Gain-of-function Pyrin mutations induce NLRP3 protein-independent interleukin-1β activation and severe autoinflammation in mice

Familial Mediterranean fever in Germany: epidemiological, clinical, and genetic characteristics of a pediatric population

Familial Mediterranean fever (FMF) is an autoinflammatory disease and belongs to the heterogeneous group of hereditary recurrent fever syndromes (HRFs).

AIMS

The aims of the study were to determine the incidence of FMF in Germany and to describe the spectrum of pyrin mutations and the clinical characteristics in children. A prospective surveillance of children with HRF including FMF was conducted in Germany during a time period of 3 years by the German paediatric surveillance unit for rare paediatric diseases (ESPED). Monthly inquiries were sent to 370 children's hospitals (Clinic-ESPED, n1) and to 23 laboratories (Laboratory-ESPED, n2). Inclusion criteria were children ≤ 16 years of age, disease-associated pyrin mutations, and more than three self-limiting episodes of fever >38.5 °C with increased inflammation markers. In n1, 122 patients with FMF and 225 pyrin mutations were identified. Ninety-two of 122 (75 %) children were of Turkish origin. The minimum incidence of FMF was estimated to be 3 (95 % CI: 2.48-3.54) per 10(6) person-years in the whole children population and 55 (95 % CI: 46-66) per 10(6) person-years in Turkish children living in Germany. N1 U n2 amounted to 593 asymptomatic and symptomatic carriers of 895 mutations (overlap of 73 cases with 134 mutations). p.Met694Val (45 %), p.Met680Ile (14 %), p.Val726Ala (12 %), and p.Glu148Gln (11.5 %) were the most common pyrin mutations.

CONCLUSIONS

Despite FMF being the most frequent of the HRFs, its incidence in Germany is low. Twenty-five to 50 FMF patients ≤ 16 years are newly diagnosed per year. The disease is most commonly observed in individuals of Turkish ancestry.

Genetic loss of murine pyrin, the Familial Mediterranean Fever protein, increases interleukin-1β levels

Familial Mediterranean Fever (FMF) is an inherited autoinflammatory disorder characterized by unprovoked episodes of fever and inflammation. The associated gene, MEFV (Mediterranean Fever), is expressed primarily by cells of myeloid lineage and encodes the protein pyrin/TRIM20/Marenostrin. The mechanism by which mutations in pyrin alter protein function to cause episodic inflammation is controversial. To address this question, we have generated a mouse line lacking the Mefv gene by removing a 21 kb fragment containing the entire Mefv locus. While the development of immune cell populations appears normal in these animals, we show enhanced interleukin (IL) 1β release by Mefv^−/− macrophages in response to a spectrum of inflammatory stimuli, including stimuli dependent on IL-1β processing by the NLRP1b, NLRP3 and NLRC4 inflammasomes. Caspase-1 activity, however, did not change under identical conditions. These results are consistent with a model in which pyrin acts to limit the release of IL-1β generated by activation and assembly of inflammasomes in response to subclinical immune challenges.

The calcium-sensing receptor regulates the NLRP3 inflammasome through Ca2+ and cAMP

Mutations in the gene encoding NLRP3 cause a spectrum of autoinflammatory diseases known as cryopyrin-associated periodic syndromes (CAPS). NLRP3 is a key component of one of several distinct cytoplasmic multiprotein complexes (inflammasomes) that mediate the maturation of the proinflammatory cytokine interleukin-1β (IL-1β) by activating caspase-1. Although several models for inflammasome activation, such as K(+) efflux, generation of reactive oxygen species and lysosomal destabilization, have been proposed, the precise molecular mechanism of NLRP3 inflammasome activation, as well as the mechanism by which CAPS-associated mutations activate NLRP3, remain to be elucidated. Here we show that the murine calcium-sensing receptor (CASR) activates the NLRP3 inflammasome, mediated by increased intracellular Ca(2+) and decreased cellular cyclic AMP (cAMP). Ca(2+) or other CASR agonists activate the NLRP3 inflammasome in the absence of exogenous ATP, whereas knockdown of CASR reduces inflammasome activation in response to known NLRP3 activators. CASR activates the NLRP3 inflammasome through phospholipase C, which catalyses inositol-1,4,5-trisphosphate production and thereby induces release of Ca(2+) from endoplasmic reticulum stores. The increased cytoplasmic Ca(2+) promotes the assembly of inflammasome components, and intracellular Ca(2+) is required for spontaneous inflammasome activity in cells from patients with CAPS. CASR stimulation also results in reduced intracellular cAMP, which independently activates the NLRP3 inflammasome. cAMP binds to NLRP3 directly to inhibit inflammasome assembly, and downregulation of cAMP relieves this inhibition. The binding affinity of cAMP for CAPS-associated mutant NLRP3 is substantially lower than for wild-type NLRP3, and the uncontrolled mature IL-1β production from CAPS patients' peripheral blood mononuclear cells is attenuated by increasing cAMP. Taken together, these findings indicate that Ca(2+) and cAMP are two key molecular regulators of the NLRP3 inflammasome that have critical roles in the molecular pathogenesis of CAPS.

