NLRP3 Inflammasome Activation by Viroporins of Animal Viruses

Slit2-Robo4 signal pathway and tight junction in intestine mediate LPS-induced inflammation in mice

BACKGROUND

Sepsis is one of the most common clinical diseases, which is characterized by a serious and uncontrollable inflammatory response. LPS-induced inflammation is a critical pathological event in sepsis, but the underlying mechanism has not yet been fully elucidated.

METHODS

The animal model was established for two batches. In the first batch of experiments, Adult C57BL/6J mice were randomly divided into control group and LPS (5 mg/kg, i.p.)group . In the second batch of experiments, mice were randomly divided into control group, LPS group, and LPS+VX765(10 mg/kg, i.p., an inhibitor of NLRP3 inflammasome) group. After 24 hours, mice were anesthetized with isoflurane, blood and intestinal tissue were collected for tissue immunohistochemistry, Western blot analysis and ELISA assays.

RESULTS

The C57BL/6J mice injected with LPS for twenty-four hours could exhibit severe inflammatory reaction including an increased IL-1β, IL-18 in serum and activation of NLRP3 inflammasome in intestine. The injection of VX765 could reverse these effects induced by LPS. These results indicated that the increased level of IL-1β and IL-18 in serum induced by LPS is related to the increased intestinal permeability and activation of NLRP3 inflammasome. In the second batch of experiments, results of western blot and immunohistochemistry showed that Slit2 and Robo4 were significant decreased in intestine of LPS group, while the expression of VEGF was significant increased. Meanwhile, the protein level of tight junction protein ZO-1, occludin, and claudin-5 were significantly lower than in control group, which could also be reversed by VX765 injection.

CONCLUSIONS

In this study, we revealed that Slit2-Robo4 signaling pathway and tight junction in intestine may be involved in LPS-induced inflammation in mice, which may account for the molecular mechanism of sepsis.

Helenine blocks NLRP3 activation by disrupting the NEK7-NLRP3 interaction and ameliorates inflammatory diseases

BACKGROUND

The involvement of NLRP3 inflammasome is associated with the progress of numerous inflammatory conditions. However, there is currently no single compound used in the clinic. Search for the inhibitor of NLRP3 inflammasome from natural products is an attractive direction. The compound Helenin (Hel), which is obtained from Inula helenium L., is reported to have anti-inflammatory activities. However, the underlying molecular mechanisms and specific inflammatory signal pathway remains not well understood.

PURPOSE

This research aims to determine the impacts of Hel on NLRP3 inflammasome and the underlying mechanism involved, meanwhile also assessing its potential as a therapeutic intervention for inflammatory diseases mediated by NLRP3 overactivation.

METHODS

Pretreated with Hel in BMDMs (bone marrow-derived macrophages), then stimulated with NLRP3 triggers and measured the expression of active caspase-1 and interleukin 1β (IL-1β). Determination of intracellular K+ and Ca2+, ASC oligomerization and mitochondrial reactive oxygen species (mtROS) production were employed to explore the preliminary mechanism of Hel on NLRP3 activation. Subsequently, Co-immunoprecipitation was used to investigate protein-protein interaction and reduction of covalent bonds of Hel was to explore the binding mode between drugs and proteins. Finally, in vivo experiments, we utilized mouse lethal sepsis and monosodium urate(MSU)-induced peritonitis models to evaluate the effectiveness of Hel in inhibiting inflammatory diseases.

RESULTS

The findings revealed that Hel exhibited a specific blocking effect on NLRP3, with no impact on the assembly of NLRC4 and AIM2 inflammasome. Through the analysis of mechanisms targeting key upstream factors in NLRP3 activation, Hel inhibited NLRP3-dependent ASC oligomerization but did not regulating inflammasome priming, K+ efflux, Ca2+ influx, or mitochondrial damage and mtROS. Moreover, Hel effectively interrupted the binding of NEK7-NLRP3, which was dependent on the active double C=C of the α,β-unsaturated carbonyl units in Hel. In mouse models, Hel showed promising therapeutic effects in the treatment of NLRP3 overactivation-associated diseases, including the lethal sepsis and acute systemic inflammation induced by lipopolysaccharide (LPS) and peritonitis induced by MSU.

