Pyrin- and CARD-only Proteins as Regulators of NLR Functions

Targeting NLRP3 Inflammasomes: A Trojan Horse Strategy for Intervention in Neurological Disorders

Recently, a growing focus has been on identifying critical mechanisms in neurological diseases that trigger a cascade of events, making it easier to target them effectively. One such mechanism is the inflammasome, an essential component of the immune response system that plays a crucial role in disease progression. The NLRP3 (nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3) inflammasome is a subcellular multiprotein complex that is widely expressed in the central nervous system (CNS) and can be activated by a variety of external and internal stimuli. When activated, the NLRP3 inflammasome triggers the production of proinflammatory cytokines interleukin-1β (IL-1β) and interleukin-18 (IL-18) and facilitates rapid cell death by assembling the inflammasome. These cytokines initiate inflammatory responses through various downstream signaling pathways, leading to damage to neurons. Therefore, the NLRP3 inflammasome is considered a significant contributor to the development of neuroinflammation. To counter the damage caused by NLRP3 inflammasome activation, researchers have investigated various interventions such as small molecules, antibodies, and cellular and gene therapy to regulate inflammasome activity. For instance, recent studies indicate that substances like micro-RNAs (e.g., miR-29c and mR-190) and drugs such as melatonin can reduce neuronal damage and suppress neuroinflammation through NLRP3. Furthermore, the transplantation of bone marrow mesenchymal stem cells resulted in a significant reduction in the levels of pyroptosis-related proteins NLRP3, caspase-1, IL-1β, and IL-18. However, it would benefit future research to have an in-depth review of the pharmacological and biological interventions targeting inflammasome activity. Therefore, our review of current evidence demonstrates that targeting NLRP3 inflammasomes could be a pivotal approach for intervention in neurological disorders.

Autophagy-enhancing ATG16L1 polymorphism is associated with improved clinical outcome and T-cell immunity in chronic HIV-1 infection

Chronic HIV-1 infection is characterized by T-cell dysregulation that is partly restored by antiretroviral therapy. Autophagy is a critical regulator of T-cell function. Here, we demonstrate a protective role for autophagy in HIV-1 disease pathogenesis. Targeted analysis of genetic variation in core autophagy gene ATG16L1 reveals the previously unidentified rs6861 polymorphism, which correlates functionally with enhanced autophagy and clinically with improved survival of untreated HIV-1-infected individuals. T-cells carrying ATG16L1 rs6861(TT) genotype display improved antiviral immunity, evidenced by increased proliferation, revamped immune responsiveness, and suppressed exhaustion/immunosenescence features. In-depth flow-cytometric and transcriptional profiling reveal T-helper-cell-signatures unique to rs6861(TT) individuals with enriched regulation of pro-inflammatory networks and skewing towards immunoregulatory phenotype. Therapeutic enhancement of autophagy recapitulates the rs6861(TT)-associated T-cell traits in non-carriers. These data underscore the in vivo relevance of autophagy for longer-lasting T-cell-mediated HIV-1 control, with implications towards development of host-directed antivirals targeting autophagy to restore immune function in chronic HIV-1 infection.

Transcriptomic analysis of large yellow croaker (Larimichthys crocea) reveals the suppression of the inflammatory response from Cryptocaryon irritans infection

Large yellow croaker (Larimichthys crocea) is the most productive marine fish in China. Cryptocaryon irritans is an extremely destructive parasite that causes great economic losses in large yellow croaker aquaculture industry. Therefore, it is very necessary to study the immune response of large yellow croaker in response to C. irritans infection. In this study, the transcriptomic profiles of large yellow croaker were sequenced and analyzed in the brain and head kidney at 72 h after C. irritans infection. Cytokines and chemokines related terms were significantly enriched based on the GO enrichment of down-regulated differentially expressed genes (DEGs) from the head kidney. Meanwhile, cytokine-cytokine receptor interaction was significantly enriched based on the KEGG enrichment of up-regulated DEGs from the brain and down-regulated DEGs from the head kidney, respectively. Moreover, the majority of inflammation-related DEGs were significantly up-regulated in the brain, but distinctly down-regulated in the head kidney. These results showed that the brain and head kidney might play different roles against C. irritans infection, and the inflammatory response of large yellow croaker may be restrained during C. irritans infection. Taken together, the transcriptomic analyses will be helpful to more comprehensively understand the immune mechanism of teleost against C. irritans infection, and provide a theoretical basis for the prevention and treatment of Cryptosporidiosis.

Bat ASC2 suppresses inflammasomes and ameliorates inflammatory diseases

Bats are special in their ability to live long and host many emerging viruses. Our previous studies showed that bats have altered inflammasomes, which are central players in aging and infection. However, the role of inflammasome signaling in combating inflammatory diseases remains poorly understood. Here, we report bat ASC2 as a potent negative regulator of inflammasomes. Bat ASC2 is highly expressed at both the mRNA and protein levels and is highly potent in inhibiting human and mouse inflammasomes. Transgenic expression of bat ASC2 in mice reduced the severity of peritonitis induced by gout crystals and ASC particles. Bat ASC2 also dampened inflammation induced by multiple viruses and reduced mortality of influenza A virus infection. Importantly, it also suppressed SARS-CoV-2-immune-complex-induced inflammasome activation. Four key residues were identified for the gain of function of bat ASC2. Our results demonstrate that bat ASC2 is an important negative regulator of inflammasomes with therapeutic potential in inflammatory diseases.

Caspase mechanisms in the regulation of inflammation

Regulated cell death is defined as genetically encoded pathways that lead towards the demise of cells. In mammals, cell demise can be either inflammatory or non-inflammatory, depending on whether the mechanism of death results in cell rupture or not. Inflammatory cell death can lead towards acute and chronic disease. Therefore, it becomes important to distinguish the mechanisms that result in these different inflammatory cell death outcomes. Apoptosis is a non-inflammatory form of cell death where cells resist rupture. In contrast, pyroptosis and necroptosis are inflammatory forms of cell death principally because of release of pro-inflammatory mediators from cells undergoing lysis. This review focusses on the mechanisms of these different cell death outcomes with specific emphasis on the caspase family of proteolytic enzymes.

Insights into inflammasome regulation: cellular, molecular, and pathogenic control of inflammasome activation

Maintenance of immune homeostasis is an intricate process wherein inflammasomes play a pivotal role by contributing to innate and adaptive immune responses. Inflammasomes are ensembles of adaptor proteins that can trigger a signal following innate sensing of pathogens or non-pathogens eventuating in the inductions of IL-1β and IL-18. These inflammatory cytokines substantially influence the antigen-presenting cell's costimulatory functions and T helper cell differentiation, contributing to adaptive immunity. As acute and chronic disease conditions may accompany parallel tissue damage, we analyze the critical role of extracellular factors such as cytokines, amyloids, cholesterol crystals, etc., intracellular metabolites, and signaling molecules regulating inflammasome activation/inhibition. We develop an operative framework for inflammasome function and regulation by host cell factors and pathogens. While inflammasomes influence the innate and adaptive immune components' interplay modulating the anti-pathogen adaptive immune response, pathogens may target inflammasome inhibition as a survival strategy. As trapped between health and diseases, inflammasomes serve as promising therapeutic targets and their modus operandi serves as a scientific rationale for devising better therapeutic strategies.

