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

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.

Human norovirus cultivation systems and their use in antiviral research

Human norovirus (HuNoV) is a major cause of acute gastroenteritis and foodborne diseases, affecting all age groups. Despite its clinical needs, no approved antiviral therapies are available. Since the discovery of HuNoV in 1972, studies on anti-norovirals, mechanism of HuNoV infection, viral inactivation, etc., have been hampered by the lack of a robust laboratory-based cultivation system for HuNoV. A recent breakthrough in the development of HuNoV cultivation systems has opened opportunities for researchers to investigate HuNoV biology in the context of de novo HuNoV infections. A tissue stem cell-derived human intestinal organoid/enteroid (HIO) culture system is one of those that supports HuNoV replication reproducibly and, to our knowledge, is most widely distributed to laboratories worldwide to study HuNoV and develop therapeutic strategies. This review summarizes recently developed HuNoV cultivation systems, including HIO, and their use in antiviral studies.

Distribution of P2Y and P2X purinergic receptor expression within the intestine

Nucleotides are potent extracellular signaling molecules during homeostasis, infection, and injury due to their ability to activate purinergic receptors. The nucleotide ATP activates P2X receptors (P2RXs), whereas the nucleotides ADP, ATP, UTP, and UDP-glucose selectively activate different P2Y receptors (P2RYs). Several studies have established crucial roles for P2 receptors during intestinal inflammatory and infectious diseases, yet the most extensive characterization of purinergic signaling has focused on immune cells and the central and enteric nervous systems. As epithelial cells serve as the first barrier against irritants and infection, we hypothesized that the gut epithelium may express multiple purinergic receptors that respond to extracellular nucleotide signals. Using the Human Protein Atlas and Gut Cell Survey, we queried single-cell RNA sequencing (RNAseq) data for the P2 purinergic receptors in the small and large intestines. In silico analysis reveals robust mRNA expression of P2RY1, P2RY2, P2RY11, and P2RX4 throughout the gastrointestinal tract. Human intestinal organoids exhibited a similar expression pattern with a prominent expression of P2RY1, P2RY2, and P2RX4, but this purinergic receptor repertoire was not conserved in T84, Caco2, and HT29 intestinal epithelial cell lines. Finally, P2YR1 and P2YR2 agonists elicited robust calcium responses in human intestinal organoids, but calcium responses were weaker or absent in the cell lines. These findings suggest that the gastrointestinal epithelia respond to extracellular purinergic signaling via P2RY1, P2RY2, P2RY11, and P2RX4 receptors and highlight the benefit of using intestinal organoids as a model of intestinal purinergic signaling.NEW & NOTEWORTHY Several studies have revealed crucial roles for P2 receptors during inflammatory and infectious diseases, however, these have largely been demonstrated in immune cells and the enteric nervous system. Although epithelial cells serve as the first barrier against infection and inflammation, the role of purinergic signaling within the gastrointestinal tract remains largely unknown. This work expands our knowledge of purinergic receptor distribution and relative expression along the intestine.

Niclosamide, but not ivermectin, inhibits anoctamin 1 and 6 and attenuates inflammation of the respiratory tract

Inflammatory airway diseases like cystic fibrosis, asthma and COVID-19 are characterized by high levels of pulmonary cytokines. Two well-established antiparasitic drugs, niclosamide and ivermectin, are intensively discussed for the treatment of viral inflammatory airway infections. Here, we examined these repurposed drugs with respect to their anti-inflammatory effects in airways in vivo and in vitro. Niclosamide reduced mucus content, eosinophilic infiltration and cell death in asthmatic mouse lungs in vivo and inhibited release of interleukins in the two differentiated airway epithelial cell lines CFBE and BCi-NS1.1 in vitro. Cytokine release was also inhibited by the knockdown of the Ca2+-activated Cl- channel anoctamin 1 (ANO1, TMEM16A) and the phospholipid scramblase anoctamin 6 (ANO6, TMEM16F), which have previously been shown to affect intracellular Ca2+ levels near the plasma membrane and to facilitate exocytosis. At concentrations around 200 nM, niclosamide inhibited inflammation, lowered intracellular Ca2+, acidified cytosolic pH and blocked activation of ANO1 and ANO6. It is suggested that niclosamide brings about its anti-inflammatory effects at least in part by inhibiting ANO1 and ANO6, and by lowering intracellular Ca2+ levels. In contrast to niclosamide, 1 µM ivermectin did not exert any of the effects described for niclosamide. The present data suggest niclosamide as an effective anti-inflammatory treatment in CF, asthma, and COVID-19, in addition to its previously reported antiviral effects. It has an advantageous concentration-response relationship and is known to be well tolerated.

