The coxsackievirus 2B protein increases efflux of ions from the endoplasmic reticulum and Golgi, thereby inhibiting protein trafficking through the Golgi

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

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.

Viroporins Manipulate Cellular Powerhouses and Modulate Innate Immunity

Viruses have a wide repertoire of molecular strategies that focus on their replication or the facilitation of different stages of the viral cycle. One of these strategies is mediated by the activity of viroporins, which are multifunctional viral proteins that, upon oligomerization, exhibit ion channel properties with mild ion selectivity. Viroporins facilitate multiple processes, such as the regulation of immune response and inflammasome activation through the induction of pore formation in various cell organelle membranes to facilitate the escape of ions and the alteration of intracellular homeostasis. Viroporins target diverse membranes (such as the cellular membrane), endoplasmic reticulum, and mitochondria. Cumulative data regarding the importance of mitochondria function in multiple processes, such as cellular metabolism, energy production, calcium homeostasis, apoptosis, and mitophagy, have been reported. The direct or indirect interaction of viroporins with mitochondria and how this interaction affects the functioning of mitochondrial cells in the innate immunity of host cells against viruses remains unclear. A better understanding of the viroporin-mitochondria interactions will provide insights into their role in affecting host immune signaling through the mitochondria. Thus, in this review, we mainly focus on descriptions of viroporins and studies that have provided insights into the role of viroporins in hijacked mitochondria.

Persistent coxsackievirus B infection and pathogenesis of type 1 diabetes mellitus

Enteroviruses are believed to trigger or accelerate islet autoimmunity in genetically susceptible individuals, thereby resulting in loss of functional insulin-producing β-cells and type 1 diabetes mellitus (T1DM). Although enteroviruses are primarily involved in acute and lytic infections in vitro and in vivo, they can also establish a persistent infection. Prospective epidemiological studies have strongly associated the persistence of enteroviruses, especially coxsackievirus B (CVB), with the appearance of islet autoantibodies and an increased risk of T1DM. CVB can persist in pancreatic ductal and β-cells, which leads to structural or functional alterations of these cells, and to a chronic inflammatory response that promotes recruitment and activation of pre-existing autoreactive T cells and β-cell autoimmune destruction. CVB persistence in other sites, such as the intestine, blood cells and thymus, has been described; these sites could serve as a reservoir for infection or reinfection of the pancreas, and this persistence could have a role in the disturbance of tolerance to β-cells. This Review addresses the involvement of persistent enterovirus infection in triggering islet autoimmunity and T1DM, as well as current strategies to control enterovirus infections for preventing or reducing the risk of T1DM onset.

Development of a Multiepitope Vaccine Against SARS-CoV-2: Immunoinformatics Study

Background

Since the first appearance of SARS-CoV-2 in China in December 2019, the world witnessed the emergence of the SARS-CoV-2 outbreak. Due to the high transmissibility rate of the virus, there is an urgent need to design and develop vaccines against SARS-CoV-2 to prevent more cases affected by the virus.

Objective

A computational approach is proposed for vaccine design against the SARS-CoV-2 spike (S) protein, as the key target for neutralizing antibodies, and envelope (E) protein, which contains a conserved sequence feature.

Methods

We used previously reported epitopes of S protein detected experimentally and further identified a collection of predicted B-cell and major histocompatibility (MHC) class II–restricted T-cell epitopes derived from E proteins with an identical match to SARS-CoV-2 E protein.

Results

The in silico design of our candidate vaccine against the S and E proteins of SARS-CoV-2 demonstrated a high affinity to MHC class II molecules and effective results in immune response simulations.

Conclusions

Based on the results of this study, the multiepitope vaccine designed against the S and E proteins of SARS-CoV-2 may be considered as a new, safe, and efficient approach to combatting the COVID-19 pandemic.

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.

Serine-ubiquitination regulates Golgi morphology and the secretory pathway upon Legionella infection

SidE family of Legionella effectors catalyze non-canonical phosphoribosyl-linked ubiquitination (PR-ubiquitination) of host proteins during bacterial infection. SdeA localizes predominantly to ER and partially to the Golgi apparatus, and mediates serine ubiquitination of multiple ER and Golgi proteins. Here we show that SdeA causes disruption of Golgi integrity due to its ubiquitin ligase activity. The Golgi linking proteins GRASP55 and GRASP65 are PR-ubiquitinated on multiple serine residues, thus preventing their ability to cluster and form oligomeric structures. In addition, we found that the functional consequence of Golgi disruption is not linked to the recruitment of Golgi membranes to the growing Legionella-containing vacuoles. Instead, it affects the host secretory pathway. Taken together, our study sheds light on the Golgi manipulation strategy by which Legionella hijacks the secretory pathway and promotes bacterial infection.

A comparative analysis of parechovirus protein structures with other picornaviruses

Parechoviruses belong to the genus Parechovirus within the family Picornaviridae and are non-enveloped icosahedral viruses with a single-stranded RNA genome. Parechoviruses include human and animal pathogens classified into six species. Those that infect humans belong to the Parechovirus A species and can cause infections ranging from mild gastrointestinal or respiratory illness to severe neonatal sepsis. There are no approved antivirals available to treat parechovirus (nor any other picornavirus) infections. In this parechovirus review, we focus on the cleaved protein products resulting from the polyprotein processing after translation comparing and contrasting their known or predicted structures and functions to those of other picornaviruses. The review also includes our original analysis from sequence and structure prediction. This review highlights significant structural differences between parechoviral and other picornaviral proteins, suggesting that parechovirus drug development should specifically be directed to parechoviral targets.