Activation and Regulation of NLRP3 by Sterile and Infectious Insults

Nod-Like Receptor (NLR) is the largest family of Pathogen Recognition Receptors (PRRs) that patrols the cytosolic environment. NLR engagement drives caspase-1 activation that cleaves pro-IL-1B which then gets secreted. Released IL-1B recruits immune cells to the site of infection/injury. Caspase-1 also cleaves Gasdermin-D (GSDM-D) that forms pores within the plasma membrane driving inflammatory cell death called pyroptosis. NLRP3 is the most extensively studied NLR. The NLRP3 gene is encoded by 9 exons, where exon 1 codes for pyrin domain, exon 3 codes for NACHT domain, and Leucine Rich Repeat (LRR) domain is coded by exon 4-9. Exon 2 codes for a highly disorganized loop that connects the rest of the protein to the pyrin domain and may be involved in NLRP3 regulation. The NLRP3 inflammasome is activated by many structurally divergent agonists of microbial, environmental, and host origin. Activated NLRP3 interacts with an adaptor protein, ASC, that bridges it to pro-Caspase-1 forming a multi-protein complex called inflammasome. Dysregulation of NLRP3 inflammasome activity is a hallmark of pathogenesis in several human diseases, indicating its highly significant clinical relevance. In this review, we summarize the existing knowledge about the mechanism of activation of NLRP3 and its regulation during activation by infectious and sterile triggers.

The Pathogenesis of COVID-19 Myocardial Injury: An Immunohistochemical Study of Postmortem Biopsies

Rationale

Myocardial injury associates significantly and independently with mortality in COVID-19 patients. However, the pathogenesis of myocardial injury in COVID-19 remains unclear, and cardiac involvement by SARS-CoV-2 presents a major challenge worldwide.

Objective

This histological and immunohistochemical study sought to clarify the pathogenesis and propose a mechanism with pathways involved in COVID-19 myocardial injury.

Methods and Results

Postmortem minimally invasive autopsies were performed in six patients who died from COVID-19, and the myocardium samples were compared to a control group (n=11). Histological analysis was performed using hematoxylin-eosin and toluidine blue staining. Immunohistochemical (IHC) staining was performed using monoclonal antibodies against targets: caspase-1, caspase-9, gasdermin-d, ICAM-1, IL-1β, IL-4, IL-6, CD163, TNF-α, TGF-β, MMP-9, type 1 and type 3 collagen. The samples were also assessed for apoptotic cells by TUNEL. Histological analysis showed severe pericardiocyte interstitial edema and higher mast cells counts per high-power field in all COVID-19 myocardium samples. The IHC analysis showed increased expression of caspase-1, ICAM-1, IL-1β, IL-6, MMP-9, TNF-α, and other markers in the hearts of COVID-19 patients. Expression of caspase-9 did not differ from the controls, while gasdermin-d expression was less. The TUNEL assay was positive in all the COVID-19 samples supporting endothelial apoptosis.

Conclusions

The pathogenesis of COVID-19 myocardial injury does not seem to relate to primary myocardiocyte involvement but to local inflammation with associated interstitial edema. We found heightened TGF-β and interstitial collagen expression in COVID-affected hearts, a potential harbinger of chronic myocardial fibrosis. These results suggest a need for continued clinical surveillance of patients for myocardial dysfunction and arrythmias after recovery from the acute phase of COVID-19.

NLRP3 Inflammasome: Checkpoint Connecting Innate and Adaptive Immunity in Autoimmune Diseases

Autoimmune diseases are a broad spectrum of human diseases that are characterized by the breakdown of immune tolerance and the production of autoantibodies. Recently, dysfunction of innate and adaptive immunity is considered to be a key step in the initiation and maintenance of autoimmune diseases. NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is a multimeric protein complex, which can detect exogenous pathogen irritants and endogenous danger signals. The main function of NLRP3 inflammasome is to promote secretion of interleukin (IL)-1β and IL-18, and pyroptosis mediated by caspase-1. Served as a checkpoint in innate and adaptive immunity, aberrant activation and regulation of NLRP3 inflammasome plays an important role in the pathogenesis of autoimmune diseases. This paper reviewed the roles of NLRP3 inflammasome in autoimmune diseases, which shows NLRP3 inflammasome may be a potential target for autoimmune diseases deserved further study.

The role of NLRP3 in lead-induced neuroinflammation and possible underlying mechanism

BACKGROUND

Neuroinflammation induced by lead exposure (Pb) is a major cause of neurotoxicity of Pb in the central nervous system (CNS). The NLR family, domain of pyrin containing 3 (NLRP3) involves in various neurological diseases, while the question of whether NLRP3 plays a role in lead-induced neuroinflammation has not yet been reported.

METHODS

Developmental and knockout (KO) NLRP3 mice were used to establish two in vivo models, and BV2 cells were used to establish an in vitro model. Behavioral and electrophysiologic tests were used to assess the neurotoxicity of Pb, and immunofluorescence staining was used to assess neuroinflammation. Real-time PCR and western blot were performed to examine the mRNA and protein levels of inflammatory cytokines and NLRP3 inflammasomes. siRNA technology was used to block NLRP3 expression.

RESULTS

Pb exposure led to neural injure and microglial activation in the hippocampus region, while minocycline intervention attenuated Pb-induced neurotoxicity by inhibiting neuroinflammation. Pb increased the expression of NLRP3 and promoted cleavage of caspase-1 in mRNA and protein levels, and minocycline partially reversed the effects of Pb on NLRP3 inflammasomes. Blocking of NLRP3 by KO mice or siRNA attenuated neural alterations induced by Pb, weakened microglial activation in vivo and in vitro as well, without affecting the accumulation of Pb. Pb increased autophagic protein levels and phosphorylation of NF-κB, while suppressing autophagy or NF-κB inhibited Pb's effects on NLRP3.

CONCLUSIONS

NLRP3 is involved in the regulation of Pb-induced neurotoxicity. These findings expand mechanism research of Pb neurotoxicity and may help establish new prevention strategies for Pb neurotoxicity.

