-layered particle (DLP)-binding domain. A previously hypothesized mechanism for this co-reassortment has been suggested to be the interaction between VP6 and NSP4 during DLP transport from viroplasms for particle maturation. In this study, we used sequence analysis, RNA secondary structure prediction, molecular dynamics and reverse genetics to form a hypothesis regarding the role of the NSP4 DLP-binding domain. Sequence analysis showed that the polarity of NSP4 DLP-binding domain amino acids 169 and 174 is clearly divided between E1 and E2 NSP4 genotypes. Viruses with E1 NSP4s had 169A/I or 169S/T with 174S. E2 NSP4s had 169R/K and 174A. RNA secondary structure prediction showed that mutation in both 545 (aa169) and 561 (aa174) causes global structure remodelling. Molecular dynamics showed that the NSP4/VP6 interaction stability is increased by mutating both aa positions 169 and 174. Using reverse genetics, we showed that an R169I mutation alone does not prevent rescue. Conversely, 174A to 174S prevented rescue, and rescue could be returned by combining 174S with 169I. When compared to rSA11 NSP4-wt, both rSA11 NSP4-R169I and rSA11 NSP4-R169I/A174S had a negligible but significant reduction in titre at specific time points. This study suggests that amino acid 174 of NSP4 may be essential in maintaining the VP6/NSP4 interaction required for DLP transport. Our results suggest that maintenance of specific polarities of amino acids at positions 169 and 174 may be required for the fitness of rotavirus field strains.

Duck STING mediates antiviral autophagy directing the interferon signaling pathway to inhibit duck plague virus infection

Migratory birds are important vectors for virus transmission, how migratory birds recognize viruses and viruses are sustained in birds is still enigmatic. As an animal model for waterfowl among migratory birds, studying and dissecting the antiviral immunity and viral evasion in duck cells may pave a path to deciphering these puzzles. Here, we studied the mechanism of antiviral autophagy mediated by duck STING in DEF cells. The results collaborated that duck STING could significantly enhance LC3B-II/I turnover, LC3B-EGFP puncta formation, and mCherry/EGFP ratio, indicating that duck STING could induce autophagy. The autophagy induced by duck STING is not affected by shRNA knockdown of ATG5 expression, deletion of the C-terminal tail of STING, or TBK1 inhibitor BX795 treatment, indicating that duck STING activated non-classical selective autophagy is independent of interaction with TBK1, TBK1 phosphorylation, and interferon (IFN) signaling. The STING R235A mutant and Sar1A/B kinase mutant abolished duck STING induced autophagy, suggesting binding with cGAMP and COPII complex mediated transport are the critical prerequisite. Duck STING interacted with LC3B through LIR motifs to induce autophagy, the LIR 4/7 motif mutants of duck STING abolished the interaction with LC3B, and neither activated autophagy nor IFN expression, indicating that duck STING associates with LC3B directed autophagy and dictated innate immunity activation. Finally, we found that duck STING mediated autophagy significantly inhibited duck plague virus (DPV) infection via ubiquitously degraded viral proteins. Our study may shed light on one scenario about the control and evasion of diseases transmitted by migratory birds.

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 protective role of prebiotics and probiotics on diarrhea and gut damage in the rotavirus-infected piglets

Rotavirus is one of the pathogenic causes that induce diarrhea in young animals, especially piglets, worldwide. However, nowadays, there is no specific drug available to treat the disease, and the related vaccines have no obvious efficiency in some countries. Via analyzing the pathogenesis of rotavirus, it inducing diarrhea is mainly due to disturb enteric nervous system, destroy gut mucosal integrity, induce intracellular electrolyte imbalance, and impair gut microbiota and immunity. Many studies have already proved that prebiotics and probiotics can mitigate the damage and diarrhea induced by rotavirus infection in hosts. Based on these, the current review summarizes and discusses the effects and mechanisms of prebiotics and probiotics on rotavirus-induced diarrhea in piglets. This information will highlight the basis for the swine production utilization of prebiotics and probiotics in the prevention or treatment of rotavirus infection in the future.

