Initiation and perpetuation of NLRP3 inflammasome activation and assembly

p38δ MAPK: A Novel Regulator of NLRP3 Inflammasome Activation With Increased Expression in Coronary Atherogenesis

OBJECTIVE

Activation of the inflammasome pathway in macrophages results in the secretion of 2 potent proinflammatory and proatherogenic cytokines, interleukin (IL)-1β, and IL-18. Atherosclerotic lesions are characterized by the presence of various endogenous activators of the NLR family pyrin domain containing 3 (NLRP3) inflammasome, including cholesterol crystals and extracellular ATP. The aim of this study was to comprehensively characterize the expression of inflammasome pathway components and regulators in human atherosclerotic lesions.

APPROACH AND RESULTS

Twenty human coronary artery RNA samples from 10 explanted hearts were analyzed using an inflammasome pathway-focused quantitative polymerase chain reaction array. Advanced atherosclerotic plaques, when compared with early-to-intermediate lesions from the same coronary trees, displayed significant upregulation of 12 target genes, including the key inflammasome components apoptosis-associated speck-like protein containing a CARD domain, caspase-1, and IL-18. Immunohistochemical stainings of the advanced plaques revealed macrophage foam cells positive for NLRP3 inflammasome components around the necrotic lipid cores. The polymerase chain reaction array target p38δ mitogen-activated protein kinase was upregulated in advanced plaques and strongly expressed by lesional macrophage foam cells. In cultured human monocyte-derived macrophages, the p38δ mitogen-activated protein kinase was activated by intracellular stress signals triggered during ATP- and cholesterol crystal-induced NLRP3 inflammasome activation and was required for NLRP3-mediated IL-1β secretion.

CONCLUSIONS

Increased expression of the key inflammasome components in advanced coronary lesions implies enhanced activity of the inflammasome pathway in progression of coronary atherosclerosis. The p38δ mitogen-activated protein kinase was identified as a novel regulator of NLRP3 inflammasome activation in primary human macrophages, and thus, represents a potential target for modulation of atherosclerotic inflammation.

Autophagy, Inflammation, and Immunity: A Troika Governing Cancer and Its Treatment

Autophagy, a cellular waste disposal process, has well-established tumor-suppressive properties. New studies indicate that, in addition to its cell-autonomous anti-tumorigenic functions, autophagy inhibits cancer development by orchestrating inflammation and immunity. While attenuating tumor-promoting inflammation, autophagy enhances the processing and presentation of tumor antigens and thereby stimulates anti-tumor immunity. Although cancer cells can escape immunosurveillance by tuning down autophagy, certain chemotherapeutic agents with immunogenic properties may enhance anti-tumor immunity by inducing autophagic cell death. Understanding the intricate and complex relationships within this troika and how they are affected by autophagy enhancing drugs should improve the efficacy of cancer immunotherapy.

The role of NLRP3 and AIM2 in inflammasome activation during Brucella abortus infection

The innate immune system is essential for the detection and elimination of bacterial pathogens. Upon inflammasome activation, caspase-1 cleaves pro-IL-1β and pro-IL-18 to their mature forms IL-1β and IL-18, respectively, and the cell undergoes inflammatory death termed pyroptosis. Here, we reviewed recent findings demonstrating that Brucella abortus ligands activate NLRP3 and AIM2 inflammasomes which lead to control of infection. This protective effect is due to the inflammatory response caused by IL-1β and IL-18 rather than cell death. Brucella DNA is sensed by AIM2 and bacteria-induced mitochondrial reactive oxygen species is detected by NLRP3. However, deregulation of pro-inflammatory cytokine production can lead to immunopathology. Nervous system invasion by bacteria of the genus Brucella results in an inflammatory disorder termed neurobrucellosis. Herein, we discuss the mechanism of caspase-1 activation and IL-1β secretion in glial cells infected with B. abortus. Our results demonstrate that the ASC inflammasome is indispensable for inducing the activation of caspase-1 and secretion of IL-1β upon infection of astrocytes and microglia with Brucella. Moreover, our results demonstrate that secretion of IL-1β by Brucella-infected glial cells depends on NLRP3 and AIM2 and leads to neurobrucellosis. Further, the inhibition of the host cell inflammasome as an immune evasion strategy has been described for bacterial pathogens. We discuss here that the bacterial type IV secretion system VirB is required for inflammasome activation in host cells during infection. Taken together, our results indicate that Brucella is sensed by ASC inflammasomes mainly NLRP3 and AIM2 that collectively orchestrate a robust caspase-1 activation and pro-inflammatory response.

