NLRP3 inflammasome assembly is regulated by phosphorylation of the pyrin domain.

NLRP3 is a cytosolic pattern recognition receptor that senses microbes and endogenous danger signals. Upon activation, NLRP3 forms an inflammasome with the adapter ASC, resulting in caspase-1 activation, release of proinflammatory cytokines and cell death. How NLRP3 activation is regulated by transcriptional and posttranslational mechanisms to prevent aberrant activation remains incompletely understood. Here, we identify three conserved phosphorylation sites in NLRP3 and demonstrate that NLRP3 activation is controlled by phosphorylation of its pyrin domain (PYD). Phosphomimetic residues in NLRP3 PYD abrogate inflammasome activation and structural modeling indicates that phosphorylation of the PYD regulates charge-charge interaction between two PYDs that are essential for NLRP3 activation. Phosphatase 2A (PP2A) inhibition or knock-down drastically reduces NLRP3 activation, showing that PP2A can license inflammasome assembly via dephosphorylating NLRP3 PYD. These results propose that the balance between kinases and phosphatases acting on the NLRP3 PYD is critical for NLRP3 activation.

Human NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) inflammasome activity is regulated by and potentially targetable through Bruton tyrosine kinase.

BACKGROUND: The Nod-like receptor NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) and Bruton tyrosine kinase (BTK) are protagonists in innate and adaptive immunity, respectively. NLRP3 senses exogenous and endogenous insults, leading to inflammasome activation, which occurs spontaneously in patients with Muckle-Wells syndrome; BTK mutations cause the genetic immunodeficiency X-linked agammaglobulinemia (XLA). However, to date, few proteins that regulate NLRP3 inflammasome activity in human primary immune cells have been identified, and clinically promising pharmacologic targeting strategies remain elusive. OBJECTIVE: We sought to identify novel regulators of the NLRP3 inflammasome in human cells with a view to exploring interference with inflammasome activity at the level of such regulators. METHODS: After proteome-wide phosphoproteomics, the identified novel regulator BTK was studied in human and murine cells by using pharmacologic and genetic BTK ablation. RESULTS: Here we show that BTK is a critical regulator of NLRP3 inflammasome activation: pharmacologic (using the US Food and Drug Administration-approved inhibitor ibrutinib) and genetic (in patients with XLA and Btk knockout mice) BTK ablation in primary immune cells led to reduced IL-1ß processing and secretion in response to nigericin and the Staphylococcus aureus toxin leukocidin AB (LukAB). BTK affected apoptosis-associated speck-like protein containing a CARD (ASC) speck formation and caspase-1 cleavage and interacted with NLRP3 and ASC. S aureus infection control in vivo and IL-1ß release from cells of patients with Muckle-Wells syndrome were impaired by ibrutinib. Notably, IL-1ß processing and release from immune cells isolated from patients with cancer receiving ibrutinib therapy were reduced. CONCLUSION: Our data suggest that XLA might result in part from genetic inflammasome deficiency and that NLRP3 inflammasome-linked inflammation could potentially be targeted pharmacologically through BTK.

A small-molecule inhibitor of the NLRP3 inflammasome for the treatment of inflammatory diseases.

The NOD-like receptor (NLR) family, pyrin domain-containing protein 3 (NLRP3) inflammasome is a component of the inflammatory process, and its aberrant activation is pathogenic in inherited disorders such as cryopyrin-associated periodic syndrome (CAPS) and complex diseases such as multiple sclerosis, type 2 diabetes, Alzheimer's disease and atherosclerosis. We describe the development of MCC950, a potent, selective, small-molecule inhibitor of NLRP3. MCC950 blocked canonical and noncanonical NLRP3 activation at nanomolar concentrations. MCC950 specifically inhibited activation of NLRP3 but not the AIM2, NLRC4 or NLRP1 inflammasomes. MCC950 reduced interleukin-1ß (IL-1ß) production in vivo and attenuated the severity of experimental autoimmune encephalomyelitis (EAE), a disease model of multiple sclerosis. Furthermore, MCC950 treatment rescued neonatal lethality in a mouse model of CAPS and was active in ex vivo samples from individuals with Muckle-Wells syndrome. MCC950 is thus a potential therapeutic for NLRP3-associated syndromes, including autoinflammatory and autoimmune diseases, and a tool for further study of the NLRP3 inflammasome in human health and disease.