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.

Recovery of Recombinant Rotaviruses by Reverse Genetics

Rotaviruses are the primary cause of severe gastroenteritis in infants and young children throughout the world. To combat rotavirus illness, several live oral vaccines have been developed, or are under development, that are formulated from attenuated human or human-animal reassortant strains of rotavirus. While the effectiveness of these vaccines is generally high in developed countries, the same vaccines are significantly less effective in many developing countries, where the need for rotavirus vaccines is greatest. Recently, reverse genetics systems have been developed that allow modification of the segmented double-stranded (ds)RNA genome of rotavirus, including reprogramming the genome to allow expression of additional proteins that may stimulate expanded neutralizing antibody responses in vaccinated children. The use of reverse genetics systems may not only lead to the development of more potent classes of vaccines but can be used to better explore the intricacies of rotavirus molecular biology and pathogenesis. In this article, we share protocols that can be used to generate recombinant rotaviruses, including modified strains that express foreign proteins.

Live-Cell Fluorescence Imaging for Virus-Host Interactions

Recent technological advances in microscopy have facilitated novel approaches to investigate host-pathogen interactions. In particular, improvements in both microscope hardware and engineered biosensors have helped to overcome barriers to live-cell imaging with fluorescence microscopy. Live fluorescent microscopy allows for the detection of discrete signaling events and protein localization, improving our ability to assess the effects of pharmacologic agents, microbes, or infection with high temporal resolution. Here we describe a protocol for long-term live-cell fluorescence imaging of virus infected cell lines.

Recombinant rotavirus expressing the glycosylated S1 protein of SARS-CoV-2

Reverse genetic systems have been used to introduce heterologous sequences into the rotavirus segmented double-stranded (ds)RNA genome, enabling the generation of recombinant viruses that express foreign proteins and possibly serve as vaccine vectors. Notably, insertion of SARS-CoV-2 sequences into the segment seven (NSP3) RNA of simian SA11 rotavirus was previously shown to result in the production of recombinant viruses that efficiently expressed the N-terminal domain (NTD) and the receptor-binding domain (RBD) of the S1 region of the SARS-CoV-2 spike protein. However, efforts to generate a similar recombinant (r) SA11 virus that efficiently expressed full-length S1 were less successful. In this study, we describe modifications to the S1-coding cassette inserted in the segment seven RNA that allowed recovery of second-generation rSA11 viruses that efficiently expressed the ~120-kDa S1 protein. The ~120-kDa S1 products were shown to be glycosylated, based on treatment with endoglycosidase H, which reduced the protein to a size of ~80 kDa. Co-pulldown assays demonstrated that the ~120-kDa S1 proteins had affinity for the human ACE2 receptor. Although all the second-generation rSA11 viruses expressed glycosylated S1 with affinity for the ACE receptor, only the S1 product of one virus (rSA11/S1f) was appropriately recognized by anti-S1 antibodies, suggesting the rSA11/S1f virus expressed an authentic form of S1. Compared to the other second-generation rSA11 viruses, the design of the rSA11/S1f was unique, encoding an S1 product that did not include an N-terminal FLAG tag. Probably due to the impact of FLAG tags upstream of the S1 signal peptides, the S1 products of the other viruses (rSA11/3fS1 and rSA11/3fS1-His) may have undergone defective glycosylation, impeding antibody binding. In summary, these results indicate that recombinant rotaviruses can serve as expression vectors of foreign glycosylated proteins, raising the possibility of generating rotavirus-based vaccines that can induce protective immune responses against enteric and mucosal viruses with glycosylated capsid components, including SARS-CoV-2.

Human intestinal organoids as models to study enteric bacteria and viruses

Laboratory studies of host-microbe interactions have historically been carried out using transformed cell lines and animal models. Although much has been learned from these models, recent advances in the development of multicellular, physiologically active, human intestinal organoid (HIO) cultures are allowing unprecedented discoveries of host-microbe interactions. Here, we review recent literature using HIOs as models to investigate the pathogenesis of clinically important enteric bacteria and viruses and study commensal intestinal microbes. We also discuss limitations of current HIO culture systems and how technical advances and innovative engineering approaches are providing new directions to improve the model. The studies discussed here highlight the potential of HIOs for studying microbial pathogenesis, host-microbe interactions, and for preclinical development of therapeutics and vaccines.

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.

