Activation of the endoplasmic reticulum calcium sensor STIM1 and store-operated calcium entry by rotavirus requires NSP4 viroporin activity

Advancing the field of viroporins-Structure, function and pharmacology: IUPHAR Review 39

Viroporins possess important potential as antiviral targets due to their critical roles during virus life cycles, spanning from virus entry to egress. Although the antiviral amantadine targets the M2 viroporin of influenza A virus, successful progression of other viroporin inhibitors into clinical use remains challenging. These challenges relate in varying proportions to a lack of reliable full-length 3D-structures, difficulties in functionally characterising individual viroporins, and absence of verifiable direct binding between inhibitor and viroporin. This review offers perspectives to help overcome these challenges. We provide a comprehensive overview of the viroporin family, including their structural and functional features, highlighting the moldability of their energy landscapes and actions. To advance the field, we suggest a list of best practices to aspire towards unambiguous viroporin identification and characterisation, along with considerations of potential pitfalls. Finally, we present current and future scenarios of, and prospects for, viroporin targeting drugs.

Role of Divalent Cations in Infections in Host-Pathogen Interaction

With increasing numbers of patients worldwide diagnosed with diabetes mellitus, renal disease, and iatrogenic immune deficiencies, an increased understanding of the role of electrolyte interactions in mitigating pathogen virulence is necessary. The levels of divalent cations affect host susceptibility and pathogen survival in persons with relative immune insufficiency. For instance, when host cellular levels of calcium are high compared to magnesium, this relationship contributes to insulin resistance and triples the risk of clinical tuberculosis. The movement of divalent cations within intracellular spaces contributes to the host defense, causing apoptosis or autophagy of the pathogen. The control of divalent cation flow is dependent in part upon the mammalian natural resistance-associated macrophage protein (NRAMP) in the host. Survival of pathogens such as M tuberculosis within the bronchoalveolar macrophage is also dependent upon NRAMP. Pathogens evolve mutations to control the movement of calcium through external and internal channels. The host NRAMP as a metal transporter competes for divalent cations with the pathogen NRAMP in M tuberculosis (whether in latent, dormant, or active phase). This review paper summarizes mechanisms of pathogen offense and patient defense using inflow and efflux through divalent cation channels under the influence of parathyroid hormone vitamin D and calcitonin.

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.

Rotavirus non-structural protein 4 usurps host cellular RIPK1-RIPK3 complex to induce MLKL-dependent necroptotic cell death

The dynamic interface between invading viral pathogens and programmed cell death (PCD) of the host is a finely regulated process. Host cellular demise at the end of the viral life cycle ensures the release of progeny virions to initiate new infection cycles. Rotavirus (RV), a diarrheagenic virus with double-stranded RNA genome, has been reported to trigger different types of PCD such as apoptosis and pyroptosis in a highly regulated way to successfully disseminate progeny virions. Recently our lab also showed that induction of MLKL-driven programmed necroptosis by RV. However, the host cellular machinery involved in RV-induced necroptosis and the upstream viral trigger responsible for it remained unaddressed. In the present study, the signalling upstream of MLKL-driven necroptosis has been delineated where the involvement of Receptor interacting serine/threonine kinase 3 (RIPK3) and 1 (RIPK1) from the host side and RV non-structural protein 4 (NSP4) as the viral trigger for necroptosis has been shown. Interestingly, RV-NSP4 was found to be an integral component of the necrosome complex by interacting with RIPK1, thereby bypassing the requirement of RIPK1 kinase activity. Subsequently, NSP4-driven elevated cytosolic Ca2+ concentration and Ca2+-binding to NSP4 lead further to RHIM domain-dependent RIPK1-RIPK3 interaction, RIPK3-dependent MLKL phosphorylation, and eventual necroptosis. Overall, this study presents the interplay between RV-NSP4 and the host cellular necrosome complex to induce necroptotic death of host cells.

Purinergic Signaling Drives Multiple Aspects of Rotavirus Pathophysiology

Rotavirus causes life-threatening diarrhea in children, resulting in ∼200,000 deaths/year. The current treatment during infection is Oral Rehydration Solution which successfully replenishes fluids but does not alleviate diarrhea volume or severity. As a result, there is an urgent need to better understand rotavirus pathophysiology and develop more effective pediatric therapeutics. Rotavirus primarily infects the tips of small intestinal villi, yet has far-reaching effects on cell types distant from infected cells. We recently identified that rotavirus infected cells release the purinergic signaling molecule ADP, which activates P2Y1 receptors on nearby uninfected cells in vitro . To elucidate the role of purinergic signaling via P2Y1 receptors during rotavirus infection in vivo , we used the mouse-like rotavirus strain D6/2 which generates a severe infection in mice. C57BL/6J mouse pups were given an oral gavage of D6/2 rotavirus and assessed over the course of 5-7 days. Beginning at day 1 post infection, infected pups were treated daily by oral gavage with saline or 4 mg/kg MRS2500, a selective P2Y1 antagonist. Mice were monitored for diarrhea severity, diarrhea incidence, and viral shedding. Neonatal mice were euthanized at days 3 and 5 post-infection and small intestine was collected to observe infection. MRS2500 treatment decreased the severity, prevalence, and incidence of rotavirus diarrhea. Viral stool shedding, assessed by qPCR for rotavirus gene levels, revealed that MRS2500 treated pups had significantly lower viral shedding starting at day 4 post infection compared to saline treated pups, which suggests P2Y1 signaling may enhance rotavirus replication. Finally, we found that inhibition of P2Y1 with MRS2500 limited transmitted rotavirus diarrhea to uninfected pups within a litter. Together, these results suggest that P2Y1 signaling is involved in the pathogenesis of a homologous murine rotavirus strain, making P2Y1 receptors a promising anti-diarrheal, anti-viral therapeutic target to reduce rotavirus disease burden.

Viral regulation of organelle membrane contact sites

At the core of organelle functions lies their ability and need to form dynamic organelle-organelle networks that drive intracellular communication and coordination of cellular pathways. These networks are facilitated by membrane contact sites (MCSs) that promote both intra-organelle and inter-organelle communication. Given their multiple functions, MCSs and the proteins that form them are commonly co-opted by viruses during infection to promote viral replication. This Essay discusses mechanisms acquired by diverse human viruses to regulate MCS functions in either proviral processes or host defense. It also examines techniques used for examining MCSs in the context of viral infections.

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.