Retinoic acid alleviates rotavirus-induced intestinal damage by regulating redox homeostasis and autophagic flux in piglets

Rotaviruses (RV) are a major cause of severe gastroenteritis, particularly in neonatal piglets. Despite the availability of effective vaccines, the development of antiviral therapies for RV remains an ongoing challenge. Retinoic acid (RA), a metabolite of vitamin A, has been shown to have anti-oxidative and antiviral properties. However, the mechanism by which RA exerts its intestinal-protective and antiviral effects on RV infection is not fully understood. The study investigates the effects of RA supplementation in Duroc × Landrace × Yorkshire (DLY) piglets challenged with RV. Thirty-six DLY piglets were assigned into six treatments, including a control group, RA treatment group with two concentration gradients (5 and 15 mg/d), RV treatment group, and RV treatment group with the addition of different concentration gradients of RA (5 and 15 mg/d). Our study revealed that RV infection led to extensive intestinal architecture damage, which was mitigated by RA treatment at lower concentrations by increasing the villus height and villus height/crypt depth ratio (P < 0.05), enhancing intestinal stem cell signaling and promoting intestinal barrier functions. In addition, 15 mg/d RA supplementation significantly increased NRF2 and HO-1 protein expression (P < 0.05) and GSH content (P < 0.05), indicating that RA supplementation can enhance anti-oxidative signaling and redox homeostasis after RV challenge. Additionally, the research demonstrated that RA exerts a dual impact on the regulation of autophagy, both stimulating the initiation of autophagy and hindering the flow of autophagic flux. Through the modulation of autophagic flux, RA influence the progression of RV infection. These findings provide new insights into the regulation of redox hemostasis and autophagy by RA and its potential therapeutic application in RV infection.

Host Subcellular Organelles: Targets of Viral Manipulation

Viruses have evolved sophisticated mechanisms to manipulate host cell processes and utilize intracellular organelles to facilitate their replication. These complex interactions between viruses and cellular organelles allow them to hijack the cellular machinery and impair homeostasis. Moreover, viral infection alters the cell membrane's structure and composition and induces vesicle formation to facilitate intracellular trafficking of viral components. However, the research focus has predominantly been on the immune response elicited by viruses, often overlooking the significant alterations that viruses induce in cellular organelles. Gaining a deeper understanding of these virus-induced cellular changes is crucial for elucidating the full life cycle of viruses and developing potent antiviral therapies. Exploring virus-induced cellular changes could substantially improve our understanding of viral infection mechanisms.

Live Calcium Imaging of Virus-Infected Human Intestinal Organoid Monolayers Using Genetically Encoded Calcium Indicators

Calcium signaling is an integral regulator of nearly every tissue. Within the intestinal epithelium, calcium is involved in the regulation of secretory activity, actin dynamics, inflammatory responses, stem cell proliferation, and many other uncharacterized cellular functions. As such, mapping calcium signaling dynamics within the intestinal epithelium can provide insight into homeostatic cellular processes and unveil unique responses to various stimuli. Human intestinal organoids (HIOs) are a high-throughput, human-derived model to study the intestinal epithelium and thus represent a useful system to investigate calcium dynamics. This paper describes a protocol to stably transduce HIOs with genetically encoded calcium indicators (GECIs), perform live fluorescence microscopy, and analyze imaging data to meaningfully characterize calcium signals. As a representative example, 3-dimensional HIOs were transduced with lentivirus to stably express GCaMP6s, a green fluorescent protein-based cytosolic GECI. The engineered HIOs were then dispersed into a single-cell suspension and seeded as monolayers. After differentiation, the HIO monolayers were infected with rotavirus and/or treated with drugs known to stimulate a calcium response. An epifluorescence microscope fitted with a temperature-controlled, humidified live-imaging chamber allowed for long-term imaging of infected or drug-treated monolayers. Following imaging, acquired images were analyzed using the freely available analysis software, ImageJ. Overall, this work establishes an adaptable pipeline for characterizing cellular signaling in HIOs.

