The p7 viroporin of the hepatitis C virus contributes to liver inflammation by stimulating production of Interleukin-1β

The role of inflammasome in chronic viral hepatitis

Infections of hepatotropic viruses cause a wide array of liver diseases including acute hepatitis, chronic hepatitis and the consequently developed cirrhosis and hepatocellular carcinoma (HCC). Among the five classical hepatotropic viruses, hepatitis B virus (HBV) and hepatitis C virus (HCV) usually infect human persistently and cause chronic hepatitis, leading to major troubles to humanity. Previous studies have revealed that several types of inflammasomes are involved in the infections of HBV and HCV. Here, we summarize the current knowledge about their roles in hepatitis B and C. NLRP3 inflammasome can be activated and regulated by HBV and HCV. It is found to exert antiviral function or mediates inflammatory response in viral infections depending on different experimental models. Besides NLRP3 inflammasome, IFI16 and AIM2 inflammasomes participate in the pathological process of hepatitis B, and NALP3 inflammasome may sense HCV infection in hepatocytes. The inflammasomes affect the pathological process of viral hepatitis through its downstream secretion of inflammatory cytokines interleukin-1β (IL-1β) and IL-18 or induction of pyroptosis resulting from cleaved gasdermin D (GSDMD). However, the roles of inflammasomes in different stages of viral infection remains mainly unclear. More proper experimental models of viral hepatitis should be developed for specific studies in future, so that we can understand more about the complexity of inflammasome regulation and multifunction of inflammasomes and their downstream effectors during HBV and HCV infections.

Inflammatory Consequences: Hepatitis C Virus-Induced Inflammasome Activation and Pyroptosis

Hepatitis C virus (HCV), despite the availability of effective direct-acting antivirals (DAAs) that clear the virus from >95% of individuals treated, continues to cause significant health care burden due to disease progression that can lead to fibrosis, cirrhosis, and/or hepatocellular carcinoma. The fact that some people who are treated with DAAs still go on to develop worsening liver disease warrants further study into the immunopathogenesis of HCV. Many viral infections, including HCV, have been associated with activation of the inflammasome/pyroptosis pathway. This inflammatory cell death pathway ultimately results in cell lysis and release of inflammatory cytokines, IL-18 and IL-1β. This review will report on studies that investigated HCV and inflammasome activation/pyroptosis. This includes clinical in vivo data showing elevated pyroptosis-associated cytokines in the blood of individuals living with HCV, studies of genetic associations of pyroptosis-related genes and development of liver disease, and in vitro studies aimed at understanding the mechanism of pyroptosis induced by HCV. Finally, we discuss major gaps in understanding and outstanding questions that remain in the field of HCV-induced pyroptosis.

Patch-clamp studies and cell viability assays suggest a distinct site for viroporin inhibitors on the E protein of SARS-CoV-2

BACKGROUND

SARS-CoV-2 has caused a worldwide pandemic since December 2019 and the search for pharmaceutical targets against COVID-19 remains an important challenge. Here, we studied the envelope protein E of SARS-CoV and SARS-CoV-2, a highly conserved 75-76 amino acid viroporin that is crucial for virus assembly and release. E protein channels were recombinantly expressed in HEK293 cells, a membrane-directing signal peptide ensured transfer to the plasma membrane.

METHODS

Viroporin channel activity of both E proteins was investigated using patch-clamp electrophysiology in combination with a cell viability assay. We verified inhibition by classical viroporin inhibitors amantadine, rimantadine and 5-(N,N-hexamethylene)-amiloride, and tested four ivermectin derivatives.

RESULTS

Classical inhibitors showed potent activity in patch-clamp recordings and viability assays. In contrast, ivermectin and milbemycin inhibited the E channel in patch-clamp recordings but displayed only moderate activity on the E protein in the cell viability assay, which is also sensitive to general cytotoxic activity of the tested compounds. Nemadectin and ivermectin aglycon were inactive. All ivermectin derivatives were cytotoxic at concentrations > 5 µM, i.e. below the level required for E protein inhibition.

CONCLUSIONS

This study demonstrates direct inhibition of the SARS-CoV-2 E protein by classical viroporin inhibitors. Ivermectin and milbemycin inhibit the E protein channel but their cytotoxicity argues against clinical application.