Association of NOD1, NOD2, PYDC1 and PYDC2 genes with Behcet's disease susceptibility and clinical manifestations

Purpose: Behçet's disease (BD) is an autoinflammatory disease with clinical manifestations such as mucocutaneous, ocular, vascular, gastrointestinal, musculoskeletal and central nervous system involvement. Features of innate and adaptive immunity and inflammasome pathways have been claimed in the pathogenesis of BD. We aimed to investigate the roles of NOD1, NOD2, PYDC1 and PYDC2 genes in the genetic predisposition of BD.Materials and Methods: Genetic variations of NOD1 (rs2075820 and rs2075818) and NOD2 (R334Q and R334W) genes were explored in 68 BD patients and 70 controls with PCR-RFLP (polymerase chain reaction-restriction fragment length polymorphism) approach. PYDC1 and PYDC2 gene variants were investigated by Sanger sequencing.Results: The polymorphism of rs2075820 (NOD1 G/A) had a statistically significant difference between the BD and controls, AA genotype was 2.460-fold protective. When compared in terms of cardiovascular involvement in BD patients, AA genotype was increased the risk of cardiovascular involvement 4.286-fold. There was a significant difference between BD and controls in rs2075818 (NOD1 G/C) polymorphism and CC genotype increased the risk of BD by 3.780-fold. In terms of rs2075818 variants, there was a statistically significant difference between BD patients with ocular lesions, joints, cardiovascular and gastrointestinal involvement and controls. There was a significant difference between the patients with joint involvement and controls and the risk increased of 3.310-fold.Conclusion: The data shed new light on the association between polymorphisms of NOD1 gene and BD and clinicial manifestations. However, NOD2, PYDC1 and PYDC2 genes were not associated with BD in the Turkish population.

Inflammasomes in Alveolar Bone Loss

Bone remodeling is tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Fine tuning of the osteoclast-osteoblast balance results in strict synchronization of bone resorption and formation, which maintains structural integrity and bone tissue homeostasis; in contrast, dysregulated bone remodeling may cause pathological osteolysis, in which inflammation plays a vital role in promoting bone destruction. The alveolar bone presents high turnover rate, complex associations with the tooth and periodontium, and susceptibility to oral pathogenic insults and mechanical stress, which enhance its complexity in host defense and bone remodeling. Alveolar bone loss is also involved in systemic bone destruction and is affected by medication or systemic pathological factors. Therefore, it is essential to investigate the osteoimmunological mechanisms involved in the dysregulation of alveolar bone remodeling. The inflammasome is a supramolecular protein complex assembled in response to pattern recognition receptors and damage-associated molecular patterns, leading to the maturation and secretion of pro-inflammatory cytokines and activation of inflammatory responses. Pyroptosis downstream of inflammasome activation also facilitates the clearance of intracellular pathogens and irritants. However, inadequate or excessive activity of the inflammasome may allow for persistent infection and infection spreading or uncontrolled destruction of the alveolar bone, as commonly observed in periodontitis, periapical periodontitis, peri-implantitis, orthodontic tooth movement, medication-related osteonecrosis of the jaw, nonsterile or sterile osteomyelitis of the jaw, and osteoporosis. In this review, we present a framework for understanding the role and mechanism of canonical and noncanonical inflammasomes in the pathogenesis and development of etiologically diverse diseases associated with alveolar bone loss. Inappropriate inflammasome activation may drive alveolar osteolysis by regulating cellular players, including osteoclasts, osteoblasts, osteocytes, periodontal ligament cells, macrophages, monocytes, neutrophils, and adaptive immune cells, such as T helper 17 cells, causing increased osteoclast activity, decreased osteoblast activity, and enhanced periodontium inflammation by creating a pro-inflammatory milieu in a context- and cell type-dependent manner. We also discuss promising therapeutic strategies targeting inappropriate inflammasome activity in the treatment of alveolar bone loss. Novel strategies for inhibiting inflammasome signaling may facilitate the development of versatile drugs that carefully balance the beneficial contributions of inflammasomes to host defense.

Roles of Inflammasomes in Epstein-Barr Virus-Associated Nasopharyngeal Cancer

Epstein-Barr virus (EBV) infection is recognised as one of the causative agents in most nasopharyngeal carcinoma (NPC) cases. Expression of EBV viral antigens can induce host's antiviral immune response by activating the inflammasomes to produce pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18. These cytokines are known to be detrimental to a wide range of virus-infected cells, in which they can activate an inflammatory cell death program, called pyroptosis. However, aberrant inflammasome activation and production of its downstream cytokines lead to chronic inflammation that may contribute to various diseases, including NPC. In this review, we summarise the roles of inflammasomes during viral infection, how EBV evades inflammasome-mediated immune response, and progress into tumourigenesis. The contrasting roles of inflammasomes in cancer, as well as the current therapeutic approaches used in targeting inflammasomes, are also discussed in this review. While the inflammasomes appear to have dual roles in carcinogenesis, there are still many questions that remain unanswered. In particular, the exact molecular mechanism responsible for the regulation of the inflammasomes during carcinogenesis of EBV-associated NPC has not been explored thoroughly. Furthermore, the current practical application of inflammasome inhibitors is limited to specific tumour types, hence, further studies are warranted to discover the potential of targeting the inflammasomes for the treatment of NPC.

In vitro complex formation of human PYRIN domain-only protein 3 prevented by self-oligomerization of ASC PYD domain

The formation of inflammasome complexes contributes inactivation of inflammatory caspases viz caspase 1, which is generally considered essential for the innate response. Three proteins constituted this inflammasome complex, such as Nod-like receptors (NLRP or AIM2), ASC possessing caspase-recruiting domain, and caspase-1. The ASC proteins comprise two domains, the N-terminal PYD domain responsible for the interaction of various proteins, including PYD only protein 3 (POP3), and the CARD domain for association with other proteins. The PYRIN Domain-Only Protein POP3 negatively regulates responses to DNA virus infection by preventing the ALR inflammasome formation. POP3 directly interacts with ASC, therefore inhibiting ASC recruitment to AIM2-like receptors (ALRs). In the current study, we designed various constructs of the PYRIN Domain-Only Protein 3 (POP3) and ASC PYD domain to find the best-overexpressed construct for biochemical characterization as well as our complex studies. We cloned, purified, and characterized the PYD domain of pyrin only protein 3 and ASC PYD domain under physiological conditions. Our in vitro study clearly shows that the ASC PYD domain of corresponding amino acid 1-96 aa with ease self-oligomerization in physiological buffer conditions, and complex formation of POP3 PYD (1-83 aa) was inhibited by ASC PYD domain. Besides, we purified the PYD of POP3 protein in low and high salt conditions and different pH values for their biochemical characterization. Our results showed that POP3 formed a dimer under normal physiological conditions and was stable under normal buffer conditions; however, the purification in extremely low pH (pH5.0) conditions shows unstable behavior, the high salt conditions (500 mM NaCl) influence the protein aggregation. SDS PAGE arbitrated the homogeneity of the PYD domain of pyrin only protein 3 and ASC PYD domain of corresponding amino acids 1-83 and 1-96, respectively. Furthermore, our native PAGE shows the PYD domain of pyrin; only protein 3 did not form a complex with ASC PYD domain because of oligomerization mediated by the PYD domain.