Host IP3R channels are dispensable for rotavirus Ca2+ signaling but critical for intercellular Ca2+ waves that prime uninfected cells for rapid virus spread

IMPORTANCE

Many viruses exploit host Ca2+ signaling to facilitate their replication; however, little is known about how Ca2+ signals from different host and viral channels contribute to the overall dysregulation of Ca2+ signaling or promote virus replication. Using cells lacking IP3R, a host ER Ca2+ channel, we delineated intracellular Ca2+ signals within virus-infected cells and intercellular Ca2+ waves (ICWs), which increased Ca2+ signaling in neighboring, uninfected cells. In infected cells, IP3R was dispensable for rotavirus-induced Ca2+ signaling and replication, suggesting the rotavirus NSP4 viroporin supplies these signals. However, IP3R-mediated ICWs increase rotavirus replication kinetics and spread, indicating that the Ca2+ signals from the ICWs may prime nearby uninfected cells to better support virus replication upon eventual infection. This "pre-emptive priming" of uninfected cells by exploiting host intercellular pathways in the vicinity of virus-infected cells represents a novel mechanism for viral reprogramming of the host to gain a replication advantage.

Rotavirus core shell protein sites that regulate intra-particle polymerase activity

IMPORTANCE

Rotaviruses are important causes of severe gastroenteritis in young children. A characteristic feature of rotaviruses is that they copy ribonucleic acid (RNA) inside of the viral particle. In fact, the viral polymerase (VP1) only functions when it is connected to the viral inner core shell protein (VP2). Here, we employed a biochemical assay to identify which sites of VP2 are critical for regulating VP1 activity. Specifically, we engineered VP2 proteins to contain amino acid changes at structurally defined sites and assayed them for their capacity to support VP1 function in a test tube. Through this work, we were able to identify several VP2 residues that appeared to regulate the activity of the polymerase, positively and negatively. These results are important because they help explain how rotavirus synthesizes its RNA while inside of particles and they identify targets for the future rational design of drugs to prevent rotavirus disease.

HDAC-Specific Inhibitors Induce the Release of Porcine Epidemic Diarrhea Virus via the COPII-Coated Vesicles

Porcine epidemic diarrhea virus (PEDV) is an alpha-coronavirus causing acute diarrhea and high mortality in neonatal suckling piglets, resulting in huge economic losses for the global swine industry. The replication, assembly and cell egression of PEDV, an enveloped RNA virus, are mediated via altered intracellular trafficking. The underlying mechanisms of PEDV secretion are poorly understood. In this study, we found that the histone deacetylase (HDAC)-specific inhibitors, trichostatin A (TSA) and sodium butyrate (NaB), facilitate the secretion of infectious PEDV particles without interfering with its assembly. We found that PEDV N protein and its replicative intermediate dsRNA colocalize with coat protein complex II (COPII)-coated vesicles. We also showed that the colocalization of PEDV and COPII is enhanced by the HDAC-specific inhibitors. In addition, ultrastructural analysis revealed that the HDAC-specific inhibitors promote COPII-coated vesicles carrying PEDV virions and the secretion of COPII-coated vesicles. Consistently, HDAC-specific inhibitors-induced PEDV particle secretion was abolished by Sec24B knockdown, implying that the HDAC-specific inhibitors-mediated COPII-coated vesicles are required for PEDV secretion. Taken together, our findings provide initial evidence suggesting that PEDV virions can assemble in the endoplasmic reticulum (ER) and bud off from the ER in the COPII-coated vesicles. HDAC-specific inhibitors promote PEDV release by hijacking the COPII-coated vesicles.