Molecular Characterization of Avian Rotaviruses F and G Detected in Brazilian Poultry Flocks

Avian rotaviruses (RVs) are important etiologic agents of gastroenteritis in birds. In general, avian RVs are understudied; consequently, there is a paucity of information regarding these viruses. Therefore, the characterization of these viral species is highly relevant because more robust information on genetic, epidemiologic, and evolutionary characteristics can clarify the importance of these infections, and inform efficient prevention and control measures. In this study, we describe partial genome characterizations of two avian RV species, RVF and RVG, detected in asymptomatic poultry flocks in Brazil. Complete or partial sequences of at least one of the genomic segments encoding VP1, VP2, VP4, VP6, VP7, NSP1, NSP4, NSP4, or NSP5 of 23 RVF and 3 RVG strains were obtained, and demonstrated that multiple variants of both RVF and RVG circulate among Brazilian poultry. In this study, new and important information regarding the genomic characteristics of RVF and RVG is described. In addition, the circulation of these viruses in the study region and the genetic variability of the strains detected are demonstrated. Thus, the data generated in this work should help in understanding the genetics and ecology of these viruses. Nonetheless, the availability of a greater number of sequences is necessary to advance the understanding of the evolution and zoonotic potential of these viruses.

Simultaneous Measurement of Changes in Mitochondrial and Endoplasmic Reticulum Free Calcium in Pancreatic Beta Cells

The free calcium (Ca2+) levels in pancreatic beta cell organelles have been the subject of many recent investigations. Under pathophysiological conditions, disturbances in these pools have been linked to altered intracellular communication and cellular dysfunction. To facilitate studies of subcellular Ca2+ signaling in beta cells and, particularly, signaling between the endoplasmic reticulum (ER) and mitochondria, we designed a novel dual Ca2+ sensor which we termed DS-1. DS-1 encodes two stoichiometrically fluorescent proteins within a single plasmid, G-CEPIA-er, targeted to the ER and R-CEPIA3-mt, targeted to mitochondria. Our goal was to simultaneously measure the ER and mitochondrial Ca2+ in cells in real time. The Kds of G-CEPIA-er and R-CEPIA3-mt for Ca2+ are 672 and 3.7 μM, respectively. Confocal imaging of insulin-secreting INS-1 832/13 expressing DS-1 confirmed that the green and red fluorophores correctly colocalized with organelle-specific fluorescent markers as predicted. Further, we tested whether DS-1 exhibited the functional properties expected by challenging an INS-1 cell to glucose concentrations or drugs having well-documented effects on the ER and mitochondrial Ca2+ handling. The data obtained were consistent with those seen using other single organelle targeted probes. These results taken together suggest that DS-1 is a promising new approach for investigating Ca2+ signaling within multiple organelles of the cell.

Mucosal and systemic neutralizing antibodies to norovirus induced in infant mice orally inoculated with recombinant rotaviruses

Rotaviruses (RVs) preferentially replicate in the small intestine and frequently cause severe diarrheal disease, and the following enteric infection generally induces variable levels of protective systemic and mucosal immune responses in humans and other animals. Rhesus rotavirus (RRV) is a simian RV that was previously used as a human RV vaccine and has been extensively studied in mice. Although RRV replicates poorly in the suckling mouse intestine, infection induces a robust and protective antibody response. The recent availability of plasmid only-based RV reverse genetics systems has enabled the generation of recombinant RVs expressing foreign proteins. However, recombinant RVs have not yet been experimentally tested as potential vaccine vectors to immunize against other gastrointestinal pathogens in vivo. This is a newly available opportunity because several live-attenuated RV vaccines are already widely administered to infants and young children worldwide. To explore the feasibility of using RV as a dual vaccine vector, we rescued replication-competent recombinant RRVs harboring bicistronic gene segment 7 that encodes the native RV nonstructural protein 3 (NSP3) protein and a human norovirus (HuNoV) VP1 protein or P domain from the predominant genotype GII.4. The rescued viruses expressed HuNoV VP1 or P protein in infected cells in vitro and elicited systemic and local antibody responses to HuNoV and RRV following oral infection of suckling mice. Serum IgG and fecal IgA from infected suckling mice bound to and neutralized both RRV and HuNoV. These findings have encouraging practical implications for the design of RV-based next-generation multivalent enteric vaccines to target HuNoV and other human enteric pathogens.