Race between virus and inflammasomes: inhibition or escape, intervention and therapy

The inflammasome is a multiprotein complex that further regulates cell pyroptosis and inflammation by activating caspase-1. The assembly and activation of inflammasome are associated with a variety of diseases. Accumulative studies have shown that inflammasome is a key modulator of the host's defense response to viral infection. Indeed, it has been established that activation of inflammasome occurs during viral infection. At the same time, the host has evolved a variety of corresponding mechanisms to inhibit unnecessary inflammasome activation. Therefore, here, we review and summarize the latest research progress on the interaction between inflammosomes and viruses, highlight the assembly and activation of inflammosome in related cells after viral infection, as well as the corresponding molecular regulatory mechanisms, and elucidate the effects of this activation on virus immune escape and host innate and adaptive immune defenses. Finally, we also discuss the potential therapeutic strategies to prevent and/or ameliorate viral infection-related diseases via targeting inflammasomes and its products.

Activation of Inflammasome during Bluetongue Virus Infection

Bluetongue virus (BTV), a double-stranded RNA virus belonging to the Sedoreoviridae family, provokes an economically important disease in ruminants. In this study, we show that the production of activated caspase-1 and interleukin 1 beta (IL-1β) is induced in BTV-infected cells. This response seems to require virus replication since a UV-inactivated virus is unable to activate this pathway. In NLRP3^-/- cells, BTV could not trigger further IL-1β synthesis, indicating that it occurs through NLRP3 inflammasome activation. Interestingly, we observed differential activation levels in bovine endothelial cells depending on the tissue origin. In particular, inflammasome activation was stronger in umbilical cord cells, suggesting that these cells are more prone to induce the inflammasome upon BTV infection. Finally, the strength of the inflammasome activation also depends on the BTV strain, which points to the importance of viral origin in inflammasome modulation. This work reports the crucial role of BTV in the activation of the NLRP3 inflammasome and further shows that this activation relies on BTV replication, strains, and cell types, thus providing new insights into BTV pathogenesis.

Channel activity of SARS-CoV-2 viroporin ORF3a inhibited by adamantanes and phenolic plant metabolites

SARS-CoV-2 has been responsible for the major worldwide pandemic of COVID-19. Despite the enormous success of vaccination campaigns, virus infections are still prevalent and effective antiviral therapies are urgently needed. Viroporins are essential for virus replication and release, and are thus promising therapeutic targets. Here, we studied the expression and function of recombinant ORF3a viroporin of SARS-CoV-2 using a combination of cell viability assays and patch-clamp electrophysiology. ORF3a was expressed in HEK293 cells and transport to the plasma membrane verified by a dot blot assay. Incorporation of a membrane-directing signal peptide increased plasma membrane expression. Cell viability tests were carried out to measure cell damage associated with ORF3a activity, and voltage-clamp recordings verified its channel activity. The classical viroporin inhibitors amantadine and rimantadine inhibited ORF3a channels. A series of ten flavonoids and polyphenolics were studied. Kaempferol, quercetin, epigallocatechin gallate, nobiletin, resveratrol and curcumin were ORF3a inhibitors, with IC₅₀ values ranging between 1 and 6 µM, while 6-gingerol, apigenin, naringenin and genistein were inactive. For flavonoids, inhibitory activity could be related to the pattern of OH groups on the chromone ring system. Thus, the ORF3a viroporin of SARS-CoV-2 may indeed be a promising target for antiviral drugs.

Significance of elevated serum and hepatic NOD-like receptor pyrin domain containing 3 (NLRP3) in hepatitis C virus-related liver disease

NOD-like receptor pyrin domain containing 3 (NLRP3) is a microbial and danger signal sensor that acts as a regulator of inflammation via activation of Caspase-1 (CASP1) and has been identified as a major contributor to human liver diseases. The present study was conducted to investigate the association between NLRP3 and the progression of hepatitis C virus (HCV)-related liver disease. Serum NLRP3 levels were analyzed in 49 patients with chronic HCV infection and 18 healthy controls and liver tissues from 34 patients were examined to assess the protein expression of NLRP3 and its activation marker CASP1 using immunohistochemical staining. The results showed that the median serum NLRP3 levels was significantly higher in HCV-infected patients compared with healthy controls (1040 pg/ml vs 695 pg/ml respectively, P < 0.001) and were positively correlated with hepatic NLRP3 and CASP1 expression (r = 0.749, P < 0.001 and r = 0.557, P = 0.001 respectively). The NLRP3 levels in serum and the liver significantly increased with worsening liver pathology and showed positive correlations with serum aminotransferases levels, HCV viremia, and albumin-bilirubin score (P < 0.05). The receiver operating characteristic curve analysis revealed a high diagnostic performance of serum NLRP3 in determining the extent of liver necroinflammation, fibrosis, and steatosis (area under the curve = 0.951, 0.971, and 0.917 respectively, P < 0.001). In conclusion, NLRP3 plays an important role in liver disease progression during HCV infection via CASP1 activation and might be a promising therapeutic target. Serum NLRP3 could be an additional biomarker for liver inflammation and fibrosis.