Rotavirus Particle Disassembly and Assembly In Vivo and In Vitro

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

The Inositol Trisphosphate Receptor (IP 3 R) is Dispensable for Rotavirus-induced Ca 2+ Signaling and Replication but Critical for Paracrine Ca 2+ Signals that Prime Uninfected Cells for Rapid Virus Spread

Rotavirus is a leading cause of viral gastroenteritis. A hallmark of rotavirus infection is an increase in cytosolic Ca 2+ caused by the nonstructural protein 4 (NSP4). NSP4 is a viral ion channel that releases Ca 2+ from the endoplasmic reticulum (ER) and the increase in Ca 2+ signaling is critical for rotavirus replication. In addition to NSP4 itself, host inositol 1,4,5- trisphosphate receptor (IP 3 R) ER Ca 2+ channels may contribute to rotavirus-induced Ca 2+ signaling and by extension, virus replication. Thus, we set out to determine the role of IP 3 R Ca 2+ signaling during rotavirus infection using IP 3 R-knockout MA104-GCaMP6s cells (MA104- GCaMP6s-IP 3 R-KO), generated by CRISPR/Cas9 genome editing. Live Ca 2+ imaging showed that IP 3 R-KO did not reduce Ca 2+ signaling in infected cells but eliminated rotavirus-induced intercellular Ca 2+ waves (ICWs) and therefore the increased Ca 2+ signaling in surrounding, uninfected cells. Further, MA104-GCaMP6s-IP 3 R-TKO cells showed similar rotavirus susceptibility, single-cycle replication, and viral protein expression as parental MA104- GCaMP6s cells. However, MA104-GCaMP6s-IP 3 R-TKO cells exhibited significantly smaller rotavirus plaques, decreased multi-round replication kinetics, and delayed virus spread, suggesting that rotavirus-induced ICW Ca 2+ signaling stimulates virus replication and spread. Inhibition of ICWs by blocking the P2Y1 receptor also resulted in decreased rotavirus plaque size. Conversely, exogenous expression of P2Y1 in LLC-MK2-GCaMP6s cells, which natively lack P2Y1 and rotavirus ICWs, rescued the generation of rotavirus-induced ICWs and enabled plaque formation. In conclusion, this study shows that NSP4 Ca 2+ signals fully support rotavirus replication in individual cells; however, IP 3 R is critical for rotavirus-induced ICWs and virus spread by priming Ca 2+ -dependent pathways in surrounding cells.

Importance

Many viruses exploit host Ca 2+ signaling to facilitate their replication; however, little is known about how distinct types of Ca 2+ signals contribute to the overall dysregulation of Ca 2+ signaling or promote virus replication. Using cells lacking IP 3 R, a host ER Ca 2+ channel, we could differentiate between intracellular Ca 2+ signals within virus-infected cells and intercellular Ca 2+ waves (ICWs), which increase Ca 2+ signaling in neighboring, uninfected cells. In infected cells, IP 3 R was dispensable for rotavirus-induced Ca 2+ signaling and replication, suggesting the rotavirus NSP4 viroporin supplies these signals. However, IP 3 R-mediated ICWs increase rotavirus replication kinetics and spread, indicating that the Ca 2+ signals from the ICWs may prime nearby uninfected cells to better support virus replication upon eventual infection. This "pre-emptive priming" of uninfected cells by exploiting host intercellular pathways in the vicinity of virus-infected cells represents a novel mechanism for viral reprogramming of the host to gain a replication advantage.

Structure-Function Relationship for a Divergent Atg8 Protein Required for a Nonautophagic Function in Apicomplexan Parasites