The adaptor ASC has extracellular and 'prionoid' activities that propagate inflammation.

Microbes or danger signals trigger inflammasome sensors, which induce polymerization of the adaptor ASC and the assembly of ASC specks. ASC specks recruit and activate caspase-1, which induces maturation of the cytokine interleukin 1ß (IL-1ß) and pyroptotic cell death. Here we found that after pyroptosis, ASC specks accumulated in the extracellular space, where they promoted further maturation of IL-1ß. In addition, phagocytosis of ASC specks by macrophages induced lysosomal damage and nucleation of soluble ASC, as well as activation of IL-1ß in recipient cells. ASC specks appeared in bodily fluids from inflamed tissues, and autoantibodies to ASC specks developed in patients and mice with autoimmune pathologies. Together these findings reveal extracellular functions of ASC specks and a previously unknown form of cell-to-cell communication.

Dual engagement of the NLRP3 and AIM2 inflammasomes by plasmodium-derived hemozoin and DNA during malaria.

Hemozoin (Hz) is the crystalline detoxification product of hemoglobin in Plasmodium-infected erythrocytes. We previously proposed that Hz can carry plasmodial DNA into a subcellular compartment that is accessible to Toll-like receptor 9 (TLR9), inducing an inflammatory signal. Hz also activates the NLRP3 inflammasome in primed cells. We found that Hz appears to colocalize with DNA in infected erythrocytes, even before RBC rupture or phagolysosomal digestion. Using synthetic Hz coated in vitro with plasmodial genomic DNA (gDNA) or CpG oligodeoxynucleotides, we observed that DNA-complexed Hz induced TLR9 translocation, providing a priming and an activation signal for inflammasomes. After phagocytosis, Hz and DNA dissociate. Hz subsequently induces phagolysosomal destabilization, allowing phagolysosomal contents access to the cytosol, where DNA receptors become activated. Similar observations were made with Plasmodium-infected RBCs. Finally, infected erythrocytes activated both the NLRP3 and AIM2 inflammasomes. These observations suggest that Hz and DNA work together to induce systemic inflammation during malaria.

ASC speck formation as a readout for inflammasome activation.

All inflammasomes require the adapter protein apoptosis associated speck-like protein containing a CARD (ASC) for the activation of caspase-1. After inflammasome activation, ASC assembles into a large protein complex, which is termed "speck". ASC specks can be observed as they reach a size of around 1 µm and in most cells only one speck forms upon inflammasome activation. Hence, ASC speck formation can be used as a simple upstream readout for inflammasome activation. Here, we describe a method for analyzing inflammasome activation by ASC speck visualization. First, we describe the generation of a clonal inflammasome reporter macrophage cell line overexpressing fluorescently tagged ASC. We then discuss stimulation conditions and the microscopic evaluation of ASC speck formation.

Activation of the NLRP3 inflammasome by IAV virulence protein PB1-F2 contributes to severe pathophysiology and disease.