CONCLUSION

Our results indicate that Hel dependent α,β-unsaturated carbonyl units interrupt the formation of the NLRP3-NEK7 interaction, thereby blocks the inflammasome assemblage and activation. These fundings would suggest that Hel is a promising inhibitor for treating diseases driven by NLRP3 overactivation.

The NLRP3 inflammasome in viral infection (Review)

The interplay between pathogen and host determines the immune response during viral infection. The Nod‑like receptor (NLR) protein 3 inflammasome is a multiprotein complex that induces the activation of inflammatory caspases and the release of IL‑1β, which play an important role in the innate immune responses. In the present review, the mechanisms of the NLR family pyrin domain containing 3 inflammasome activation and its dysregulation in viral infection were addressed.

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.

The SARS-CoV-2 envelope protein disrupts barrier function in an in vitro human blood-brain barrier model

Patients with coronavirus disease 2019 (COVID-19) have been frequently reported to exhibit neurological manifestations and disruption of the blood-brain barrier (BBB). Among the risk factors for BBB breakdown, the loss of endothelial cells and pericytes has caused widespread concern. Recent studies have revealed that severe acute respiratory syndrome coronavirus 2 envelope (S2E) protein caused cell death. We tested the hypothesis that the S2E protein alone could induce BBB dysfunction. The S2E protein bound to human BBB-related cells and inhibited cell viability in a dose- and time-dependent manner. Importantly, the S2E protein disrupted barrier function in an in vitro BBB model composed of HCMEC/D3 (brain endothelial cell line), HBVP (brain vascular pericyte), and U87MG (astrocyte cell line) cells and suppressed the expression of major genes involved in maintaining endothelial permeability and function. In addition, the S2E protein crossed the HCMEC/D3 monolayer. The S2E protein triggered inflammatory responses in HCMEC/D3 and U87MG cells. Taken together, these results show for the first time that the S2E protein has a negative impact on the BBB. Therapies targeting the S2E protein could protect against and treat central nervous system manifestations in COVID-19 patients.

Cytoplasmic RNA sensors and their interplay with RNA-binding partners in innate antiviral response: theme and variations

Sensing of pathogen-associated molecular patterns including viral RNA by innate immunity represents the first line of defense against viral infection. In addition to RIG-I-like receptors and NOD-like receptors, several other RNA sensors are known to mediate innate antiviral response in the cytoplasm. Double-stranded RNA-binding protein PACT interacts with prototypic RNA sensor RIG-I to facilitate its recognition of viral RNA and induction of host interferon response, but variations of this theme are seen when the functions of RNA sensors are modulated by other RNA-binding proteins to impinge on antiviral defense, proinflammatory cytokine production and cell death programs. Their discrete and coordinated actions are crucial to protect the host from infection. In this review, we will focus on cytoplasmic RNA sensors with an emphasis on their interplay with RNA-binding partners. Classical sensors such as RIG-I will be briefly reviewed. More attention will be brought to new insights on how RNA-binding partners of RNA sensors modulate innate RNA sensing and how viruses perturb the functions of RNA-binding partners.

Coordinated regulation of interferon and inflammasome signaling pathways by SARS-CoV-2 proteins

Type I and III interferons (IFNs) and the nucleotide-binding domain (NBD) leucine-rich repeat (LRR)-containing receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasome play pivotal roles in the pathogenesis of SARS-CoV-2. While optimal IFN and inflammasome responses are essential for limiting SARS-CoV-2 infection, aberrant activation of these innate immune responses is associated with COVID-19 pathogenesis. In this review, we focus our discussion on recent findings on SARS-CoV-2-induced type I and III IFNs and NLRP3 inflammasome responses and the viral proteins regulating these mechanisms.

Therapeutic Interventions Targeting Innate Immune Receptors: A Balancing Act

The innate immune system is an organism's first line of defense against an onslaught of internal and external threats. The downstream adaptive immune system has been a popular target for therapeutic intervention, while there is a relative paucity of therapeutics targeting the innate immune system. However, the innate immune system plays a critical role in many human diseases, such as microbial infection, cancer, and autoimmunity, highlighting the need for ongoing therapeutic research. In this review, we discuss the major innate immune pathways and detail the molecular strategies underpinning successful therapeutics targeting each pathway as well as previous and ongoing efforts. We will also discuss any recent discoveries that could inform the development of novel therapeutic strategies. As our understanding of the innate immune system continues to develop, we envision that therapies harnessing the power of the innate immune system will become the mainstay of treatment for a wide variety of human diseases.