A viroporin-like 2B protein of duck hepatitis A virus 1 that induces incomplete autophagy in DEF cells

Duck hepatitis A virus 1 (DHAV-1) can cause high morbidity and fatal acute infectious hepatitis in ducklings, which seriously endangers animal husbandry. Viroporin is a small molecular weight hydrophobic transmembrane protein encoded by the virus, that has been suggested to induce autophagy in host cells by increasing the membrane permeability through disturbing the ion balance. In this study, we aimed to investigate whether the DHAV-1 2B protein can induce autophagy in DEF cells with a viroporin-like function. Bioinformatics analysis has indicated that the 2B protein is characterized by a viroporin domain, which is consistent with the type IA viroporin transmembrane protein. We experimentally confirmed that the 2B protein disturbed the Ca2+ balance of infected cells by elevating the intracellular Ca2+ concentration. Eukaryotic expression of the 2B protein upregulates the expression of microtubule-associated protein 1 light chain 3 II (LC3-II) and the number of autophagosomes in the cell. Interestingly, the Western Blot (WB) results showed that 2B protein expression induced less protein degradation of the autophagic substrate sequestosome 1 (SQSTM1/p62) than the positive control, while microscopy observations showed that the autophagosomes did not colocalize with the lysosomes. In summary, 2B protein expression induced autophagy in host cells, but the autophagic flow was incomplete. The results of this experiment are expected to provide reference scientific data for elucidating the infective and pathogenic mechanism of DHAV-1.

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.

Coxsackievirus B3-Its Potential as an Oncolytic Virus

Oncolytic virotherapy represents one of the most advanced strategies to treat otherwise untreatable types of cancer. Despite encouraging developments in recent years, the limited fraction of patients responding to therapy has demonstrated the need to search for new suitable viruses. Coxsackievirus B3 (CVB3) is a promising novel candidate with particularly valuable features. Its entry receptor, the coxsackievirus and adenovirus receptor (CAR), and heparan sulfate, which is used for cellular entry by some CVB3 variants, are highly expressed on various cancer types. Consequently, CVB3 has broad anti-tumor activity, as shown in various xenograft and syngeneic mouse tumor models. In addition to direct tumor cell killing the virus induces a strong immune response against the tumor, which contributes to a substantial increase in the efficiency of the treatment. The toxicity of oncolytic CVB3 in healthy tissues is variable and depends on the virus strain. It can be abrogated by genetic engineering the virus with target sites of microRNAs. In this review, we present an overview of the current status of the development of CVB3 as an oncolytic virus and outline which steps still need to be accomplished to develop CVB3 as a therapeutic agent for clinical use in cancer treatment.

Endoplasmic reticulum & mitochondrial calcium homeostasis: The interplay with viruses

Calcium ions (Ca²⁺) act as secondary messengers in a plethora of cellular processes and play crucial role in cellular organelle function and homeostasis. The average resting concentration of Ca²⁺ is nearly 100 nM and in certain cells it can reach up to 1 µM. The high range of Ca²⁺ concentration across the plasma membrane and intracellular Ca²⁺ stores demands a well-coordinated maintenance of free Ca²⁺ via influx, efflux, buffering and storage. Endoplasmic Reticulum (ER) and Mitochondria depend on Ca²⁺ for their function and also serve as major players in intracellular Ca²⁺ homeostasis. The ER-mitochondria interplay helps in orchestrating cellular calcium homeostasis to avoid any detrimental effect resulting from Ca²⁺ overload or depletion. Since Ca²⁺ plays a central role in many biological processes it is an essential component of the virus-host interactions. The large gradient across membranes enable the viruses to easily modulate this buffered environment to meet their needs. Viruses exploit Ca²⁺ signaling to establish productive infection and evade the host immune defense. In this review we will detail the interplay between the viruses and cellular & ER-mitochondrial calcium signaling and the significance of these events on viral life cycle and disease pathogenesis.

The Golgi Calcium ATPase Pump Plays an Essential Role in Adeno-associated Virus Trafficking and Transduction

Adeno-associated viruses (AAVs) have proven to be effective gene transfer vectors. However, our understanding of how the host cell environment influences AAV transduction is still evolving. In the present study, we investigated the role of ATP2C1, which encodes a membrane calcium transport pump, SPCA1, essential for maintaining cellular calcium homeostasis on AAV transduction. Our results indicate that cellular calcium is essential for efficient intracellular trafficking and conformational changes in the AAV capsid that support efficient genome transcription. Further, we show that pharmacological modulation of cellular calcium levels can potentially be applied to improve the AAV gene transfer efficiency.

Adeno-associated viruses (AAVs) are dependoparvoviruses that have proven useful for therapeutic gene transfer; however, our understanding of host factors that influence AAV trafficking and transduction is still evolving. Here, we investigated the role of cellular calcium in the AAV infectious pathway. First, we demonstrated a critical role for the host Golgi compartment-resident ATP-powered calcium pump (secretory pathway calcium ATPase 1 [SPCA1]) encoded by the ATP2C1 gene in AAV infection. CRISPR-based knockout (KO) of ATP2C1 decreases transduction by different AAV serotypes. ATP2C1 KO does not appear to inhibit AAV binding, cellular uptake, or nuclear entry; however, capsids within ATP2C1 KO cells demonstrate dispersed and punctate trafficking distinct from the perinuclear, trans-Golgi pattern observed in normal cells. In addition, we observed a defect in the ability of AAV capsids to undergo conformational changes and support efficient vector genome transcription in ATP2C1 KO cells. The calcium chelator BAPTA-AM, which reduces cytosolic calcium, rescues the defective ATP2C1 KO phenotype and AAV transduction in vitro. Conversely, the calcium ionophore ionomycin, which disrupts calcium gradients, blocks AAV transduction. Further, we demonstrated that modulating calcium in the murine brain using BAPTA-AM augments AAV gene expression in vivo. Taking these data together, we postulate that the maintenance of an intracellular calcium gradient by the calcium ATPase and processing within the Golgi compartment are essential for priming the capsid to support efficient AAV genome transcription.

IMPORTANCE Adeno-associated viruses (AAVs) have proven to be effective gene transfer vectors. However, our understanding of how the host cell environment influences AAV transduction is still evolving. In the present study, we investigated the role of ATP2C1, which encodes a membrane calcium transport pump, SPCA1, essential for maintaining cellular calcium homeostasis on AAV transduction. Our results indicate that cellular calcium is essential for efficient intracellular trafficking and conformational changes in the AAV capsid that support efficient genome transcription. Further, we show that pharmacological modulation of cellular calcium levels can potentially be applied to improve the AAV gene transfer efficiency.