Enzymatically Inactive Procaspase 1 stabilizes the ASC Pyroptosome and Supports Pyroptosome Spreading during Cell Division

Caspase-1 is a key player during the initiation of pro-inflammatory innate immune responses, activating pro-IL-1β in so-called inflammasomes. A subset of patients with recurrent febrile episodes and systemic inflammation of unknown origin harbor mutations in CASP1 encoding caspase-1. CASP1 variants result in reduced enzymatic activity of caspase-1 and impaired IL-1β secretion. The apparent paradox of reduced IL-1β secretion but systemic inflammation led to the hypothesis that CASP1 mutations may result in variable protein interaction clusters, thus activating alternative signaling pathways. To test this hypothesis, we established and characterized an in vitro system of transduced immortalized murine macrophages expressing either WT or enzymatically inactive (p.C284A) procaspase-1 fusion reporter proteins. Macrophages with variant p.C284A caspase-1 did not secrete IL-1β and exhibited reduced inflammatory cell death, referred to as pyroptosis. Caspase-1 and apoptosis-associated speck-like protein containing a CARD (ASC) formed cytosolic macromolecular complexes (so-called pyroptosomes) that were significantly increased in number and size in cells carrying the p.C284A caspase-1 variant compared with WT caspase-1. Furthermore, enzymatically inactive caspase-1 interacted with ASC longer and with increased intensity compared with WT caspase-1. Applying live cell imaging, we documented for the first time that pyroptosomes containing enzymatically inactive variant p.C284A caspase-1 spread during cell division. In conclusion, variant p.C284A caspase-1 stabilizes pyroptosome formation, potentially enhancing inflammation by two IL-1β-independent mechanisms: pyroptosomes convey an enhanced inflammatory stimulus through the recruitment of additional proteins (such as RIP2, receptor interacting protein kinase 2), which is further amplified through pyroptosome and cell division.

Pathogen perception by NLRs in plants and animals: Parallel worlds

Intracellular NLR (Nucleotide-binding domain and Leucine-rich Repeat-containing) receptors are sensitive monitors that detect pathogen invasion of both plant and animal cells. NLRs confer recognition of diverse molecules associated with pathogen invasion. NLRs must exhibit strict intramolecular controls to avoid harmful ectopic activation in the absence of pathogens. Recent discoveries have elucidated the assembly and structure of oligomeric NLR signalling complexes in animals, and provided insights into how these complexes act as scaffolds for signal transduction. In plants, recent advances have provided novel insights into signalling-competent NLRs, and into the myriad strategies that diverse plant NLRs use to recognise pathogens. Here, we review recent insights into the NLR biology of both animals and plants. By assessing commonalities and differences between kingdoms, we are able to develop a more complete understanding of NLR function.

ASC filament formation serves as a signal amplification mechanism for inflammasomes

A hallmark of inflammasome activation is the ASC speck, a micrometre-sized structure formed by the inflammasome adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD), which consists of a pyrin domain (PYD) and a caspase recruitment domain (CARD). Here we show that assembly of the ASC speck involves oligomerization of ASC^PYD into filaments and cross-linking of these filaments by ASC^CARD. ASC mutants with a non-functional CARD only assemble filaments but not specks, and moreover disrupt endogenous specks in primary macrophages. Systematic site-directed mutagenesis of ASC^PYD is used to identify oligomerization-deficient ASC mutants and demonstrate that ASC speck formation is required for efficient processing of IL-1β, but dispensable for gasdermin-D cleavage and pyroptosis induction. Our results suggest that the oligomerization of ASC creates a multitude of potential caspase-1 activation sites, thus serving as a signal amplification mechanism for inflammasome-mediated cytokine production.