The Dynamic Immune Response of Yellow Catfish (Pelteobagrus fulvidraco) Infected With Edwardsiella ictaluri Presenting the Inflammation Process

Edwardsiella ictaluri is a highly destructive pathogen in cultured yellow catfish, thus it was very necessary to study the immune response of yellow catfish against bacterial infection. In this study, RNA-Seq technology was used to study the immune response in two distinct tissues of yellow catfish at eight different time points (h) after E. ictaluri infection. The number of differentially expressed genes (DEGs) in the spleen and liver was low at 3 h and 6 h post-infection, respectively. Afterwards, the most number of DEGs in the spleen was detected at 72 h, while the number of DEGs in the liver maintained a high level from 24 h to 120 h. The GO and KEGG enrichment analyses of DEGs at different time points uncovered that cytokines were continuously transcribed at 6 h to 120 h; whereas the liver is the main organ that secretes the components of the complement system, and metabolic regulation was activated from 12 h to 120 h. Moreover, an overview of the inflammation response of yellow catfish was exhibited including pattern-recognition receptors, inflammatory cytokines, chemokines, complements, and inflammation-related signal pathways. The similar expression tendency of nine genes by qRT-PCR validated the accuracy of transcriptome analyses. The different transcriptomic profiles obtained from the spleen and liver will help to better understand the dynamic immune response of fish against bacterial infection, and will provide basic information for establishing effective measures to prevent and control diseases in fish.

Involvement of Oxidative Stress and the Innate Immune System in SARS-CoV-2 Infection

The emergence of the novel coronavirus in December 2019 in China marked the beginning of a pandemic that impacted healthcare systems and economic life all over the world. The virus primarily targets the respiratory system causing severe acute respiratory syndrome (SARS) in some patients, and therefore received the name of SARS-CoV-2. The pathogen stands out among other coronaviruses by its rapid transmission from human to human, with the majority of infected individuals being asymptomatic or presenting with only minor illness, therefore facilitating the pathogen spread. At the same time, people from the risk groups, such as the elderly, patients suffering from chronic diseases, or obese individuals, have increased chances of developing a severe or even fatal disease. The search for risk factors explaining this phenomenon continues. In this review, we focus on the known mechanisms of SARS-CoV-2 infection affecting the functioning of the immune system and discuss potential risk factors responsible for the severe disease course. Oxidative stress is one of such factors, which plays a prominent role in innate immunity activity, and recent research has revealed its tight involvement in SARS-CoV-2 infection. We discuss these recent findings and the development of excessive inflammation and cytokine storm observed during SARS-CoV-2 infection. Finally, we consider potential use of antioxidant drugs for alleviating the severe symptoms in affected patients.

Structure, Activation and Regulation of NLRP3 and AIM2 Inflammasomes

The inflammasome is a three-component (sensor, adaptor, and effector) filamentous signaling platform that shields from multiple pathogenic infections by stimulating the proteolytical maturation of proinflammatory cytokines and pyroptotic cell death. The signaling process initiates with the detection of endogenous and/or external danger signals by specific sensors, followed by the nucleation and polymerization from sensor to downstream adaptor and then to the effector, caspase-1. Aberrant activation of inflammasomes promotes autoinflammatory diseases, cancer, neurodegeneration, and cardiometabolic disorders. Therefore, an equitable level of regulation is required to maintain the equilibrium between inflammasome activation and inhibition. Recent advancement in the structural and mechanistic understanding of inflammasome assembly potentiates the emergence of novel therapeutics against inflammasome-regulated diseases. In this review, we have comprehensively discussed the recent and updated insights into the structure of inflammasome components, their activation, interaction, mechanism of regulation, and finally, the formation of densely packed filamentous inflammasome complex that exists as micron-sized punctum in the cells and mediates the immune responses.

Inflammasome dysregulation in human gingival fibroblasts in response to periodontal pathogens

OBJECTIVE

Uncontrolled production of Interleukin-1β (IL-1β), a major proinflammatory cytokine, is associated with tissue destruction in periodontal disease. IL-1β production is controlled by inflammasomes which are multiprotein regulatory complexes. The current study aimed to elucidate potential regulatory pathways by monitoring the effects of periodontal pathogens Fusobacterium nucleatum (Fn) and Porphyromonas gingivalis (Pg) on inflammasomes and their regulators in human gingival fibroblasts (HGFs) in vitro.

METHODS

HGFs were exposed to Fn and Pg alone or in combination for 24 hr at a multiplicity of infection of 100, ±30 min exposure with 5 mM adenosine triphosphate (ATP) incubation. Gene expression of NLRP3 and AIM2, inflammasome regulatory proteins POP1, CARD16 and TRIM16, and inflammasome components ASC and CASPASE 1, and IL-1β, were evaluated by RT-PCR. Pro- and mature IL-1β levels were monitored intracellularly by immunocytochemistry and extracellularly by ELISA.

RESULTS

Fn + ATP significantly upregulated NLRP3, AIM2, IL-1β, ASC, and CASPASE 1; however, it downregulated POP1 and TRIM16. Pg + ATP downregulated NLRP3, ASC, POP1, but upregulated IL-1β and CARD16. Pg + Fn+ATP significantly upregulated AIM2, IL-1β and CARD16, and downregulated POP1, TRIM16, and CASPASE 1. Pg + ATP exposure significantly increased pro- and mature IL-1β production.

CONCLUSION

Bacterial exposure with ATP may deregulate IL-1β by dysregulating inflammasomes and their regulators in HGFs.

Inflammasomes: a preclinical assessment of targeting in atherosclerosis

INTRODUCTION

Inflammasomes are central to atherosclerotic vascular dysfunction with regulatory effects on inflammation, immune modulation, and lipid metabolism. The NLRP3 inflammasome is a critical catalyst for atherogenesis thus highlighting its importance in understanding the pathophysiology of atherosclerosis and for the identification of novel therapeutic targets and biomarkers for the treatment of cardiovascular disease.

AREAS COVERED

This review includes an overview of macrophage lipid metabolism and the role of NLRP3 inflammasome activity in cardiovascular inflammation and atherosclerosis. We highlight key activators, signal transducers and major regulatory components that are being considered as putative therapeutic targets for inhibition of NLRP3-mediated cardiovascular inflammation and atherosclerosis.

EXPERT OPINION

NLRP3 inflammasome activity lies at the nexus between inflammation and cholesterol metabolism; it offers unique opportunities for understanding atherosclerotic pathophysiology and identifying novel modes of treatment. As such, a host of NLRP3 signaling cascade components have been identified as putative targets for drug development. We catalog these current discoveries in therapeutic targeting of the NLRP3 inflammasome and, utilizing the CANTOS trial as the translational (bench-to-bedside) archetype, we examine the complexities, challenges, and ultimate goals facing the field of atherosclerosis research.