The first versatile human iPSC-based model of ectopic virus induction allows new insights in RNA-virus disease

A detailed description of pathophysiological effects that viruses exert on their host is still challenging. For the first time, we report a highly controllable viral expression model based on an iPS-cell line from a healthy human donor. The established viral model system enables a dose-dependent and highly localized RNA-virus expression in a fully controllable environment, giving rise for new applications for the scientific community.

Ins and Outs of Reovirus: Vesicular Trafficking in Viral Entry and Egress

Cell entry and egress are essential steps in the viral life cycle that govern pathogenesis and spread. Mammalian orthoreoviruses (reoviruses) are nonenveloped viruses implicated in human disease that serve as tractable models for studies of pathogen-host interactions. In this review we discuss the function of intracellular vesicular transport systems in reovirus entry, trafficking, and egress and comment on shared themes for diverse viruses. Designing strategic therapeutic interventions that impede these steps in viral replication requires a detailed understanding of mechanisms by which viruses coopt vesicular trafficking. We illuminate such targets, which may foster development of antiviral agents.

Enteroviral Pathogenesis of Type 1 Diabetes: The Role of Natural Killer Cells

Enteroviruses, especially group B coxsackieviruses (CV-B), have been associated with the development of chronic diseases such as type 1 diabetes (T1D). The pathological mechanisms that trigger virus-induced autoimmunity against islet antigens in T1D are not fully elucidated. Animal and human studies suggest that NK cells response to CV-B infection play a crucial role in the enteroviral pathogenesis of T1D. Indeed, CV-B-infected cells can escape from cytotoxic T cells recognition and destruction by inhibition of cell surface expression of HLA class I antigen through non-structural viral proteins, but they can nevertheless be killed by NK cells. Cytolytic activity of NK cells towards pancreatic beta cells persistently-infected with CV-B has been reported and defective viral clearance by NK cells of patients with T1D has been suggested as a mechanism leading to persistence of CV-B and triggering autoimmunity reported in these patients. The knowledge about host antiviral defense against CV-B infection is not only crucial to understand the susceptibility to virus-induced T1D but could also contribute to the design of new preventive or therapeutic approaches for individuals at risk for T1D or newly diagnosed patients.

An in-silico approach to develop of a multi-epitope vaccine candidate against SARS-CoV-2 envelope (E) protein

Since the first appearance of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS- CoV-2) in China on December 2019, the world has now witnessed the emergence of the SARS- CoV-2 outbreak. Therefore, due to the high transmissibility rate of virus, there is an urgent need to design and develop vaccines against SARS-CoV-2 to prevent more cases affected by the virus. In this study, a computational approach is proposed for vaccine design against the envelope (E) protein of SARS-CoV-2, which contains a conserved sequence feature. First, we sought to gain potential B-cell and T-cell epitopes for vaccine designing against SARS-CoV-2. Second, we attempted to develop a multi-epitope vaccine. Immune targeting of such epitopes could theoretically provide defense against SARS-CoV-2. Finally, we evaluated the affinity of the vaccine to major histocompatibility complex (MHC) molecules to stimulate the immune system response to this vaccine. We also identified a collection of B-cell and T-cell epitopes derived from E proteins that correspond identically to SARS-CoV-2 E proteins. The in-silico design of our potential vaccine against E protein of SARS-CoV-2 demonstrated a high affinity to MHC molecules, and it can be a candidate to make a protection against this pandemic event.

Pancreatic beta cells persistently infected with coxsackievirus B4 are targets of NK cell-mediated cytolytic activity

It has been suggested that the persistence of coxsackieviruses-B (CV-B) in pancreatic beta cells plays a role in the pathogenesis of type 1 diabetes (T1D). Yet, immunological effectors, especially natural killer (NK) cells, are supposed to clear virus-infected cells. Therefore, an evaluation of the response of NK cells to pancreatic beta cells persistently infected with CV-B4 was conducted. A persistent CV-B4 infection was established in 1.1B4 pancreatic beta cells. Infectious particles were found in supernatants throughout the culture period. The proportion of cells containing viral protein VP1 was low (< 5%), although a large proportion of cells harbored viral RNA (around 50%), whilst cell viability was preserved. HLA class I cell surface expression was downregulated in persistently infected cultures, but HLA class I mRNA levels were unchanged in comparison with mock-infected cells. The cytolytic activities of IL-2-activated non-adherent peripheral blood mononuclear cells (PBMCs) and of NK cells were higher towards persistently infected cells than towards mock-infected cells, as assessed by an LDH release assay. Impaired cytolytic activity of IL-2-activated non-adherent PBMCs from patients with T1D towards infected beta cells was observed. In conclusion, pancreatic beta cells persistently infected with CV-B4 can be lysed by NK cells, implying that impaired cytolytic activity of these effector cells may play a role in the persistence of CV-B in the host and thus in the viral pathogenesis of T1D.

Coxsackievirus B Persistence Modifies the Proteome and the Secretome of Pancreatic Ductal Cells

The group B Coxsackieviruses (CVB), belonging to the Enterovirus genus, can establish persistent infections in human cells. These persistent infections have been linked to chronic diseases including type 1 diabetes. Still, the outcomes of persistent CVB infections in human pancreas are largely unknown. We established persistent CVB infections in a human pancreatic ductal-like cell line PANC-1 using two distinct CVB1 strains and profiled infection-induced changes in cellular protein expression and secretion using mass spectrometry-based proteomics. Persistent infections, showing characteristics of carrier-state persistence, were associated with a broad spectrum of changes, including changes in mitochondrial network morphology and energy metabolism and in the regulated secretory pathway. Interestingly, the expression of antiviral immune response proteins, and also several other proteins, differed clearly between the two persistent infections. Our results provide extensive information about the protein-level changes induced by persistent CVB infection and the potential virus-associated variability in the outcomes of these infections.