14th International dsRNA Virus Symposium, Banff, Alberta, Canada, 10-14 October 2022

This triennial International dsRNA Virus Symposium covered original data which have accrued during the most recent five years. In detail, the genomic diversity of these viruses continued to be explored; various structure-function studies were carried out using reverse genetics and biophysical techniques; intestinal organoids proved to be very suitable for special pathogenesis studies; and the potential of next generation rotavirus vaccines including use of rotavirus recombinants as vectored vaccine candidates was explored. 'Non-lytic release of enteric viruses in cloaked vesicles' was the topic of the keynote lecture by Nihal Altan-Bonnet, NIH, Bethesda, USA. The Jean Cohen lecturer of this meeting was Polly Roy, London School of Hygiene and Tropical Medicine, who spoke on aspects of the replication cycle of bluetongue viruses, and how some of the data are similar to details of rotavirus replication.

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.

Computational Modeling of Virally-encoded Ion Channel Structure

Viroporins are ion channels encoded within a virus's genome, that facilitate a range of devastating infectious diseases such as COVID-19, HIV, and rotavirus. The non-structural protein 4 (NSP4) from rotavirus includes a viroporin domain that disrupts cellular Ca2+ homeostasis, initiating viral replication, and leading to life-threatening vomiting and diarrhea. Though the structure of soluble segments of NSP4 has been determined, membrane-associated regions, including the viroporin domain, remain elusive when utilizing well-established available experimental methods such as x-ray crystallography. However, two recently published protein folding algorithms, AlphaFold2 and trRosetta, demonstrated a high degree of accuracy, when determining the structure of membrane proteins from their primary amino acid sequences, though their training datasets are known to exclude proteins from viral systems. We tested the ability of these non-viral algorithms to predict functional molecular structures of the full-length NSP4 from SA11 rotavirus. We also compared the accuracy of these structures to predictions of other experimental structures of eukaryotic proteins from the Protein Data Banks (PDB), and show that the algorithms predict models more similar to corresponding experimental data than what we saw for the viroporin structure. Our data suggest that while AlphaFold2 and trRosetta each produced distinct NSP4 models, constructs based on either model showed viroporin activity when expressed in E. coli, consistent with that seen from other historical NSP4 sequences.

Screening of candidate genes associated with high titer production of oncolytic measles virus based on systems biology approach

The number of viral particles required for oncolytic activity of measles virus (MV) can be more than a million times greater than the reported amount for vaccination. The aim of the current study is to find potential genes and signaling pathways that may be involved in the high-titer production of MV. In this study, a systems biology approach was considered including collection of gene expression profiles from the Gene Expression Omnibus (GEO) database, obtaining differentially expressed genes (DEGs), performing gene ontology, functional enrichment analyses, and topological analyses on the protein-protein interaction (PPI) network. Then, to validate the in-silico data, total RNA was isolated from five cell lines, and full-length cDNA from template RNA was synthesized. Subsequently, quantitative reverse transcription-PCR (RT-qPCR) was employed. We identified five hub genes, including RAC1, HSP90AA1, DNM1, LTBP1, and FSTL1 associated with the enhancement in MV titer. Pathway analysis indicated enrichment in PI3K-Akt signaling pathway, axon guidance, proteoglycans in cancer, regulation of actin cytoskeleton, focal adhesion, and calcium signaling pathways. Upon verification by RT-qPCR, the relative expression of candidate genes was generally consistent with our bioinformatics analysis. Hub genes and signaling pathways may be involved in understanding the pathological mechanisms by which measles virus manipulates host factors in order to facilitate its replication. RAC1, HSP90AA1, DNM1, LTBP1, and FSTL1 genes, in combination with genetic engineering techniques, will allow the direct design of high-throughput cell lines to answer the required amounts for the oncolytic activity of MV.

The ins and outs of virus trafficking through acidic Ca2+ stores

Many viruses exploit host-cell Ca2+ signaling processes throughout their life cycle. This is especially relevant for viruses that translocate through the endolysosomal system, where cellular infection is keyed to the microenvironment of these acidic Ca2+ stores and Ca2+-dependent trafficking pathways. As regulators of the endolysosomal ionic milieu and trafficking dynamics, two families of endolysosomal Ca2+-permeable cation channels - two pore channels (TPCs) and transient receptor potential mucolipins (TRPMLs) - have emerged as important host-cell factors in viral entry. Here, we review: (i) current evidence implicating Ca2+ signaling in viral translocation through the endolysosomal system, (ii) the roles of these ion channels in supporting cellular infection by different viruses, and (iii) areas for future research that will help define the potential of TPC and TRPML ligands as progressible antiviral agents.

Enterotoxigenic Escherichia coli Heat-Stable Toxin and Ebola Virus Delta Peptide: Similarities and Differences

Enterotoxigenic Escherichia coli (ETEC) STb toxin exhibits striking structural similarity to Ebola virus (EBOV) delta peptide. Both ETEC and EBOV delta peptide are enterotoxins. Comparison of the structural and functional similarities and differences of these two toxins illuminates features that are important in induction of pathogenesis by a bacterial and viral pathogen.