Staphylococcus aureus α-toxin impairs early neutrophil localization via electrogenic disruption of store-operated calcium entry

The pore-forming S. aureus α-toxin (Hla) contributes to virulence and disease pathogenesis. While high concentrations of toxin induce cell death, neutrophils exhibit relative resistance to lysis, suggesting that the action of Hla may not be solely conferred by lytic susceptibility. Using intravital microscopy, we observed that Hla disrupts neutrophil localization and clustering early in infection. Hla forms a narrow, ion-selective pore, suggesting that Hla may dysregulate calcium or other ions to impair neutrophil function. We found that sub-lytic Hla did not permit calcium influx but caused rapid membrane depolarization. Depolarization decreases the electrogenic driving force for calcium, and concordantly, Hla suppressed calcium signaling in vitro and in vivo and calcium-dependent leukotriene B4 (LTB4) production, a key mediator of neutrophil clustering. Thus, Hla disrupts the early patterning of the neutrophil response to infection, in part through direct impairment of neutrophil calcium signaling. This early mis-localization of neutrophils may contribute to establishment of infection.

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.

Targeting host calcium channels and viroporins: a promising strategy for SARS-CoV-2 therapy

Despite passing the pandemic phase of the COVID-19, researchers are still investigating various drugs. Previous evidence suggests that blocking the calcium channels may be a suitable treatment option. Ca2+ is required to enhance the fusion process of Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Also, some important inflammatory factors during SARS-CoV-2 infection are dependent on Ca2+ level. On the other hand, viroporins have emerged as attractive targets for antiviral therapy due to their essential role in viral replication and pathogenesis. By inhibiting the host calcium channels and viroporins, it is possible to limit the spread of infection. Therefore, calcium channel blockers (CCBs) and drugs targeting Viroporins can be considered an effective option in the fight against SARS-CoV-2.

Differential Cellular Sensing of Fusion from within and Fusion from without during Virus Infection

The physical entry of virus particles into cells triggers an innate immune response that is dependent on both calcium and nucleic acid sensors, with particles containing RNA or DNA genomes detected by RNA or DNA sensors, respectively. While membrane fusion in the absence of viral nucleic acid causes an innate immune response that is dependent on calcium, the involvement of nucleic acid sensors is poorly understood. Here, we used lipoplexes containing purified reovirus p14 fusion protein as a model of exogenous or fusion from without and a cell line expressing inducible p14 protein as a model of endogenous or fusion from within to examine cellular membrane fusion sensing events. We show that the cellular response to membrane fusion in both models is dependent on calcium, IRF3 and IFN. The method of sensing fusion, however, differs between fusion from without and fusion from within. Exogenous p14 lipoplexes are detected by RIG-I-like RNA sensors, whereas fusion by endogenous p14 requires both RIG-I and STING to trigger an IFN response. The source of nucleic acid that is sensed appears to be cellular in origin. Future studies will investigate the source of endogenous nucleic acids recognized following membrane fusion events.

Understanding the role of Ca2+ via transient receptor potential (TRP) channel in viral infection: Implications in developing future antiviral strategies

Transient receptor potential (TRP) channels are a superfamily of cation-specific permeable channels primarily conducting Ca2+ions across various membranes of the cell. The perturbation of the Ca2+ homeostasis is the hallmark of viral infection. Viruses hijack the host cell Ca2+ signaling, employing tailored Ca2+ requirements via TRP channels to meet their own cellular demands. This review summarizes the importance of Ca2+ across diverse viruses based on the Baltimore classification and focuses on the associated role of Ca2+-conducting TRP channels in viral pathophysiology. More emphasis has been given to the role of the TRP channel in viral life-cycle events such as viral fusion, viral entry, viral replication, virion maturation, and egress. Additionally, this review highlights the TRP channel as a store-operated channel which has been discussed vividly. The TRP channels form an essential aspect of host-virus interaction by virtue of its Ca2+ permeability. These channels are directly involved in regulating the viral calcium dynamics in host cells and thereby affect the viral infection. Considering its immense potential in regulating viral infection, the TRP channels may act as a target for antiviral therapeutics.

Gastrointestinal organoids in the study of viral infections

Viruses are among the most prevalent enteric pathogens. Although virologists historically relied on cell lines and animal models, human intestinal organoids (HIOs) continue to grow in popularity. HIOs are nontransformed, stem cell-derived, ex vivo cell cultures that maintain the cell type diversity of the intestinal epithelium. They offer higher throughput than standard animal models while more accurately mimicking the native tissue of infection than transformed cell lines. Here, we review recent literature that highlights virological advances facilitated by HIOs. We discuss the variations and limitations of HIOs, how HIOs have allowed for the cultivation of previously uncultivatable viruses, and how they have offered insight into tropism, entry, replication kinetics, and host-pathogen interactions. In each case, we discuss exemplary viruses and archetypal studies. We discuss how the speed and flexibility of HIO-based studies contributed to our knowledge of SARS-CoV-2 and antiviral therapeutics. Finally, we discuss the current limitations of HIOs and future directions to overcome these.

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.

Calcium-sensing receptor acts as an antiviral factor for rotavirus infections and participates in cellular antiviral response

OBJECTIVES

Rotavirus (RV) is one of the most significant pathogens associated with childhood diarrhoeal deaths worldwide. Elevated cytoplasmic calcium is required for RV replication, but the underlying mechanisms responsible for calcium influx remain poorly understood. The Calcium-sensing receptor (CaSR) is an important Ca²⁺ sensor that regulates the transport of Ca²⁺ into or out of the extracellular space by affecting the status of Ca²⁺ ion channels on the membrane of cells. Currently, the function of CaSR in RV replication is unclear.

MATERIALS AND METHODS

We evaluated the mRNA and protein levels of CaSR in RV-infected cells using qRT-PCR and Western blotting, respectively. Furthermore, we silenced or overexpressed CaSR in Caco-2 cells using siRNA or a CaSR gene contained adenovirus (Adv-CaSR). qRT-PCR, plaque assay, and Western blotting were used to determine the synthesis of virus genomic RNA, production of progeny virion, and the levels of viral proteins. The content of Ca²⁺ in cells was observed under confocal microscopy.

RESULTS

Compared with control cells, the RV-infected cells presented significantly decreased CaSR expression. Moreover, adenoviral-mediated over-expression or induction of CaSR by R568 greatly inhibited the RV RNA synthesis, protein expression, and formation of viroplasm plaques, thereby suppressing RV replication. In contrast, CaSR-silenced cells exhibited significantly enhanced RV replication. Compared with the Adv-Control group, the concentration of cytosolic Ca²⁺ significantly decreased in the Adv-CaSR group.

CONCLUSION

These findings demonstrated that CaSR is a potential target for inhibition of RV replication. Therefore, enhancing the expression of CaSR might protect hosts from RV infections.

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.

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.