Potassium viroporins as model systems for understanding eukaryotic ion channel behaviour

Ion channels are membrane proteins essential for a plethora of cellular functions including maintaining cell shape, ion homeostasis, cardiac rhythm and action potential in neurons. The complexity and often extensive structure of eukaryotic membrane proteins makes it difficult to understand their basic biological regulation. Therefore, this article suggests, viroporins - the miniature versions of eukaryotic protein homologs from viruses - might serve as model systems to provide insights into behaviour of eukaryotic ion channels in general. The structural requirements for correct assembly of the channel along with the basic functional properties of a K+ channel exist in the minimal design of the viral K+ channels from two viruses, Chlorella virus (Kcv) and Ectocarpus siliculosus virus (Kesv). These small viral proteins readily assemble into tetramers and they sort in cells to distinct target membranes. When these viruses-encoded channels are expressed into the mammalian cells, they utilise their protein machinery and hence can serve as excellent tools to study the cells protein sorting machinery. This combination of small size and robust function makes viral K+ channels a valuable model system for detection of basic structure-function correlations. It is believed that molecular and physiochemical analyses of these viroporins may serve as basis for the development of inhibitors or modulators to ion channel activity for targeting ion channel diseases - so called channelopathies. Therefore, it may provide a potential different scope for molecular pharmacology studies aiming at novel and innovative therapeutics associated with channel related diseases. This article reviews the structural and functional properties of Kcv and Kesv upon expression in mammalian cells and Xenopus oocytes. The mechanisms behind differential protein sorting in Kcv and Kesv are also thoroughly discussed.

SARS-CoV-2 Viroporins: A Multi-Omics Insight from Nucleotides to Amino Acids

COVID-19 is caused by SARS-CoV-2 which has so far affected more than 500 million people worldwide and killed over 6 million as of 1 May 2022. The approved emergency-use vaccines were lifesaving in such a devastating pandemic. Inflammation-related pathways have been well documented to be upregulated in the case of SARS-CoV-2 in rodents, non-human primates and human samples. We reanalysed a previously published dataset to understand if certain molecular components of inflammation could be higher in infected samples. Mechanistically, viroporins are important players in the life cycle of SARS-CoV-2 and are primary to its pathogenesis. We studied the two prominent viroporins of SARS-CoV-2 (i) Orf3a and (ii) envelope (E) protein from a sequence and structural point of view. Orf3a is a cation-selective viral ion channel which has been shown to disrupt the endosomal pathways. E protein is one of the most conserved proteins among the SARS-CoV proteome which affects the ERGIC-related pathways. The aqueous medium through the viroporins mediates the non-selective translocation of cations, affecting ionic homeostasis in the host cellular compartments. We hypothesize a possible mechanistic approach whereby the ionic imbalance caused by viroporin action could potentially be one of the major pathogenic drivers leading to the increased inflammatory response in the host cell. Our results shed light into the transcriptomic, genomic and structural proteomics aspects of widely studied SARS-CoV-2 viroporins, which can be potentially leveraged for the development of antiviral therapeutics.

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.

Viroporins: Structure, function, and their role in the life cycle of SARS-CoV-2

Viroporins are indispensable for viral replication. As intracellular ion channels they disturb pH gradients of organelles and allow Ca²⁺ flux across ER membranes. Viroporins interact with numerous intracellular proteins and pathways and can trigger inflammatory responses. Thus, they are relevant targets in the search for antiviral drugs. Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) underlies the world-wide pandemic of COVID-19, where an effective therapy is still lacking despite impressive progress in the development of vaccines and vaccination campaigns. Among the 29 proteins of SARS-CoV-2, the E- and ORF3a proteins have been identified as viroporins that contribute to the massive release of inflammatory cytokines observed in COVID-19. Here, we describe structure and function of viroporins and their role in inflammasome activation and cellular processes during the virus replication cycle. Techniques to study viroporin function are presented, with a focus on cellular and electrophysiological assays. Contributions of SARS-CoV-2 viroporins to the viral life cycle are discussed with respect to their structure, channel function, binding partners, and their role in viral infection and virus replication. Viroporin sequences of new variants of concern (α–ο) of SARS-CoV-2 are briefly reviewed as they harbour changes in E and 3a proteins that may affect their function.