Effects of Porphyromonas gingivalis and Fusobacterium nucleatum on inflammasomes and their regulators in H400 cells

INTRODUCTION

Inflammasomes are multiprotein complexes that regulate immune processes in response to infections and tissue damage. They modulate Interleukin-1beta (IL-1β) expression, a major proinflammatory cytokine. The inflammasome/IL-1β pathway is involved in head and neck squamous cell carcinoma (HNSCC) progression and the periodontal pathogens Fusobacterium nucleatum (Fn) and Porphyromonas gingivalis (Pg) have been reported to cause chronic inflammation in HNSCC. The aim of this study was to characterise the role of these pathogens in regulating inflammasome activity and the IL-1β response in HNSCC in vitro.

METHODS

An HNSCC cell line (H400) was exposed to Fn and Pg individually or in combination for 24h, ± incubation for 30 min with 5 mM adenosine triphosphate (ATP). Transcript levels of inflammasomes, NLRP3 and AIM2; inflammasome-regulatory proteins, POP1, CARD16 and TRIM16; and inflammasome-component, ASC and caspase 1 and IL-1β, were assayed by RT-PCR. Expression of IL-1β was by immunocytochemistry and ELISA.

RESULTS

NLRP3 expression was significantly upregulated in response to Pg, Fn + Pg, Pg + ATP and Fn + Pg + ATP. AIM2 was significantly upregulated by Fn, Pg and Fn + Pg + ATP exposure. All conditions significantly upregulated IL-1β gene expression. POP1 expression was significantly downregulated by Pg or Fn exposure but not by Fn + Pg. Intracellular pro- and mature IL-1β were significantly higher following Fn and Pg + ATP exposure.

CONCLUSION

Pg alone increased IL-1β by upregulating AIM2, NLRP3 and downregulating POP1. Fn promoted IL-1β by increasing AIM2 and downregulating POP1. Pg + ATP with or without Fn upregulated NLRP3, IL-1β by downregulating POP1. Periodontal pathogens may contribute to HNSCC pathogenesis by increasing the IL-1β response due to inflammasome dysregulation.

A Membrane Potential- and Calpain-Dependent Reversal of Caspase-1 Inhibition Regulates Canonical NLRP3 Inflammasome

The NLRP3 inflammasome senses a range of cellular disturbances, although no consensus exists regarding a common mechanism. Canonical NLRP3 activation is blocked by high extracellular K⁺, regardless of the activating signal. We report here that canonical NLRP3 activation leads to Ca²⁺ flux and increased calpain activity. Activated calpain releases a pool of Caspase-1 sequestered by the cytoskeleton to regulate NLRP3 activation. Using electrophysiological recording, we found that resting-state eukaryotic membrane potential (MP) is required for this calpain activity, and depolarization by high extracellular K⁺ or artificial hyperpolarization results in the inhibition of calpain. Therefore, the MP/Ca²⁺/calpain/ Caspase-1 axis acts as an independent regulatory mechanism for NLRP3 activity. This finding provides mechanistic insight into high K⁺-mediated inhibition of NLRP3 activation, and it offers an alternative model of NLRP3 inflammasome activation that does not involve K⁺ efflux.

Zhang et al. find that, in canonical NLRP inflammasome activation, calpain activity is essential for releasing caspase-1 from flightless-1 and the cytoskeleton. Membrane depolarization, such as under high extracellular K⁺ or hyperpolarization, impairs this activity. This work provides insight into extracellular K⁺ -mediated inhibition of the NLRP3 inflammasome.

Inflammasomes and their regulation in periodontal disease: A review

Interleukin-1β (IL-1β), which is secreted by host tissues leading to periodontal tissue inflammation, is a major pro-inflammatory cytokine in the pathogenesis of periodontal disease. The conversion of pro-IL-1β into its biologically active form is controlled by multiprotein complexes named as inflammasomes, which are key regulator of host defense mechanisms and inflammasome involved diseases, including the periodontal diseases. Inflammasomes are regulated by different proteins and processes, including pyrin domain (PYD)-only proteins (POPs), CARD-only proteins (COPs), tripartite motif family proteins (TRIMs), autophagy, and interferons. A review of in vitro, in vivo, and clinical data from these publications revealed that several inflammasomes including (NOD)-like receptor (NLR) pyrin domain-containing 3 (NLRP3) and absent in melanoma 2 (AIM2) have been found to be involved in periodontal disease pathogenesis. To the best of our knowledge, the current article provides the first review of the literature focusing on studies that evaluated both inflammasomes and their regulators in periodontal disease. An upregulation for inflammasomes and a downregulation of inflammasome regulator proteins including POPs, COPs, and TRIMs have been reported in periodontal disease. Although interferons (types I and II) and autophagy have been found to be involved in periodontal disease, their possible role in inflammasome activation has not evaluated yet. Modulating the excessive inflammatory response by the use of inflammasome regulators may have potential in the management of periodontal disease.

Differential expression of inflammasome regulatory transcripts in periodontal disease

BACKGROUND

The inflammasome modulates the release of key proinflammatory cytokines associated with periodontal disease pathogenesis. The aim of this study was to evaluate the expression of proteins that regulate the inflammasome, namely pyrin domain-only proteins (POPs), caspase activation recruitment domain (CARD)-only proteins, and tripartite motif-containing (TRIM) proteins, in periodontal diseases.

METHODS

A total of 68 participants (34 males and 34 females) were divided into four groups, including periodontal health (H), gingivitis (G), chronic periodontitis (CP), and aggressive periodontitis (AgP) based on clinical parameters. Gingival tissue samples were obtained from all participants for reverse transcription polymerase chain reaction (RT-PCR)-based gene expression analyses of molecules that regulate the inflammasome, including apoptosis-associated speck-like protein (ASC) containing CARD, caspase-1, interleukin-1β (IL-1β), interleukin-18 (IL-18), nucleotide-binding domain, leucine rich family (NLR) pyrin domain containing 3 (NLRP3), NLR family pyrin domain containing 2 (NLRP2), AIM2 (absent in melanoma 2), POP1, POP2, CARD16, CARD18, TRIM16, and TRIM20 by RT-PCR.

RESULTS

NLRP3 and IL-1β were upregulated in the G, CP, and AgP groups compared with group H (P < 0.05). AIM2 was downregulated in the CP group compared with the H, G, and AgP groups (P < 0.05). TRIM20, TRIM16, and CARD18 were downregulated in the G, CP, and AgP groups compared with the H group (P < 0.05). POP1 and POP2 were downregulated in the CP and AgP, and AgP and G groups, respectively (P < 0.05).

CONCLUSION

Active periodontal disease may result in downregulation of inflammasome regulators that may increase the activity of NLRP3 and IL-1β in periodontal disease.