Opposite effects of apoptotic and necroptotic cellular pathways on rotavirus replication

Group A rotavirus (RVA), one of the leading pathogens causing severe acute gastroenteritis in children and a wide variety of young animals worldwide, induces apoptosis upon infecting cells. Though RVA-induced apoptosis mediated via the dual modulation of its NSP4 and NSP1 proteins is relatively well studied, the nature and signaling pathway(s) involved in RVA-induced necroptosis are yet to be fully elucidated. Here, we demonstrate the nature of RVA-induced necroptosis, the signaling cascade involved, and correlation with RVA-induced apoptosis. Infection with the bovine NCDV and human DS-1 RV strains was shown to activate receptor-interacting protein kinase 1 (RIPK1)/RIPK3/mixed lineage kinase domain-like protein (MLKL), the key necroptosis molecules in virus-infected cells. Using immunoprecipitation assay, RIPK1 was found to bind phosphorylated RIPK3 (pRIPK3) and pMLKL. pMLKL, the major executioner molecule in the necroptotic pathway, was translocated to the plasma membrane of RVA-infected cells to puncture the cell membrane. Interestingly, transfection of RVA NSP4 also induced necroptosis through the RIPK1/RIPK3/MLKL necroptosis pathway. Blockage of each key necroptosis molecule in the RVA-infected or NSP4-transfected cells resulted in decreased necroptosis but increased cell viability and apoptosis, thereby resulting in decreased viral yields in the RVA-infected cells. In contrast, suppression of RVA-induced apoptosis increased necroptosis and virus yields. Our findings suggest that RVA NSP4 also induces necroptosis via the RIPK1/RIPK3/MLKL necroptosis pathway. Moreover, necroptosis and apoptosis-which have proviral and antiviral effects, respectively-exhibited a crosstalk in RVA-infected cells. These findings significantly increase our understanding of the nature of RVA-induced necroptosis and the crosstalk between RVA-induced necroptosis and apoptosis. IMPORTANCE Viral infection usually culminates in cell death through apoptosis, necroptosis, and rarely, pyroptosis. Necroptosis is a form of programmed necrosis that is mediated by signaling complexes of the receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL). Although apoptosis induction by rotavirus and its NSP4 protein is well known, rotavirus-induced necroptosis is not fully understood. Here, we demonstrate that rotavirus and also its NSP4 protein can induce necroptosis in cultured cells through the activation of the RIPK1/RIPK3/MLKL necroptosis pathway. Moreover, rotavirus-induced necroptosis and apoptosis have opposite effects on viral yield, i.e., they function as proviral and antiviral processes, respectively, and counterbalance each other in rotavirus-infected cells. Our findings provide important insights for understanding the nature of rotavirus-induced necroptosis and the development of novel therapeutic strategies against infection with rotavirus and other RNA viruses.

Dysregulation of host cell calcium signaling during viral infections: Emerging paradigm with high clinical relevance

Viral infections are one of the leading causes of human illness. Viruses take over host cell signaling cascades for their replication and infection. Calcium (Ca2+) is a versatile and ubiquitous second messenger that modulates plethora of cellular functions. In last two decades, a critical role of host cell Ca2+ signaling in modulating viral infections has emerged. Furthermore, recent literature clearly implicates a vital role for the organellar Ca2+ dynamics (influx and efflux across organelles) in regulating virus entry, replication and severity of the infection. Therefore, it is not surprising that a number of viral infections including current SARS-CoV-2 driven COVID-19 pandemic are associated with dysregulated Ca2+ homeostasis. The focus of this review is to first discuss the role of host cell Ca2+ signaling in viral entry, replication and egress. We further deliberate on emerging literature demonstrating hijacking of the host cell Ca2+ dynamics by viruses. In particular, a variety of viruses including SARS-CoV-2 modulate lysosomal and cytosolic Ca2+ signaling for host cell entry and replication. Moreover, we delve into the recent studies, which have demonstrated the potential of several FDA-approved drugs targeting Ca2+ handling machinery in inhibiting viral infections. Importantly, we discuss the prospective of targeting intracellular Ca2+ signaling for better management and treatment of viral pathogenesis including COVID-19. Finally, we highlight the key outstanding questions in the field that demand critical and timely attention.

Partial glycosylation of the Ibaraki virus NS3 protein is sufficient to support virus propagation

Ibaraki virus (IBAV) causes Ibaraki disease. We have previously shown that IBAV NS3 protein is highly glycosylated and that tunicamycin, an inhibitor of N-linked glycosylation, suppressed NS3 glycosylation and viral propagation. Since tunicamycin is known to cause endoplasmic reticulum (ER) stress, we explored the effects of ER stress and NS3 glycosylation on IBAV infection using tunicamycin and thapsigargin. These reagents both induced ER stress and NS3 glycosylation inhibition in a concentration-dependent manner, and as in our previous report, high concentrations of tunicamycin and thapsigargin suppressed IBAV propagation. However, lower concentrations of these reagents produced limited differences in IBAV propagation, despite their ability to suppress NS3 glycosylation and induce ER stress. These findings suggest that a considerable degree of NS3 glycosylation inhibition and ER stress induction does not suppress IBAV propagation. Conversely, lower concentrations of thapsigargin enhanced IBAV propagation, suggesting that moderate ER stress could benefit IBAV.

Rotavirus A Genome Segments Show Distinct Segregation and Codon Usage Patterns

Reassortment of the Rotavirus A (RVA) 11-segment dsRNA genome may generate new genome constellations that allow RVA to expand its host range or evade immune responses. Reassortment may also produce phylogenetic incongruities and weakly linked evolutionary histories across the 11 segments, obscuring reassortment-specific epistasis and changes in substitution rates. To determine the co-segregation patterns of RVA segments, we generated time-scaled phylogenetic trees for each of the 11 segments of 789 complete RVA genomes isolated from mammalian hosts and compared the segments’ geodesic distances. We found that segments 4 (VP4) and 9 (VP7) occupied significantly different tree spaces from each other and from the rest of the genome. By contrast, segments 10 and 11 (NSP4 and NSP5/6) occupied nearly indistinguishable tree spaces, suggesting strong co-segregation. Host-species barriers appeared to vary by segment, with segment 9 (VP7) presenting the weakest association with host species. Bayesian Skyride plots were generated for each segment to compare relative genetic diversity among segments over time. All segments showed a dramatic decrease in diversity around 2007 coinciding with the introduction of RVA vaccines. To assess selection pressures, codon adaptation indices and relative codon deoptimization indices were calculated with respect to different host genomes. Codon usage varied by segment with segment 11 (NSP5) exhibiting significantly higher adaptation to host genomes. Furthermore, RVA codon usage patterns appeared optimized for expression in humans and birds relative to the other hosts examined, suggesting that translational efficiency is not a barrier in RVA zoonosis.

Functions of Coronavirus Accessory Proteins: Overview of the State of the Art

Coronavirus accessory proteins are a unique set of proteins whose genes are interspersed among or within the genes encoding structural proteins. Different coronavirus genera, or even different species within the same coronavirus genus, encode varying amounts of accessory proteins, leading to genus- or species-specificity. Though accessory proteins are dispensable for the replication of coronavirus in vitro, they play important roles in regulating innate immunity, viral proliferation, and pathogenicity. The function of accessory proteins on virus infection and pathogenesis is an area of particular interest. In this review, we summarize the current knowledge on accessory proteins of several representative coronaviruses that infect humans or animals, including the emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with an emphasis on their roles in interaction between virus and host, mainly involving stress response, innate immunity, autophagy, and apoptosis. The cross-talking among these pathways is also discussed.