Biological Function and Application of Picornaviral 2B Protein: A New Target for Antiviral Drug Development

Picornaviruses are associated with acute and chronic diseases. The clinical manifestations of infections are often mild, but infections may also lead to respiratory symptoms, gastroenteritis, myocarditis, meningitis, hepatitis, and poliomyelitis, with serious impacts on human health and economic losses in animal husbandry. Thus far, research on picornaviruses has mainly focused on structural proteins such as VP1, whereas the non-structural protein 2B, which plays vital roles in the life cycle of the viruses and exhibits a viroporin or viroporin-like activity, has been overlooked. Viroporins are viral proteins containing at least one amphipathic α-helical structure, which oligomerizes to form transmembrane hydrophilic pores. In this review, we mainly summarize recent research data on the viroporin or viroporin-like activity of 2B proteins, which affects the biological function of the membrane, regulates cell death, and affects the host immune response. Considering these mechanisms, the potential application of the 2B protein as a candidate target for antiviral drug development is discussed, along with research challenges and prospects toward realizing a novel treatment strategy for picornavirus infections.

Coronavirus envelope protein: current knowledge

BACKGROUND

Coronaviruses (CoVs) primarily cause enzootic infections in birds and mammals but, in the last few decades, have shown to be capable of infecting humans as well. The outbreak of severe acute respiratory syndrome (SARS) in 2003 and, more recently, Middle-East respiratory syndrome (MERS) has demonstrated the lethality of CoVs when they cross the species barrier and infect humans. A renewed interest in coronaviral research has led to the discovery of several novel human CoVs and since then much progress has been made in understanding the CoV life cycle. The CoV envelope (E) protein is a small, integral membrane protein involved in several aspects of the virus' life cycle, such as assembly, budding, envelope formation, and pathogenesis. Recent studies have expanded on its structural motifs and topology, its functions as an ion-channelling viroporin, and its interactions with both other CoV proteins and host cell proteins.

MAIN BODY

This review aims to establish the current knowledge on CoV E by highlighting the recent progress that has been made and comparing it to previous knowledge. It also compares E to other viral proteins of a similar nature to speculate the relevance of these new findings. Good progress has been made but much still remains unknown and this review has identified some gaps in the current knowledge and made suggestions for consideration in future research.

CONCLUSIONS

The most progress has been made on SARS-CoV E, highlighting specific structural requirements for its functions in the CoV life cycle as well as mechanisms behind its pathogenesis. Data shows that E is involved in critical aspects of the viral life cycle and that CoVs lacking E make promising vaccine candidates. The high mortality rate of certain CoVs, along with their ease of transmission, underpins the need for more research into CoV molecular biology which can aid in the production of effective anti-coronaviral agents for both human CoVs and enzootic CoVs.

Mitochondria Redistribution in Enterovirus A71 Infected Cells and Its Effect on Virus Replication

Enterovirus A71 (EV-A71) is one of the main causative agents of hand, foot and mouth disease (HFMD) and it also causes severe neurologic complications in infected children. The interactions between some viruses and the host mitochondria are crucial for virus replication and pathogenicity. In this study, it was observed that EV-A71 infection resulted in a perinuclear redistribution of the mitochondria. The mitochondria rearrangement was found to require the microtubule network, the dynein complex and a low cytosolic calcium concentration. Subsequently, the EV-A71 non-structural protein 2BC was identified as the viral protein capable of inducing mitochondria clustering. The protein was found localized on mitochondria and interacted with the mitochondrial Rho GTPase 1 (RHOT1) that is a key protein required for attachment between the mitochondria and the motor proteins, which are responsible for the control of mitochondria movement. Additionally, suppressing mitochondria clustering by treating cells with nocodazole, EHNA, thapsigargin or A23187 consistently inhibited EV-A71 replication, indicating that mitochondria recruitment played a crucial role in the EV-A71 life cycle. This study identified a novel function of the EV-A71 2BC protein and provided a potential model for the regulation of mitochondrial motility in EV-A71 infection.

The Saffold Virus-Penang 2B and 3C Proteins, but not the L Protein, Induce Apoptosis in HEp-2 and Vero Cells

Our previous work has shown that Saffold virus (SAFV) induced several rodent and primate cell lines to undergo apoptosis (Xu et al. in Emerg Microb Infect 3:1-8, 2014), but the essential viral proteins of SAFV involved in apoptotic activity lack study. In this study, we individually transfected the viral proteins of SAFV into HEp-2 and Vero cells to assess their ability to induce apoptosis, and found that the 2B and 3C proteins are proapoptotic. Further investigation indicated the transmembrane domain of the 2B protein is essential for the apoptotic activity and tetramer formation of the 2B protein. Our research provides clues for the possible mechanisms of apoptosis induced by SAFV in different cell lines. It also opens up new directions to study viral proteins (the 2B, 3C protein), and sets the stage for future exploration of any possible link between SAFV, inclusive of its related uncultivable genotypes, and multiple sclerosis.

Interactions Between Enteroviruses and the Inflammasome: New Insights Into Viral Pathogenesis

Enteroviruses (EVs) have emerged a substantial threat to public health. EVs infection range from mild to severe disease, including mild respiratory illness, diarrhea, poliomyelitis, hand, foot, and mouth disease, aseptic meningitis, and encephalitis. In the Asia-Pacific region, for example, one of the best studied enterovirus 71 (EV71) has been associated with pandemics of hand, foot, and mouth disease (HFMD) in children, particularly those under the age of five. Serious HFMD cases are associated with neurological complications, such as aseptic meningitis, acute flaccid paralysis, brainstem encephalitis, and have been associated with as many as 1000s of deaths in children and infants from 2008 to 2017, in China. More than 90% of laboratory confirmed deaths due to HMFD are associated with EV71. However, little is known about the pathogenesis of EVs. Studies have reported that EVs-infected patients with severe complications show elevated serum concentrations of IL-1β. The secretion of IL-1β is mediated by NLRP3 inflammasome during EV71 and CVB3 infection. Enteroviruses 2B and 3D proteins play an important role in activation of NLRP3 inflammasome, while 3C and 2A play important roles in antagonizing the activation of NLRP3 and the secretion of IL-1β. In this review, we summarize current knowledge regarding the molecular mechanisms that underlie the activation and regulation of the NLRP3 inflammasome, particularly how viral proteins regulate NLRP3 inflammasome activation. These insights into the relationship between the NLRP3 inflammasome and the pathogenesis of EVs infection may ultimately inform the development of novel antiviral drugs.