Type I IFN-mediated regulation of IL-1 production in inflammatory disorders

Although contributing to inflammatory responses and to the development of certain autoimmune pathologies, type I interferons (IFNs) are used for the treatment of viral, malignant, and even inflammatory diseases. Interleukin-1 (IL-1) is a strongly pyrogenic cytokine and its importance in the development of several inflammatory diseases is clearly established. While the therapeutic use of IL-1 blocking agents is particularly successful in the treatment of innate-driven inflammatory disorders, IFN treatment has mostly been appreciated in the management of multiple sclerosis. Interestingly, type I IFNs exert multifaceted immunomodulatory effects, including the reduction of IL-1 production, an outcome that could contribute to its efficacy in the treatment of inflammatory diseases. In this review, we summarize the current knowledge on IL-1 and IFN effects in different inflammatory disorders, the influence of IFNs on IL-1 production, and discuss possible therapeutic avenues based on these observations.

After GWAS: mice to the rescue?

The genetic basis of human autoimmune diseases remains incompletely understood, despite significant progress from genome-wide association studies (GWAS). In this review we outline how studies in mice may help filling these knowledge gaps. Forward genetic approaches including mutagenesis screens and quantitative trait locus (QTL) mapping studies can identify candidate genes for in depth analysis in human patient populations. Reverse genetic approaches utilize genetically engineered mice to analyze the function of disease-associated genes and their variants. Inbred strains are a distinctive feature of mouse genetics and we discuss their history, advantages and disadvantages. Three factors need to be considered when comparing experimental results from studies in mice and humans: In addition to species-specific differences, phenotypes are affected by the genetic background of the mouse strain being analyzed, and by microbial factors. Despite of these complexities, mice are essential discovery tools in the post GWAS era.

In vitro analysis of the functional effects of an NLRP3 G809S variant with the co-existence of MEFV haplotype variants in atypical autoinflammatory syndrome

PURPOSE

Hereditary periodic fever syndromes have been considered monogenic diseases. However, some recent reports have described patients with co-existence of recurrent fever responsible genes. This study assessed whether a rare variant, found in Japanese children showing atypical autoinflammatory syndrome, located in the leucine-rich repeat domain of Nod-like receptor family, pyrin domain containing 3 (NLRP3) with co-existence of Mediterranean fever (MEFV) haplotype variants may contribute to a proinflammatory phenotype using a systematic approach.

METHODS

Cytokine production in serum or from peripheral blood monocytes was measured by ELISA. DNA sequence analysis of genes including NLRP3, MEFV, mevalonate kinase (MVK), and tumor necrosis factor receptor superfamily, member 1A (TNFRSF1A) were performed on patient samples. In vitro functional assays determined the effects of the NLRP3 variants and pyrin using NF-κB activation and speck formation assays.

RESULTS

A heterozygous genetic variant of NLRP3, G809S, was found in samples from both patients. Additionally the previously reported heterozygous MEFV variants (P369S-R408Q or E148Q-P369S-R408Q) were also detected in both patients. Serum IL-1ra and sTNFR1 levels increased in the attack phase of the disease in both patients. The production levels of IL-1β from monocytes isolated from both cases were elevated following LPS and IFN-γ stimulation. The NLRP3 G809S variant demonstrated no increase of NF-κB activity following monosodium urate stimulation, whereas it significantly increased speck formation by interacting with apoptosis-associated speck-like protein with caspase recruitment domain.

CONCLUSIONS

The phenotype of atypical autoinflammatory disease in patients could be modified by a synergistic effect with two other variants of autoinflammatory-associated genes.