Airway Delivery of Hydrogel-Encapsulated Niclosamide for the Treatment of Inflammatory Airway Disease

Repurposing of the anthelminthic drug niclosamide was proposed as an effective treatment for inflammatory airway diseases such as asthma, cystic fibrosis, and chronic obstructive pulmonary disease. Niclosamide may also be effective for the treatment of viral respiratory infections, such as SARS-CoV-2, respiratory syncytial virus, and influenza. While systemic application of niclosamide may lead to unwanted side effects, local administration via aerosol may circumvent these problems, particularly when the drug is encapsulated into small polyethylene glycol (PEG) hydrospheres. In the present study, we examined whether PEG-encapsulated niclosamide inhibits the production of mucus and affects the pro-inflammatory mediator CLCA1 in mouse airways in vivo, while effects on mucociliary clearance were assessed in excised mouse tracheas. The potential of encapsulated niclosamide to inhibit TMEM16A whole-cell Cl- currents and intracellular Ca2+ signalling was assessed in airway epithelial cells in vitro. We achieved encapsulation of niclosamide in PEG-microspheres and PEG-nanospheres (Niclo-spheres). When applied to asthmatic mice via intratracheal instillation, Niclo-spheres strongly attenuated overproduction of mucus, inhibited secretion of the major proinflammatory mediator CLCA1, and improved mucociliary clearance in tracheas ex vivo. These effects were comparable for niclosamide encapsulated in PEG-nanospheres and PEG-microspheres. Niclo-spheres inhibited the Ca2+ activated Cl- channel TMEM16A and attenuated mucus production in CFBE and Calu-3 human airway epithelial cells. Both inhibitory effects were explained by a pronounced inhibition of intracellular Ca2+ signals. The data indicate that poorly dissolvable compounds such as niclosamide can be encapsulated in PEG-microspheres/nanospheres and deposited locally on the airway epithelium as encapsulated drugs, which may be advantageous over systemic application.

Ebola virus delta peptide is an enterotoxin

During the 2013-2016 West African (WA) Ebola virus (EBOV) outbreak, severe gastrointestinal symptoms were common in patients and associated with poor outcome. Delta peptide is a conserved product of post-translational processing of the abundant EBOV soluble glycoprotein (sGP). The murine ligated ileal loop model was used to demonstrate that delta peptide is a potent enterotoxin. Dramatic intestinal fluid accumulation follows injection of biologically relevant amounts of delta peptide into ileal loops, along with gross alteration of villous architecture and loss of goblet cells. Transcriptomic analyses show that delta peptide triggers damage response and cell survival pathways and downregulates expression of transporters and exchangers. Induction of diarrhea by delta peptide occurs via cellular damage and regulation of genes that encode proteins involved in fluid secretion. While distinct differences exist between the ileal loop murine model and EBOV infection in humans, these results suggest that delta peptide may contribute to EBOV-induced gastrointestinal pathology.

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.

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.

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.

Recent advances in rotavirus reverse genetics and its utilization in basic research and vaccine development

Rotaviruses are segmented double-stranded RNA viruses with a high frequency of gene reassortment, and they are a leading cause of global diarrheal deaths in children less than 5 years old. Two-thirds of rotavirus-associated deaths occur in low-income countries. Currently, the available vaccines in developing countries have lower efficacy in children than those in developed countries. Due to added safety concerns and the high cost of current vaccines, there is a need to develop cost-effective next-generation vaccines with improved safety and efficacy. The reverse genetics system (RGS) is a powerful tool for investigating viral protein functions and developing novel vaccines. Recently, an entirely plasmid-based RGS has been developed for several rotaviruses, and this technological advancement has significantly facilitated novel rotavirus research. Here, we review the recently developed RGS platform and discuss its application in studying infection biology, gene reassortment, and development of vaccines against rotavirus disease.

Mechanisms of Nausea and Vomiting: Current Knowledge and Recent Advances in Intracellular Emetic Signaling Systems

Nausea and vomiting are common gastrointestinal complaints that can be triggered by diverse emetic stimuli through central and/or peripheral nervous systems. Both nausea and vomiting are considered as defense mechanisms when threatening toxins/drugs/bacteria/viruses/fungi enter the body either via the enteral (e.g., the gastrointestinal tract) or parenteral routes, including the blood, skin, and respiratory systems. While vomiting is the act of forceful removal of gastrointestinal contents, nausea is believed to be a subjective sensation that is more difficult to study in nonhuman species. In this review, the authors discuss the anatomical structures, neurotransmitters/mediators, and corresponding receptors, as well as intracellular emetic signaling pathways involved in the processes of nausea and vomiting in diverse animal models as well as humans. While blockade of emetic receptors in the prevention of vomiting is fairly well understood, the potential of new classes of antiemetics altering postreceptor signal transduction mechanisms is currently evolving, which is also reviewed. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide potential answers.

