The NLRP3 inflammasome is released as a particulate danger signal that amplifies the inflammatory response

Nedd8 regulates inflammasome-dependent caspase-1 activation

Caspase-1 is activated by the inflammasome complex to process cytokines like interleukin-1β (IL-1β). Pro-caspase-1 consists of three domains, CARD, p20, and p10. Association of pro-caspase-1 with the inflammasome results in initiation of its autocatalytic activity, culminating in self-cleavage that generates catalytically active subunits (p10 and p20). In the current study, we show that Nedd8 is required for efficient self-cleavage of pro-caspase-1 to generate its catalytically active subunits. Nedd8 silencing or treating cells with the neddylation inhibitor MLN4924 led to diminished caspase-1 processing and reduced IL-1β maturation following inflammasome activation. Coimmunoprecipitation and mass spectrometric analysis of 293 cells overexpressing pro-caspase-1 (and CARD) and Nedd8 suggested possible neddylation of caspase-1 CARD. Following inflammasome activation in primary macrophages, we observed colocalization of endogenous Nedd8 with caspase-1. Similarly, interaction of endogenous Nedd8 with caspase-1 CARD was detected in inflammasome-activated macrophages. Furthermore, enhanced autocatalytic activity of pro-caspase-1 was observed following Nedd8 overexpression in 293 cells, and such activity in inflammasome-activated macrophages was drastically diminished upon treatment of cells with MLN4924. Thus, our studies demonstrate a role of Nedd8 in regulating caspase-1 activation following inflammasome activation, presumably via augmenting autoprocessing/cleavage of pro-caspase-1 into its corresponding catalytically active subunits.

Structural mechanisms of inflammasome assembly

Inflammasomes are supramolecular signaling complexes that activate a subset of caspases known as the inflammatory caspases, an example of which is caspase 1. Upon stimulation by microbial and damage-associated signals, inflammasomes assemble to elicit the first line of host defense via the proteolytic maturation of cytokines interleukin-1β and interleukin-18, and by induction of pyroptotic cell death. Inflammasome assembly requires activation of an upstream sensor, a downstream effector and, in most cases, an adaptor molecule such as apoptosis-associate speck-like protein containing a caspase recruitment domain (ASC). Depending on whether ASC is required, inflammasomes can be categorized into ASC-dependent and ASC-independent inflammasomes. Here, we review current understandings of the structures of inflammasomes, as probed using traditional structural methods, as well as biochemical, biophysical and single-molecule methods. The key structural scaffold for inflammasome assembly is composed of filaments of Pyrin domains and caspase recruitment domains (CARD) in the sensor, adaptor and effector components. Nucleated polymerization appears to govern the ordered assembly process from activation of a Pyrin domain-containing sensor such as AIM2 by dsDNA or NLRP3 by extracellular particulates, to recruitment of the Pyrin domain and CARD-containing adaptor ASC, and finally to activation of CARD-containing caspase 1. The underlying filamentous architecture of inflammasomes and the cooperativity in the assembly may explain the 'all-or-none' response in inflammasome activation. Inflammasomes are tightly regulated by a number of cytosolic inhibitors, which may change the morphology and assembly kinetics of inflammasomes. Biochemical and cellular studies suggest that Pyrin domain and CARD filaments possess prion-like properties in propagating inflammasome activation within and between cells.

The inflammasomes and autoinflammatory syndromes

Inflammation, a vital response of the immune system to infection and damage to tissues, can be initiated by various germline-encoded innate immune-signaling receptors. Among these, the inflammasomes are critical for activation of the potent proinflammatory interleukin-1 cytokine family. Additionally, inflammasomes can trigger and maintain inflammatory responses aimed toward excess nutrients and the numerous danger signals that appear in a variety of chronic inflammatory diseases. We discuss our understanding of how inflammasomes assemble to trigger caspase-1 activation and subsequent cytokine release, describe how genetic mutations in inflammasome-related genes lead to autoinflammatory syndromes, and review the contribution of inflammasome activation to various pathologies arising from metabolic dysfunction. Insights into the mechanisms that govern inflammasome activation will help in the development of novel therapeutic strategies, not only for managing genetic diseases associated with overactive inflammasomes, but also for treating common metabolic diseases for which effective therapies are currently lacking.

Prion-like polymerization as a signaling mechanism

The innate immune system uses pattern recognition receptors such as RIG-I and NLRP3 to sense pathogen invasion and other danger signals. Activation of these receptors induces robust signal transduction cascades that trigger the production of cytokines important for host protection. MAVS and ASC are essential adaptor proteins downstream of RIG-I and NLRP3, respectively, and both contain N-terminal domains belonging to the death domain superfamily. Recent studies suggest that both MAVS and ASC form functional prion-like fibers through their respective death domains to propagate downstream signaling. Here, we review these findings, and in this context discuss the emerging concept of prion-like polymerization in signal transduction. We further examine the potential benefits of this signaling strategy, including signal amplification, host evolutionary advantage, and molecular memory.