Lighting the fires within: the cell biology of autoinflammatory diseases

Autoinflammatory diseases are characterized by seemingly unprovoked pathological activation of the innate immune system in the absence of autoantibodies or autoreactive T cells. Discovery of the causative mutations underlying several monogenic autoinflammatory diseases has identified key regulators of innate immune responses. Recent studies have highlighted the role of misfolding, oligomerization and abnormal trafficking of pathogenic mutant proteins in triggering autoinflammation, and suggest that more common rheumatic diseases may have an autoinflammatory component. This coincides with recent discoveries of new links between endoplasmic reticulum stress and inflammatory signalling pathways, which support the emerging view that autoinflammatory diseases may be due to pathological dysregulation of stress-sensing pathways that normally function in host defence.

Critical role for mast cells in interleukin-1β-driven skin inflammation associated with an activating mutation in the nlrp3 protein

Cryopyrin-associated periodic syndromes (CAPS) are caused by aberrant interleukin-1β (IL-1β) production induced by mutations in the NLRP3 protein in humans, but the mechanisms involved remain poorly understood. Using a mouse model, we show a role for the indigenous microbiota and mast cells (MCs) in skin disease associated with mutant Nlrp3 protein. Unlike normal cells, MCs expressing mutant Nlrp3 produced IL-1β in response to lipopolysaccharide or tumor necrosis factor-α (TNF-α). In neonatal mice, the microbiota induced TNF-α and IL-1β and promoted skin disease. MC deficiency greatly reduced disease in Nlrp3 mutant mice, and reconstitution of MC-deficient mice with mutant MCs restored skin disease, which required the expression of IL-1β in MCs. Surprisingly, neutralization of TNF-α abrogated IL-1β production and skin disease in neonatal Nlrp3 mutant mice, but not in affected adult mice. Thus, the microbiota and MCs initiate cellular events leading to dysregulated IL-1β production and skin inflammation in neonatal mice with the CAPS-associated Nlrp3 mutation.

TRIM27 negatively regulates NOD2 by ubiquitination and proteasomal degradation

NOD2, the nucleotide-binding domain and leucine-rich repeat containing gene family (NLR) member 2 is involved in mediating antimicrobial responses. Dysfunctional NOD2 activity can lead to severe inflammatory disorders, but the regulation of NOD2 is still poorly understood. Recently, proteins of the tripartite motif (TRIM) protein family have emerged as regulators of innate immune responses by acting as E3 ubiquitin ligases. We identified TRIM27 as a new specific binding partner for NOD2. We show that NOD2 physically interacts with TRIM27 via the nucleotide-binding domain, and that NOD2 activation enhances this interaction. Dependent on functional TRIM27, ectopically expressed NOD2 is ubiquitinated with K48-linked ubiquitin chains followed by proteasomal degradation. Accordingly, TRIM27 affects NOD2-mediated pro-inflammatory responses. NOD2 mutations are linked to susceptibility to Crohn's disease. We found that TRIM27 expression is increased in Crohn's disease patients, underscoring a physiological role of TRIM27 in regulating NOD2 signaling. In HeLa cells, TRIM27 is partially localized in the nucleus. We revealed that ectopically expressed NOD2 can shuttle to the nucleus in a Walker A dependent manner, suggesting that NOD2 and TRIM27 might functionally cooperate in the nucleus.

We conclude that TRIM27 negatively regulates NOD2-mediated signaling by degradation of NOD2 and suggest that TRIM27 could be a new target for therapeutic intervention in NOD2-associated diseases.

The role of TNF-α and PAI-1 gene polymorphisms in familial Mediterranean fever

OBJECTIVES

Familial Mediterranean fever (FMF) is one of the most serious inherited inflammatory disorders among Jewish, Armenian, Turkish and Arab populations. The imbalance between pro- and anti-inflammatory cytokines may play a role in its etiology. We have investigated whether tumor necrosis factor-alpha (TNF-α) and plasminogen activator inhibitor 1 (PAI-1) gene polymorphisms are associated with FMF and evaluated the relationship between these polymorphisms and genotypic manifestation of FMF.

METHODS

We investigated single nucleotide polymorphisms of the TNF-α promoter at positions -308 G/A and the PAI-1 4G/5G gene polymorphism in peripheral blood leukocytes collected from 177 individuals with FMF with different genotype combinations. All of the polymorphisms of TNF-α and PAI-1 were detected by PCR and restriction fragment length polymorphism analysis.

RESULTS

There were no association between the TNF-α/308 genotypes and mutations in FMF. In contrast, the PAI-1 4G/5G gene polymorphism may have a significant effect in FMF disease.