Inflammasomes regulate IL-1β family maturation and pyroptosis. Here the authors show that ASC oligomerization and the formation of ASC specks are needed for IL-1β processing, but are not required for pyroptosis, indicating distinct inflammasome regulatory pathways.

Host Resistance and Immune Aging

Human immune system aging results in impaired responses to pathogens or vaccines. In the innate immune system, which mediates the earliest pro-inflammatory responses to immunologic challenge, processes ranging from Toll-like Receptor function to Neutrophil Extracellular Trap formation are generally diminished in older adults. Dysregulated, enhanced basal inflammation with age reflecting activation by endogenous damage-associated ligands contributes to impaired innate immune responses. In the adaptive immune system, T and B cell subsets and function alter with age. The control of cytomegalovirus infection, particularly in the T lineage, plays a dominant role in the differentiation and diversity of the T cell compartment.

Reparative inflammation takes charge of tissue regeneration

Inflammation underlies many chronic and degenerative diseases, but it also mitigates infections, clears damaged cells and initiates tissue repair. Many of the mechanisms that link inflammation to damage repair and regeneration in mammals are conserved in lower organisms, indicating that it is an evolutionarily important process. Recent insights have shed light on the cellular and molecular processes through which conventional inflammatory cytokines and Wnt factors control mammalian tissue repair and regeneration. This is particularly important for regeneration in the gastrointestinal system, especially for intestine and liver tissues in which aberrant and deregulated repair results in severe pathologies.

The MUC1 mucin specifically inhibits activation of the NLRP3 inflammasome

MUC1 is a cell membrane-associated mucin, expressed ubiquitously on the mucosal epithelia as well as by immune cells, that limits the inflammatory response to multiple pathogens. We have recently shown that MUC1 controls inflammation resulting from Helicobacter pylori infection by suppressing interleukin-1β (IL-1β) produced via the NLRP3 inflammasome. Here, we demonstrate that MUC1 also regulates IL-1β secretion induced by the NLRP3-activating bacteria Haemophilus influenzae but not bacteria that activate other inflammasomes. Using purified ligands, we further demonstrate that MUC1 regulation of NLRP3 is specific, as it has no effect on the NLRP1b, NLRC4 and AIM2 inflammasomes. This indicates a unique role for MUC1 in the regulation of NLRP3-activating bacterial infections.

NF-κB Restricts Inflammasome Activation via Elimination of Damaged Mitochondria

Nuclear factor κB (NF-κB), a key activator of inflammation, primes the NLRP3-inflammasome for activation by inducing pro-IL-1β and NLRP3 expression. NF-κB, however, also prevents excessive inflammation and restrains NLRP3-inflammasome activation through a poorly defined mechanism. We now show that NF-κB exerts its anti-inflammatory activity by inducing delayed accumulation of the autophagy receptor p62/SQSTM1. External NLRP3-activating stimuli trigger a form of mitochondrial (mt) damage that is caspase-1- and NLRP3-independent and causes release of direct NLRP3-inflammasome activators, including mtDNA and mtROS. Damaged mitochondria undergo Parkin-dependent ubiquitin conjugation and are specifically recognized by p62, which induces their mitophagic clearance. Macrophage-specific p62 ablation causes pronounced accumulation of damaged mitochondria and excessive IL-1β-dependent inflammation, enhancing macrophage death. Therefore, the "NF-κB-p62-mitophagy" pathway is a macrophage-intrinsic regulatory loop through which NF-κB restrains its own inflammation-promoting activity and orchestrates a self-limiting host response that maintains homeostasis and favors tissue repair.