Help wanted: helper NLRs and plant immune responses

Plant nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins function as intracellular receptors in response to pathogens and activate effector-triggered immune responses (ETI). The activation of some sensor NLRs (sNLR) by their corresponding pathogen effector is well studied. However, the mechanisms by which the recently defined helper NLRs (hNLR) function to transduce sNLR activation into ETI-associated cell death and disease resistance remains poorly understood. We briefly summarize recent examples of sNLR activation and we then focus on hNLR requirements in sNLR-initiated immune responses. We further discuss how shared sequence homology with fungal self-incompatibility proteins and the mammalian mixed lineage kinase domain like pseudokinase (MLKL) proteins informs a plausible model for the structure and function of an ancient clade of plant hNLRs, called RNLs.

The Canonical Inflammasome: A Macromolecular Complex Driving Inflammation

The inflammasome is a multi-molecular platform crucial to the induction of an inflammatory response to cellular danger. Recognition in the cytoplasm of endogenously and exogenously derived ligands initiates conformational change in sensor proteins, such as NLRP3, that permits the subsequent rapid recruitment of adaptor proteins, like ASC, and the resulting assembly of a large-scale inflammatory signalling platform. The assembly process is driven by sensor-sensor interactions as well as sensor-adaptor and adaptor-adaptor interactions. The resulting complex, which can reach diameters of around 1 micron, has a variable composition and stoichiometry. The inflammasome complex functions as a platform for the proximity induced activation of effector caspases, such as caspase-1 and caspase-8. This ultimately leads to the processing of the inflammatory cytokines pro-IL1β and pro-IL18 into their active forms, along with the cleavage of Gasdermin D, a key activator of cell death via pyroptosis.

The signaling adaptor BCAP inhibits NLRP3 and NLRC4 inflammasome activation in macrophages through interactions with Flightless-1

B cell adaptor for phosphoinositide 3-kinase (PI3K) (BCAP) is a signaling adaptor that activates the PI3K pathway downstream of B cell receptor signaling in B cells and Toll-like receptor (TLR) signaling in macrophages. BCAP binds to the regulatory p85 subunit of class I PI3K and is a large, multidomain protein. We used proteomic analysis to identify other BCAP-interacting proteins in macrophages and found that BCAP specifically associated with the caspase-1 pseudosubstrate inhibitor Flightless-1 and its binding partner leucine-rich repeat flightless-interacting protein 2. Because these proteins inhibit the NLRP3 inflammasome, we investigated the role of BCAP in inflammasome function. Independent of its effects on TLR priming, BCAP inhibited NLRP3- and NLRC4-induced caspase-1 activation, cell death, and IL-1β release from macrophages. Accordingly, caspase-1-dependent clearance of a Yersinia pseudotuberculosis mutant was enhanced in BCAP-deficient mice. Mechanistically, BCAP delayed the recruitment and activation of pro-caspase-1 within the NLRP3/ASC preinflammasome through its association with Flightless-1. Thus, BCAP is a multifunctional signaling adaptor that inhibits key pathogen-sensing pathways in macrophages.

SERPINB1-mediated checkpoint of inflammatory caspase activation

Inflammatory caspases (caspase-1, caspase-4, caspase-5 and caspase-11 (caspase-1/-4/-5/-11)) mediate host defense against microbial infections, processing pro-inflammatory cytokines and triggering pyroptosis. However, precise checkpoints are required to prevent their unsolicited activation. Here we report that serpin family B member 1 (SERPINB1) limited the activity of those caspases by suppressing their caspase-recruitment domain (CARD) oligomerization and enzymatic activation. While the reactive center loop of SERPINB1 inhibits neutrophil serine proteases, its carboxy-terminal CARD-binding motif restrained the activation of pro-caspase-1/-4/-5/-11. Consequently, knockdown or deletion of SERPINB1 prompted spontaneous activation of caspase-1/-4/-5/-11, release of the cytokine IL-1β and pyroptosis, inducing elevated inflammation after non-hygienic co-housing with pet-store mice and enhanced sensitivity to lipopolysaccharide- or Acinetobacter baumannii-induced endotoxemia. Our results reveal that SERPINB1 acts as a vital gatekeeper of inflammation by restraining neutrophil serine proteases and inflammatory caspases in a genetically and functionally separable manner.

Caspase recruitment domains for protein interactions in cellular signaling (Review)

The caspase recruitment domain (CARD), a well-known protein interaction module, belongs to the death domain (DD) superfamily, which includes DDs, death effector domains, and pyrin domains. The DD superfamily mediates the protein interactions necessary for apoptosis and immune cell signaling pathways. Among these domains, the CARD has been studied extensively as it mediates important cellular signaling events that are associated with various human diseases including cancer, neuro-degenerative diseases and immune disorders. Homo-type and hetero-type CARD-CARD interactions mediate the formation of large signaling complexes, including caspase-activating complexes and downstream signaling complexes. The present review summarizes and discusses the results of structural studies of various CARDs and their complexes. These studies shed light on the mechanisms that control the assembly and disassembly of signaling complexes and provide an improved understanding of cellular signaling processes.

Epigenetic hypomethylation and upregulation of NLRC4 and NLRP12 in Kawasaki disease

INTRODUCTION

Kawasaki disease (KD) is a type of childhood febrile systemic vasculitis. Inflammasomes control inflammatory signaling and are related with the development of KD. In this study, we performed a survey of transcripts and global DNA methylation levels of inflammasome sensors of NOD-like receptors (NLRs) and the downstream interleukin 1β (IL-1β).

MATERIALS AND METHODS

In this study, for the chip studies, we recruited a total of 18 KD patients, who we analyzed before receiving intravenous immunoglobulin (IVIG) and at least 3 weeks after IVIG treatment, as well as 36 non-fever controls by Illumina HumanMethylation 450 BeadChip and Affymetrix GeneChip® Human Transcriptome Array 2.0. A separate group of 78 subjects was performed for real-time quantitative PCR validations.

RESULTS

The expressions of mRNA levels of NLRC4, NLRP12, and IL-1β were significantly upregulated in KD patients compared to the controls (p<0.05). Once KD patients underwent IVIG treatment, these genes considerably decreased. In particular, the methylation status of the CpG sites of these genes indicated a significant opposite tendency between the KD patients and the controls. Furthermore, mRNA levels of IL-1β represented a positive correlation with NLRC4 (p=0.002). We also observed that the mRNA levels of NLRP12 were lower in KD patients who developed coronary arterial lesions (p<0.005).

CONCLUSION

This study is among the first to report epigenetic hypomethylation, increased transcripts, and the upregulation of NLRC4, NLRP12 and IL-1β in KD patients. Moreover, a decreased upregulation of NLRP12 was related to coronary arterial lesion formation in KD patients.