Different residues in the SARS-CoV spike protein determine cleavage and activation by the host cell protease TMPRSS2

The spike (S) protein of severe acute respiratory syndrome coronavirus (SARS-CoV) mediates viral entry into target cells. Cleavage and activation of SARS S by a host cell protease is essential for infectious viral entry and the responsible enzymes are potential targets for antiviral intervention. The type II transmembrane serine protease TMPRSS2 cleaves and activates SARS S in cell culture and potentially also in the infected host. Here, we investigated which determinants in SARS S control cleavage and activation by TMPRSS2. We found that SARS S residue R667, a previously identified trypsin cleavage site, is also required for S protein cleavage by TMPRSS2. The cleavage fragments produced by trypsin and TMPRSS2 differed in their decoration with N-glycans, suggesting that these proteases cleave different SARS S glycoforms. Although R667 was required for SARS S cleavage by TMPRSS2, this residue was dispensable for TMPRSS2-mediated S protein activation. Conversely, residue R797, previously reported to be required for SARS S activation by trypsin, was dispensable for S protein cleavage but required for S protein activation by TMPRSS2. Collectively, these results show that different residues in SARS S control cleavage and activation by TMPRSS2, suggesting that these processes are more complex than initially appreciated.

Evidence that Processing of the Severe Fever with Thrombocytopenia Syndrome Virus Gn/Gc Polyprotein Is Critical for Viral Infectivity and Requires an Internal Gc Signal Peptide

The severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging, highly pathogenic bunyavirus against which neither antivirals nor vaccines are available. The SFTSV glycoproteins, Gn and Gc, facilitate viral entry into host cells. Gn and Gc are generated from a precursor protein, Gn/Gc, but it is currently unknown how the precursor is converted into the single proteins and whether this process is required for viral infectivity. Employing a rhabdoviral pseudotyping system, we demonstrate that a predicted signal sequence at the N-terminus of Gc is required for Gn/Gc processing and viral infectivity while potential proprotein convertase cleavage sites in Gc are dispensable. Moreover, we show that expression of Gn or Gc alone is not sufficient for host cell entry while particles bearing both proteins are infectious, and we provide evidence that Gn facilitates Golgi transport and virion incorporation of Gc. Collectively, these results suggest that signal peptidase liberates mature Gc from the Gn/Gc precursor and that this process is essential for viral infectivity and thus constitutes a potential target for antiviral intervention.

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.

Endoplasmic reticulum stress in beta cells and autoimmune diabetes

Type 1 diabetes results from the autoimmune destruction of pancreatic β cells, leading to insulin deficiency and hyperglycemia. Although multiple attempts have been made to slow the autoimmune process using immunosuppressive or immunomodulatory agents, there are still no effective treatments that can delay or reverse the progression of type 1 diabetes in humans. Recent studies support endoplasmic reticulum (ER) as a novel target for preventing the initiation of the autoimmune reaction, propagation of inflammation, and β cell death in type 1 diabetes. This review highlights recent findings on ER stress in β cells and development of type 1 diabetes and introduces potential new treatments targeting the ER to combat this disorder.

Recombinant Dengue virus protein NS2B alters membrane permeability in different membrane models

Background

One of the main phenomena occurring in cellular membranes during virus infection is a change in membrane permeability. It has been observed that numerous viral proteins can oligomerize and form structures known as viroporins that alter the permeability of membranes. Previous findings have identified such proteins in cells infected with Japanese encephalitis virus (JEV), a member of the same family that Dengue virus (DENV) belongs to (Flaviviridae). In the present work, we investigated whether the small hydrophobic DENV protein NS2B serves a viroporin function.

Methods

We cloned the DENV NS2B sequence and expressed it in a bacterial expression system. Subsequently, we evaluated the effect of DENV NS2B on membranes when NS2B was overexpressed, measured bacterial growth restriction, and evaluated changes of permeability to hygromycin. The NS2B protein was purified by affinity chromatography, and crosslinking assays were performed to determine the presence of oligomers. Hemolysis assays and transmission electron microscopy were performed to identify structures involved in permeability changes.

Results

The DENV-2 NS2B protein showed similitude with the JEV viroporin. The DENV-2 NS2B protein possessed the ability to change the membrane permeability in bacteria, to restrict bacterial cell growth, and to enable membrane permeability to hygromycin B. The NS2B protein formed trimers that could participate in cell lysis and generate organized structures on eukaryotes membranes.

Conclusions

Our data suggest that the DENV-2 NS2B viral protein is capable of oligomerizing and organizing to form pore-like structures in different lipid environments, thereby modifying the permeability of cell membranes.

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.

Mechanisms of Beta Cell Dysfunction Associated With Viral Infection

Type 1 diabetes (T1D) results from genetic predisposition and environmental factors leading to the autoimmune destruction of pancreatic beta cells. Recently, a rapid increase in the incidence of childhood T1D has been observed worldwide; this is too fast to be explained by genetic factors alone, pointing to the spreading of environmental factors linked to the disease. Enteroviruses (EVs) are perhaps the most investigated environmental agents in relationship to the pathogenesis of T1D. While several studies point to the likelihood of such correlation, epidemiological evidence in its support is inconclusive or in some instances even against it. Hence, it is still unknown if and how EVs are involved in the development of T1D. Here we review recent findings concerning the biology of EV in beta cells and the potential implications of this knowledge for the understanding of beta cell dysfunction and autoimmune destruction in T1D.