Rotavirus as an Expression Platform of Domains of the SARS-CoV-2 Spike Protein

Among vaccines administered to children are those targeting rotavirus, a segmented double-stranded RNA virus that represents a major cause of severe gastroenteritis. To explore the feasibility of establishing a combined rotavirus-SARS-CoV-2 vaccine, we generated recombinant (r)SA11 rotaviruses with modified segment 7 RNAs that contained coding cassettes for NSP3, a translational 2A stop-restart signal, and a FLAG-tagged portion of the SARS-CoV-2 spike (S) protein: S1 fragment, N-terminal domain (NTD), receptor-binding domain (RBD), extended RBD (ExRBD), or S2 core (CR) domain. Generation of rSA11 containing the S1 coding sequence required a sequence insertion of 2.2 kbp, the largest such insertion yet introduced into the rotavirus genome. Immunoblotting showed that rSA11 viruses containing the smaller NTD, RBD, ExRBD, and CR coding sequences expressed S-protein products of expected size, with ExRBD expressed at highest levels. These rSA11 viruses were genetically stable during serial passage. In contrast, the rSA11 virus containing the full-length S coding sequence (rSA11/NSP3-fS1) failed to express its expected 80 kDa fS1 product, for unexplained reasons. Moreover, rSA11/NSP3-fS1 was genetically unstable, with variants lacking the S1 insertion appearing during serial passage. Nonetheless, these results emphasize the potential usefulness of rotavirus vaccines as expression vectors of immunogenic portions of the SARS-CoV-2 S protein, including NTD, RBD, ExRBD, and CR, that have sizes smaller than the S1 fragment.

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.

Egress of non-enveloped enteric RNA viruses

A long-standing paradigm in virology was that non-enveloped viruses induce cell lysis to release progeny virions. However, emerging evidence indicates that some non-enveloped viruses exit cells without inducing cell lysis, while others engage both lytic and non-lytic egress mechanisms. Enteric viruses are transmitted via the faecal-oral route and are important causes of a wide range of human infections, both gastrointestinal and extra-intestinal. Virus cellular egress, when fully understood, may be a relevant target for antiviral therapies, which could minimize the public health impact of these infections. In this review, we outline lytic and non-lytic cell egress mechanisms of non-enveloped enteric RNA viruses belonging to five families: Picornaviridae, Reoviridae, Caliciviridae, Astroviridae and Hepeviridae. We discuss factors that contribute to egress mechanisms and the relevance of these mechanisms to virion stability, infectivity and transmission. Since most data were obtained in traditional two-dimensional cell cultures, we will further attempt to place them into the context of polarized cultures and in vivo pathogenesis. Throughout the review, we highlight numerous knowledge gaps to stimulate future research into the egress mechanisms of these highly prevalent but largely understudied viruses.

Rotavirus as an Expression Platform of the SARS-CoV-2 Spike Protein

Rotavirus, a segmented double-stranded RNA virus, is a major cause of acute gastroenteritis in young children. The introduction of live oral rotavirus vaccines has reduced the incidence of rotavirus disease in many countries. To explore the possibility of establishing a combined rotavirus-SARS-CoV-2 vaccine, we generated recombinant (r)SA11 rotaviruses with modified segment 7 RNAs that contained coding sequences for NSP3 and FLAG-tagged portions of the SARS-CoV-2 spike (S) protein. A 2A translational element was used to drive separate expression of NSP3 and the S product. rSA11 viruses were recovered that encoded the S-protein S1 fragment, N-terminal domain (NTD), receptor-binding domain (RBD), extended receptor-binding domain (ExRBD), and S2 core (CR) domain (rSA11/NSP3-fS1, -fNTD, -fRBD, -fExRBD, and -fCR, respectively). Generation of rSA11/fS1 required a foreign-sequence insertion of 2.2-kbp, the largest such insertion yet made into the rotavirus genome. Based on isopycnic centrifugation, rSA11 containing S sequences were denser than wildtype virus, confirming the capacity of the rotavirus to accommodate larger genomes. Immunoblotting showed that rSA11/-fNTD, -fRBD, -fExRBD, and -fCR viruses expressed S products of expected size, with fExRBD expressed at highest levels. These rSA11 viruses were genetically stable during serial passage. In contrast, rSA11/NSP3-fS1 failed to express its expected 80-kDa fS1 product, for unexplained reasons. Moreover, rSA11/NSP3-fS1 was genetically unstable, with variants lacking the S1 insertion appearing during serial passage. Nonetheless, these results emphasize the potential usefulness of rotavirus vaccines as expression vectors of portions of the SARS-CoV-2 S protein (e.g., NTD, RBD, ExRBD, and CR) with sizes smaller than the S1 fragment.