CONCLUSIONS

Screening with PAI-1 gene polymorphism tests may be beneficial for tracing future FMF patients. However, further investigations are needed to reach a conclusion on the association between PAI-1 polymorphisms and FMF.

Immunology in clinic review series; focus on autoinflammatory diseases: inflammasomes: mechanisms of activation

OTHER THEMES PUBLISHED IN THIS IMMUNOLOGY IN THE CLINIC REVIEW SERIES Allergy, Host Responses, Cancer, Type 1 diabetes and viruses, Metabolic diseases.

SUMMARY

Initiation of a successful immune response requires a working set of sensors that detect any noxious agent within the cellular microenvironment and molecular platforms that process this signal to trigger an appropriate effector response. Pattern recognition receptors can engage different signalling cascades that lead to proinflammatory gene expression. At the same time, transcription-independent events such as activation of proteases and/or phagocytosis are also initiated. The inflammasome pathway constitutes a signalling platform that leads to the activation of so-called inflammatory caspases, most notably caspase-1, which plays a pivotal role in the cleavage and thus maturation of proinflammatory cytokines, but also in the induction of pyroptosis, a special type of cell death. In this review we elaborate on the currently known inflammasome complexes with a special focus on the mechanism behind their activation. Understanding these mechanisms could provide important information regarding the potential signalling nodes that might be targeted for therapeutic intervention.

Regulation of inflammasome signaling

Innate immune responses have the ability to both combat infectious microbes and drive pathological inflammation. Inflammasome complexes are a central component of these processes through their regulation of interleukin 1β (IL-1β), IL-18 and pyroptosis. Inflammasomes recognize microbial products or endogenous molecules released from damaged or dying cells both through direct binding of ligands and indirect mechanisms. The potential of the IL-1 family of cytokines to cause tissue damage and chronic inflammation emphasizes the importance of regulating inflammasomes. Many regulatory mechanisms have been identified that act as checkpoints for attenuating inflammasome signaling at multiple steps. Here we discuss the various regulatory mechanisms that have evolved to keep inflammasome signaling in check to maintain immunological balance.

Immunology in clinic review series; focus on autoinflammatory diseases: role of inflammasomes in autoinflammatory syndromes

OTHER THEMES PUBLISHED IN THIS IMMUNOLOGY IN THE CLINIC REVIEW SERIES Allergy, Host Responses, Cancer, Type 1 diabetes and viruses, Metabolic diseases.

SUMMARY

Autoinflammatory syndromes are disorders characterized by the hyperactivation of the innate immune system in the absence of microbial infection or autoantibody production. Some autoinflammatory syndromes are associated with recurrent episodes of fever and systemic inflammation that are caused by dysregulated activation of inflammasomes, molecular platforms responsible for the activation of caspase-1 and the production of interleukin (IL)-1β. In this review we will discuss the role of IL-1β and the inflammasomes in host defence and how mutations of two genes, NLRP3 and PYRIN, leads to the autoinflammatory syndromes, cryopyrin-associated periodic syndromes (CAPS) and familial Mediterranean fever (FMF). Both CAPS and FMF are characterized by increased inflammasome activity and overproduction of IL-1β which is ultimately responsible for disease manifestations. Importantly, understanding the molecular mechanisms of these syndromes has led to effective treatment for these rare diseases with biological drugs that target IL-1β-mediated signalling.

[Clinical aspects of Familial Mediterranean fever]

Familial Mediterranean fever (FMF) is a hereditary autoinflammatory disease characterized by recurrent and short duration (1-3 days) of fever, and serositis. Based on the nationwide survey of FMF in Japan, the estimated number of Japanese FMF patients is about three hundred. High grade fever was observed in 95.5%, chest pain in 35.8% abdominal pain in 62.7% and arthritis in 31.3% among Japanese FMF patients. AA amyloidosis was confirmed in 5 patients (3.7%). Colchicine was effective in 91.8% of Japanese FMF patients. A significant number of FMF patients exist in Japan, and early diagnosis and treatments should be required to prevent AA amyloidosis.