Microarray and whole-exome sequencing analysis of familial Behçet's disease patients

Behçet’s disease (BD), a chronic systemic inflammatory disorder, is characterized by recurrent oral and genital mucous ulcers, uveitis, and skin lesions. We performed DNA microarray analysis of peripheral blood mononuclear cell (PBMC) mRNA from 41 Japanese BD patients and revealed elevated levels of interleukin (IL) 23 receptor (IL23R) mRNA in many BD patients. DNA sequencing around a SNV (Rs12119179) tightly linked to BD revealed an elevated frequency of the C genotype, consistent with a previous report that IL23R is a susceptibility locus for BD. Notably, four of these BD patients are members of familial BD; a whole-exome sequencing (WES) of these BD patients identified 19 novel single-nucleotide variations (SNVs) specific to these patients. They include heterozygous SNVs in the genes encoding IL-1 receptor-associated kinase 4 (IRAK4), nucleotide-binding oligomerization domain (NOD)-like receptor family pyrin domain-containing 14 (NRP14) and melanoma antigen-encoding gene E2 (MAGEE2); IRAK4 harbors a missense mutation, whereas NRP14 and MAGEE2 harbor nonsense mutations. These SNVs may serve as genetic markers that characterize BD.

Extracellular ATP mediates inflammatory responses in colitis via P2 × 7 receptor signaling

Extracellular purinergic products, particularly ATP, have recently been implicated to regulate immune cell functions and contribute to aberrant inflammatory responses of immune diseases. However, regulation of immune responses of colitis by extracellular ATP and its main receptor, P2 × 7, remains to be elucidated. In the study, we induced murine colitis by feeding mice with 4% dextran sulfate sodium (DSS), and noted dramatically heightened extracellular ATP levels in colon tissues during the progression of experimental colitis. Blockade of ATP release by carbenoxolone (CBX) treatment, or promoting ATP degradation by ATP diphosphohydrolase (apyrase), decreased extracellular ATP levels in colon tissues, attenuated DSS-induced colitis, whereas inhibition of extracellular ATP degradation by sodium metatungstate (POM-1) exacerbated tissue damage in the mice with colitis. Moreover, treatment with inhibitor of P2 × 7 receptor, A438079, decreased NFκB activation and active caspase-1 expression in lamina propria immune cells, downregulated proinflammatory cytokine production in colon tissues, and attenuated murine colitis. Collectively, these data suggest extracellular ATP participates in regulation of inflammatory responses of experimental colitis, through P2 × 7 receptor and inflammasome and NFκB signaling, which provides potential alternatives to the current clinical approaches to suppress extracellular ATP-mediated immune responsiveness.

Bacterial Exotoxins and the Inflammasome

The inflammasomes are intracellular protein complexes that play an important role in innate immune sensing. Activation of inflammasomes leads to activation of caspase-1 and maturation and secretion of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18. In certain myeloid cells, this activation can also lead to an inflammatory cell death (pyroptosis). Inflammasome sensor proteins have evolved to detect a range of microbial ligands and bacterial exotoxins either through direct interaction or by detection of host cell changes elicited by these effectors. Bacterial exotoxins activate the inflammasomes through diverse processes, including direct sensor cleavage, modulation of ion fluxes through plasma membrane pore formation, and perturbation of various host cell functions. In this review, we summarize the findings on some of the bacterial exotoxins that activate the inflammasomes.

Synthetic Oligodeoxynucleotides Containing Multiple Telemeric TTAGGG Motifs Suppress Inflammasome Activity in Macrophages Subjected to Oxygen and Glucose Deprivation and Reduce Ischemic Brain Injury in Stroke-Prone Spontaneously Hypertensive Rats

The immune system plays a fundamental role in both the development and pathobiology of stroke. Inflammasomes are multiprotein complexes that have come to be recognized as critical players in the inflammation that ultimately contributes to stroke severity. Inflammasomes recognize microbial and host-derived danger signals and activate caspase-1, which in turn controls the production of the pro-inflammatory cytokine IL-1β. We have shown that A151, a synthetic oligodeoxynucleotide containing multiple telemeric TTAGGG motifs, reduces IL-1β production by activated bone marrow derived macrophages that have been subjected to oxygen-glucose deprivation and LPS stimulation. Further, we demonstrate that A151 reduces the maturation of caspase-1 and IL-1β, the levels of both the iNOS and NLRP3 proteins, and the depolarization of mitochondrial membrane potential within such cells. In addition, we have demonstrated that A151 reduces ischemic brain damage and NLRP3 mRNA levels in SHR-SP rats that have undergone permanent middle cerebral artery occlusion. These findings clearly suggest that the modulation of inflammasome activity via A151 may contribute to a reduction in pro-inflammatory cytokine production by macrophages subjected to conditions that model brain ischemia and modulate ischemic brain damage in an animal model of stroke. Therefore, modulation of ischemic pathobiology by A151 may have a role in the development of novel stroke prevention and therapeutic strategies.