Unravelling the Immunomodulatory Effects of Viral Ion Channels, towards the Treatment of Disease

The current COVID-19 pandemic has highlighted the need for the research community to develop a better understanding of viruses, in particular their modes of infection and replicative lifecycles, to aid in the development of novel vaccines and much needed anti-viral therapeutics. Several viruses express proteins capable of forming pores in host cellular membranes, termed "Viroporins". They are a family of small hydrophobic proteins, with at least one amphipathic domain, which characteristically form oligomeric structures with central hydrophilic domains. Consequently, they can facilitate the transport of ions through the hydrophilic core. Viroporins localise to host membranes such as the endoplasmic reticulum and regulate ion homeostasis creating a favourable environment for viral infection. Viroporins also contribute to viral immune evasion via several mechanisms. Given that viroporins are often essential for virion assembly and egress, and as their structural features tend to be evolutionarily conserved, they are attractive targets for anti-viral therapeutics. This review discusses the current knowledge of several viroporins, namely Influenza A virus (IAV) M2, Human Immunodeficiency Virus (HIV)-1 Viral protein U (Vpu), Hepatitis C Virus (HCV) p7, Human Papillomavirus (HPV)-16 E5, Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) Open Reading Frame (ORF)3a and Polyomavirus agnoprotein. We highlight the intricate but broad immunomodulatory effects of these viroporins and discuss the current antiviral therapies that target them; continually highlighting the need for future investigations to focus on novel therapeutics in the treatment of existing and future emergent viruses.

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.

Elucidating the Neuropathologic Mechanisms of SARS-CoV-2 Infection

The current pandemic caused by the new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has become a public health emergency. To date, March 1, 2021, coronavirus disease 2019 (COVID-19) has caused about 114 million accumulated cases and 2.53 million deaths worldwide. Previous pieces of evidence suggest that SARS-CoV-2 may affect the central nervous system (CNS) and cause neurological symptoms in COVID-19 patients. It is also known that angiotensin-converting enzyme-2 (ACE2), the primary receptor for SARS-CoV-2 infection, is expressed in different brain areas and cell types. Thus, it is hypothesized that infection by this virus could generate or exacerbate neuropathological alterations. However, the molecular mechanisms that link COVID-19 disease and nerve damage are unclear. In this review, we describe the routes of SARS-CoV-2 invasion into the central nervous system. We also analyze the neuropathologic mechanisms underlying this viral infection, and their potential relationship with the neurological manifestations described in patients with COVID-19, and the appearance or exacerbation of some neurodegenerative diseases.

Nucleus distribution of cathepsin B in senescent microglia promotes brain aging through degradation of sirtuins

Cathepsin B (CatB) leakage from the lysosome into the cytosol in senescent microglia is associated with cognitive impairment. However, whether cellular compartmental translocation of CatB is associated with brain aging remains unclear. In the present study, increased CatB was found in the nucleus of CatB-overexpressed microglia followed by L-leucyl-L-leucine methyl ester, a lysosome-destabilizing reagent, and in the nuclear fraction of the cortex and hippocampus from aged mice. Moreover, CatB enzymatic activity examination showed the nuclear CatB exhibited the proteolytic activity to cleave its specific substrates. The amount of sirtuin1 (Sirt1), Sirt6, Sirt7, and p-Sirt1 was decreased in the cortical lysates from aged mice, in parallel with increased expression of proinflammatory mediators, which were diminished by CatB deficiency. Furthermore, intralateral ventricle administration of microglia overexpressed CatB, and treatment with L-leucyl-L-leucine methyl ester induced cognitive impairment in middle-aged mice. These observations suggest that the increase and nucleus translocation of CatB in senescent microglia were involved in the degradation of nuclear Sirts, which induced proinflammatory responses, resulting in cognition impairment.