The ability for a host to recognize infection is critical for virus clearance and often begins with induction of inflammation. The PB1-F2 of pathogenic influenza A viruses (IAV) contributes to the pathophysiology of infection, although the mechanism for this is unclear. The NLRP3-inflammasome has been implicated in IAV pathogenesis, but whether IAV virulence proteins can be activators of the complex is unknown. We investigated whether PB1-F2-mediated activation of the NLRP3-inflammasome is a mechanism contributing to overt inflammatory responses to IAV infection. We show PB1-F2 induces secretion of pyrogenic cytokine IL-1ß by activating the NLRP3-inflammasome, contributing to inflammation triggered by pathogenic IAV. Compared to infection with wild-type virus, mice infected with reverse engineered PB1-F2-deficient IAV resulted in decreased IL-1ß secretion and cellular recruitment to the airways. Moreover, mice exposed to PB1-F2 peptide derived from pathogenic IAV had enhanced IL-1ß secretion compared to mice exposed to peptide derived from seasonal IAV. Implicating the NLRP3-inflammasome complex specifically, we show PB1-F2 derived from pathogenic IAV induced IL-1ß secretion was Caspase-1-dependent in human PBMCs and NLRP3-dependent in mice. Importantly, we demonstrate PB1-F2 is incorporated into the phagolysosomal compartment, and upon acidification, induces ASC speck formation. We also show that high molecular weight aggregated PB1-F2, rather than soluble PB1-F2, induces IL-1ß secretion. Furthermore, NLRP3-deficient mice exposed to PB1-F2 peptide or infected with PB1-F2 expressing IAV were unable to efficiently induce the robust inflammatory response as observed in wild-type mice. In addition to viral pore forming toxins, ion channel proteins and RNA, we demonstrate inducers of NLRP3-inflammasome activation may include disordered viral proteins, as exemplified by PB1-F2, acting as host pathogen 'danger' signals. Elucidating immunostimulatory PB1-F2 mediation of NLRP3-inflammasome activation is a major step forward in our understanding of the aetiology of disease attributable to exuberant inflammatory responses to IAV infection.

Activation and regulation of the inflammasomes.

Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1ß (IL-1ß) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform - the apoptosome - that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy.

Chemical genetics reveals a kinase-independent role for protein kinase R in pyroptosis.

Formation of the inflammasome, a scaffolding complex that activates caspase-1, is important in numerous diseases. Pyroptotic cell death induced by anthrax lethal toxin (LT) is a model for inflammasome-mediated caspase-1 activation. We discovered 7-desacetoxy-6,7-dehydrogedunin (7DG) in a phenotypic screen as a small molecule that protects macrophages from LT-induced death. Using chemical proteomics, we identified protein kinase R (PKR) as the target of 7DG and show that RNAi knockdown of PKR phenocopies treatment with 7DG. Further, we show that PKR's role in ASC assembly and caspase-1 activation induced by several different inflammasome stimuli is independent of PKR's kinase activity, demonstrating that PKR has a previously uncharacterized role in caspase-1 activation and pyroptosis that is distinct from its reported kinase-dependent roles in apoptosis and inflammasome formation in lipopolysaccharide-primed cells. Remarkably, PKR has different roles in two distinct cell death pathways and has a broad role in inflammasome function relevant in other diseases.

NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice.

Alzheimer's disease is the world's most common dementing illness. Deposition of amyloid-ß peptide drives cerebral neuroinflammation by activating microglia. Indeed, amyloid-ß activation of the NLRP3 inflammasome in microglia is fundamental for interleukin-1ß maturation and subsequent inflammatory events. However, it remains unknown whether NLRP3 activation contributes to Alzheimer's disease in vivo. Here we demonstrate strongly enhanced active caspase-1 expression in human mild cognitive impairment and brains with Alzheimer's disease, suggesting a role for the inflammasome in this neurodegenerative disease. Nlrp3(-/-) or Casp1(-/-) mice carrying mutations associated with familial Alzheimer's disease were largely protected from loss of spatial memory and other sequelae associated with Alzheimer's disease, and demonstrated reduced brain caspase-1 and interleukin-1ß activation as well as enhanced amyloid-ß clearance. Furthermore, NLRP3 inflammasome deficiency skewed microglial cells to an M2 phenotype and resulted in the decreased deposition of amyloid-ß in the APP/PS1 model of Alzheimer's disease. These results show an important role for the NLRP3/caspase-1 axis in the pathogenesis of Alzheimer's disease, and suggest that NLRP3 inflammasome inhibition represents a new therapeutic intervention for the disease.