Atg8 family proteins are highly conserved eukaryotic proteins with diverse autophagy and nonautophagic functions in eukaryotes. While the structural features required for conserved autophagy functions of Atg8 are well established, little is known about the molecular changes that facilitated acquisition of divergent, nonautophagic functions of Atg8. The malaria parasite Plasmodium falciparum offers a unique opportunity to study nonautophagic functions of Atg8 family proteins because it encodes a single Atg8 homolog whose only essential function is in the inheritance of an unusual secondary plastid called the apicoplast. Here, we used functional complementation to investigate the structure-function relationship for this divergent Atg8 protein. We showed that the LC3-interacting region (LIR) docking site (LDS), the major interaction interface of the Atg8 protein family, is required for P. falciparum Atg8 (PfAtg8) apicoplast localization and function, likely via Atg8 lipidation. On the other hand, another region previously implicated in canonical Atg8 interactions, the N-terminal helix, is not required for apicoplast-specific PfAtg8 function. Finally, our investigations at the cellular level demonstrate that the unique apicomplexan-specific loop, previously implicated in interaction with membrane conjugation machinery in recombinant protein-based in vitro assays, is not required for membrane conjugation nor for the apicoplast-specific effector function of Atg8 in both P. falciparum and related Apicomplexa member Toxoplasma gondii. These results suggest that the effector function of apicomplexan Atg8 is mediated by structural features distinct from those previously identified for macroautophagy and selective autophagy functions. IMPORTANCE The most extensively studied role of Atg8 proteins is in autophagy. However, it is clear that they have other nonautophagic functions critical to cell function and disease pathogenesis that are so far understudied compared to their canonical role in autophagy. Mammalian cells contain multiple Atg8 paralogs that have diverse, specialized functions. Gaining molecular insight into their nonautophagic functions is difficult because of redundancy between the homologs and their role in both autophagy and nonautophagic pathways. Malaria parasites such as Plasmodium falciparum are a unique system to study a novel, nonautophagic function of Atg8 separate from its role in autophagy: they have only one Atg8 protein whose only essential function is in the inheritance of the apicoplast, a unique secondary plastid organelle. Insights into the molecular basis of PfAtg8's function in apicoplast biogenesis will have important implications for the evolution of diverse nonautophagic functions of the Atg8 protein family.

Sneaking into the viral safe-houses: Implications of host components in regulating integrity and dynamics of rotaviral replication factories

The biology of the viral life cycle essentially includes two structural and functional entities—the viral genome and protein machinery constituting the viral arsenal and an array of host cellular components which the virus closely associates with—to ensure successful perpetuation. The obligatory requirements of the virus to selectively evade specific host cellular factors while exploiting certain others have been immensely important to provide the platform for designing host-directed antiviral therapeutics. Although the spectrum of host-virus interaction is multifaceted, host factors that particularly influence viral replication have immense therapeutic importance. During lytic proliferation, viruses usually form replication factories which are specialized subcellular structures made up of viral proteins and replicating nucleic acids. These viral niches remain distinct from the rest of the cellular milieu, but they effectively allow spatial proximity to selective host determinants. Here, we will focus on the interaction between the replication compartments of a double stranded RNA virus rotavirus (RV) and the host cellular determinants of infection. RV, a diarrheagenic virus infecting young animals and children, forms replication bodies termed viroplasms within the host cell cytoplasm. Importantly, viroplasms also serve as the site for transcription and early morphogenesis of RVs and are very dynamic in nature. Despite advances in the understanding of RV components that constitute the viroplasmic architecture, knowledge of the contribution of host determinants to viroplasm dynamicity has remained limited. Emerging evidence suggests that selective host determinants are sequestered inside or translocated adjacent to the RV viroplasms. Functional implications of such host cellular reprogramming are also ramifying—disarming the antiviral host determinants and usurping the pro-viral components to facilitate specific stages of the viral life cycle. Here, we will provide a critical update on the wide variety of host cellular pathways that have been reported to regulate the spatial and temporal dynamicity of RV viroplasms. We will also discuss the methods used so far to study the host-viroplasm interactions and emphasize on the potential host factors which can be targeted for therapeutic intervention in the future.