Bioinformatics Analysis Identifies a Small ORF in the Genome of Fish Nidoviruses of Genus Oncotshavirus Predicted to Encode a Novel Integral Protein

Genome sequence analysis of Atlantic salmon bafinivirus (ASBV) revealed a small open reading frame (ORF) predicted to encode a Type I membrane protein with an N-terminal cleaved signal sequence (110 aa), likely an envelope (E) protein. Bioinformatic analyses showed that the predicted protein is strikingly similar to the coronavirus E protein in structure. This is the first report to identify a putative E protein ORF in the genome of members of the Oncotshavirus genus (subfamily Piscavirinae, family Tobaniviridae, order Nidovirales) and, if expressed would be the third family (after Coronaviridae and Arteriviridae) within the order to have the E protein as a major structural protein.

Inhibition of SARS CoV Envelope Protein by Flavonoids and Classical Viroporin Inhibitors

Severe acute respiratory syndrome coronavirus (SARS-CoV), an enveloped single-stranded positive-sense RNA virus, is a member of the genus Betacoronavirus, family Coronaviridae. The SARS-CoV envelope protein E is a small (∼8.4 kDa) channel-forming membrane protein whose sequence is highly conserved between SARS-CoV and SARS-CoV-2. As a viroporin, it is involved in various aspects of the virus life cycle including assembly, budding, envelope formation, virus release, and inflammasome activation. Here, SARS-CoV E protein was recombinantly expressed in HEK293 cells and channel activity and the effects of viroporin inhibitors studied using patch-clamp electrophysiology and a cell viability assay. We introduced a membrane-directing signal peptide to ensure transfer of recombinant E protein to the plasma membrane. E protein expression induced transmembrane currents that were blocked by various inhibitors. In an ion-reduced buffer system, currents were proton-dependent and blocked by viroporin inhibitors rimantadine and amantadine. I-V relationships of recombinant E protein were not pH-dependent in a classical buffer system with high extracellular Na⁺ and high intracellular K⁺. E-protein mediated currents were inhibited by amantadine and rimantadine, as well as 5-(N,N-hexamethylene)amiloride (HMA). We tested a total of 10 flavonoids, finding inhibitory activity of varying potency. Epigallocatechin and quercetin were most effective, with IC₅₀ values of 1.5 ± 0.1 and 3.7 ± 0.2 nM, respectively, similar to the potency of rimantadine (IC₅₀ = 1.7 ± 0.6 nM). Patch-clamp results were independently verified using a modified cell viability assay for viroporin inhibitors. These results contribute to the development of novel antiviral drugs that suppress virus activity and proliferation.

Innate Immunity in Hepatitis C Virus Infection

Activation and viral control of the innate immune response are hallmarks of hepatitis C virus (HCV) infection and are major determinants of spontaneous clearance or progression to chronic infection and liver disease. In this review, we provide a contemporary overview of how HCV is sensed by the host cell to trigger innate immune activation and the mechanisms deployed by the virus to evade this response. Type I and III interferons (IFNs) are crucial mediators of antiviral innate immunity against HCV, and we specifically highlight the importance of IFN-λ host genetics for the outcome of HCV infection. Last, we focus on the proinflammatory responses elicited by HCV infection and describe our current understanding of how interleukin (IL)-1β signaling and cross talk between the IL-1β and IFN signaling pathways lead to sustained inflammation and increased risk of liver pathology.

The Network of Interactions Between Classical Swine Fever Virus Nonstructural Protein p7 and Host Proteins

Classical swine fever (CSF) is a highly contagious viral disease causing severe economic losses to the swine industry. As viroporins of viruses modulate the cellular ion balance and then take over the cellular machinery, blocking the activity of viroporin or developing viroporin-defective attenuated vaccines offers new approaches to treat or prevent viral infection. Non-structural protein p7 of CSF virus (CSFV) is a viroporin, which was highly involved in CSFV virulence. Deciphering the interaction between p7 and host proteins will aid our understanding of the mechanism of p7-cellular protein interaction affecting CSFV replication. In the present study, seven host cellular proteins including microtubule-associated protein RP/EB family member 1 (MAPRE1), voltage-dependent anion channel 1 (VDAC1), proteasome maturation protein (POMP), protein inhibitor of activated STAT 1 (PIAS1), gametogenetin binding protein 2 (GGNBP2), COP9 signalosome subunit 2 (COPS2), and contactin 1 (CNTN1) were identified as the potential interactive cellular proteins of CSFV p7 by using yeast two-hybrid (Y2H) screening. Plus, the interaction of CSFV p7 with MAPRE1 and VDAC1 was further evaluated by co-immunoprecipitation and GST-pulldown assay. Besides, the p7-cellular protein interaction network was constructed based on these seven host cellular proteins and the STRING database. Enrichment analysis of GO and KEGG indicated that many host proteins in the p7-cellular protein interaction network were mainly related to the ubiquitin-proteasome system, cGMP-PKG signaling pathway, calcium signaling pathway, and JAK-STAT pathway. Overall, this study identified potential interactive cellular proteins of CSFV p7, constructed the p7-cellular protein interaction network, and predicted the potential pathways involved in the interaction between CSFV p7 and host cells.