Reactive oxygen species and mitochondria: A nexus of cellular homeostasis

Reactive oxygen species (ROS) are integral components of multiple cellular pathways even though excessive or inappropriately localized ROS damage cells. ROS function as anti-microbial effector molecules and as signaling molecules that regulate such processes as NF-kB transcriptional activity, the production of DNA-based neutrophil extracellular traps (NETs), and autophagy. The main sources of cellular ROS are mitochondria and NADPH oxidases (NOXs). In contrast to NOX-generated ROS, ROS produced in the mitochondria (mtROS) were initially considered to be unwanted by-products of oxidative metabolism. Increasing evidence indicates that mtROS have been incorporated into signaling pathways including those regulating immune responses and autophagy. As metabolic hubs, mitochondria facilitate crosstalk between the metabolic state of the cell with these pathways. Mitochondria and ROS are thus a nexus of multiple pathways that determine the response of cells to disruptions in cellular homeostasis such as infection, sterile damage, and metabolic imbalance. In this review, we discuss the roles of mitochondria in the generation of ROS-derived anti-microbial effectors, the interplay of mitochondria and ROS with autophagy and the formation of DNA extracellular traps, and activation of the NLRP3 inflammasome by ROS and mitochondria.

Intracellular survival ofBurkholderia cepaciacomplex in phagocytic cells

Burkholderia cepacia complex (Bcc) species are a group of Gram-negative opportunistic pathogens that infect the airways of cystic fibrosis patients, and occasionally they infect other immunocompromised patients. Bcc bacteria display high-level multidrug resistance and chronically persist in the infected host while eliciting robust inflammatory responses. Studies using macrophages, neutrophils, and dendritic cells, combined with advances in the genetic manipulation of these bacteria, have increased our understanding of the molecular mechanisms of virulence in these pathogens and the molecular details of cell-host responses triggering inflammation. This article discusses our current view of the intracellular survival of Burkholderia cenocepacia within macrophages.

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

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

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

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

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Inflammasome derived exacerbated proinflammation causes SARS worsening.

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

A new pharmacological effect of levornidazole: Inhibition of NLRP3 inflammasome activation

Levornidazole, which was originally used to inhibit anaerobic and protozoal infections, is currently known to possess a novel pharmacological effect. In this study, we investigated the possible modulation by levornidazole of NOD-like receptor protein 3 (NLRP3) inflammasome-mediated IL-1β and IL-18 release from macrophages. The NLRP3 inflammasome could be activated by lipopolysaccharide (LPS) plus ATP or monosodium urate (MSU) in PMA-pretreated THP-1 macrophages. Surprisingly, an in vitro study showed that levornidazole suppressed IL-1β and IL-18 secretion by blocking the activation of the NLRP3 inflammasome. However, dextrornidazole barely suppressed the NLRP3 inflammasome. Levornidazole displays activity similar to that of dextrornidazole against clinical anaerobic bacteria, and they possess the same pharmacokinetic properties. Moreover, both of these compounds were unable to ameliorate T cell-mediated inflammation. Therefore, we used the widely applied NLRP3 inflammasome-related models of dextran sodium sulfate (DSS)-induced colitis and LPS-induced endotoxin shock to confirm the novel pharmacological effect of levornidazole in vivo. The in vivo studies verified the novel activity of levornidazole because the inhibition of NLRP3 inflammasome by levornidazole contributed to a better ameliorating effect than that of dextrornidazole in the in vivo models tested. Furthermore, this inhibitory effect of levornidazole was found to be at least partially achieved by decreasing the mitochondrial ROS generation without inhibiting NF-κB activation. In summary, these data describe a new pharmacological effect of levornidazole as an inhibitor of NLRP3 inflammasome activation.

Relevance of Viroporin Ion Channel Activity on Viral Replication and Pathogenesis

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