Inflammasome Fuels Dengue Severity

Dengue is an acute febrile disease triggered by dengue virus. Dengue is the widespread and rapidly transmitted mosquito-borne viral disease of humans. Diverse symptoms and diseases due to Dengue virus (DENV) infection ranges from dengue fever, dengue hemorrhagic fever (life-threatening) and dengue shock syndrome characterized by shock, endothelial dysfunction and vascular leakage. Several studies have linked the severity of dengue with the induction of inflammasome. DENV activates the NLRP3-specific inflammasome in DENV infected human patients, mice; specifically, mouse bone marrow derived macrophages (BMDMs), dendritic cells, endothelial cells, human peripheral blood mononuclear cells (PBMCs), keratinocytes, monocyte-differentiated macrophages (THP-1), and platelets. Dengue virus mediated inflammasome initiates the maturation of IL-1β and IL-18, which are critical for dengue pathology and inflammatory response. Several studies have reported the molecular mechanism through which (host and viral factors) dengue induces inflammasome, unravels the possible mechanisms of DENV pathogenesis and sets up the stage for the advancement of DENV therapeutics. In this perspective article, we discuss the potential implications and our understanding of inflammasome mechanisms of dengue virus and highlight research areas that have potential to inhibit the pathogenesis of viral diseases, specifically for dengue.

Is There a Link Between the Pathogenic Human Coronavirus Envelope Protein and Immunopathology? A Review of the Literature

Since the severe acute respiratory syndrome (SARS) outbreak in 2003, human coronaviruses (hCoVs) have been identified as causative agents of severe acute respiratory tract infections. Two more hCoV outbreaks have since occurred, the most recent being SARS-CoV-2, the causative agent of coronavirus disease 2019 (COVID-19). The clinical presentation of SARS and MERS is remarkably similar to COVID-19, with hyperinflammation causing a severe form of the disease in some patients. Previous studies show that the expression of the SARS-CoV E protein is associated with the hyperinflammatory response that could culminate in acute respiratory distress syndrome (ARDS), a potentially fatal complication. This immune-mediated damage is largely caused by a cytokine storm, which is induced by significantly elevated levels of inflammatory cytokines interleukin (IL)-1β and IL-6, which are partly mediated by the expression of the SARS-CoV E protein. The interaction between the SARS-CoV E protein and the host protein, syntenin, as well as the viroporin function of SARS-CoV E, are linked to this cytokine dysregulation. This review aims to compare the clinical presentation of virulent hCoVs with a specific focus on the cause of the immunopathology. The review also proposes that inhibition of IL-1β and IL-6 in severe cases can improve patient outcome.

The Role of NLRP3 Inflammasome in the Pathogenesis of Traumatic Brain Injury

Traumatic brain injury (TBI) represents an important problem of global health. The damage related to TBI is first due to the direct injury and then to a secondary phase in which neuroinflammation plays a key role. NLRP3 inflammasome is a component of the innate immune response and different diseases, such as neurodegenerative diseases, are characterized by NLRP3 activation. This review aims to describe NLRP3 inflammasome and the consequences related to its activation following TBI. NLRP3, caspase-1, IL-1β, and IL-18 are significantly upregulated after TBI, therefore, the use of nonspecific, but mostly specific NLRP3 inhibitors is useful to ameliorate the damage post-TBI characterized by neuroinflammation. Moreover, NLRP3 and the molecules associated with its activation may be considered as biomarkers and predictive factors for other neurodegenerative diseases consequent to TBI. Complications such as continuous stimuli or viral infections, such as the SARS-CoV-2 infection, may worsen the prognosis of TBI, altering the immune response and increasing the neuroinflammatory processes related to NLRP3, whose activation occurs both in TBI and in SARS-CoV-2 infection. This review points out the role of NLRP3 in TBI and highlights the hypothesis that NLRP3 may be considered as a potential therapeutic target for the management of neuroinflammation in TBI.

Micheliolide Attenuates Lipopolysaccharide-Induced Inflammation by Modulating the mROS/NF-κB/NLRP3 Axis in Renal Tubular Epithelial Cells