Immune system control of hepatitis C virus infection

Hepatitis C virus (HCV) remains a global public health problem even though more than 95% of infections can be cured by treatment with direct-acting antiviral agents. Resolution of viremia post antiviral therapy does not lead to protective immunity and therefore reinfections can occur. Immune cell detection of HCV activates signaling pathways that produce interferons and trigger the innate immune response against the virus, preventing HCV replication and spread. Cells in the innate immune system, including natural killer, dendritic, and Kupffer cells, interact with infected hepatocytes and present viral antigens to T and B cells where their effector responses contribute to infection outcome. Despite the immune activation, HCV can evade the host response and establish persistent infection. Plans to understand the correlates of protection and strategies to activate proper innate and adaptive immune responses are needed for development of an effective prophylactic vaccine that stimulates protective immunity and limits HCV transmission.

COVID-19-associated cardiovascular morbidity in older adults: a position paper from the Italian Society of Cardiovascular Researches

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects host cells following binding with the cell surface ACE2 receptors, thereby leading to coronavirus disease 2019 (COVID-19). SARS-CoV-2 causes viral pneumonia with additional extrapulmonary manifestations and major complications, including acute myocardial injury, arrhythmia, and shock mainly in elderly patients. Furthermore, patients with existing cardiovascular comorbidities, such as hypertension and coronary heart disease, have a worse clinical outcome following contraction of the viral illness. A striking feature of COVID-19 pandemics is the high incidence of fatalities in advanced aged patients: this might be due to the prevalence of frailty and cardiovascular disease increase with age due to endothelial dysfunction and loss of endogenous cardioprotective mechanisms. Although experimental evidence on this topic is still at its infancy, the aim of this position paper is to hypothesize and discuss more suggestive cellular and molecular mechanisms whereby SARS-CoV-2 may lead to detrimental consequences to the cardiovascular system. We will focus on aging, cytokine storm, NLRP3/inflammasome, hypoxemia, and air pollution, which is an emerging cardiovascular risk factor associated with rapid urbanization and globalization. We will finally discuss the impact of clinically available CV drugs on the clinical course of COVID-19 patients. Understanding the role played by SARS-CoV2 on the CV system is indeed mandatory to get further insights into COVID-19 pathogenesis and to design a therapeutic strategy of cardio-protection for frail patients.

Viroporins and inflammasomes: A key to understand virus-induced inflammation

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The article provides a summary on cellular receptors involved in virus immunity.

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It summarizes key findings on viroporins, a novel class of viral proteins and their role in the virus life cycle and host cell interactions.

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It presents an overview of the current understanding of inflammasomes complex activation, with special focus on NLRP3.

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It discusses the correlation between viroporins and inflammasomes activation and aggravated inflammatory cytokines production.

Viroporins are virus encoded proteins that alter membrane permeability and can trigger subsequent cellular signals. Oligomerization of viroporin subunits results in formation of a hydrophilic pore which facilitates ion transport across host cell membranes. These viral channel proteins may be involved in different stages of the virus infection cycle. Inflammasomes are large multimolecular complexes best recognized for their ability to control activation of caspase-1, which in turn regulates the maturation of interleukin-1 β (IL-1β) and interleukin 18 (IL-18). IL-1β was originally identified as a pro-inflammatory cytokine able to induce both local and systemic inflammation and a febrile reaction in response to infection or injury. Excessive production of IL-1β is associated with autoimmune and inflammatory diseases. Microbial derivatives, bacterial pore-forming toxins, extracellular ATP and other pathogen-associated molecular patterns trigger activation of NLRP3 inflammasomes. Recent studies have reported that viroporin activity is capable of inducing inflammasome activity and production of IL-1β, where NLRP3 is shown to be regulated by fluxes of K⁺, H⁺ and Ca²⁺ in addition to reactive oxygen species, autophagy and endoplasmic reticulum stress. The aim of this review is to present an overview of the key findings on viroporin activity with special emphasis on their role in virus immunity and as possible activators of inflammasomes.