NLR-Dependent Regulation of Inflammation in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS) associated with inappropriate activation of lymphocytes, hyperinflammatory responses, demyelination, and neuronal damage. In the past decade, a number of biological immunomodulators have been developed that suppress the peripheral immune responses and slow down the progression of the disease. However, once the inflammation of the CNS has commenced, it can cause serious permanent neuronal damage. Therefore, there is a need for developing novel therapeutic approaches that control and regulate inflammatory responses within the CNS. Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are intracellular regulators of inflammation expressed by many cell types within the CNS. They redirect multiple signaling pathways initiated by pathogens and molecules released by injured tissues. NLR family members include positive regulators of inflammation, such as NLRP3 and NLRC4 and anti-inflammatory NLRs, such as NLRX1 and NLRP12. They exert immunomodulatory effect at the level of peripheral immune responses, including antigen recognition and lymphocyte activation and differentiation. Also, NLRs regulate tissue inflammatory responses. Understanding the molecular mechanisms that are placed at the crossroad of innate and adaptive immune responses, such as NLR-dependent pathways, could lead to the discovery of new therapeutic targets. In this review, we provide a summary of the role of NLRs in the pathogenesis of MS. We also summarize how anti-inflammatory NLRs regulate the immune response within the CNS. Finally, we speculate the therapeutic potential of targeting NLRs in MS.

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.

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.

AIM2 inflammasome activation and regulation: A structural perspective

Absent in melanoma 2 (AIM2) inflammasome is a multi-protein platform that recognizes aberrant cytoplasmic dsDNA and induces cytokine maturation, release and pyroptosis. It is composed of AIM2, apoptosis-associated speck-like protein containing a CARD (ASC), and caspase-1. Recent X-ray crystallographic and high resolution cryo-electron microscopic (cryo-EM) studies have revealed a series of structures in AIM2 inflammasome activation and regulation. One prominent feature common in multiple steps is the assembly of high-order structures, especially helical filaments nucleated by upstream molecules, rather than stoichiometric complexes. In this review, we track the AIM2 inflammasome activation process step by step, using high-resolution structures to illustrate the overall architecture of AIM2 inflammasome and its assembly and regulatory mechanisms.

The Role of Interferons in Inflammation and Inflammasome Activation

Inflammation is an essential physiological process, which enables survival during infection and maintains tissue homeostasis. Interferons (IFNs) and pro- and anti-inflammatory cytokines are crucial for appropriate response to pathogens, damaged cells, or irritants in inflammatory response. The inflammasom is multiprotein complex, which initiates cleavage of pro-inflammatory cytokines IL-1β and IL-18 into active forms. In addition, inflammasomes initiate pyroptotic cell death. In the present review, I summarize and analyze recent findings regarding the cross talk of IFNs and inflammasomes.

Pyrin-only protein 2 limits inflammation but improves protection against bacteria

Pyrin domain-only proteins (POPs) are recently evolved, primate-specific proteins demonstrated in vitro as negative regulators of inflammatory responses. However, their in vivo function is not understood. Of the four known POPs, only POP2 is reported to regulate NF-κB-dependent transcription and multiple inflammasomes. Here we use a transgenic mouse-expressing POP2 controlled by its endogenous human promotor to study the immunological functions of POP2. Despite having significantly reduced inflammatory cytokine responses to LPS and bacterial infection, POP2 transgenic mice are more resistant to bacterial infection than wild-type mice. In a pulmonary tularaemia model, POP2 enhances IFN-γ production, modulates neutrophil numbers, improves macrophage functions, increases bacterial control and diminishes lung pathology. Thus, unlike other POPs thought to diminish innate protection, POP2 reduces detrimental inflammation while preserving and enhancing protective immunity. Our findings suggest that POP2 acts as a high-order regulator balancing cellular function and inflammation with broad implications for inflammation-associated diseases and therapeutic intervention.

Pyrin-only proteins (POPs) are primate-specific negative regulators of inflammasome activation. Here the authors generate transgenic mice expressing POP2 under the control of the human promoter, and show that POP2 is important for balancing antibacterial inflammatory responses in vivo.

Assembly and regulation of ASC specks

The inflammasome adapter ASC links activated inflammasome sensors to the effector molecule pro-caspase-1. Recruitment of pro-caspase-1 to ASC promotes the autocatalytic activation of caspase-1, which leads to the release of pro-inflammatory cytokines, such as IL-1β. Upon triggering of inflammasome sensors, ASC assembles into large helical fibrils that interact with each other serving as a supramolecular signaling platform termed the ASC speck. Alternative splicing, post-translational modifications of ASC, as well as interaction with other proteins can perturb ASC function. In several inflammatory diseases, ASC specks can be found in the extracellular space and its presence correlates with poor prognosis. Here, we review the role of ASC in inflammation, and focus on the structural mechanisms that lead to ASC speck formation, the regulation of ASC function during inflammasome assembly, and the importance of ASC specks in disease.

The NLRP3 and Pyrin Inflammasomes: Implications in the Pathophysiology of Autoinflammatory Diseases

Inflammasomes are multiprotein complexes that critically control different aspects of innate and adaptive immunity. Among them we could highlight the release of pro-inflammatory cytokines that induce and maintain the inflammatory response. Usually, inflammasomes result from oligomerization of a nucleotide-binding domain-like receptor (NLR) after sensing different pathogenic or endogenous sterile dangerous signals; however, other proteins such as absent in melanoma 2, retinoic acid-inducible gene I, or pyrin could also form inflammasome platforms. Inflammasome oligomerization leads to caspase-1 activation and the processing and release of the pro-inflammatory cytokines, such as interleukin (IL)-1β and IL-18. Mutations in different inflammasomes are causative for multiple periodic hereditary syndromes or autoinflammatory diseases, characterized by acute systemic inflammatory flares not associated with infections, tumors, or autoimmunity. This review focuses on germline mutations that have been described in cryopyrin-associated periodic syndrome (CAPS) for NLRP3 or in familial Mediterranean fever (FMF) and pyrin-associated autoinflammation with neutrophilic dermatosis (PAAND) for MEFV. Besides the implication of inflammasomes in autoinflammatory syndromes, these molecular platforms are involved in the pathophysiology of different illnesses, including chronic inflammatory diseases, degenerative processes, fibrosis, or metabolic diseases. Therefore, drug development targeting inflammasome activation is a promising field in expansion.