Host Calcium Channels and Pumps in Viral Infections

Ca²⁺ is essential for virus entry, viral gene replication, virion maturation, and release. The alteration of host cells Ca²⁺ homeostasis is one of the strategies that viruses use to modulate host cells signal transduction mechanisms in their favor. Host calcium-permeable channels and pumps (including voltage-gated calcium channels, store-operated channels, receptor-operated channels, transient receptor potential ion channels, and Ca²⁺-ATPase) mediate Ca²⁺ across the plasma membrane or subcellular organelles, modulating intracellular free Ca²⁺. Therefore, these Ca²⁺ channels or pumps present important aspects of viral pathogenesis and virus–host interaction. It has been reported that viruses hijack host calcium channels or pumps, disturbing the cellular homeostatic balance of Ca²⁺. Such a disturbance benefits virus lifecycles while inducing host cells’ morbidity. Evidence has emerged that pharmacologically targeting the calcium channel or calcium release from the endoplasmic reticulum (ER) can obstruct virus lifecycles. Impeding virus-induced abnormal intracellular Ca²⁺ homeostasis is becoming a useful strategy in the development of potent antiviral drugs. In this present review, the recent identified cellular calcium channels and pumps as targets for virus attack are emphasized.

Porcine deltacoronavirus (PDCoV) modulates calcium influx to favor viral replication

Ionic calcium (Ca²⁺) is a versatile intracellular second messenger that plays important roles in cellular physiological and pathological processes. Porcine deltacoronavirus (PDCoV) is an emerging enteropathogenic coronavirus that causes serious vomiting and diarrhea in suckling piglets. In this study, the role of Ca²⁺ to PDCoV infection was investigated. PDCoV infection was found to upregulate intracellular Ca²⁺ concentrations of IPI-2I cells. Chelating extracellular Ca²⁺ by EGTA inhibited PDCoV replication, and this inhibitory effect was overcome by replenishment with CaCl₂. Treatment with Ca²⁺ channel blockers, particularly the L-type Ca²⁺ channel blocker diltiazem hydrochloride, inhibited PDCoV infection significantly. Mechanistically, diltiazem hydrochloride reduces PDCoV infection by inhibiting the replication step of the viral replication cycle. Additionally, knockdown of CACNA1S, the L-type Ca²⁺ voltage-gated channel subunit, inhibited PDCoV replication. The combined results demonstrate that PDCoV modulates calcium influx to favor its replication.

Recovirus NS1-2 Has Viroporin Activity That Induces Aberrant Cellular Calcium Signaling To Facilitate Virus Replication

Enteric viruses in the Caliciviridae family cause acute gastroenteritis in humans and animals, but the cellular processes needed for virus replication and disease remain unknown. A common strategy among enteric viruses, including rotaviruses and enteroviruses, is to encode a viral ion channel (i.e., viroporin) that is targeted to the endoplasmic reticulum (ER) and disrupts host calcium (Ca2+) homeostasis. Previous reports have demonstrated genetic and functional similarities between the nonstructural proteins of caliciviruses and enteroviruses, including the calicivirus NS1-2 protein and the 2B viroporin of enteroviruses. However, it is unknown whether caliciviruses alter Ca2+ homeostasis for virus replication or whether the NS1-2 protein has viroporin activity like its enterovirus counterpart. To address these questions, we used Tulane virus (TV), a rhesus enteric calicivirus, to examine Ca2+ signaling during infection and determine whether NS1-2 has viroporin activity that disrupts Ca2+ homeostasis. We found that TV increases Ca2+ signaling during infection and that increased cytoplasmic Ca2+ levels are important for efficient replication. Further, TV NS1-2 localizes to the endoplasmic reticulum, the predominant intracellular Ca2+ store, and the NS2 region has characteristics of a viroporin domain (VPD). NS1-2 had viroporin activity in a classic bacterial functional assay and caused aberrant Ca2+ signaling when expressed in mammalian cells, but truncation of the VPD abrogated these activities. Together, our data provide new mechanistic insights into the function of the NS2 region of NS1-2 and support the premise that enteric viruses, including those within Caliciviridae, exploit host Ca2+ signaling to facilitate their replication.IMPORTANCE Tulane virus is one of many enteric caliciviruses that cause acute gastroenteritis and diarrheal disease. Globally, enteric caliciviruses affect both humans and animals and amass >65 billion dollars per year in treatment and health care-associated costs, thus imposing an enormous economic burden. Recent progress has resulted in several cultivation systems (B cells, enteroids, and zebrafish larvae) to study human noroviruses, but mechanistic insights into the viral factors and host pathways important for enteric calicivirus replication and infection are still largely lacking. Here, we used Tulane virus, a calicivirus that is biologically similar to human noroviruses and can be cultivated by conventional cell culture, to identify and functionally validate NS1-2 as an enteric calicivirus viroporin. Viroporin-mediated calcium signaling may be a broadly utilized pathway for enteric virus replication, and its existence within caliciviruses provides a novel approach to developing antivirals and comprehensive therapeutics for enteric calicivirus diarrheal disease outbreaks.

Rotavirus Calcium Dysregulation Manifests as Dynamic Calcium Signaling in the Cytoplasm and Endoplasmic Reticulum

Like many viruses, rotavirus (RV) dysregulates calcium homeostasis by elevating cytosolic calcium ([Ca²⁺]cyt) and decreasing endoplasmic reticulum (ER) stores. While an overall, monophasic increase in [Ca²⁺]cyt during RV infection has been shown, the nature of the RV-induced aberrant calcium signals and how they manifest over time at the single-cell level have not been characterized. Thus, we generated cell lines and human intestinal enteroids (HIEs) stably expressing cytosolic and/or ER-targeted genetically-encoded calcium indicators to characterize calcium signaling throughout RV infection by time-lapse imaging. We found that RV induces highly dynamic [Ca²⁺]cyt signaling that manifest as hundreds of discrete [Ca²⁺]cyt spikes, which increase during peak infection. Knockdown of nonstructural protein 4 (NSP4) attenuates the [Ca²⁺]cyt spikes, consistent with its role in dysregulating calcium homeostasis. RV-induced [Ca²⁺]cyt spikes were primarily from ER calcium release and were attenuated by inhibiting the store-operated calcium entry (SOCE) channel Orai1. RV-infected HIEs also exhibited prominent [Ca²⁺]cyt spikes that were attenuated by inhibiting SOCE, underlining the relevance of these [Ca²⁺]cyt spikes to gastrointestinal physiology and role of SOCE in RV pathophysiology. Thus, our discovery that RV increases [Ca²⁺]cyt by dynamic calcium signaling, establishes a new, paradigm-shifting understanding of the spatial and temporal complexity of virus-induced calcium signaling.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