Cytokines and serum amyloid A in the pathogenesis of hepatitis C virus infection

Expression of the acute phase protein serum amyloid A (SAA) is dependent on the release of the pro-inflammatory cytokines IL-1, IL-6 and TNF-α during infection and inflammation. Hepatitis C virus (HCV) upregulates SAA-inducing cytokines. In line with this, a segment of chronically infected individuals display increased circulating levels of SAA. SAA has even been proposed to be a potential biomarker to evaluate treatment efficiency and the course of disease. SAA possesses antiviral activity against HCV via direct interaction with the viral particle, but might also divert infectivity through its function as an apolipoprotein. On the other hand, SAA shares inflammatory and angiogenic activity with chemotactic cytokines by activating the G protein-coupled receptor, formyl peptide receptor 2. These latter properties might promote chronic inflammation and hepatic injury. Indeed, up to 80 % of infected individuals develop chronic disease because they cannot completely clear the infection, due to diversion of the immune response. In this review, we summarize the interconnection between SAA and cytokines in the context of HCV infection and highlight the dual role SAA could play in this disease. Nevertheless, more research is needed to establish whether the balance between those opposing activities can be tilted in favor of the host defense.

Modulation of calcium signaling pathway by hepatitis C virus core protein stimulates NLRP3 inflammasome activation

Hepatitis C virus (HCV) infection remains a major cause of hepatic inflammation and liver disease. HCV triggers NLRP3 inflammasome activation and interleukin-1β (IL-1β) production from hepatic macrophages, or Kupffer cells, to drive the hepatic inflammatory response. Here we examined HCV activation of the NLRP3 inflammasome signaling cascade in primary human monocyte derived macrophages and THP-1 cell models of hepatic macrophages to define the HCV-specific agonist and cellular processes of inflammasome activation. We identified the HCV core protein as a virion-specific factor of inflammasome activation. The core protein was both necessary and sufficient for IL-1β production from macrophages exposed to HCV or soluble core protein alone. NLRP3 inflammasome activation by the HCV core protein required calcium mobilization linked with phospholipase-C activation. Our findings reveal a molecular basis of hepatic inflammasome activation and IL-1β release triggered by HCV core protein.

This study deciphers the molecular mechanism of Hepatitis C virus (HCV)-induced hepatic inflammation. HCV triggers NLRP3 inflammasome activation and IL-1β release from hepatic macrophages, thus driving liver inflammation. Using biochemical, virological, and genetic approaches we identified the HCV core protein as the specific viral stimulus that triggers intracellular calcium signaling linked with phospholipase-C activation to drive NLRP3 inflammasome activation and IL-1β release in macrophages.

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.

Severe Acute Respiratory Syndrome Coronavirus Viroporin 3a Activates the NLRP3 Inflammasome

Nod-like receptor family, pyrin domain-containing 3 (NLRP3) regulates the secretion of proinflammatory cytokines interleukin 1 beta (IL-1β) and IL-18. We previously showed that influenza virus M2 or encephalomyocarditis virus (EMCV) 2B proteins stimulate IL-1β secretion following activation of the NLRP3 inflammasome. However, the mechanism by which severe acute respiratory syndrome coronavirus (SARS-CoV) activates the NLRP3 inflammasome remains unknown. Here, we provide direct evidence that SARS-CoV 3a protein activates the NLRP3 inflammasome in lipopolysaccharide-primed macrophages. SARS-CoV 3a was sufficient to cause the NLRP3 inflammasome activation. The ion channel activity of the 3a protein was essential for 3a-mediated IL-1β secretion. While cells uninfected or infected with a lentivirus expressing a 3a protein defective in ion channel activity expressed NLRP3 uniformly throughout the cytoplasm, NLRP3 was redistributed to the perinuclear space in cells infected with a lentivirus expressing the 3a protein. K⁺ efflux and mitochondrial reactive oxygen species were important for SARS-CoV 3a-induced NLRP3 inflammasome activation. These results highlight the importance of viroporins, transmembrane pore-forming viral proteins, in virus-induced NLRP3 inflammasome activation.

The NS1 Protein of Influenza A Virus Participates in Necroptosis by Interacting with MLKL and Increasing Its Oligomerization and Membrane Translocation