Severe acute respiratory syndrome coronavirus E protein transports calcium ions and activates the NLRP3 inflammasome

Severe acute respiratory syndrome coronavirus (SARS-CoV) envelope (E) protein is a viroporin involved in virulence. E protein ion channel (IC) activity is specifically correlated with enhanced pulmonary damage, edema accumulation and death. IL-1β driven proinflammation is associated with those pathological signatures, however its link to IC activity remains unknown. In this report, we demonstrate that SARS-CoV E protein forms protein–lipid channels in ERGIC/Golgi membranes that are permeable to calcium ions, a highly relevant feature never reported before. Calcium ions together with pH modulated E protein pore charge and selectivity. Interestingly, E protein IC activity boosted the activation of the NLRP3 inflammasome, leading to IL-1β overproduction. Calcium transport through the E protein IC was the main trigger of this process. These findings strikingly link SARS-CoV E protein IC induced ionic disturbances at the cell level to immunopathological consequences and disease worsening in the infected organism.

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SARS-CoV E protein forms calcium ion channels, a novel highly relevant function.

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Transport of calcium ions through E protein channel stimulates the inflammasome.

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Inflammasome derived exacerbated proinflammation causes SARS worsening.

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E protein ion channel and its driven proinflammation may be targets to treat SARS.

Identification of the interaction of VP1 with GM130 which may implicate in the pathogenesis of CVB3-induced acute pancreatitis

Coxsackievirus B3 (CVB3) is a causative agent of viral myocarditis, pancreatitis, and meningitis in humans. Although the susceptibility of CVB3-induced acute pancreatitis is age-dependent, the underlying mechanisms remain unclear. Here we identified the host factor Golgi matrix protein 130 (GM130) as a novel target of CVB3 during CVB3-induced acute pancreatitis. The viral protein VP1 interacted with GM130, disrupted GM130-GRASP65 complexes, and caused GM130 degradation, which may lead to disruption of the Golgi ribbon and development of acute pancreatitis in mice. Interestingly, the expression level of GM130 in mouse pancreas was age-dependent, which was nicely correlated with the age-associated susceptibility of CVB3-induced acute pancreatitis. Furthermore, interference RNA-mediated knockdown of GM130 significantly reduced CVB3 replication in HeLa cells. Taken together, the study identified GM130 as a novel target of CVB3, which may implicate in the pathogenesis of CVB3-induced acute pancreatitis.

Replication and Inhibitors of Enteroviruses and Parechoviruses

The Enterovirus (EV) and Parechovirus genera of the picornavirus family include many important human pathogens, including poliovirus, rhinovirus, EV-A71, EV-D68, and human parechoviruses (HPeV). They cause a wide variety of diseases, ranging from a simple common cold to life-threatening diseases such as encephalitis and myocarditis. At the moment, no antiviral therapy is available against these viruses and it is not feasible to develop vaccines against all EVs and HPeVs due to the great number of serotypes. Therefore, a lot of effort is being invested in the development of antiviral drugs. Both viral proteins and host proteins essential for virus replication can be used as targets for virus inhibitors. As such, a good understanding of the complex process of virus replication is pivotal in the design of antiviral strategies goes hand in hand with a good understanding of the complex process of virus replication. In this review, we will give an overview of the current state of knowledge of EV and HPeV replication and how this can be inhibited by small-molecule inhibitors.

Relevance of Viroporin Ion Channel Activity on Viral Replication and Pathogenesis

Modification of host-cell ionic content is a significant issue for viruses, as several viral proteins displaying ion channel activity, named viroporins, have been identified. Viroporins interact with different cellular membranes and self-assemble forming ion conductive pores. In general, these channels display mild ion selectivity, and, eventually, membrane lipids play key structural and functional roles in the pore. Viroporins stimulate virus production through different mechanisms, and ion channel conductivity has been proved particularly relevant in several cases. Key stages of the viral cycle such as virus uncoating, transport and maturation are ion-influenced processes in many viral species. Besides boosting virus propagation, viroporins have also been associated with pathogenesis. Linking pathogenesis either to the ion conductivity or to other functions of viroporins has been elusive for a long time. This article summarizes novel pathways leading to disease stimulated by viroporin ion conduction, such as inflammasome driven immunopathology.

Recent progress in understanding coxsackievirus replication, dissemination, and pathogenesis

Coxsackieviruses (CVs) are relatively common viruses associated with a number of serious human diseases, including myocarditis and meningo-encephalitis. These viruses are considered cytolytic yet can persist for extended periods of time within certain host tissues requiring evasion from the host immune response and a greatly reduced rate of replication. A member of Picornaviridae family, CVs have been historically considered non-enveloped viruses - although recent evidence suggest that CV and other picornaviruses hijack host membranes and acquire an envelope. Acquisition of an envelope might provide distinct benefits to CV virions, such as resistance to neutralizing antibodies and efficient nonlytic viral spread. CV exhibits a unique tropism for progenitor cells in the host which may help to explain the susceptibility of the young host to infection and the establishment of chronic disease in adults. CVs have also been shown to exploit autophagy to maximize viral replication and assist in unconventional release from target cells. In this article, we review recent progress in clarifying virus replication and dissemination within the host cell, identifying determinants of tropism, and defining strategies utilized by the virus to evade the host immune response. Also, we will highlight unanswered questions and provide future perspectives regarding the potential mechanisms of CV pathogenesis.

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.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Viroporin Activity of the Foot-and-Mouth Disease Virus Non-Structural 2B Protein

Viroporins are a family of low-molecular-weight hydrophobic transmembrane proteins that are encoded by various animal viruses. Viroporins form transmembrane pores in host cells via oligomerization, thereby destroying cellular homeostasis and inducing cytopathy for virus replication and virion release. Among the Picornaviridae family of viruses, the 2B protein encoded by enteroviruses is well understood, whereas the viroporin activity of the 2B protein encoded by the foot-and-mouth disease virus (FMDV) has not yet been described. An analysis of the FMDV 2B protein domains by computer-aided programs conducted in this study revealed that this protein may contain two transmembrane regions. Further biochemical, biophysical and functional studies revealed that the protein possesses a number of features typical of a viroporin when it is overexpressed in bacterial and mammalian cells as well as in FMDV-infected cells. The protein was found to be mainly localized in the endoplasmic reticulum (ER), with both the N- and C-terminal domains stretched into the cytosol. It exhibited cytotoxicity in Escherichia coli, which attenuated 2B protein expression. The release of virions from cells infected with FMDV was inhibited by amantadine, a viroporin inhibitor. The 2B protein monomers interacted with each other to form both intracellular and extracellular oligomers. The Ca(2+) concentration in the cells increased, and the integrity of the cytoplasmic membrane was disrupted in cells that expressed the 2B protein. Moreover, the 2B protein induced intense autophagy in host cells. All of the results of this study demonstrate that the FMDV 2B protein has properties that are also found in other viroporins and may be involved in the infection mechanism of FMDV.