Re-Examining Rotavirus Innate Immune Evasion: Potential Applications of the Reverse Genetics System

Rotaviruses represent one of the most successful pathogens in the world, with high infectivity and efficient transmission between the young of many animal species, including humans. To overcome host defenses, rotaviruses have evolved a plethora of strategies to effectively evade the innate immune response, establish initial infection in the small intestine, produce progeny, and shed into the environment. Previously, studying the roles and relative contributions of specific rotaviral factors in innate immune evasion had been challenging without a plasmid-only reverse genetics system. Although still in its infancy, current reverse genetics technology will help address important research questions regarding rotavirus innate immune evasion, host range restriction, and viral pathogenesis. In this review, we summarize the current knowledge about the antiviral host innate immune defense mechanisms, countermeasures of rotavirus-encoded factors, and strategies to better understand these interactions using the rotavirus reverse genetics system.

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.

Rotavirus-Induced Lipid Droplet Biogenesis Is Critical for Virus Replication

A variety of pathogens, including viruses, bacteria and parasites, target cellular lipid droplets for their replication. Rotaviruses (RVs) infect the villous epithelium of the small intestine and are a major cause of acute gastroenteritis in infants and young children worldwide. RVs induce and require lipid droplets for the formation of viroplasms, sites of virus genome replication, and nascent particle assembly. Here we review the role of lipid droplets in RV replication. Inhibitors of fatty acid synthesis or chemicals that interfere with lipid droplet homeostasis decrease the number and size of viroplasms and the yield of infectious virus. We used a genetically engineered RV, delayed in viroplasm assembly, to show an early interaction of RV nonstructural protein NSP2 and the lipid droplet-associated protein phospho-PLIN1. The interaction between NSP2 and phospho-PLIN1 suggests that we have identified part of the mechanism of RV-induced lipid droplet formation. These studies demonstrate that RV is an excellent model to dissect the cellular process of lipid droplet formation and to determine how RV induces and usurps lipid droplet biogenesis to form viroplasm/lipid droplets for virus replication.

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.

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.

HCV egress - unconventional secretion of assembled viral particles

It is believed that hepatitis C virus (HCV) particles are released through the canonical secretory route: from the endoplasmic reticulum (ER), via the Golgi, to the plasma membrane. While the Golgi is important for HCV release per se, its direct involvement in the trafficking of assembled virions has not yet been established. In fact, data from studies analyzing HCV egress are compatible with several potential pathways of HCV secretion. Here, we summarize and discuss the current knowledge related to the HCV export pathway. Apart from the prototypical anterograde transport, possible routes of HCV release include ER-to-endosomal transport, secretory autophagy, and poorly described mechanisms of unconventional protein secretion. Studying HCV egress promises to shed light on unconventional cellular trafficking and secretory routes.

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.

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.

Autophagosome biogenesis: From membrane growth to closure

Autophagosome biogenesis involves de novo formation of a membrane that elongates to sequester cytoplasmic cargo and closes to form a double-membrane vesicle (an autophagosome). This process has remained enigmatic since its initial discovery >50 yr ago, but our understanding of the mechanisms involved in autophagosome biogenesis has increased substantially during the last 20 yr. Several key questions do remain open, however, including, What determines the site of autophagosome nucleation? What is the origin and lipid composition of the autophagosome membrane? How is cargo sequestration regulated under nonselective and selective types of autophagy? This review provides key insight into the core molecular mechanisms underlying autophagosome biogenesis, with a specific emphasis on membrane modeling events, and highlights recent conceptual advances in the field.

Viral use and subversion of membrane organization and trafficking

Membrane trafficking is an essential cellular process conserved across all eukaryotes, which regulates the uptake or release of macromolecules from cells, the composition of cellular membranes and organelle biogenesis. It influences numerous aspects of cellular organisation, dynamics and homeostasis, including nutrition, signalling and cell architecture. Not surprisingly, malfunction of membrane trafficking is linked to many serious genetic, metabolic and neurological disorders. It is also often hijacked during viral infection, enabling viruses to accomplish many of the main stages of their replication cycle, including entry into and egress from cells. The appropriation of membrane trafficking by viruses has been studied since the birth of cell biology and has helped elucidate how this integral cellular process functions. In this Review, we discuss some of the different strategies viruses use to manipulate and take over the membrane compartments of their hosts to promote their replication, assembly and egress.