Caspase crosstalk: integration of apoptotic and innate immune signalling pathways

The caspase family of cysteine proteases has been functionally divided into two groups: those involved in apoptosis and those involved in innate immune signalling. Recent findings have identified 'apoptotic' caspases within inflammasome complexes and revealed that 'inflammatory' caspases are capable of inducing cell death, suggesting that the earlier view of caspase function may have been overly simplistic. Here, I review evidence attributing nonclassical functions to many caspases and propose that caspases serve as critical mediators in the integration of apoptotic and inflammatory pathways, thereby forming an integrated signalling system that regulates cell death and innate immune responses during development, infection, and homeostasis.

Mislocalization of the interferon inducible protein IFI16 by environmental insults: implications in autoimmunity

The nuclear DNA sensor IFI16, a member of PYHIN family of proteins, was previously studied for its role in cell cycle regulation, tumor suppression, apoptosis and DNA damage signaling. Autoantibodies against IFI16 are prevalent in the sera of patients with systemic autoimmunity, thus depicting physiological significance as an autoantigen. At present, the nuclear IFI16 protein has been thoroughly investigated for its role as an innate immune sensor involved in inflammasome signaling and viral restriction. While the sub-cellular localization of IFI16 during such events has been known, very little knowledge about its presence and significance in the extracellular space is available. Recently our group has discovered the presence of circulating IFI16 in the sera from systemic autoimmune patients indicating that in this setting it may be mislocalized form its nuclear site and secreted in the extracellular milieu. In this review, we will discuss the leakage of endogenous IFI16 that has been experimentally proved using in vivo and in vitro models. Also we will comment on the significance of mislocalized inflammasome components in the extracellular space and how it can be responsible for chronic inflammation.

Cell death and autophagy in tuberculosis

Mycobacterium tuberculosis has succeeded in infecting one-third of the human race though inhibition or evasion of innate and adaptive immunity. The pathogen is a facultative intracellular parasite that uses the niche provided by mononuclear phagocytes for its advantage. Complex interactions determine whether the bacillus will or will not be delivered to acidified lysosomes, whether the host phagocyte will survive infection or die, and whether the timing and mode of cell death works to the advantage of the host or the pathogen. Here we discuss cell death and autophagy in TB. These fundamental processes of cell biology feature in all aspects of TB pathogenesis and may be exploited to the treatment or prevention of TB disease.

Post-ischemic inflammation regulates neural damage and protection

Post-ischemic inflammation is important in ischemic stroke pathology. However, details of the inflammation process, its resolution after stroke and its effect on pathology and neural damage have not been clarified. Brain swelling, which is often fatal in ischemic stroke patients, occurs at an early stage of stroke due to endothelial cell injury and severe inflammation by infiltrated mononuclear cells including macrophages, neutrophils, and lymphocytes. At early stage of inflammation, macrophages are activated by molecules released from necrotic cells [danger-associated molecular patterns (DAMPs)], and inflammatory cytokines and mediators that increase ischemic brain damage by disruption of the blood–brain barrier are released. After post-ischemic inflammation, macrophages function as scavengers of necrotic cell and brain tissue debris. Such macrophages are also involved in tissue repair and neural cell regeneration by producing tropic factors. The mechanisms of inflammation resolution and conversion of inflammation to neuroprotection are largely unknown. In this review, we summarize information accumulated recently about DAMP-induced inflammation and the neuroprotective effects of inflammatory cells, and discuss next generation strategies to treat ischemic stroke.

The multifaceted nature of NLRP12

NLRs are a class of cytoplasmic PRRs with various functions, ranging from pathogen/damage sensing to the modulation of inflammatory signaling and transcriptional control of MHC and related genes. In addition, some NLRs have been implicated in preimplantation and prenatal development. NLRP12 (also known as RNO, PYPAF7, and Monarch-1), a member of the family containing an N-terminal PYD, a NBD, and a C-terminal LRR region, is one of the first described NLR proteins whose role remains controversial. The interest toward NLRP12 has been boosted by its recent involvement in colon cancer, as well as in the protection against some severe infections, such as that induced by Yersinia pestis, the causative agent of plague. As NLRP12 is mainly expressed by the immune cells, and its expression is down-regulated in response to pathogen products and inflammatory cytokines, it has been predicted to play a role as a negative regulator of the inflammatory response. Herein, we present an overview of the NLR family and summarize recent insights on NLRP12 addressing its contribution to inflammatory signaling, host defense, and carcinogenesis.