CRAC channel regulation of innate immune cells in health and disease

Calcium is a major intracellular signaling messenger in innate immune cells. Similar to other immune cell subsets, the majority of calcium entry into innate immune cells is induced by cell surface receptors that stimulate store-operated calcium entry through calcium-release activated calcium (CRAC) channels. Since the molecular description of the STIM family of calcium sensors and the ORAI family of CRAC channel proteins, the majority of studies support a dominant role for these proteins in calcium signaling in innate cells. In reviewing the literature on CRAC channel function in innate cells, several general themes emerge. All innate cells express multiple members of the STIM and ORAI family members, however the ratio and relative contribution of individual isoforms changes depending on the cell type and activation state of the cell. It is evident that study of functional roles for STIM molecules is clearly ahead of studies of specific ORAI family members in all innate cell types, and that studies of CRAC channels in innate cells are not nearly as advanced as studies in lymphocytes. However, taken together, evidence from both STIM calcium sensors and ORAI channels in innate cells indicates that deficiency of STIM and ORAI proteins tends not to affect the development of any innate cell lineage, but certainly affects their function, in particular activation of the neutrophil oxidase and mast cell activation via IgE receptors. Furthermore, there are clearly hints that therapeutic targeting of CRAC channels in innate cells offers a new approach to various inflammatory and allergic diseases.

Amelioration of bleomycin-induced pulmonary fibrosis by chlorogenic acid through endoplasmic reticulum stress inhibition

To investigate the inhibitory effects of chlorogenic acid on pulmonary fibrosis and the internal mechanisms in vivo and in vitro. 30 male BALB/C mice were randomized into 5 groups: control group, pulmonary fibrosis model group, low, middle and high dose of chlorogenic acid groups. Mice in pulmonary fibrosis model group were administered 5.0 mg/kg bleomycin with intracheal instillation and mice in 3 chlorogenic acid groups were treated with chlorogenic acid every day for 28 days after bleomycin administration. Lung tissue histology was observed using HE staining. Primary pulmonary fibroblasts were isolated and cultured. The expressions of fibrosis related factors (α-SMA and collagen I), as well as ER stress markers (CHOP and GRP78) were determined by both real-time PCR assay and Western blotting, while the expressions of other ER stress signaling pathway factors PERK, IRE-1, ATF-6 and protein levels of caspase-12, caspase-9, caspase-3, PARP were determined by Western blotting. RLE-6TN cell line induced by TGF-β1 was also used to verify the amelioration effects in vitro study. In both in vivo and in vitro studies, TUNEL staining was used to evaluate cell apoptosis. Expressions of collagen I, α-SMA, GRP78, and CHOP were significantly inhibited by chlorogenic acid in dose-dependent manner. Similarly, decreasing levels of cleaved caspase-12, caspase-9, caspase-3 and increasing level of uncleaved PARP were observed in chlorogenic acid groups compared with those in the fibrosis group both in vivo and in vitro. Chlorogenic acid could also significantly down-regulate the level of phosphorylation of PERK and cleaved ATF-6 in vivo study. Moreover, MTT assay demonstrated chlorogenic acid could enhance proliferation of RLE-6TN cells induced by TGFβ1 in vitro. And the apoptosis assays indicated that chlorogenic acid could significantly inhibit cell apoptosis both in vivo and in vitro studies. Chlorogenic acid could inhibit the pulmonary fibrosis through endoplasmic reticulum stress inhibition in vivo and in vitro.

Beyond Channel Activity: Protein-Protein Interactions Involving Viroporins

Viroporins are short polypeptides encoded by viruses. These small membrane proteins assemble into oligomers that can permeabilize cellular lipid bilayers, disrupting the physiology of the host to the advantage of the virus. Consequently, efforts during the last few decades have been focused towards the discovery of viroporin channel inhibitors, but in general these have not been successful to produce licensed drugs. Viroporins are also involved in viral pathogenesis by engaging in critical interactions with viral proteins, or disrupting normal host cellular pathways through coordinated interactions with host proteins. These protein-protein interactions (PPIs) may become alternative attractive drug targets for the development of antivirals. In this sense, while thus far most antiviral molecules have targeted viral proteins, focus is moving towards targeting host proteins that are essential for virus replication. In principle, this largely would overcome the problem of resistance, with the possibility of using repositioned existing drugs. The precise role of these PPIs, their strain- and host- specificities, and the structural determination of the complexes involved, are areas that will keep the fields of virology and structural biology occupied for years to come. In the present review, we provide an update of the efforts in the characterization of the main PPIs for most viroporins, as well as the role of viroporins in these PPIs interactions.

Rotaviral nonstructural protein 4 triggers dynamin-related protein 1-dependent mitochondrial fragmentation during infection

Dynamic equilibrium between mitochondrial fission and mitochondrial fusion serves as an important quality control system within cells ensuring cellular vitality and homeostasis. Viruses often target mitochondrial dynamics as a part of their obligatory cellular reprogramming. The present study was undertaken to assess the status and regulation of mitochondrial dynamics during rotavirus infection. Distinct fragmentation of mitochondrial syncytia was observed during late hours of RV (SA11, Wa, A5-13) infection. RV nonstructural protein 4 (NSP4) was identified as the viral trigger for disrupted mitochondrial morphology. Severance of mitochondrial interconnections was found to be a dynamin-related protein 1 (Drp1)-dependent process resulting synergistically from augmented mitochondrial fission and attenuated mitochondrial fusion. Cyclin-dependent kinase 1 was subsequently identified as the cellular kinase responsible for fission-active Ser616 phosphorylation of Drp1. In addition to its positive role in mitochondrial fission, Drp1 also resulted in mitochondrial translocation of E3-ubiquitin ligase Parkin leading to degradation of mitochondrial fusion protein Mitofusin 1. Interestingly, RV-NSP4 was found to interact with and be involved in recruiting fission-active pool of Serine 616 phosphoDrp1 (Ser616 pDrp1) to mitochondria independent of accessory adaptors Mitochondrial fission factor and Fission protein 1 (Fis1). Inhibition of either Drp1 or Ser616 pDrp1 resulted in significant decrease in RV-NSP4-induced intrinsic apoptotic pathway. Overall, this study underscores an efficient strategy utilised by RV to couple apoptosis to mitochondrial fission facilitating dissemination of viral progeny.