Chronic kidney disease is a common disease closely related to renal tubular inflammation and oxidative stress, and no effective treatment is available. Activation of the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome is an important factor in renal inflammation, but the mechanism remains unclear. Micheliolide (MCL), which is derived from parthenolide, is a new compound with antioxidative and anti-inflammatory effects and has multiple roles in tumors and inflammatory diseases. In this study, we investigated the effect of MCL on lipopolysaccharide- (LPS-) induced inflammation in renal tubular cells and the related mechanism. We found that MCL significantly suppressed the LPS-induced NF-κB signaling and inflammatory expression of cytokines, such as tumor necrosis factor-α and monocyte chemoattractant protein-1 in a rat renal proximal tubular cell line (NRK-52E). MCL also prevented LPS- and adenosine triphosphate-induced NLRP3 inflammasome activation in vitro, as evidenced by the inhibition of NLRP3 expression, caspase-1 cleavage, and interleukin-1β and interleukin-18 maturation and secretion. Additionally, MCL inhibited the reduction of mitochondrial membrane potential and decreases the release of reactive oxygen species (ROS). Moreover, MCL can prevent NLRP3 inflammasome activation induced by rotenone, a well-known mitochondrial ROS (mROS) agonist, indicating that the mechanism of MCL's anti-inflammatory effect may be closely related to the mROS. In conclusion, our study indicates that MCL can inhibit LPS-induced renal inflammation through suppressing the mROS/NF-κB/NLRP3 axis in tubular epithelial cells.

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.

Impact of Respiratory Syncytial Virus Infection on Host Functions: Implications for Antiviral Strategies

Respiratory syncytial virus (RSV) is one of the leading causes of viral respiratory tract infection in infants, the elderly, and the immunocompromised worldwide, causing more deaths each year than influenza. Years of research into RSV since its discovery over 60 yr ago have elucidated detailed mechanisms of the host-pathogen interface. RSV infection elicits widespread transcriptomic and proteomic changes, which both mediate the host innate and adaptive immune responses to infection, and reflect RSV's ability to circumvent the host stress responses, including stress granule formation, endoplasmic reticulum stress, oxidative stress, and programmed cell death. The combination of these events can severely impact on human lungs, resulting in airway remodeling and pathophysiology. The RSV membrane envelope glycoproteins (fusion F and attachment G), matrix (M) and nonstructural (NS) 1 and 2 proteins play key roles in modulating host cell functions to promote the infectious cycle. This review presents a comprehensive overview of how RSV impacts the host response to infection and how detailed knowledge of the mechanisms thereof can inform the development of new approaches to develop RSV vaccines and therapeutics.

Dengue Virus Serotype 2 and Its Non-Structural Proteins 2A and 2B Activate NLRP3 Inflammasome

Dengue is the most prevalent and rapidly transmitted mosquito-borne viral disease of humans. One of the fundamental innate immune responses to viral infections includes the processing and release of pro-inflammatory cytokines such as interleukin (IL-1β and IL-18) through the activation of inflammasome. Dengue virus stimulates the Nod-like receptor (NLRP3-specific inflammasome), however, the specific mechanism(s) by which dengue virus activates the NLRP3 inflammasome is unknown. In this study, we investigated the activation of the NLRP3 inflammasome in endothelial cells (HMEC-1) following dengue virus infection. Our results showed that dengue infection as well as the NS2A and NS2B protein expression increase the NLRP3 inflammasome activation, and further apoptosis-associated speck-like protein containing caspase recruitment domain (ASC) oligomerization, and IL-1β secretion through caspase-1 activation. Specifically, we have demonstrated that NS2A and NS2B, two proteins of dengue virus that behave as putative viroporins, were sufficient to stimulate the NLRP3 inflammasome complex in lipopolysaccharide (LPS)-primed endothelial cells. In summary, our observations provide insight into the dengue-induced inflammatory response mechanism and highlight the importance of DENV-2 NS2A and NS2B proteins in activation of the NLRP3 inflammasome during dengue virus infection.

NLRP3 inflammasome activation by Foot-and-mouth disease virus infection mainly induced by viral RNA and non-structural protein 2B