Elimination of infected cells by programmed cell death is a well-recognized host defense mechanism to control the spread of infection. In addition to apoptosis, necroptosis is also one of the mechanisms of cell death that can be activated by viral infection. Activation of necroptosis leads to the phosphorylation of mixed-lineage kinase domain-like protein (MLKL) by receptor-interacting protein kinase 3 (RIPK3) and results in MLKL oligomerization and membrane translocation, leading to membrane disruption and a loss of cellular ion homeostasis. It has recently been reported that influenza A virus (IAV) infection induces necroptosis. However, the underlying mechanism of the IAV-mediated necroptosis process, particularly the roles of IAV proteins in necroptosis, remains unexplored. Here, we report that IAV infection induces necroptosis in macrophages and epithelial cells. We demonstrate that the NS1 protein of IAV interacts with MLKL. Coiled-coil domain 2 of MLKL has a predominant role in mediating the MLKL interaction with NS1. The interaction of NS1 with MLKL increases MLKL oligomerization and membrane translocation. Moreover, the MLKL-NS1 interaction enhances MLKL-mediated NLRP3 inflammasome activation, leading to increased interleukin-1β (IL-1β) processing and secretion.IMPORTANCE Necroptosis is a programmed cell death that is inflammatory in nature owing to the release of danger-associated molecular patterns from the ruptured cell membrane. However, necroptosis also constitutes an important arm of host immune responses. Thus, a balanced inflammatory response determines the disease outcome. We report that the NS1 protein of IAV participates in necroptosis by interacting with MLKL, resulting in increased MLKL oligomerization and membrane translocation. These results reveal a novel function of the NS1 protein and the mechanism by which IAV induces necroptosis. Moreover, we show that this interaction enhances NLRP3 inflammasome activation and IL-1β processing and secretion. This information may contribute to a better understanding of the role of necroptosis in IAV-induced inflammation.

Hepatitis C: From inflammatory pathogenesis to anti-inflammatory/hepatoprotective therapy

Hepatitis C virus (HCV) infection commonly causes progressive liver diseases that deteriorate from chronic inflammation to fibrosis, cirrhosis and even to hepatocellular carcinoma. A long-term, persistent and uncontrolled inflammatory response is a hallmark of these diseases and further leads to hepatic injury and more severe disease progression. The levels of inflammatory cytokines and chemokines change with the states of infection and treatment, and therefore, they may serve as candidate biomarkers for disease progression and therapeutic effects. The mechanisms of HCV-induced inflammation involve classic pathogen pattern recognition, inflammasome activation, intrahepatic inflammatory cascade response, and oxidative and endoplasmic reticulum stress. Direct-acting antivirals (DAAs) are the first-choice therapy for effectively eliminating HCV, but DAAs alone are not sufficient to block the uncontrolled inflammation and severe liver injury in HCV-infected individuals. Some patients who achieve a sustained virologic response after DAA therapy are still at a long-term risk for progression to liver cirrhosis and hepatocellular carcinoma. Therefore, coupling with anti-inflammatory/hepatoprotective agents with anti-HCV effects is a promising therapeutic regimen for these patients during or after treatment with DAAs. In this review, we discuss the relationship between inflammatory mediators and HCV infection, summarize the mechanisms of HCV-induced inflammation, and describe the potential roles of anti-inflammatory/hepatoprotective drugs with anti-HCV activity in the treatment of advanced HCV infection.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

Adaptive mutation F772S-enhanced p7-NS4A cooperation facilitates the assembly and release of hepatitis C virus and is associated with lipid droplet enlargement

Hepatitis C virus (HCV) infection is a major cause of chronic hepatitis and liver cancer worldwide. Adaptive mutations play important roles in the development of the HCV replicon and its infectious clones. We and others have previously identified the p7 mutation F772S and the co-presence of NS4A mutations in infectious HCV full-length clones and chimeric recombinants. However, the underlying mechanism of F772S function remains incompletely understood. Here, we investigated the functional role of F772S using an efficient JFH1-based reporter virus with Core-NS2 from genotype 2a strain J6, and we designated J6-p7/JFH1-4A according to the strain origin of the p7 and NS4A sequences. We found that replacing JFH1-4A with J6-4A (wild-type or mutated NS4A) or genotype 2b J8-4A severely attenuated the viability of J6-p7/JFH1-4A. However, passage-recovered viruses that contained J6-p7 all acquired F772S. Introduction of F772S efficiently rescued the viral spread and infectivity titers of J6-p7/J6-4A, which reached the levels of the original J6-p7/JFH1-4A and led to a concomitant increase in RNA replication, assembly and release of viruses with J6-specific p7 and NS4A. These data suggest that an isolate-specific cooperation existed between p7 and NS4A. NS4A exchange- or substitution-mediated viral attenuation was attributed to the RNA sequence, and no p7-NS4A protein interaction was detected. Moreover, we found that F772S-enhanced p7-NS4A cooperation was associated with the enlargement of intracellular lipid droplets. This study therefore provides new insights into the mechanisms of adaptive mutations and facilitates studies on the HCV life cycle and virus–host interaction.

Symmetric dimeric adamantanes for exploring the structure of two viroporins: influenza virus M2 and hepatitis C virus p7

Background

Adamantane-based compounds have been identified to interfere with the ion-channel activity of viroporins and thereby inhibit viral infection. To better understand the difference in the inhibition mechanism of viroporins, we synthesized symmetric dimeric adamantane analogs of various alkyl-spacer lengths.