Coronavirus virulence genes with main focus on SARS-CoV envelope gene

Coronavirus (CoV) infection is usually detected by cellular sensors, which trigger the activation of the innate immune system. Nevertheless, CoVs have evolved viral proteins that target different signaling pathways to counteract innate immune responses. Some CoV proteins act as antagonists of interferon (IFN) by inhibiting IFN production or signaling, aspects that are briefly addressed in this review. After CoV infection, potent cytokines relevant in controlling virus infections and priming adaptive immune responses are also generated. However, an uncontrolled induction of these proinflammatory cytokines can lead to pathogenesis and disease severity as described for SARS-CoV and MERS-CoV. The cellular pathways mediated by interferon regulatory factor (IRF)-3 and -7, activating transcription factor (ATF)-2/jun, activator protein (AP)-1, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and nuclear factor of activated T cells (NF-AT), are the main drivers of the inflammatory response triggered after viral infections, with NF-κB pathway the most frequently activated. Key CoV proteins involved in the regulation of these pathways and the proinflammatory immune response are revisited in this manuscript. It has been shown that the envelope (E) protein plays a variable role in CoV morphogenesis, depending on the CoV genus, being absolutely essential in some cases (genus α CoVs such as TGEV, and genus β CoVs such as MERS-CoV), but not in others (genus β CoVs such as MHV or SARS-CoV). A comprehensive accumulation of data has shown that the relatively small E protein elicits a strong influence on the interaction of SARS-CoV with the host. In fact, after infection with viruses in which this protein has been deleted, increased cellular stress and unfolded protein responses, apoptosis, and augmented host immune responses were observed. In contrast, the presence of E protein activated a pathogenic inflammatory response that may cause death in animal models and in humans. The modification or deletion of different motifs within E protein, including the transmembrane domain that harbors an ion channel activity, small sequences within the middle region of the carboxy-terminus of E protein, and its most carboxy-terminal end, which contains a PDZ domain-binding motif (PBM), is sufficient to attenuate the virus. Interestingly, a comprehensive collection of SARS-CoVs in which these motifs have been modified elicited full and long-term protection even in old mice, making those deletion mutants promising vaccine candidates. These data indicate that despite its small size, E protein drastically influences the replication of CoVs and their pathogenicity. Although E protein is not essential for CoV genome replication or subgenomic mRNA synthesis, it affects virus morphogenesis, budding, assembly, intracellular trafficking, and virulence. In fact, E protein is responsible in a significant proportion of the inflammasome activation and the associated inflammation elicited by SARS-CoV in the lung parenchyma. This exacerbated inflammation causes edema accumulation leading to acute respiratory distress syndrome (ARDS) and, frequently, to the death of infected animal models or human patients.

Topology and biological function of enterovirus non-structural protein 2B as a member of the viroporin family

Viroporins are a group of transmembrane proteins with low molecular weight that are encoded by many animal viruses. Generally, viroporins are composed of 50–120 amino acid residues and possess a minimum of one hydrophobic region that interacts with the lipid bilayer and leads to dispersion. Viroporins are involved in destroying the morphology of host cells and disturbing their biological functions to complete the life cycle of the virus. The 2B proteins encoded by enteroviruses, which belong to the family Picornaviridae, can form transmembrane pores by oligomerization, increase the permeability of plasma membranes, disturb the homeostasis of calcium in cells, induce apoptosis, and cause autophagy; these abilities are shared among viroporins. The present paper introduces the structure and biological characteristics of various 2B proteins encoded by enteroviruses of the family Picornaviridae and may provide a novel idea for developing antiviral drugs.

Severe acute respiratory syndrome coronavirus envelope protein ion channel activity promotes virus fitness and pathogenesis

Deletion of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) envelope (E) gene attenuates the virus. E gene encodes a small multifunctional protein that possesses ion channel (IC) activity, an important function in virus-host interaction. To test the contribution of E protein IC activity in virus pathogenesis, two recombinant mouse-adapted SARS-CoVs, each containing one single amino acid mutation that suppressed ion conductivity, were engineered. After serial infections, mutant viruses, in general, incorporated compensatory mutations within E gene that rendered active ion channels. Furthermore, IC activity conferred better fitness in competition assays, suggesting that ion conductivity represents an advantage for the virus. Interestingly, mice infected with viruses displaying E protein IC activity, either with the wild-type E protein sequence or with the revertants that restored ion transport, rapidly lost weight and died. In contrast, mice infected with mutants lacking IC activity, which did not incorporate mutations within E gene during the experiment, recovered from disease and most survived. Knocking down E protein IC activity did not significantly affect virus growth in infected mice but decreased edema accumulation, the major determinant of acute respiratory distress syndrome (ARDS) leading to death. Reduced edema correlated with lung epithelia integrity and proper localization of Na⁺/K⁺ ATPase, which participates in edema resolution. Levels of inflammasome-activated IL-1β were reduced in the lung airways of the animals infected with viruses lacking E protein IC activity, indicating that E protein IC function is required for inflammasome activation. Reduction of IL-1β was accompanied by diminished amounts of TNF and IL-6 in the absence of E protein ion conductivity. All these key cytokines promote the progression of lung damage and ARDS pathology. In conclusion, E protein IC activity represents a new determinant for SARS-CoV virulence.