Foot-and-mouth disease virus (FMDV) is a positive-strand RNA virus of the family Picornaviridae. Early studies show that some viruses of Picornaviridae, such as EMCV and EV71, induce NLRP3 inflammasome activation. Our current study demonstrates that FMDV induces the secretion of caspase-1 and interleukin 1 beta (IL-1β), as well as activates the NLRP3 inflammasome in a dose- and time-dependent manner. Meanwhile, NLRP3 inflammasome can suppress FMDV replication during virus infection. Both FMDV RNA and viroporin 2B stimulate NLRP3 inflammasome activation. FMDV RNA triggers NLRP3 inflammasome through p-NF-κB/p65 pathway not dependent on RIG-I inflammasome. FMDV 2B activates NLRP3 inflammasome through elevation of intracellular ion, but not dependent on mitochondrial reactive oxygen species (ROS) and lysosomal cathepsin B. It further demonstrates that 2B viroporin activates NLRP3 inflammasome and induces IL-1β in mice, which enhances the specific immune response against FMDV as an ideal self-adjuvant for FMD VLPs vaccine in guinea pigs. The results reveal a series of regulations between NLRP3 inflammasome complex and FMDV. Amino acids 140-145 of 2B is essential for forming an ion channel. By mutating the amino acid and changing the hydrophobic properties, the helical transmembrane region of the viroporin 2B is altered, so that the 2B is insufficient to trigger the activation of NLRP3 inflammasome. This study demonstrates the functions of FMDV RNA and 2B viroporin activate NLRP3 inflammasome and provides some useful information for the development of FMD vaccine self-adjuvant, which is also helpful for the establishment of effective prevention strategies by targeting NLRP3 inflammasome.

Fusogenic Reoviruses and Their Fusion-Associated Small Transmembrane (FAST) Proteins

With no limiting membrane surrounding virions, nonenveloped viruses have no need for membrane fusion to gain access to intracellular replication compartments. Consequently, nonenveloped viruses do not encode membrane fusion proteins. The only exception to this dogma is the fusogenic reoviruses that encode fusion-associated small transmembrane (FAST) proteins that induce syncytium formation. FAST proteins are the smallest viral membrane fusion proteins and, unlike their enveloped virus counterparts, are nonstructural proteins that evolved specifically to induce cell-to-cell, not virus-cell, membrane fusion. This distinct evolutionary imperative is reflected in structural and functional features that distinguish this singular family of viral fusogens from all other protein fusogens. These rudimentary fusogens comprise specific combinations of different membrane effector motifs assembled into small, modular membrane fusogens. FAST proteins offer a minimalist model to better understand the ubiquitous process of protein-mediated membrane fusion and to reveal novel mechanisms of nonenveloped virus dissemination that contribute to virulence.

Virome and Inflammasomes, a Finely Tuned Balance with Important Consequences for the Host Health

BACKGROUND

The virome is a network of viruses normally inhabiting humans. It forms a conspicuous portion of the so-called microbiome, once generically referred to as resident flora. Indeed, viruses infecting humans without leading to clinical disease are increasingly recognized as part of the microbiome and have an impact on the development of our immune system. In addition, they activate inflammasomes, multiprotein complexes that assemble in cells and that are responsible for the downstream effects of sensing pathogens.

OBJECTIVE

This review aims at summarizing the evidence on the role of the virome in modulating inflammation and emphasizes evidence for Anelloviruses as useful molecular markers to monitor inflammatory processes and immune system competence.

METHOD

We carried out a review of the literature published in the last 5 years and summarized older literature to take into account ground-breaking discoveries concerning inflammasome assembly and virome.

RESULTS

A massive amount of data recently emerging demonstrate that the microbiome closely reflects what we eat, and many other unexpected variables. Composition, location, and amount of the microbiome have an impact on innate and adaptive immune defences. Viruses making up the virome contribute to shaping the immune system. Anelloviruses, the best known of such viruses, are present in most human beings, persistently without causing apparent disease. Depending on their interplay with such viruses, inflammasomes instruct host defences to tolerate or forfeit a specific microorganism.

CONCLUSION

The virome plays an important role in shaping human immune defences and contributes to inflammatory processes by quenching or increasing them.

Severe acute respiratory syndrome coronavirus ORF3a protein activates the NLRP3 inflammasome by promoting TRAF3-dependent ubiquitination of ASC