Methods

Symmetric dimeric adamantane derivatives were synthesized where two amantadine or rimantadine molecules were linked by various alkyl-spacers. The inhibitory activity of the compounds was studied on two viroporins: the influenza virus M2 protein, expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique, and the hepatitis C virus (HCV) p7 channels for five different genotypes (1a, 1b, 2a, 3a, and 4a) expressed in HEK293 cells using whole-cell patch-clamp recording techniques.

Results

Upon testing on M2 protein, dimeric compounds showed significantly lower inhibitory activity relative to the monomeric amantadine. The lack of channel blockage of the dimeric amantadine and rimantadine analogs against M2 wild type and M2-S31N mutant was consistent with previously proposed drug-binding mechanisms and further confirmed that the pore-binding model is the pharmacologically relevant drug-binding model. On the other hand, these dimers showed similar potency to their respective monomeric analogs when tested on p7 protein in HCV genotypes 1a, 1b, and 4a while being 700-fold and 150-fold more potent than amantadine in genotypes 2a and 3a, respectively. An amino group appears to be important for inhibiting the ion-channel activity of p7 protein in genotype 2a, while its importance was minimal in all other genotypes.

Conclusion

Symmetric dimeric adamantanes can be considered a prospective class of p7 inhibitors that are able to address the differences in adamantane sensitivity among the various genotypes of HCV.

Role of Inflammasomes in the Development of Gastrointestinal Diseases

Many diseases of the gastrointestinal tract have been attributed to chronic inflammation, and a few have identified the role of inflammasomes in their pathogenesis. Inflammasomes are a group of protein complexes comprising of several intracellular proteins that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines. Recent studies have implicated activation of several families of NOD-like receptors (NLRs) which are major components of inflammasomes in the development and exacerbation of many diseases of human systems. In this chapter, we discuss the role of inflammasomes in some of the most prevalent diseases of the gastrointestinal tract and highlight potential targets for treatment.

The amino-terminus of the hepatitis C virus (HCV) p7 viroporin and its cleavage from glycoprotein E2-p7 precursor determine specific infectivity and secretion levels of HCV particle types

Viroporins are small transmembrane proteins with ion channel activities modulating properties of intracellular membranes that have diverse proviral functions. Hepatitis C virus (HCV) encodes a viroporin, p7, acting during assembly, envelopment and secretion of viral particles (VP). HCV p7 is released from the viral polyprotein through cleavage at E2-p7 and p7-NS2 junctions by signal peptidase, but also exists as an E2p7 precursor, of poorly defined properties. Here, we found that ectopic p7 expression in HCVcc-infected cells reduced secretion of particle-associated E2 glycoproteins. Using biochemical assays, we show that p7 dose-dependently slows down the ER-to-Golgi traffic, leading to intracellular retention of E2, which suggested that timely E2p7 cleavage and p7 liberation are critical events to control E2 levels. By studying HCV mutants with accelerated E2p7 processing, we demonstrate that E2p7 cleavage controls E2 intracellular expression and secretion levels of nucleocapsid-free subviral particles and infectious virions. In addition, our imaging data reveal that, following p7 liberation, the amino-terminus of p7 is exposed towards the cytosol and coordinates the encounter between NS5A and NS2-based assembly sites loaded with E1E2 glycoproteins, which subsequently leads to nucleocapsid envelopment. We identify punctual mutants at p7 membrane interface that, by abrogating NS2/NS5A interaction, are defective for transmission of infectivity owing to decreased secretion of core and RNA and to increased secretion of non/partially-enveloped particles. Altogether, our results indicate that the retarded E2p7 precursor cleavage is essential to regulate the intracellular and secreted levels of E2 through p7-mediated modulation of the cell secretory pathway and to unmask critical novel assembly functions located at p7 amino-terminus.

Activation and Role of NACHT, LRR, and PYD Domains-Containing Protein 3 Inflammasome in RNA Viral Infection

NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome activation and effects during ribonucleic acid (RNA) viral infection are the focus of a wide range of research currently. Both the pathogen-associated molecule pattern derived from virions and intracellular stress molecules involved in the process of viral infection lead to activation of the NLRP3 inflammasome, which in turn triggers inflammatory responses for antiviral defense and tissue healing. However, aberrant activation of the NLRP3 inflammasome can instead support viral pathogenesis and promote disease progression. Here, we summarize and expound upon the recent literature describing the molecular mechanisms underlying the activation and effects of the NLRP3 inflammasome in RNA viral infection to highlight how it provides protection against RNA viral infection.