Several highly pathogenic viruses encode small transmembrane proteins with ion-conduction properties named viroporins. Viroporins are generally involved in virus production and maturation processes, which many times are achieved by altering the ion homeostasis of cell organelles. Cells have evolved mechanisms to sense these imbalances in ion concentrations as a danger signal, and consequently trigger the innate immune system. Recently, it has been demonstrated that viroporins are inducers of cytosolic macromolecular complexes named inflammasomes that trigger the activation of key inflammatory cytokines such as IL-1β. The repercussions of this system in viral pathogenesis or disease outcome are currently being explored. SARS-CoV infection induces an uncontrolled inflammatory response leading to pulmonary damage, edema accumulation, severe hypoxemia and eventually death. In this study, we report that SARS-CoV E protein ion channel activity is a determinant of virulence, as the elimination of this function attenuated the virus, reducing the harmful inflammatory cytokine burst produced after infection, in which inflammasome activation plays a critical role. This led to less pulmonary damage and to disease resolution. These novel findings may be of relevance for other viral infections and can possibly be translated in order to find therapies for their associated diseases.

Coxsackievirus B exits the host cell in shed microvesicles displaying autophagosomal markers

Coxsackievirus B3 (CVB3), a member of the picornavirus family and enterovirus genus, causes viral myocarditis, aseptic meningitis, and pancreatitis in humans. We genetically engineered a unique molecular marker, “fluorescent timer” protein, within our infectious CVB3 clone and isolated a high-titer recombinant viral stock (Timer-CVB3) following transfection in HeLa cells. “Fluorescent timer” protein undergoes slow conversion of fluorescence from green to red over time, and Timer-CVB3 can be utilized to track virus infection and dissemination in real time. Upon infection with Timer-CVB3, HeLa cells, neural progenitor and stem cells (NPSCs), and C2C12 myoblast cells slowly changed fluorescence from green to red over 72 hours as determined by fluorescence microscopy or flow cytometric analysis. The conversion of “fluorescent timer” protein in HeLa cells infected with Timer-CVB3 could be interrupted by fixation, suggesting that the fluorophore was stabilized by formaldehyde cross-linking reactions. Induction of a type I interferon response or ribavirin treatment reduced the progression of cell-to-cell virus spread in HeLa cells or NPSCs infected with Timer-CVB3. Time lapse photography of partially differentiated NPSCs infected with Timer-CVB3 revealed substantial intracellular membrane remodeling and the assembly of discrete virus replication organelles which changed fluorescence color in an asynchronous fashion within the cell. “Fluorescent timer” protein colocalized closely with viral 3A protein within virus replication organelles. Intriguingly, infection of partially differentiated NPSCs or C2C12 myoblast cells induced the release of abundant extracellular microvesicles (EMVs) containing matured “fluorescent timer” protein and infectious virus representing a novel route of virus dissemination. CVB3 virions were readily observed within purified EMVs by transmission electron microscopy, and infectious virus was identified within low-density isopycnic iodixanol gradient fractions consistent with membrane association. The preferential detection of the lipidated form of LC3 protein (LC3 II) in released EMVs harboring infectious virus suggests that the autophagy pathway plays a crucial role in microvesicle shedding and virus release, similar to a process previously described as autophagosome-mediated exit without lysis (AWOL) observed during poliovirus replication. Through the use of this novel recombinant virus which provides more dynamic information from static fluorescent images, we hope to gain a better understanding of CVB3 tropism, intracellular membrane reorganization, and virus-associated microvesicle dissemination within the host.

Enteroviruses are significant human pathogens, causing myocarditis, aseptic meningitis and encephalitis. The mechanisms of enterovirus dissemination in the host and cell-to-cell spread may be critical factors influencing viral pathogenesis. Here, we have generated a recombinant coxsackievirus expressing “fluorescence timer” protein (Timer-CVB3) which assists in following the progression of infection within the host. Unexpectedly, we observed the shedding of microvesicles containing virus in partially-differentiated progenitor cells infected with Timer-CVB3. These extracellular microvesicles (EMVs) were released in high levels following cellular differentiation, and may play a role in virus dissemination. Timer-CVB3 will be a valuable tool in monitoring virus spread in the infected host.

Viroporin activity of the JC polyomavirus is regulated by interactions with the adaptor protein complex 3

Viroporins, which are encoded by a wide range of animal viruses, oligomerize in host cell membranes and form hydrophilic pores that can disrupt a number of physiological properties of the cell. Little is known about the relationship between host cell proteins and viroporin activity. The human JC polyomavirus (JCV) is the causative agent of progressive multifocal leukoencephalopathy. The JCV-encoded agnoprotein, which is essential for viral replication, has been shown to act as a viroporin. Here we demonstrate that the JCV agnoprotein specifically interacts with adaptor protein complex 3 through its δ subunit. This interaction interrupts adaptor protein complex 3-mediated vesicular trafficking with suppression of the targeting of the protein to the lysosomal degradation pathway and instead permits the transport of agnoprotein to the cell surface with resulting membrane permeabilization. The findings demonstrate a previously undescribed paradigm in virus-host interactions allowing the host to regulate viroporin activity and suggest that the viroporins of other viruses may also be highly regulated by specific interactions with host cell proteins.

Viruses as modulators of mitochondrial functions

Mitochondria are multifunctional organelles with diverse roles including energy production and distribution, apoptosis, eliciting host immune response, and causing diseases and aging. Mitochondria-mediated immune responses might be an evolutionary adaptation by which mitochondria might have prevented the entry of invading microorganisms thus establishing them as an integral part of the cell. This makes them a target for all the invading pathogens including viruses. Viruses either induce or inhibit various mitochondrial processes in a highly specific manner so that they can replicate and produce progeny. Some viruses encode the Bcl2 homologues to counter the proapoptotic functions of the cellular and mitochondrial proteins. Others modulate the permeability transition pore and either prevent or induce the release of the apoptotic proteins from the mitochondria. Viruses like Herpes simplex virus 1 deplete the host mitochondrial DNA and some, like human immunodeficiency virus, hijack the host mitochondrial proteins to function fully inside the host cell. All these processes involve the participation of cellular proteins, mitochondrial proteins, and virus specific proteins. This review will summarize the strategies employed by viruses to utilize cellular mitochondria for successful multiplication and production of progeny virus.