Low serum calcium: a new, important indicator of COVID-19 patients from mild/moderate to severe/critical

BACKGROUND

 Coronavirus disease 2019 (COVID-19) virus is still spreading, finding out the initial hits of viral infection is important to minimize the mild/moderate population, prevent disease aggravation and organs dysfunction. Objective: We investigated COVID-19 patients with different serum calcium levels.

DESIGN

 We checked the serum calcium level of the patients based on days after symptom onset as well as the severity of COVID-19. We also checkeed multi-organ injuries and immune cytokines level in their blood.

RESULTS

 Both mild/moderate and severe critical cases we observed showed low calcium level in the early stage of viral infection, while the severe/critical cases showed significant lower calcium level than mild/moderate cases in the early stage. We also found that low calcium level related to severe/critical multi-organ injuries especially in the mild/moderate population. Proinflammatory cytokine IL-6 also correlated to calcium change in both mild/moderate and severe/critical cases.

CONCLUSIONS

 Our finding indicates that calcium balance is a primal hit of COVID-19 and a biomarker of clinical severity at the beginning of symptom onset. Calcium is closely associated with virus-associated multiple organ injuryes and the increase of inflammatory cytokines. Our results provide a new, important indicator of COVID-19 patients from mild/moderate to severe/critical: serum calcium.

Rotavirus induces intercellular calcium waves through ADP signaling

Rotavirus causes severe diarrheal disease in children by broadly dysregulating intestinal homeostasis. However, the underlying mechanism(s) of rotavirus-induced dysregulation remains unclear. We found that rotavirus-infected cells produce paracrine signals that manifested as intercellular calcium waves (ICWs), observed in cell lines and human intestinal enteroids. Rotavirus ICWs were caused by the release of extracellular adenosine 5'-diphosphate (ADP) that activated P2Y1 purinergic receptors on neighboring cells. ICWs were blocked by P2Y1 antagonists or CRISPR-Cas9 knockout of the P2Y1 receptor. Blocking the ADP signal reduced rotavirus replication, inhibited rotavirus-induced serotonin release and fluid secretion, and reduced diarrhea severity in neonatal mice. Thus, rotavirus exploited paracrine purinergic signaling to generate ICWs that amplified the dysregulation of host cells and altered gastrointestinal physiology to cause diarrhea.

Put Some Guts into It: Intestinal Organoid Models to Study Viral Infection

The knowledge about enteric viral infection has vastly increased over the last eight years due to the development of intestinal organoids and enteroids that suppose a step forward from conventional studies using cell lines. Intestinal organoids and enteroids are three-dimensional (3D) models that closely mimic intestinal cellular heterogeneity and organization. The barrier function within these models has been adapted to facilitate viral studies. In this review, several adaptations (such as organoid-derived two-dimensional (2D) monolayers) and original intestinal 3D models are discussed. The specific advantages and applications, as well as improvements of each model are analyzed and an insight into the possible path for the field is given.

Expression of Separate Heterologous Proteins from the Rotavirus NSP3 Genome Segment Using a Translational 2A Stop-Restart Element