The Biophysical Characterisation and SAXS Analysis of Human NLRP1 Uncover a New Level of Complexity of NLR Proteins

NOD-like receptors represent an important class of germline-encoded pattern recognition receptors that play key roles in the regulation of inflammatory signalling pathways. They function as danger sensors and initiate inflammatory responses and the production of cytokines. Since NLR malfunction results in chronic inflammation and auto-immune diseases, there is a great interest in understanding how they work on a molecular level. To date, a lot of insight into the biological functions of NLRs is available but biophysical and structural studies have been hampered by the difficulty to produce soluble and stable recombinant NLR proteins. NLRP1 is an inflammasome forming NLR that is believed to be activated by binding to MDP and induces activation of caspase 1. Here, we report the identification of a soluble fragment of NLRP1 that contains the NACHT oligomerization domain and the putative MDP-sensing LRR domain. We describe the biophysical and biochemical characterization of this construct and a SEC-SAXS analysis that allowed the calculation of a low resolution molecular envelope. Our data indicate that the protein is constitutively bound to ATP with a negligible ability to hydrolyse the triphosphate nucleotide and that it adopts a monomeric extended conformation that is reminiscent of the structure adopted by NLRC4 in the inflammasome complex. Furthermore, we show that the presence of MDP is not sufficient to promote self-oligomerization of the NACHT-LRR fragment suggesting that MDP may either bind to regions outside the NACHT-LRR module or that it may not be the natural ligand of NLRP1. Taken together, our data suggest that the NLRP1 mechanism of action differs from that recently reported for other NLRs.

[Interleukin-1, inflammasome and autoinflammatory diseases]

Interleukin-1 is a major cytokine of innate immunity and inflammation. It exerts various systemic effects during the inflammatory response, such as fever induction, thrombopoiesis and granulopoiesis, or leukocyte recruitment. Its involvement has been demonstrated in many inflammatory-mediated diseases, such as diabetes or gout. Moreover, interleukin-1 plays a pivotal role in some autoinflammatory diseases, such as cryopyrinopathies or familial Mediterranean fever. In these diseases, a constitutional defect of the inflammasome, a protein complex responsible for the activation of interleukin-1, explains the hypersecretion of interleukin-1. Other autoinflammatory diseases have a more complex pathophysiology involving deregulation of the interleukin-1 pathway, upstream or downstream of the inflammasome, or through more complex mechanisms. In this review, we are detailing the synthesis, the activation, the signalling, and the regulation of interleukin-1. We then describe the autoinflammatory diseases or related-diseases where the pathological role of interleukin-1 has been demonstrated.

Molecular basis of caspase-1 polymerization and its inhibition by a new capping mechanism

Inflammasomes are cytosolic caspase-1-activation complexes that sense intrinsic and extrinsic danger signals, and trigger inflammatory responses and pyroptotic cell death. Homotypic interactions among Pyrin domains and caspase recruitment domains (CARDs) in inflammasome-complex components mediate oligomerization into filamentous assemblies. Several cytosolic proteins consisting of only interaction domains exert inhibitory effects on inflammasome assembly. In this study, we determined the structure of the human caspase-1 CARD domain (caspase-1(CARD)) filament by cryo-electron microscopy and investigated the biophysical properties of two caspase-1-like CARD-only proteins: human inhibitor of CARD (INCA or CARD17) and ICEBERG (CARD18). Our results reveal that INCA caps caspase-1 filaments, thereby exerting potent inhibition with low-nanomolar Ki on caspase-1(CARD) polymerization in vitro and inflammasome activation in cells. Whereas caspase-1(CARD) uses six complementary surfaces of three types for filament assembly, INCA is defective in two of the six interfaces and thus terminates the caspase-1 filament.

An updated view on the structure and function of PYRIN domains

The PYRIN domain (PYD) is a protein-protein interaction domain, which belongs to the death domain fold (DDF) superfamily. It is best known for its signaling function in innate immune responses and particularly in the assembly of inflammasomes, which are large protein complexes that allow the induced proximity-mediated activation of caspase-1 and subsequently the release of pro-inflammatory cytokines. The molecular mechanism of inflammasome assembly was only recently elucidated and specifically requires PYD oligomerization. Here we discuss the recent advances in our understanding of PYD signaling and its regulation by PYD-only proteins.

Macrophage cell death upon intracellular bacterial infection

Macrophage-pathogen interaction is a complex process and the outcome of this tag-of-war for both sides is to live or die. Without attempting to be comprehensive, this review will discuss the complexity and significance of the interaction outcomes between macrophages and some facultative intracellular bacterial pathogens as exemplified by Francisella, Salmonella, Shigella and Yersinia. Upon bacterial infection, macrophages can die by a variety of ways, such as apoptosis, autophagic cell death, necrosis, necroptosis, oncosis, pyronecrosis, pyroptosis etc, which is the focus of this review.

The inflammasome: Learning from bacterial evasion strategies

The innate immune system plays a critical role in defense against microbial infection and employs germline-encoded pattern recognition receptors to detect broadly conserved microbial structures or activities. Pattern recognition receptors of the nucleotide binding domain/leucine rich repeat (NLR) family respond to particular microbial products or disruption of cellular physiology, and mediate the activation of an arm of the innate immune response termed the inflammasome. Inflammasomes are multiprotein complexes that are inducibly assembled in response to the contamination of the host cell cytosol by microbial products. Individual NLRs sense the presence of their cognate stimuli, and initiate assembly of inflammasomes via the adaptor protein apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and the effector pro-enzyme caspase-1. Inflammasome activation leads to rapid release of pro-inflammatory mediators of the IL-1 family as well as the release of intracellular alarmins due to a lytic form of programmed cell death termed pyroptosis. Over the past 15 years, a great deal has been learned about the mechanisms that drive inflammasome activation in response to infection by diverse pathogens. However, pathogens have also evolved mechanisms to evade or suppress host defenses, and the mechanisms by which pathogens evade inflammasome activation are not well-understood. Here, we will discuss emerging evidence on how diverse pathogens evade inflammasome activation, and what these studies have revealed about inflammasome biology. Deeper understanding of pathogen evasion of inflammasome activation has the potential to lead to development of novel classes of immunomodulatory factors that could be used in the context of human inflammatory diseases.

Monogenic autoinflammatory diseases: Cytokinopathies

Rapid advances in genetics are providing unprecedented insight into functions of the innate immune system with identification of the mutations that cause monogenic autoinflammatory disease. Cytokine antagonism is profoundly effective in a subset of these conditions, particularly those associated with increased interleukin-1 (IL-1) activity, the inflammasomopathies. These include syndromes where the production of IL-1 is increased by mutation of innate immune sensors such as NLRP3, upstream signalling molecules such as PSTPIP1 and receptors or downstream signalling molecules, such as IL-1Ra. Another example of this is interferon (IFN) and the interferonopathies, with mutations in the sensors STING and MDA5, the upstream signalling regulator AP1S3, and a downstream inhibitor of IFN signalling, ISG15. We propose that this can be extended to cytokines such as IL-36, with mutations in IL-36Ra, and IL-10, with mutations in IL-10RA and IL-10RB, however mutations in sensors or upstream signalling molecules are yet to be described in these instances. Additionally, autoinflammatory diseases can be caused by multiple cytokines, for example with the activation of NF-κB/Rel, for which we propose the term Relopathies. This nosology is limited in that some cytokine pathways may be degenerate in their generation or execution, however provides insight into likely autoinflammatory disease candidates and the cytokines with which newly identified mutations may be associated, and therefore targeted.