Dengue virus induced changes in Ca2+ homeostasis in human hepatic cells that favor the viral replicative cycle

The role of Ca²⁺ during dengue virus (DENV) replication is unknown; thus, changes in Ca²⁺ homeostasis in DENV infected human hepatic HepG2 and Huh-7 cells were analyzed. Infected HepG2 cells, but not Huh-7 cells, showed a significant increase in plasma membrane permeability to Ca²⁺, while both cell lines showed marked reduced levels of Ca²⁺ stored in the endoplasmic reticulum. While the expression levels of STIM1 and ORAI1 showed no changes, STIM1 and ORAI1 were shown to co-localized in infected cells, indicating activation of the store-operated Ca²⁺ entry (SOCE) pathway. Finally, manipulation in the infected cells of the intra and extracellular Ca²⁺ levels by chelators (BAPTA-AM and EGTA), SOC inhibitor (SKF96365), IP3 Receptor antagonist (2APB) or increase of extracellular [Ca²⁺], significantly reduced DENV yield, but not vesicular stomatitis virus yield, used as a control. These results show that DENV infection alters cell Ca²⁺ homeostasis and that such changes favor viral replication.

A Systems Survey of Progressive Host-Cell Reorganization during Rotavirus Infection

Pathogen invasion is often accompanied by widespread alterations in cellular physiology, which reflects the hijacking of host factors and processes for pathogen entry and replication. Although genetic perturbation screens have revealed the complexity of host factors involved for numerous pathogens, it has remained challenging to temporally define the progression of events in host cell reorganization during infection. We combine high-confidence genome-scale RNAi screening of host factors required for rotavirus infection in human intestinal cells with an innovative approach to infer the trajectory of virus infection from fixed cell populations. This approach reveals a comprehensive network of host cellular processes involved in rotavirus infection and implicates AMPK in initiating the development of a rotavirus-permissive environment. Our work provides a powerful approach that can be generalized to order complex host cellular requirements along a trajectory of cellular reorganization during pathogen invasion.

Ebola Virus Delta Peptide Is a Viroporin

The Ebola virus (EBOV) genome encodes a partly conserved 40-residue nonstructural polypeptide, called the delta peptide, that is produced in abundance during Ebola virus disease (EVD). The function of the delta peptide is unknown, but sequence analysis has suggested that delta peptide could be a viroporin, belonging to a diverse family of membrane-permeabilizing small polypeptides involved in replication and pathogenesis of numerous viruses. Full-length and conserved C-terminal delta peptide fragments permeabilize the plasma membranes of nucleated cells of rodent, dog, monkey, and human origin; increase ion permeability across confluent cell monolayers; and permeabilize synthetic lipid bilayers. Permeabilization activity is completely dependent on the disulfide bond between the two conserved cysteines. The conserved C-terminal portion of the peptide is biochemically stable in human serum, and most serum-stable fragments have full activity. Taken together, the evidence strongly suggests that Ebola virus delta peptide is a viroporin and that it may be a novel, targetable aspect of Ebola virus disease pathology.IMPORTANCE During the unparalleled West African outbreak of Ebola virus disease (EVD) that began in late 2013, the lack of effective countermeasures resulted in chains of serial infection and a high mortality rate among infected patients. A better understanding of disease pathology is desperately needed to develop better countermeasures. We show here that the Ebola virus delta peptide, a conserved nonstructural protein produced in large quantities by infected cells, has the characteristics of a viroporin. This information suggests a critical role for the delta peptide in Ebola virus disease pathology and as a possible target for novel countermeasures.

Rice Bran and Probiotics Alter the Porcine Large Intestine and Serum Metabolomes for Protection against Human Rotavirus Diarrhea

Human rotavirus (HRV) is a leading cause of severe childhood diarrhea, and there is limited vaccine efficacy in the developing world. Neonatal gnotobiotic pigs consuming a prophylactic synbiotic combination of probiotics and rice bran (Pro+RB) did not exhibit HRV diarrhea after challenge. Multiple immune, gut barrier protective, and anti-diarrheal mechanisms contributed to the prophylactic efficacy of Pro+RB when compared to probiotics (Pro) alone. In order to understand the molecular signature associated with diarrheal protection by Pro+RB, a global non-targeted metabolomics approach was applied to investigate the large intestinal contents and serum of neonatal gnotobiotic pigs. The ultra-high performance liquid chromatography-tandem mass spectrometry platform revealed significantly different metabolites (293 in LIC and 84 in serum) in the pigs fed Pro+RB compared to Pro, and many of these metabolites were lipids and amino acid/peptides. Lipid metabolites included 2-oleoylglycerol (increased 293.40-fold in LIC of Pro+RB, p = 3.04E-10), which can modulate gastric emptying, andhyodeoxycholate (decreased 0.054-fold in the LIC of Pro+RB, p = 0.0040) that can increase colonic mucus production to improve intestinal barrier function. Amino acid metabolites included cysteine (decreased 0.40-fold in LIC, p = 0.033, and 0.62-fold in serum, p = 0.014 of Pro+RB), which has been found to reduce inflammation, lower oxidative stress and modulate mucosal immunity, and histamine (decreased 0.18-fold in LIC, p = 0.00030, of Pro+RB and 1.57-fold in serum, p = 0.043), which modulates local and systemic inflammatory responses as well as influences the enteric nervous system. Alterations to entire LIC and serum metabolic pathways further contributed to the anti-diarrheal and anti-viral activities of Pro+RB such as sphingolipid, mono/diacylglycerol, fatty acid, secondary bile acid, and polyamine metabolism. Sphingolipid and long chain fatty acid profiles influenced the ability of HRV to both infect and replicate within cells, suggesting that Pro+RB created a protective lipid profile that interferes with HRV activity. Polyamines act on enterocyte calcium-sensing receptors to modulate intracellular calcium levels, and may directly interfere with rotavirus replication. These results support that multiple host and probiotic metabolic networks, notably those involving lipid and amino acid/peptide metabolism, are important mechanisms through which Pro+RB protected against HRV diarrhea in neonatal gnotobiotic pigs.

The Rotavirus NSP4 Viroporin Domain is a Calcium-conducting Ion Channel

Viroporins are small virus-encoded ion channel proteins. Most viroporins are monovalent selective cation channels, with few showing the ability to conduct divalent cations, like calcium (Ca2+). Nevertheless, some viroporins are known to disrupt host cell Ca2+ homeostasis, which is critical for virus replication and pathogenesis. Rotavirus nonstructural protein 4 (NSP4) is an endoplasmic reticulum transmembrane glycoprotein that has a viroporin domain (VPD), and NSP4 viroporin activity elevates cytosolic Ca2+ in mammalian cells. The goal of this study was to demonstrate that the NSP4 VPD forms an ion channel and determine whether the channel can conduct Ca2+. Using planar lipid bilayer and liposome patch clamp electrophysiology, we show that a synthetic peptide of the NSP4 VPD has ion channel activity. The NSP4 VPD was selective for cations over anions and channel activity was observed to have both well-defined "square top" openings as well as fast current fluctuations, similar to other viroporins. Importantly, the NSP4 VPD showed similar conductance of divalent cations (Ca2+ and Ba2+) as monovalent cations (K+), but a viroporin defective mutant lacked Ca2+ conductivity. These data demonstrate that the NSP4 VPD is a Ca2+-conducting viroporin and establish the mechanism by which NSP4 disturbs host cell Ca2+ homeostasis.