Cell invasion and strain dependent induction of suppressor of cytokine signaling-1 by Toxoplasma gondii.

Toxoplasma gondii is an intracellular parasite that has to cope with the microbicidal actions of IFNγ. Previously we reported that parasite-mediated induction of suppressor of cytokine signaling protein 1 (SOCS1) contributes to inhibition of IFNγ signaling. However, the signaling requirements remained elusive. We now show that induction of SOCS1 and inhibition of nitric oxide production by IFNγ was independent of stimulation of Toll-like receptors. Instead, infection by T. gondii resulted in induction of egr transcription factors which have been reported to regulate SOCS expression. Indeed, induction of egr2 as well as SOCS1 was dependent on p38 MAP kinase and blockade of egr inhibited SOCS1 expression. Moreover, we found that Mic8, a previously identified invasion factor of T. gondii, was necessary for SOCS1 regulation and escape of IFNγ mediated nitric oxide secretion within macrophages. Surprisingly, when further analyzing Mic8 deficient parasites we noted that inhibition of IFNγ mediated up-regulation of MHC-class II and ICAM1 molecules was independent of cell invasion. Furthermore, these inhibitory effects were equally observed in type I and II strains of T. gondii and were dependent on excreted and secreted antigens. In contrast, only the virulent RH type I strain additionally induced SOCS1 and efficiently inhibited nitric oxide secretion by IFNγ. The results show that T. gondii makes use of two different mechanisms to escape from IFNγ activity with one mode being strain dependent and relying on active cell invasion and SOCS1 induction.

Innate immune receptors for nucleic acids.

The innate immune system has evolved to detect microbes and sterile tissue damage with the help of a series of signaling receptors. One key strategy is to detect infectious microbes or host cell damage by recognizing nucleic acids that are modified or appear in compartment normally devoid of nucleic acids. Here, we describe two methods that allow studying the molecular interaction between various nucleic acid recognizing signaling receptors with their ligands. A ligand pull-down assay can be used to show a known interaction between a ligand and its receptor or the method can be utilized as a discovery approach to identify an unknown receptor to a given ligand. An AlphaScreen experiment can be set up to assess the ligand binding affinity to a given receptor.

Inflammasomes: too big to miss.

Inflammation is the coordinated immune response to harmful stimuli that appear during infections or after tissue damage. Cells of the innate immune system are the central players in mediating inflammatory tissue responses. These cells are equipped with an array of signaling receptors that detect foreign molecular substances or altered endogenous molecules that appear under situations of stress. This review provides an overview of recent progress in elucidating the molecular mechanisms that lead to inflammatory reactions. We discuss the current knowledge of the mechanisms leading to the activation of cytoplasmic, multimolecular protein complexes, termed "inflammasomes," which regulate the activity of caspase-1 and the maturation and release of IL-1beta.

Cutting edge: NF-kappaB activating pattern recognition and cytokine receptors license NLRP3 inflammasome activation by regulating NLRP3 expression.

The IL-1 family cytokines are regulated on transcriptional and posttranscriptional levels. Pattern recognition and cytokine receptors control pro-IL-1beta transcription whereas inflammasomes regulate the proteolytic processing of pro-IL-1beta. The NLRP3 inflammasome, however, assembles in response to extracellular ATP, pore-forming toxins, or crystals only in the presence of proinflammatory stimuli. How the activation of gene transcription by signaling receptors enables NLRP3 activation remains elusive and controversial. In this study, we show that cell priming through multiple signaling receptors induces NLRP3 expression, which we identified to be a critical checkpoint for NLRP3 activation. Signals provided by NF-kappaB activators are necessary but not sufficient for NLRP3 activation, and a second stimulus such as ATP or crystal-induced damage is required for NLRP3 activation.