Severe acute respiratory syndrome coronavirus (SARS-CoV) is capable of inducing a storm of proinflammatory cytokines. In this study, we show that the SARS-CoV open reading frame 3a (ORF3a) accessory protein activates the NLRP3 inflammasome by promoting TNF receptor-associated factor 3 (TRAF3)-mediated ubiquitination of apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC). SARS-CoV and its ORF3a protein were found to be potent activators of pro-IL-1β gene transcription and protein maturation, the 2 signals required for activation of the NLRP3 inflammasome. ORF3a induced pro-IL-1β transcription through activation of NF-κB, which was mediated by TRAF3-dependent ubiquitination and processing of p105. ORF3a-induced elevation of IL-1β secretion was independent of its ion channel activity or absent in melanoma 2 but required NLRP3, ASC, and TRAF3. ORF3a interacted with TRAF3 and ASC, colocalized with them in discrete punctate structures in the cytoplasm, and facilitated ASC speck formation. TRAF3-dependent K63-linked ubiquitination of ASC was more pronounced in SARS-CoV-infected cells or when ORF3a was expressed. Taken together, our findings reveal a new mechanism by which SARS-CoV ORF3a protein activates NF-κB and the NLRP3 inflammasome by promoting TRAF3-dependent ubiquitination of p105 and ASC.-Siu, K.-L., Yuen, K.-S., Castaño-Rodriguez, C., Ye, Z.-W., Yeung, M.-L., Fung, S.-Y., Yuan, S., Chan, C.-P., Yuen, K.-Y., Enjuanes, L., Jin, D.-Y. Severe acute respiratory syndrome coronavirus ORF3a protein activates the NLRP3 inflammasome by promoting TRAF3-dependent ubiquitination of ASC.

Immune Ecosystem of Virus-Infected Host Tissues

Virus infected host cells serve as a central immune ecological niche during viral infection and replication and stimulate the host immune response via molecular signaling. The viral infection and multiplication process involves complex intracellular molecular interactions between viral components and the host factors. Various types of host cells are also involved to modulate immune factors in delicate and dynamic equilibrium to maintain a balanced immune ecosystem in an infected host tissue. Antiviral host arsenals are equipped to combat or eliminate viral invasion. However, viruses have evolved with strategies to counter against antiviral immunity or hijack cellular machinery to survive inside host tissue for their multiplication. However, host immune systems have also evolved to neutralize the infection; which, in turn, either clears the virus from the infected host or causes immune-mediated host tissue injury. A complex relationship between viral pathogenesis and host antiviral defense could define the immune ecosystem of virus-infected host tissues. Understanding of the molecular mechanism underlying this ecosystem would uncover strategies to modulate host immune function for antiviral therapeutics. This review presents past and present updates of immune-ecological components of virus infected host tissue and explains how viruses subvert the host immune surveillances.

ATP-activated P2X7 receptor in the pathophysiology of mood disorders and as an emerging target for the development of novel antidepressant therapeutics

Mood disorders are a group of psychiatric conditions that represent leading global disease burdens. Increasing evidence from clinical and preclinical studies supports that innate immune system dysfunction plays an important part in the pathophysiology of mood disorders. P2X7 receptor, belonging to the ligand-gated ion channel P2X subfamily of purinergic P2 receptors for extracellular ATP, is highly expressed in immune cells including microglia in the central nervous system (CNS) and has a vital role in mediating innate immune response. The P2X7 receptor is also important in neuron-glia signalling in the CNS. The gene encoding human P2X7 receptor is located in a locus of susceptibility to mood disorders. In this review, we will discuss the recent progress in understanding the role of the P2X7 receptor in the pathogenesis and development of mood disorders and in discovering CNS-penetrable P2X7 antagonists for potential uses in in vivo imaging to monitor brain inflammation and antidepressant therapeutics.

Inflammasomes and its importance in viral infections

A complex interplay between pathogen and host determines the immune response during viral infection. A set of cytosolic sensors are expressed by immune cells to detect viral infection. NOD-like receptors (NLRs) comprise a large family of intracellular pattern recognition receptors. Members of the NLR family assemble into large multiprotein complexes, termed inflammasomes, which induce downstream immune responses to specific pathogens, environmental stimuli, and host cell damage. Inflammasomes are composed of cytoplasmic sensor molecules such as NLRP3 or absent in melanoma 2 (AIM2), the adaptor protein ASC (apoptosis-associated speck-like protein containing caspase recruitment domain), and the effector protein procaspase-1. The inflammasome operates as a platform for caspase-1 activation, resulting in caspase-1-dependent proteolytic maturation and secretion of interleukin (IL)-1β and IL-18. This, in turn, activates the expression of other immune genes and facilitates lymphocyte recruitment to the site of primary infection, thereby controlling invading pathogens. Moreover, inflammasomes counter viral replication and remove infected immune cells through an inflammatory cell death, program termed as pyroptosis. As a countermeasure, viral pathogens have evolved virulence factors to antagonise inflammasome pathways. In this review, we discuss the role of inflammasomes in sensing viral infection as well as the evasion strategies that viruses have developed to evade inflammasome-dependent immune responses. This information summarises our understanding of host defence mechanisms against viruses and highlights research areas that can provide new approaches to interfere in the pathogenesis of viral diseases.