STIM1 Phosphorylation at Y361 Recruits Orai1 to STIM1 Puncta and Induces Ca2+ Entry

Store-operated Ca²⁺ entry (SOCE) mediates the increase in intracellular calcium (Ca²⁺) in endothelial cells (ECs) that regulates several EC functions including tissue-fluid homeostasis. Stromal-interaction molecule 1 (STIM1), upon sensing the depletion of (Ca²⁺) from the endoplasmic reticulum (ER) store, organizes as puncta that trigger store-operated Ca²⁺ entry (SOCE) via plasmalemmal Ca²⁺-selective Orai1 channels. While the STIM1 and Orai1 binding interfaces have been mapped, signaling mechanisms activating STIM1 recruitment of Orai1 and STIM1-Orai1 interaction remains enigmatic. Here, we show that ER Ca²⁺-store depletion rapidly induces STIM1 phosphorylation at Y361 via proline-rich kinase 2 (Pyk2) in ECs. Surprisingly, the phospho-defective STIM1-Y361F mutant formed puncta but failed to recruit Orai1, thereby preventing. SOCE Furthermore, studies in mouse lungs, expression of phosphodefective STIM1-Y361F mutant in ECs prevented the increase in vascular permeability induced by the thrombin receptor, protease activated receptor 1 (PAR1). Hence, Pyk2-dependent phosphorylation of STIM1 at Y361 is a critical phospho-switch enabling recruitment of Orai1 into STIM1 puncta leading to SOCE. Therefore, Y361 in STIM1 represents a novel target for limiting SOCE-associated vascular leak.

Hepatitis B virus modulates store-operated calcium entry to enhance viral replication in primary hepatocytes

Many viruses modulate calcium (Ca²⁺) signaling to create a cellular environment that is more permissive to viral replication, but for most viruses that regulate Ca²⁺ signaling, the mechanism underlying this regulation is not well understood. The hepatitis B virus (HBV) HBx protein modulates cytosolic Ca²⁺ levels to stimulate HBV replication in some liver cell lines. A chronic HBV infection is associated with life-threatening liver diseases, including hepatocellular carcinoma (HCC), and HBx modulation of cytosolic Ca²⁺ levels could have an important role in HBV pathogenesis. Whether HBx affects cytosolic Ca²⁺ in a normal hepatocyte, the natural site of an HBV infection, has not been addressed. Here, we report that HBx alters cytosolic Ca²⁺ signaling in cultured primary hepatocytes. We used single cell Ca²⁺ imaging of cultured primary rat hepatocytes to demonstrate that HBx elevates the cytosolic Ca²⁺ level in hepatocytes following an IP₃-linked Ca²⁺ response; HBx effects were similar when expressed alone or in the context of replicating HBV. HBx elevation of the cytosolic Ca²⁺ level required extracellular Ca²⁺ influx and store-operated Ca²⁺ (SOC) entry and stimulated HBV replication in hepatocytes. We used both targeted RT-qPCR and transcriptome-wide RNAseq analyses to compare levels of SOC channel components and other Ca²⁺ signaling regulators in HBV-expressing and control hepatocytes and show that the transcript levels of these various proteins are not affected by HBV. We also show that HBx regulation of SOC-regulated Ca²⁺ accumulation is likely the consequence of HBV modulation of a SOC channel regulatory mechanism. In support of this, we link HBx enhancement of SOC-regulated Ca²⁺ accumulation to Ca²⁺ uptake by mitochondria and demonstrate that HBx stimulates mitochondrial Ca²⁺ uptake in primary hepatocytes. The results of our study may provide insights into viral mechanisms that affect Ca²⁺ signaling to regulate viral replication and virus-associated diseases.

Pathophysiological Consequences of Calcium-Conducting Viroporins

Eukaryotic cells have evolved a myriad of ion channels, transporters, and pumps to maintain and regulate transmembrane ion gradients. As intracellular parasites, viruses also have evolved ion channel proteins, called viroporins, which disrupt normal ionic homeostasis to promote viral replication and pathogenesis. The first viral ion channel (influenza M2 protein) was confirmed only 23 years ago, and since then studies on M2 and many other viroporins have shown they serve critical functions in virus entry, replication, morphogenesis, and immune evasion. As new candidate viroporins and viroporin-mediated functions are being discovered, we review the experimental criteria for viroporin identification and characterization to facilitate consistency within this field of research. Then we review recent studies on how the few Ca(2+)-conducting viroporins exploit host signaling pathways, including store-operated Ca(2+) entry, autophagy, and inflammasome activation. These viroporin-induced aberrant Ca(2+) signals cause pathophysiological changes resulting in diarrhea, vomiting, and proinflammatory diseases, making both the viroporin and host Ca(2+) signaling pathways potential therapeutic targets for antiviral drugs.

Rhesus rotavirus VP6 regulates ERK-dependent calcium influx in cholangiocytes

The Rhesus rotavirus (RRV) induced murine model of biliary atresia (BA) is a useful tool in studying the pathogenesis of this neonatal biliary obstructive disease. In this model, the mitogen associated protein kinase pathway is involved in RRV infection of biliary epithelial cells (cholangiocytes). We hypothesized that extracellular signal-related kinase (ERK) phosphorylation is integral to calcium influx, allowing for viral replication within the cholangiocyte. Utilizing ERK and calcium inhibitors in immortalized cholangiocytes and BALB/c pups, we determined that ERK inhibition resulted in reduced viral yield and subsequent decreased symptomatology in mice. In vitro, the RRV VP6 protein induced ERK phosphorylation, leading to cellular calcium influx. Pre-treatment with an ERK inhibitor or Verapamil resulted in lower viral yields. We conclude that the pathogenesis of RRV-induced murine BA is dependent on the VP6 protein causing ERK phosphorylation and triggering calcium influx allowing replication in cholangiocytes.

Virus and calcium: an unexpected tandem to optimize insecticide efficacy

The effective control of insect pests is based on the rational use of the most efficient and safe insecticide treatments. To increase the effects of classical insecticides and to avoid the ability of certain pest insects to develop resistance, it is essential to propose novel strategies. Previous studies have shown that calcium-dependent phosphorylation/dephosphorylation is now considered as a new cellular mechanism for increasing the target sensitivity to insecticides. Because it is known that virus entry is correlated with intracellular calcium concentration rise, this report attempts to present the most important data relevant to the feasibility of combining an insect virus such as baculovirus or densovirus with an insecticide. In this case, the insect virus is not used as a bioinsecticide but acts as a synergistic agent able to trigger calcium rise and to activate calcium-dependent intracellular signalling pathways involved in the increase of the membrane receptors and/or ion channels sensitivity to insecticides. This virus-insecticide mixture represents a promising alternative to optimize the efficacy of insecticides against insect pests while reducing the doses.