PSPH induces cell autophagy and promotes cell proliferation and invasion in the hepatocellular carcinoma cell line Huh7 via the AMPK/mTOR/ULK1 signaling pathway

Phosphoserine phosphatase (PSPH), a key enzyme of the l-serine synthesis pathway, has been involved in cancer progression and survival. However, limited evidence revealed the PSPH influence on hepatocellular carcinoma (HCC). Herein, we observed that PSPH expression was upregulated in both HCC tissues and cell lines, which was determined by western blotting. TCGA database showed that the PSPH protein levels were significantly upregulated and affected patient survival rates in HCC. Then gain- and loss-of-function manipulations were performed by transfection with a pcDNA-PSPH expression vector or a specific short interfering RNA against PSPH in Huh7 cells. Huh7 cell proliferation, stemness, invasion, and apoptosis were assessed by using CCK-8 test, colony formation assay, Transwell assay, and Flow cytometry analysis, respectively, and levels of autophagy-related proteins were detected by using western blotting. The results showed that PSPH could induce Huh7 cell autophagy, promote cell proliferation and invasion, and inhibit apoptosis. The knockdown of PSPH could inhibit Huh7 cell proliferation, invasion, and autophagy. Furthermore, PSPH activated Liver kinase B1 (LKB1) and TGF beta-activated kinase 1 (TAK1), affected the adenosine 5'-monophosphate-activated protein kinase (AMPK)/mTOR/ULK1 signaling pathway, but could not activate calcium/calmodulin-dependent protein kinase kinase (CaMKK) in Huh7 cells. Inhibition of either LKB1, TAK1, or AMPK could eliminate the effect of PSPH overexpression on Huh7 cell behaviors. However, inhibition of CaMKK could not influence the effect of PSPH overexpression on Huh7 cell behaviors. In conclusion, PSPH could induce autophagy, promote proliferation and invasion, and inhibit apoptosis in HCC cells via the AMPK/mTOR/ULK1 signaling pathway.

A Calcium Sensor Discovered in Bluetongue Virus Nonstructural Protein 2 Is Critical for Virus Replication

After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Many viruses use specific viral proteins to bind calcium ions (Ca²⁺) for stability or to modify host cell pathways; however, to date, no Ca²⁺ binding protein has been reported in bluetongue virus (BTV), the causative agent of bluetongue disease in livestock. Here, using a comprehensive bioinformatics screening, we identified a putative EF-hand-like Ca²⁺ binding motif in the carboxyl terminal region of BTV nonstructural phosphoprotein 2 (NS2). Subsequently, using a recombinant NS2, we demonstrated that NS2 binds Ca²⁺ efficiently and that Ca²⁺ binding was perturbed when the Asp and Glu residues in the motif were substituted by alanine. Using circular dichroism analysis, we found that Ca²⁺ binding by NS2 triggered a helix-to-coil secondary structure transition. Further, cryo-electron microscopy in the presence of Ca²⁺ revealed that NS2 forms helical oligomers which, when aligned with the N-terminal domain crystal structure, suggest an N-terminal domain that wraps around the C-terminal domain in the oligomer. Further, an in vitro kinase assay demonstrated that Ca²⁺ enhanced the phosphorylation of NS2 significantly. Importantly, mutations introduced at the Ca²⁺ binding site in the viral genome by reverse genetics failed to allow recovery of viable virus, and the NS2 phosphorylation level and assembly of viral inclusion bodies (VIBs) were reduced. Together, our data suggest that NS2 is a dedicated Ca²⁺ binding protein and that calcium sensing acts as a trigger for VIB assembly, which in turn facilitates virus replication and assembly.

IMPORTANCE After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Modification of the Mammalian Endomembrane System in Healthy and Diseased Cells

One remarkable characteristic of eukaryotic cells is the complexity of their membrane systems [...].