Cholesterol, Inflammasomes, and Atherogenesis

Plasma cholesterol levels have been strongly associated with atherogenesis, underscoring the role of lipid metabolism in defining cardiovascular disease risk. However, atherosclerotic plaque is highly dynamic and contains elements of both the innate and adaptive immune system that respond to the aberrant accumulation of lipids in the subendothelial space. Previous research has focused on defining how proinflammatory cytokines synthesized by macrophages, such as interleukin-1β (IL-1β), modulate the progression of atherosclerosis, supporting the notion that chronic inflammation accelerates atherogenesis. More recently, emphasis has been placed on the elucidation of the mechanisms that contribute to pro-IL-1β production and finally its processing via multiprotein complexes termed the inflammasomes, a family of cytosolic multiprotein complexes that serve as sensors of either pathogen invasion or cellular stress (ie, cholesterol crystals) and work via triggering caspase-1-mediated processing of pro-IL-1β to IL-1β. Based on this link between cholesterol metabolism, NLRP3 inflammasome activation, and IL-1β release, it is important to re-evaluate how the atherogenic environment stimulates immune cells to produce IL-1β.

Rare hereditary autoinflammatory disorders: towards an understanding of critical in vivo inflammatory pathways

Hereditary autoinflammatory syndromes are monogenic disorders with an inborn error of innate immunity, and include periodic fever syndromes such as familial Mediterranean fever (FMF), tumor necrosis factor receptor-associated periodic syndrome and cryopyrin-associated periodic syndromes (CAPS), pyogenic diseases such as pyogenic arthritis, pyoderma gangrenosum and acne syndrome (PAPAS), and granulomatous diseases such as Blau syndrome. By identifying the genetic abnormalities and subsequent analyses of the molecular mechanisms underlying these disorders, several critical in vivo pathways for inflammatory processes have been discovered. In this review, three categories of autoinflammatory disorders are discussed: inflammasomopathies, receptor antagonist deficiencies and proteasome disability syndromes. Inflammasomopathies are diseases with dysregulated NLRP3 inflammasome activation, and include CAPS with NLRP3, FMF with MEFV, and PAPAS with PSTPIP1 mutations. Analyses of these diseases have clarified some critical pathways regulating NLRP3 inflammasome signaling. Receptor antagonist deficiencies include the newly defined deficiency for interleukin-1 receptor antagonist resulting in sterile multifocal osteomyelitis with periostosis and pustulosis, and deficiency for interleukin-36 receptor antagonist resulting in generalized pustular psoriasis. The identification of these genetic abnormalities has revealed a critical role for receptor antagonists of IL-1 family cytokines in regulating neutrophil activation/recruitment. Finally, proteasome disability syndromes with PSMB8 mutations include Nakajo-Nishimura syndrome and related disorders distributed globally. Analyses of these diseases have unexpectedly shown a critical role of the ubiquitin-proteasome system in the regulation or homeostasis of inflammation/metabolism. Since there still remain a number of predicted but undefined hereditary autoinflammatory syndromes, further clinical and genetic approaches are required to discover novel in vivo critical inflammatory pathways.

The inflammasome: in memory of Dr. Jurg Tschopp

A decade ago, Jurg Tschopp introduced the concept of the inflammasome. This exciting discovery of a macromolecular complex that senses 'danger' and initiates the inflammatory response contributed to a renaissance in the fields of innate immunity and cell death. Jurg led the biochemical characterization of the inflammasome complex and demonstrated that spontaneous hyperactivation of this interleukin (IL)-1β processing machinery is the molecular basis of a spectrum of hereditary periodic fever syndromes, caused by mutated forms of the inflammasome scaffolding receptor, NLRP3. The identification of the underlying mechanism in these disorders has led to their now successful therapy, with the use of the IL-1 receptor antagonist in the clinic. Jurg's pioneering work has subsequently defined a number of inflammasome agonists ranging from microbial molecules expressed during infection, to triggers of sterile inflammation, most notably gout-associated uric acid crystals, asbestos, silica and nanoparticles. More recently, Jurg introduced the critical new concept of the metabolic inflammasome, which senses metabolic stress and contributes to the onset of the metabolic syndrome associated with obesity and type 2 diabetes. Jurg was an outstanding and skillful biochemist, an elegant and rigorous researcher often far ahead of his peers. He was a truly amiable person, fair, generous and inspiring, and will be most remembered for his infectious enthusiasm. We write this review article on the inflammasome in his honor and dedicate it to his memory.

Autophagy: cellular defense to excessive inflammation

Autophagy can orchestrate a variety of cellular responses to dangerous stimuli. Our understanding of the physiologic roles of autophagy has recently expanded; in addition to its other roles, autophagy now appears to play an essential role in regulating inflammatory responses. This review describes recent findings concerning the roles and mechanisms of autophagy in controlling excessive inflammation.