Disentangling the Frames, the State of Research on the Alphavirus 6K and TF Proteins

For 30 years it was thought the alphavirus 6K gene encoded a single 6 kDa protein. However, through a bioinformatics search 10 years ago, it was discovered that there is a frameshifting event and two proteins, 6K and transframe (TF), are translated from the 6K gene. Thus, many functions attributed to the 6K protein needed reevaluation to determine if they properly belong to 6K, TF, or both proteins. In this mini-review, we reevaluate the past research on 6K and put those results in context where there are two proteins, 6K and TF, instead of one. Additionally, we discuss the most cogent outstanding questions for 6K and TF research, including their collective importance in alphavirus budding and their potential importance in disease based on the latest virulence data.

Uncoupling protein 2 modulation of the NLRP3 inflammasome in astrocytes and its implications in depression

Mitochondrial uncoupling protein 2 (UCP2) has been well characterized to control the production of reactive oxygen species (ROS) and astrocytes are the major cells responsible for the ROS production and the inflammatory responses in the brain. However, the function of UCP2 in astrocytes and the contribution of astrocytic UCP2 to depression remain undefined. Herein, we demonstrated that UCP2 knockout (KO) mice displayed aggravated depressive-like behaviors, impaired neurogenesis, and enhanced loss of astrocytes in the chronic mild stress (CMS)-induced anhedonia model of depression. We further found that UCP2 ablation significantly enhanced the activation of the nod-like receptor protein 3 (NLRP3) inflammasome in the hippocampus and in astrocytes. Furthermore, UCP2 deficiency promoted the injury of mitochondria, the generation of ROS and the physical association between thioredoxin-interacting protein (TXNIP) and NLRP3 in astrocytes. Moreover, transiently expressing exogenous UCP2 partially rescued the deleterious effects of UCP2 ablation on the astrocytes. These data indicate that UCP2 negatively regulates the activation of NLRP3 inflammasome and inhibited the ROS-TXNIP-NLRP3 pathway in astrocytes. Collectively, our findings reveal that UCP2 regulates inflammation responses in astrocytes and plays an important role in the pathogenesis of depression and that UCP2 may be a promising therapeutic target for depression.

Targeting the Channel Activity of Viroporins

Since the discovery that certain small viral membrane proteins, collectively termed as viroporins, can permeabilize host cellular membranes and also behave as ion channels, attempts have been made to link this feature to specific biological roles. In parallel, most viroporins identified so far are virulence factors, and interest has focused toward the discovery of channel inhibitors that would have a therapeutic effect, or be used as research tools to understand the biological roles of viroporin ion channel activity. However, this paradigm is being shifted by the difficulties inherent to small viral membrane proteins, and by the realization that protein-protein interactions and other diverse roles in the virus life cycle may represent an equal, if not, more important target. Therefore, although targeting the channel activity of viroporins can probably be therapeutically useful in some cases, the focus may shift to their other functions in following years. Small-molecule inhibitors have been mostly developed against the influenza A M2 (IAV M2 or AM2). This is not surprising since AM2 is the best characterized viroporin to date, with a well-established biological role in viral pathogenesis combined the most extensive structural investigations conducted, and has emerged as a validated drug target. For other viroporins, these studies are still mostly in their infancy, and together with those for AM2, are the subject of the present review.

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.

Emerging Roles of Viroporins Encoded by DNA Viruses: Novel Targets for Antivirals?

Studies have highlighted the essential nature of a group of small, highly hydrophobic, membrane embedded, channel-forming proteins in the life cycles of a growing number of RNA viruses. These viroporins mediate the flow of ions and a range of solutes across cellular membranes and are necessary for manipulating a myriad of host processes. As such they contribute to all stages of the virus life cycle. Recent discoveries have identified proteins encoded by the small DNA tumor viruses that display a number of viroporin like properties. This review article summarizes the recent developments in our understanding of these novel viroporins; describes their roles in the virus life cycles and in pathogenesis and speculates on their potential as targets for anti-viral therapeutic intervention.