The segmented 18.5-kbp dsRNA genome of rotavirus expresses 6 structural and 6 nonstructural proteins. We investigated the possibility of using the recently developed plasmid-based rotavirus reverse genetics (RG) system to generate recombinant viruses that express a separate heterologous protein in addition to the 12 viral proteins. To address this, we replaced the NSP3 open reading frame (ORF) of the segment 7 (pT7/NSP3) transcription vector used in the RG system with an ORF encoding NSP3 fused to a fluorescent reporter protein (i.e., UnaG, mRuby, mKate, or TagBFP). Inserted at the fusion junction was a teschovirus translational 2A stop-restart element designed to direct the separate expression of NSP3 and the fluorescent protein. Recombinant rotaviruses made with the modified pT7/NSP3 vectors were well growing and generally genetically stable, and they expressed NSP3 and a separate fluorescent protein detectable by live cell imaging. NSP3 made by the recombinant viruses was functional, inducing nuclear accumulation of cellular poly(A)-binding protein. Further modification of the NSP3 ORF showed that it was possible to generate recombinant viruses encoding 2 heterologous proteins (mRuby and UnaG) in addition to NSP3. Our results demonstrate that, through modification of segment 7, the rotavirus genome can be increased in size to at least 19.8 kbp and can be used to produce recombinant rotaviruses expressing a full complement of viral proteins and multiple heterologous proteins. The generation of recombinant rotaviruses expressing fluorescent proteins will be valuable for the study of rotavirus replication and pathogenesis by live cell imagining and suggest that rotaviruses will prove useful as expression vectors.IMPORTANCE Rotaviruses are a major cause of severe gastroenteritis in infants and young children. Recently, a highly efficient reverse genetics system was developed that allows genetic manipulation of the rotavirus segmented double-stranded RNA genome. Using the reverse genetics system, we show that it is possible to modify one of the rotavirus genome segments (segment 7) such that virus gains the capacity to express a separate heterologous protein in addition to the full complement of viral proteins. Through this approach, we have generated wild-type-like rotaviruses that express various fluorescent reporter proteins, including UnaG (green), mRuby (far red), mKate (red), and TagBFP (blue). Such strains will be of value in probing rotavirus biology and pathogenesis by live cell imagining techniques. Notably, our work indicates that the rotavirus genome is remarkably flexible and able to accommodate significant amounts of heterologous RNA sequence, raising the possibility of using the virus as a vaccine expression vector.

Repurposing calcium channel blockers as antiviral drugs

The current pandemic caused by the SARS-CoV-2 has claimed over a half a million lives within a very short span of time. A therapeutic drug which could prevent the entry and propagation of the virus is the need of the hour. Several lines of evidence collected from experimental studies older than three decades have pointed out the fact that inhibiting calcium entry into cells can affect vital steps in the lifecycle of viruses. Hence, calcium channel blockers may be considered as an effective measure in the containment of the viruses. This commentary throws light two scientific papers although with divergent facts converging at a point by suggesting a promising treatment option for CoVID-19 (Fang et al. Lancet Respir Med 8:e21, 2020; Straus et al. J Virol 94:e00426, 2020).

Ion Channels as Therapeutic Targets for Viral Infections: Further Discoveries and Future Perspectives

Ion channels play key roles in almost all facets of cellular physiology and have emerged as key host cell factors for a multitude of viral infections. A catalogue of ion channel-blocking drugs have been shown to possess antiviral activity, some of which are in widespread human usage for ion channel-related diseases, highlighting new potential for drug repurposing. The emergence of ion channel–virus interactions has also revealed the intriguing possibility that channelopathies may explain some commonly observed virus induced pathologies. This field is rapidly evolving and an up-to-date summary of new discoveries can inform future perspectives. We herein discuss the role of ion channels during viral lifecycles, describe the recently identified ion channel drugs that can inhibit viral infections, and highlight the potential contribution of ion channels to virus-mediated disease.

Network Pharmacology-Based Strategy for Predicting Active Ingredients and Potential Targets of Gegen Qinlian Decoction for Rotavirus Enteritis

Materials and Methods

In this study, a network pharmacology-based strategy was used to elucidate the mechanism of GGQLD for the treatment of RVE. Oral bioavailability and drug-likeness were taken as the judgment criteria to search the active ingredients of GGQLD in traditional Chinese medicine systems pharmacology database and analysis platform (TCMSP). The affinity between protein and ingredients was further determined using the similarity ensemble approach to find the corresponding targets. According to the genes related to enteritis in GeneCards database, the key targets were screened by intersections between drug and disease targets. And the therapeutic mechanism was predicted using the protein-protein interactions (PPIs), the Gene Ontology (GO), and the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, which was verified by detecting calcium ion concentration with the fluorescent probe.

Result

130 active ingredients were screened from GGQLD, including (R)-canadine, moupinamide, formononetin, and other flavonoids. They act on a total of 366 targets, which is mainly distributed in the biological process of hormone binding or signaling pathways of neuroactive ligand receptor interaction, serotonergic synapse, and calcium signaling pathway. Furthermore, serotonin receptors, adrenergic receptors, cholinergic receptors, and dopamine receptors in the enteric nervous system may be the key targets of RVE treatment by GGQLD.

Conclusion

This study demonstrated that the potential mechanism that GGQLD can effectively improve the symptoms of RVE may depend on the regulation of calcium ions, serotonin, and gastrointestinal hormone ion that could mutually affect the intestinal nervous system.

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.

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.