Use of genetically-encoded calcium indicators for live cell calcium imaging and localization in virus-infected cells

Calcium signaling is a ubiquitous and versatile process involved in nearly every cellular process, and exploitation of host calcium signals is a common strategy used by viruses to facilitate replication and cause disease. Small molecule fluorescent calcium dyes have been used by many to examine changes in host cell calcium signaling and calcium channel activation during virus infections, but disadvantages of these dyes, including poor loading and poor long-term retention, complicate analysis of calcium imaging in virus-infected cells due to changes in cell physiology and membrane integrity. The recent expansion of genetically-encoded calcium indicators (GECIs), including blue and red-shifted color variants and variants with calcium affinities appropriate for calcium storage organelles like the endoplasmic reticulum (ER), make the use of GECIs an attractive alternative for calcium imaging in the context of virus infections. Here we describe the development and testing of cell lines stably expressing both green cytoplasmic (GCaMP5G and GCaMP6s) and red ER-targeted (RCEPIAer) GECIs. Using three viruses (rotavirus, poliovirus and respiratory syncytial virus) previously shown to disrupt host calcium homeostasis, we show the GECI cell lines can be used to detect simultaneous cytoplasmic and ER calcium signals. Further, we demonstrate the GECI expression has sufficient stability to enable long-term confocal imaging of both cytoplasmic and ER calcium during the course of virus infections.

Calcium Regulation of Hemorrhagic Fever Virus Budding: Mechanistic Implications for Host-Oriented Therapeutic Intervention

Hemorrhagic fever viruses, including the filoviruses (Ebola and Marburg) and arenaviruses (Lassa and Junín viruses), are serious human pathogens for which there are currently no FDA approved therapeutics or vaccines. Importantly, transmission of these viruses, and specifically late steps of budding, critically depend upon host cell machinery. Consequently, strategies which target these mechanisms represent potential targets for broad spectrum host oriented therapeutics. An important cellular signal implicated previously in EBOV budding is calcium. Indeed, host cell calcium signals are increasingly being recognized to play a role in steps of entry, replication, and transmission for a range of viruses, but if and how filoviruses and arenaviruses mobilize calcium and the precise stage of virus transmission regulated by calcium have not been defined. Here we demonstrate that expression of matrix proteins from both filoviruses and arenaviruses triggers an increase in host cytoplasmic Ca²⁺ concentration by a mechanism that requires host Orai1 channels. Furthermore, we demonstrate that Orai1 regulates both VLP and infectious filovirus and arenavirus production and spread. Notably, suppression of the protein that triggers Orai activation (Stromal Interaction Molecule 1, STIM1) and genetic inactivation or pharmacological blockade of Orai1 channels inhibits VLP and infectious virus egress. These findings are highly significant as they expand our understanding of host mechanisms that may broadly control enveloped RNA virus budding, and they establish Orai and STIM1 as novel targets for broad-spectrum host-oriented therapeutics to combat these emerging BSL-4 pathogens and potentially other enveloped RNA viruses that bud via similar mechanisms.

Filoviruses (Ebola and Marburg viruses) and arenaviruses (Lassa and Junín viruses) are high-priority pathogens that hijack host proteins and pathways to complete their replication cycles and spread from cell to cell. Here we provide genetic and pharmacological evidence to demonstrate that the host calcium channel protein Orai1 and ER calcium sensor protein STIM1 regulate efficient budding and spread of BSL-4 pathogens Ebola, Marburg, Lassa, and Junín viruses. Our findings are of broad significance as they provide new mechanistic insight into fundamental, immutable, and conserved mechanisms of hemorrhagic fever virus pathogenesis. Moreover, this strategy of targeting highly conserved host cellular protein(s) and mechanisms required by these viruses to complete their life cycle should elicit minimal drug resistance.

The effect of bovine rotavirus and its nonstructural protein 4 on ER stress-mediated apoptosis in HeLa and HT-29 cells

Endoplasmic reticulum (ER) plays important roles in multiple cellular processes as well as cell survival and apoptosis. Perturbation of ER functions leads to ER stress and unfolded protein response (UPR). The primary goal of this response is cell survival, but severe ER stress can trigger apoptosis signaling. In tumor cells, chronically activated UPR response provides tumor growth. So, apoptosis induced by the ER stress has been the target for anti-cancer therapy. In this in vitro study, we examined the apoptotic effect associated with ER stress of bovine rotavirus and its nonstructural protein 4 (NSP4) alone in two cancer cell lines. The plasmid pcDNA3.1 encoding NSP4 protein of bovine rotavirus transfected with lipofectamine 2000 into the HeLa and HT-29 cells for protein production. MTT, flow cytometry, and Western blot were used to evaluate the cell viability, apoptosis, and expression level of C/EBP-homologous protein (CHOP) and activated caspase-4. In parallel, the apoptotic effect of the bovine rotavirus associated with ER stress in the infected cells was examined too. The cytotoxic and apoptotic effect of NSP4 protein on the cells were statistically significant compared to the control groups. However, Western blot showed that the expression of the NSP4 protein by recombinant plasmid did not lead to high expression of CHOP and activation of caspase-4. Interestingly, rotavirus not only induced significant apoptosis but also caused an increase in CHOP expression and caspase-4 activation in the infected cells compared to control. As a result, NSP4 protein and bovine rotavirus can be considered a potential novel bio-therapeutic strategy for cancer treatment.

NLRP3 Inflammasome Activation by Viroporins of Animal Viruses

Viroporins are a group of low-molecular-weight proteins containing about 50–120 amino acid residues, which are encoded by animal viruses. Viroporins are involved in several stages of the viral life cycle, including viral gene replication and assembly, as well as viral particle entry and release. Viroporins also play an important role in the regulation of antiviral innate immune responses, especially in inflammasome formation and activation, to ensure the completion of the viral life cycle. By reviewing the research progress made in recent years on the regulation of the NLRP3 inflammasome by viroporins of animal viruses, we aim to understand the importance of viroporins in viral infection and to provide a reference for further research and development of novel antiviral drugs.

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.

The Emerging Roles of Viroporins in ER Stress Response and Autophagy Induction during Virus Infection

Viroporins are small hydrophobic viral proteins that oligomerize to form aqueous pores on cellular membranes. Studies in recent years have demonstrated that viroporins serve important functions during virus replication and contribute to viral pathogenicity. A number of viroporins have also been shown to localize to the endoplasmic reticulum (ER) and/or its associated membranous organelles. In fact, replication of most RNA viruses is closely linked to the ER, and has been found to cause ER stress in the infected cells. On the other hand, autophagy is an evolutionarily conserved "self-eating" mechanism that is also observed in cells infected with RNA viruses. Both ER stress and autophagy are also known to modulate a wide variety of signaling pathways including pro-inflammatory and innate immune response, thereby constituting a major aspect of host-virus interactions. In this review, the potential involvement of viroporins in virus-induced ER stress and autophagy will be discussed.